25

3

Local area networks

A local area network (LAN) is a number of computers and computer
peripherals (disc storage devices, printers) connected by high speed datalines within a building or adjacent buildings.
LANs originated as ways of interconnecting diverse electronic equipment within an organisation, which would permit local processing while
also giving access to other devices connected to the network (inter-station
message transmission without the use of a central host computer, accessto various file stores). Potentially, LAN traffic is not restricted tocharacter transmission: some architectures would permit the transmission of voice and video signals. The total distance spanned by a LAN can
be up to a few kilometres.
Although LANs are conceptually very different from the conventional"star" data processing network of terminals connected to a powerfulcentral host computer, in practice there is often still very little perceived
difference to users or even to applications designers.
Many LANs, and certainly those of the type which were used for thisproject, are intended mainly for office automation applications such as
word processing, small-scale management information systems and"electronic mail". Their user stations are normally fairly conventional
microcomputers of the personal computer type. Typically, theirapplications would involve multi-access to a large number of small files.
Nestar, the supplier of the networks used for the current project, wereamazed when they discovered that we intended to store, process andsearch files of ten, twenty or more megabytes.
Some of the terminology which is used may be unfamiliar: computersused as terminals on a network are often referred to as user stations;computers which control disc storage are called file servers, and thosewhich control hard copy output are print servers. Networks may beinterconnected by means of gateways.
There is a large and growing literature on the subject of LANs. Cheongand Hirschheim [1] gave a not too technical introduction to the subject as3. Local area networks

26

it stood in 1982. There is a more recent brief survey of the field in [2]. Mel
Collier [3] has given an introduction with an emphasis on potential
library uses.
A brief summary follows of those aspects of LAN design and architecture
which have a bearing on the present project.
3.1

Types of LAN

LANs may be compared with respect to the method of interconnection of
the participating devices (topology and connection medium) and to their
methods of data transmission and control.
Topologically, LANs may best be classified as being either rings or trees;{star and bus topology are often distinguished, but both of these may beconsidered as trees). In a ring network devices are connected like beads ona necklace, the most significant examples being LANs of the Cambridge
ring type. The distinguishing property of tree networks is that there are noclosed loops.
Independently of the topology, the connection medium may be coaxial
cable (as used for television signal transmission), twisted pairs of
conductors as used for telephone signal transmission, ribbon cable ofperhaps ten conductors, and fibre optics (where light is used as the carrier
instead of electrical signals). Most currently available LANs use either
coaxial cable or twisted pair. Coaxial cable allows higher data rates, but issomewhat more expensive and less robust than twisted pair.
Data transmission is generally in "packets" of a few hundred to a few
thousand bits. Each packet contains source and destination addresses aswell as "rear' data. Some LANs use packets of fixed size: the size chosen
has to be a compromise between wasting capacity by splitting longmessages into multiple packets, and wasting it by having to transmit morethan is required when a message is very short. The speed of datatransmission is between a few hundred thousand and several million bits
per second. It should be noted that the total amount of data which can betransmitted over a network is considerably less than that given by the"raw" data transmission rate, because of the necessity for transmitting
control information and for avoiding collisions. A network with a raw
data speed of five megabits per second might be able to achieve amaximum effective overall capacity of two to three megabits per second.

3. Local area networks

27

However, transmission capacity is rarely a limiting factor. Transmission
accuracy is generally high, particularly in ring networks; in any case,error detection codes are incorporated in the packets, so that retransmission can take place automatically in the event of an error.
As far as control is concerned, a general philosophy has been to avoid
centralisation. A good LAN installation should continue to work if anumber of components fail or are disconnected: in some systems this isalmost an inherent property; in others it can be achieved by, for example,
the duplication of devices such as cables and file servers. In ring systems
the failure of a device is likely to cause the entire network to fail unlesssuch precautions have been taken.
There are two widely used methods of controlling access by individual
stations to the network.
One, used in the Cambridge ring, is to circulate a "token" or "slots"
round the ring: a station can transmit when, and only when, it holds thetoken. This is simple and efficient, and should not be thought of as greatly
decreasing the capacity of the network, because the token circulates
extremely fast.
The other method is used by LANs of the Ethernet type. {Ethernet is a
public specification which resulted from preliminary work by the Xerox
Corporation; details were published jointly by Xerox, Digital Equipment
and Intel [4].) With Ethernet-type networks, access is controlled by amethod known as Carrier Sense Multiple Access with Collision Detection(CSMA/CD). It is analogous to confrontation avoidance in felines[5, p219], and works roughly like this: a station wishing to transmit
listens to the network; if it detects no other signal, it transmits. It thenreads its own data back to check that no collision with another packet has
occurred. If there has been a collision, it emits a "jamming" signal, waits
for a random period (up to a few milliseconds), and then attempts toretransmit. The process is repeated until a successful transmission has
been achieved. Clearly CSMA/CD is far less simple than token passing,but it works very satisfactorily. It imposes somewhat more of a limitation
on network size than does token passing, but this can be minimised byusing a fairly large packet size.

