axe-10(a)

J.T.O. Phase II (Switching Specialisation) : AXE-10

AXE-10

Contents Page

1.0 Introduction to AXE 1

1.1 What is AXE? 1

1.2 AXE as viewed by the subscriber 3

1.3 AXE as viewed by the telecom administration 5

1.4 Flexibility - the be-all and end-all 8

2.0 AXE system structure 8

2.1 Processors in the AXE system-basic principles 8

2.2 System structure 14

2.3 Internal interworking and hardware in APT 21

2.4 The digital group switch 34

2.5 The digital subscriber stage 44

2.6 APZ 211 and APZ 212-control parts of the AXE system 55

2.7 The I/O (Input/Output) system in AXE 69

2.8 Addressing principles and the operating system 74

2.9 Traffic handling 83

Appendix 97

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1. Introduction to AXE

1.1 What is AXE?

This question may be answered in many different ways. Some would say, “A

telephone exchange”, while others might be more specific and say, “A telephone

system capable of serving all types of telecom networks- national as well as

international”. And many of the answers given would be right.

But if the question reads, “What do the three letters ‘AXE’ stand for?”, there

will usually be no answer.

What, then, does “AXE” mean? - The answer is that it is just a three-letter

code denoting an Ericsson product.

All products, instruments, tools, etc. made or used by Ericsson are identified

by a three-letter code.

The three letters are usually also followed by a number to indicate product

variants.

We will discuss this matter in more detail later on in this book, Section 4.2.

Let us now revert to the first question, “What is AXE?”

To be able to give a comprehensive answer we are going to use a

comparative example: we will compare an AXE exchange installed today with one of

the first AXE exchanges ever installed, that is, the Sodertalje Exchange just south of

Stockholm, which was cut over in 1976.

If we could place these two exchanges side by side, we would find that they

look quite different. And if we take a closer look, the differences will become even

more manifest. The older version uses relay-based technique for some of its

functions, whereas relays are very rare in the newer one. The modern exchange

features a wide range of facilities for clients to choose among, whereas the old one

can offer only a limited number.

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Yet both are called AXE. Where is the logic in this?

The answer is as follows: Even though the two versions differ as far as

external characteristics are concerned, they are very similar in terms of internal

structure because the same system structure has been used. Furthermore, the same

type of design aids have been used in designing the two exchanges.

Since this internal structure is in no way dependent on the technology used,

the AXE system is sometimes referred to as “future-proof”.

Another ten years from now new technology will be available, resulting

perhaps in new AXE versions.

1.2 AXE as viewed by the Subscriber

A subscriber will make certain demands on his telephone as well as on the

telecom network as a whole. These demands are usually more or less unreasonable:

“My telephone should function at all times, and it is a must that I should

always be connected to the number I have dialled”.

Of course, such a demand is excessive, but on the other hand reality is not

many steps behind. In most countries, the portion of unsuccessful calls due to

technical faults and congestion, can be far below 1 per cent.

Another demand is that a telephone that is out of service should be quickly

repaired. In these situations, subscribers will receive better service if the exchange

itself can decide whether the telephone or the line is faulty.

These types of demands- together with the demand for quick set-up of

connections- have always been made by subscribers.

The introduction of computer-controlled telephone exchanges also meant the

introduction of a new concept- SUBSCRIBER FACILITIES. An AXE exchange can be

provided with a variety of subscriber facilities, which means that subscribers can be

offered better service.

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We are now going to take a look at some of the facilities offered and see how

they can be used.

Subscriber Facilities in AXE

Wake-up and Reminder Service

The subscriber can dial the hour for automatic wake-up on his telephone.

Call Transfer (“Follow me” or Temporary Call Transfer)

The subscriber can divert calls intended for his number to any other number

within a specified area.

Abbreviated Dialling

A short code replaces a long number or a number used frequently by the

subscriber. The capacity is up to 100 numbers per subscriber.

Non-dialled Connection (“Hot Line”)

The subscriber need only lift the handset (receiver) to be connected to a given

number, either directly or after, say, 5 seconds. If the subscriber dials a digit

during these 5 seconds, he can use his telephone in the usual manner.

Alternation on Inquiry

The subscriber presses a button to alternate between two calls.

Add-on Conference (Three-party Conference)

Three subscribers can converse with each other simultaneously.

Call Waiting

The subscriber hears a weak tone if called by a third party during a conversation

in progress. This facility also includes alternation on inquiry.

Diversion

This facility is available in two variants: diversion on busy and diversion on no

reply. A common characteristic of both variants is that diversion takes place to

some other number programmed by the subscriber.

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These are some of the subscriber facilities offered by the AXE system today.

Future AXE facilities are dealt within section 3.5, ISDN.

1.3 AXE as VIEWED by the TELECOM ADMINISTRATION

Who buys an AXE exchange? In most cases the buyers are national telecom

administrations, but some countries have private telephone companies- Finland and

the USA, for example.

Of course, the buyers also make demands on the telephone systems they are

going to purchase.

The administration usually makes a so-called CHOICE OF SYSTEM, which

means that it decides to buy a large number of exchanges from one and the same

supplier.

In this way, maintenance, spare parts handling, training, etc. will be easier to

organize as compared with a purchase comprising various types of exchanges from

different suppliers.

Considering the fact that the service life of an exchange is very long, we

realize that this kind of decision is a very important one.

It is essential that the administration should choose the “right” system from

the beginning.

We will now mention some of the factors that an administration must take into

account before adopting a new system.

As readers, you should have these factors in mind when studying the system

structure later on in this book.

Does the system include basic functions (coin telephones, private exchange

functions, etc.)?

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Can the system handle operator-controlled traffic, for instance, to a local

exchange?

What other facilities does the system offer? Note that subscriber facilities can be

profitable to an administration.

EXAMPLE: The “Call Waiting” facility results in a larger portion of successful

calls, thus increasing the number of charged calls as well as the administration’s

business earnings.

Will future extensions be costly? (Is “spare capacity” for future extensions

available?)

Does the system include concentrators? (Can the administration offer subscriber

facilities to subscribers in rural areas?)

Can the system provide the administration with adequate statistical information?

Such information constitutes a useful tool when dimensioning the network, which

in turn results in a higher grade of service for the subscribers.

Is the system capable of handling digital transmission?

How many alternative routes (number of routes and number of lines per route)

can the system handle?

