ERICSSON REVIEW

4 1984

Control System for AXE 10 Field Trial with CCITT No. 7 in Sweden Fault Location System ZAN 201 Subscriber Line Management System, SLMS160 SDS 80 - A Standardized Computing System AXT121 - A Digital Stored-Program-Controlled Exchange for Special Telecommunication Networks

ERICSSON REVIEW Number 4 1984 Volume 61

Responsible publ isher Gosta Lindberg

Editor Gosta Neovius

Editorial staff Martti Viitaniemi

Address S-126 25 S tockho lm, Sweden

Subscr ipt ion one year $ 16

Published in Swedish, Engl ish, French and Spanish with four issues per year

Copyright Telefonakt iebolaget LM Ericsson

Contents 146

156

162

170

178

186

Control System for AXE 10

Field Trial with CCITT No. 7 in Sweden

Fault Location System ZAN201

Subscriber Line Management System, SLMS 160

SDS80 - A Standardized Computing System

AXT 121 - A Digital Stored-Program-Controlled Exchange

for Special Telecommunication Networks

Cover A printed board assembly in the central proces­sor APZ 210 in the AXE 10 control system

Control System for AXE 10

Ingmar Jonsson

The basic system for AXE 10 was developed during the 1970s, and the first exchange was put into operation in 1977. Its control system, which consisted of duplicated central and regional processors, was designed to facilitate further development. A new central processor puts AXE 10 among the top systems on the market as regards capacity. A small processor intended for small and medium exchanges has also been developed. The author gives a summary of the functions included in the operating system and describes the development of the central and regional processors.

UDC 621 395 722:681 3 326 The control system, APZ, which forms part of the telephone exchange system AXE 10, consists of a powerful, syn­chronously duplicated central pro­cessor, a number of simpler regional processors and an operating system. The same control system is also used for telex and data exchanges (AXB20 and AXB30 respectively) and for mobile tele­phone applications.

The central processor in thefirst AXE 10 system, APZ210, which was taken into service in 1977, has a traffic handling capacity of 144000BHCA (Busy Hour Call Attempts). Two new central pro­cessors, APZ211 and APZ212, belong­ing to the same processor family, have been introduced. This has greatly in­creased the span between the lower economical limit of the system and its upper capacity limit. APZ211 is inten­ded for small and medium-size ex­changes. Its maximum traffic handling capacity is approximately 150000 BHCA. A complete central processor consists of 60 printed board assemblies, as against approximately 1500 equiv­alent printed board assemblies in the first APZ210 exchanges. The second new processor, APZ 212, raises the ca­pacity limit of the system to 800000 BHCA, which makes AXE 10 one of the most powerful telephone exchange sys­tems on the market. All central pro­cessors are fully compatible with all AXE 10 application software and hard­ware. In addition large parts of the oper­ating systems are common to all central processors. This makes the handling of the system largely independent of the central processor. New processor gen­erations can therefore easily be intro­duced into the system in step with new development. The operating system is programmed mainly in the high-level language PLEX.

Control system APZ 210 and the modu­lar structure of the AXE 10 software have been described previously.1 3

System structure AXE 10 comprises two systems, the ap­plication system APT and the control system APZ.

The purpose of the control system is to supply the application system with ap­propriate data processing power in a de­manding real-time environment. The basic aim when designing the system was to create a robust and flexible con­trol system that was easy to handle. This has resulted in a system having the fol­lowing characteristics: - A distributed processor structure

with an optional number of small re­gional processors connected to a powerful central processor.

- Synchronous duplication of the cen­tral processor. Regional processors with single or duplicated control of the application system hardware.

- A flexible and advanced I/O system.

The APZ processor has been structured with a view to combining high capacity and high reliability with good cost adap­tation by modular design. Fig 1 shows the system structure.

The powerful central processor, CP, is designed to execute complex functions, often of an analyzing or administrative nature. The regional processors, which are designed for simple, frequently used functions, are used for the immediate control of the hardware units in the ap­plication system. This structure permits a gradual extension of the processor ca­pacity as the AXE 10 system is extended.

Any processor can only read and write in its own memory. The communication between the central and regional pro­cessors is by means of transmitted mes­sages, signals.2

The application system hardware is organized in a number of extension modules, EM. The number of hardware units (devices) in different EMs varies depending on the type of device. There are two alternative methods for the con­trol of EM. A regional processor, RP, connected to the regional processor

147

INGMAR JONSSON

Public Telecomunications Division Telefonaktiebolaget LM Ericsson

Fig. 1 System s t ruc ture

CP MAU RPB RPBC ST-C ST-R EMRP, IO ETC RSS LSM

RP

Central processor Maintenance unit Regional processor bus Bus converter Central signal terminal Remote signal terminal Regional processor Input and output unit Exchange terminal circuit Remote subscriber stage Line switch module

bus RPB, can be used. RP is common for several EMs, which are connected to the processorviaabus, EMB. Alternatively a single or duplicated regional processor, EMRP, is used for each EM. Each EM is then connected to a bus, EMPRB, which is connected to RPB via a signal termi­nal. The signal terminal can be divided into two units, which can be connected via a data link. As a result control power can easily be obtained at a distance from the central processor, for example for controlling a concentrator.

The high reliability of APZ is ensured by duplicating all hardware down to the EM level, fig. 2. This means that a hardware fault in APZ will not normally affect the controlled equipment.

The central processor works in parallel synchronous mode with comparison of

the two CP sides. The maintenance unit MAU receives error signals from the two processor sides and determines, to­gether with the two sides, which side is to be the executive and which the stand­by.

Theduplicated RPbus(RPB) hasafixed connection to the two CP sides Each CP side transmits its operating status dur­ing signalling to a regional processor or signal terminal. The hardware in the RP or the terminal compares the operating status of the CP sides with its own status in order to determine with which CP side it is to interwork.

The regional processors normally work in pairs, so that each one usually con­trols half of the connected EMs. If a fault occurs in one RP, the other unit takes over all the connected EMs. However,

148

Fig. 2 Processor subsystem structure

duplication is not necessary for the con­trol of equipment with lower availability requirements.

The software in a function block can be divided into programs and data. One di­rect result of the system structure is that programs in a function block can only write and read data belonging to that particular block. All interworking be­tween different function blocks is in the form of strictly defined messages, sig­nals.2

Functions of the operating system The operating system in the central pro­cessor contains a number of general functions that provide efficient support for the execution of application pro­grams. The operating and maintenance functions for the central and regional processors are based on the extremely reliable hardware of the system. Its good handling properties are a result of the modular hardware and software struc­ture. Essential functions are imple­mented in microprograms, which makes it possible to optimise the system performance in the demanding real­time environment of telecommunica­tion applications.

Control of program execution The control of the program execution in the system is based on messages (sig­nals) transmitted between software

units. The signals are given different pri­ority levels when the software unit in question is being compiled. The system has four priority levels, which corre­spond to queues (job buffers) in the pro­cessor, with separate sets of registers for each level.

Powerful microprograms handle the signals when reading data into and out from the job buffers Each software unit can only reach its own data. Interwork­ing with other units can only take place via signals. Address calculations, and checks that writing and reading is not done outside the own data area are car­ried out with the aid of hardware. This ensures high reliability without loss of capacity.

Handling of modifications Extensions and addition or modification of functions often occur in telecom­munication plant. The operating system is equipped with extensive support functions for such alterations.

Each software unit with its associated data can be handled independently of other software units, since all refer­ences in the program are relative to the start of the program, and data are ad­dressed via a reference table, fig 3.

The size of data files as well as physical units (memories, regional processors etc.) can be changed during operation without causing any disturbance of nor­mal operation.

149

Fig. 3 Logic structure

(T) Active function block ® Instruction address ® Variable data W Instruction tor addressing data elements (S) Addressed data CPU Central processing unit PS Program store RS Reference store DS Data store ^ — Absolute address

Relative address

Function modifications mean the re­placement, removal or addition of one or more software units, or alternatively the whole software system. Regional software units and the central and re­gional hardware can also be modified. The system duplication ensures a high degree of safety also during such modi­fications. The central processor sides are separated before a modification is carried out on one side, and the other side then constitutes a standby which is quickly and automatically switched in, if a fault occurs in the new software or hardware units. It is also possible to transfer automatically unique exchange data from an old software unit to the new unit.

The procedure for changing a whole central processor, for example from APZ210toAPZ212, is very simple and is similar to the changing of a large soft­ware package, because all CPs in AXE 10 have similar structure and to a large extent contain the same functions.

In the case of software faults which must be cleared quickly, corrections defined at the assembly level can easily be input and activated by means of commands, both in central and regional software units. However, software faults are nor­mally cleared by updating and recompil­ing faulty source programs and then changing the software units in question.

Program testing Basic testing functions are built into APZ, so that the control system itself is the main aid for testing both software and hardware.

A system of tracer bits in the memory, together with special sensing circuits and microprograms, makes possible se­lective tracing of signal transmissions, data changes and instruction execu-tionsetc. in the central processor. When the preset conditions are met the infor­mation is stored without the tested pro­cess being affected. Aids are also avail­able for tracing data transmissions over the internal processor bus and for auto­matic recording of program jumps, sig­nal and interrupts.

Processor statistics Queue times and the load at different levels etc. can be measured and pre­sented to facilitate optimization of the system performance.

Handling of software faults The microprogram contains a large number of software execution checks. It also includes hardware monitoring to ensure that the program runs its normal course. If a fault is detected, the micro­program initiates software system res­toration, i.e. system restart. Special hardware ensures that the restoration process is started.

There are three types of system restart: - Minor system restart. The program

system is started from a defined point. Certain dynamic data are retained. This means that established connec­tions are not lost.

Fig. 4 Transparen t hardware fault handl

EX SB WO UP HA

Executive Standby Parallel operation Updating mode Stopped in the idle state

ng

- Major system restart. All dynamic data are reset and the program system is restarted.

- Major system restart with reloading. All memory information is reloaded from an external storage medium. A major system restart is then executed.

Handling of hardware faults The hardware in CP and RP includes an extensive system of supervision circuits for checking parity, time, voltages etc. The synchronous duplication of CP, to­gether with test programs that guaran­tee the activation of all hardware, also ensures that the probability of detecting intermittent or permanent hardware faults is very high. All hardware faults in CP can be dealt with by the system itself without any disturbance to the applica­tion system (transparent recovery). The procedure is as follows:

In the normal situation, the central pro-cssors are operating in parallel. This means that the two CP sides (CP-A and CP-B) execute exactly the same pro­gram at the microprogram level. CP-A (executive) normally executes a micro-cycle ahead of CP-B (standby). The two sides thereto re con tain exactly the same information in each register and all memory positions all the time. All data and a number of control signals (ad­dresses), which during the program ex­ecution are transmitted via the central processor bus (CPB) are compared by hardware in the standby side (CP-B).

When a hardware fault occurs in one CP side, the two sides no longer contain exactly the same information. The fault is detected because the execution of an application or test program involves the faulty hardware. The fault handling pro­cess illustrated in fig. 4 is then initiated. For most faults, supervision circuits in

the faulty CP side, such as parity and time supervision circuits, generate and send a fault signal to MALI which indi­cates the faulty CP side. If only one mis­match indication is obtained, MAU or­ders the CP sides to carry out a special test program independently. As soon as an indication of a faulty side is obtained, MAU orders the faulty CP side to stop. The side that is not faulty continues as the executive side. The whole process normally takes a few milliseconds, which from the point of view of traffic handling is negligible. If a fault occurs in CP-A, which is the executive side at the time, the system will therefore imme­diately switch over to CP-B as the execu­tive side. The execution of the applica­tion program continues, with the next program instruction being executed without any disturbance to the traffic handling. The regional processors auto­matically switch over the signalling to the new executive CP side.

At a low priority level the faultless CP side carries out comprehensive fault lo­cation in the faulty side. Powerful func­tions for indicating the faulty printed board assembly can locate both perma­nent and intermittent hardware faults. Parallel synchronousoperation isthena great advantage, since it is not possible to anticipate all possible fault situations when designing the diagnosis pro­grams. Data from parallel start attempts can instead be used for exact fault loca­tion.

After the diagnosis, an alarm printout is automatically initiated containing a list of a few possible faulty units (printed board assemblies). The system also con­tains functions that facilitate the subse­quent repair work. The maintenance functions have been described in detail previously.5

Fig. 5 Block diagram of APZ210

RPB Regional processor bus CPU Central processing unit CPB Central processor bus DS Data store PS Program store RS Reference store ALU Arithmetic and logic unit MIG Microinstruction generator DSH Data store handler PSH Program store handler RSH Reference store handler RPH Regional processor handler UPM Updating and matching unit BAM Buffer for MAU MAU Maintenance unit

IO-functions The man-machine language in AXE 10 is designed in accordance with CCITT-M M L 5 The communicat ion takes place via printers, visual display units or a data link to the maintenance centre. In order to simplify the man-machine communi ­cation further, a system has been de­veloped which is based on a personal computer, wi th a menu-orientated inter­face to the user

The I/O system handles alarm signals to the operating staff, such as the internal system alarms for operation and mainte­nance, and external alarms for, for ex­ample, ambient temperature.

Different types of files can be stored on magnetic tape or discs. The system can be equipped for the storage of data amount ing to between a few Mbytes and a Gbyte.

The data communicat ion funct ions are designed in accordance with the OSI principles formulated by ISO. Data links with speeds of up to 64kbit /s are sup­ported by the system.

Portable operating system The central processors in the APZ21 family are fully compat ib le with all ap­plication software. No source code modif icat ions have to be made in ap­plication programs when a change is made from one central processor to an­other. There is also ful l compatibi l i ty in the interface to regional processors and thus the appl icat ion system hardware. The CP hardware also has structural compatibi l i ty, i.e. all CPs are syn­chronously dupl icated and control led by a maintenance unit (MAU).

The operating system of the central pro­cessors consists of a large part which is independent of the hardware and a small core which is dependent on the central processor. However, the func­tions included in the processor-depen­dent part, e.g. programs for fault detec­t ion in the CP hardware, are based on the same principles in all operating sys­tems. The interface towards the opera­tors is thus kept uni form. This is illus­trated by the fact that more than 9 0 % of the operating procedures, commands and printouts for the control system are independent of the processor type.

Central processors APZ210 Central processor APZ210 is built up around a central processor bus, which distributes control signals and 16-bit data, f ig.5. A microprogrammed unit (MIG) controls the interworking of the other units via the data bus. The pro­gram store (PS), reference store (RS) and data store (DS) are placed in sepa­rate memory units. They are used in par­allel by the microprogram via autono­mous handlers, PSH, RSH and DSH: - PSH fetches machine instruct ions in

advance, divides them into suitable parameters for the microprogram and initiates RSH to calculate addresses for data in the data store

- RSH calculates addresses to data fields and checks the addressing so that illegal data access is prevented

- DSH reads or writes 16-bit data in DS and extracts or inserts shorter sub-variables.

The microprogram in APZ210 com­prises approximately 8 kwords of 24 bits each. The microprogram unit carries out

Fig. 6 Block d iagram of APZ 211

RPB Regional processor bus RPH Regional processor handler CPB Central processor bus BAC Bus control unit MS Main store (containing program, reference and

data store) CPU Central processing unit UPM Updating and matching unit MAU Maintenance unit

all work in CP, the execution of machine instructions as well as job administra­tion, and the signalling, both internally in CP and also to RP.

Parity control is used to check the mem­ory contents by means of one bit per 16 data bits. The ability of the system to withstand the effects of memory errors has been increased. The program and data stores each contain a spare bit, which can be switched in if an error oc­curs in one of the data bits in the store.

Central processor for AXE 10 APZ 210

Call handling capacity, BHCA

Mean instruction time, us

Number of software blocks

Block size in kwords of 16 bits per word

Number of signals/block

Number of RPs

Memory capacity DS (Mwords of 16 bits) PS (Mwords of 16 bits) RS

Per printed board assembly (kwords)

Relative floor space

Power consumption, W, for a typical exchange

Fan cooling

Construction practice

Printed circuit boards

144 000

3.2

4096

16

4095

512

8 4 256 kwc

64

1.0

3200

yes

row

ROF13

APZ 211

150 000

2.9

4096

16

4095

512

16 (DS + PS + RS)

256

0.12

350

no

row, cabinet

ROF13

Glass epoxy 4-layer printed boards Hole mounting

TTL74F, CMOS Gate matrices TTL bit-slice-processors Microprocessors

DRAM 64 (256) kbits

SRAM, PROM, EPROM

APZ 212

800 000

0.75

4096

32

4095

1024

64 32 1 024 kwords 40 bits

512

0.42

1700

yes

cabinet

ROF13

Metal base 4-layer printed boards Surface mounting

TTL74F Gate matrices

Microprocessors

DRAM 64 (256) kbits SRAM, PROM, EPROM

The central processor sides work semi-synchronously, which means that the standby side works with a lag of a vary­ing number of bus cycles. This is neces­sary because the CP sides work with separate oscillators and because the re­freshing of the memory is not syn­chronized between the CP sides.

