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1144 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. COM-27, NO. 7, JULY 1979 Digital Switching-Forces Which Shape Its Future c. DENIS HALL, SENIOR MEMBER, IEEE Abstroct-Whqe the theories relevyt to digital communication switching have been understood for some time, it is only now econom- ically feasible to piade a digital switchinto an analog environment. This permits the transition from an analog-xiented' communications world into a digitally oriented communications world hthout the need for costly interfaces or for *e building of special overlay networks. This paper fist examines the current state of the relevant tech- nologies. It then exam4es thewaysinwhich these ,technologies wiU interact with the exiseg networks, the terhealsto those existing networks and economic forces. It is shown that we can expect evolution of the .world wide communications netwoiks; the terminals of those netwqrks and the digital switching machines at the he+ of those networks-all in consort. ' It is concluded that the real opportunity presented by the advent and evolution of digital switching technology is that of almost un- im~aginable' flexibiliw'.' @I networking and end-user features. Digital switching is indeed the keystone to a telecommunications revolution and the attendant marriagtfof computing with communications. D I I .. IGITAL switching is an idea whose time has come. The theories of '.$me divided speech transmission, digital speech encoding,, electronic ' control logic and data error control hive been married in thought during the past several years. The arrival of the digital computer, the subsequent maturing 'of'th& ,:software art, the growing usefulness of data transmission, and- the availability -of large scale integrated circuits on planarsgic,on 'now maki these theoretical combina- tions economically' ,..deliverable as. a general purpose digital communications switch. It is important to our understanding of,future possibilities that we understand that we came to our present situation through the clever combination of many ideas. Neither a sinde theory nor a single technolob made digital communications switching economically attractive. Likewise, we aie unlikely to find a single theory or technology breakthrough in the .future which will, by itself, be capable of significantly altering the' course of an' evolving digital commhnications network. ' Although the communications network is dependent upon all of'its parts for 'its successful functioning, it can be readily seen that ,the switch lies at its heart. In the past the switch has provided much of the- network's ruggedness and flexibility. Clearly, this will continue to be the case, but now digital technology permits us to design a general purpose switch for a general purpose network. ' The simple replacement of an analog communications facility with a digitally based facility produces some interest- ing .potential operating cost reductions. .If that was all there was to it,' I could not predict 'anytliing' more exciting than the gradual evolution of the world's communications net- ILJ Manuscript received February 2,1978;revised July 2,1978. The author is with Bell-Northern Research, Ottawa, Ont;, Canada. work toward realization in digital technology spurred by the worldwide demand for increased communication capacity, and gated by the availability of capital to provide the equipment. However, this .is notthe case. Many aspects ofthe analog world have been very constraining. The analog electrical signal itself is fragile. The editing of analog signalsis difficult and the procgssing of these signals on' any appreciable scale is almost impossible. Analog storage media of any significant capacity is mechanically based and shlows 'little prospect of being amenable to significant cost reductions. Many of today's communication signals, including tho!je for the control of the network itself, are fundamentally digital and are today convertedinto analog formthrough 1.he use of modems at high cost. It is clear that major newdemands for flexibility are being placed upon worldwide communications netwoiks. More than any other factor, this dennand for flexibility is the one which will drive the communications networks of the world to digital format and' digital switching. Digital transmission was introduced as a cost reduction, but digital switching will be introduced both as a cost reduction and as an 'essential to the provision of the demanded flexibility. Thisflexibility will beprovided through the removal of the constraints to message' storage, editing, multiplexing and processing which are'inhere:nt in' the anilog world. Major new communications technologies can be introduced only in two different ways. Either new technologies are imbedded in an existing network or an overlay network is built using only the new technology. In this latter case, inter- faces witli the existing network are initially nonexistent. In the case of digital communications switching, we have seen the establishment of some clverlay networks primarily for the transmission and switching of data. However, we have now reached -'that point in time when a digital communication switching machine can be made to function economically in a totally analog transmission environment. This constitutes a major breakthrough.It is now possible to offerthe flexibility , inherent in a digital communication switch while imbedding it in an existing analog network. With this breakthrough, digital switching can be'installed with impunity in-the analog world with some immediate benefits.. The future benefits of this imbedding include the re-use of much of the existing analog network hardware to carry digital traffic and.the shedding of theconstraintsofthe analog world as new digital facilities and terminals are installed. This paper explores the forces which will shape the future of digital switching under three major headings. After looking at the Current State of the Art, this paper addresses Telecom- munications Network Evolution and Important Technology Developments-the two forces which will inieract to shape I . .. 0090-6778/79/0700-1144$00.75 0 1979 IEEE ; ,