Will the system be able to satisfy present and future demands as regards

numbering? (A numbering plan often covers a period of 30-50 years into the

future).

Will it be easy to change the numbering of subscriber lines? (A subscriber who

moves to a new address within the same exchange area usually wants to keep

his old number).

Is the system capable of handling present and future call metering methods?

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Does the system incorporate facilities for time-differentiated call metering?

(Lower rates in the evening than during office hours).

Can the system handle call metering for coin telephones and special facilities?

Is the system compatible with all existing and planned signalling systems? (For

instance, CCITT’s Signalling System No. 7).

Will the system be easy to operate and maintain?

Operation and maintenance activities are performed by personnel who (1) cost

money and (2) need training. Reduction in the number of personnel and/or

training time will, of course, reduce costs.

Will centralized operation and maintenance be possible? (Unattended local

exchanges are supervised from a central point. This means less personnel and

lower total cost of operation and maintenance).

Is automatic testing of system equipment provided? (Such testing will facilitate

fault tracing, thus reducing repair time).

Is the system easy to communicate with? (Shorter personnel training time).

As we can see, a great many factors influence the purchase of telephone

exchanges. Since today’s systems are beginning to reach a very high degree of

complexity - a fact which makes them difficult to evaluate - some administrations find

it convenient to buy one exchange from each of a number of suppliers. This gives the

administration time to evaluate the different systems and to compare them with one

another before deciding on one or, perhaps, two systems.

Can we then say that AXE satisfies these requirements? YES, INDEED. Its

designers took them into account even at the “drawing board stage”.

Since the development of the system was controlled at all times by the

demands made on its performance, the solutions to the problems resulting from

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these demands form an integral part of the system. Or in other words: there are no

temporary solutions in AXE.

FLEXIBILITY- The Be-all and End-all

Does a telephone system have to be flexible? Yes, a telephone system must

be flexible from two different points of view.

First, flexibility is a prerequisite when producing and selling the system. It

must be possible to use one and the same system in different parts of the world and

to satisfy different requirements with regard to system operation.

Second, the system must be flexible for telecom administrations to operate. In

this context, it is of particular importance to remember that an exchange cannot just

be shut down for extension or repair.

All modifications, repairs or changes must be made while the exchange is in

service, and without disturbing the traffic handling. These factors, too, have been

taken into consideration when designing the AXE system. Only very extensive

changes in the exchange will interfere with the traffic, though still to a very small

degree.

2. AXE SYSTEM STRUCTURE

2.1 PROCESSORS in the AXE SYSTEM- BASIC PRINCIPLES

The AXE system is referred to as an SPC system. Here, SPC stands for

Stored Program Control, which means that programs stored in a computer control the

operation of the exchange. (Note that exchange is used generally to denote either

the plant as a whole - i.e. including the means of control employed - or that part of

the plant which performs the telephony or switching functions).

All operations to be performed by the exchange are stored in the computer

memory. To modify a function we must consequently modify the computer memory.

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Figure. 2.1.1An SPC Exchange

The memory contains a large number of instructions which tell the computer

what to do in different situations. To illustrate this, we may compare an AXE

exchange with an old manual exchange.

A manual exchange is controlled by an operator. During the decades

immediately before and after the turn of the century this was the most common type

of exchange, but even today manual exchanges are used (small company PBXs,

hotel PBXs, etc.; PBX = Private Branch Exchange).

Figure 2.1.2 shows a manual exchange used in Vasa (Finland) in 1890.

Figure 2.1.2Manual Exchange in 1890

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Putting it somewhat simply, we might say that in AXE the operators have

been replaced by a powerful computer. The computer memory contains all the

information and skills previously possessed by operators.

In those days, “reprogramming” the operator meant telling her how to change

her procedures. Thus, to change something in AXE we must reprogram the

computer, i.e. modify the list of instructions. There are many other similarities

between manual exchanges and AXE.

For instance, what would happen in the manual exchange if the operator was

taken ill? It would, of course, “stop”.

To improve the reliability of a manual exchange we may have two operators,

one of whom is standby. And this is also a principle used in AXE: the switching

equipment is controlled by two computers, one of which is standby. We will revert to

this duplication concept later on.

APT and APZ

As has been said, AXE consists of two main parts: switching equipment for

switching telephone calls, and a computer for controlling the switching equipment.

These two parts have been given designations resembling the AXE letter code. The

switching equipment is called APT, and the computer is called APZ.

But not just what we can see and touch in the exchange is called APT. APT

also has programs, which are stored in the computer (APZ) but which belong to the

exchange (switching) part (APT).

To illustrate this correlation we are going to design a simple system for traffic

signals to be used at an intersection, and these signals will be controlled by a

computer. Let us assume that we buy a computer consisting of a central processing

unit containing the processor and the memory, and that we supplement this computer

with a DISPLAY UNIT, a KEYBOARD and a FLOPPY-DISK UNIT. These last three

units are known under the collective term of INPUT/OUTPUT DEVICES.

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Figure 2.1.3Personal Computer

We assemble our computer equipment, connect it, and switch on the power.

What will happen?

A beep is heard, and something is printed out on the display. Obviously, the

computer already contains some kind of program. And this is called the operating

program because it handles the work performed in the computer. What we now have

in front of us on the desk corresponds to the APZ part of AXE. Thus, APZ consists of

hardware (the computer, the memory, the input/output devices, etc.) and software for

handling memories and input/output devices, and for administering the work done by

the computer.

We are now going to take a look at the functions to be controlled by our

computer.

The traffic signal system will be of modern type, with dug-in sensors for

detecting motor-cars. In addition, the traffic signal posts will have buttons to be

pressed by pedestrians before crossing the street.

Figure 2.1.4Traffic Signals Controlled by a Computer

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To control these traffic signals we must write a program which tells the

computer how to act in different situations.

And the program that we write must have certain data to work with.

The data in our program will be, for instance, what the signals indicate at any

given moment. The computer must “remember” what the signals indicate to enable

the program to work satisfactorily. We provide the computer with two kinds of

material: a program and data. The program will not change when the system is

started up, but the data will.

We will now compare our traffic signal system with the AXE system and

define some common concepts.

The program we have written is intended for a specific application. Hence, as

opposed to general programs, this type of program is called Application Program.

Our application program consists of program and data, or Software.

