Journal of Microcomputer Applications (1985) 8, 51-61

TUTORIAL

Networking micros-principles and practice

John D. Rice

University of Manchester Regional Computer Centre, Oxford Road, Manchester Ml3 9PL, UK

Microcomputers and local area networks are subjects of considerable topical interest. The current status of local area networking is reviewed and the suitability of various networking systems for the solution of the problems of interconnecting microcomputers is examined.

1. Introduction The explosive growth over the last five years in the number of microcomputers used in commerce, industry and education has created several problems. Many of these result from the fact that whilst cheap processing facilities have been distributed, the data used by each processing element has not: either as a direct consequence of the processing application (e.g. local processing of data in a corporate database) or as a result of cost constraints (e.g. the need to share filestore). The freestanding microcomputer, widely welcomed initially as the harbinger of user liberation from the tyrannical control of the computing centre, has been found to be unequal to the task of providing a complete computing service. The primary reason for this is that with the state of the art in both software and hardware technology it is still more cost-effective to share certain resources. True distributed database systems are rare in the field and rotating mass storage is generally cheaper per bit the larger the capacity of the device. In most computing environments there are some resources which people will agree to share. In IBM mainframe environments few MVS programmers have their own copy of the full set of about 80 manuals; in the micro field, whilst on-line documentation is a useful facility, the amount of disc space it can occupy may result in the user retaining it on removable media, and media removed is not on-line. Good quality printed output is a definite requirement in many applications, but matters seem out of proportion if a printer costs more than the microcomputer to which it is connected, especially when it is used relatively infrequently. Sharing such a device has obvious attractions, but with whom, and how?

The answer to this kind of problem is generally recognized to be to associate distributed resources by creating a computer network. Despite the sterling efforts of the technocrats to drive towards standardization in networking, terminology is used as loosely in this area as it is in the microcomputer field. The subject of microcomputer networks often appears in the literature simply as ‘Micronets’. A micronet is a network in which all the processing devices attached to it are microcomputers. This conveys

51 0745-7138/85/010051+ 11 003.00/0 0 1985 Academic Press Inc. (London) Limited

52 J. D. Rice

information about one attribute of such a network, but it is not usually the only significant one. Considering the attribute of geographical area served by a network, the field was once split simply into wide area networks (WANs) and local area networks (LANs) with the dividing line set at about 4 km. In these terms, this paper is concerned primarily with microLANs. The IEEE 802 project, which is considered in more detail in a later section, has clarified the definitions of a LAN with the following statement:

Although the exact parametic values for the area, length, number of stations and topology can- and have-been debated at great length, suffice it to say that the committee is basically concerned with a cable up to several kilometres in length, supporting several hundred stations in a variety of topologies at speeds of, tentatively, 1, 5, 10 and 20 Mbit/s (Middleboe, 1982).

However, they have since made a further distinction between LANs and Metropolitan area networks (MANS) the latter defined in IEEE standard 802.6 as operating at speeds greater than or equal to 1 Mbit/s, generally using CATV technology over 5 to 50 km. MANS are seen as normally being used by many individuals and organizations and often provide the means for internetworking of LANs (IEEE Standard 802.1, 1983).

It is useful to create another distinction at the opposite end of the spectrum. The definition of a LAN given by IEEE Project 802, has been further refined so that it is now generally considered to relate to networks operating on a cable of about 500 metres to 5 km in length at speeds between 1 Mbit/s and 20 Mbit/s, although ANSI have a similar committee examining LANs at speed up to 50 Mbit/s. We are then left with networks with cable lengths under 500m, such as typically might occur in a floor of a building or a single laboratory. In some literature these have been referred to as small area networks (SANs) (Goldberger, Kaplinsky & Moelands, 1982; Moelands, 1983; Dang, Diaz Nava & Husovic, 1983), albeit in a somewhat specialized context. Whilst the scope of this paper is not limited to networks of the scale of the three SAN examples in these references (100 kbit/s over 4 m; 64 kbit/s over 150 m; 100 kbit/s over 25 m), in the remainder of this paper all references to LANs may be read as ‘LANs with a bias towards SANs’.

