International review

USA

cast communications systems contrast with communications systems that use frequency, time or statistical time-division multiplexing, or even circuit, message or packet switching, to provide point-to-point communications). In the USA in recent years, there has been considerable work on a new form of broadcast communication based on the standard specifications has been the concept of 'contention'. An early contention broadcast sysspecification of the MIL-STD (military tem is Aloha, developed at the Universtandard) 1553 multiplex interface. sity of Hawaii. In this system, each This is essentially the specification of terminal attempts to send messagesto an intercomputer communications bus the central computer as soon as the and an interface to that bus. For inmessage (i.e. a line of typed text) is stance, in the computer system being ready, without any synchronization built for a major military aeroplane between the terminals or between the development, the several functionally terminal and the central computer. The distinct computers, each with its own program, each provided by a different terminals are scattered through the Hacontractor or subcontractor, communiwaiian islands, and communicate with cate using the 1553-standard bus. The the central computer using radio transstandard bus has proved itself to be ceivers. Each message includes a check exceptionally useful, permitting the sum. If a messagesent by a terminal implementation of the various comreaches the central computer correctly, puter systems to begin at once without it is acknowledged by the central comit first being necessary to specify the puter, and the terminal is free to accept possible intercomputer communications another line of typing. If the message interfaces for all the pairs of computers is in error, for instance if it collides that might need to communicate. It with a messagefrom another terminal permits the maintenance of individual and therefore its check sum does not computer systems typically without check, no acknowledgement is sent, simultaneous investigation of all the and the terminal will retransmit the other computer systems with which the message, after a short random length failing system communicates. The (to avoid recollison with the message specification of a standard interface is, from the other terminal). Messages sent from the central computer to the of course, simply good system design terminals cannot collide with other practice. It is a major step, however, messages, as they are sent on their own for the US military to attempt to require its contractors to use such a speci- frequency and there is only one source of traffic in that direction. The Aloha fication, and it is an indication of the system has great appeal because of its reasonableness of the specification that simplicity, i.e. no scheduling of the comcontractors appear more and more munications channel is required, but it willing to conform with it. suffers from the fact that, unless the channel is quite lightly used, it becomes clogged with competing transmissions and retransmissions. Nonetheless, there are circumstances in which the Aloha system is ideal. A variation on the Aloha system has been suggested in which terminals transmit without coordination with other Broadcast communications systems terminals; rather, the channel is logically have long been possible. They have typically involved a number of terminals divided into message-length time slots, and terminals transmit either within a 'multidropped' off a single circuit to a slot or not at all, but not over a slot central computer which can broadcast boundary. This system, called slotted a message to all the terminals simultaneously or poll them in turn to receive Aloha, is slightly more complex than the Aloha system, in that each terminal messages from the terminals. (Broad-

Package services over networks There has long been a market in the USA for software products and package services. Software products are proprietary programs rented out for use on customers' computers, e.g. a financial management package to run on an IBM 370 or a database management system to run on a PDP-IO. A package service is a proprietary program that customers can use on the owner's machine or on the machine of a third party. For some time, the major US time-sharing bureaux have provided package services via their own networks. Common carrier networks have provided a greatly increased facility for the distribution of package services; now any one-city or regional supplier of such services can reach a nationwide market simply by connecting to a common carrier network. For instance, the directory of services currently available over one US common carrier network includes the following: 836 different package services, 179 different interactive data bases and 39 programming languages from 82 companies on 100 computers. I think it likely that the US package service industry (which is already of substantial size) will grow much larger with the expansion of the US computer communications industry; one can envisage a market for package services complete with brokers, wholesalers, retailers, speciality shops etc.

MIL-STD 1553 In its quest for more effective procurement of computer programs (i.e. for less risk that the program ever fails to work), the US Defense Department has been trying to specify some standards to be used by contractors providing computer systems to the Defence Department. The attempts include a standard high-level programming language and a formal design review procedure. One of the apparently more successful

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Contention broadcast communications systems

computer communications

must be aware of time to the extent of being able to determine the logical slot boundaries, but it permits twice the throughput before the channel becomes clogged. This approach has been implemented in an experiment arranged by the Advanced Research Projects Agency (ARPA) of the US Department of Defense and carried out with the cooperation of relevant authorities and research institutions in Norway and the UK using and Intelsat satellite over the Atlantic. A series of variations on the basic contention broadcast technology has been developed. When it is used over a high-speed bus, e.g. a coaxial cable, one can use the basic Aloha technique with 'carrier sense' (the simple enhancement of a terminal listening before it transmits and not transmitting if the channel is already in use). This enhancement significantly increases potential channel throughput. The technique is used in the Ethernet system developed by Xerox, and the system provided commercially by Network Systems Corporation of Minneapolis. Some of these variations have been developed as part of an ARPA-sponsored experiment to build a radio-based packet broadcast contention system for command and

control purposes. The basic technology has also been developed in the direction of scheduling the channel so that the various users coordinate their use of the channel to minimize or eliminate conflicts. There is a fundamental tradeoff involved in this. Unless the channel is permanently scheduled (as with a conventional timedivision multiple-access system in which each channel user simply transmits in turn), one must either add delay as a result of exchanging control messages or lose throughput by not scheduling and having conflicts. Particularly with a satellite channel, where the round trip delay to a synchronous satellite is about 0.25 s, the question of how to schedule the channel without exchanging many control messagesbecomes paramount. A number of systems have been developed for scheduling a contention broadcast channel. Experiments with these systems are being carried out as part of the above-mentioned efforts of ARPA and groups in the UK and in Norway, and an ARPA-sponsored experiment is to use megabit channels over a domestic US satellite. Contention broadcast technology is particularly appealing for use over satellite channels, because the

alternatives are so expensive; providing all the combinations of point-to-point channels is financially very expensive and forwarding from one user to another via a third inserts delays that are intolerable for some applications.* Contention broadcast technology is proving to be a viable alternative to more traditional broadcast technologies. The technology is novel, and there may therefore be problems, both in the USA and Europe, in getting it tariffed by the authorities that control public communications. However, I predict that its use will dramatically increase in the next few years in everyday applications outside the control of the telecommunications authorities, such as office automation systems, energy monitoring and control systems, and vehicle control systems; and it is only a matter of time before it achieves widespread use in tariffed communications. David Walden

* A fairlydetailedtechnicalcomparisonof contentionbroadcastmethodscan befound in Lam,S 'Satellitemulti-accessschemesfor

data traffic', Conf. Rec. 1977 Int. Conf. Communications Chicago (June 1977) pp 37. 119-37.1-24

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