APZ 210 has a 250 ns cycle on the central processor bus. The memory access time is 800 ns. 1 kbit memory capsules were used when APZ210 was originally de­veloped. More modern memory compo­nents have subsequently been sub­stituted, and 64kbit/s memory capsules are used at present.

CPU is built up on 28 printed circuit boards of type ROF16 (343x399mm). The printed circuit boards used for the memories are of type ROF13 (221x178 mm). The boards have sepa­rate foils for earth and voltage layers.

A number of limiting parameters in the system have been increased compared with the first version of APZ210, for ex­ample: Store volume for PS

Store volume for DS

Number of software blocks Block size

from 1 to 4 Mwords from 1 to 8 Mwords from 256 to 4096 from 4 to 16 kwords

APZ 211 The main aims when developing APZ211 were to obtain low cost, small volume and high reliability. Few repairs, and planned corrective maintenance are very important characteristics for exchanges in rural networks.

In APZ 211 the work at the lowest level is divided between a microprogrammed unit (CPU) and an intelligent regional processor handler, RPH, fig 6.

CPU carries out all instruction execu­tion, job administration and interrupt signal handling. RPH handles signalling to and from RP and loading. RPH has also been equipped with extended pro­tection against overloading in CP. In ad­dition RPH contains powerful support fuctions for fault tracing in an unloaded system.

153

Fig. 7 APZ211

CPU is built up of bit slice components. The microprogram has a width of 40 bits and a total volume of approximately 14kwords. Assistance with address cal­culation and the processing of variables (corresponding to DSH and RSH in APZ210) has been introduced in the form of gate array circuits in CPU. CPU also includes a rapid access store, which is used for reference store infor­mation that is frequently used.

RPH is based on a commercial 16-bit microprocessor. Each RPH can control up to 128 RPs. This is sufficient for most systems. If necessary, more RPHs can be added

APZ211 works with a 122 ns microcycle. The memory access t ime is 610 ns.

A complete dupl icated central pro­cessor contains 60 printed board as­semblies, all on boards of type ROF13, fig. 7.

The small quantity of hardware involved, together with the error correct ing code in the memory, means that there is no need to repair individual faults in CP im­mediately. Planned maintenance with a delay of up to one week before a repair is made gives an MTBSF (Mean Time Be­tween System Failures) value of 1000 years.

Fig. 8 Block diagram of APZ212

RPB Regional processor bus RPH Regional processor handler SPU Signal processor unit IPU Instruction processor unit PS Program store RS Reference store DS Data store MAU Maintenance unit

Since both CPU and RPH can control the central processor bus (CPB) the bus access logic is placed in a separate unit, BAC. CPB transmits 16bits of data and 32 bits of address information and is su­pervised by means of parity checks.

The three logic stores, PS, RS and DS are placed in a common physical main store, MS, in orderto minimizethequan-tity of hardware required. MS is built up of 64 kbit memory components. Both the hardware and the software are adapted for the introduction of 256 kbit DRAM.

APZ211 is built up of printed circuit boards with DIL components inserted in holes in the board in the conventional way. The boards contain four electrical layers. Heat dissipation is by means of natural convection.

The error correct ing code in the memo­ry gives error-free operation regardless of any permanent or intermitten single-bit errors in memory words. The system reliability is therefore only marginally af­fected by memory extensions, since the majority of memory errors are single-bit errors, which are completely concealed by the error correct ion funct ion.

When fetching RP signals, RPH checks the amount of data in the job buffers in order to protect CP in temporary over­load situations. If the amount exceeds a certain limit, RPH interrups the signal input until CP has cleared sufficient traf­fic to reduce the contents of the buffers.

RPH and the microprogram contain a number of hardware test programs (stored in a PROM), which are used dur­ing normal operation for cont inuous testing of the CP when there is spare capacity. These tests can also be init i­ated by an operator, by means of an I/O device, dur ing the product ion and in­stallation testing of CP and dur ing man­ual start of the system.

Repairs in APZ 211 are control led by one simple operating instruct ion for all types of faults in CP and MAU. Considerable support for the repair procedure is pro­grammed in CP.

APZ 212 APZ212 offers a very high traffic hand­ling capacity for present and future AXE 10 appl icat ions.

Fig. 9 Printed board assembly in APZ212 with memory unit for 1 Mbyte and ALU

Fig. 10 A CP side in APZ212

the CP capacity is used for administra­tion rather than for the execut ion of ma­chine instructions. The job administra­tion consists, mainly of signal and job buffer handl ing.

As a result APZ212 is divided into two processor units, a signal processor (SPU) and an instruct ion processor (IPU), f ig.8.

The instruct ion processor is fed jobs from the signal processor. While IPU ex­ecutes one job SPU prepares the next one in accordance with a set order of priority. IPU can therefore work solely with instruct ion execut ion.

IPU has three separate access paths to the stores PS, DS and RS. The bus width for IPU has been increased to 32 bits.

Another reason for the increased capac­ity compared with APZ210 is the exten­sive internal parallel and synchronous interworking between the funct ion units in IPU.

Signal processor unit. SPU SPU uses a specially designed 16-bit arithmetic and logic unit, ALU Fast op­eration is ensured by equipping SPU with several internal data buses which are used in parallel. SPU is micro­programmed. The microprogram has a width of 80 bits in order to facil itate par­allel operat ion.

Since SPU has few special administra­tive tasks, its activities are control led en­tirely by microprograms. SPU does not have separate program and data stores. The total microprogram volume is ap­proximately 2.5kwords.

Job buffers are located in a store con­nected direct to SPU. The store access time is 200 ns, which corresponds to one microcycle.

Instruction processor unit, IPU The purpose of the instruction pro­cessor is to execute machine instruc­tions as fast as possible. The limiting factor is the memory access times. The memory bandwidth has been increased by a factor of 2.5 compared with APZ210, by increasing the width from 16 to 32 bits and the clock frequency from 4 to 5MHz.

IPU uses the same ALU circuit as SPU Two circuits are connected in cascade, however, in order to obtain a width of 32 bits. This has made it possible to in­troduce a number of new machine in­struct ions which operate directly on 32 bits.

I P U i s b u i l t u p a r o u n d a number of inter­nal data buses. A high degree of parallel operation is supported by a wide micro­program (104 bits).

On average the machine instructions are exeuted four t imes faster in APZ212 than in APZ210. With the increase in capacity provided by SPU the total in­crease in capacity is approximately six­fold.

IPU contains hardware which pre-pro­cesses the machine instructions in three stages (pipelining). The execution of ordinary machine instructions takes only two cycles (400 ns).

Mechanical construction APZ212 contains surface mounted chip-carrier components. Bipolar gate array circuits of the TTL type are used in order to achieve a high packing density and the opt imum distr ibut ion of func­tions between the printed board assem­blies. The printed circui t boards are of the multi-layer type (metallic base), f ig. 9. This type has several advantages: - high immunity f rom interference be­

tween outer signal layers because of the inner layers used for voltage feed­ing and earth

- high packing density - short conductor lengths - small overall volume - few printed board assemblies.

The mechanical construct ion has re­sulted in a complete APZ212 central processor consist ing of less than 100 printed board asemblies, f ig. 10. The

155

Fig. 11 Hardware block diagram for the regional pro­cessor RP EMB EM bus PRO Processor unit MEU Memory unit RPBU Butter towards RPB RPB-A Regional processor bus towards CP-A RPB-B Regional processor bus towards CP-B

Fig. 12 Hardware block diagram for the regional pro­cessor EMRP EMRPB-A EMRPB-B DEVCB PRO MEU

Regional processor bus Regional processor bus Device control bus Processor unit Memory unit

References 1. Eklund, M. et al.: AXEW-System De­

scription. Ericsson Rev. 53 (1976):2, pp. 70-89.

2. Hemdal, G.: AXE 10-Software Struc­ture and Features. Ericsson Rev. 53 (1976):2, pp. 90-99.

3. Nilsson, B. A. and Sorme, K.: AXE10-A Review. Ericsson Rev. 57 (1980):4, pp. 138-148.

4. Ossfeldt, B. and Jonsson, I.: Recovery and Diagnostics in the Central Control of AXE. IEEE Transactions on Compu­ters. Vol.C-29, No. 6, June 1980, pp. 482-491.

5. CCITT Rec. Z.311-341. Yellow Book, Geneva 1980.

memory boards contain 1 Mbyte (in­cluding error correct ion) with 64 kbit DRAMs. The design is adapted for 256 kbit DRAMs.

Regional processors RP The RP executive system contains sup­port funct ions for - loading and al locat ion - dumping data - t ime-control led init iations of applica­

t ion programs - support for program testing - signall ing to CP via RPB - signall ing via EMB to the devices con­

cerned.

Connection to CP-A and CP-B is made via two simple printed board assemblies (RPBU), f ig. 11. RP also includes a mem­ory board for 256kwords and a pro­cessor board. RP is microprogrammed. Advanced gate array circuits in CMOS technology are used for interfaces to­wards EMB and RPB, and for ari thmetic funct ions, data manipulat ion and ad­dress calculat ion. The memory is built up of 256 kbit DRAMs.

Compared with the previous RP genera­t ion this means a considerable reduc­t ion in the amount of hardware, and at the same time the processor capacity is more than doubled. The new RP genera­tion is fully compatible with the applica­tion hardware and software.

EMRP and signal terminals ST The regional processor EMRP can con­trol either equipment placed in the ex­change or remote equipment. It is con­nected to CP via a signal terminal, f ig. 1. ST-C and ST-R are connected via the signal channel in a PCM system. When the equipment is placed in the exchange ST-C and ST-R form a unit, RPBC. The signal terminals contain commercial mi­croprocessors.

The fuct ions of EMRP are similar to those of RP. The programming is in the same high-level language as in CP, i.e. PLEX.

EMRP comprises two printed board as­semblies, f ig. 12. They can be placed in the same magazine as the control led

equipment. The processor contains a gate array circuit for connect ion to the bus EMRPB The program execut ion and control of devices is handled by two microprocessors. The memory consists of a separate printed board assembly for 128 kbytes.

Summary The two new central processors have greatly enlarged the range of AXE 10 while at the same t ime full compatibi l i ty towards the appl icat ion system has been retained and also the same system structure. APZ212 makes AXE 10 one of the most powerful systems on the mar­ket. APZ211, together with the reduc­tion in the volume of the regional pro­cessor hardware, extends the range of appl ication to include even very small exchanges.

This proves the effectiveness of the or ig­inal system principles, which were based on wide experience from earlier SPC system generations developed by Ericsson and the Swedish Telecom­municat ions Administrat ion. The con­trol system, with a synchronously dupl i ­cated pair of central processors which control a number of regional pro­cessors, ensures very good characteris­tics as regards robustness, capacity, re­liability and handl ing.

Further development of regional and central processors permits a reduct ion of the power, volume and number of printed circuit boards in the processors, and at the same time its reliability and capacity can be increased. Studies have shown that an increase in capacity by at least a factor of three is quite feasible if the need should arise. Moreover, the cost of a small system can be reduced even further. From the points of view of economy and capacity AXE 10 wil l there­fore be a competi t ive alternative for telephone exchanges from 1000 to 2 000000BHCA with the present-day range of funct ions. The latter f igure is merely given as a capacity reference. In­creases in capacity wil l be exploi ted pri­marily for large long-distance and tan­dem exchanges and for more complex funct ions, above all those that may be required in integrated services net­works.

Field Trial with CCITT No. 7 in Sweden

Alf Heidermark and Bengt Nordberg

CCITT signalling system No. 7 has been introduced in AXE 10. The system represents a new way of transmitting signals in telecommunications networks. The signalling for a very large number of circuits takes place over a signalling network built for this purpose. This network can also be used for other purposes than just the setting up of connections, for example for transmitting operation and maintenance information. The signalling system, which is in service in Saudi Arabia and Finland, will also be introduced in the Swedish telecommunications network. The Swedish Telecommunications Administration and Ericsson started a joint trial with the system in 1983 in order to obtain operational experience of the system at an early stage.

The authors describe the trial and the experience gained. They also give a summary of the Administration's plans for the introduction and development of the system in the Swedish telecommunications network.

UDC 621.395.38.001.55(485)

Fig. 2 A partly equipped magazine group for MTP, containing three signalling terminals, two multi­plexers and one RP pair. This equipment was used in the field trial exchanges. A fully equipped magazine group contains eight signalling termi­nals, two multiplexers and two RP pairs

ST Signalling terminal for signalling system No . 7 PCD-D Multiplexer for the connection of four STs to the

group switch RP Regional processor POU Power unit for the regional processor

Modern telephone networks with digital exchanges and digital transmission links (IDN) open the way to integration of services and functions (ISDN). Such ser­vices and functions, in their turn, result in the need for the transmission of great­er quantities of information between the nodes in the network. This demands effi­cient and flexible signalling. One signal­ling system that has the desired proper­ties is CCITT signalling system No.7, which was standardized by CCITT in 1980.2 The system and its implementa­tion in AXE 10 have been described in a previous issue of Ericsson Review.1

Signalling system No. 7 Signalling system No. 7 has a functional structure based on the separation of a common message transfer part, MTP, from a number of user parts, UP.

A user part, for example the telephone user part, TUP, can communicate with the corresponding user part in another exchange by means of MTP in order to establish a connection. MTP thus con­stitutes a communication network (sig­nalling network) in which the connected exchanges form nodes.

In AXE 10 the common channel signal­ling subsystem, CCS, includes the MTP functions. The TUP functions are in­cluded in the trunk and signalling sub­system, TSS, fig. 1.

Field trial with CCITT No. 7 Background In 1978 Ericsson and the Swedish Tele­communications Administration started a development project for introducing signalling system No. 7 in AXE 10. The main part of the design work was carried out by Ericsson. It soon became appar­ent that a field trial with the signalling system was desirable. In view of the large scale of the introduction of the sys­tem in the Swedish telecommunications network it was considered necessary to carry out afield trial in order to verify the design under operational conditions. In the autumn of 1981 a joint decision was made to start the field trial at the begin­ning of 1983.

Basic aims The basic aims of the field trial were to ensure safe introduction of the signal­ling system in the Swedish network and to gain experience of the actual com­missioning of the system.

These basic aims were divided into a number of concrete jobs: - verifying the design against the

CCITT recommendations for MTP and TUP, and verifying the interwork-ing with older signalling systems

- testing the system in actual operation - evaluation of the operation and main­

tenance functions of the system and operation manuals by the Administra­tion's regular operation and mainte­nance staff

ALF HEIDERMARK

Networks Department Swedish Te lecommunicat ions Admin is t ra t ion BENGTNORDBERG Public Te lecommunica t ions Div is ion Telefonaktiebolaget LM Ericsson

Fig. 1 Signalling system No. 7 in AXE 10 TUP in one exchange communica tes with TUP in another exhange via the signalling links in MTP. A signalling link is set up by a signalling terminal, which is connected through the group switch to a time slot in a PCM sys tem

ST Signalling terminal for signalling system No. 7 PCD-D Multiplexer with digital inputs for 64 kbit s ETC Exchange Terminal Circuit (for the connection of

PCM systems) TS Time slot in the PCM system RP Regional processor CP Central processor MSU Message Signal Unit SSS Subscriber Switching Subsystem GSS Group Switching Subsystem CCS Common Channel Signalling Subsystem TSS Trunk and Signalling Subsystem

157

- studying the availability of the signal­ling links

- verifying the calculated value for the capacity of a signal l ing link

- studying the effect of the STP (signal­l ing transfer point) funct ion on the processor load

- testing the operation of the system over different transmission media (cable and radio relay links)

- training operators and demonstrat ing the system with the field trial plant.

Preparations A working group with members f rom both Ericsson and the Swedish Tele­communicat ions Administrat ion was set up to prepare and conduct the field trial.