Digital Switching--Forces Which Shape Its Future

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1144 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. COM-27, NO. 7 , JULY 1979

Digital Switching-Forces Which Shape Its Future c. DENIS HALL, SENIOR MEMBER, IEEE

Abstroct-Whqe the theories relevyt to digital communication switching have been understood for some time, it is only now econom- ically feasible to piade a digital switch into an analog environment. This permits the transition from an analog-xiented' communications world into a digitally oriented communications world hthout the need for costly interfaces or for *e building of special overlay networks.

This paper f ist examines the current state of the relevant tech- nologies. It then exam4es the ways in which these ,technologies wiU interact with the e x i s e g networks, the terheals to those existing networks and economic forces. It is shown that we can expect evolution of the .world wide communications netwoiks; the terminals of those netwqrks and the digital switching machines at the he+ of those networks-all in consort. ' It is concluded that the real opportunity presented by the advent and evolution of digital switching technology is that of almost un- im~aginable' flexibiliw'.' @I networking and end-user features. Digital switching is indeed the keystone to a telecommunications revolution and the attendant marriagtfof computing with communications.

D I I ..

IGITAL switching is an idea whose time has come. The theories of '.$me divided speech transmission, digital

speech encoding,, electronic ' control logic and data error control hive been married in thought during the past several years. The arrival of the digital computer, the subsequent maturing 'of'th& ,:software art, the growing usefulness of data transmission, and- the availability -of large scale integrated circuits on planarsgic,on 'now maki these theoretical combina- tions economically' ,..deliverable as. a general purpose digital communications switch. It is important to our understanding of,future possibilities that we understand that we came to our present situation through the clever combination of many ideas. Neither a sinde theory nor a single technolob made digital communications switching economically attractive. Likewise, we aie unlikely to find a single theory or technology breakthrough i n the .future which will, by itself, be capable of significantly altering the' course of an' evolving digital commhnications network. ' Although the communications network is dependent upon all of'its parts for 'its successful functioning, it can be readily seen that ,the switch lies at its heart. In the past the switch has provided much of the- network's ruggedness and flexibility. Clearly, this will continue to be the case, but now digital technology permits us to design a general purpose switch for a general purpose network. '

The simple replacement of an analog communications facility with a digitally based facility produces some interest- ing .potential operating cost reductions. .If that was all there was to it, ' I could not predict 'anytliing' more exciting than the gradual evolution of the world's communications net-

I L J

Manuscript received February 2,1978;revised July 2,1978. The author is with Bell-Northern Research, Ottawa, Ont;, Canada.

work toward realization in digital technology spurred by the worldwide demand for increased communication capacity, and gated by the availability of capital to provide the equipment. However, this .is not the case. Many aspects of the analog world have been very constraining. The analog electrical signal itself is fragile. The editing of analog signals is difficult and the procgssing of these signals on' any appreciable scale is almost impossible. Analog storage media of any significant capacity is mechanically based and shlows 'little prospect of being amenable to significant cost reductions. Many of today's communication signals, including tho!je for the control of the network itself, are fundamentally digital and are today converted into analog form through 1.he use of modems at high cost. It is clear that major new demands for flexibility are being placed upon worldwide communications netwoiks. More than any other factor, this dennand for flexibility is the one which will drive the communications networks of the world to digital format and' digital switching. Digital transmission was introduced as a cost reduction, but digital switching will be introduced both as a cost reduction and as an 'essential to the provision of the demanded flexibility. This flexibility will be provided through the removal of the constraints to message' storage, editing, multiplexing and processing which are'inhere:nt in' the anilog world.

Major new communications technologies can be introduced only in two different ways. Either new technologies are imbedded in an existing network or an overlay network is built using only the new technology. I n this latter case, inter- faces witli the existing network are initially nonexistent. In the case of digital communications switching, we have seen the establishment of some clverlay networks primarily for the transmission and switching of data. However, we have now reached -'that point in time when a digital communication switching machine can be made to function economically in a totally analog transmission environment. This constitutes a major breakthrough. It is now possible to offer the flexibility ,

inherent in a digital communication switch while imbedding it in an existing analog network. With this breakthrough, digital switching can be'installed with impunity in-the analog world with some immediate benefits.. The future benefits of this imbedding include the re-use of much of the existing analog network hardware to carry digital traffic and.the shedding of the constraints of the analog world as new digital facilities and terminals are installed.

