CONSUMER TELEPHONE

The buzzwords and acronyms are all around us: digital phones, digital switching, PCM, toname a few. The era of sending digital telephone information by digital means isaccelerating. The benefits of lower operating costs, higher reliability, better quality ofservice, and more performance. features are enhanced through the power of the computer.The day when a tremendous amount of information is accessed via a single telephone call ina short time is not very far away. Some would argue that the day is already here.

Given the aura of digital telephony, it isn't surprising to see some developmentsoverplayed, while significant ones get little or no mention. For digital transmission oftelephone information is not new. Back in 1962 the Bell System put into service the firstlarge digital transmission system, known as TI. This system multiplexed telephoneconversations by pulse-code modulation. With PCM, specific digital codes are assigned tothe analog variations of the transmitted telephone message. These codes are thenreconstructed back to their original analog forms at the receiving end of the telephonesystem. Today about 40 percent of the trunks in the Bell System employ digital transmission.

What is new is the rate of penetration of digital transmission and switching techniques Intelephone systems. The majority of independent suppliers of switching equipment in theUnited States are installing only digital equipment in new offices. Many modern telephonesystems will incorporate digital switching and transmission into all-digital networks.

el n 9III

I..I I

lower cost, higher performance and greater flexibilityfavor the digital approach over analog switching

It is only a matter of time before the largely analog telephonesystem becomes all-digital. One big reason is cost. Deviceslike microprocessors, memory, and logic are now no moreexpensive than analog devices. Another reason is perfor­mance. Digital switching networks are easier to maintainthan analog; they have fewer components for higher­reliabilitylevels, require less floor space, dissipate less powerand are easier to install. In addition they can handle moretraffic per equipment frame and provide uniform transmis­sion capabilities, Yet a third big reason is versatility. Whenput into digital form, information from a variety ofsources-video, audio, and facsimile-can be multiplexedand transmitted over conventional and coaxial cable, radiowaves, and other media. Digital information can also beeasily coded for privacy, a must for such modern telecom­munications services as electronic funds transfer.

Semiconductor technology is behind the swing to digitaltelephony. Increasing complexities on smaller and smaller icchips are making possible complex functions on a single chipat low costs (Fig. 1). Examples include LSIIC filters, time-slotgenerators, subscriber-line interface circuits, telephone­relay drivers, and codecs (coder-decoders). As shown in Fig.2, the cost per circuit for digital switching is now approx­imately equal to the cost per circuit of analog and electro­mechanical systems. In time analog and electromechanicalsystems will become more expensive than digital systems,because of reduct:d volumes of production and increases in

Sam G. Pitroda Wascom SWitching Inc.

raw materials and labor costs.There are some problems with digital, to be sure! The

greater complexity of line circuits results in a higher failurerate than that for analog line circuits. But with built-inredundancy, overall system failures are significantly re­duced, because fewer components are needed than in anequivalent analog switching network. Table I briefly com-pares both types of switching systems. .

It should be emphasized that in both analog and digitalswitching systems, control remains the same. As a result,features and facilities related to stored-program control arethe same. Often stored-program control is confused withdigital switching.

Digital evolution is under wayAlthough the vast majority of telephone systems still use

analog equipment, digital hardware is steadily replacing theanalog. The evolution is occurring with minimum servicedisruption during changeovers. Thus the first digital equip­ment was used in pulse-code-modulation carriers for interof­fice trunks, followed by its use for PCM carriers for inter­tandem trunks. Carriers for the subscriber lines followedsuit, then came digital PABXs (private automatic branch ex­changes), and central offices. Line concentrators and remoteswitching then took hold. Soon the all-digital telephone willfollow. Digital switching techniques that were in thelaboratory in the 1960s are now the techniques of futuretelecommunications.

Digital switching of telephone information has become amultimillion-dollar industry, providing digital hardware for

IEEE spectrum OCTOBER 1979 OO18-9235/79/1000-00S1$OO.75©1979 IEEE 51

5 cents per gate(TTL)

Microprocessors and 'memories with over100 gates per device

2

1098765

4

3

x 10 Pricepergate,collars

2/1L98765

4 Standard SSI with4 gates per device

3

tions shows why digital switching and transmission are theelements of future telecommunications systems.