The traffic signals, the sensors, the lines and the program that we have written to

control these correspond to APT in AXE.

Consequently, APT in AXE consists of the exchange (printed board

assemblies, lines, etc.) and of software stored in the computer (APZ).

APT = Telephony part of AXEAPZ = Control part of AXE

Figure 2.1.5The Two Parts of an AXE Exchange

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Let us now take a closer look at the computer that controls the exchange.

TWO TYPES of PROCESSORS

As you will understand, we cannot use a personal computer like the one used

in our traffic signal system.

The work to be performed in a telephone exchange can be said to fall into two

main groups:

1. Routine scanning of equipment to detect changes. An example is the

checking performed to see if a subscriber has lifted his handset. This is

done several times every second.

2. Complex analyses and diagnostics requiring high computing capacity and

large volumes of data. Examples are the selection of outgoing routes or

traffic measurements.

These two chief tasks have one thing in common: the importance of the TIME

factor.

Here, TIME refers to the moment at which something is done or happens.

(When a subscriber lifts his handset he expects to receive dial tone directly - not

after, say, 10 seconds).

A computer designed to cope with such time requirements is usually called a

real time processor or just processor. The solution is to have two different types of

processor to control the system: one Central Processor (CP) and a number of

Regional Processors (RP). The RPs assist the CP in performing routine tasks and

report important events occurring in the exchange to the CP.

All decisions are made by the central processor.

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Figure 2.1.6The Architecture of the Control-system

Figure 2.1.7RP Handles Simple but Frequent Tasks, whereas CP Handles Complex Tasks

This type of configuration permits simple modification of the system capacity

by just increasing or decreasing the number of regional processors. This rule applies

up to the capacity limit of the central processor.

2.2 SYSTEM STRUCTURE

As we have already seen, the AXE system consists of two main parts: APT,

which is the telephony part, and APZ, which is the control part. Both APT and APZ

use hardware (printer board assemblies) and software (programs and data). We will

now take a closer look at the telephony part, APT, and see what it includes. Later on

in this book we will also discuss the control part, APZ.

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APT

To facilitate the handling of a system the size of AXE, APT has also been

divided into a number of Subsystems.

The division into subsystems is function-related, and below we will briefly

discuss some of the many reasons why such a division is necessary.

DESIGN : The responsibility for the design of a subsystem rests with a

department or section at Ericsson.

DOCUMENTATION : The fact that the division into sub-systems is function-

related facilitates the locating of the documents involved.

SYSTEM DESCRIPTION : Some subsystems are needed only in certain

applications. The names of the subsystems included in a particular exchange

give a condensed description of the tasks to be performed by the exchange

concerned.

The name of a given subsystem reflects the function of that subsystem. Some

subsystems contain only software whereas others contain both software and

hardware. We will now briefly discuss all the subsystems presently used in APT (the

telephony part). Some of them will be studied in more detail later on.

SUBSYSTEMS in APT

TCS, TRAFFIC CONTROL SUBSYSTEM: Only software. TCS is a central

part of APT and can be said to replace the operator of a manual system.

Examples of the subsystem’s functions are:

– Set-up, supervision and clearing of calls.

– Selection of outgoing routes.

– Analysis of incoming digits.

– Storage of subscriber categories.

TSS TRUNK and SIGNALLING SUBSYSTEM: Software and hardware. The

subsystem handles the signalling over and the supervision of connections to

other exchanges.

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GSS GROUP SWITCHING SUBSYSTEM: Software and hardware. GSS

sets up, supervises and clears connections through the group switch.

Selection of a path through the switch takes place in the software.

OMS OPERATION and MAINTENANCE SUBSYTEMS: Software and

hardware. The subsystem contains various functions related to statistics and

supervision. OMS is one of the largest subsystems in APT.

SSS SUBSCRIBER SWITCHING SUBSYSTEM: Software and hardware.

The subsystem handles traffic to and from subscribers connected to the

exchange.

CHS CHARGING SUBSYSTEM: Only software. The subsystem handles call

metering (call charging) functions. Two call metering methods are available:

pulse metering and toll ticketing.

SUS SUBSCRIBER SERVICES SUBSYSTEM: Only software. Subscriber

facilities (services), such as abbreviated dialling, are implemented in SUS.

OPS OPERATOR SUBSYSTEM: Only software. The subsystem handles the

connection and disconnection of operators. OPS cooperates with OTS

(Operator Terminal System), which includes the operator positions.

CCS COMMON CHANNEL SIGNALLING SUBSYSTEM: Software and

hardware. Two variants exist: one for CCITT No. 6 and one for CCITT No. 7.

CCS contains functions for signalling, routing, supervision and correction of

messages sent in accordance with CCITT No. 6 or No. 7.

MTS MOBILE TELEPHONY SUBSYSTEM: Software and hardware. The

subsystem handles traffic to and from mobile subscribers.

MNS NETWORK MANAGEMENT SUBSYSTEM: Only software. The

subsystem contains functions for supervising the traffic flow through the

exchange, and for introducing temporary changes in that flow.

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APT = Telephony Part of AXECCS = Common Channel Signalling SubsystemCHS = Charging SubsystemGSS = Group Switching SubsystemMTS = Mobile Telephony SubsystemNMS = Network Management SubsystemOMS = Operation and Maintenance SubsystemOPS = Operator SubsystemSSS = Subscriber Switching SubsystemSUS = Subscriber Services SubsystemTCS = Traffic Control SubsystemTSS = Trunk and Signalling Subsystem

Figure 2.2.1Subsystems in APT

As has been said, the control part consists of one central processor and a

number of regional processors.

The task of the software allocated to a subsystem is to control the hardware

of that subsystem.

Since the hardware (the telephony devices) is controlled by the regional

processors, these must, of course, also contain programs belonging to the

subsystem concerned. Consequently, the software for a subsystem can be divided

into one central part (programs + data which are stored in the central processor) and

one regional part (programs + data which are stored in the regional processors).

Naturally, this applies only to subsystems containing hardware.

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APT = Telephony Part of AXECCS = Common Channel Signalling SubsystemCHS = Charging SubsystemGSS = Group Switching SubsystemMTS = Mobile Telephony SubsystemNMS = Network Management SubsystemOMS = Operation and Maintenance SubsystemOPS = Operator SubsystemSSS = Subscriber Switching SubsystemSUS = Subscriber Services SubsystemTCS = Traffic Control SubsystemTSS = Trunk and Signalling Subsystem

Figure 2.2.2The Structure of Subsystems in APT

Structuring of Subsystems

Each subsystem is in turn divided into a number of parts called FUNCTION

BLOCKS. At this level, too, the division is function-related.