This paper explores the forces which will shape the future of digital switching under three major headings. After looking at the Current State of the Art, this paper addresses Telecom- munications Network Evolution and Important Technology Developments-the two forces which will inieract to shape

I . . .

0090-6778/79/0700-1144$00.75 0 1979 IEEE ; ,

HALL: DIGITAL SWITCHING-FORCES WHICH SHAPE FUTURE 1145

communication switching’s future. Important points are summarized at the end of each of these three sections.

CURRENT STATE OF THE ART Perusal of the other papers in this publication will thor-

oughly ground the reader in the current state of the art on digital switching. However, it might 6e wise to emphasize here some of the key state of the art situations upon which the future of digital switching rests.

The Voice Encoder

Since the widespread implementation of digital switching in voice telecommunications had to await the arrival of economi- cally viable designs which would syitch digitally in a totally analog world, the most crucial element in the near term is the circuit which interfaces the telephpne set station apparatus. The telecommunication trunking intercace is of some importance, but because there are five to ten times as many telephones as there are trunking interfaEes, economics dictate that the telephone set interface is the deteimining factor. One element crucial to the line circuit is the cost of the encoding/decoding function including its inherent cost of signal filtering. Other functions in the line circuit are essentially those required in an analog voice switching machine. At the state of the art, we find full integration of filter and codec functions in planar silicon. Group codecs have been used in some designs. We will continue to see implementations using group codecs for some time, but I feel that in the long run the economies of a group codec will be marginal or even negative, and that failure mode considerations and the m,&sion flexibility available with a per line codec architecture will clearly win. We can also expect to see further integration of functions to reduce the component count per line. Innovative solutions to the provision of high voltage ringing and the performance of test functions on analog lines can be expected.

Memory Communications switching without computerized control is

today unthinkable. Crucial t o the evolution of the computer controls, is the cost of memory. Silicon based memories with 16 K bits per chip are in widespread use today and practical chips as large as 64 K have been announced. Widespread pre- dictions place the cost of random access memory in the mid 1980’s at $2,000 per megabite. Future architectures in both hardware and ‘software will obviously be heavily influenced by these numbers.

So fhvare

Software is clearly becoming a science. The economic essence of software ,is that it has no manufactured cost of any significance on a per copy basis. The architectural signifi- cance of software is that it is a means of implementing very complex logic functions. But this is not enough. Flexibility of future mission is the cardinal contribution of digital switching to communications networks. Software’s full potential in flexibility has yet to be demonstrated.

The software which controls telecommunications switchers today at the state of the art is very robust software. It is not

only robust against hardware failures within the, contiol complex, but is robust against its own internal failures, pr bugs. Automatic software restart, with remarkably (little dis- ruption of switching,activity within the peripherals under +e control of that software, has been with us since the 1960’s. So, robustness in software design is not only the state of the art, but it is an accepted standard. However, it is only recently that real operating systems have been intioduced,to the world of communication switching. Not oily does the decreasing cost of memory make large operating systems economically possible in the teleciimmunications environment, but it is clear that only software packages based on powerful oper- ating systems will produce the flexibility of software mission needed in the near future.

Transmission Parameters . . In the,digital Znvironment, the switching machine is not as

easily defined functionally as it was in the analog voice world. There are two voice transmission parameters which will be key to the future of digital switching in the near term. First, in the local switching environment a question of line to line loss has to be addressed. In the analog world, a nominal 0.5 dB loss as the analog signal crossed the switching matrix was easy to achieve, and thus became the accepted standard. In a digital switching environment, this parameter is not readily achievable at low, cost because of the necessity to guarantee adequate singing margin around the transmission loop involving the separate transmit and receive paths, coders, decoders, and the hybrids in the two wire/four wire interfaces. This interface occurs on every analog voice line. The state of the art is at the nominal 0 dB level, but most switchers delivered today achieve anywhere from 2 dB to 6 dB nominal line to line loss as this requires less precision in matching the hybrid balance imped- ance to the line impedance. With 2 dB readily achievable, it is reasonable to question on several grounds whether lower figures are worth the extra cost and effort.