2. Local area networks: the facts

This section provides ‘basic’ background information in the four areas of LAN topology, technology, technocracy and tokens.

Topology

There are two fundamental LAN topologies: bus and ring. A bus network shares a single physical channel, giving full connection between passively attached nodes without centralized routing or switching control, and permitting easy reconfiguration and expansion. Ethernet networks use a bus topology (IEEE Standard 802.3) as does the recently announced IBM Personal Computer Cluster (IBM, 1984~).

A ring network consists of a physical loop in which each node is actively connected to the next in the ring. Messages pass through the nodes, so in simple rings it is possible for both ring medium and ring node faults to disrupt network traffic. These problems are addressed by medium duplication and/or failure bypass mechanisms in nodes, though

Networking micros 53

these increase component costs. Control of rings may be distributed or vested in a designated (though not necessarily fixed) controller. The Cambridge Ring is the best known example in the UK of a LAN with ring topology (Cambridge Ring, 1982).

A star topology is a degenerate case of a ring in which all nodes and controller (if any) are compressed to a single point. Such systems suffer the disadvantages of high cabling requirements for large sites and dependence upon a single switching point which poses

potential reliability, throughput and resilience problems. Networks centred on terminal switches, such as the Gandalf PACX IV, or digital PABXs, are examples of star

topology. Tree topologies, which may physically resemble interconnected stars, normally have a

logical topology of integrated interconnected buses, though in some cases with ring features. The Datapoint ARCNET has a tree topology (Bertness, 1983) as does the recently announced IBM Personal Computer Network (IBM, 1984b).

Technology

Whilst the length and type of cabling required to install a LAN is important (per unit

length installation costs are higher than cable costs), the nature of the signals traversing the cable and the equipment required to enable attached devices to transmit and receive them is of much greater interest. We therefore need to consider access mechanisms, transmission methods and transmission media (Hart, 1984; Frith, 1983; King, 1984).

At least three basic access mechanisms can be distinguished. Jntegrated PABXs will allow voice, data, image and video communications to take place concurrently using

circuit switching techniques which allocate a dedicated path through the switch for a particular connection. Routing is generally restricted to point-to-point communication and the PABX performs the functions of switch processing, applications processing and data conversion processing, with a relatively low level of complexity required at the equipment interface (Wurzburg & Kelley, 1984; Mokhoff, 1984).

The second basic access mechanism is a random scheme, carrier sensed multiple access (CSMA), in which all nodes on a network listen to the carrier on a network (carrier sensing). In the event of multiple transmissions taking place simultaneously, arbitration according to a pre-defined algorithm takes place to determine recovery action to be taken. The algorithm may be based on collision detection (CD) or the lack of it, collision avoidance (CA). The best known occurrence of the CSMA/CD mechanism is in Ethernet. Among the disadvantages of such a mechanism are generally the lack of priority (nodes have equal priorities) and the inability to guarantee maximum delay times for data transmission.

The third basic mechanism is a polling scheme involving token passing in which a ‘token’ is circulated around the network and possession of the token is required in order to transmit. A controller in a token passing network can provide a priority mechanism, performance is predictable and a maximum for transmission delay can be guaranteed. Token passing networks may be either bus (e.g. Datapoint ARCNET) or ring [e.g. IBM token ring (Terrell, 1983)]. The Cambridge Ring in a ‘slotted ring’ variant of the token ring.

Transmission methods normally used by LANs are baseband, in which the cable carries a single channel, and broadband, in which multiple channels are carried and for which a higher quality transmission media is required. Transmission media include

54 J. D. Rice

twisted pair cable, baseband coaxial cable, broadband coaxial cable and fibre optic cable.

