Circuit switching systems predate computers. Partly for thatreason, computer engineers may learn something from a look

at their architecture.

Circuit Switching:Unique Architecture andApplicationsAmos E. Joel, Jr.

Bell Telephone Laboratories

Switching techniques have been essential to thedevelopment of modern computers and computernetworks. These techniques include the use of logic,memory, signaling, and connectives, all ofwhichwerehighly developed before the advent of modern com-puters. Connectives are the major functional ele-ments that distinguish circuit switching from theforms of switching that are used where messages canbe delayed. Although there are many ways to defineor identify a circuit switch, it is difficult to couple thedefinition only to the concept of delay, since all tele-communications inherently involve some delay, nomatter how small. Therefore, circuit switching is alsodefined as a form of switching capable of servingsimultaneous two-way conversational communica-tions of the desired bandwidth in real time.Figure 1(a) shows the three major functions of a cir-

cuit switch. These include the so-called "switchingnetwork," on which terminate, for purpose of inter-connection, the circuits from outside the switch; thesignal-processing function, for receiving and trans-mitting signals from outside the switch that indicatethe connections desired of this or other switches in abroader network of switches; and the information-processing function, for interpreting and controllingthe other two functions.In circuit switching, the term "network" is used in

two different senses. One is that of the network ofswitches (in the broader sense) tied together withrelatively long-distance transmission facilities; apublic telephone network is a network of switches,where there is a circuit switch or "office" at eachnode. The other sense is that of a network of switch-ing devices employing metallic contacts or semicon-ductor logic gates. In this article, I refer to the prin-cipal switching network required for a circuit switchas the "switching center network," or SCN.

Actually, any switching network is an assemblageof connectives. AND gates with common inputs andcapable of being enabled exclusively (one at a time)are basic connective elements. For example, the cir-cuit switching system's functions are connectedtogether through connectives or "accessnetworks"-NCAN, SAN, and CAN in Figure 1(b).These might be used for access between the inputsand outputs and the signal-processing function; thesignal-processing function and the information-pro-cessing function; and the information-processing andthe SCN-control functions. Each of these networks isa circuit switch by itself with control-that is, infor-mation processing-and even short-distance signal-ing. (One of the problems in understanding circuitswitching is that different system manufacturersand technologies may use different nomenclatures. Asignal-access network, for example, is a "register con-nector" in electromechanical systems and a "scan-ner" in electronic systems.)

Switching-network topologies

Within a computer, the same type of switching net-work may be found, for example, connecting memorysections to CPUs or I/O units to multiprocessors (seeFigure 2). Most applications of connectives, as inmany modern telecommunication switching systemsand computers, are specified and designed to be"nonblocking," but other "blocking" configurationsare equally important.

Nonblocking networks. The simplest circuit switchis one with a rectangular nXm array of devices,called, generically, "crosspoints" (see Figure 3(a)).This is known as a two-sided, single-stage, nonblock-

0018-9162/79/0600-0010S00.75 © 1979 IEEE COMPUTER10

ing switching.network. Figure 3(b) is a one-sided,single-stage, nonblocking network.Two-sided networks are also known as "permuta-

tion" networks when n=m, since the outputs can beany permutation of the input. Typically, in a largelocal switching office, lines or stations (terminals incomputer technology) would be on one side of the net-work and trunks to other offices on the other side. In amultiprocessor computer, memory units might be onone side and processors on the other.The single-switch stages may also be used to illus-

trate the three basic functions of a circuit-switchingnetwork, as shown in Figure 4. If l>m, we speak of"concentration"; if n<t, we speak of "expansion"; ifn=m, we speak of "distribution."As n becomes large, it is more economical in cross-

points to divide a network into more than one stage.Multistage nonblocking networks are named aftertheir discoverer, Clos,' who showed that such net-works are not only possible but require fewer cross-points than single-stage distribution networks whenn>16 (see Figure 5). (n'12 is used to optimize thenumber of switches relative to the total inputs, n).Clos networks are permutation networks thatemploy matrices of crosspoints in each stage.

Figure 1. (a) The basic functions in circuit switching. (b)Placement of connectives (access networks) in a switch-ing center network: network control access network, sig-nal access networks, and control access network.

Figure 2. Switching network topologies in computersystems. (a) Multiple memories connected to multipleCPUs. (b) Multiple l/O units connected to multipleprocessors.

Figure 3. Nonblocking switching networks. (a) Two-sidednetwork. (b) Single-sided network.

June 1979 1 1

Figure 4. Circuit switching network functions.

Figure 5. Nonblocking network configuration, after Clos.'