Almost instantaneously, the telecommunication networkinterconnects over 90 percent of the approximately 400million telephones in the world through clusters of equip­ment that the telephone user rarely sees. This equipment isthe switching system. There are two basic functions of anytelephone switching system:

1

1960 1965 1970 1975 1980

[1] Over the last 20 years digital semiconductor device corn­plexity has increased about a hundredfold, while the price pergate has decreased a hundredfold.

such applications as P ABXs, central offices, and tandems.Today over a dozen manufacturers produce digital switchingproducts worldwide, for systems with as few as 100lines to asmany as 110000 lines. Table II lists some of the commercialdigital switching systems in operation in the world, In addi­tion a significant amount of design and field testing is occur­ring on systems not shown in the table. These include the No.S EAX from the GTE Automatic Electric Co. and theSystem X, which subcontractors are building for the BritishPost Office. Manufacturers that have digital-switchingtelephone systems under investigation, either in the field orin the laboratory, include L.M. Ericsson, Siemens, Oki, Fu­jitsu, Philips, and the Plessey Co. Ltd.

CNET introduced the first digital switching network forpublic telephones with its ElO system. Today there are aboutone million EIO lines in operation worldwide.

By far, the Bell Systems' No.4 ESS (electronic switchingsystem) is the largest digital-switching system in operation. Itwas put into service in Chicago in 1976. It has 107 000 ter­minals and capacity for 1.7 million CCS (a telephone trafficunit meaning one hundred-call seconds) and 550 000BHCAs (busy-hour call attempts).

It is estimated that more than one million lines of digitalcentral-office equipment and 500 000 lines of digital PABXequipment are in service in the world today.

The only element in a telephone switching system that isnot within sight of being digital is the telephone itself. Once atelephone set that can economically deliver a direct binary bitstream of information becomes available-and that day maynot be too far off, in view of the rapid pace of digitaltelephony developments-a total digital network, withpoint-to-point digital transmission capabilities, will becomea reality (Fig. 3).

From manual to electronic switchingAn examination of the architecture of telephone systems,

their modes of operation, and their capabilities and limita-

I. Comparison of analog and digital switching

-lI

Digital

Year

I

1990I

1985

Electromechanical

1980

0.5

[2] Although digital switching cost per circuit line is now equalto the cost of analog switching, it will be less expensive in thefuture. And electromechanical switching costs are risingrapidly.

2.5

2.0

i 1.5cuEo.s.~ 1.0L-_r:::::::::;...----------_03a.W8

Switches digital waveformand requires logic gate ormemory as crosspoint element

Digital (PCM)

Crosspoints are miniature andinexpensive, and there arehundreds or thousands perdevice in high packagingdensity.

Line interfaces are complex .because of AID conversion

Line circuit costs are highNetwork costs are lowNonblocking and control com-

ponents are economical, sincethey are alike and consistentwith future component trends

Bandwidth-limitedProvides foundation for the

integration of transmissionand switching

Requires less. space than analogEasy to install because of

plug-in construction

Requires substantial spaceDifficult to install because of

multistage configurations

Nonbandwidth-I imitedCannot be integrated with

major carrier concepts

Line circuit costs are lowNetwork costs are highNonblocking configurations

are uneconomical and incon­sistent with future trends

Line interfaces are simple

Analog

Switches analog waveformand requires analog com­ponents and crosspoints,such as pnpn transistors,relays, etc.

Crosspoints are expensiveand bulky (low packagingdensity)

52 IEEE spectrum QCTOBER 1979

[3] A typical telecommunications environment as it changestoward an all-digital network.

[4] A basic PCM system involves encoding analog inputs(speech signals) at the transmitting end of 8 telephone con­versation. The signals are then decoded at the receiving endfor reconstruction to their original analog form.

Receiver sendingunit

Lineconcentrator

®~..n..n..W·

PCM: two advantages over analogPCM, invented by A. H. Reeves in the 1930s, wasn't used

commercially until the 1960s, when Bell introduced it forpoint-to-point digital transmission in metropolitan areas. Itwas then that the necessary reliable, low-cost, high­performance semiconductor components became available.PCM has provided two main advnatages over analog switch­ing methods: performance and versatility.

Since a signal is presented in digital form that can beregenerated (Fig. 4), the noise, stability, and accuracy prob-

to discrete binary symbols. This digital information is thenswitched through semiconductor logic gates and/ormemories, where it is transmitted over telephone lines in atime-sharing scheme known as TDM (time-divisionmultiplex­ing). One common method of digital information processingis pulse-code modulation.