3. Local area networks

28
3.2

LAN or multi-user minicomputer?

In choosing between a LAN and a multi-user minicomputer there are
two factors to be taken into account: cost and performance.
3.2.1

Relative cost

It had been hoped to give here some comparative hardware and software
prices to help prospective purchasers in their choice of configurations forthe automation of small libraries. For a combination of reasons it is notpossible to do this realistically.
HARDWARE

Although suppliers of LANs and of minicomputer installations will often
quote "list prices", these may bear little relation to what a corporate
customer would have to pay. For example, a customer who can qualify as
an original equipment manufacturer (OEM), may obtain equipment at
perhaps half the list price; some suppliers would also give "educational
discounts". It is likely that at the present time there is little difference
between the cost of LAN (or other multiprocessor) and single-processor
hardware to carry out data storage and retrieval on the scale needed bysmall to medium libraries. The current list price for a LAN installation of
the type and size on which the Okapi prototype operates is given in
Section 3.3.1.
SOFTWARE

Here also it is not possible to make a comparison, but the reason is that,with one exception, there is no commercially available library automation
software which is designed to operate on a LAN. (The exception is thelarge Geac installation at New York University [6].) There is a certain
amount of library software for stand-alone microcomputers, and some ofthis might be adaptable for LAN configurations.
3.2.2

Performance considerations: LAN advantages

The potential advantages of LANs stem mainly from the fact that eachuser has a station which is very much like a dedicated desk-top computer.
Multi-processor systems — a number of processors in one box — are very
similar, but may not provide the same high data transmission speed touser terminals.

3. Local area networks

29

The implication of the single-user-single-processor (e.g. LAN) configuration is that highly interactive programs can operate. When theprocessing is time-shared by a single processor, as in a typicalminicomputer environment, it is not possible to operate programs which
involve instant responses and finely tuned dialogue, such as computer
games with elaborate interactive graphics, or even screen-based word
processing programs.
This is well illustrated by the process of searching conventional online
reference retrieval systems such as DIALOG or Datastar where, apartfrom problems with noise, and losing telephone contact, the system's
response time is highly variable and often very slow, and datatransmission speed rarely attains a hundred characters per second: theycannot engage in the kind of rapid exchanges which occur incommunication between humans, or between a human and a computer
game on a micro. Interaction has to be by means of a well definedcommand language, as any other approach requires a much higher degreeof responsiveness.
For an online catalogue, the local processing power of a LAN allows
instant response to some user needs — help messages or prompts can be
stored in local memory and displayed instantly when needed. The
importance of this lies in the fact that programs can be made to respondmuch more finely to specific user needs and to the current state of asearch: for example, when the user is entering a specific item search in
Okapi the message which gives possible next actions changes instantly
according to whether the user is entering a title or an author andaccording to the amount of text entered (Section 7.5).
It must be emphasised that such highly interactive programs are difficult
to design and to write (Section 9.4.4).
It is of course true that an IR system on a LAN will still have search
speeds which vary according to the load on the system, because of varying
loads on the network and, particularly, on the file server, but much of thehuman-machine dialogue takes place without any access being made to
the network or disc store. User input can be validated for acceptablestructure, instructions or help can be given rapidly and automatically. To
some extent, search programs can be made so that network access takes

3. Local area networks

30

place when the user is occupied reading what is on the screen or keying
something in.
3.2.3

