Platon: a university local area network

Platon : a university local area network The implementation and operation of a 1 Mbit/s bus-structured LAN operating under CSMA/CD are discussed by Nicholas Georganas* and Rama Mwikalot

Platon is a local area network set up in the Department of Electrical Engineering, University of Ottawa, operating at 1 Mbit/s and interconnecting departmental computers, terminals, peripherals, various instruments and the university mainframe . The primary objective of Platon is to facilitate network evolution and provide resource sharing by supporting various application protocols such as database access and file transfer. In addition, Platon is intended to serve as a research vehicle for distributed systems studies. To this end, a packet-voice system and a packet-radio gateway are concurrently being developed. Platon has adopted the bus architecture, using a coaxial cable with CSMA/CD as its access protocol . Retransmissions of collided packets are scheduled using a linear incremental backoff (LIB) algorithm.

Keywords : local area networks, protocols, packet switching, CSMA/CD, LIB The advent of VLSI devices, in particular microprocessors, and their equally complex supporting ICs has made it possible to construct low-cost, very powerful local computer networks . Local networks can be broadly categorized into three main topologies : the star topology, which is characterized by a central controller managing communications and other resources ; the ring topology, which is characterized by a sequential medium and has completely decentralized control ; and the bus topology, which is characterized by a broadcast medium and, like the ring, uses distributed control. To date, several local networks have been 'Department of Electrical Engineering, University of Ottawa, Ottawa, Ontario K1 N 6N5, Canada tComputer Communications Group, Bell Canada, 220 LaurierAvenue W, Ottawa, Ontario K1G 3J4, Canada

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implemented using these particular topologies or slight variations . While some early local networks were of the star type, most recently developed systems are based on either ring or bus topologies' -4,a . This paper describes one such network, Platon, a bus-type local area computer network operating at 1 Mbit/s, currently being developed at the University of Ottawa . Initially, Platon will provide interconnection between the university and departmental computers, consisting of two PDP-1 1/34 minicomputers, a Microan processor Development System (MDS-800) AMDAHL 470/V 7A mainframe and several computer peripherals, terminals and instruments (Figure 1) . The reasons for undertaking this project were based on the following :

• providing a vehicle for a smooth network evolution which includes phasing out old equipment and installing or replacing with new equipment, • providing the means for sharing expensive peripherals as exemplified by directing output from different computers systems to the same printing facilities, • allowing efficient use of expensive resources such as terminals, hardware and software, by providing a standard network access to all interconnected computers rather than providing dedicated or hard-wired connections to these computers, • above all, this design and implementation exercise is intended to provide hands-on design experience in computer communications in general, and distributed systems in particular. Networking problems such as protocol performance, throughputdelay characteristics and buffer management can be investigated by conducting various tests on the operating network . To this end, a packet-voice system and a packet-radio gateway are concurrently being developed . 1982 Butterworth & Co . (Publishers) Ltd . computer communications



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that pierces the cable should not cause any physical damage to it . The 50 S2 CATV-type cable has been selected because of its relative abundance and its lower price . The passive tap on the cable is intended to prevent a malfunction in the active devices from degrading the signals on the main bus, which could affect the operation of the rest of the network . The broadcast medium interface unit (BMIU) consists of the coaxial cable drivers and receivers, the encoding and decoding logic, isolation transformers and the collision-detect circuitry (Figure 3) . The baseband Manchester encoding/decoding scheme was adopted mainly because of its self-clocking ability, which eliminates the need for a separate network synchronization clock. Also, because of its simplicity, the cost of the hardware required to implement this scheme is cheaper than other competing schemes, such as FSK . The standard network interface processor (SNIP) is an intelligent module that supports the user protocols as well as the network access protocols . Functions performed by the SNIP include channel multiplexing

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NETWORK ARCHITECTURE The bus-type topology has been adopted for three major reasons : the high reliability and availability achieved through simplicity, the ease of network expansion made possible by the single bus topology and, most important, the low start-up cost . Platon system components

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I Platon consists of three basic components (see Figure 2) . The main transmission medium is a 50 S2, low-loss coaxial cable (RC-58) . The drop cable to the station is connected to the main cable through a passive tap which makes mechanical and electrical contact through a clamp . Contact to the coaxial centre conductor is achieved through a thin spring-loaded probe which pierces the cable insulation . This arrangement is necessary if no service interruption is to result during attachingand disconnecting stations from the cable . Of at least equal importance, the thin probe

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The Ottawa packet radio system was designed to provide computer networking to serve the requirements of club members in the Ottawa community. The network's 25 kHz bandwidth is allocated in the 220 MHz band and the system was designed to conform to Canadian packet radio regulations'. The standard interface between the computer or terminal and the radio equipment is RS232C, and the modulation scheme used is frequency shift keying (FSK) . Operation of the network is controlled by a central controller and repeater station named the 'digipeater' . The control program in the digipeater contains a list of armature license call-signs permitted to access the network . During normal operation, the primary station, the digipeater, polls each secondary station in its list using SDLC's normal response mode (N RM) . In this mode of operation a secondary station can only transmit in response to a command frame from the primary, i .e . the digipeater. A secondary station cannot talk to any other secondary station directly . Communication between two secondary stations is achieved by routing messages through the digipeater, where a higher level protocol is invoked to perform a store-and-forward function . I n response to a poll, a secondary station must indicate which is the last frame of the response . Timeouts, retransmissions and all recovery functions are performed by the digipeater .