Control and network are fundamentalModern telephone switching systems consists of two fun­

damental building blocks: control and network elements. Acontrol complex provides all the functions for the personmaking a phone call, and it organizes the required connec­tions. Control can be implemented through electro­mechanical hardware or through sophisticated stored­program digital computer systems. The network, which isthe heart of any switching system, provides a physical trans­mission path between any two or more telephone systemusers.

Most switching networks have long been implementedwith such electromechanical devices as step-by-step andcrossbar switches and relays. These networks are known asanalog networks, because they process voice and data thatare represented by analog signals. The signals continuallyvary in voltage or current magnitude, corresponding to theamplitude variation ofthe input voice or data information.

In digital networks, on the other hand, analog informa­tion is processed by sampling, quantizing, and converting it

1. Signaling-to detect service requests, ringing, and dial­ing, etc.2. Connection-to set up communication paths.

For several years after the invention of the telephone,these two switching functions were performed manually.The introduction of electromechanical switching equipment(the Strowger switch) during the early 1900srevolutionizedtelephone communications by opening the door to auto­matic switching. Through electromechanical switching, atelephone user exercises direct control over the switchingequipment. As a subscriber dials a telephone number, relaysand switches, in a step-by-step manner, hunt for connectionscorresponding to the dialed numbers. Today about 30 per­cent of the Bell System and the majority of other telephonesystems still use this switching technique.

During the 1940s crossbar switching was introduced,followed byelectronic switching. Thesetwo concepts furtherautomated telephone communications by establishing com­mon control, in which electronic pulses corresponding todialed numbers were stored for switching by relays andswitches later. About 70 percent of the Bell Systemtelephones use common control switching, as do some in­dependent networks.

PAMsamples

Decoder

Pitroda-Telephones go digital 53

II. Commercial digital sWitching products in operationSystem Application Capacity Network Manufacturer

Northern TelecomNorthern TelecomNippon ElectricBell System

GTE AutomaticElectric

TRWVidarNorthern Telecom

L. M. EricssonPhilips

SiemensThompson-CSFITT

Stromberg CarlsonITT NorthCNET

GTE AutomaticElectric

STS switch at 1.544 Mb/sTTTT switch at 2.048 Mb/s

TST switch at 2.048 Mb/sTST switch at 2.048 Mb/sTor TST switch at 2.048 Mb/sTST switch at 2.048 Mb/sTST switch at 2.048 Mb/s

32-channel compatible. TST switch at 2.048 Mb/s24-channel, T1-compatible. TST switch at 1.544 Mb/s32-channel compatible. Time modules only, with

concentrator at 2.048 Mb/s

32-channel compatible. TST switch at 2.048 Mb/sTTTT switch at 2.048 Mb/sTST switch at 2.048 Mb/sTSSSST switch. 24-channel, T1-compatible24-channel, T1-compatible

100-800 lines100-5000 lines

100-2400 lines

PABX

PABX

PABX

ITS-5

ROLMSL1

580

ITS Local/tandem 12 000 linesOMS 200 Tandem 60 000 terminals

AXE Local/tandem 200 000 linesPRX Local/tandem 25 000 linesEWSD Local/tandem 100000 lines

MT Local/tandem 64 000 linesSystem 12 Local/tandem 60 000 lines

Central office 6384 lines,expandableto 12 768 lines

Century Central office 7680 linesOSS(210) Central office Averages 32 000 linesE10A Central office 10 000 lines

GTD PABX 120 to over 4000 lines 24-channel, T1-compatibleTime switch for 120 and 1000channels

TST switch for 4600 channels at 1.544 Mb/s12-kHz sampling. 12-bit linear code. Unibus concept Aolm Corp.Time-time, 32-channel compatible network at 2.048 Mb/s Northern Telecom

24-channel, T1-compatible. Time modules only. Non- Wescomblocking at 1.544 Mb/s. Can be converted to 2.048 Mb/s

24-channel, T1-compatible. TST switch at 1.544 Mb/s TRWVidar

OMS 10 Central office Up to 6000 linesOMS 100 Local/tandem 100 000 linesNEAX Central office Up to 32 000 linesNo.4 ESS Tandem 107000 terminalsNo.3 EAX Tandem 60 000 terminals

lems of analog transmission are eliminated. And because ofpeM's digital nature, information from a variety ofsources-voice, video, facsimile-can be multiplexed fortime-sharing. PCM is compatible with various transmissionmedia, can be easily coded for privacy, and iseconomical foruse in existing cable facilities, because of the increased chan­nel capacity possible through TDM. peM offers hope for in­tegrated carrier and switching sytems.