In order to reduce the cost of LAN access and to increase the speed of access and transmission, considerable efforts have been made by the semiconductor manufacturers to produce LAN LSI/VLSI components. The functions performed by such components are controllers (handling the data link interface to the connected device), serial interface adaptors and transceivers (handling the physical medium). Table 1 compares the characteristics of various chips. Complete three-chip sets are expected to retail at approximately $75 in quantity by 1985. For details of current chip features see (Mokhoff, 1984; Fontaine, 1984; Baker, 1984). It is evident that the major activity at present is concerned with the production of chip sets for Ethernet. Some exceptions to this will be considered later.

Table 1. VLSI components (a) 10 Mbit/s Ethernet components

Part Manufacturer Function

8001 EDLC 8003 EDLC (DMA) 8002 MB502 MB61301 DLC R68802 8390 8391 8392 i82586 i82501 AM7990 MK68590 AM7991 MK3891

Seeq Technology Seeq Technology Seeq Technology Ungermann-Bass/Fujitsu Ungermann-Bass/Fujitsu Rockwell National Semiconductor National Semiconductor National Semiconductor Intel Intel Advanced Micro Devices Mostek Advanced Micro Devices Mostek

Data link controller

Serial interface Serial interface Data link controller Data link controller (LNET) Data link controller Serial interface Transceiver interface Local comms controller Ethernet serial interface Controller (LANCE) Controller (LANCE) Serial interface Serial interface (bipolar)

(b) Token passing components

Part Manufacturer Function Speed

(Mbit/s) Protocol

? IBM/Texas Instruments Controller 10 token passing (IBM) COM9026 Standard Microsystems Controller 2.5 token passing (ARC) WD2840 Western Digital Token access 1.1 token passing/HDLC

controller

(c> ‘Cheaper Net’: Mirlan (1 Mbit/s; reduced facility Ethernet)

Part Manufacturer Function

i82586 ?

Intel National Semiconductor

Controller Controller

Networking micros 55

Technocracy

One of the fastest growing occupations is that of network technocrat. Large numbers of national and international committees meet regularly to define and ratify standards for networking hardware and software. In the local area networking field, the IEEE Standards Project 802 has made the most significant contribution. Its purpose was to define a single LAN standard for the benefit of equipment manufacturers. Such a standard could benefit users who should be able to obtain equipment at lower cost, and should benefit system designers in enabling them to specify standard products. Not surprisingly, despite having such a high ideal as its driving force, the committee failed to achieve its goal, though it has concluded proposals for three major categories: CMSA/

CD access on a bus (P802.3), token passing access on a bus (P802.4), and token passing on a ring (P802.5). The structure of P802’s division of work is given in Table 2. The European Computer Manufacturers Association (ECMA) has adopted IEEE 802‘s proposals as its standard and, prior to international acceptance by the International

Standards Organization (ISO), the UK Department of Industry IT Standards Unit endorsed them as ‘intercept standards’ (Norton, 1983) along with the Joint Network

Team’s CR82 proposal (Cambridge Ring, 1982). However, standards for media access (IEEE Standard 802.3; IEEE Standard 802.4; IEEE Standard 802.5) are only part of the story. A single logical link control protocol (IEEE Standard 802.2) is defined by IEEE for use in conjunction with all media access standards, for connectionless (datagram), acknowledged connectionless (transaction), and connection-based services. In the UK,

the recognized ‘intercept standard’ for the Cambridge Ring at the data link level is the CR82 Byte Stream Protocol.