The second transmission related problem of importance to future switch design is that of digital synchronization. Although there are several proposed methods of dealing wi$ this prob- lem, there is at this time no accepted standard arrangement. As usual, the question is one of. economics. Virtually slip free transmission can be obtained through elaborate and costly synchronization schemes, whereas significant sums of money can be saved by agreeing to accept finite but nevertheless low digital error rates due to slip. The technology is available today to implement either scheme. Digital switching machines being designed today should contain no architectural constraint which would prevent full synchronization of the digital switch to an external timing source. It is equally clear that many digital switching machines placed into the analog public network within the next few years will not be so synchronized initially.

Peripheral Architecture If the future of digital switching is to be characterized as

“feature rich”, it can be concluded that the nature of h e peripheral architecture of the switch is important to its future evolution. it should be remembered that the last generation of

1146 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. CbOM-27, NO. 7 , JULY 1979

stored progr+m ,controlled telecommunications switches relied almost entirely upon’ centralized computing facilities for the provision of fea’tures’.through the peripheral circuits because of the ,relatively ,’high cost of computing capacity and its associated memory requirements. At the state of the art today, we find peripheral systems controlled by microprocessors. These microprocessors are in some cases down-loaded with Program from i central .processing compiex. We see in this aspect of machine design the seeds for future evolution.

Summaj

At the present state ,of the art the encodingldecoding problem for voice signals has been solved. The foreseeable transmission system interface problems are solvable. Peripheral circuits tend to be microprocessor controlled and many design approaches are predicated on further significant reductions in the cost of random access memory.

.TELECOMMUNICATIONS NETWORK EVOLUTION Although digital switching nodes can now be provided in a

fully analog network, the future usefulness of the flexibility providable by the digital switching machine is obviously tied to the evolution of the telecommunications network. The digital switching machine is a crucial enabler of change but it will not. of itself force change. New network offerings such as combined voice/data, made feasible by the introduction of digital switching, will only materialize as potential services are aggressively marketed and the .using public is convinced of the usefulness of these services at the offered prices.

Integration of Overlay Networks

Repeatedly in the past, new services have first been offered on. “overlay” networks. An overlay network has the advantage of being able to offer new services to a selected few customers relatively quickly. The overlay technique has the distinct disadvantages of significantly higher cost in the long run and long delays in providing new services to the population at large. In the past, overlay networks have not survived, but have been integrated progressively into evolving general purpose networks. I expect that we will witness this s h e integrative phenomena with the advent of digitally provided network services.

The combination of special overlay networks with the evolving public’network in the past has been most marked in the trahsmission areas. The reason for this is that many of the special services offered through overlay networks in the past have been non-switched services. For the reasons mentioned earlier in this paper, analog signals emanating from various sources could not be combined during their passage through an analog switch, However, with the.advent of digital switching the combination of signals emanating digitally from various services is a simple matter. The control of these combinations in real time through computer techniques becomes attractive. Thus, I predict that many sources,of service will be offered to end users via a digital switch. The switch will facilitate the accessing of these services by interfacing a new breed of termi- nals which can be characterized as allowing continual control access to the switch by the end user. In most communications

networks today, control functions other than the ability to disconnect are lost to the end user as; soon a r the requested connection has been estalblished. Full ‘ability, of the end user to control network ,actions will force the rapid evolution of a public combined services n.etwork.

In the meantime, the same factors which have made digital switching practical-integiated silicon circuitry and software control-will ease the problems of interfacing overlay networks to each other and to the p,ublic network. In the past, early standardization of telecdrnmunication protocols was essential to the economic well-being of worldwide networks. While it is obvious that such standardization minimizes interfacing costs, and that a single network minimizes provisioning costs overall, the interfacing costs are not nearly as important to the total economic picturl: in the digit.al world as they were in the analog world.

Effects of Terminai Apparatus

For several years, the evolution of the terminal apparatus (telephone sets and their successors) will force the evolutipn of the public communications networks and not visa versa. We already see considerable evidence of this phenomena in the packet switched networks. The purvejrors of packet switched networks are scrambling to;provide the multitude of access protocols demanded by existing terminal (subscriber) appara- tus. The economic forces of network integration,will produce demands for the interfacing of many of the sgne data oriented terminals to switchers wh.ich were designed primarily to carry voice traffic. The old sty1.e ,switchers will be made t o comply to the most pressing demands but their designs lack the flexibility for anything be.tter than minimal and awkward com- pliance. However, the ne’w general purposes digital switchers will eventually start to dic.tate interfacing-standards. There will no longer be the strong (economic forces rooted in inflexible designs to prevent rapid agreement 017 standards. Almost any reasonable proposal, will be acceptable. End user two-way control interaction with -the switcher ;through the connection will be, inherent in the standards. The telephone set of today becomes a control and display terminal which, incidently, is capable of transmitting voice.