The group decided to carry out the trial at two AXE 10 exchanges already in op­eration, Ulriksdal and Djursholm in the Stockholm area. Digital transmission

systems between these exchanges could easily be made available for the tests. From these exchanges it was also easy to reach the Administrat ion's sys­tem test plant at Nynashamn, which was used as a signal transfer point (STP) over a 34Mbit /s digital radio relay link.

Separate AXE 10 exchanges were in­stalled for the tr ial, both at Ulriksdal and Djursholm, in order not to interfere with the ordinary operat ion of these ex­changes, and also to provide greater freedom of action for the tests. The trial exchanges were placed on the same premises as the ordinary exchanges, and they consisted of standard equip­ment both as regards hardware and soft­ware. This also applied to the equipment that is specif ic for signal l ing system No. 7, f ig. 2. For reasons of economy only the hardware needed for the trial was installed. Hence subscriber switch­es, for example, were not included.

Fig. 3 Link break generator A break generator was connected into the PCM system so that controlled breaks could be gener­ated in signalling links. The duration of the break could also be controlled

Fig. 4 Printout from the AXE 10 program test system The printout shows a CCITT7 message (MSU) which is sent from MTP (function block C7DR) to TUP (function block C7LABT). The message, which has a length of 13 octets, belongs to the user having the service information H'84, i.e. national TUP. The message is an initial address message, IAM (HO = 1, H1 = 1) which carries the six B-number digits 611001

The Swedish Telecommunications Ad­ministration was firmly of the opinion that the field trial ought to be based on an ordinary system package (applica­tion system) designed for the Swedish market, in order to subsequently facili­tate the general introduction of the sig­nalling system. This made the field trial more complicated and prolonged, since the application system was successively added to and modified.

Test specifications Separate test specifications were pre­pared for MTP and TUP in order to test that the CCITT recommendations2 were met. The MTP tests comprised: - Signalling link functions, such as ini­

tial alignment and signal unit error rate monitor, SUERM.

- All signalling network functions.

The TUP tests comprised: - Control and supervision of the con­

nection in accordance with Recom­mendation Q.724

- Interworking with older signalling systems.

The operation and maintenance instruc­tions were evaluated in simulated typi­cal operating situations, in which the in­structions were used.

Tests were carried out to check the effect of various types of system faults, for example a link break and restart in the interworking exchange.

The calculated values of the capacity and availability of the signalling link were verified, and also the effect of the STP function on the processor load.

Test aids Two simple test aids were developed for the field trial, a test user part and a link break generator.

The test user part, which is implemented in software for the AXE 10 central pro­cessor, acts as a user part (UP) towards the message transfer part (MTP). It can generate messages (MSU) to the test user part in the interworking exchange. The user data in the message consist of a sequential number, which is analyzed in the test user part in the receiving ex­change. A printout is obtained if a mess­age is received out of order or is dupli­cated or lost. This aid was most useful for verifying the signalling network functions and link capacity.

The link break generator was connected into the PCM system between the ex­changes. A break of optional duration

To the telecommunications network 159

could be generated. The generator, fig. 3, was used to test the error rate monitor, SUERM.

The program test system built into AXE 10 was also used in the field trial. With this system, commands can be used to record a number of events dur­ing the program execution, for example the transmission of software signals be­tween function blocks or the changes in the data store contents of a function block. Fig.4 shows an example of the facilities provided by the test system. It shows the printout obtained when an initial address message (IAM) is trans­mitted from the message transfer part (MTP) to the telephone user part (TUP) for an incoming call in to the exchange.

Trial network Fig. 5 shows the network used in the tri­al.

Most tests were carried out in the basic network, which consisted of the signal­ling points Ulriksdal-F and Djursholm-F, connected via two PCM systems con­taining two signalling links. Thefield tri­al exchanges were also connected to the ordinary AXE 10 exchanges via two PCM systems. These connections were used, for example, when the ordinary telephone-traffic between the Ulriksdal and Djursholm AXE 10 exchanges was

routed over the CCITT No. 7 route be­tween the field trial exchanges and when the interworking with older signal­ling systems was tested.

Signalling links were established be­tween the trial exchanges and an AXE 10 exchange in the Swedish Telecom­munications Administration's system test plant at Nynashamn in order to be able to test the signalling network man­agement functions that are used when the signalling traffic between two sig­nalling points is routed via a signalling transfer point (STP).

Results The field trial showed that the system has the operational reliability required for large-scale introduction into the Swedish telecommunications network. The Swedish AXE 10 exchanges will be supplemented with signalling system CCITT No. 7 during 1985. The implemen­tation plans and the structure of the sig­nalling network are described in the next section.

The trial provided valuable experience of the system. Some software faults were found, of course, but they have been very few in view of the large vol­ume and great complexity of the pro­grams. This indicates that the quality of the product was already high at the start of the trial.

A number of programming errors were found that had not been detected during the design verification in the laboratory. However, it was possible to eliminate these errors by means of program cor­rections.

The CCITT recommendations for signal­ling system No. 7 were interpreted dif­ferently by the designers and the au­thors of the specifications in three re­spects. The first of these was at which moment changeover to a standby link should take place if a failure should oc­cur in a PCM system which contains a signalling link that carries traffic. The second concerned the action to be taken in case of abnormal incidents in the TFP (transfer prohibited) procedure, and the third referred to the method of blocking the speech channels if a failure should occur in the PCM system con­cerned.

Fig. 5 The trial network

Speech circuits

Signalling links

Regular AXE 10 exchange

Field trial exchange

Signalling point1

Signalling transfer point1

Fig. 6 The s t ruc ture of the Swed ish s igna l l ing network

The availability of the signalling links was measured during a long-term test lasting 60days. The measurement was carried out on the two signalling links between Ulriksdal-F and Djursholm-F. The availability during the 60 days (and nights) was 0.999902, which is well with­in the limit of 0.99936. At least one of the links was always available during the test period, which means that the pos­sibility of communicating with the inter-working exchange was never lost.

The ability of a signalling link to transfer messages generated by users (MSU) varies depending on the length of the messages. The capacity is 0.4Erlang (1 Erlang = 64 kbit/s) for short messages and 0.2 Erlang for long. On the link the difference between 0.4 and 0.2 Erlang and 1 Erlang is made up with "Fill In Signal Units", FISU.

It was verified during the field trial that the transiting in an STP of 25 message signal units(MSU) persecond increases the load on the central processor (APZ210) by approximately 1 %. This load increase corresponds to approx­imately the traffic generated from 600 circuits, depending partly on the signal­ling procedure and the mean load per circuit.

The ordinary operation and mainte­nance instructions for the system were used in the trial. The comments made by the Administration's regular operating staff regarding the instructions pro­vided valuable information for subse­quent improvements of the manuals.

Introduction in Sweden Background In 1981 preparations were startedforthe introduction of signalling system No.7 in the Swedish telephone network. An investigation had shown that this was the most suitable signalling system for the modernized telephone network. The work comprised the preparation of a structure, dimensioning rules and rout­ing rules for the signalling network and drawing up a plan for the introduction.

Signalling network The structure chosen for the Swedish signalling network is based on non-as­sociated signalling, fig.6. Non-associ­ated signalling means that the signals associated with a connection are trans­mitted over a different path from that used for the transmission of the speech or data.

The present Swedish signalling network contains STP at two levels. The upper level contains four national STPs, placed in Stockholm and Gothenburg. Regional STPs, which constitute the lower level, are connected to the na­tional STPs. The regional STPs work to­gether in pairs, but each has the capaci­ty to handle all the signal ling traffic if the other STP fails. There are two alterna­tives for the connection of a regional STP pair to the national STPs: - The regional STP pair is connected to

all national STPs - The regional STP pair is connected to

two national STPs.

The first alternative is used when the STP pair transits signalling traffic that is destined for signalling points (ex­changes) throughout the country. The other alternative is used when the ma­jority of the traffic handled by the re­gional pair is destined for two national STPs. Each signalling point is normally connected to two STPs via one signal­ling link to each.

Plan of introduction The signalling network will be put into operation early in 1985. The procedure will be as follows: - Signalling links are established be­

tween four national STPs. - Regional STPs are connected to na­

tional STPs.

National STP

Regional STP

Signalling point

Link between a signalling point and an STP

Link between a regional and a national STP

Link between two national STPs

Link between two regional STPs

161

Fig. 7 Members of the field trial group at work in the Djursholms field trial exchange

- The signalling points are connected to their regional STPs.

Table 1 shows the planned rate of exten­sion for the signalling network.

Initially the signalling network will only be used for telephony. The range of functions provided by the Swedish tele­phone user part is similar to that provided by the CCITT TUP. The intro­duction of the signalling network will make it possible to introduce new func­tions into the telephone network in the next few years. First a number of sub­scriber services that are now only avail­able for use within the subscribers own area, for example call back, will be of­fered also outside the local area. Func­tions will also be introduced to support early ISDN services.

At the end of 1985 the mobile telephone exchanges (MTX) will be connected in as signalling points in the signalling net­work. Initially the signalling system will be used in the interworking between the mobile and the fixed telephone net­works. Later on the system will also be used for the roaming function in the mobile telephone network.

Some of the applications mentioned above and several others, for example those that are built up with databases, require information to be transmitted between nodes without physical con­nections having been established be­tween them. The requirements made on such applications are summerized in the recommendations for SCCP (Signalling Connection Control Part). These func­tions, which are included in the CCITT recommendations of 1984 (Red Book), will be introduced in AXE 10.

Summary A field trial with the CCITT signalling system No. 7 in AXE 10 has been carried out by the Swedish Telecommunica­tions Administration and Ericsson. The experience gained from the trial shows that the system has the operational re­liability required for large-scale intro­duction. In the Swedish telecom­munications network it will constitute the regular signalling system between AXE 10 exchanges An extensive signal­ling network will thereby be built up in Sweden as a step towards an integrated services digital network, ISDN.

Table 1 The rate of extension of the Swedish signalling network

Date 1985-06 1986-06 1987-06

Number of STPs 8

12 14

Number of SPs 43 57 71

References 1. Du Rietz, J. and Giertz, H.: CCITT Sig­

nalling System No. 7 in AXE 10. Erics­son Rev. 59 (1982):2 pp. 100-105 .

2. CCITT Yellow Book Vol. VI. 3. Bi l ls t rom, O. and Troi l i , B.: A Public

Automatic Mobile Telephone System. Ericsson Rev. 57(1980):1, pp. 2 1 - 3 6 .

Fault ion System ZAN 201

Ulf Silvergran and Kidane Woldegiorgis

Fault location system ZAN201 is used for the supervision of transmission

networks, for fault location in these networks and for making measurements on

the transmission links. The system can either be used autonomously or it can be

connected to the transmission maintenance system ZAN 101, the operation and

maintenance system AOM 101 or direct to the operation and maintenance

subsystem in AXE 10.

The authors explain the need for such a system and describe its applications and

functions and the design of the equipment.

UDC 621 395 74 004 5

Fig. 1 Centralized fault location in a digital transmission network

ZAM565 Line system for 565 Mbit/s over optical fibre ZAM 140 Line system for 140 Mbit/s over optical fibre ZAD 2 Line system for 2 Mbit/s over pair cable

Line systems containing electronics in the form of geographical ly scattered re­peaters have always required some form of equipment that makes possible the location of faulty units. In Er icssons first digital line system the trio method was used, which was based on the trans­mission of special pulse patterns (trio pulses) that contained both address in­formation and a test signal. The main disadvantage of the method was that it could not be used unless the line system was taken out of service.

A fault location system which permits measurements on line systems while they are in service was developed for Er icssons digital line systems in the M5 construct ion practice. The fault detec­tion system was developed for the 8Mbit /s line system for pair cable and was later adapted for 2Mbit /s on pair cable and 140 Mbit/s on coaxial cable. The system can also be used for fault location on the 34 Mbit/s line system on optical fibre cable as long as the opto cable contains copper pairs.

The new fault location system ZAN201 has been developed primarily as a gen­eral fault location system for all digital line systems in the BYB construction practice. It can also be used for certain older line systems in the M5 con­struct ion practice. The system includes a number of new funct ions and particu­lar importance has been attached to user-orientated man-machine commu­nication. Due to its large address capac­ity and special equipments for building up complex fault location networks the system can be used for supervising large digital transmission networks from a central work station.

A centralized fault location system like ZAN 201 has the fo l lowing advantages: - The maintenance personnel can be

employed more efficiently since they can handle the maintenance of large networks. Furthermore the personnel retain their competence since the fault location system is used more often because of the size of the net­work.

- Owing to the eff iciency of the system, quality control can be carried out when services other than pure speech transmission, for example data trans­mission, become more common. Such services demand higher quality than that provided by the alarm limits specified by CCITT

Reasons for developing the system The development of ZAN 201 was prompted by the need to have a fault location system that - could be used with any type of line

system, both metallic over separate pairs or coaxial tube and optical over fibres

- uses the same construct ion practice as the line system, so that they can be mounted together

- has suff iciently large address capaci­ty, range and resolut ion (bit error rate)

- has simple and efficient man-ma­chine dialogue

- could be connected to ZAN 101 and AXE10

- includes such funct ions as alarm col­lection and remote control

- could be used as a measuring instru­ment when making measurements on line systems.

163

\

ULFSILVERGRAN

KIDANE WOLDEGIORGIS Fibre Opt ics and Line Transmiss ion Telefonakt iebolaget LM Ericsson

Field of application ZAN201 can be used in different types ofdig i ta l transmission networks. The system can work autonomously or be connected to ZAN101 or AXE 10. The system can also be connected to main­tenance systems of different manu­facture since it has a CCITT interface (V.24/V.28).

How the system is uti l ized depends on the type of digital transmission network and the adminictrat ion's maintenance organization. However, there are two basic types of appl icat ion: - Fault location in a hierarchic digital

metropol i tan network or part of the digital long-distance network. When the responsibil i ty for the mainte-

Fig. 2 ZAN201 in an AXE 10 network

2 Mbn s intermediate repeater station with a fault detector and fault location pair

FLM Fault location magazine RSM Remote subscriber multiplexer RSS Remote subscriber switch

nance of such a network is divided among many regions and the fault lo­cation systems overlap several re­gions, each region can be equipped with a terminal giv ing access to the relevant system.

- Fault location in digital transmission networks in an AXE 10 network.

ZAN201 used in a hierarchic digital transmission network A fault location system ZAN201 with a centrally sited fault location magazine, FLM, and a work station can supervise an extensive digital transmission net­work having a hierarchic structure, f ig. 1. Central sit ing of FLM means that the operator has access to alarm infor­mation from the terminal exchanges. Alarm col lect ion units, ACU, can be placed in such exchanges in order to forward alarms to the work stat ion.

FLM can be control led either direct f rom its own control unit or f rom one or more operator terminals connected to stan­dardized (V.24/V.28) interfaces. Up to five operator terminals in the form of display units or typewriters can be con­nected.

A modem repeater magazine, MRM, can be used at suitable posit ions in the net­work in order to exploit the large ad­dress capacity of FLM and to give com­plicated fault location networks a f lexi­ble structure. MRM contains a number of funct ions, which are described later.

ZAN201 in AXE 10 networks An AXE 10 network for digital transmis­sion and compris ing an AXE 10 parent exchange and remote units, RSS and RSM, can be supervised by ZAN201, f ig. 2. An FLM connected to the AXE10 parent exchange can be control led f rom the AXE 10 operator terminal by means of the same command language as is used for AXE10 and AOM101. The ex­change staff can therefore carry out fault location on the transmission equip­ment without having to be transmission experts. If AXE 10 is connected to AOM101 the FLM can be control led f rom an AOM101 work station, f ig. 3. FLM is connected to AXE 10 via a V.24 interface. In such a network the alarm col lect ion can either be performed by AXE 10 (external alarm funct ion EXALl) or with the aid of ACU.

Fig. 3 ZAN201 in an AXE 10 network

Control interlace

Fault location interlace

Fig. 4 ZAN 201 as a subsys tem of ZAN 101

ZAN 201 used as a subsystem in ZAN 101 ZAN201 is a subsystem of ZAN 101, fig. 4. A ZAN 201 system can at a later stage be upgraded to ZAN 101 operation without any equipment becoming re­dundant or any reallocations having to be made to previously installed equip­ment or in the main distribution frame.

Mobile FLM A mobile variant of FLM can be con­nected anywhere in a fault location net­work. For example, such an FLM can be connected to a fault location channel via an FDU in an intermediate repeater housing. It is therefore possible for the repairmen to verify that a repair has been successful before they leave the site. It will then not be necessary to send a person to a terminal exchange in order to verify the repair.