Figure 6. A three-stage blocking network.

Figure 7. Five-stage8 x 8 Benes-type network of beta elements.

Blocking networks. Multistage networks may bedesigned with fewer crosspoints by allowing theintroduction of blocking. Public telecommunicationsswitching systems are generally not economical ifthey are designed to provide SCN and trunk capacityto handle all calls that might be submitted at anytime. Blocking, particularly by the introduction ofconcentrators, is most common in local offices, whichconnect lines to trunks.

Blocking characteristically distinguishes circuitswitching from store-and-forward switching, fre-quently considered for data messages; the latter formof'-switching systems are generally engineered todelay, rather than block, the delivery of messages.However, delay is intolerable for real-time conversa-tional messages.One simple and usual form of introducing blocking

into a network is to change the first-stage and last-stage matrices of the Clos network to square switches(n1/2Xn1/2) instead of expansion switches(n112X2n112-1) and use n1/2 instead of 2n1/2-1switches in the center stage of a three-stage network(see Figure 6). As n (the total number of terminals tobe served) increases, it becomes more economical touse more stages rather than increase the size of theswitch matrices. In fact, it is possible to limit thedesign of networks to matrices composed only of 2X 2switches, known as beta elements (see Figure 7).2 Inelectromechanical switching systems, the sizes ofswitch matrices have been fixed by mechanicaldevices such as 10X20 or 1OX10 crossbar switches.Individual sealed-reed relay or semiconductor cross-point matrices of different sizes are somewhat easierto realize. Blocking in multistage networks with afixed number of switches per stage may be reducedby expansion, as in the Clos structure, or by addingmore stages.Figure 8 shows a typical multistage switching net-

work used in the Bell System's No. 1A ESS.3 Thesymbolism denotes the number of matrices per stageand their interconnection. This network can growfrom n = 2048 ton = 128,000.

Other design considerations. The various factors indesigning multistage switching networks are beyondthe scope of this article. There are many excellenttreatises on this subject.2,4 There are special factorsin some circuit-switching networks for computers.5Some of the more important of these to be taken intoaccount, besides the physical limitation on matrices,are the growth characteristics (if any), the overloadcharacteristics (for blocking networks), and the con-trol complexity.Networks generally are thought of as having full

accessibility; that is, every input can reach every out-put under some set of circumstances. But in somesystems, there is limited accessibility, where some in-put cannot reach every output or every input cannotreach some output. The first concentrator stage ofsome No. 1 ESS networks employs such a "grading,"as it is sometimes called. There is some correlationbetween the number of interstage links in a networkand the complexity and cost of the control.6

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Figure 8. Typical space-division network (from No. 1A ESS).

Space, frequency, and time division

The networks described above are used for circuitswitching and are generally thought of as maintain-ing circuit paths for the entire message, call, or con-nection. Networks with exclusive paths for each useby each message are known as "space-division" net-works.The advent of transmission systems that carry

more than onemessage on a given facility prompted adesire-and possibly created an economic incen-tive-to design switching networks to interface suchfacilities directly, without returning the messagesignals to baseband or individual paths or channels inspace.The earliest carrier systems used frequency separa-

tion of message channels. Frequency-division switch-ing requires, in principle, space-division networks ateach end of the carrier system to apply the modu-lating and demodulating frequencies (see Figure 9).Such networks are rarely used in circuit switchingsystems.By the late 1950's, time-division transmission

became practical, particularly using binary-codedsampled signals. This digital time-division transmis-sion7 is now economically competitive with frequen-cy carriers for voice transmission over intermediatedistances (see Figure 10) and is being extensively de-ployed. It is also important for digital data transmis-sion. Circuit switching of digital signals may takeplace with space-division switching-for example,through crossbar switches. However, the digitaltechnology has proven most attractive in integratingdigital switching with digital transmission.Analog time-division techniques have also been

used in switching, using sample pulses ofvarying am-plitude8 or width. With sampling techniques appliedto voice signals, the bandwidth of the switched signalis limited to approximately half the sample rate.

Figure 9. Frequency-division network (requires space-division-net-work control access).

Figure 10. Cost picture for wire-line digital transmission.

June 1979 13

Space-division switching with sampled signals islimited in bandwidth by the physical arrangement ofthe crosspoints and wiring. Figure 11 shows the vari-ous types of space-divisidn and time-division switch-ing using metallic or nonmetallic (semiconductor)crosspoints; systems using all possible combinationsthrough which a radial line may be drawn have beenproduced and used commercially.In some respects, the freedom available to de-

signers of time-division networks is much greaterthan that available to designers of space-division net-works. These factors have been described in moredetail elsewhere.9 There are three principal switchingelements in a time-division network: (1) multiplexing,(2) time-slot interchange, and (3) space divisionstages.

