COMMUNICATIONS

PABX as a future off ice control ler by JAMES BATES

In the electronic offices being promised for tomorrow it could well be that the PABX systems of today will be in control.

A variety of technology is available to computerise office tasks. This is f'me as far as processing infor-

marion is concerned, but information is only useful if it actually informs i.e. it needs to be communicated. As the amount of information to be processed increases so the range of computing sys- tems to process it also increases, but not always in an integrated or even compat- ible fashion.

Many managers are now realising that the modern telephone exchange or Pri- vate Automatic Branch eXchange (PABX) could represent a potential solu- tion to their information communica- tions problems. The PABX has only reached such a position recently due to advances in switching and distribution technology.

Today's digital, stored program- controlled PABX is a far cry from its electro-mechanical predecessors. But before discussing the capabilities of the modern PABX it is worth considering how it evolved, and from what. This will help to establish its limitations in a mod- ern business envi ronment before describing its potential role as the hub of tomorrow's office, switching and dis- tributing electronic information bet- ween systems and linking them with the outside world.

The first telephone service, intro- duced about 100 years ago, consisted simply of te lephones hard-wired (directly connected) to each other in pairs. In other words there was no switching, each telephone could only access one other telephone.

As the number of te lephones increased it became apparent that a switching function would be required in order to expand the network. Thus a manual operator-controlled telephone switching exchange was introduced and the beginnings of today's public switched telephone network (PSTN)

James Bates is a senior consultant in information technology with Logica.

emerged. Each telephone still required a direct connection to the exchange (as it still does in today's residential service).

As the size of office buildings increased and with them the number of business telephones, it became apparent that an internal switching function was required for the high percentage of internal calls, i.e. calls between indi- viduals located in the same office build- ing. In offices with over 30 telephones 30-50 per cent of business calls are inter- nal. This resulted in the introduction of the PBX, a manual operator controlled private exchange.

The next important upgrade in busi- ness telephone service was the facility to allow users to automatically select out- side lines (by dialling "9"). With this feature, the system was redesiguated as a PABX.

A PABX basically consists of a switch- ing matrix and interfaces to internal and external lines. The primary functions of a PABX are: to provide private switching for intra- office calls to provide concentration (large number of telephones accessing relatively small number of outside lines).

The modem PABX employs digital technology for switching while all ser- vices, defined by software programs, are held in stored program control (SPC). Distribution for voice transmission bet- ween the telephone and the PABX is analogue. At the interface to the PABX the analogue voice signals are passed through a group coder (a multiple port analogue-to-digital/digital-to-analogue coder/decoder) and converted to digital pulse code modulation (PCM). The digi- tal PCM voice signals are then passed to the switching matrix via a time sl6t in a time division multiplex (TDM) loop.

These modem systems support both conventional telephone sets ("700-type sets" in British Telecom terminology) and proprietary sets ("feature-phones" in BT terminology).

The 700-type sets utilise single-pair cable distribution with voice and signal- ling information passed over the same wires. The feature-phone sets utilise two-pair cable dis t r ibut ion, with analogue voice transmission on one pair and digital signalling on the other (for

The Plessey Vutei terminal integrates voice and data communication in a single desk- top unit.

dialling, voice feature activation and call manipulation).

Feature-phones provide access to additional lines and/or voice features. These "features" are all related to originating or manipulating voice calls (establishing connections, transferring connections, etc.). Examples include third-party add on, call transfer, call forward, ring again, abbreviated dialling plus many others.

The PABX is a system for switching and distributing interactive voice com- munications in real time. It is designed to connect telephone sets and distribute analogue signals of the human voice. Most modem systems are designed with these as the most important criteria.

In digital SPC PABXs analogue voice signals are digitised simply to allow software rather than hardware to pro- vide the switching function. This means they are designed primarily to carry analogue voice, though they obviously have the potential to carry digital data. Indeed PABX systems now exist that do just that.

The problem in designing a PABX to carry computer originated (or destined) digital data is twofold and concerns the bandwidth limitations of the distribu- tion system and the common equipment associated with the switching function.

Voice and data traffic With regard to distribution, there is no problem in digitising the signal right out to the telephone set and therefore to associated data terminals. The Rolm CBX and Northern Telecom's SL-I support digital transmissions for data terminals on their existing digital signal-

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ling path via a data module that plugs into the telephone set.

This converts the parallel outputs of an RS232 interface from the data termi- nal to the required serial PCM bit stream for distribution to and from the PABX. These two systems carry digital data and analogue voice over conventional (two- pair) telephone cabling.

There are bandwidth and distance limitations, due to attentuation and cross-talk on telephone cable pairs. However, the potential for data trans- mission rates of up to 64 Kb/s on existing cabling is possible. Also, of course, dis- tribution need not be restricted to con- ventional telephone cables in the future.