To illustrate this we are going to study the Trunk and Signalling Subsystem

(TSS).

TSS contains a function block called BT (Both-way Trunk). The function of the

BT function block is to handle both-way digital links between exchanges. (A both-way

trunk is a trunk that can carry traffic in both directions). Of course, there is hardware

to which the digital link is connected. In this case, the hardware consists of a printed

board assembly containing circuits and logic for clocking the digital signals.

A regional processor contains software to control and supervise the

hardware. The software belongs to the BT function block. If a change occurs in the

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hardware, this will be detected by the regional software, which scans the hardware at

regular intervals.

The regional software (BTR) will then inform the central software (BTU) in the

BT function block.

After that, the central software can interwork with other function blocks in the

central processor. The interworking between function blocks always takes place at

the central level, i.e. in the central processor. See Figure 2.2.3.

BT = Bothway trunkBTR = Regional software of block BTBTU = Central software of block BT

Figure 2.2.3Examples of Function Blocks

As shown in the figure, function block Y has neither hardware nor regional

software, and this is just as frequent a solution as any other combination, taking into

account that entire subsystems may consist exclusively of central software.

The data belonging to a function block can only be addressed by the block’s

own programs. If a block needs data from some other block, it must make a

“request”.

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WHY FUNCTION BLOCKS?

The basic idea of function blocks can be explained as follows:

– Well-defined processes with data of their “own”.

– Borders between function blocks where the exchange of information is

least frequent.

– A function block need not know what other blocks do.

– Standardized signals between the function blocks.

To summarize this section we are going to study Figure 2.2.4, which shows

the structure of the AXE system.

Remember: The division into different units at different levels is always function-

related.

APT = Telephony Part of AXEAPZ = Control Part of AXEBT = Bothway TrunkBTR = Regional software of block BTBTU = Central software of block BTCPS = Central Processor SubsystemCS = Code SenderFMS = File Management SubsystemHW = HardwareMCS = Man-machine Communication SubsystemOMS = Operation and Maintenance SubsystemOT = Outgoing TrunkSUS = Subscriber Services SubsystemTSS = Trunk and Signalling Subsystem

Figure 2.2.4The Structure of the AXE System

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2.3 INTERNAL INTERWORKING and HARDWARE in APT

We are now going to have a closer look at some central system parts. To

describe the operation of an AXE exchange we will study how TCS (Traffic Control

Subsystem) interworks with the other subsystems.

As has been said TCS is the central part from the traffic-handling point of

view. TCS in AXE corresponds to the operators in a manual system.

Remember that TCS consists only of central software.

TCS = Traffic Control SubsystemFigure 2.3.1

A Comparison

The TCS subsystem consists of 9 important function blocks; see Figure 2.3.2.

CL = Call supervisionCOF = Coordination of Flash servicesDA = Digit AnalysisRA = Route AnalysisRE = Register functionsSC = Subscriber CategoriesTCS = Traffic Control SubsystemTOD = Trunk Offering DataTOM = Trunk Offering ManagementSECA = Semi-permanent Connections

Figure 2.3.2Some of the TCS Function Blocks

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RE REGISTER FUNCTION

This block stores the incoming digits and handles the set-up of calls.

CL CALL SUPERVISION

This block supervises calls in progress and clears them.

DA DIGIT ANALYSIS

This block contains tables for digit analysis. Such analysis is ordered by RE.

RA ROUTE ANALYSIS

This block contains tables for selecting outgoing routes (including alternative

routes). Such selection is ordered by RE.

SC SUBSCRIBER CATEGORIES

This block stores subscriber categories for all subscribers connected to the

exchange.

TOM TRUNK OFFERING MANAGEMENT

This block takes over the functions of RE or CL when a busy subscriber is to be

supervised by an operator.

TOD TRUNK OFFERING DATA

Like TOM, this block takes over the functions of RE or CL when a busy

subscriber is to be supervised by an operator.

COF COORDINATION OF FLASH SERVICES

This block takes over the functions of CL when more than two subscribers are to

take part in one and the same speech connection. (This applies to certain

subscriber facilities).

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SECA SEMI-PERMANENT CONNECTIONS

This block permits the setting-up of semi-permanent connections through the

group switch.

As we can see, the TCS subsystem occupies a central position in the AXE

system. As its name indicates (Traffic Control Subsystem), TCS’s tasks

include controlling the set-up and clearing phases. Figure 2.3.3 shows where

in the system TCS is positioned.

CCS = Common Channel Signalling SubsystemGSS = Group Switching SubsystemMTS = Mobile Telephony SubsystemSSS = Subscriber Services SubsystemTCS = Traffic Control SubsystemTSS = Traffic and Signalling Subsystem

Figure 2.3.3A Central Part of APT (The figure does not include all subsystems)

SIGNALLING

To set up a call to another exchange, the operator of an old-type manual

system exchanged verbal information (“signals”) with other operators. When

automatic exchanges were introduced, these, too, needed to exchange signals.

Different electrical signals were given different meanings. Signalling can be divided

into two main groups: line signalling and register signalling.

Line signals control the set-up and clearing of a speech connection. Register

signals contain information such as the number to which a call is to be connected.

Register signals are only used in the set-up phase. Let us compare automatic

signalling with the operator’s way of communicating.

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To set-up a call to another exchange the operator sends a current through the

line by turning the handle of a generator. The current causes an indicator to react at

the receiving operator’s desk, thus indicating that a call is coming. This is a line

signal. The receiving operator connects her headset to the line and says, “Hello”. The

other operator hears this and says, “Please connect me to number 1234”. These are

examples of register signals.

This was one of the first procedures for interexchange signalling. During the

hundred years of telephony history, a great many signalling systems have been

developed. These systems have naturally been dependent on the technology

available, and consequently the “history of signalling” covers a wide range of means -

from uncomplicated currents and tones to today’s high-capacity digital signalling

systems.

This development process has resulted in a mixture of new and old

technology in telecom networks. An exchange must often be capable of handling

many different signalling systems simultaneously.