Figure 11. Transmission and switching relationships.

Multiplexing. This function, denoted by M, is usedboth to convert channels from space division to timedivision and to concentrate or expand tine-separat-ed channels.

Time-slot interchange. This involves the use ofmemory to hold the digital samples for no more thanone frame (repetitive group of channels), so that theymay be sent out in a different channel assignment ortime slot. (These are sometimes known as time or Tstages.)

Space-division stages. These stages, designated byS, are designed like the stages for space-division net-works, except that in time-division networks theychange the combinations of operated crosspoints oneach time slot. Here, for the first time, it is practical tochange the assignment of a particular call or connec-tion from one crosspoint to another. Therefore, onemay speak ofa "rearrangeable" switching network. 10The network of Figure 6 is blocking when used in aspace-division mode, but, when used as a space-division stage in a time-division switching system, itcan be nonblocking if existing connections are re-switched between time slots. (This characteristicdoes not maintain for all space-division networksused in time-division switching.)

Figure 12 shows a typical message path through adigital time-division switch, the No. 4 ESS of the BellSystem," which uses all three of these switching ele-ments. Time-division networks are described as com-binations of time-slot interchanges and space-divi-sion stages; this one being TSSSST. The networksare generally one-sided, with symmetry about thecenter. Transmission in each direction requires aseparate path.

Network control

This brings us to the subject ofmemory associatedwith the control of switching networks. All switchingnetworks require memory to keep track of the connec-

Figure 12. Path of a message through a Bell System No. 4 ESS digital time-division switch.

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tions that have been established in space or timestages. In some electromechanical switches, memoryis inherent in the mechanical or magnetic latching ofthe devices, and separate actions are generally re-quired to restore the crosspoints to normal. For elec-trically held switches, the flow ofholding current pro-vides the memory function. Modern space-divisionsystems maintain in memory a "map" of the occu-pied links or operated crosspoints and employalgorithms to assign new connections to the network.

In time-division systems, memory is always need-ed to keep track of the assignmient of connections totime slots (see Figure 13); the same memory issometimes also used to control the space-divisionstages of a time-division system."The control of switching networks involves more

than just calling up information from memory; it in-volves findingan idle terminal with which to connect;hunting for and selecting paths; and establishing(and sometimes releasing) connections. These func-tions are generally distinct enough to use separatecontrols for the network function alone, in which casethis function is shown separately on a system blockdiagram. A popular term for the network controller inspace-division systems is "marker."

System control

The broader control of the entire switching systemis the information-processing function, which in-cludes transferring, gathering, interpreting, andtranslating the information the system receives,generally from its signal-processing functions.A broad spectrum of technology has been used to

implement system controls, ranging from a pluralityof distributed electromechanical devices-generally,relays and sequence (programmable) switches-tohigh-speed, highly integrated, general-purposeswitching processors used as centralized stored-program controls. (See Figure 14.)

Figure 13. Time-multiplex switch control.

There are two general classes of switching-systemcontrols: wired logic and stored-program logic.(Stored-program logic may be inPROM orRAM withtape or disk backup.) Either type of control may becentralized or distributed. A study of the softwareused in stored-program controls shows a wide differ-ence in the size of the programs for various systemsdepending on the degree of flexibility desired and thefeatures and services provided.One class of systems, using macroinstructions pe-

culiar to the basic circuit-switching system and ac-cessed through a lookup table, has been called"action-translator" control systems. As comparedwith stored-program-controlled systems (Figure 15),the action translator has special-purpose, wired-logiccontrol (Figure 16). In most distributed-logicsystems, control has been implemented in wired logic

j: t ;:: :\ : : | ~~~~~~~~~~~~~~~~~~~~iFigur 14 Moer swthn syte poe so (dpicatd) caalifpoesn ,,0 al e or

June 1979 15

Figure 15. Stored-program system.

Figure 16. Action-translator system.

Figure 17. Distributed wired-logic system.