The Impact of Pair Gain Systems We have had “pair gain” systems with us since the advent of

analog voice telephony. They have always been clumsy devices with economic prove-in ljossible only for geographically dis- persed pocket developments of telecommunication users or for the serving of subscribers at extreme distances from switching facilities. However, digital communication is inherently “pair gain” because of its time multiplexing basis. T h s fact will revolutionize the physical topology of communication net- works. Economics will ,force loop lengths to shorten as remote switching units are estab1:ished closer i o the subscriber appara- tus. In the digital world there is no transmission cost penalty for this arrangement as there was in the analog world, arid there are significant electrical benefits. With the shortened loop length there is the potential for bandwidth increase. Within the next few years we will see the widespread imple- mentation of switch units which are controlled remotely from

HALL: DIGITAL SWITCHING-FORCES WHICH SHAPE FUTURE 1147

their geographic location. We can expect to see significant intelligence built into these remote units because the cost of doing so will not be significant in concentrations above a few hundred lines. The future of digital switching is very much tied to the effective utilization of the flexibility in physical net- work topology and end user features made available through remote switching.

The Emergence of Virtual Circuits

A large proportion of the communications network invest- ment in the North American urban environment consists of end user dedicated non-switched transmission facilities. Furthermore, this type of facility has a higher percentage growth rate today than does the switched facility. The intro- duction of digital communication switching at the private branch exchange level alone will start to curtail this develop- ment. Virtual circuits are easily formulated in the digital environment. Such facilities are already offered by some of the packet switch networks. As digital switching capability is extended in the switching hierarchy, I predict the widespread use of virtual circuits. As the voice, data and video communi- cating needs of businesses increase in the office of the future environment, both total traffic and the opportunity for brief pre-emption of some traffic within the total mix increases. The eventual extension of the same concepts to the non- business environment leads me quickly to the conclusion that the present rapid growth of non-switched network will be significantly curtailed by the availability of digital switching. The effect upon digital switching is to increase its economic attractiveness as an alternative to continued rapid growth of customer dedicated transmission facilities.

The Effects of Transmission Developments The development of the future telecommunications net-

work cannot be oblivious to transmission costs even though with time these costs are a decreasing proportion of the total networking costs. We can expect to see a series of attempts to reduce the bandwidth requirements for the transmission of digitized human speech. Digital Speech Interpolation (DSI) methods would seem to hold considerable promise. Again the cost of interfacing a new system to the existing networks becomes a dominant item. The intermixing of various DSI schemes and other bandwidth conserving mechanisms (which to me appears .inevitable) will necessitate the digital equivalent of the traveling’classmark. This is, of course, already contained in the proposed CCITT No. 7 signaling system. Using this method of communicating between stored program controlled digital switching machines, the control centers can establish the speech encoding scheme used on incoming traffic and thus appropriately treat the incoming messages individually. The intermixing of various transmission schemes will be demanded by their required co-existance due to evolutionary improve- ment of these schemes. The digital switcher must co-exist with all of these schemes. This constitutes no great burden because of the current availability of microprocessor controlled periph- erals which can take care of the details. The requirement to communicate control signals digitally with other machines is a natural in the digital network.

Usage Sensitive Pricing

The “quality” of transmission is readily and dynamically variable in the digital environment. Specifications on tolerable delay (including multiple hour delay), perceived analog band- width, packetized service versus pseudo circuit switched service, and a host of other possibilities introduce the necessity for usage sensitive pricing. In the past there has been a socio- political requirement for universality of service. This has created striking uniformity in service offerings with the attendant economies of scale. As long as all users were offered essentially the same service, overall savings could be produced by offering flat rate services. The inequities thus forced upon some users were small compared to the overall benefit. How- ever, this will no longer be the case in a world characterized by the provision of a wide variety of digitized services from a digital switching machine. The network cost burden of the “book keeping” necessary for usage sensitive pricing will have to be borne because of the large pricing inequities otherwise forced upon the subscribers. The telecommunication networks and their nerve centers, the switchers, will thus make provision for elaborate usage sensitive pricing schemes. The principle effect upon the switchers themselves will be in the consump- tion of the computer real time needed to gather the necessary data for appropriate billing.