Packet radio gateway A packet radio gateway (Figure 4) for Platon is currently being developed . This specialized interface unit would allow users of the amateur packet radio system to access services provided by Platon and vice versa. A packet addressed to Platon from the radio network is transformed into the correct format before being transmitted to its destination . The two major functions of the packet radio gateway are:

• to provide protocol conversion between the two networks and to make available a wide variety of other services offered outside a user's particular network, • to facilitate internetworking by providing for address translation and store-and-forward services .

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ACCESS PROTOCOL To increase network efficiency while maintaining high throughput with minimum delay, several access schemes for broadcast systems have been proposed by various authors . I n Platon, the access protocol adopted is carrier sense multiple access with collision detection (CSMA/CD) . This access mechanism is used primarily because it is adequate for the intended applications, and because of its simplicity of implementation . The CSMA/CD protocol used is of the persistent type, which requires the transmitting station to sense a silent channel (carrier sense) before attempting a packet transmission . However, if the channel is sensed busy, the station must wait until the channel goes idle before attempting a packet transmission . When more than one station attempts to transmit at the same time, a collision results . The contention situation is resolved by a linear incremental backoff (LIB) algorithm' ,' implemented in the BMIU hardware . This algorithm changes the uniform random delay distribution by modifying the mean of the distribution as follows : Mean =(2+3(i-1))t where i = 1-8 (collision count) and t = 8 us . Thus, the lower bound of the distribution is 0 and the upper bound is (4 + 6(i - 1))t . The random delay numbers are stored in PROM .

LINK-LEVEL PROTOCOL AND SERVICES Platon's link-level protocol is based on HDLG with source and destination addressing the same elements of procedure . HDLG was chosen for the following reasons :

• the availability of chips (for example, Intel 8274), • the link-error detection capability provided by a 16 bit CRC,

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• its data transparency, • bit-stuffing simplifies clock recovery by guaranteeing phase transitions on the datastream .

Link services Two types of services are offered by the link layer to the higher layers . Connectionless service This service permits the exchange of frames between stations without the need for prior establishment of a logical datalink (virtual circuit) . The exchanged information units are self contained and can only be delivered with a high probability . In other words, frames may possibly be lost . At the link level, the exchanged frames are not acknowledged, and, in addition, error recovery and flow control are not provided . A connectionless service trades perfect reliability for reduced delays . It thus lends itself to real-time applications . In Platon, the connectionless service is intended to facilitate testing and operating the packet voice system . Its implementation is relatively simple, since recoding and updating of state information is not required . Connection service In this type of service a virtual circuit must be established prior to any frame exchange . The link level supports acknowledgements, flow control and error control . Sequential delivery of frames is guaranteed . Connection service is a requirement in applications which depend upon a fully reliable transmission system . Such applications include file transfer and time sharing. Unlike its connectionless counterpart, this service is slightly more complicated to implement because of the requirement to record and update state information .

SNIP HARDWARE AND SOFTWARE A standard microprocessor-based node (SNIP) that interfaces to computers, terminals and peripherals has been developed . Functionally, the SNIP is divided into four main blocks :

and a read-only memory (ROM) of 2 kbyte, holding the control program for SNIP, • the local interface module provides an RS232C interface between the user equipment and the SNIP; the interface is controlled by a USART, which supports both synchronous and asynchronous operation, • lastly, the network interface module provides an RS422 interface to the BMIU to support the higher datarates involved ; I/O operation in this module is controlled by an HDLC/SDLC protocol controller chip (Intel 8274) . The program residing in SNIP performs several tasks, the most important of which are :

• packet processing, which involves packet assembly and disassembly, • port multiplexing and demultiplexing, • scheduling processes based on parameters supplied by the users, • command interpretation for locally generated as well as remotely originating commands .

CONCLUSIONS The network described in this paper, Platon, is still under development . Several SNIP prototypes have been constructed and successfully tested . A simple file-transfer protocol has been developed for use in testing the prototypes . Still under development for Platon are :

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file-transfer service, remote database access service, packet-voice service, time-sharing service.

Future development plans for Platon will concentrate on making the network services available to a broader community of users . The following specific development areas are under consideration .

• Extending coverage of Platon over the rest of the university community . • Developing an X .25 gateway for gaining access to Datapac, Canada's public packet-switched network . • Developing a centralized filing system to allow file archiving. • Developing a word-processing access service.

REFERENCES

• The control module, which consists of a CPU and DMA controller ; the DMA controller has been incorporated in order to speed up the I/O operation between the node and the network, • the memory module consists of two types of memory- random access memory (RAM) of 2 kbyte, which is used both as a scratchpad and as a storage buffer for incoming and outgoing packets,

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Anderson, E W'A microprocessor-based controller for a loop switching system' Proc. /CC Toronto, Canada (June 1978) Metcalfe, RMand Boggs, D R'Ethernet: distributed packet switching for local computer networks' Commun . ACM Vol 19 No 7 (July 1976) pp 395-404

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Moura, J 'Loops and Ethernets : evaluation and comparison of performance and complexity' MASc Thesis, University of Waterloo, Canada (April 1978) Tokoro, MandTamaru, K'Acknowledging Ethernet' Digest of papers, IEEE COMPCON (September 1977) pp 320-325 Matteau, M and Pinard, J'Development of a local computer network', BA Sc Thesis, University of Ottawa, Canada (April 1979)

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Beaudoin, D and Joyce, D' Platon : development of a local computer network' BASc Thesis, University of Ottawa, Canada (April 1980) Bertine, H V 'Physical level protocols' IEEE Trans . Commun. Vol COM 28 No 9 (April 1980) Tobagi, F A'Multiaccess protocols in packet communications systems' IEEE Trans . Commun . Vol COM 28 No 9 (April 1980) Kazmiruk, S, Kayser, L S and Pett, H Personal communication

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