Establishing logical links between devices on a network is not an end in itself and higher level protocols are necessary to provide for meaningful message interchange between applications. At this point the water begins to become somewhat murky. Xerox, who designed Ethernet in the first place, have a set of higher level protocols of which

Table 2. IEEE 802 Committee Work Areas

P802.3 CSMAjCD Baseband (Coaxial cable: 1, 5, 10 and 20 Mbit/s) Broadband (Coaxial cable: 10 Mbit/s (6 MHz channel)

P802.4 Token Bus Broadband (Coaxial cable: 1.544 Mbit/s (4 and 6 MHz channel)

5 and 10 Mbit/s (6 MHz channel) 10 and 20 Mbit/s (12 MHz channel)

Baseband Phase continuous (Coaxial cable: 1 Mbit/s) Phase coherent (Coaxial cable: 5 and 10 Mbit/s)

Baseband P802.5 Token Ring

Coaxial cable: 4, 20 and 40 Mbit/s Shielded twisted pair wiring: 1.4 Mbit/s

56 J. D. Rice

details are publicly available:* Courier, the Remote Procedure Call Protocol, and

Internet Transport Protocols. IS0 has a standard for Transport Service and Protocol (DP8702, DP8703) (Berry, 1984), and the UK universities are encouraged to adopt the so-called ‘Yellow Book’ Transport Service. In the latter two cases at least these protocols were derived originally for WANs. They are seen as basic building blocks to support messages and file transfer or remote peripheral device access. Nelson comments,

However, this view falls short of being able to provide the comprehensive computing environment that most timesharing users are accustomed to, particularly if a community of users desire sharing of programs, data files, and other resources. While the design of local area networks may be interesting, they are not very useful in the absence of higher level semantic forms (virtual memory, data streaming, etc), which relate network resources directly to real applications (Nelson, 1983).

In justification of their own network layered model Blair, Hutchison & Shepherd (1983) state that

the best known example of a network layering model is the IS0 OS1 model which is recommended as a standard architecture for all distributed systems. However, problems can be encountered with this model when it is applied, specificially, to local area network based systems. Although layers 1 and 2 (the physical layer and data link layer) correspond well with typical LAN architectures, as evidenced by their use in the models proposed by ECMA and IEEE, difficulty can be encountered in relating layers 3 and 4 (the network layer and transport layer) to such systems. Furthermore, higher layers are too ill-defined to be of any practical assistance in the design stage of a LAN application.

Many others take a similar view and little regard is given to standards in the creation of products designed for SANs or in the construction of SAN experiments.

University computer centres in the UK have in the recent past been encouraged to develop local area networks based on X25 packet-switching exchanges or Cambridge Rings.

Tokens

It cannot be denied that the major reason for the current uncertainty in the LAN marketplace, and the less than total acceptance of Ethernet as the sole LAN standard, apart from the current relatively high cost of Ethernet products, is the fact that IBM has not yet released a product based on the token-passing baseband ring technology which it proposed as a standard to IEEE 802.5 and of which it publicly demonstrated a prototype, the ‘Zurich Ring’, at Telecom ‘83 in Geneva (Mini-Micro Systems, 1984). IBM has various LANs amongst its product line (8100 SDLC Loop; Series l/LCC for example), and has used LANs in special purpose systems in the past, the SDLC loop in POS systems for example. Recently IBM has announced two networking products for the IBM PC marketplace. The Personal Computer Cluster is a baseband bus network, using CSMA/CA access protocol, running at 375 kbit/s over coaxial cable and supporting a maximum of 64 stations, one of which may be a disc server (IBM, 1984~). For larger networks the Personal Computer Network is a broadband tree network, using CSMA/CD access protocol, running at 2 Mbit/s over coaxial cable and supporting, in its fully expanded state, up to 1000 workstations, any of which with hard discs may be

*Ethernet Administrator Xerox Office Products Division, 3333 Coyote Road, Palo Alto, CA 94304, USA.

Networking micros 57

server stations (IBM, 1984b). These networks address existing IBM PC users’ needs. However, the proposal of a standard LAN has led the computer industry to assume that a token ring product will eventually emerge which will have as far-reaching effects as the introduction of the IBM PC microcomputer, for which it was recently estimated that half the computer programs in the world are now being written.