In the AXE system, this problem has been solved by letting the TSS

subsystem (Trunk and Signalling Subsystem) adapt different signalling systems to

TCS. In other words, TCS can be said to be unchanging.

GSS = Group Switching SubsystemRP = Regional ProcessorTCS = Traffic Control SubsystemTSS = Trunk and Signalling Subystem

Figure 2.3.4Adaptation to Different Signalling Systems is made in TSS

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To see how TCS works we will study a small portion of an incoming call to an

AXE exchange.

A STUDY CASE

The register signalling system used in our example is MFC (Multi Frequency

Code). MFC sends register signals by combining two tones. A special piece of

equipment is required to handle these tones. This equipment is called the CR (Code

Receiver) and is connected via the group switch.

CR = Code ReceiverDA = Digit AnalysisGSS = Group Switching SubsystemIT = Incoming TrunkRE = Register FunctionRP = Regional ProcessorTCS = Traffic Control SubsystemTSS = Trunk and Signalling Subsystem

Figure 2.3.5Hardware and Software for an Incoming Call

The sequence of events is as follows:

(i) The other exchange wants to set–up a call to “our” exchange, and

selects a free line to interconnect the two exchanges.

(ii) The other exchange sends a line signal to our exchange simultaneously

with the sending of the first digit by means of MFC signals.

(iii) The line signal is detected by the regional processor scanning the

incoming line (IT, Incoming Trunk). The regional processor sends a

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message to the central software of the IT block, telling it that a call

attempt is in progress.

(iv) As IT’s central software (ITU) receives the message, it consults its data

and finds that the line concerned uses MFC signalling. ITU now

requests a CR from the central software (CRU) of the CR block.

(v) CRU selects a free CR device and orders GSS (Group Switching

Subsystem) to connect the CR device to the IT device.

(vi) ITU informs the RE block in TCS that a call is coming. RE reserves a

data area in the memory to be used exclusively for this call.

(At this point, all arrangements have been made for the reception of

digits from the other exchange).

(vii) The first digit is received by the CR device. The regional processor

scans the CR device and sends the digit to CRU. CRU sends the digit

on to ITU, which forwards it to the register, RE.

(viii) RE sends the digit to the DA block for analysis. The DA block contains a

number of tables for digit analysis. The result of the analysis is stored in

RE. Depending on the result of the analysis, the register can now take

different kinds of action.

CR = Code ReceiverDA = Digit AnalysisGSS = Group Switching SubsystemIT = Incoming TrunkRE = Register FunctionRP = Regional ProcessorTCS = Traffic Control SubsystemTSS = Trunk and Signalling Subsystem

Figure 2.3.6The Digit is Transferred from the CR Device to the Register

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As has been said, it is the register that controls the set-up phase. This control

is based on the result obtained in the digit analysis.

The following data may come from the DA block on completion of the digit

analysis (one digit at a time is analysed - not the whole B-number in one go).

– Send the next digit.– Routing case (the analysis in the Route Analysis Block, RA, indicates an

outgoing route). – Charging case.– Number length.– Terminating Call.– Modification of B-number.– End of analysis.

We have now studied the processing of a call in AXE, and we will revert to

this subject later on in this book.

HARDWARE in TSS and CCS

We will now study some of the TSS and CCS hardware in APT. It is important

to remember that all hardware is controlled by its own software both in the central

processor and in the regional processors.

INCOMING and OUTGOING TRUNKS (TSS)

ETC = Exchange Terminal CircuitGSS = Group Switching SubsystemIT = Incoming TrunkOT = Outgoing TrunkPCD = Pulse Code Modulation Device~ = Analog Signal

= Digital Signal

Figure 2.3.7Hardware for Incoming and Outgoing Trunks

27


ETC (Exchange Terminal Circuit) is the hardware of the BT blocks. An ETC

consists of a printed board assembly housed in a magazine. For examples of

magazines, see Section 2.10, “Construction Practice”.

The printed board assembly is illustrated in Figure 2.3.8.

Figure 2.3.8Exchange Terminal Circuit (ETC)

Each channel in the digital connection is regarded as a BT device. If a 32-

channel system is used, only 30 of the channels can be utilized for speech. Channel

0 is always used for synchronization and alarm information while channel 16 is used

for signalling (Channel 16 is primarily used for line signalling, but some signalling

systems can also use it for register signals).

The USA and South Korea are examples of countries using 24-channel

systems. In these systems, all 24 channels can be used for speech (Line signals are

sent by “stealing” one bit from every six samples).

OT (Outgoing Trunk) is the block used to handle outgoing analog

connections.

The hardware consists of a magazine containing 32 devices, and an analog-

to-digital converter. The converter, which is called PCD (Pulse-Code Modulation

Device), has no software and no signalling function.

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IT (Incoming Trunk) is the block used to handle incoming analog connections.

The hardware is almost identical with that of OT.

To distinguish between different variants, the “trunk blocks” are given

numbers: BT1, BT2 …. Here the term “variant” refers to different signalling systems.

Exchanges installed today are almost exclusively equipped with ETCs. In

applications with analog transmission, the digital signals sent by ETCs are converted

into analog signals.

The equipment used to do the conversion is called a Multiplexer (MUX). A

multiplexer thus converts signals from digital to analog form, but it can also multiplex

several analog signals on one and the same line (FDM, Frequency Division

Multiplex). Note that the MUX does not belong to the AXE system; it is transmission

equipment.

ETC = Exchange Terminal CircuitGSS = Group Switching SubsystemMUX = Multiplexer

~ = Analog signal= Digital signal

Figure 2.3.9A Multiplexer (MUX)

29


CODE SENDERS and CODE RECEIVERS (TSS)

CR = Code ReceiverCS = Code SenderCSR = Code Sender/ReceiverETC = Exchange Terminal CircuitGSS = Group Switching SubsystemMUX = Multiplexer~ = Analog Signal

= Digital Signal

Figure 2.3.10Analog and Digital Code Senders/Receivers

Code Senders (CS) and Code Receivers (CR) are used for sending MFC

register signals.

CR/CS are connected by means of the group switch when a device (IT, OT or

BT) needs to send register signals by MFC.

AXE has two types of CR/CS:

(i) Analog Devices: 4 CR or 4 CS in each magazine. Analog-to-digital

conversion takes place in the PCD (Pulse Code Modulation Device).