(Figure 17); but the availability of low-costmicroprocessors is making distributed stored-program-control, and action-translator-controlsystems with PROM storage, popular in newlydeveloped systems (Figure 18).9

Reliability and service continuity, which are ex-tremely important in circuit-switching systems serv-ing the public, make redundancy of controls essen-tial. Redundancy is also required toensure continuityof service when additional memory or new processorsare being installed; service must continue during thechangeover.The most common method of providing redundan-

cy is to duplicate the controls. Some duplicated con-trols share the load. This assumes that, to avoid over-load, the engineered use ofeach control should not ex-ceed 50 percent of its real-time capacity; this isknownas "traffic division." The capacity of systems is mea-sured by a specified mix of the number of calls of alltypes the controls can process without significantdelay.Another method used to increase system capacity

is "functional division," where the processing of callsis divided into distinct call actions, and the allocationof work for each call is distributed amongtwo or moreprocessors. One such functional division is signal pro-cessing, which is common in many switchingsystems.",Duplicated controls may imply synchronous opera-

tion with matching. Multiprocessing may haveduplicated pairs or some other checking means, in-cluding self-checking. An important goal is to detecttrouble before it fails to process calls. Load-sharingmay be extended to multiprocessing with self-check-ing. This technique is used for the markers in elec-tromechanical systems.

Real time and software

LSI technology has prompted the wide adoption ofSPC-stored-program control-of theprocessors andmemory used in modern circuit-switching systems.However, two factors in the use of these techniquesdistinguish circuit switches from computers. Unlikestore-and-forward switches, a circuit switch operateson a no-delay basis; therefore, real-time processing isessential. Initially, programs were written inmachine language, and the instruction set includedorders particularly needed for switching algorithms,such as for hunting idle links in network maps. Thiswas essential not only to minimize memory cost(memory was relatively expensive), but also to mini-mize instruction execution cycles. With lower-costmemory and somewhat faster processors, these fac-tors became less important.As shown in Figure 19 for the No. 1 and No. 2 ESSs

of the Bell System, the number of operating and ad-ministrative features and customer services provid-ed by these systems has continued to grow. Thenumber of words required in the machine-languageprogram has now more than tripled. More than halfofthe continuing development effort is devoted to soft-

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ware. Systemprograms arechanged frequently whilethe offices in which they are placed continue to pro-vide service. Special high-level languages have beendevised to ensure that the processing remains cost-effective in real time. For many years, there has beenexperimentation with high-level languages, aimed atefficient execution in real time as well as optimizingprogramming effort and portability. Acceptance ofsuch languages has been slow, but the CCITT* isabout to standardize on one.'2 Preservation of real-time capability remains important as the programsgrow (and the the programs have grown con-siderably).

Circuit switch applications

So far, this article has discussed some design char-acteristics of circuit switch architecture. But, as im-plied from the discussion of growth of features andservices, there are many unique requirements thatcircuit switches must meet to be viable in commercialapplications. They are used for local central officesthat directly serve individual lines; for intermediateoffices-between local (end) offices-for interlocal(tandem), intertoll (toll) operation; for internationalgateway exchanges, and for operator services; andfor business applications (PBXs). See Figure 20 for anexample of a national network. In most countries,there is a national center through which most interna-tional calls pass and which is the last alternate routepoint in the country's toll hierarchy. In the NorthAmerican network, there are four levels of toll hier-archy with no national center."3 There are several in-ternational gateways at various levels in the hier-archy.

"Plain old telephone service." Most of us arefamiliar only with what has been called "POTS" or"plain old telephone service," but over time thenature of POTS changes. Many years ago, toll callingwas a very special activity requiring operators towrite one or more tickets and to recall you when tollcircuits to your destination were available. Nowa-days, the direct dialing of calls not only within a coun-try, but also to other countries, is part of POTS.

Domestic US services. The complex requirementsfor modern circuit switches for application in the USpublic network have developed with the sophistica-tion of the services offered and of the methods used toadminister, and maintain them. While many ser-vices-such as party line, coin, PBX, and Centrex-were developed using electromechanical switching, ithas only been since the general development of SPC,or stored-program-control, systems for all types ofcircuit-switching applications that new services forthe public and features for administration and main-tenance have flourished.

Overseas services. Outside of North America, SPCsystems have been developed, installed, and accept-

*Consultative Comnittee for International Telephone and Tele-graph.

Figure 18. Distributed stored-program system.

Figure 19. Growth in features and services.

Figure 20. Structure of a national telephone network.

June 1979 17

ed for a number of different reasons. In particular,they have been deployed for international gatewayexchanges to provide flexibility in meeting changingCCITT requirements and to provide for newly emerg-ing international signaling systems.14 In other coun-tries, SPC local exchanges have been installed toenable the introduction of new customer billingmethods, andmodern crossbar offices are beingretro-

Figure 21. TOUCH-A-MATIC repertory station set.

Figure 22. Signaling network concept.