Two commercial PCM formats have evolved (Table III):

[5] Digital as well as analog signals canbe used as inputs to adigital (PCM)switch. Digital inputs require less hardware thananalog inputs.

the North American 24-channel format and the European32-channel. The North American format was introducedfirst. It involves multiplexing 24 analog voice channels,which are sampled at a frequency of 8 kHz and encoded intoan 8-b nonlinear digital signal of 193 b (one bit is added forsynchronization and framing). A resulting transmission rateof 1.544 Mb/s (193 b/125 us) has been standardized for theBell System's T 1carriers, the first commercial telephone car­rier system employing peM. Signaling is performed by bor­rowing the least-significant bit every sixth frame from eachchannel.

The European format involves multiplexing 32 analog

s~Digital

Lineinterface

Synchronizer

__ 7---'~'7 JlSLOIAconverter ~--1.DernU1t:lpJelx6r .••~~

Encoder

54 IEEE spectrumOCTOBER 1979

Out

(6] Space switching involves connecting data buses through amultiplexer. In a switching system with eight data buses, forexample, all eight buses are fed in'to an 8:1 multiplexer, wheretwo or more buses are connected within a specific time zone.

Reliability

Controlcomplexity

Expansion

Packaging

SIT

S TITS

STSIIST

STTS/TSST

STSSTS I TSSSST

3

4

5

2

voice channels, which are also sampled at a frequency of 8kHz and encoded into an 8-b nonlinear digital signal of 256b. This results in a transmission rate of 2.048 Mb/s (256b/125 us). Signaling and synchronization are performed intwo channels, while the remaining 30 channels are used forvoice communications.

Digital switching is thus the switching of analog informa­tion that has been converted to a PCM format (24- or32-channel) through electronic digital devices (logic and/ormemory elements),

The phenomenal rate at which both PCM formats havebeen applied worldwide has heightened interest in a totallyintegrated digital network, in which both transmission andswitching are performed digitally. The integration oftransmission and switching would eliminate unnecessaryPCM-to-analog conversion at the receiving end of a telecom­munications line. Such conversion is required to interfacewith a predominantly analog switching environment, and itis not only expensive but also introduces quantizing noise ateach analog transmission node.

(8)Various combinations of time and space s"Jitchin~can beorganized in a pyramid arrangement, starting with time andspace atthe top,Levelof SWitching Areasof concern

Inputs may be analog or digitalAs shown in Fig. 5, the inputs to any digital switching

system may be analog or digital. Digital conversion ofanaloginputs requires a two-to-four-wire hybrid amplifier, a filterto limit bandwidth over a range of 300 Hz to 3.4 kl-lz, and an8-kHz sampling gate. An AID converter then sends the digitalsignal to a multiplexer for tranmsission. This hardware ar­rangement is repeated, in reverse order, for the receiving endof transmission.

Digital input signals, on the other hand, need no conver­sion circuitry and are fed directly into a circuit that syn­chronizes them with the communication system's switchingclock and separates them into appropriate control and net­work channels (signaling and voice functions). These digitalinputs are then multiplexed by a TDM scheme to a higher­order bus structure that has an integral multiple of the 24- or32-channel format.

Unlike analog input signals, digital input signals must besynchronized with a system clock to separate the control andnetwork channels. Furthermore synchronization is needed,because peM data arriving at the switching system may bederived from two or more independent clocks whose speedsmay vary. And even if the clocks are identical in speed, phasevariations due to loop lengths, jitter, and temperaturechanges can cause differences in clock performance.

Nonstandard switching formats can be used for digitalswitching. For example, l-b and differential peM as well as12-b encoding, are sometimes used. However, if a digitalswitch is to become part of a totally digital network, it should

Control

Amn

Control

Dataoutput

AmIXrn-----~I--------.~

,~ "1 1 ",;, "'.

"~

5 111,1. UU 0'1 5 .. ;....'. ,;. ,,,,-lI4:<',

18 ','10 '11'1'01,1': 18 .::;" :) ;,. ,) .'

24 ,.".' 24 ')" ,:,; '\

SLJL..JL......,.

Dataselect

(7] Time switching involves connecting different time slots ondifferent data buses.

24-channel PCM bus

.ri.ri.ri.,

Pitroda-Telephones go digital 55