(ii) Digital Devices: 16 devices in a magazine, CSR, that can be used on

both CR and CS.

ANNOUNCING MACHINE (TSS)

30


ASD = Auxilliary Service DeviceDAM = Digital Announcing MachineGSS = Group Switching SubsystemPCD = Pulse Code Modulation DeviceRD = Recorder Device

Figure 2.3.11Analog and Digital Announcing Machines (DAM)

The announcing machine is a subscriber facility which uses recorded

messages to inform calling subscribers why they cannot reach dialled numbers.

Announcing machines are also necessary in combination with certain

subscriber facilities where the subscriber can control the facility by dialling

predetermined codes (The announcing machines inform subscribers whether they

have used the right or wrong procedure).

Two different types of announcing machine can be connected to AXE: a

digital machine of recent design, or a “conventional” analog machine.

As its name indicates, the Digital Announcing Machine (DAM) is fully digital.

Recorded verbal messages and tones are stored in digital form on two types of

storage boards: one with PROMs and one with RAMs. The messages stored in

PROMs are seldom changed and special external recording equipment is required to

make changes in them. But no external equipment is needed to change messages

stored in RAMs. In fact, uses can change them by dialling procedures on an ordinary

telephone. Consequently, these messages are best suited for the Weather Line,

sports results, news, etc.

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The maximum message length is 32 seconds for “permanent” messages and

64 seconds for information that is frequently changed. Verbal messages from an

external analog announcing machine can be connected to DAM, and external

messages can be combined with messages stored in DAM. An example of how this

type of message is used is the subscriber facility “automatic wake-up service”. When

woken up by the ringing signal, the called subscriber hears a message, for example:

“You have ordered automatic wake-up. The time is ………”. (Here a speaking clock

can be activated to give the hour).

Verbal messages can also be combined with various types of tones.

As appears from Figure 2.3.11, the analog machine requires a great deal of

peripheral equipment.

Announcing machine messages are recorded on magnetic disks, which

repeat the message as the disk rotates. To prevent subscribers from being

connected up in the middle of a message, the announcing machines send

synchronizing pulses when a message starts. These pulses are sent to a magazine

called RD (Recording Device). RD sees to it that ASD (Auxiliary Service Device)

connects the subscriber at the right moment. The ASD magazine also operates as a

“mini-switch”, as each input from the group switch must be connectable to any of the

recorded messages.

SIGNALLING TERMINALS in CCS

Signalling terminals (ST) for signalling according to CCITT No. 7 are

connected to the group switch via a PCD-D. Since the signalling terminals are digital

devices, the PCD-D equipment includes no conversion function but merely serves as

an adaptation device towards the group switch.

The signalling information from a signalling terminal is sent through the group

switch to a certain channel in an ETC.

32


This channel is then used exclusively for signalling. The advantage of

connecting the signalling terminals via the group switch is that some devices can be

kept in reserve and automatically replace inoperative devices.

ETC = Exchange Terminal CircuitGSS = Group Switching SubsystemPCD-D = Pulse Code Device - DigitalST-7 = Signalling Terminal for CCITT No. 7

Figure 2.3.12Signalling Terminals for CCITT No. 7

Figure 2.3.13Signalling Terminal for CCITT No. 7

CCITT No. 6 is a signalling system used for international connections. The

basic principle is the same as for CCITT No. 7, but the system design is adapted to

suit analog signalling links. This means that the transmission rate is somewhat lower

(2400 bit/s), that is in comparison to 56 or 64 kbit/s when CCITT No.7 is used.

33


Figure 2.3.14 shows the hardware used for CCITT No. 6.

GSS = Group Switching SubsystemPCD = Pulse Code Modulation DeviceST-6 = Signalling Terminal for CCITT No. 6

Figure 2.3.14Signalling Terminals for CCITT No. 6

2.4 The Digital Group Switch

Before studying the structure of the digital group switch in AXE we will touch

upon some of the basic principles of digital switching.

The introduction of digital switching gave birth to a new concept:

TIME SWITCH

Let us first see what a time switch is made up of and how it operates.

A/D = Analog/Digital converter~ = Analog signal

= Digital signal

Figure 2.4.1A Simplified Time Switch

34


A time switch is made up of:

a Speech Store for temporary storage of the speech samples. Each

channel in the time switch has a position of its own in the Speech

Store.

a Control Store which controls the read-out from the Speech Store.

This means that we can change the sequence of speech samples in a time

switch.

Assume that we are going to read out samples from the speech store in the

following order: 3, 2, 1, 4 (the read-in order is 1, 2, 3, 4). The control store would then

have the following contents (see Figure 2.4.2).

A/D = Analog/Digital ConverterD/A = Digital/Analog Converter~ = Analog Signal

= Digital Signal

Figure 2.4.2Control Information in the Control Store

This small-size time switch has only 4 inputs. How, then, do we go about

designing a digital group switch with tens of thousands of inputs?

In theory we could use a single time switch having the required number of

inputs. But then the following question arises: “How often would we have to ‘empty’ a

given position in the speech store?” The answer is 8,000 times every second for

each position (the sampling frequency is 8,000 Hz). Consequently, for a 20,000 input

switch the read-in/read-out rate would be 20,000 x 8,000 Hz = 160 MHz.

35


Today’s market does not offer any circuits that can cope with these speeds.

The solution to the problem is to divide the time switch into suitable sub-units. To set-

up connections from one time switch to another we use a SPACE SWITCH.

The capacity of each time switch in AXE is 512 inputs. A maximum of 32 time

switches can be connected to one space switch.

Terminology : Time Switch Module (TSM)

Space Switch Module (SPM)

PCM = Pulse Code ModulationSPM = Space Switch ModuleTSM = Time Switch Module

Figure 2.4.3The Fundamental Parts of the Digital Group Switch

A connection will pass through a TSM - via SPM - to the same or another

TSM.

All calls are set-up via SPM, including those which return to the original TSM.

We say that the switch has a T-S-T (Time-Space-Time) structure.

TIME SWITCH MODULE (TSM)

Since a TSM handles samples in both directions, we need two speech stores:

one for samples entering the TSM [Speech Store A (SSA)] and another for samples

36


leaving the TSM [Speech Store B (SSB)]. Each speech store has a separate control

store: CSA and CSB, respectively (in this case, CS stands for Control Store).

TSM also has a control store for SPM called CSC.