18

fitted with SPC for the same reason.'5 (In the BellSystem, many crossbar toll offices were retrofittedwith SPC starting in 1969 to provide improved flexi-bility in routing control and network management.16)Now that SPC systems are economically competitivewith other types of systems, they are being installedmore generally. However, only in a few countries arethe new services made possible with SPC, such as ab-breviated dialing and call waiting, being offered tothe public.

Effects of SPC. The growth of features for all typesof systems was not due solely to the invention ofSPC;the growth process first developed with the flexibili-ty improvements made available by the architectureof the No. 5 crossbar system.'7 SPC is a good solu-tion not only for the growth of requirements, but alsotheir changing nature. More important, however, isthat SPC has enabled the economic introduction ofnew signaling methods and new services.

Signaling. Switching systems are remotely con-trolled-that is, the information processingneeded todetermine'what connections to establish starts withinformation transmitted to the switch. Unfortunate-ly, public networks have become tied to signaling sys-tems that were first implemented many years ago.With the large investment in subscriber equipment,and now with customer-owned equipment in theUnited States, it becomes increasingly difficult tomake changes in this portion of the signaling re-quirements.There are three aspects of signaling: addressing,

alerting, and supervising. Addressing usually com-prises pulses generated by the dial (DC pulses orACdual-tone signals). Alerting is generally done withhigh-power (90-135-volt) ringing signals at between16 Hz and 67 Hz. Supervising consists of sensing re-quests for service and sensing that line signaling oran established connection should be cancelled. A flowof 25 or more milliamperes has been the standard for"off-hook" signals and lack of flow for "on-hook"signals.'8

Address signaling From an information-process-ing point of view, the address signaling-that is, theinformation conveyed by pulsing-is the most impor-tant. The information is organized into a "numberingplan," and the use of the calling device (dial) is deter-mined by a "dialing plan" (for example, waiting for asecond dial tone). In a public network, the numberingplan should be universal; that is, the same algorithmshould be used everywhere so that a user knows howto place a call from any location in the network. Mostnumbering plans are "open," requiring differentnumbers of digits for different purposes, such as"611" for repair service and "0" for assistance. Aninternational numbering plan has been agreed to andis in use.'9For signaling between offices, the dial-pulse and

multifrequency concepts for address pulsing were ex-tended, and tone supervisory signaling was alsointroduced. 18With an expanding worldwidenetwork,more digits are required to address a particular dis-

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tant station. Since letters are in different places onthe dials in each country, they cannot be used tobreakup long strings ofnumericals thatbecome diffi-cult to remember and to dial without error. For-tunately, the availability of modern memory tech-nology, accessible either in the central office or in spe-cial telephone sets such as TOUCH-A-MATIC20 (seeFigure 21), makes it possible for users to prestoretheir own repertory of numbers so that they may besent automatically by touching a button or dialing avery short code.

Interoffice signaling. The interoffice signaling net-work has served well during the evolution oftelephone services, including the growth of direct-distance dialing by customers. However, with the in-troduction of SPC offices and the many new servicesthey offer, it was recognized that a signaling systembetter adapted to communication between SPC of-fices should be developed. This effort started in thelate 1950's. The method chosen was one resurrectedfrom the early days of switching, when operatorscommunicated between offices over separate "order"or "call" wires. This technique, using signals of 2400and 4800 bits per second, is now called "commonchannel signaling." It has been adapted by theCCITT as the international standard (known as Sys-tem No. 621) and by the Bell System and independenttelephone companies as the US domestic standard(known as "CCIS" for "common channel interofficesignaling"). Initial international service was in-augurated in 197822 between the US, Japan,Australia, and the United Kingdom. Other countriesare also planning to use such systems, includingJapan, where a network is in use.23

The CCIS network. The introduction of CCIS intothe US network will bring many noticeable improve-ments and opportunities fornew services. Since thereare fewer toll offices and the majority of them arealready stored-program controlled, CCIS is being in-troduced initially into the toll network.24 Since allfuture toll calling will depend on this form of signal-ing, it is necessary that it be highly reliable. With theUS toll network consisting of more than 1000 offices,it is not economical for each office pair to be directlyconnected by data links. Therefore, a subtending sig-naling network, or "CCIS network" is being imple-mented.For the routing of long-distance calls, the US has

been divided into 10 regions, with a hierarchy of tollswitching offices in each region. (There is no nationalcenter.) The new CCIS network takes advantage ofthe toll regions and establishes in each region dupli-cated SPC processors as redundant signal transferpoints.25 For CCIS, each SPC-type toll office in aregion has duplicated links to the STPs (see Figure22). The STPs in and between the 10 regions are fullyinterconnected. Initially, the SPC processors for theSTPs are used jointly with switching offices; as traf-fic builds, separate highly reliable switching pro-cessors will be used for this function.The CCIS network was started in the US in 1976,