Flexibility and reliability in the fault location network Up to six fault location channels can be connected to FLM. Each channel can be branched so that star and tree-shaped fault location networks can be estab­lished. Each fault location channel can have up to six terminated branches, fig. 5a. However, further branches can be arranged using a modem repeater magazine.

The availability can be increased by loop connecting the fault location channels in accordance with fig. 5b, thus provid­ing an alternative path to any point in the case of a break. Figs. 5c and 5d show the use of a modem repeater magazine for automatic routing in two different net­works, where the alternative paths may consist of channels in two parallel line systems or channels in two different routes.

Several FLMs can be connected to the same fault location network without any risk of mutual interference. Before oc­cupying a channel, an FLM will first check that the channel is free.

Man-machine communication In an autonomous ZAN 201 system there are two alternative operator interfaces: - Simple control display unit (CDU),

mounted on the front of the FLM - Visual display unit (VDU) ortypewriter

(TTY).

Both types of interface have simple and fast man-machine communication. This has been achieved by - using a dialog procedure for the input

of data, whereby the system requests the necessary input data from the op­erator

- retaining previously input parameter data in the system, so that they need not be fed in again in cases where they ar used repeatedly

- positive diagnosis of user errors, i.e. the system states exactly what is ex­pected in the form of input data, not just that a parameter is faulty.

The operators work is made easier when operating via CDU by the use of a special logging function, which is auto­matically connected in for commands that can generate a large amount of out­put data. The operator can then com­plete the measurements and thereafter request the results at a suitable speed, and hence there is no need to watch the result panel all the time that work is in progress.

Functions The main functions of ZAN 201 are - fault location - bit error rate measurement - alarm supervision - remote control.

Fault location The fault location function is designed to locate faulty units in a line system. The unit can be either a terminal or an intermediate repeater. Fig.6 shows a fault location order with its result printout. The input data consist of chan­nel number, bit error rate threshold and an address specification for the re­generators to be checked. The bit rate rate threshold defines the lowest rela-

165

Fig. 5b Looped fault location channel

Fig. 5c Channel with a modern repeater magazine con­nected to give alternative path selection inwards

Fig. 5d Modem repeater magazines connected up to provide alternative path selection in both direc­tions

hold defines the lowest relative contri­bution to the bit error rate that has to be exceeded before a fault indication is given. The type of equipment placed in the specified address area is identified before fault location work starts. The system then calculates the measure­ment time per unit that is needed in order to obtain reliable measurement re­sults, based on data unique for the type of equipment, such as bit rate and the relevant bit error rate threshold.

In the case of fault location in line sys­tems on optical fibres and coaxial ca­bles, certain other fault criteria (in addi­tion to the bit error rate threshold) also apply. This is because the fault location signalling uses the same transmission medium as the supervised signal.

Fault location can be carried out in two different ways, either as a single sweep through the specified address area or as a cyclic procedure in which the address area is scanned until a fault is found. The second method makes it easier to detect intermittent faults. In this mode the sys­tem also gives the date and time when the fault is located.

Bit error rate (BER) measurement The BER measurement function is used to measure the bit error rate of a number of terminal or intermediate regenera­tors, but not for the purpose of locating an existing fault. This function is there­fore well suited for quality and mainte­nance measurements on line systems. There are three different measurement modes: - normal mode - simultaneous mode - supervision mode.

In the normal mode, the absolute and relative bit error rates are measured by ordering two fault detector units to mea­sure simultaneously on one regenerator each. This provides a true relative bit error measurement for one test object at a time.

In the simultaneous mode, a special ad­dressing procedure gives simultaneous

Fig. 6 An example of cyclic fault location

Fig. 5a Branch ing of a faul t locat ion channel

FLM Fault location magazine MRM Modem repeater magazine

166

Fig. 7 An example of BER measurements in the supervi­sion mode

Fig. 8 Printout from manual alarm collection

Fig. 9 An example of remote control

measurement of all test objects. The si­multaneity gives a higher degree of re­liability of the relative error rate results. In both the normal and simultaneous modes it is possible to request inter­mediate results during lengthy mea­surements.

Bit error measurements in the supervi­sion mode make it possible to check one specific regenerator, fig. 7. The mea­surements comprise absolute bit error

rate, bit error rate relative to a specified reference object and the number of de­tected bit errors during an optional re­porting interval. Printout can also be suppressed when the number of bit er­rors is smaller than a certain, optional value. This "filter function" ensures that interesting information, such as inter­mittent error bursts from a regenerator, is not submerged in a large mass of in­formation. The printout also shows the time when detection takes Dlace.

Fig. 10 A printout trom an auto test. IN LOOP MODE indicates that the FDU is set to test a loop connected repeater

END

Alarm supervision Supervision of alarms with ZAN201 can take place in three different ways: - manual alarm collection - single alarm location - cyclic alarm location.

With manual alarm collection the opera­tor requests the current alarm picture from a specified alarm collection unit (ACU), fig. 8. The alarm picture is dis­played in a way that is closely connected with the physical appearance of the con­nectors in the alarm collection unit.

With single och cyclic alarm location the oparator gives an address specification for the alarm collection units to be scan­ned, after which the system locates the units that report a change in alarm sta­tus. The alarm picture from the located unit is displayed to the operator, who acknowledges the alarm report by de­pressing a button.

All alarm collection units are also con­nected via a special alarm report chan­nel to an alarm report receiver (AIU), which by means of optical and acoustic signals informs the operator that a change of alarm status has taken place.

Remote control Remote operation of control points can be carried out via ZAN 201 if one or more remote control units (RCU) are con­nected to the fault location channel. Each RCU operates 4x4 relay contacts.

Fig. 9 shows an example of remote con­trol. The operator indicates the RCU and the group of four contacts to be oper­ated, and the system displays the cur­rent relay status and requests a control command. The status after operation is then displayed for the operator, who is requested to give an acknowledgement if the command is to be executed.

Other functions In addition to the main functions de­scribed above the system contains cer­

tain special functions, such as: - diary-controlled measurements - auto test - restart of the fault location channel.

Diary-controlled measurements An internal system clock in FLM is used to start measurements automatically at a preset time, and at an optional interval if the measurements are to be repeated. This makes it possible to carry out regu­lar performance measurements auto­matically on a number of line systems at daily, weekly or monthly intervals.

The diary function can also be used to start time-consuming routine measure­ments at night, and thereby increase the availability of the system in daytime.

Auto test ZAN201 monitors a number of internal hardware and software functions. Any faults in the FLM write and read stores are immediately reported to the opera­tor, whereas faults in peripheral units, such as FDU and ACU, are reported in connection with routine communica­tion with the units. Equipment faults in FLM are indicated at the time of system restarts, for example when the power is switched on.

A special auto test function can also be used to check the status of a number of specified peripheral units, fig. 10. Since the auto test printout also contains in­formation regarding the type of super­vised equipment it is possible to verify the supervised network picture against current documentation. This could be of importance, since ZAN201, unlike ZAN1012, has no network data stored internally.

Restart of the fault location channel There are two alternative methods for restarting all the peripheral units along a fault location channel from a single op­erator's position: - soft restart - hard restart.

Fig. 11 The fault location magazine, FLM

Fig 12 A fault detector unit for 2 Mbit s over pair cable

Fig. 13 The fault detector unit for 140 Mbit/s over coaxial cable, mounted on an intermediate repeater

With soft restart a col lective restart com­mand is sent to the channel, and it is assumed that all the units addressed are funct ioning correctly.

Hard restart means that a tone is sent out on the channel for approximately 3s. The tone initiates a physical restart of all the units connected to the channel.

Equipment The fault location system consists of a fault location magazine, FLM, which constitutes the central equipment. Fault detection units, FDU, are installed in each station containing repeaters. Dif­ferent types of FDU are used for different line system hierarchies. All premises containing supervised equipment are each equipped with an alarm col lect ion unit (ACU), to which all the alarm signals are connected. Remote control of dif­ferent funct ions is arranged by equip­ping remote stations with remote con­trol units, RCU. Modem repeater maga­zines, MRM, are also important network components.

FAULT LOCATION MAGAZINE, FLM The fault location magazine, FLM, fig. 11, constitutes the central equip­ment in ZAN201 and is preferably placed centrally in the network. Up to six independent communicat ion channels can be connected to FLM. Each channel can handle up to 255 FDU, ACU and RCU.

Up to five operator terminals can be con­nected via V.24 interfaces to an FLM. Current loop interfaces can also be ob­tained f o r two terminals, which increase the range within the station. One of these interfaces is intended for a 300 baud dial-up connect ion. FLM has a robust communicat ion interface to­wards the fault location channel. A 300 baud modem is used, which works in the halfduplex mode, with 600/ 1200Hz frequency shift. Channels capa­ble of transmitt ing the speech band can thereby be used.

MODEM REPEATER MAGAZINE, MRM MRM is used for different purposes in a fault location network, for example to ensure flexibil i ty and greater avail­ability. MRM can be used to provide - signal ampl i f icat ion of the fault loca­

t ion signal, if it is subjected to high attenuation

- signal regeneration of the fault loca­t ion signal, if it is subjected to at­tenuation or pulse distort ion

- automatic choice between two sig­nall ing paths as regards first-choice and second-choice route

- reduction of the effect of impedance mismatching when the fault location channel is branched

- power feeding of FDU - separation of physical pairs and chan­

nels to give protect ion against induc­ed overvoltages

- conversion of 300 baud fault location interfaces to V.24 interfaces, in order to make possible transmission of fault location signals over leased or switched circuits.

FAULT DETECTOR UNIT, FDU Different types of FDUs serve different hierarchies and variants of line systems.

FDU for 2 Mbit/s over pair cable An FDU for 2 Mbit/s over pair cable uses a separate physical fault location pair. Two types of FDUs are used. An FDU that is power fed from FLM or MRM is used in repeater housings, f ig. 12. For terminal repeaters the FDU funct ion is incorpo­rated in the supervision unit SU and serves all terminal repeaters in a maga­zine. Each such FDU has a unique ad­dress among the 255 available. FDUs in intermediate repeaters also give access to a service telephone for communica­tion with terminal stations. Loading coils for both the fault location pair and the service pair can be connected in by strapping in the FDU.

FDU for 140 Mbit/s over coaxial cable An FDU for 140 Mbit/s over coaxial cable communicates wi th FLM by means of the same coaxial tube that is used to transmit the digital signal. However, it is necessary to convert the 600/1200 Hz signal f rom FLM to a 14/16kHz band. The conversion is carried out in an inter­face adapter (IA), mounted in a line ter­minating magazine, which can serve up to three line systems.

One FDU is required for each two-way repeater. All FDUs in a housing have one and the same address out of the 255 available. The FDU is screwed to te top of the repeater, f ig. 13.

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Fig. 14 A fault detector unit for 140 Mbit/s over optical fibre

Techn i ca l data FLM Maximum number of fault location channels per FLM Maximum number of FDUs. ACUs, RCUs per fault location channel Terminating impedance of the fault location channel

Communication FLM-FDU, ACU, RCU Transmission rate Modulation Frequencies Transmission of informa­tion

255 600 or 1200 ohms or high-im­pedance

300 baud FSK 600/1200 Hz asynchronous halfduplex

FDU for 2 Mbit/s over pair cable Maximum number of one­way repeaters per FDU 96 Loading of fault location 118 mH, can be pair strapped out

FDU for 140Mbit's over coaxial cable Maximum number of one­way repeaters per FDU 2 Maximum number of FDUs that can share the same FDU address 8 Fault location channel over the coaxial tube Type of modulation FSK Frequencies 14/16 kHz

FDU for optical fibre line systems An FDU for optical f ibre line systems communicates with the FLM over the same fibre that is used to transmit the digital signal. Two types of FDUs are used. The FDU installed in the terminal repeater, f ig. 14, utilizes the FLM600/ 1200 Hz baseband signal for low-fre­quency modulat ion of the optical signal. The FDU used in the intermediate re­peaters is of another type, in which the fault location signals do not have to be converted to 600/1200 Hz. Each FDU serves one two-way repeater.

ALARM COLLECTION UNIT, ACU Alarms from the transmission equip­ment and other equipment are con­nected to the alarm interface in ACU. The unit scans the alarm inputs cont inu­ously, and if a change in alarm status occurs, an alarm report signal is sent to the terminal station and forwarded to the operator's terminal. The alarm re­port signal is transmitted over a sepa­rate channel. In optical f ibre systems this signal can be transmitted over the fibre. When the operator receives the alarm report signal, he can address the ACU and obtain information concerning the changed alarm status.

4 x 7 (28) alarms can be connected to an ACU, which occupies one address out of 255. Both simple relay or t rans is torear th closures and alarms at TTL levels can be connected to an ACU

In order to increase the availability of an ACU it can be reached via two alterna­tive paths, with automatic path selection in the unit.

ALARM REPORT RECEIVER, AIU The alarm report receiver, AIU, provides optical and acoustic indicat ion of any change in alarm status detected in any ACU. AIU can also be used as a con­centrator for several alarm report chan­nels and to transfer signals to other AlUs.

REMOTE CONTROL UNIT, RCU RCU can be used for such operations as starting diesel generators or carrying out changeovers in o rder to facil i tate re­mote control . An RCU provides up to 16 relay closures, organized in four groups of four relay contacts each. Like ACU the RCU can be control led via two alterna­tive paths, with automatic path selection built into the RCU.

FDU for 140 Mbit/s and 565 Mbit/s over optical fibre Maximum number of one­way repeaters per FDU 2 Maximum number of FDUs that can share the same FDU address 32

ACU Number of alarm inputs per ACU, max. organized into Alarm interface (for a group of 8) - general

- alarm interface for trans­mission equipment in construction practice BYB

- alarm interface for 2Mbit/s line terminal

RCU Number of contacts per RCU, max.

28 four groups of 8

7 closures to earth

1 closure to earth 6 TTL levels 2 closures to earth 5 TTL levels

16

References 1. Eneborg, M. et al.: Integrated Mainte­

nance of Transmission Systems in AXE 10 Networks. Ericsson Rev. 61 (1984):1, pp. 10-17.

2. Eneborg, M. and Johansen, B.: Trans­mission Maintenance System ZAN 101. Ericsson Rev. 61 (1984):1, pp. 18-25.

Subscriber Line Management System, SLMS160

Jan Kjonigsen

A/S Elektrisk Bureau, has been working on matters relating to the operation and maintenance of the telecommunication network for a long time, and has supplied O&M equipment to the Norwegian Telecommunications Administration. By the end of the 1970s a need had arisen for new and more modern equipment, preferably computerized, for remote measurements on subscriber lines. Development work was started on SLMS 16, which is merely a measurement system. In a few years this system will be in use throughout Norway. SLMS 16 has also been sold abroad and. particularly in the foreign markets, demands have been made for facilities for storing subscriber and line data, fault histories etc. These demands resulted in further development of a database for SLMS 16. The new system is designated SLMS 160

The author summarizes first the problems that are encountered with a traditional fault complaints service and then the demands that should be made on a modern subscriber line management system. This is followed by a description of SLMS 160.

UDC621 395 74 004:681 3 During the last decade, the wor ld has experienced a very large growth in tele­communicat ions. This applies in most countries regardless of their level of de­velopment. As a result all countr ies be­come aware of the need for more relia­ble networks and the importance of im­proving or introducing operation and maintenance systems in their networks in order to obtain better - service quality - operating economy - administrative routines.

Problems with a traditional fault complaints service As the telephone network in a country grows larger and more complex, the problems associated with the operation and maintenance of the subscriber line network grow more acute. Briefly the main problems are that - an increasing number of highly skill­

ed employees are tied up with the maintenance of the subscriber line network

- repairmen are often dispatched un­necessarily

- a large quantity of paper is required to administer the operation

- it is very dif f icult to keep the informa­tion concerning the network up to date.

As a result it is becoming more difficult for administrat ions to maintain the quality of service for the subscribers without the operating costs growing too large. At the same time improvements resulting f rom development in switch­ing technology have led to greater de­mands on administrat ions for better op­eration and maintenance functions.

It is therefore obvious that the tradi­tional methods of subscriber line man­agement wil l not be able to meet these new demands or to cope with the in­crease in the number of subscribers that most countr ies will experience.