One variation of the conventional star network distribution with a centralised PABX is typified by the Intecom IBX system. This has a distributed switching capability with digitised voice and data distribution (digital telephone sets) and high speed (40 Mb/s) optical fibre con- nections back to a centralised control unit (see diagram), providing connec- tions between telephones and data ter- minals of up to three miles on the same system.

It is in the area of switching that the more serious implications exist for PABX systems carrying data. Most cur- rent PABX systems are designed to carry voice communications. The switching equipment design is based on empirical voice traffic data regarding average hold- ing times (call duration) and the number of call attempts (requests for service) during specific time periods. The sys- tems are designed to carry these traffic patterns which, while not allowing every user of the system to make a simultane- ous call, provide an acceptable grade of service with imperceptible call delays at all but the busiest periods i.e. if one telephone set is required to connect to another idle telephone set or outside line, a path between the two is usually, but not always, available this is known as "blocking".

While this approach is acceptable for voice traffic, it does introduce problems when carrying data traffic. Data traffic does not behave like voice traffic, rang- ing from the one extreme of very long holding times (e.g. batch transmissions) to the other extreme of numerous call attempts (e.g. interactive applications). The former ties up equipment and the latter makes heavy demands on the switching matrix. Both can contribute to a degraded level of service in a blocking system designed to switch voice traffic.

These problems can be overcome by the introduction of non-blocking sys-

tems designed to carry voice and data traffic. (The IBX system previously mentioned is a non-blocking system).

With the advent of cheaper and more powerful technology, the PABX is evolving towards a system specifically designed to handle both voice and data traffic. The integration of both voice and data within the system is also leading to the development of a new generation of integrated voice/data terminals, typified by the Plessey's Vutel terminal and Northern Telecom's Displayphone.

Both these devices incorporate a VDU with a telephone handset and built-in modems for external communications via the PSTN. The Displayphone comes with an optional keyboard .for full alphanumeric input capability.

These terminals were primarily developed for viewdata applications, which in itself is a development of dis- play technology in conjunction with the telephone network. The popularity, and therefore success, of any new system is dependent in part on user acceptance.

Viewdata enjoys a similar advantage in this respect to the PABX in that they both provide access to their services via familiar use~ terminals- the telephone in the case of the PABX and the television set in the case of viewdata.

The integration of viewdata with the PABX seems a natural step in the evolu- tion towards fully integrated electronic office systems.

New services The PABX has the potential to pro-

vide computer terminals access to their host computer. It also has the more sig- nificant potential, via its switching func- tion, to "free" terminals from their hosts - to allow single terminal access to a variety of computing facilities.

An important development in the external network is the introduction of British Telecom's System X digital tele- phone exchange. This opens the door for a full digital communications network in the UK. Plans exist for the introduction of such a network by the middle of this

decade- the public switched integrated services digital network (ISDN). T h e digital integrated PABX will have an important role to play in switching digi- tal voice and data on to this network.

In addition, BT already provides a packet switching service (PSS) with full duplex transmission capability for both analogue and digital connections. The PABX can connect to PSS via a packet switching exchange- PSE with a packet assembler/disassembler (PAD). Direct connections will be made available for packet mode terminals as developments continue, and digital PABXs also have the potential to be developed for direct connection.

Once a PABX is supporting the exter- nal distribution of digital data, it can also support internal distribution. The range of devices and applications will only be limited by the supply of required inter- faces. As long as a terminal can talk to a computer in a language it can under- stand, the PABX could provide the intermediate transparent link.

However, the role of the PABX will not be restricted to the level of "dumb" switching. User requirements for com- municating information are changing as fast as the range of methods increase. Electronic mail (EM) is a prime example of this. Traditional methods of informa- tion were based on, and indeed limited by, the available media: face-to-face, telephone, mail, telex and facsimile. The latter two (telex and facsimile) are exam- ples of very limited forms of EM.

Communicating information via these methods is often costly, time-consuming and inefficient. The modern forms of EM can allow information to be com- municated to individuals in remote loca- tions relatively cheaply, quickly and without the necessity for the recipient to be present at the time of connection. EM can take advantage of PABX switching, via PSTN, private lines, PSS and even- tually ISDN. The PABX also has the potential to provide the processing as well as the distribution function.

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MARCH 1982 29

COMMUNICATIONS

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The Rolm CBX provides a limited EM facility (known as R E M S - Rolm Electronic Message System) by utilising its redundant processor as an applica- tions processor to send and receive mes- sages. User requirements for EM extend to more than just messages. A full func- tional EM system will duplicate a paper mail system, with the capability for text, image, graphics and handwritten infor- mation in any quantity, plus it will pro- v ide the added value of near - instantaneous transmission, high stor- age capacity and a variety of delivery options.