Performance considerations: shared processor advantages

From the point of view of software development and file maintenancethere are certain advantages in the multi-user mini if it is not too heavily
loaded. Both the processor and disc access are likely to be very much fasterthan those available with microcomputer based LANs. The mini isunquestionably superior for batch processes such as source file and index
maintenance, and in some respects also for program development.
In the Okapi system file inversion (creation of indexes) for a file of 90,000
bibliographic records takes many hours, although the inversion programs
are written so that certain processes can be done concurrently using
several stations simultaneously (Section 5.7.6). In program development
we became inured to quite small compiling and linking jobs taking many
minutes.
The mini's superiority for software development stems also from the factthat most minicomputer suppliers will be able to provide well-proven andsophisticated system software such as compilers, operating systems andfile management systems, together with libraries of high level routines,
sorting procedures for example, which can be included in applications
programs. Micro-based LANs intended for office automation are notsupplied with such system software, and it was not possible for the teamto make much use of the software which was provided. To obtainsatisfactory performance from the Okapi programs it was necessary to
work in assembly language, and to write an extensive library ofprocedures for input and output, comparison, sorting and file
management.
3.3

The project LANs

3.3.1

General description and cost

The LAN system originally chosen for this project was a Nestar(formerly Zynar) c 'Cluster/One". At an early stage in the project this was
replaced by a "PLAN 4000" from the same supplier. Each of the two
PLAN 4000 installations which were used consists of a file server, anumber of user stations, a Winchester disc drive of 137 megabytes, a

3. Local area networks

31

printer, and also a 40 megabyte cartridge tape drive for archiving thecontents of the Winchester.
The file server is a microcomputer with a 68000 CPU, provided with anumber of system programs enabling applications programs, which
reside in the user stations, to use the network to read from and write to thecentral Winchester disc store.
The user stations are Apple He microcomputers. The PLAN system also
allows IBM PCs to be used as stations, but the Apples were very muchcheaper and have a keyboard which is more suitable for people unfamiliar
with computers (almost the same layout and number of keys as an electric
typewriter). On the other hand, Apples are extremely slow and primitive
compared with more recent microcomputer designs. As it turned out, thishas scarcely been a constraint. The Apples were supplied equipped with
Z80 processors enabling them to run much of the software written for the
widely used CP/M operating system. A version of CP/M is supplied by
Nestar, as well as other operating systems for standard Apples and for
IBM PCs.
Stations can be connected to the network in a fairly arbitrary way,provided that there are no closed loops. Data transmission is by coaxial
cable. Line isolation devices (LIDs) are used for connecting stations to the
network. These are of two types, active and passive. Each active L I D has
six ports, so it can be used either to connect five additional stations to the
network or to connect four stations and also provide a cable to anotherpart of the building; thus an economical configuration might resemble asequence of stars emanating from LIDs on a bus. (The passive LIDs havefour ports, and do not need a power supply, but are of limited use since
they can only be used as the final node in a branch of the tree.) A station
can be connected or disconnected, or can fail, without affecting otherstations.
A typical layout is indicated in Fig. 3.1.
Cost
Based on January 1985 list prices from Nestar, and average figures for thecost of the Apple He user stations, the cost of the hardware for an
installation of the type on which Okapi was tested breaks down as follows:

3. Local area networks

32

file server, with 137 megabyte disc drive(a 60 megabyte disc drive costs £3000 less)10 network interface cards
10 Apple He's, with monitors, 80-column
cards and Z-80 cards
4 active LIDs
2 passive LIDs
1000 feet of coaxial cable and connectors
printer£18500total£33180

4950
7000
1980
100
150
500

The file server price includes file server system software, but the cost ofoperating systems, compilers etc, for user stations is not included. To thiswould have to be added the cost of installation, and about 15% per annum
for maintenance.
3.3.2 Performance
Early in 1983 some tests were done on the Zynar Cluster/One network
which involved a number of stations making random reads from a fairly
large file. It was found that over the whole network, almost regardless ofthe number of stations (one to four), only about six kilobytes per second
were being transmitted. The raw data transmission speed was almost
certainly irrelevant, but it was difficult to determine whether the fileserver software or the physical speed of disc access was to blame. (Itturned out later that the limiting factor had been using the operating
system CP/M to control network access.) This low data transmission ratewould have been an important factor in the design of the IR system(Section 1.3).
The PLAN 4000 is somewhat faster than the Cluster/One, but the fileserver software was at first a limiting factor. Whenever several stations
were carrying out processes which made intensive disc access via the
network, from time to time there would be a pause of a second or so while
the whole system seemed to be sorting itself out. By the time the second
PLAN 4000 was obtained there was a later release of the file server
software in which network and disc access seem quite nicely balanced.
Nestar later informed us that there had been a bug in the network
queuing procedure. It now became possible to feel more confident about