Fig. 1 SLMS 160 consists of three subsystems: SLMC, to which the operators are connected and which contains equipment for communication with SLTU and DBS; SLTU, which carries out the tests and/or measurements and which is installed in each local exchange; DBS, which contains data about the individual subscribers and subscriber lines, fault records, network data and various reports and statistics

DBS Database. Uses X.25 for communication with SLMC

_ _ _ _ Switched line

Complaints line

Leased line

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JAN KJONIGSEN A/S Elektrisk Bureau

SLMS functions Receiving fault complaints from subscribers Screening which means that the system inves­tigates a subscriber number reported as faulty, regarding whether - it is a valid number - it is a recovery number - it is temporarily disconnected - it is a repeated complaint, i.e. a fault report

has already been prepared - the fault is likely to have been caused by a

previously known fault - nobody was at home when the repairman

came to repair the fault. Centralized testing and verification Presenting the test result (OK/not OK) Presenting detailed measurement results Distribution and printing of fault reports Standardized operator procedures for all types of exchanges All functions (both measurement and tele­phony) are accessible from one and the same terminal Automatic call distribution Database system comprising - data concerning subscribers and subscriber

lines - fault reports and fault records - extensive report and statistics functions Automatic proposals for appointment times to carry out repairs at subscribers' premises Automatic routine measurements Automatic follow-up tests Automatic detection of fault patterns Handling of known faults, which means that information concerning generally known faults in the network can be input into the system. This facility is used by, for example, the operators when carrying out screening.

Requirements of a modern subscriber line management system The major demands made on a modern subscriber line management system are: - Cost effectiveness as regards the ser­

vice organization, system location and utilization of the manpower

- Improvement of the service quality, so that the subscriber can be told the cause of the fault the first time he con­tacts the fault complaints service

- The establishment of a computerized database for the subscriber lines

- Compatibility with existing and future switching technology

- Ability to adapt to other existing and new services

- Increased subscriber line availability through improved maintenance using automatic routine tests

- Improved test accuracy, with all tests and measurements being carried out by automatic test equipment, using the same procedures, thereby giving more reliable measurement data.

Description of SLMS 160 The development of SLMS 160 was based on the requirements imposed on modern subscriber line management

system given above. SLMS consists of three subsystems, SLMS, SLTU and DBS, fig 1 A maximum of 16 operators' positions can be connected to each SLMC module, either direct or remotely Another SLMC may be connected in par­allel, via the dtabase system DBS, if more than 16 operators are needed. Ei­ther leased or switched lines in the tele­communication network can be used for communication with SLTU (Subscriber Line Test Unit), one of which is placed in each local exchange.

Subscriber Line Management Centre SLMC is equipped with an automatic call distributor (ACD) which distributes the incoming calls on the A-lines to the operators who have been free longest. It also contains an internal switch, which makes it possible to make inquiries and transfer calls to another operator or the supervisor. Conference calls can also be set up between a complainant, the oper­ator or repairman concerned, the super­visor or another operator.

Each operator is provided with a visual display terminal, VDU, and a specially designed keyboard with function keys for easy operation and a headset for communication with subscribers and

Fig. 2 An SLMS operator position comprises an "intel­ligent" VDU, a specially designed keyboard with a number of function keys and a headset for communication with subscribers and repairmen

172

Fig. 3 A display screen showing a fault report. The report contains parts of the subscriber line record (SLR) and also the results of a test just made. The results indicate a break on the subscriber line, since the line status is given as "open" and the capacitance between the a and b branches is shown in inverse video, which is a fault indication

repairmen, f ig. 2. Each operator 's posi­t ion is also connected to an individual telephone line, which is used for con­nection to SLTU or which can be used by the repairmen to call the operator for verif ication of a repair. The operators' terminals are " inte l l igent ' ' VDU termi­nals. They are adapted for use in the system and meet the special ergonomic requirements by such systems. Particu­lar attention was paid to the fo l lowing requirements when developing the ter­minals:

- All information on the screen should be based on fixed forms, f ig. 3, which guide the operator through the hand­ling of a complaint call

- The communicat ion should be inter­active, i.e. the operator must be kept informed regarding what takes place during the setting up of a test call

- All funct ions in SLMC can be carried out from a keyboard designed in ac­cordance with ergonomic principles. This means that funct ion keys and se­lection via menus is used extensively. The procedures used for making measurements on different types of exchanges are the same. This further simplif ies the communicat ion with the system.

The system offers many possibilities for dif ferentiat ing between different opera­tor categories. The various operators' posit ions are described here, with the most important funct ions mentioned briefly: - Answer posit ion, which the operator

uses to receive calls and complaints from subscribers, carry out screening (see the fact panel "SLMS functions") of the telephone number in question, generate fault reports, carry out nec­essary tests, make appointments with subscribers for repairs, monitor busy lines and route fault reports.

- Test posit ion, which is used to carry out more detailed tests than can be made from the answer position, measure subscriber line parameters, send r inging signals or howler tone, talk to the subscriber, test the sub­scriber dial or push-button set and monitor busy lines.

- Dispatch posit ion, which is used to distr ibute the jobs to the repairmen, receive calls from them and verify completed repairs.

- Analyser posit ion, which is used to initiate automatic routine measure­ments, handle faults that are already known (see the fact panel "SLMS

173

Fig. 4 SLMS is installed in BYB cabinets that hold five magazines. The picture shows an SLMS for eight operators

functions") and collect and analyse fault statistics

- Supervisors position, from which the supervisor can monitor the operation of the system, change the operator's tasks, handle fault reports etc. which have been waiting too long in the sys­tem without being handled, initiate a number of operational statistics and management reports, such as on-line status reports, complaint statistics, fault statistics, subscriber statistics and administrative reports.

- Operation and maintenance position, which is used to process fault reports that concern faults in exchanges, dis­tribute fault reports and to deal with faults that are already known.

- Position for recording subscriber data, which is used to input subscrib­er line records (SLR) for new sub­scribers and update existing SLRs.

In addition some of the operators' posi­tions may be used for training. A training position does not have access to the subscriber database, but can work with a "minidatabase" of its own which has been specially installed for training pur­poses.

SLMC consists of nine subsystems:

OPS, Operator Position Subsystem OPS contains the individual line circuit for each operator, which is used to con­nect the headset and the individual line (B-line), which is also connected to the subscriber multiple in the local ex­change. OPS also includes functions that enable any operator to set up con­ference calls. Incoming complaint lines (A-lines) and data communication equipment are also connected to each line circuit, via SWS and HDU respec­tively.

HDU, High level Data link Unit HDU handles the communication be­tween SLMC and the database system (DBS). The communication is in accor­dance with CCITT Recommendation X.25.

AMS, Alarm Subsystem AMS supervises all d.c./d.c. converters in SLMC by means of voltage monitor­ing. AMS also receives alarms from other equipment and presents these to­gether with the other alarms on the alarm panel.

POS, Power Subsystem POS provides SLMC with d.c. voltages. POS consists of a number of d.c./d.c. converters, which convert 48 V d.c. to the necessary working voltages. POS also contains the fuses required for SLMC and the d.c./d.c. converters and distributes 48 V d.c. to the rest of SLMC

SWS, Switching Subsystem SWS comprises line equipment for the detection of calls and a congestion-free switch for the distribution of incoming calls to the operators. SWS also in­cludes a switch for internal connections between the operators.

DSS, D-line Switching Subsystem DSS consists of a congestion-free switch for connections between the op­erators and all D-lines. DSS can within each SLMC connect 16 operators to 384 D-lines.

TSS. Test and Signalling Subsystem TSS includes MFC equipment and equipment for sending and analysing push-button dialling tones. These equipments are connected to OPS via a

174

switch. TSS also contains equipment for detecting recalls from SLTU, as well as a program system for the administrat ion of all these funct ions.

RPS, Regional Processing Subsystem RPS contains a central processing unit, memory modules, a unit for interrupt handl ing and a unit for extending the system bus to the hardware to be con­trolled or scanned. RPS also includes the basic software units of the operat ing system, which work in close association with the hardware and whose main task is to administer the work of the pro­cessor, and also programs and tables for the administrat ion of RPS in SLMC.

CTE, Cartridge Tape Equipment CTE consists of a cassette recorder which is used to load programs into RPS (program back up).

SLTU, Subscriber Line Test Unit SLTU, f ig.5, which is installed in each local exchange, is the same for all types of exchange It is adapted to the ex­change in question by means of a test

trunk. SLTU carries out the measure­ments on the subscriber line concerned on command from SLMC. The measure­ment results are sent to SLMC, where they are presented on the operators VDU terminal.

The connect ion between SLMC and SLTU can either be a leased line or a switched circuit. Speech and data are transmitted on the same line. The data communicat ion is asynchronous, with a speed of 300 bauds.

A test of the exchange status is automat­ically done when the connection be­tween SLMC and SLTU has been estab­lished. This means that SLTU can scan two test outputs: - FIR-P/test t runk blocked - Exchange alarm.

This is done in order to provide the oper­ator with a status report from the ex­change (urgent alarms on or off) before measurements are started.

Other SLTU functions The other funct ions performed by SLTU are control led f rom SLMC and consist of:

Fig. 5 SLTU is usually installed in a single-magazine cabinet that can easily be mounted on a wall

175

Test of subscriber status Various methods are used for testing subscriber status (idle, busy or cannot be measured) depending on the type of exchange, i.e. test points are used, test information is contained in the signall­ing system (MFC) or tone information (busy tone etc.) is provided.

Monitoring of a busy line When the subscriber status is given as engaged, the operator can monitor the line in order to check whether a call is in progress, or talk to the subscriber.

Disconnection of blocking (line lock­out) When measurements cannot be made on the subscriber line the operator can disconnect the line unit in order to check whether the line is blocked. This function is dependent on the exchange system, since it can only be performed in exchanges where there is galvanic through-connection to the subscriber line units. The measurements can then start when the subsriber line is found to be free.

Recall to SLMC By recall is meant that a test number in SLMC is called from the subscriber number which has been reported as faulty, in order to test the multiple posi­tion and line unit of that subscriber and the part of the common exchange equipment (registers etc.) needed to set

SLTU, test and measurement functions AC voltage a-earth, b-earth a n d a - b 0-190 V DC voltage a-earth, b-earth and a -b 0 - ±190 V Resistance a-earth, b-earth anda -b 0-1,9Mohm Capacitance a-earth, b-earth a n d a - b 0-7 (iF Measurement of the feeding current a - b 0-100 mAd.c. Test of dialling tone Test of dial as regards the number of transmit­ted pulses and the make/break ratio Test of push-button set Test of the subscriber's private meter Polarity inversion of the a- and b-wires Sending howler tone Sending ringing signal

up the call. The SLMC test number is transmitted to SLTU, where it is convert­ed to dial impulses. SLMC checks that a ringing signal is received. If the SLMC test number is to be sent in the form of push-button tones, the appropriate sig­nal is transmitted from SLMC to the ex­change in which the SLTU is installed.

Ringing towards the subscriber SLMC gives the order to send a ringing signal to the subscriber, and the signal is sent out by SLTU. If the subscriber answers, this is immediately reported to SLMC, and the operator can talk to the subscriber.

Howler tone to the subscriber The order to send a howler tone to the subscriber is received from SLMC. SLTU then connects up a howler tone to the subscriber for a preset time. When the time has elapsed, the subscriber status checked and the result, microphone on hook or off hook, is transmitted to SLMC.

Dial test When testing the dial the operator asks the subscriber to dial a certain digit, e.g. 0, and at the same time a command is sent to SLTU to connect a dial pulse re­ceiver. The received pulses and the make/break ratio are transmitted to SLMC. SLTU then sets up a speech con­nection between the subscriber and the operator again.

Test of subscribers' private meters The subscriber's private meter can only be tested when a speech connection has been set up between the operator and the subscriber. The operator asks the subscriber to check the private meter and then orders a test of the meter. SLTU sends out a specified frequency with a set level and a certain duration. The sub­scriber's private meter should step once.

Polarity reversal A command for polarity reversal is sent from SLMC to SLTU. SLTU tests the po­larity reversal, sends the result back to SLMC and then restores the polarity.

Measuring the feeding current The feeding current can be measured during conversation with the subscrib­er.

176

Fig. 6 A database system consists of at least two processing units in addition to the necessary system terminals and mirrored disc-pairs for storing, for example, data concerning subscribers and subscriber lines

SLTU units SLTU consists of the following units.

CPU. Central Processor Unit The central processor unit in SLTU con­tains an Intel 8080A microprocessor. The program is stored in programmable read only memories (EPROM). CPU also includes watchdog circuits for monitor­ing the program, a generator for data communication and a V.24/V.28 modem interface.

MLU, Measuring Line Unit MLU contains a measuring system and its own power supply unit. The measur­ing system is based upon an analog/dig­ital converter. The separate power unit ensures that the input to the measuring system is floating in relation to the ex­change earth.

STS. Switching Telephony Subsystem STS contains all telephony functions and an oscillator for howler tone tests. The functions include: - terminating the line - sending ringing signals and receiving

the B-answers - receiving dial pulses - connecting of measurement func­

tions.

COU, Communication Unit The modem is frequency shift keyed and has a transmission rate of 300 bauds.

TTI. Test Trunk Interface TTI includes functions for operating a test trunk in the exchange.

DBS, Data Base System DBS, fig. 6, is normally built up around a computer from Tandem Computer Inc. This is a commercial computer specially developed to meet the demands for a terminal-orientated, transaction-driven real-time system. Some of the require­ments for such a system are: - high availability - high handling capacity - unlimited extension possibilities - flexible communication facilities.

The smallest system consists of two in­dependent but interworking processor units, which are connected together via two high-speed buses. Each processor unit consists of a main memory, a CPU, an I/O processor, a bus control unit (in­cluding cache buffers for high-speed transmission) and an I/O channel. Up to 16 processor units can be included in a system (node), which in turn can form

177

Technical data Maximum capacity Incoming complaint lines per SLMC module 24 Number of operators per SLMC module 16 Number of printers directly connected to each SLMC module 9 Number of printers connected to each operator terminal 1 Number of different forms available 80 Number of SLTUs that can be connected to an SLMS module via leased lines 384

Power consumption 48 V d.c.

SLTU 50 W SLMC with 8 operators 400W SLMC with 16 operators 600 W

230Va .c .+10%, -15% 100VA VDU

Dimensions SLMC

5-magazine cabinet 1860x600x300mm Number of cabinets required for 8 operators 2 Number of cabinets required for 16 operators 3

SLTU (mounted in a single-magazine cabinet) 470x600x300 mm

part of a network with up to 255 nodes. Within these limits, from two up to 1000 terminals can be connected in. The soft­ware consists of standard Tandem soft­ware and SLMS application software For example, a powerful program pack­age in DBS makes it easy for an admin­istration to prepare reports that are tai­lored exactly to its own needs. Such re­ports are an effective aid in improving the fault complaints service, making the repair activities more efficient, provid­ing proof of the quality of service offered to the subscribers etc.

The data base contents are divided into ten groups: - subscriber line data - subscriber line data history - network component data - data concerning the handling of com­

plaints - data concerning personnel status

and activity - system parameter file - file for the maintenance of telephone

exchanges - statistics log - training files - files for faults that are already known

Summary When the telephone network in a coun­try grows both in size and complexity, the problems regarding the operation and maintenance of the subscriber net­work also grow. It therefore becomes more difficult for the fault complaints centres to maintain their previous

quality of service to the subscribers without incurring an unproportionaiiy large increase in operating costs. The extensive development in the field of switching has resulted in a correspond­ing increase in the administrations' de­mands for maintenance functions and their scope. It is therefore evident that the traditional fault complaints service will not be able to solve the problems that will confront administrations re­garding their existing and future tele­communication networks.

SLMS 160 provides administrations with a tool for improving the mainte­nance of the subscriber line networks. It contains many administrative, test and measurement functions, giving the fol­lowing advantages and savings: - A combined administrative and meas­

urement system for handling fault complaints and fault location, and for sending out repairmen

- Possibilities for delegating more of the work to less qualified personnel

- Better and quicker subscriber ser­vice, which enhances the administra­tion's reputation

- Greatly increased personnel prod­uctivity

- Reduction of the cost of maintaining the subscriber line network

- More than 40 different administrative, performance and operational reports for the management

- A drastic reduction in the amount of paperwork since SLMS 160 is a paper­less system

- Facilities for round-the-clock opera­tion.