Network Traffic The existing public telecommunications network caters pri-

marily to speech. As new services are added to this network- many of them not centered on the transmission of speech in any form-total network traffic will increase. Assuming that we eventually arrive at a common services public network, one must be driven to the conclusion that the traffic statistics of such a network will in time diverge significantly from those of the existing speech network. More compute power will be required per call and there will be more calls and more call types. Traffic patterns will thus become more difficult to predict. This in turn will require that the switching machines at the heart of the network become more adaptive to quickly changing traffic patterns than are existing machines. Traffic predictive algorithms will thus be built into future digital switching machines.

Fortunately, within digital communications technology are the remedies for the problems i t is about to create. Because of the time multiplexed nature of both the switching machine matrix and the transmission facilities which interconnects these machines, Erlang capacity is cheaper than in the analog case. Techniques such as DSI give the network some elasticity in capacity at the slight expense of quality of speech under overload conditions, thus reducing the cost exposures intro- duced by increasing uncertainty of traffic parameter values.

Network Maintenance As pointed out earlier, the switching machine has always

been at the heart of any communications network. In the past, the control bandwidth available from the switch to the end user terminals and from the switch to the transmission facilities has been truly minimal. There is every indication

1148 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. COM-27, NO. 7 , JULY 1979

that this will not be the case as digital switching machines evolve.

There is clear evidence that analog facilities can be success- fully tested using digitally driven test sets and appropriate compyter analysis of mechanistically gathered test results. That digital transmission facilities could be maintained by a digital computer is almost a foregone conclusion. Appro- priately designed station apparatus can also be tested and maintained by the digital switching machine to which it is connected. This point has already been demonstrated in existing digital private branch exchanges.

The digital switch will thus evolve as the true nerve center of combined analog and digital networks. Most networking ele- ments will be tested and have their status controlled directly by the digital communications switching machine.

The Econo-Political Considerations

This paper states that a major evolution in communica- tions technology makes possible the rapid introduction of almost unimaginable communications flexibility. Small initial installations of units located remotely from their powerful computer controls are key to the rapid introduction. However, the question of cash availability will be the gating item. New communication carriers must either convince their financers of rapid return on investment due to the acquisition of new paying customers for the new services or established communi- cations suppliers must be allowed to change their depreciation rates on existing equipment to generate the necessary cash.

In much of the North American environment, the political question as to who has the right to deliver the broad spectrum of services made possible by digital switching must be solved. I feel certain that the predicted evolution will occur, but will it be significantly delayed in some political jurisdictions?

Summary The physical topology of existing analog networks will be

greatly affected by the pair gain capabilities inherent in a digital switching machine. Existing overlay networks will be gradually integrated into a single public combined services network due to the economies thus provided and the great flexibility for accommodating the transient configurations necessary to execute the integration of these overlay networks. Subscriber apparatus, while initially forcing its multitude of protocols upon the network, will eventually become sub- servient to the digital switching machines which will monitor apparatus serviceability and dictate its service status. Because of the multiple grades of service available easily within a digital communications network, usage sensitive pricing will be forced upon the network. With its combined computing and communicating abilities, the digital switch is quite able to handle this complication too.

IMPORTANT TECHNOLOGY DEVELOPMENTS

In these days of fast moving technological evolution, no one technology thrust can significantly alter the evolutionary path of the design of any major system. The evolution of digital switching will be characterized by increased complexity of system mission. The technology combinations which permit

the design'ers, operators and end users to handle this complex- ity are the ones which will be the forcing functions in future digital switching designs. The next few sections of this paper deal with the more important forcing functions individually.

Silicon Technology My readers will be familiar with the rapid advances being

made in silicon technology. Changes in lithography, moves toward dry processing and away from the wet chemistry of wafer 'fabrication combine with new test methods to permit both h i g h density of cixcuitry on the chip and the fabrica- tion of larger chips.

The most profound application of this technology to switching has been manifested in the encoder/decoder func- tion which simply was not economically feasible without these technical advances. Recent success in high voltage silicon technologies spur hopes that a large part of a very difficult interface problem between the voice switcher and the tele- phone set can be solved in silicon technology. Cheaply imple- mented high density logic will also facilitate a solution to the line testing problem. In brief, we can expect significant further advances in silicon tech:nology applicable directly to the analogjdigitallvoice interface.

