LOCAL AREA NETWORKS

Success will depend on software quality and range by JOHN McCARTNEY

Problems of standards, protocols and network control will have to be resolved before local area networks will be able to fulfil their promise.

hough it is still likely to be some time before local area networks make a significant impact upon

data processing in general, much of the excitement about the hardware has clouded the issue of software.

As is well recognised nowadays, it is the software in a system which is likely to be the most important factor in achiev- ing a workable sy s t em- no matter how advanced the hardware may be. So the success of local area networks will rely upon the quality and range of the soft- w a re - as in all other computer systems. Where the local area network is used simply as a means of connecting a number of computers together, the problems are at least as manageable as those associated with established com- puter systems. But the real potential of local area networks lies in much more ambitious projects.

A group of computers connected together by such technology can be viewed as a complete system and for ap- plications which lend themselves to this there are a whole new group of prob- lems to be considered.

According to Dr Pamela Geisler of Zilog there are two levels of possible complexity. Firstly, applications such as real time control systems and electronic mail require the facility to interrupt at operating system level. This means that the software in each machine will need to take account of more than simple pro- tocol conversion. The second and more ambitious type of system requires shar- ing of resources at operating system level.

In this case, the local area network acts as a means of tightly coupling pro- cessors and leads to a whole new area of complexity. Such software has to take concurrent processing into account. Some progress has been made in this direction on the fringes of data proces- sing already. The new Ada program-

John McCartney isa technical journalist.

ming language, for example, includes facilities to control parallel processing and a special version of Pascal, concur- rent Pascal, has been used to address this problem.

In addition to this, the Royal Signals and Radar Establishment has developed a programming system called Mascot which contains parallel processing facilities.

But all of this effort has been directed at the specialist problems of military sys- tems and has made little impact on commercial systems development as yet.

Professor Edsger Dijkstra b~heves that the problems likely to be encoun- tered in systems which involve parallel processes could lead to disaster. He recently condemned operational think- ing as the curse of commercial pro- gramming. "People who are used to oper- ational thinking try to understand com- putational histories and try and impose cause and effect structures in their think- ing. Imagine the immense mental prob- lems those people will have when you confront them with the fact that as soon as you start arguing about networks and communicating processes the arguments are independent of the direction of the communication."

At present most of the efforts of the local area network builders are directed at the problems of controlling the net- work at a physical level and attaching devices to it. Viewed in the context of the International Standards Organisation's seven layer reference model, this means that the effort is being directed at the top l aye r s - the physical layer and the link layer.

Whilst these problems are not trivial, they do not begin to approach the com- plexity of parallel processing in networks.

Even here there is a danger that prob- lems could be forthcoming because of the lack of standardisation. The well publicised conflict between technologies has already contributed to this.

IBM's reticence certainly has not helped. Although it made some sort of commitment to local area net technology with its Series/1 announcements a year ago, it has not made its preferences clear in terms of whether it will go for an Ethernet style broadcast system or some

form of Cambridge style "r ing" net- work.

Perhaps IBM's coyness in this area displays a level of wisdom that other manufacturers should take heed of.

IBM probably has more experience of network systems than any other com- pany in the world. It has a deep apprecia- t ion o f the types o f p r o b l e m s involved in complex networking soft- ware. It has stressed a form of central- ised control in its network s y s t e m s - although there is evidence of a change of heart in recent years. It is no surprise, therefore, that IBM has been reluctant to commit itself to a particular type of technology before it needs to.

But there can be no doubt at all that IBM is certainly looking into the prob- lem. '

In many ways its future mainframe strategy of offering tightly coupled mul- tiple processor machines makes exam- ination of such problems a necessity. Connect ing mainf rame processors together poses the same problems as connecting devices to a local area net- work. The success of the Network Sys- tems' Hyperchannel in this area serves to confirm the comparison.

But whereas the connection of proces- sors built by the same manufacturer, and therefore under its control, poses a for-

DECEMBER 1981/JANUARY 1982 35

LOCAL AREA NETWORKS

midable set of problems when it comes to distribution of tasks across a number of resources, the problem increases exponentially when applied to the gen- eral area of local area networks.