3. Local area networks

33

including functions, like Boolean OR, which give rise to intensive discactivity.
Nestar supplied routines for disc access via the network which by-passed
the CP/M operating system. These were incorporated in the search
program when the prototype was nearly ready and gave a further very
considerable improvement in the speed of operations involving discaccess.

file server

Figure 3.1

LAN

diagram

3. Local area networks

VI
3.3.3

Reliability

Things which can fail are the disc drive, the file server, user stations andconnecting cable and LIDs. All hardware and system software wascovered by a maintenance agreement at 15% of purchase price per year,
and this gave service usually within about half a day.
The Cluster/One disc drive had timing slots which were exposed to theatmosphere and gradually filled with dust, and these had to be cleaned
every few months.
After running almost continuously for more than a year the disc drive of
one of the PLAN 4000s failed. Zynar replaced the entire disc unit and all
files were restored from backup tapes, losing almost nothing. The user
stations have been surprisingly reliable — with the ten Apple He's usedwith the PLAN 4000s there has been one keyboard failure and one failureof a Z80 card. The power supply to one of the LIDs failed. Failure of acable will only result from physical damage, and the coaxial cable isrobust unless bent vigorously.
There have been failures of the mains supply. There is no automatic
detection of incipient power failure; when power is restored the disc driveand file server start operating again, but some new data may have beenlost. For our purposes it was certainly not considered necessary to have an
emergency power supply. A normal, fairly "dirty", mains supply has
been used for one of the installations, although the second one wasconnected directly to the incoming cable on a separate circuit. There has
been no problem with static electricity, and one of the networks has
functioned in ambient temperatures between about 5 and 35 degrees
Celsius.
3.3.4

Some technical information on the PLAN 4000 LAN

The PLAN 4000 has layers of network protocols following theInternational Standards Organization (ISO) guidelines for Open System
Interconnection (OSI). The physical layers use a token passing protocol(Arcnet), and the intermediate layers are like Ethernet. The network has
a "distributed star" topology, but the stations behave as if they wereconnected in a logical ring. The raw data transmission speed is 2.5
megabits per second, and the maximum effective network capacity is
about 1.6 megabits per second (roughly 200,000 characters per second).
Packet size is variable up to 256 bytes.

3. Local area networks

35

The network capacity considerably exceeds the amount which a fileserver can read and transmit when ''random" disc accesses are being
made. The 137 megabyte disc drive has a mean access time of 42
milliseconds, and a "cache" or buffer of four kilobytes. Up to four suchdisc drives can be controlled by a single file server station. It is possible touse more than one file server on a network. Direct communication
between user stations is also possible, as is the use of local floppy or harddisc drives at individual stations.
Stations are connected to the network by means of a network interfacecard. This has a two kilobyte I/O buffer, and contains PROMs which
control network access by the station. The theoretical maximum numberof stations on a single network is 255, but for most practical applications it
would not be advisable to exceed about twenty to thirty stations for eachfile server [7,8].
3.4

Data security

This section applies to any computer installation. We have thought itworth including, because security is normally the responsibility of acomputer operations manager, and as installations are becoming cheapermany are being operated without a professional manager.
It is of course vitally important to ensure that there are always backup
tapes of all important data. With mainframe computer installations there
is normally some archiving done every night and a complete dump every
few weeks, organised in such a way that in the event of a complete discfailure not more than a day's work will be lost. With a small disc drive thesimplest and most reliable procedure is to take two complete backups
daily (if some file has been altered on that day). This is rarely practicablewith discs of forty megabytes or more. One recommended method is tomake a weekly dump of the entire disc and a daily backup of every file
(pathname) that has been written to since the last complete dump. Nestar
provide a program which enables selective archiving, and the systemtimestamps pathnames with the first time they are written to on a givenday.
All the networks which the project has used (Cluster/One and PLAN
4000s) are equipped with their own built-in tape drives. The cartridge
tapes (similar to audio cassette tapes but somewhat larger and able to run
at a very high speed) are expensive, but fast and reliable. It usually takes