and all STPs were in service by the end of 1977. By

May 1979, about 100 toll offices were connected tothis signalingnetwork. A noticeable difference on callsserved by these facilities is the reduced time from theend of dialing until the ringing of the distant stationbegins. Since, with CCIS, supervisory and addresssignals will eventually no longer be sent over thetrunks between offices, "talk-off" (simulation ofsupervisory signals by speech) and so-called "bluebox" signal simulation will no longer affect interof-fice signalingMore important than these and other improve-

ments in the rendering of POTS are thenew customerservices that will be introduced. CCIS applies notonly for intertoll signaling, but alsobetween local andtoll, between operator and toll, and between local of-fices. These capabilities will be among the featuresthat will extend the curves of Figure 19 into thefuture.The network of offices capable of working with the

CCIS network and bringing new services to custom-ers and service benefits to customers and telephonecompanies alike is known as the SPC network (seeFigure 23). It consists initially of toll SPC circuitswitching offices.26With the SPC network, it will be possible to offer,

across the entire network of circuit-switching offices,services that are possible now only within SPC of-fices. The list of possible services is very long andgrowing. The marketing and administration costs forsuch a large network will ultimately determine whatservices are offered. Principal among the direct costsis the increase in memory requirements for programsand data bases in SPC offices and STPs. Some of theservices that are being considered have been de-scribed elsewhere27 and will not be covered here.The SPC network permits the nationwide public

circuit-switched network to appear not only as a col-lection of nodes representing traffic sources andsinks, but also as a gigantic nationwide circuitswitch, with centralized SPC controls at STPs anddistributed SPC controls at each node. Eventually,the network will include local offices and Traffic Ser-vice Position Systems.

Figure 23. Stored-program-controlled network.

June 1979 19

Administration and maintenance

The realization of the type of nationwide switch al-luded to above would not be possible without the pro-vision of many features less obvious than the essen-tial basic feature of signaling. The importance ofredundancy to provide growth and reliability hasalready been mentioned. But this is only the tip of theiceberg when we are discussing the design of circuitswitches for public communication. More than half ofthe words of the programs in Bell System offices aredevoted to features that relate to the operation of theservices rather than to the basic switching, call-sig-naling, and information-processing functions. In ad-dition to the on-site programs for these features,there are also very large programs devoted to thedesign of the on-site programs and to their d,ploy-ment to the many offices in the field.28'29Portions of programs are devoted to many func-

tions associated with the overall provision of serviceby the circuit switches in a network that is growing,that faces changing service needs, and that is ex-periencing improvements in engineering and man-agement techniques. The following is a list of some ofthese functions, implemented in both hardware andsoftware.

(1) Obtaining and "crunching" information tomeasure theusage of the service so that the adequacyof facilities {including such varied items as trunks,common-control equipment, and software registers)may be determined under actual usage. This informa-tion is disseminated in what are known as "traffic en-gineering reports."

(2) Obtaining and recording information for billingpurposes include information about cash services(such as those from coin telephones, a highly real-time activity) as well as providing for the billing ofcalls by calling-line or account number. These areknown as AMA-automatic message accounting-records.

(3) When matching or self-checking circuits orfault-sensitive programs detect that any portion of aswitching system is not functioning correctly, thesystem is designed to stop processing new calls, re-tain records for all calls in progress, and locate thetrouble automatically by switching redundant unitsinto service. Here, again, the complexity is muchgreater than the obvious.actions. Such acitivities arebroadly related to service maintenance and musteventually call for human intervention to correct thefault, whether it is in hardware or in software.

(4) Even when the system is functioning preciselyas planned, certain routine activities must be provid-ed for in the design. As I have already mentioned, avery complex activity is providing for growth, of theSPC network itself as well as the control and pro-grams, while the system is functioning and providingservice. This is a requirement unique to public circuitswitches. Sufficient equipment is installed to meetthe needs over a rather short engineering interval ofonly one or two years, it being assumed by the com-munications companies and regulators, on the basisof past experience, that service needs may be pre-dicted accurately for such an interval. This conservescapital investment. In some circuit switch applica-tions, where a government agency administers thetelecommunication network, frequent additions toexisting offices are avoided due to factors in com-petitive bidding; new offices are built of a standardsize that may provide growth formany years into thefuture.While system designs provide for obvious main-

tenance routines, a man-machine interface or"maintenance control center" is usually provided topermit the maintenance personnel to take overridingmanual actions on the system (see Figure 24). This in-cludes the ability to make minor as well as completechanges in the operating programs and other storeddata. Programs that include newly developed fea-tures and other improvements have been changed inBell System offices about every one to two years. Insystems that offer only POTS, such changes are notrequired as frequently.