All these functions could be provided via a digital PABX with access to an information storage and retrieval system and associated communicating word processors (via a local area network, for example). International standards are now being developed for this type of capability. The open systems intercon- nect ion (OSI) model provides a framework for the various standards required, with the objective of allowing any compatible system to be connected to any other.

Perhaps the most significant standard currently under development is teletex. This is a standard for communicating word processors to provide a memory- to-memory "super telex" service via PSTN and, eventually, PSS and ISDN. As more of these detailed standards are produced, so manufacturers have the opportunity to produce equipment that is compatible with the integrated ser- vices that are being introduced.

When considering the costs involved in producing these new devices, and the resulting cost to the user, they can seem very high. A digital PABX designed for voice and data with advanced EM ser- vices and integrated workstations could easily cost two to three times as much as a conventional PABX equipped only with telephones. This to many managers may look inordinately expensive. How- ever, as any good manager knows, the cost of providing a service is not just the cost of the equipment.

User communications behaviour pat- terns change as new services are intro- duced and this can have a significant effect on costs. For example, only 28 per cent of telephone calls reach the intended recipient: 72 per cent of all call attempts are unsuccessful. How much does that represent in lost time, lost pro- ductivity, unavailable equipment for other would-be callers, etc.?

When looking at the costs involved in an integrated system approach there are

many factors to take into consideration, and each case is different. The costs of the equipment have to be balanced against network usage (e.g. delayed computer communications during cheap t a r i f f pe r iods ) , special o p e r a t o r requirements versus desk-to-desk ser- vice, the speed and timing of delivery options, and the storage and access capabilities of electronic based systems. Also, the under-utilisation of existing computer equipment or the over- provision of terminal devices can be alleviated by the switching function of a digital PABX.

It is not yet clear how far down the road tO full integration the digital PABX should be taken. It is envisaged that the PABX has a major role to play in switch- ing and distributing electronic informa- tion both internally and externally, and providing single terminal access to a var- iety of services for users.

The next generation of PABX's will be designed to handle the whole range of information t r a f f i c - voice, image and t e x t - and to interface with compatible office equipment and networks. How- ever, the PABX will continue as a vehi- cle designed to carry people-oriented information, the telephone, telex, EM traffic of the future.

Whether it will evolve into a vehicle for machine-oriented information (i.e. data generated by computers and passed directly to other computers) is question- able. This seems to be an area where customised private data networks and interfaces will continue to serve the requirements for the foreseeable future. Nevertheless, even machine-oriented information is eventually processed for human consumption.

The first of these new integrated PABX systems are now available in North America, and with the relaxation of the BT monopoly will soon be avail- able in the UK. As competition increases between suppliers, and user require- ments become more sophisticated the traditional role of the PABX will disap- pear. We can look forward to a range of integrated voice/data systems being introduced to the UK market within the next few years.

For further information circle the following numbers on the enquiry card. Plessey Vutel 102 Rolm CBX 103

PABX in the office To ICL, office networking means linking together components of a product range that runs from per- sonal computing facilities to all sizes of mainframe.

Some systems require the high speeds available from specialised area networks, such as those based on Ethernet or the Cambridge ring. For example, Ethernet is used to connect the Perq high resolution graphics sys- tem. Similarly DRS 20 Microlan cable, l inking the D i s t r i bu t ed Resource System workstations, will carry up to one million bits of infor- mation per second.

In ICL's view, though, this sort of approach is not cost-effective for the executive's desk. So the existing tele- phone wiring has been used in the office networking strategy. To this end ICL introduced a PABX system into its product range. The system is the DNX-2000, made by the Cana- dian company, Mitel. ICL chose the DNX-2000 because of its up-to-date technology, its low power and space requirements.

At 256 K-bytes per second, the data t ransmiss ion rate of the DNX-2000 is claimed by ICL to be four times as fast as that of its nearest rival.

ICL expects the digital network based on the PABX to provide the cement for all office systems, linking terminals, and providing an interface with local area networks. Tele- phones, DNX-workstations and ter- minals, such as the Sinclair digital telephone terminal, can be connected over telephone lines, and then into local area networks. So the LAN and the PABX will be complementing each other in the office networking system.

Interfaces within the office net- work are to be standardised under the Information Processing Architec- ture. A gateway to IBM's SNA is also to be provided.

Cost control and administrative functions, electronic mail, and selec- tive access to facilities are all available in the standard product. Future developments are to include ICL or joint ICL/Mitel specialised applica- tion software.

Workstations linked through the PABX can now, says ICL, be regarded as extensions of the tele- phone system.

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