CSA = Control Store A CSB = Control Store BCSC = Control Store CSPM = Space Switch ModuleSSA = Speech Store ASSB = Speech Store BTSM = Time Switch Module

Figure 2.4.4Speech Stores and Control Stores in TSM

SPACE SWITCH MODULE (SPM)

The SPM structure is very simple and can be drawn as an ordinary matrix

with cross points.

Of course, in reality, the cross points represent logic gates that open and

close very rapidly.

37


CSC = Control Store CSPM = Space Switch ModuleTSM = Time Switch Module

Figure 2.4.5Space Switch Module (SPM)

As appears from Figure 2.4.5, the CSC of each TSM controls a row of “cross

points”. Thus, CSC in TSM-0 controls all “cross points” leading to TSM-0.

When a call is to be set-up in the switch, it is the central software of the GS

block (Group Switch) that selects the path through the switch. In this case, path

selection refers to the moment when a sample is to be transferred. This is called

“selection of an internal time slot”.

After the central software (GSU) of the GS block has selected a path, the

regional software (GSR) is ordered to write information to this effect in the control

stores of the TSMs concerned.

From now on, GSU will not pay any attention to the connection until the call is

to be cleared.

64K GROUP SWITCH

As we know, 32 TSMs can be connected to each SPM, providing a total

capacity of 32 x 512 = 16,384 inputs (This type of group switch is often called 16K).

38


What can we do, then, to build a larger switch?

We can interconnect several SPMs to form a large matrix as illustrated in

Figure 2.4.6.

PCM = Pulse Code ModulationSPM = Space Switch ModuleTSM = Time Switch Module

Figure 2.4.6A Fully Equipped Group Switch

This gives a total switch capacity of 128 x 512 = 65,536 inputs (This type is

often called 64K).

SYNCHRONIZATION

All types of digital equipment require some form of clocking. The clock rate

determines the rate at which samples are read from or written into the speech stores.

The accuracy of this clock is of great importance in networks containing

several interconnected digital exchanges. The whole network must be synchronized.

It is also important that the clock does not stop, as this would stop the whole

group switch.

39


To prevent this happening, the group switch has three clocks, or Clock

Modules (CLM).

CLM = Clock ModuleETC = Exchange Terminal CircuitSPM = Space Switch ModuleTSM = Time Switch Module

= Digital Signal

Figure 2.4.7Clock Modules to Synchronize the Group Switch

The operation of the group switch will be trouble-free even if only one clock is

used, i.e. in emergency situations.

As has been said, the whole network must be synchronized if it contains

several digital exchanges.

There are various ways of doing this. The simplest method is perhaps the

MASTER-SLAVE configuration, which means that one of the exchanges has a

control (master) function, while the others (the slave exchanges) try to follow the

operating pattern of the master.

Figure 2.4.8The Master-slave Principle

40


The master exchange has a number (usually 3) of more sophisticated and

accurate clocks called Reference Clock Modules (RCM). Figure 2.4.9 shows the

hardware included in the master and slave exchanges.

CLM = Clock ModuleETC = Exchange Terminal CircuitRCM = Reference Clock ModuleSPM = Space Switch ModuleTSM = Time Switch Module

= Digital Signal

Figure 2.4.9Hardware in Master and Slave Exchanges

The photograph in Figure 2.4.10 shows an RCM magazine (left) and a CLM

magazine.

The CLM magazine has hardware for operating a switch containing 8 TSMs

(4,000 inputs, often written as 4K). For larger switches, a larger version of the CLM

magazine is available.

CLM = Clock ModuleRCM = Reference Clock Module

Figure 2.4.10RCM and CLM for 4K Switch

41


There is also another way of synchronizing a network, Mutual

Synchronization. This method is to be preferred in national transit networks.

The basic principle of mutual synchronization is that one of the exchanges

operates according to a mean value based on all incoming frequencies.

Consequently, the network has no “master”. In order to prevent the whole network

from “drifting” as a result of frequency displacement, one of the exchanges is locked

to a fixed frequency value. This reference exchange is called a SINK and has three

highly stable clocks called CCMs (Cesium Clock Modules) which are connected in

the same way as RCM in Figure 2.4.9.

It is thus common practice to use two types of synchronization in a network. A

fully built-up digital network may use the configuration shown in Figure 2.4.11.

Figure 2.4.11Network Synchronization

EQUIPMENT for THREE-PARTY CALLS

Since the digital group switch is only capable of interconnecting two inputs,

external equipment must be used to set up a three-party call (for example operator

intervention or “Add-on conference”). This equipment is called Multi-Junctor Circuit

(MJC).

An MJC magazine can handle 10 simultaneous three-party calls.

42


MJC, which also has regional and central software, forms part of the GSS

subsystem.

MJC = Multi-Junctor CircuitSPM = Space Switch ModuleTSM = Time Switch Module

Figure 2.4.12A Multi-Junctor Circuit (MJC)

RELIABILITY

Since the group switch forms a vital part of an AXE exchange, exacting

demands are, of course, made on its functional reliability.

What would happen if, for instance, an SPM broke down? Well, as many as

16,000 calls would “collapse”. And, of course, this must not happen.

To solve this problem, AXE is equipped with two complete group switches:

one called the A-plane and the other the B-plane.

A speech sample is always sent through both planes but it is only fetched

from one of them, usually the A-plane.

To supervise the hardware, a number of parity check functions are provided

for checking the speech samples sent through the switch. A hardware fault will

immediately be detected by these functions. The faulty equipment is blocked, and

43


corresponding equipment in the other plane takes over the traffic handling. All these

measures are taken without disturbing calls in progress.

2.5 THE DIGITAL SUBSCRIBER STAGE

As mentioned before, there is a subsystem for handling the traffic between

subscribers: the Subscriber Switching Subsystem (SSS). The subscriber stage in

AXE is digital, which means that the analog signal from the subscriber line is

converted into digital form. This is done in the subscriber’s Line Interface Circuit (LIC)

and all switching is digital. To be able to understand the structure of the subscriber

stage we will first discuss its tasks.

BASIC FUNCTIONSA subscriber stage includes the following functions:

– Feed current to the subscriber line.

– Concentrate the traffic towards the group switch.

– Receive digits from dial telephones (pulses).

– Receive digits from keyset telephones (tones).

– Send ring signals to the subscriber.