3. Local area networks for microcomputers: the issues

Having reviewed the current ‘official status’ of LANs we now consider some practical issues such as cost, configuration, capacity and communication.

cost

Microcomputer networks require both a low entry cost and a low cost per unit attached to the LAN. Flint (1983) states that

the only international standard at present is Ethernet. Regrettably Ethernet remains too expensive to be used with most microcomputers. There is no official standard for micros; neither has the market created a de facto standard.

Table 3 includes a list of manufacturers of Ethernet network products. In general the cost of connection of a microcomputer directly to such a network is between &700 and &2300. The lower prices tend to be for such microcomputers as the IBM PC, and it would appear that currently an Ethernet connection to a particular microcomputer costs

Table 3. Vendors of network products interconnecting products from more than one computer manufacturer

I. Ethernet 3 COM Interlan Bridge Communications Associated Computer Consultants Ungermann-Bass : Net/One (also broadband) Excelan Perex : SIOO controller Camtec : Ether-PAD Intel

2. Other Baseband Network Systems Corporation

3. Broadband Sytek

4. Cambridge Ring Racal Milgo Logical VTS SEEL

: PLANET : Polynet : Transring

58 J. D. Rice

approximately one third of the microcomputer’s cost. It is expected that, with the advent

of VLSI implementations during 1984, the cost of Ethernet connection will halve, reducing it to perhaps one sixth of the cost for any microcomputer, with a bottom-line price of about &350.

The cost to connect a microcomputer to Ethernet can be reduced if it is connected via a terminal server such as one produced by Interlan, Bridge, Camtec or Ungermann-Bass. The cost for connection via an RS232 interface then reduces to somewhere between &200 and &500. At this price, considering that in future connection costs will fall and that standard software is available, this form of connection can be recommended. If several hundred microcomputers are to be interconnected, a broadband network with connection via terminal servers may be viable,

The CAMTEC JNT-PAD provides similar terminal concentration on to X25 networks and may be used for local switching at a cost of about &300 per terminal.

It is unfortunate that the cost of connection to a Cambridge Ring is little different from that to Ethernet and that fact, plus the smaller number of vendors, is likely to limit the extent to which it is used in a service environment as opposed to the research laboratory.

The Datapoint ARCNET Technology is available from its chip supplier Standard Microsystems in the form of its ARC SIOO board, and the same technology is used by Tandy in its TRS-80 network. Whilst ARCNET has the property of offering an intermediate transmission speed of 2.5 Mbit/s, it is almost manufacturer-specific, despite the recent availability of an interface to the IBM PC, and therefore not likely to be widely used.

Turning to lower cost networks, a number of networks exist which are essentially one- brand micronets, namely those produced by Altos, Cromenco (C-NET), Digital Micro Systems (HINET), Orchid Technology (PCNET), Northstar (Northnet) and Wordcraft Systems (HYDRA:PET). All of these are of interest, as they generally cost under 6500 per connection, if one has standardized on the use of a particular microcomputer.

The remaining microcomputer networking facilities can be divided into two groups,

those costing between &200 and &400 per connection, and those costing under &200. In the former category are Omninet, from Corvus Systems which runs at 1 Mbit/s and

supports, amongst others, the Corvus Concept, IBM PC, Apples II and III and Dee Rainbow 100 at a cost of approximately 5400 per microcomputer. A range of associated products are available for usual facilities such as a print server (&800) and a file system disc server (&700) together with disc file systems at around &3000. Another product in

this category is the PLAN 4000 system for Nestar Systems which supports Apples II and III and the IBM PC (and possibly others). A print and file server on this system costs around &16,000, but that includes a 60 Mbyte disc and 20 Mbit tape backing unit. The cost of connection per microcomputer is approximately the same as to Omninet. More recently, the PLAN 3000 and PLAN 2000 systems have become available at lower cost.