(5) While much effort and many precautions aretaken to ensure the continuity of service in each of-fice, it is still impossible to prevent an office or a routeof traffic (trunking) facilities from going out of ser-vice unexpectedly. Central offices have gone up insmoke, microwave towers have been blown up, andcables have been plowed up.An important set of features in switching systems

are those related to continuity of service despite suchfailures. They constitute what is known as networkmanagement30 and include things as simple as

Figure 24. Control consoles for a maintenance control aborting slowly dialed calls and as sophisticated ascenter. the automatic rerouting of calls from Atlanta to New

20 COMPUTER

York by way of Seattle if the latter route is not in atraffic peak. In the United States, network manage-ment centers are established in all regions, many sec-tional centers, metropolitan areas, and a nationalcenter for the entire network (Figure 25).

These are only a few of the functions a circuitswitch provides in addition to basic switching. Butthey are all examples of what is required to providebasic communication service in a modern society.The detailed requirements for these elements of theservice have been growing faster than the basic ser-vice requirements. Their application has permittedservice to grow within rather stringent economic con-straints. The overal cost of switching has changedvery little over many decades, but the aforemen-tioned functions and others have greatly improvedthe productivity of the capital invested in switches.

Future trends

I see five major trends for the future of circuitswitching: (1) distributed processing, (2) consolida-tion of offices, (3) integration of analog and digitalprocessing, (4) centralization of operations, and (5)the further expansion of SPC technology.

Distributed processing. In the realm of architec-ture, there wil be a return to distributed processing,as provided originaly in the step-by-step system, ex-cept that the distributed microprocessing elementscan now be stored-program controlled and can beloaded and changed from central and remote sources.Centralized executive control wil always be required.

Office consolidation. There will be a trend towardconsolidation of offices, due to the availability oflarger-capacity systems, and toward remote controlof smal offices by SPC hosts. This latter wil con-tinue until SPC controls can be made economical foreven the smallest community dial offices.

Digital/analog integration. The SCN networks wiUbe designed to serve effectively both analog and digitalsignals. Once the services that use each of these typesof signals can be defined and standardized, it wiUbecome clearer where space-division and time-divisionnetworks may be best applied. So long as the com-munications require a bandwidth no greater thanvoice, time-division networks wil probably prove to beeconomicaUly effective.

Centralization of operations. The traffic, mainte-nance, biling, network management, and other ad-ministrative functions are being centralized insystems designed to serve many offices. In the BelSystem, these are known as OSS-operations sup-port systems-and represent a major trend in pro-viding high-quality switched telecommunication ser-vice. Abroad, some centralization of maintenance aswel as service processors is discernible.31

June 1979

SPC expansion. Finally, and most important, theintroduction of SPC will continue to be used to ex-pand, consolidate, and replace existing switching en-tities. As a result, the meaning of POTS will change,and it will be possible to introduce new signalingtechniques to offer services more in keeping with thespecific needs of the users. In the US, there wil be anetwork bf 15,000 or more SPC offices (nodes) tiedtogether with CCIS for signaling and network-wideservices, and a network of OSS nodes for administer-ing the service rapidly and efficiently. The US net-work, and the networks in other countries demandingsophisticated communications, wiUl set this trend. Incountries requiring less sophisticated services, it islikely that POTS as we have known it wiU continue tobe the principal service offering, and that it wiU besome time before users wiU be able to request servicesnow possible with modern circuit switches. B

Figure 25. National Operations Center displays network status.

21

References

1. C. Clos, "A Study of Non-Blocking Switching Net-works," Bell System Technical Journal, Vol. 32,March 1953, pp. 406-424.

2. V. Benei, Mathematical Theory of Connecting Net.works and Telephone Traffic, Academic Press, NewYork, 1965.

3. A. Feiner and W. S. Hayward, "No. 1 ESS SwitchingNetwork Plan," Bell System Technical Journal, Vol.43, Sept. 1964, pp. 2193-2220.

4. M. J. Marcus, "A Theory of Connecting Networks andTheir Complexity: A Review," IEEE Proc., Vol. 65,No. 9, Sept. 1977, pp. 1263-1271.

5. Y. Mima and M. Kato, "Development of the CircuitSwitching System for Digital Data Networks," Rev.Elect. Comm. Lab., Vol. 25, July/August 1977, p. 629.

6. A. Feiner, and J. Kappel, "A Method of Deriving Effi-cient Switching-Network Configurations," Proc. NEC1970, pp. 818-823.