– Send different tones to the subscriber.

– Carry out measurements on the subscriber line.

Some of the above mentioned functions are common to many subscribers,

others are individual. All individual functions are concentrated in the subscriber’s line

interface circuit.

These functions are: current feed, polarity reversal, reception of dial pulses,

relay for connecting ring signals, relay for connecting test equipment, and analog-to-

digital conversion.

Each printed board assembly has 8 line interface circuits; see Figure 2.5.1.

44


Figure 2.5.1Board with 8 Line Interface Circuits (LIC)

The board is equipped with components of special Ericsson design called

SLIC and SLAC (Subscriber Line Interface Circuit and Subscriber Line Audio

processing Circuit, respectively).

The flexibility of the circuits makes it easy to adapt them to varying

requirements in different countries. This goes in particular for power supply, speech

levels and balance.

As we have seen, the line interface circuit has no equipment for the reception

of digits from keyset telephones (tones). The equipment, for this receiving function is

common to several subscribers and is called Keyset code Reception Circuit (KRC).

This device is digital, and each printed board assembly can accommodate 8

KRCs. To connect the KRCs to calling subscribers we need a switch- the Extension

Module Time Switch (EMTS).

All three equipment units dealt with above (LIC, KRC and EMTS) have both

regional and central software; see Figure 2.5.2.

45


EMTS = Extension Module Time SwitchKRC = Keyset Code Reception CircuitKRR = Regional software of block KRKRU = Central software of block KRLIC = Line Interface CircuitLIR = Regional software of block LILIU = Central software of block LITSR = Regional software of block TSTSU = Central software of block TS

Figure 2.5.2The Basic Part of the Subscriber Switch

Additional equipment is required to connect subscribers to the group switch.

This equipment, which handles the 32 digital channels to the group switch, is called

the Exchange Terminal Board (ETB).

ETB is the hardware of a function block called the Remote Terminal (RT). It is

the central software of the RT block which reserves channels to the exchange.

CJ, A CO-ORDINATING FUNCTION BLOCK

A function block called Combined Junctor (CJ) is provided to co-ordinate all

functions in the SSS subsystem.

In addition to co-ordinating the set-up and clearing phases, CJ serves as an

interface with TCS and, in particular, with the RE block. See Figure 2.5.3.

46


CJU = Central software of block CJEMTS = Extension Module Time SwitchETB = Exchange Terminal BoardKRC = Keyset Code Reception CircuitKRR = Regional software of block KRKRU = Central software of block KRLIC = Line Interface CircuitLIR = Regional software of block LILIU = Central software of block LIRTR = Regional software of block RTRTU = Central software of block RTTCS = Traffic Control SubsystemTSR = Regional software of block TSTSU = Central software of block TS

Figure 2.5.3CJ - The Central Block of SSS

How many subscribers can be connected to an EMTS?

The answer is 128 subscribers, 8 KRCs and one 32-channel ETB. All this is

referred to as an Extension Module (EM) or an LSM (Line and Switch Module).

REGIONAL SOFTWAREThe regional software for the subscriber stage is stored and executed in a

processor incorporated in the magazine: the Extension Module Regional Processor

(EMRP).

The routine scanning of the hardware is done by small, simple

microprocessors located in different parts of the hardware. These are called Device

Processors (DP) and are in their turn scanned by an EMRP.

47


The program in DP has no decision-making functions; it just reports hardware

changes to EMRP.

DP = Device ProcessorEM = Extension ModuleEMRP = Extension Module Regional ProcessorGSS = Group Switching SubsystemKRC = Keyset Code Reception CircuitLIC = Line Interface CircuitLSM = Line Switch Module

Figure 2.5.4EMRP - DP Interwork

LSM is illustrated in Figure 2.5.5.

EMRP = Extension Module Regional ProcessorEMTS = Extension Module Time SwitchETB = Exchange Terminal BoardKRC = Keyset Code Reception CircuitLIC = Line Interface CircuitRG = Ringing GeneratorSLCT = Subscriber Line Circuit Tester

Figure 2.5.5An LSM Magazine

48


The primary advantage of using a digital subscriber stage is that it can be

detached from the exchange and installed closer to the subscribers. This will imply

less cost and less maintenance.

REMOTE SUBSCRIBER SWITCH (RSS)

But before this can be done, two problems must be solved:

(i) The 128-subscriber capacity is too small. It must be possible to combine

several LSMs to obtain the required size.

(ii) How can EMRP communicate with the central processor over distances of

tens of kilometres?

Let us see how a subscriber stage for 512 subscribers is designed.

EMRP = Extension Module Regional ProcessorEMTS = Extension Module Time SwitchETB = Exchange Terminal BoardGSS = Group Switching SubsystemKRC = Keyset Code Reception CircuitLIC = Line Interface CircuitTSB-A = Time Switch Bus, plane ATSB-B = Time Switch Bus, plane B

Figure 2.5.6Remote Subscriber Stage for 512 Subscribers

49


As appears from the figure, the topmost LSM has no direct contact with the

parent exchange, and calls coming from this LSM must therefore use the bus which

interconnects all the LSMs. This bus is called Time Switch Bus (TSB) and is thus

used for speech data. The bus is duplicated for reliability reasons.

At first sight, TSB may seem “unnecessary”, but a closer study will reveal

three very important advantages:

(a) The number of PCM links to the parent exchange can be adapted to the

traffic volume. Thus, all LSMs do not need a separate PCM link.

(b) If the “own” PCM link has no free channels, another PCM link can be used

instead. This makes the subscriber stage immune to situations with

unbalanced traffic load (full availability).

(c) If the contact with the parent exchange is broken, this will not affect the

internal traffic within the subscriber stage.

How many simultaneous calls can be handled by a detached subscriber

stage?

Let us study the example in Figure 2.5.6. Obviously, the traffic is handled by 3

PCM links, and channel 16 of the first two links is used for signalling. For reasons of

reliability, we normally have two signalling channels, which means that channels 0

and 16 cannot be used for speech transmission over these two links. In the third link,

on the other hand, channel 16 is available for speech. Consequently, a maximum of

91 simultaneous calls are possible in this example.

Up to 16 LSMs can be interconnected.

In this way, the number of subscribers served by a detached unit can be

varied between 128 and 2048.

The second task to solve is the communication between one or more EMRPs

and the central processor of the parent exchange.

50

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