36

3. Local area networks

about ten minutes to write thirty or forty megabytes to a tape: the exact
time depends on the number of pathnames. The archiving programprovides a verification option, in which the tape is re-read and its
contents compared with the contents of those parts of the disc of which it
is a copy. Verification is much slower than writing since a rather small
buffer is used and the tape has to keep stopping and restarting. Wheneverpossible, tapes were verified, and there have been several "verify errors".
It is not possible to do anything with the tape drive while the file server isrunning, so backup has to be carried out when the network is not being
used.(1)Apart from magnetic tape, other media which might be offered for
backup by suppliers of microcomputer installations include videotape
and floppy discs. The former is satisfactory provided that the amount of
stored data is not too large: the drives are cheaper than those for magnetic
tape cartridges, but it is not possible to do selective saving and restoring.
Floppy discs are still only suitable for the smallest installations, if only
because the capacity of a disc is unlikely to be more than a megabyte or so.
3.5

The design of applications software for LANs

A major advantage of LAN distributed processing has been mentioned
above: it is possible to use programs which are highly interactive in thesense that they involve consistently fast responses to user input. Thus,
software designed for single user computers, like word processing
systems and "spreadsheet" programs will usually work almost as well on
a LAN as on a dedicated microcomputer. Except when disc access is being
made, the stations are single user computers.
However, library applications, except on a very small scale (when it isdoubtful whether computers are of any practical use anyway), needaccess to comparatively large files. Then, it is likely that, just as withmulti-user-single-processor systems, processing may become "discbound". That is, the time required to read from or write to the central
disc store may often be more important than the speed of the processors
and the data transmission capacity of the network. Thus the design of thedisc access hardware and software will be a very important factor, andthis is something over which the purchaser has no control.
(1) By using a second file server on the PLAN network a high degree of automatic security
can be provided. This would be important if a LAN were to be used for, say, circulation
control.

3. Local area networks

37

This project entailed building an IR system in which up to twenty or sousers could simultaneously retrieve information from a file or files of up
to several hundred thousand bibliographic records. Such a system isclosely analogous to online and in-house reference retrieval systems,
almost all of which use multi-access to single mainframe or minicomputers. There has been some disagreement over whether these IR systems
tend to become disc bound or processor bound, but undoubtedly discaccess is an important factor in affecting response times. Although datatransmission over LANs is very much faster than the transmission
between a mainframe and a terminal, regarded as devices for allowing
terminals to access disc files, microcomputer based LANs are considerably slower than many mainframe installations. Consequently the projectsoftware had to be designed so that disc access was reduced to a minimum.
T H E LANAS A PROGRAM DEVELOPMENT ENVIRONMENTAs mentioned in Section 3.2.3, system software for microcomputers is
rudimentary in comparison to that available for mini and mainframe
installations. In particular, there are few, if any, high level programming
language systems which can be used to write programs for applications
which call for high processing speed and fine control over memory
resources and timing. Had we been working with a minicomputer system
we should undoubtedly have used an implementation of one of the widely
available high level languages which are suitable for applications
involving character processing, such as C or PL/I (or possibly Pascal,
which is easier to read but somewhat restrictive). This would have had
the advantages of saving development time, improving portability andreducing the effort of documentation. There are, of course, high level
language systems for micros, but we did not consider any of them to besuitable unless used in combination with a considerable amount ofassembly language. (One of the Okapi programs — the second stage of thefile inversion process (Section 5.7.6) — is written in a mixture ofcompiled MicroSoft BASIC and Z80 assembler, but this is a batch
operation and a very simple process which can afford to be fairly
inefficient.) All the remaining programs have been written entirely in
Z80 assembly language. No doubt this unsatisfactory situation willchange in the fairly near future.
On the positive side, the highly interactive facilities made possible by the
micro environment meant that the team could use a word processing
system, WordStar, for almost all keyboarding of program source code38

3. Local area networks

and other textual data. This is very much easier than using the line editors
which are usually provided in time sharing systems.
References
1 Cheong V E and Hirschheim R A. Local area networks: issues,
products, and developments. Wiley, 1983.
2 Local area networks — how computers talk to each other. Institution ofElectrical Engineers. Digest number 1984/50. 1984.
3 Collier M. Local area networks. British Library. Library andInformation Research Report 19. 1984.
4 Foot B. Ethernet — basic principles. Institution of Electrical Engineers.
(Undated).
5 Leyhausen P. Cat behaviour. Chapter 19: Territorial behaviour andrank. Garland Press, 1979.
6 Persky G et al. A Geac local area network for the Bobst Library.
Library HI TECH (6), 1984, p37-45.
7 [Nestar Systems]. Unpublished communications from staff of
Nestar Systems, 1983-1985.
8 Frith W R. A layered approach to local area networks. New
Electronics 17 (24), 1984, p64-66.