To some extent, the large scale cu!;tom integrated circuit and the emerging microprbcessor technologies are competitors for prominence in the design of digital communications switching machines. We can expect to :see further advances in microprocessor technology which will simplify the use of these units in hierarchical proce:ssor structuxes. In the evolution of microprocessor technology, the designers will be looking for decreased power dissipation and increased input/output flexibility. Both of these wishes will be met and will result in the continued evolution of flexible switching peripherals. The same comments will become applicable to station appa- ratus. We will see microprocessor technology applied to digital station apparatus with the result that this apparatus will behave more and more as a true peripheral to a computing/communicating complex.

The third manifestation of continued improvements in silicon technology is the continually decreasing costs of memory systems.

Software Systems Large memory systems are essenlial t o the solution of

system architectural problems in systems destined for increas- ing mission complexity. This statement is equally true of both analog and digital switching machines. However, as the digital machine is capable of a much more complex mission than is the analog machine, so the question of memory economics, machine memory capacity and the attendant software genera- tion problems are more crucial in the digital case. But, ample memory systems at affordable prices, while necessary for cost effective ,design, are not sufficent. These memories must be filed with manageable code and mana.geable data to produce viable system designs. Methods of software code generation, control and change must evolve. This evolution is a require- ment for the future mission development of digital switching and I believe that the necessary evolution is happening.

HALL: DIGITAL SWITCHING-FORCES WHICH SHAPE FUTURE 1149

Robust operating systems which will permit the on-line modification of executable code, with little chance of causing system mission disruption, will be developed. The “software factor” will become a working reality. In this environment, software packages will be assembled, tested and released in a manner similar to that used today to produce customized electronic packages from standard electronic modules. Many of the academics who talk of the software factory today do not understand the essence of the quality problem which must be solved to make a software factory into a commercially viable proposition. However, as usual, the academics are pointed in the right direction and they only underestimate the complexity of the systems which must be in place in order to do such a series of custom jobs at predictable costs. Because of the logical complexity of the software systems which will be assembled by a software factory, advances have to be made in the practical testing for logical completeness of software packages. Advances must also be made in the methods for on line system testing before we find software factory generated packages routinely loaded into high system integrity units such as digital communications switching machines.

will be loaded through the switched matrix under the control of the machine at the top of the hierarchy. In the future, we cannot continue to expect traffic to consist primarily of voice encoded at 64 kbits/s. Multiples and submultiples of this number should be easily catered to and traffic from all of these varying speed sources will in the future be mixed as they pass through the switched matrix. A minor extension of this principle encompasses the concepts of packet switching.

Evolution of the Computing Complex

Switching machines have been driven by multiple processor complexes operating in a variety of load sharing modes for more than a decade. However, the more complex of these structures have been bedeviled by problems with fault isola- tion in both hardware and software-software in particular. Advances in computer architecture have brought us to the point that hardware fault isolation in such complexes is tractable. Hardware architectures which permit the trapping of transient hardware malfunctions are available. The software science has not yet solved the general problem of fault tracing in a highly dynamic resource assignment system. To our

Architectural Evolution present software skills we must add the ability to detect and

The architectural evolution of digital switching machines will be driven by the increasing demands for flexibility and accomplished as reduced technical uncertainties make it fea- sible. The designers will become increasingly more uncertain as to the eventual missions of the switches which they design. Since advances in software technology and in silicon technol- ogy will place decreasing penalties on open ended design, the move toward open endedness in design is inevitable. The architecture of the future digital communications switch will follow the same general pattern of evolution, but will move one step further. It will permit the generalization of the peripherals to the point that they become quite general purpose units permitting data and voice entry, data and voice output.

Use of Digital Matrix Digital computing has in the past struggled with the prob-

lems of switched communication between the processor, its memories and its peripheral subsystems. The concept of the “channel” provided considerable flexibility but with message’ capacities which pale to insignificance compared to the mes- sage capacity available in a digital communications switching machine. Thus, digital switching machine architectures of the future will make heavy use of their own switched matrices to provide flexibility of future mission to both the control complex and to their own peripheral subsystems.