SDS 80 - A Standardized Computing System

Jorgen Olsson

Standardized Computing System SDS80 has been developed by Ericsson, in close collaboration with the Swedish Defence Materiel Administration, in order to break the trend of rising costs of development and maintenance of defence systems containing computers. SDS80 is an integrated system, comprising a high-level language based on Pascal, advanced software development aids and modular hardware, which can be used to build up computers adapted to different reguirements. SDS80 will be used in the Swedish Armed Forces' new aircraft JAS39 (JAS = fighter-attack-reconnaissance), in the radar system and the display system, as the system computer and as the computer in the counter-measure system. SDS80 will also be used in other aircraft projects. The author gives the background of the project and describes the various parts of SDS 80 and some applications.

UDC 681 324:623 At the beginning of the 1970s an increas­ing number of subsystems in military aircraft were equipped with computers. At the same time the software costs were rising to become the greater part of the overall costs of the computing systems.

These facts were noted at an early stage by the Swedish Defence Materiel Admin­istration. For example, the fighter-at­tack plane JA37 contained no less than six large subsystems (air data, inertial navigation, central computer, display, radar and automatic pilot) with compu­ters.

The computers in these subsystems were all different. None of them con­tained any hardware module that was interchangeable with a module in any other computer. Neither could the as­sembler language of any one computer be used in any of the others. Each of the computers had its own software de­velopment system in different host com­puter systems.

Fig. 1 shows that the resources devoted to software development constituted a considerable part of the total cost of the subsystems.

For the maintenance personnel the sit­uation meant that a number of different test equipments, spare parts and spe­cialists were required. In the mid-1970s the Defence Materiel Administration therefore initiated a project aimed at de­veloping a standardized computing sys­tem that could be used in different sub­systems in any future aircraft.

The project was designated SDS80, and three large Swedish companies in the defence industry were contracted to carry out the work. The companies were Telefonaktiebolaget LM Ericsson (the Ml Division), SRA CommunicationsAB and DatasaabAB. After purchases and company mergers the whole project has now been broughttogetherwithin Erics­son Radio SystemsAB.

Fig. 1 The software development work devoted to AJ37, the attack version of aircraft Viggen, and JA37, the fighter version

179

JORGEN OLSSON Ericsson Radio Systems AB

Fig. 2 The three basic elements in a standardized com­puting system

Requirements During the initial stage of project SDS 80 much effort was put into establishing and formulating the demands to be made on a standardized computing sys­tem.

One major requirement was to reduce the costs of handling the software dur­ing the whole of its life, from the de-velopmenttothe maintenance and mod­ification. During the development stage it is essential to have access to high-quality software development tools, and to be able to work with a language that is well suited to the application. This in­creases the productivity of the develop­ment staff and the quality of the soft­ware, and reduces the test time.

During the software production stage aids are needed to handle and generate different editions of software systems. The maintenance stage, which is often responsible for a very large part of the cost of the software, requires tools for efficient fault tracing. It must also be easy to train new maintenance staff, since the software systems will be used for several decades, and during thistime the staff will have changed several times.

Another important requirement for the standardized computing system was that the hardware should have a modu­lar structure. This gives such a high de­gree of flexibility that the standardized computers can be adapted to suit a wide range of different requirements in dif­ferent applications. Certain applica­tions need large memory volume, whereas others require large calcula­tion capacity.

A modular structure also makes it easier to keep abreast of technical develop­ment, for example to exploit new, faster component technology as soon as it be­comes available.

The use of a standardized computing system in aeronautical applications will of course make stringent demands on both performance and environment. The performance must meet the need for, for example, advanced real-time data processing which is often encoun­tered in aeroplanes.

The environmental requirements mean that the equipment must be able to oper­ate under adverse conditions and with­stand vibrations, high g values, large temperature differences and moisture and salt mists.

Aeronautical applications also demand small volume and low power consump­tion.

On the basis of the above requirements a standardized system was developed, containing three fundamental elements, fig.2: - An efficient high-level language suit­

ed to real-time applications - An extensive, high-quality software

development system - A family of hardware modules that

can be combined in different ways, all designed for the execution of a high-level language.

In the middle of the 1970s there were no languages, software development sys­tems or computers available that to­gether satisfied the demands. It was therefore decided to develop the three elements within the project, and to de­sign them to interwork in an efficient system.

Characteristics and design of the standardized computing system The three basic elements took firmer shape as project SDS80 progressed. The language was designated Pascal/ D80, the software development system PUS80 and the hardware modules D80. Together they now form the integrated system that was the original aim of the project.

A brief description of the most important characteristics and the design of the three parts in Standardized Computing System SDS80 is given below.

High-level language Pascal/D80 The high-level language Pascal/D80 is based on the original Pascal definition by N.Wirth. Since Pascal was originally intended for training and not for pro­gramming real-time systems, Pascal/ D80 is an extended and modified vari­ant.

180

Fig. 3 Top down design of Pascal D80

The fundamental part, with control structures and declarations of data, is to a large extent similar to the original Pas­cal version, which means that the lan­guage is very easy to learn and under­stand. This in its turn means that the programs become efficient and reliable.

In real-time applications it is essential to be able to deal with fault situations. Con­cepts for fault handling have therefore been included. The system can handle faults detected by the hardware, by com­piler-generated code and by applica­tions software.

The character of real-time systems often makes it necessary to handle the inte­gral functions as parallel processes. The language must therefore include spe­cial concepts for this purpose, and also for synchronization and for the transfer of data between these processes.

Pascal/D80 contains concepts for hand­ling not only parallel processes but also parallel processors. Many applications require so many calculations or the tim­ing is so crucial that they must be divid­ed up between several processors (CPU).

When there is a large amount of soft­ware, it must be handled in small parts, modules. These modules must be com­pilable units, i.e. it must be possible to translate them individually. Conse­quently Pascal/D80 contains four dif­ferent types of modules, all of which are compatible, fig.3. This gives the soft­ware system designerfull freedom in the structuring of a system.

Software development system PUS 80 The main task of the software develop­ment system is to provide the tools and aids needed to translate the software system from Pascal/D80 to machine code that can be input directly into the D80 computer for execution. The system must also supply aids for testing and fault tracing, and for handling program versions and data specific to individual projects.

PUS80 consists of three parts, fig.4: - A host computer equipped to store

large software volumes (files) on discs or magnetic tapes and having a large number of terminals (VDUs) dis­tributed over a large area. The host computer in PUS 80 is the DEC VAX 11 with operating system VMS.

- A control panel, which forms a link between the host computer and one or more D80 computers. The com­pleted machine code, produced in the test computer, is loaded into the D80 computer by means of a command via the control panel.

- Development aids, consisting of pro­grams in the host computer, for pro­ducing and testing Pascal/D80 pro­grams.

The development aids are numerous and are used at different stages of the software development.

The Delta handler is used to handle source texts. This aid can store a text and all subsequent modifications. Older versions of programs can easily be re­trieved, and also different versions that have been developed in parallel for dif­ferent test stations.

Other aids are compilers, linkers and symbolic debuggers for the develop­ment of Pascal/D80 programs that are to be run on the D80 computer, as well as programs that are to be run on the host computer.

A special tool, called channel program handler, has been developed for the pro­gramming of the I/O channel of the D80 computer.

Other tools that simplify the work on large program systems are the system variable handler and the software sys­tem generator. The system variable han­dler has a database where global system variables can be stored, so that later on it will be possible to generate a global declaration for the Pascal/D80 system automatically.

The software system generator is used, together with the other PUS80 tools, to build up previously defined versions of software systems automatically.

PUS 80 also includes tools for standard­ized formatting of Pascal/D80 source text.

181

Technical data for D80 Word length Stack-oriented machine Floating point representation Separate stores for data, operand stack and process administration Extensive hardware and microprogram support tor the execution of high-level language Instruction times down to Execution speed per CPU approx. Number ot parallel processes per CPU Program store per CPU, max.

32 bits

32 bits

0.125 us 1 Mops

15 4 Mbytes

Software development system PUS80 is designed so that the work can be carried out interactively from host computer ter­minals. This means that the operator has access to all development tools simul­taneously from the work station. From there the operator can also handle the D80 computer with which he is working at the moment, even if it is located elsewhere. The interactive working also reduces the time required for tests, since the result of the operator's com­mand is immediately displayed on the visual display unit. It also permits an ex­tensive support function, which enables the operator to enquire of the software development system what possibilities are available in different situations. This is of great assistance to new users.

D80 hardware modules The hardware for standardized comput­ing system SDS80 is in the form of dif­ferent modules instead of a single stan­dard computer. The result is a flexible system of building blocks, which can be combined into D80 computers that are suitable for different applications and requirements.

The modular structure also makes it very easy to replace old technology (e.g. memory components) in any module with new. Such modifications do not af­fect other modules. The modularity also makes it possible to add new hardware modules as the need arises.

All modules are implemented using the standardizd construction practice BYB 601, which is based on the interna­tional standard ARINC600.

Module types The communication unit, CU, controls the traffic on the inter-module (IM) bus The unit includes a real-time clock. CU also handles the communication with the host computer, either direct or via a control panel, as in fig. 4.

A memory, consisting of universal mem­ory (UM) modules, can be connected to CU to provide a common store for the different processes.

A CPU module, together with a program store, constitutes a processor. The pro­gram store consists of one or more UM modules and contains both programs and static data (constants). CPU has extensive support forexecution of high-level languages, in particular Pascal/ D80.

Comprehensive support functions have been built into the system in order to enable it to handle parallel processes in real time. This means that most operat­ing system functions are implemented in hardware and in microprograms. This design gives superior performance compared with conventional compu­ters, in which the corresponding func­tions are carried out by software in the operating system.

The communication between a D80 computer and the environment takes place via MIL-STD 1553 B data buses. I/O channel modes (CH) provide the inter­face to these buses and can work as either master, slave or monitor units. Master CH modules contain a channel program that controls the traffic on the 1553B bus.

Fig. 4 PUS 80 software development environment

182

Fig. 5 D80 computer consisting of four processors, two I/O channels and one CU module

Fig. 6 D80 computer comprising two processors, one non-volatile bulk store (bubble memory), one I/O channel and one CU module

Fig. 7 D80 computer comprising a single processor, one CU module and one I/O channel

Non-volatile storage in the form of both bulk storage (BS) and EEPROM mod­ules is provided for the storage of data and programs. The BS modulesare bub­ble memories and hold up to 8 Mbytes. They are connected directly to the IM bus. The EEPROM modules are con­nected to the CPU modules as program stores.

The module structure described above makes it possible to build up a number of different D80 computers according to different requirements, see f igs.5-7.

Applications The idea behind standardized comput­ing system SDS80 is to reduce the cost of the computers included in, for exam­ple, avionics systems. It was therefore only natural that in 1982 SDS80 was chosen for the new Swedish multi-pur­pose aircraft JAS 39 Gripen. The system was selected in the face of stiff interna­tional competition from some of the largest computer manufacturers in the world

SDS 80 was chosen for the radar, display (two D80 computers) and counter-mea­sure subsystems and as the system com­puter, fig. 8. The same types of hardware modules are used in all these systems in spite of the fact that each has its own requirements as regards calculation ca­pacity and memory volume. All compu­ters use the same language, Pascal/D80, and the same software development system, PUS80

The various systems in aircraft JAS39 communicate with each other via MTL-STD 1553B buses. The system compu­ter controls the traffic on all three buses. Only two buses were planned originally, but a third was soon added in order to increase the capacity. This presented no problems thanks to the flexible modular structure of SDS80.

The system computer contains three CH modules for the I/O function, see fig.8, all of which are programmed to work as masters. The system computer also con­tains a non-volatile bulk store, BS, for storing the software for all D80 compu­ters in the aircraft. This store is used in, for example, "cold start" of the aircraft.

The two D80 computers in the display system are somewhat smaller, with fewer storage modules. On the other hand they contain special I/O units (SK modules) towards other parts of the dis­play. The SK modules do not form part of SDS80.

The D80 computer in the radar system also contains a special I/O channel to the other radar equipment.

The flexibility and modular structure of SDS 80 makes it possible to use the sys­tem in other applications in ships, tanks, helicopters etc. The environmental and

performance requirements for such ap­plications are similar.

Experience of SDS80 Certain subprojects wi th in project JAS39 have progressed so far that equipment with D80 computers in­cluded has already been fl ight tested. These subprojects have shown consid­erable improvement in the productivi ty of the programmers. Four t imes more produced code per unit of t ime has been achieved in PUS80 than in the old de­

velopment system, which is based on assembler language.

The test time has been greatly reduced thanks to the interactive environment and the short cycle t ime for the compi l ­ing, l inking and loading process. The short cycle time has also el iminated the need for patching, i.e. making modif ica­tions in the computer program store wi thout also changing the source text. Now all modif icat ions are made direct in the source text, which gives better con­trol of the development work.

Fig. 8 SDS80 in the new Swedish aircraft J AS 39 Gripen

184

Fig. 9 The different elements In a fully bullt-out standard computer system comprising the languages Pas­cal D80 and Ada

When high-level languages were intro­duced intoavionics, many peoplefeared that this would entail an unnecessary amount of code, and that a part of the program would therefore have to be written in assembler code. Fortunately this proved to be incorrect. Less than 1 % of the software system has had to be coded in assembler. The efficiency of the compilers has resulted in the in­crease in the input code being less than 10%.

Whereas the expected disadvantages have not materialized, several advan­tages have instead been gained in the form of easily readable and comprehen­sible programs. The language has proved very easy to learn, partly be­cause Pascal is a common language that is taught in many universities and colleges.

However, the major advantages of a standard computer system are not ex­pected to be apparent until the mainte­nance stage in the life of the applica­tions systems is reached. Traditionally a large part of the cost is incurred during this stage, and the standardization could lead to a considerable reduction in cost.

Fig. 10 Comparison of the performance of D80 and D80M

Simple assignment statements A:=A+B where A and B are integers where A and B are reals

Calling a process and return

Change of procedure

D80

1.2 tis 2.5 MS

4.1 Ms

3-5 us

D80M

2.8 /xs 20.0 MS

6.8 MS

30.0 MS

Future potential The positive experience gained from standardized computing system SDS80 means that the idea will be taken a stage further, by introducing new computer types and the new language Ada (regis­tered trade mark), which is expected to become predominant in the nearfuture, fig.9.

The purpose of introducing new compu­ter types is to achieve an even higher degree of flexibility. In certain applica­tions the high performance of D80 is not necessary, a lower level would be suffi­cient A reduction of the performance requirements would mean that the com­puter could be made simpler and there­by cheaper.

A computer designated D80M will there­fore be available for applications that do not require the high performance level of D80. D80M will be based on a micro­processor having a word length of 16 bits. The computer will be built up using a CPU from Intel, APX286, and CMOS memories.

The computer family described above is designed for use with the Pascal/D80 language and software development system PUS80. However, the interna­tional development trend means that an increasing number of new computer systems will use Ada (MIL-STD 1815) as the programming language. As a con­sequence the Swedish Defence Materiel Administration and Ericsson Radio Sys­tems, together with other companies within the defence industry, have start­ed a project called SDS80/A with the aim of extending SDS80 to include Ada.

The project includes two important sub-projects. One is to design an Ada Inte­grated Development Environment, AIDE, of at least the same quality and

Fig. 11 Standardized computing system SDS80 has been supplied to several different Swedish enterprises. The picture shows some of these computers

Fig. 14 A CPU module in micro-wire technology

Fig. 12 A D80 computer in a version with forced cooling. The cooling air enters at the rear edge of the box and passes through small channels in the printed boards. The unit has a width of 4MCU (Modular Concept Units) and contains a power unit in addition to the processor modules

Fig. 13 A fan-cooled version of the D80 computer. The construction practice is BYB601, adapted to ARINC600 standard. The box in the picture is 6MCU wide and holds processor modules and a power unit

scope as PUS 80. The second is to define and verify an Ada Instruction Set Archi­tecture, AISA, which is to form a stan­dardized interface between the com­piler and different computers. The ver­ification of AISA is intended to prove that it supports the execution of Ada in an efficient manner, as D80 does for Pascal/D80.

Different machines are adapted to AISA, corresponding to those in the original SDS80 family. In the case of D80 the adaptation consists of the computers being modified to execute AISA code. The performance of the Ada computers is expected to be only slightly lower than that of the Pascal/D80 machines thanks to the complexity of the Ada language.

Project SDS 80/A is now in progress and is expected to be completed during 1986.

Summary Standardized computing system SDS80 has been developed for the purpose of reducing the increasing cost of using

embedded computers, for example in aircrafts.