7. M. R. Aaron, "Digital Communications-The Silent(R)evolution," IEEE Communications Magazine,Jan. 1979, pp. 18-26.

8. J. A. Herndon and F. H. Tendick, "Time DivisionSwitch for an Electronic PBX (ESS 101), IEEE Trans.Comm. Elect., Vol. 83, July 1964, pp. 338-345.

9. A. E. Joel, Jr., "Digital Switching-How It HasDeveloped," IEEE Trans. on Comm., July 1979, PartI.

10. M. C. Paull, "Reswitching of Connection Networks,"BeU System Technical Journal, May 1962, p. 833.

11. "No. 4 ESS," Bell System Technical Journal, Sept.1977.

12. R. H. Bourgonjon, "A High Level ProgrammingLanguage for SPC Software Systems," ISS Proc.1976, pp. 222-231.

13. "Operations and Engineering in the Bell System,"Bell Telephone Laboratories, 1977, pp. 41-43.

14. J. Meurling, L. 0. Noren, and B. Svedberg, "TransitExchange SystemAKE 132," Ericsson Review, No.2,1973, pp. 34-57.

15. 0. Ryberg, "ARE 11 in Australia," Ericsson Review,No. 2, 1977, pp. 90-92.

16. B. T. Fought and C. J. Funk, "Electronic TranslatorSystem for Toll Switching," IEEE Trans. Comm.Tech., Vol. 18, June 1970, p. 168.

17. J. W. Dehn and R. E. Hersey, "Recent New Featurofor No. 5 Crossbar Switching System," Trans. AIEJP1, 1956, p. 457.

18. C. Breen and C. A. Dahlbom, "Signaling Systems f4Control of Telephone Systems," Bell SysteJTechnical Journal, Vol. 39, Nov. 1960, p. 1381.

19. L. J. Scott, "Overseas Dialing Yesterday, Today ayTomorrow," Bell Labs Record, May 1970, p. 142.

20. V. Gust, D. Huizinga, and T. Pass, "Call Anywherethe Touch of a Button," Bell Labs Record, Jan. 197pp. 3-8.

21. J. J. Bernard, "CCITT International Signaling No.6,Telecommunications Journal, Vol. 41, No. 2, 1974,69.

22. "CCITT Signaling System No. 6-Inaugural Call,Telecommunications Journal, Vol. 45, No. 10, 1978,l536.

23. S. Suzuki, Y. Korenaga, M. Useda, and K. Hasui,"Control Method for the Quasi Associated Mode ofOperation in the Common Channel SignalingSystem," ISS Proc, 1974, pp. 413-423.

24. P. K. Bohacek, "The Implementation of CCIS in theBell System," IEEE National Telecomm. Conf, Dec.1978, p. 31.1.

25. S. L. Johnson and R. S. Little, "Logistics of Implemen-tation (SPC Network)," Proc. IEEE Inter. Conf. onComms., June 1977, p. 35.3.

26. P. K. Bohacek, "The Systems Engineering Role (SPCNetwork)," Proc. IEEE Inter. Conf on Comms., June1977, p. 35.2.

27. C. R. Jacobsen, "Planninig for SPC NetworkFeatures," Proc. IEEE National Telecomm. Conf,Dec. 1978, p. 39.1.

28. R. W. Ketchledge, "Development of DevelopmentMethods," Proc. Inter. Switching Symposium, 1974,p. 412.1.

29. W. 0. Fleckenstein, "Development of Telecommuni-cation Switching in the USA," Inter. Switching Sym-posium, 1976, p. 121-126.

30. W. B. Macurdy, "Network Management in the UnitedStates," Proc. 6th Inter. Teletraffic Congress Record,1973, p. 621/1.

31. H. E. Binder and H. Eberl, "Safeguarding and Opera-tional Tasks of the Service Computer Employed in theEWS Electronic Switching System," Telecomm.Report (Special Issue), 1978, pp. 44-52.

MAmos E. Joel, Jr., is a switchingconsultant with Bell Laboratories,cid r:t; ;Xs ^ 0 Holmdel, New Jersey. He has been de-partment head for development plan-

at __ ning of the Bell System's First ESS,6, --and in 1976 was corecipient of the Alex-

ander Graham Bell medal for his workin electronic switching systems. He isthe author of many works on switching,

p. r |i 1 _ including the 1976 IEEE Press BookElectronic Switching. Central Office Systems ofthe World

" He holds more than 65 patents in the field of telecom-p. munications. He is a Fellow of the IEEE and a Licensed

Professional Engineer in the state of New Jersey.

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