Finally, there are a number of systems at under &200 per microcomputer. Three provide support for RS232 interfaces only, at 9.6 kbit/s, namely the Midlectron VNET, essentially a circuit switch, and Real Time Developments Clearway, which uses a register insertion technique on a ring running at 56 kbit/s. VNET cost less than &200 per connection and Clearway about &150. In both cases software is available for micro-to- micro communication. VNET software is available for CP/M, CP/M 86, PC DOS, MS DOS, Apple DOS and BBC DOS, whilst Clearway software, by Peachtree, is available

Networking micros 59

for CP/M, CP/M 86 and MS DOS. If the path from the microcomputer to the switch is short and the switch already exists, then circuit switches such as the Gandalf PACX IV provide an RS232 interconnection facility at comparable cost.

The Acorn Econet provides interconnection at 210 kbit/s for BBC microcomputers, though other manufacturers now supply compatible cards for other systems. This is again a cheap solution, but with a relatively limited number of different manufacturers’

systems supported. The Nine Tiles Multilink network, manufactured by Hawker Siddeley, has network

interfaces for the Epson QXlO, Apple II, Multibus, Q-BUS, SlOO bus, UNIBUS and BBC Micro, with a connection cost of under &lo0 for the BBC Micro. CP/Net and a network filing system are available. This new development is clearly an indication that network interfaces at about one tenth of the cost of the microcomputer to which they connect are likely to become commonplace in the very near future.

Cost information was derived from manufacturers’ literature and Middleboe (1982);

Anon. (1984); Communications Management (1984) and Local Netter Designer’s Handbook (1983).

Configuration

From examining a number of manufacturers’ network products it is clear that one of the most significant factors in determining cost is the question as to whether or not it is necessary to have a dedicated network controller. That requirement on the Cambridge Ring significantly affects the overall cost of the network. However, other networks may not require a dedicated controller, but nevertheless require that one attached microcomputer performs this task, perhaps in addition to acting as a file server. In evaluating low-cost networks it is therefore essential to determine whether a file server is required initially. If not, then this may affect network selection. Space does not permit a detailed analysis of this aspect of the networks mentioned in this paper, nor of the question as to whether the network can be dynamically reconfigured whilst continuing to operate, nor of the distances, both overall and between connections, over which the network can operate. The significance of those factors depends entirely upon the application environment envisaged.

Capacity

In general, low cost networks operate at a transmission speed of less than 250 kbit/s with up to about 100 attached devices. Again, the acceptability of a particular network depends on the nature of the applications envisaged. The requirements for data transmission speed on the network and across the interface between the microprocessor and the network differ, for instance, if the application is one of periodic file transfer to local discs from the requirements if the application is for on-line distributed database access.

Communication

Once the networking bug has struck and microcomputers are neatly connected in a LAN it is likely that further connections both to larger and smaller networks will be desired. A

60 J. D. Rice

whole hierarchy of SANs, LANs, MANS and WANs may evolve. It is therefore important when selecting a network to consider whether it is necessary to obtain one with bridge facilities (linking similar networks), and/or gateway facilities (linking different networks). At present, as a general rule, the cheaper the network the less such facilities are available.

4. Conclusion

Networking products, like microcomputers, are continually being improved. Cheap standard network products, with interconnection costs per microcomputer of about &loo, are on the horizon and should have a revolutionary effect in their field as the announcement of the IBM PC had on microcomputing. Until then, the situation, sadly is, to quote Flint (1983) again, that, when selecting a LAN

despite the apparent plethora of products, a careful study of the requirements often leaves very few choices.

References

Anon. 1984. Znformatics, 5 (3), 8&86. Baker, S. 1984 Falling costs forecast for Ethernet as chip-makers shift into gear. Electronics

Weekly, I1 January, 16. Berry, D. 1984. Standardizing upper-level network protocols. Computer Design, 23 (2) 175-185. Bertness, E. A. 1983. Baseband LAN fine tunes token-passing technique. Computer Design, 22

(12), 95-99. Blair, G. S., Hutchison, D. & Shepherd, D. Implementation of a local network operating system.

In Local Networks: Strategy and Systems, Proceedings of Localnet ‘83 (Europe), pp. 387-398. Online.