Far-reaching modularity in the hardware structure provides a high degree of flex­ibility, which makes it possible to build up different computers to meet different performance requirements.

A high-level language and an extensive software development system have meant that the productivity of program­mers has increased by a factor of four, the test times have been reduced and the quality of the software has in­creased.

SDS80 has been chosen, in the face of stiff international competition, as the standard computer in the new Swedish multi purpose aircraft JAS39 Gripen. It has also been chosen for other aircraft.

SDS80 is now being developed for the new language Ada on the basis of the positive experience already gained. At the same time new computer types are being introduced in the family of stan­dard computers.

AXT121 - A Digital Stored-Program-Controlled Exchange for Special Telecommunication Networks Torbjorn Nilsson

AXT is a digital stored-program-controlled system developed for exchanges in special analog or digital telecommunication networks. The design is based on the public telephone exchange system AXE 10. The AXT exchanges are characterized by high traffic handling capability, sophisticated traffic routing, functions for ensuring high availability in the network and efficient operation and maintenance functions. AXT is available in the form of a number of application systems, such as PABXs for telephony or for integrated telephony and radio, AXT 101, combined local exchanges and PABXs, transit exchanges or combined transit exchanges and PABXs, AXT 121. AXT 101 is used mainly in civil or military operation centres, for example for air traffic control. The author describes AXT 121 and its use in, for example, nation-wide telecommunication networks.

UDC 621 395 2 621.395.722

In addition to the large, public telecom­munication networks, certain public and private organizations need their own, special telecommunication net­works, with limited capacity but provid­ing sophisticated traffic and network functions. Such networks are used by the defence establishment, the police, oil companies and power companies, and by airlines for air traffic control. These networks often have to meet strin­gent requirements as regards:

Operational reliability and robustness Most of these networks suffer inten­tional jamming and sabotage. However, it is in just such situations that these special telecommunication networks must function satisfactorily.

Traffic handling capability The volume of traffic can become high in parts of the network because of great activity or jamming in certain areas. This can quickly lead to congestion and loss of service in an ordinary network, with serious consequences for the organiza­tion concerned.

Communication paths between sub­scribers in the network In many cases there is a requirement of very short setting-up times in the net­work, forexamplefor military communi­cation, airtraffic control, regulation pro­cesses and foreign exchange trading. This requirement still applies even if the network is partly or wholly knocked out.

Information security Both military and private networks are subjected to external monitoring, at­tempts at tracing communication cen­tres, and the introduction of misleading information. It is therefore necessary to safeguard important information, intro­duce authorization checks and reduce electromagnetic radiation from the communication equipment.

Interworking with other networks The networks must be able to interwork and to a certain extent be integrated with other telecommunication net­works, for example defence networks with public networks. This normally means that CCITT and CEPT rec­ommendations must be followed.

Economy It is an important requirement that the costs of the developed system remain low during the whole of its life. Hence it must be possible to introduce new func­tions, carry out installation and also op­eration and maintenance at a low cost.

System AXT 121 System AXT 121 hasa modular structure as regards functions, technology and capacity.

Fig. 1 The functional levels of system AXT 121

187

TORBJORN NILSSON Ericsson Radio Systems AB

The functional modularity means that the system has been structured, de­signed and documented in such a way that functions can be added, removed or modified with only the minimum of effect on other functions in the system. The technical modularity ensures that new technology can successively be in­troduced into the system. Modular ca­pacity makes it easy to change the size and traffic capacity within the overall ca­pacity limits of the exchange system.

The system structure is hierarchic with the three levels; system, subsystem and function unit, fig. 1.

The highest level includes switching system APT and control system APZ. Both are divided into a number of sub­systems. They in their turn are divided into a number of function units, which are realized in hardware, regional and central software or only central soft­ware.

disconnects circuits through the ex­change. The main tasks of the switching subsystem also include the synchro­nization of the exchange clock frequen­cy towards internal or external refer­ence sources. The digital time multiplex switch TSM normally has 512 multiple positions. The switch has full availability with negligible internal congestion and OdB attenuation through the switch. 16 groups of 32 lines each can be con­nected to the switch. Such a group can consist of, for example, a 30/32-channel first-order PCM link in accordance with CCITT recommendations, connected via terminal units ETCA or ETCC, or a group of up to 32 analog lines, con­nected via terminal units PCD or SMD or via a conference device MJC-D. The conference device occupies 32 multiple positions in the switch, 30 of which can be used for various conference group­ings. If necessary, the capacity of the digital switch can be extended to about 2000 multiple positions.

Switching system The switching system consists of six subsystems, fig. 2, and contains the traf­fic processing functions and operation and maintenance functions of the ex­change. The system is built up around a duplicated or single digital selector. Ter­minal units and other equipment for ad­aptation to different types of lines are connected to this selector, fig.3.

The switching subsystem, SWS, which is controlled by the traffic control sub­system, TCS, sets up, supervises and

The subsystem for line matching and signalling, TSS, includes functions for signalling and for the supervision of analog and digital lines to the exchange. It contains equipment for the matching and connection of analog and digital lines, and also common devices, e.g. tone senders and receivers.

The common channel signalling sub­system, CCS, handles the functions for such signalling (CC). The CC signalling is based on CCITT signalling system No. 7 and is used primarily between AXT

Fig. 2

Subsystems in sw i t ch ing sys tem APT

TSS Trunk and signalling subsystem SWS Switching subsystem OPS Operator panel subsystem CCS Common channel signalling subsystem TCS Traffic control subsystem for sending, receiving

and analyzing digits, choice of route and line etc. TCS also handles the setting up of point-to-point circuits

OMS Operation and maintenance subsystem for load­ing software, administering data modifications and transmitting data between the exchanges

Hardware

™ Central software

Regional software

Fig. 3 The hardware structure of sys tem AXT121 used as a combined PABX and t rans i t exchange

RSM Remote subscriber multiplexer PCD PCM terminal device (for analog lines) SMD Terminal device for analog subscriber lines TP/OP Test and operation point unit ETCA Exchange terminal unit for PCM lines with chan­

nel associated signalling ETCC Exchange terminal unit for PCM lines with com­

mon channel signalling CS/CR Tone sender and tone receiver OPT Operator panel terminal unit CLM Clock module RCM Reference clock module RP Regional processor BIM Bus interface module CCSA Terminal unit for analog lines with common

channel signalling

exchnges. CC signalling transmits infor­mation that replaces register and line signalling, and alsooperation and main­tenance information.

The information on the signalling chan­nel is transmitted in the form of signal units on a two-way connection. ETCC is the terminal unit for PCM links with CC signal ling, fig. 3. In the unit hardware the contents of time slot 16 is extracted and formed into a continuous 64 kbit/s bit stream, which is transmitted to a signal processor containing a micropro­cessor. The signal processor carries out the signalling procedure autonomously, including acknowledgements and re­transmission. The contents of a mes­sage is extracted by the microprocessor and sent to the central processor for processing. CC signalling for analog cir­cuits takes place over a two-way signall­ing channel equipped with a modem. The channel is connected to signalling terminal CCSA in AXT 121, fig,3.

The traffic control subsystem, TCS, con­trols and coordinates the activities in the different parts of the system during the setting up, progress and disconnection

of a call. Such functions as digit recep­tion and analysis, route selection, digit sending, call supervision and discon­nection are either carried out entirely within TCS or are coordinated by TCS.

The operation and maintenance sub­system, OMS, contains functions for op­eration and maintenance, starting up the system, authorization checking and the administration of exchange data.

The operator panel subsysten, OPS, handles the funcions for operator assis­tance. It is possible to connect up to four operators' panels to an exchange. Each panel is connected via a control unit, OPC, to the selector via PCD, and to the central processor via the RP bus, fig.3. The operator's panel comprises a visual display unit, on which the various call states are indicated by means of sym­bols, a digit indicator, a set of function keys for setting up calls and a push-but­ton set for dialling, fig. 4.

Control system The control system consists of four sub­systems, fig. 5.

Operator

Analog subscriber lines

PCM lines with channel-associated signalling

PCM lines with common channel signalling

Common channel signalling for analog lines

189

Fig. 4 The operator 's pos i t ion in the AXT121 exchange

Fig. 5 Subsys tems in con t ro l sys tem APZ

Central processor subsystem Regional processor subsystem Input/output subsystem Maintenance subsystem

CPS RPS IOS MAS

Hardware

Central software

Regional software

The central processor subsystem, CPS, comprises a duplicated real-time pro­cessor and the associated program and data stores. The analysis and decision functions of the switching system soft­ware are stored in and executed by CPS.

The regional processor subsystem, RPS, carries out the routine and capaci­ty-demanding functions of the switch­ing system software. RPS includes a number of microprocessors. The pro­gram store consists of EPROM, a pro­grammable memory that is erased by ul­traviolet light. This means that the pro­grams are not lost in the case of a mains break.

The input/output subsystem, IOS, han­dles the man-machine communication and the input and output of programs and data via VDUs, line printers and a cassette tape recorder. The tape rec­order is used for the storage and back­up of the exchange programs and soft­ware.

The maintenance subsystem, MAS, su­pervises the control system and takes

appropriate action in the case of a fault. The subsystem also includes functions for starting and restarting the pro­cessors. MAS also contains circuits for alarm transmission.

Software The central software has a modular structure. Each subsystem contains a number of function units consisting of one or more programs and the associ­ated data. Each program deals with a separate function, with strictly defined interfaces towards other programs. All interworking between programs is by means of formal messages, so-called signals, with the associated data.

When the system is started up the pro­grams and data are loaded from a cas­sette tape recorder. The parts of the memory that contain programs and data specific to the exchange are protected against writing in order to prevent any errors from spreading.

The central software is written in a high-level assembler language in orderto ob­tain, for example, a high traffic handling capability and small memory volume. The language combines the advantages of an assembler language, i.e. generat­ing a compact machine code and providing rapid execution, with the pos­sibilities provided by a high-level lan­guage for writing programs that are well-structured and hence easy to test and maintain. A software development system, APS, has been produced for the design and production of software.

The regional software is written in an assembler language, and aids have also been developed for the production and testing of this type of software.

Mechanical construction The function units in AXT121 are built up of printed board assemblies. These are installed in magazines, which in theirturn are mounted in cabinets. Each cabinet can be equipped with up to six magazines. The intermediate distribu­tion frame and the power unit for the exchange are usually also mounted in the cabinets. Connections between units in different magazines are made via cables connected to the magazine fronts.

190

Fig. 6 Special cabinet for AXT 121 systems that have to operate in adverse environmental conditions

A special cabinet with the associated mechanics and cooling fans, fig. 6, has been produced in order to ensure that the AXT121 exchange can operate in adverse environments. For example, the cabinet satisfies military requirements (MILSTD461B/462) as regards elec­tromagnetic compatibility (EMC). In ad­dition the cables between the maga­zines in the cabinet are screened. The protection against electromagnetic pulses can be increased considerably by connecting up AXT 121 via military opti­cal fibre cables. Furthermore the AXT121 equipment in the cabinet can be mounted on various types of me­chanical dampers, thereby making it possible to use the exchange in a mobile environment, for example in a transpor­table container.

In stationary installations, where such special environmental protection is not required, the magazines are mounted in the standard Ericsson BYB201 cabi­nets.

Power supply The exchange is normally supplied from the mains via rectifiers, which convert the mains voltage to -48 Vd.c. Standby batteries ensure an uninterrupted sup­ply in cases of mains failure.

Each magazine has its own d.c./d.c. con­verter, which converts -48 V to suitable voltages. This type of power distribution increases the reliability of AXT 121 by limiting to a single magazine the amount of equipment that can be knocked out by a faulty power unit.

Operation and maintenance of AXT 121 Man-machine communication Communication between AXT121 and the operation and maintenance staff takes place via a visual display terminal. The terminal is connected to AXT 121 via a CCITT V.24/V.28 interface or via a cur­rent loop. The terminal can also be re­motely connected via a modem and in­stalled in, for example, a control centre. In an AXT 121 network it is quite possible for a VDU terminal connected to one exchange to communicate with all other AXT121 exchanges as if it was con­nected locally to each. This is made pos­

sible by the use of common channel sig­nalling between the exchanges.

Authorization checking is used, thereby ensuring that only authorized staff can carry out operations in the system. The check comprises two parts, identifica­tion and authorization, and results in the operator being authorized to carry out certain tasks. Basically the same check is performed for all operators, whether placed at a control centre or at the local exchange terminal. For example, an al­arm is given after three unsuccessful at­tempts to reach the system.

The man-machine communication in AXT 121 is based on the use of pre­programmed menus displayed on the VDU terminal. The menus can be con­sidered as forms, the contents of which specify the exchange data, and thus also how the system is operating at that par­ticular moment, in a simple and user-oriented way. The operator calls up a menu on the screen by depressing a function key, F1, F2 etc., followed by the menu number. The menus are arranged in groups according to their fields of use.

New exchange data, for example line or subscriber data, are entered by writing or changing data in the appropriate menus. The changes can be docu­mented by printing the menus on the printer connected to the system.

This method of administering the opera­tional and exchange data makes it easy to change line, subscriber or routing data in AXT 121 exchanges or networks. The new data are distributed throughout a network by means of common channel signalling between the exchanges.

The central program and the specific ex­change data are stored on a cassette tape. When the system is to be started up there is a choice between a number of previously prepared exchange data al­ternatives for different types of opera­tion. If the need arises, the exchange data can be altered during operation with the aid of the VDU terminal. After such changes the exchange data should be transferred to the cassette tape, so that the correct data are used in subse­quent restarts.

191

Fig. 7 A fault menu in the AXT121 exchange. The left-hand side gives the faulty unit, alarm class and fault code. Previous faults and the time and date when they occurred are shown to the right, where it can also be seen that the unit has been blocked automatically (A). The lower part of the menu shows which of the duplicated units in the AXT 121 exchange are executive at the moment. At the bottom of the fault menu are listed the function menus that are available for the opera­tion and maintenance of the exchange.

Operational supervision and fault location Supervision, fault detection and fault lo­cation in the system are carried out with the aid of extensive, built- in rout inetests of the various units, parity checks, plau­sibility checks, t ime supervision etc. The supervision includes hardware, soft­ware and power supply faults.

The purpose of the supervision is to de­tect faults and to indicate, wi th a high probabil i ty, the faulty funct ion unit.

Detection and presentation of faults When a fault occurs in the system, an alarm is initiated by MAS. The faults are grouped into different alarm classes de­pending on their severity: A The fault requires immediate action B Action should betaken promptly dur­

ing daytime C Action should be taken at a suitable

time.

System redundancy The central parts of exchange AXT 121 are normally dupl icated in order to in­crease reliability, f ig .3.

The t ime switch module, TSM, is dupl i ­cated, with both sides work ing. With normal operation all call connect ions are set up in both sides. One of the sides is used as the executive side, and if this develops a fault, a changeover is made to the other side.

A duplicated clock unit, CLM, provides the clock frequencies for the switch and the terminal equipments. One SLM is connected in at a time and provides the clock frequencies for both sides of the time switch.

A dupl icated, extremely stable refer­ence clock module, RCM, takes over the control of the CLM frequency if a fault should occur in the network reference clock.

The central processor and maintenance system, CPS and MAS, are dupl icated, and also the processor buses with their associated equipment. The dupl icat ion method used is active-passive with hot passive. By this is meant that the active processor handles all the traffic. All in­formation regarding established call connect ions is passed on to the passive processor. Hence a change of active processor, for example if a fault occurs, has no effect on established connec­tions. The funct ion of the processors is supervised by means of test programs. The interworking between central and regional processors is tested regularly.

The three alarm classes A, B and C are also shown on a lamp indicator. The alarms are accessible in the main dis­tr ibut ion frame, for transmission to other operation and maintenance cen­tres. An acoustic alarm can also be initi­ated for each new fault, to be acknowl­edged either locally or centrally.

The fault situation for the exchange, i.e. which units have been or are faulty, al­arm classes, types of fault and the time when the faults occurred, can be shown on request as a fault menu on the VDU, see f ig. 7. A separate information menu is used to translate the fault digit codes into clear text.

Maintenance and repairs A fault in a certain funct ion unit wil l af­fect the funct ion of a group of lines. In such cases the faulty unit is automat­ically blocked, with automatic deblock­ing when the unit is work ing correctly again. Devices and lines can also be blocked and deblocked manually with the aid of the fault menu and the key­board.

When a fault is detected, a changeover is normally made to the standby unit. In the case of hardware faults the indicated unit is replaced by a spare and the faulty unit is sent for repair.