Now that I have dealt with the future use of the switched matrix by the control complex itself, let us turn to matters affecting the use of the switched matrix for its prime mission. The switched matrix itself must be designed to serve a loosely defined mission. This flexibility is probably best built in by moving software control very close to the detailed switching functions. This, of course, requires a hierarchical architecture of computing machines. Using the concepts discussed in the next section, the software for the machines in this hierarchy

contain software malfunction in a reasonably random task assigned multiple processor environment. The software mal- function must be readily separable from hardware malfunction and, once detected, the software malfunction must be trace- able to its source. We must remember that in the general environment of which I speak the software malfunction and its detection may be several seconds apart and that processor task assignment may have changed many hundreds of times in the interval. I predict that all of these problems will be overcome. The force which encourages us to find a solution to these problems is the fact. that mission flexibility requires undefined compute power. The ability to add compute power

ever, the attendant hardware and software problems must be solved first, since to do otherwise is to produce a design of undefined mission integrity.

- on-line as required at future dates is the designer’s goal. How-

Terminal Apparatus

The terminal apparatus associated with a digital switch will evolve in the direction of true computer peripherals. Computer peripherals, it should be remembered, are characterized as having access both to a data stream which is to be “displayed” (listened to!) and to a second two-way data stream which provides both for control by the central processor and for status reporting by the peripheral to its master in the hier- archy. Today, the bandwidth in the “display” stream of all computer peripherals is considerably higher than that in the control stream. I expect that this will remain the case but it is obvious that the bandwidth of the control stream must significantly exceed that which is available to the present telephone set. Future architectures will provide for these data streams and will provide through software means for con- siderable flexibility in the formatting of these streams in order that significant peripheral evolution is possible during the life of the digital switch.

1150 IEEE TRANSACTIONS ON COMMUNICATI[ONS, VOL. COM-27, NO. 7 , JULY 1979

End User Services Since I am firm in my belief that increasing uncertainty

of end use awaits the future designer of digital switching machines, I am hardly in a good position to expound con- vincingly on the details of the future services which will be offered by these flexible switches. However, we can be sure that all of these services produce increased traffic for both the switching matrix and the control complex. We can be reason- ably certain that automatic identification of terminals in use (Automatic Number Identification is the telephony analogy) will form an inherent part of most of these services. This identity will often be .necessarily forwarded to other switching machines.

Provision of a robust computing environment will encour- age the generation and sale of software for these machines. This software, coupled with a public combined services corn- munications network will provide a quickly evolving service offering.

As hinted at earlier in this paper, we all expect a major change in the communicating needs of the business office environment. It is in this environment that a marriage of computers and communications will first be seen. The natural mode for all of this communication is digital. The digital communication switch thus becomes a very natural gateway to the business office environment. The digital switch in the public communication network must appear transparent to the office environment user. Great flexibility while main- taining transparency to the offered traffic is a mission which can only be filled by a digital switch.

CONCLUSIONS

Light can be switched and there is no reason why encoded scent cannot be transmitted, switched and reproduced by subscriber apparatus! Silicon technology will provide solutions

to interface problems at all levels of design while retaining flexibility of mission through software control. Software tech- nology will evolve into a well understood science handled in software factories and married to on-line hardware through robust on-line operating systems. The flexibility introduced by digital communications switching initially imbedded in an analog network, is the keystone to .a telecommunications revolution and the attendant marriage of computing with communications.

ACKNOWLEDGMENT I wish to thank the many people who have discussed the

future of digital switching with me and have thus helped me to form the foregoing opinions. Spec:ial thanks are due to Dr. D. A. Chisholm, Mr. R. Kenedi and to Mr. J. S. Bomba for their constructive criticism of my unc1ea.r prose.

C. Denis Hall (S’S8-M’64-SM177) graduated from McGill University, Montreal, with a B.Eng. in Physics in 1960. He obtained his M.SC. (Elec. Eng.) in 1961 from the Uni- versity of Saskatchewan, and his Ph.D. (Elec. Eng.) from the University of Saskatchewan in 1964.

In 1964 he joine,d the R&D Laboratories of Northern Electric a s a member of the scien- tific staff to work a’n the initial design con- cepts of the SP-1 Electronic Switching System.

During the next few years, he held several managerial positions respon- sible for switching software development.

In early 1973, Dr. Hall was appointed Vicie-President of the Switch- ing Division at Bell-Northern Research, and in February 1974 Executive Vice-president, Development, responsible for the total development program o f ’ the laboratories. On Septembcr 9, 1976 Dr. Hall was appointed President of Bell-Northern Research.

Dr. Hall is the author of many technical articles and papers relating to stored program controlled switching. He is a member of the Associa- tion of Professional Engineers of Ontario, the Engineering Institute of Canada, and a Director of the Youth Science Foundation.