Cambridge Ring. 1982. Cambridge Ring 82 Interface Specijications. SERC. Communications Management, February 1984, 40-41. Dang, Ng. X., Diaz Nava, M. & Husovic, N. 1983. Small local area network with a collision-free

technique for connecting heterogeneous microcomputer systems. Microprocessing and Microprogramming, 12, 167-174.

Flint, D. C. 1983. The selection of a local communications network. Information Technology Training, 1 (2), 65-68.

Fontaine, J. A. 1984. Controller and micro team up for smart Ethernet node. Computer Design, 23, (2), 215-223.

Frith, R. 1983. Local area networks-the basic principles. DP International, Summer, 21-26. Goldberger, A., Kaplinsky, C. & Moelands, A. P. M. 1982. Small area networks for serial data

transfers. Electronic Components and Applications, 5 (l), 15-22. Hart, M. 1984. Should you wish upon a star or take a bus? Communications Management,

February, 36-39. IBM. 1984a. Personal Computer Cluster, GU20-4022. IBM. 19846. Personal Computer Network, GU20-4021. IEEE Standard 802.1, Architecture and Znternetworking, Revision A. March 1983. IEEE Standard802.2, Logical Link Controt (Class I: Connectionless Logical Link Control; Class

II: Connection Oriented and Connectionless Logical Link Control). IEEE Standard 802.3, CSM.4ICD Access Method and Physical Layer Specifications. IEEE Standard 802.4, Token-Passing Bus Access Method and Physical Layer Specifications. IEEE Standard 802.5, Token-Passing Ring Access Method and Physical Layer Specifications. King, P. J. B. 1984. Performance comparison of local area network architectures. In Information

Technology and the Computer Network, pp. 125-135. Berlin: Springer-Verlag.

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Local Netter Designer’s Handbook, 2nd Ed., March 1983. Minneapolis: Architecture Technology Corporation.

Middelboe, S. 1982. Local area networks. Microprocessors and Microsystems, 6 (I), 25-32. Mini-Micro Systems. 1984. IBM demonstrates ring proposed as LAN standard. Mini-Micro

Systems, January, 31-33. Moelands, A. 12C Bus in consumer applications. Electronic Components and Applications, 5 (4),

214-221. Mokhoff, N. Networks expand as PBXs get smarter. Computer Design, 23 (2), 149-168. Nelson, D. Network protocols for Apollo’s DOMAIN System. In Local Networks: Strategy and

Systems, Proceedings of Localnet ‘83 (Europe), pp. 3433348. Online. Norton, M. J. 1983. The Do1 FOCUS Committee: a status report on activities and results. In

Local Networks: Strategy and Systems, Proceedings of Localnet ‘83 (Europe), pp. 4255436, Online.

Terrell, P. 1983. A local area network. In Local Network: Strategy and Systems, Proceedings of Localnet ‘83 (Europe), pp. 251-259. Online.

Wurzburg, H. & Kelley, S. 1984. PBX-Based LANs: lower cost per terminal connections. Computer Design, 23 (2), 198-199.

General references

Flint, D. C. 1983. The Data Ring Main. Chichester: John Wiley. Local Networks: Strategy and Systems, Proceedings of Localnet ‘83 (Europe), Online, 1983.

John Rice holds a BSc in mathematics (1967) and an MSc in electronic computation (1968) from the University of Leeds, and a PhD in computer science (1976) from the University of Bradford, where his research was on software for linked computer systems. He is currently director of the local computing service at the University of Manchester Regional Computer Centre (UMRCC) where he is responsible, amongst other services, for the Microprocessor Unit, which offers information and laboratory facilities to the University of Manchester and UMIST. Prior to taking up his present post he held posts as deputy director of the Computer Laboratory (University of Liverpool), as assistant director (Systems) in the Computing Laboratory (University of Salford) and as a lecturer in computer science (University of Bradford). His research interests are in the management of computer software development and the use of microprocessors in communication networks.