Network and Subscriber by means of the subscriber classifica-f unctions t i o n " i s P ° s s i D ' e t 0 block either out­

going or incoming traffic to a sub­scriber.

- Long-term conections, for example semi-permanent circuits, which can be set up between two subscribers in the AXT121 network by means of the VDU terminal.

- Special facilities, for example the connection of an announcing ma­chine. These connections are ob­tained by dialling a special number.

AXT 121 as a combined PABX and transit exchange provides a number of traffic functions. Some of these are included in the basic version, whereas others are optional features. Some functions give more rapid connection or a larger num­ber of connection paths, and others make for a more robust network. The system offers the following traffic func­tions:

- Automatic connection of local and transit traffic.

- Direct inward dialling to PABXs. - Concealed through-dialling, i.e. a

subscriber can be connected to a PABX but still be reached by means of just the subscriber number in the AXT 121 network, instead of the nor­mal procedure of dialling the sub­scriber number of the PABX, followed by the internal extension number.

- Group number, i.e. a subscriber num­ber corresponds to more than one line. Each subscriber line can also have an individual number.

- Multi-connection, i.e. a subscriber can be connected to several AXT 121 exchanges in the network and be reached via the same number.

- Party line, i.e. a subscriber line can have more than one number.

- Hot line, i.e. a connection can be set up automatically between two sub­scribers without dialling. This can ap­ply to just one or both directions.

- Priority traffic is possible in the AXT 121 network. The system has three priority classes in addition to traffic without priority. Facilities for priority with disconnection are also available in the AXT121 network.

- Conference calls, which are arranged in accordance with the so-called pro­gressive conference method, i.e. the subscriber who initiates the con­ference calls in the members one by one.

- Closed user groups can be arranged. The size is optional. A subscriber can belong to different groups and can call ordinary subscribers. Normally ten subscriber groups are available.

- Enquiry and transport of calls, which makes it possible for a called sub­scriber to transfer the call to a third person.

- Blocking of traffic, which is arranged

Up to four operators' positions can be connected to the AXT 121 exchange. The operator's panel gives access to the following facilities: - Setting up calls between different

subscribers, closed user groups and telephone networks (the public net­work, PABXs and various military net­works).

- Setting up conferences. - Automatic and time supervised call

back to engaged subscribers. - Call back to operators. - Series calls, i.e. call back to the opera­

tor when a call has been completed. - Connecting up to an engaged line. - Camp-on.

Technical network plans with AXT 121 The technical network plans for traffic routing, numbering, signalling, trans­mission, synchronization etc. form the basis for the operation of the whole tele­communication network. A brief de­scription is therefore given below of the more important basic plans for net­works using AXT 121 exchanges.

Traffic routing and numbering The traffic routing in AXT 121 networks is designed to operate in simple star-shaped as well as complex mesh-shaped networks.

The routing in AXT121 can exploit the possibilities offered by a mesh-shaped network as regards alternative connec­tion paths in order to create a network that is less vulnerable to damage. This feature means, for example, that when an inaccessible path is encountered in an exchange, new paths can be sought in the previous exchange. A new route selection is then made in the latter ex-

192

193

Technical Data Signalling data Line signalling - On-off signalling, to automatic or manual

PBXs, local battery telephones etc. - Continuous signalling, to public exchanges,

PABXs, telephone sets etc.

Register signalling - MFC. to other exchanges - Dual tone multi-frequency, to push-button

telephones - Decadic pulses, to telephone sets and ex­

changes

Common channel signalling, CC - CC is based on CCITT signalling system No. 7

and is used over digital PCM lines and also over analog lines via modem. CC replaces the conventional line and register signalling and is used primarily between AXT exchanges.

Transmission data Frequency band Nominal impedance Output level Regulation steps Clipping level Attenuation through the selector

300-3400 Hz 600 ohms, balanced -7.5 to 8dBr 0.5dB 3.14dBmO.

OdB

change, with the route just used ex­cluded from the selection If the new route selection is also unsuccessful, the path chosen through the network is abandoned and the original exchange starts to set up an entirely new path through the network. The whole traffic routing process is time supervised, with a preset maximum time.

One prerequisite for this rather complex routing method is that intelligent sig­nalling can take place between the AXT 121 exchanges. This is made possi­ble by the use of common channel or MFC signalling.

Both fixed and free numbering schemes can be used in an AXT 121 exchange. In the fixed numbering scheme used in most telephone networks, the subscrib­er number is associated with a geo­graphical area, or more specifically with the local exchange. Hence, if the sub­scriber moves and is connected to an­other exchange, his telephone number will have to be changed.

also used on analog lines, but then via modems over an analog four-wire line between exchanges.

The main advantages of this type of sig­nalling, compared with traditional chan­nel-associated signalling, are: - Savings as regards signalling equip­

ment, since it is common for several lines, hence the cost and volume of the equipment are reduced.

- Higher signalling rates, which result in shorter setting-up and disconnec­tion times, and hence more efficient utilization of the network

- Possibilities of readily introducing new services and functions thanks to the great flexibility and high capacity of CC signalling.

In AXT121 networks the signalling channel can also be used to change sub­scriber and line data and to read out maintenance information. Communica­tion can be established between any two AXT121 exchanges in the network for this purpose.

Free numbering means that a subscrib­er in an AXT 121 network can retain his telephone number if he moves. This provides a degree of flexibility in the telecommunication service that sim­plifies organizational changes and moves. In military networks the sub­scriber numbers can be associated with positions in the organization, and there is then no need to know where a person is in order to be able to call him.

Signalling and connection The AXT 121 exchanges can be con­nected to and interwork with different types of exchanges and devices with dif­ferent signalling diagrams and connec­tion methods, see the Technical Data. The connections can be digital systems, 30/32-channel PCM in accordance with CCITT recommendations, or analog lines with four wires for speech and two for signalling. The matching to two-wire and four-wire lines is done via terminal circuits connected to the selector.

The signalling between AXT 121 ex­changes is common channel signalling, based on CCITT signalling system No. 7. The signalling then takes place in time slot 16 at a data speed of 64 kbit/s on the digital PCM systems. CC signalling is

Network synchronization Digital technology in the telecom­munication network requires network synchronization. This means that all clocks in the digital exchanges must have the same frequency or timing in order to be able to control incoming bit streams and connect the information through the digital selector in the ex­change with as little disturbance as pos­sible. When the clock frequencies of the exchanges deviate too much from each other, slips occur, i.e. some PCM sam­ples are lost or repeated during the transmission between exchanges. This is experienced by the subscribers as clicks

The network synchronization equip­ment in AXT121 includes a clock modu­le, CLM, which contains an oscillator with adjustable frequency. This fre­quency is compared with frequencies from one or several external reference sources, either the main reference clock for the network or the frequencies ob­tained over links from surrounding ex­changes. The result of the comparison forms the input data to a regulation al­gorithm in the regional software. The resultant output regulates the oscillator in the clock module.

Fig. 8 An example of an integrated defence network with common switching technology in different ap­plications

Network exchange, transit/local (AXT)

Operationsl exchange, telephony/radio (AXT>

PABX (AXT)

Remote subscriber multiplexer (RSM)

Transmission link

Radio station

Each AXT 121 exchange contains a number of reference clocks that are connected to the clock module. The clock or clocks to be used for the syn­chronizat ion are determined with the aid of the exchange data in the central software. It is therefore possible to re­move or add reference sources by means of a command from the ex­change VDU terminal or f rom a control centre.

A digital AXT 121 network can be oper­ated either plesiochronously or syn­chronously in the way described above. Plesiochronous operation means that the clocks that control the bit rate in the exchanges are independent of each other, but at the same time they deviate very little f rom each other. Extremely ac­curate atomic clocks are normally used for this purpose.

Network applications with AXT 121 Introduction The te lecommunicat ion networks are planned and dimensioned to give the opt imum solut ion taking into considera­t ion the specif ied condi t ions and re­quirements.

The basic technical plans, together with such factors as reliability requirements, costs, existing networks, traffic require­ments, subscriber numbers, growth rate etc., consti tute the foundat ion for the planning and dimensioning The final result should comprise the network structure, dimensions of exchanges and routes, the number of exchanges and their posit ions, transmission paths etc.

In special te lecommunicat ion networks the avilability and the ability of the net­work to withstand jamming are gener­ally increased by making the network mesh-shaped and giving it a certain amount of extra capacity, and also by a judic ious choice of transmission media and transmission paths.

With mesh-shaped networks several dif­ferent paths should be available from each publ ic and private exchange. The extra capacity ensures that the network can operate even if a certain number of exchanges and lines have been knocked out. It also takes care of any uneven loading and high traffic volumes with short average call lengths (less than 60 seconds).

Telecommunication network for defence forces Fig.8 shows an example of a modern digital te lecommunicat ion network for defence forces. The command centres and the headquarters can belong to air force, naval or army units. At the com­mand centres information is collected f rom different sensor systems (e.g. radar systems), and then processed and pre­sented for evaluation and decisions. Commands are transmitted via the net­work to various defence units, such as air bases, ships, aeroplanes etc.

The command centres, sensors and de­fence units are connected together by a te lecommunicat ion network that satis­fies the special requirements. Such net­works can be regional, nation-wide or global.

The network may consist of, for exam­ple, transmission systems (cable, radio relay link and optical fibre systemsetc), radio systems in different frequency bands, exchanges, subscriber equip­ment and encrypt ion systems.

The exchanges in the network consist of the AXT system in different applications, such as PABXs, line and transit ex­changes or combinat ions of these. The AXT exchange at the command centre handles internal and external tele­phony. Radio traffic can also be inte­grated into the exchange for control and supervisory purposes. Subscribers with integrated telephony and radio can also be offered facil i t ies for remote control of

195

Fig. 9 An example of a country-wide special private network

Transit exchange (AXT)

PABX (AXT, ASB etc.)

Transmission

the telecommunication functions of dif­ferent radio and radar stations This is made possible by installing small AXT exchange modules at the radio and radar sites. Such modules would give greater flexibility as regards connec­tion, and would enable changes in the network structure to be carried out from command or control centres connected to the network.

The use of small, remotely installed units, such as subscriber multiplexers, RSM, provides greater flexibility both as regards the geographical positioning and the connection of subscribers or other telecommunication networks to the basic network. Thirty subscribers can be connected to an RSM, and a 30/32-channel PCM system is therefore required. The introduction of PCM re­sults in fewer cable pairs and thereby in reduced costs.

The AXT121 exchanges in the network offer a number of functions designed to handle a high traffic load or traffic in damaged networks, e.g. automatic re­routing, multiple connection of sub­scribers to different AXT 121 ex­changes, priority facilities, high traffic handling ability and traffic regulation fa­cilities via visual display terminals in AXT 121.

As can be seen from fig. 8 it is possible to install AXT 121 exchanges in containers for transport to and operation at places where damaged exchanges have to be replaced or where the network needs to be augmented locally.

Rapid telecommunication facilities be­tween different centres, radar stations and other information sources are an

essential requirement for defence sys­tems, particularly for the command and control of split-second operations This requires automated, congestion-free telecommunication systems with short connection times and a high traffic handling capacity. It must also be possi­ble to set up hot lines and semi-perma­nent connections through the digital se­lector, controlled from a visual display terminal. Priority facilities are essential. The AXT exchanges provide all these facilities and also permit rapid changeover to different types of opera­tion.

Information security in the network is obtained through the encryption of whole PCM systems or of individual point-to-point circuits

Special telecommunications networks AXT 121, with its high traffic handling capability and large range of functions, is suitable for special country-wide net­works when it is desired to keep the cost of the transit network low, or when the above-mentioned special requirements have to be met.

Compared with ordinary stored-pro-gram-controlled PABXs of the same size and equipped with transit functions, AXT 121 gives a call capacity that is five to ten times larger. This is of course ex­tremely important when planning a tele­communication network, as probably fewer transit exchanges will be re­quired, with a consequent reduction in costs. Since AXT 121 is also a combined transit exchange and PABX, it is possi­ble to combine the connection of sub­scribers and transit lines in one and the same exchange, which also increases the efficiency of the exchange.

Such AXT 121 networks can then be equipped to provide ability to withstand disturbances, fast connection pro­cedures and centralized operation and maintenance of the exchanges.

Fig. 9 shows an example of such a net­work, which connects different organi­zational units. The terminals then con­sist of PABXs (e.g. AXT, ASB). This net­work also makes it possible to carry out the central control functions for the or­ganization from more than one place in the network.

196

Fig. 10 Operational changes are carried out by means of the local video display terminal in the exchange

The types of organizations for which such networks would besu i tab leare. for example, oil companies, the police, the customs, national civil aviation admin­istrations, banks, power companies and other large companies.

Operation and maintenance of AXT121 networks The operation and maintenance costs consti tute a large part of the total costs incurred in a te lecommunicat ion net­work. It is therefore important to mini­mize these costs when the technology and system are decided. It is normally also essential to keep the number of dif­ferent types of equipment low in order to minimize the adaptat ion and service costs that arise in connect ion with main­tenance, stores administrat ion, docu­mentat ion, t raining, development of dif­ferent support systems etc.

Stored program control of the ex­changes simplif ies the operation and maintenance routines. System AXT has been designed with eff icient operation and maintenance funct ions, combined with high quality. The AXT exchanges are designed and produced for 2 0 - 4 0 years of operat ion.

To summarize, the high operational re­liability of the AXT121 exchanges and networks has been achieved by means of such features as: - modular structure - dupl icat ion of the central exchange

equipment - low fault rates for boards and compo­

nents - long-term testing of hardware and

testing of cabl ing and software - automatic fault detect ion and loca­

t ion, together with short repair times - sophist icated network funct ions,

such as automatic rerout ing, mult i -connect ion of subscribers, priority fa­cilities.

Maintenance organization for AXT 121 exchanges The operation and maintenance can be organized at different levels, e.g. locally and centrally. The staff in an operation and maintenance centre, OMC, must be kept informed about the situation in the exchanges by means of alarm messages and other information on video display units. When a fault has been located the staff can dispatch a repairman to the exchange. The repairman is told in ad­vance which unit is faulty and has to be exchanged. Repair centres and stores are organized in the most suitable way for the network, with regard to the geo­graphical condit ions, security, costs etc. Faulty printed board assemblies can normally be repaired and tested by Ericsson, since the number of faulty units is small and centralized repair makes for more efficient uti l ization of the repair support systems.

The operation and maintenance staff are often also responsible for changes in the operational data, for example ex­tensions and modif icat ions of line and subscriber data. Most operational changes can be carried out f rom the OMC video display terminals or f rom the local VDU terminal in the exchange, fig. 10.

Operational statistics are col lected for use in short-term and long-term plan­ning, for example for the dimensioning of routes.

The operating staff is also responsible for any changes in the network struc­ture, for example in the case of overload or damage to the network. The ex­

changes can then be loaded with alter­native exchange data, preferably data that has been prepared and tested in advance.

Summary Some of the main characteristics of sys­tem AXT 121 are summarized below: - The system has been designed for use

in different applications in telecom­municat ion networks, such as transit and local exchanges and PABXs, or combinat ions of these.

- The system is f lexible, having a modu­lar structure which greatly simplifies changes as regards technology, ca­pacity and funct ion.

- High traffic handl ing capability and sophist icated traffic routing provide high availability in the network.

- High reliability is ensured by the use of modular structure, duplication of central units in the exchanges, auto­matic supervision, short repair times and low error rates.

- All units are supervised continuously and an alarm is given if a fault occurs. The cause of the fault is indicated on a visual display unit.

- Addit ions and modif ications in the ex­change data are easily made with the aid of preprogrammed menus dis­played on a visual display terminal.

The combined PABX and transit ex­change AXT 121 offers several advan­tageous traffic facil i t ies: - local and transit traffic - direct inward dial l ing to PABXs - mult iple connect ions - group numbers - free number ing scheme - conferences - closed subscriber groups - long-term connect ions - direct connect ions - enquiries - transport - operator handl ing with special facili­

ties.

References 1. Brunberg, G. and Widen, B.: A digital

Telecommunication System for Opera­tional Centres. Ericsson Rev. 57 (1980):1, pp. 16-25.

2. Larsson. C. and Olsson, E.: Remote Subscriber Multiplexer-RSM. Erics­son Rev. 60 (1983):2, pp. 58-65.

ERICSSON

ISSN 0014-0171 Teletonaktiebolaget LM Ericsson 13785 Liungforetagen. orebro 1985