Here the devices which are sup- posedly going to connect into the system can theoretically embrace each end of the computing scale. The ultimate dream of the local area network builders is to develop a general purpose method of connecting vastly different devices together and enable them to communi- cate.

"Ethernet is a simple and powerful way of connecting a number of devices such as terminals, computers, word pro- cessors, printer and files within a build- hag or site," wrote Neil Blake of Xerox in his paper to the Online conference on data networks in 1980.

Hasler 's Silk system is even more ambitious in extending the concept to include speech and videotex as well as the list above.

Whilst the network is considered as a means of connecting these devices t o g e t h e r - acting simply as a c o n d u i t - the problems of building software are restricted to the format of data on the circuit and access control to the com- munications channel.

It is highly likely that users will be expecting something more than this however.

Supercomputer network Following the announcement of the

ICL ME29 in 1980, Basil Cousins of Computel speculated on a system which would involve several ME29's connected together in a type of local area net con- figuration. He also suggested that ICL's more esoteric products, the Distributed Array Processor (DAP) and the Content Addressable Filestore (CAFS) could be connected to the same network. He saw the system as a kind of "supercompu- ter", when viewed from the outside at least.

The system would be capable of scheduling tasks to the appropriate pro- cessor when required. Database access would all be handled through the CAFS and number crunching calculations, for an applications like financial modelling, through the DAP. The ME29s would act as access p o i n t s - with work being shared evenly among them.

At present there is no software offered by ICL which could cope with the prob- lem and although it is highly likely that the company (like IBM) views this as a strong direction to take in the future, it, too, has been cautious about actually

making a lot of noise about it. Examples which actually exist are

Tandems ' Nonstop system and the Datapoint Attached Resource Comput- ing (ARC) system.

The former emphasises reliability rather than device flexibility and uses multiple processors to provide an unin- ferrupted service to its users. The latter is probably closer to the type of system that would emerge through the use of local net technology. But all of these examples defeat the basic object of local area networks.

The strong appeal of local area nets is the ability to hang devices of almost any type from any manufacturer. Although gateways to other networks or other materials are provided as a matter of course on these systems - so a Tandem, for example, may be connected to a 370 via H y p e r c h a n n e l - the gateway is a gateway to another system. It is not poss- ible in these cases to consider the system as a single entity.

Perhaps a clue to how these problems can be solved and a true general purpose local area network built, can be seen in the concept of virtual circuits, as used in the Ungermann Bass Net/One system. Ungermann Bass has chosen to emulate the physical circuit switching system favoured by the telecommunications industry, only it uses software to achieve this.

"Using software controlled processes as the end points for virtual circuits pro- vides enormous flexibility in the types of service that can be provided for any par- ticular connection," writes John David- son of Ungermann Bass in a paper pub- lished last October. "The use of the pro- cess allows devices to establish circuits to network services as well as to other devices," he continues.

He goes on to describe three types of virtual circuit. The first is used for con- necting two dev ices - a sort of point to point communica t ion - where the send- ing device is aware of the identity of a remote device it wants to communicate with.

" T h e analogy here is again just like the telephone s y s t e m - you can make an outgoing call whenever you want but you can only receive incoming calls once you've hung up the phone," he explains.

The second circuit can be related to a permanent connection between two devices. As the circuit is built into the software through the use of tables, it can be altered by the network administrator using a utility program. The entry in the table is altered to suit the changing cir- cumstances.

The third type are called "administra- tive circuits" and are dynamically allo- cated by the network administrator. "Administrative circuits are extremely powerful because the administrator can actually be a c o m p u t e r " explains Davidson. In this case the circuit can be established between two remote devices.

As Davidson points out, the circuits are, in reality, physically the same. It is only the means used to establish them which differentiates them.

The use of a logical structure to the network brings the advantage of being able to forget what the actual devices are. This is similar to the way in which a user of the Unix timesharing service views peripherals. Net/One does not stop there, however. It provides the virtual circuit service to ease the operation of " d u m b " terminals specifically.

An additional service is also provided for "smart" devices such as host compu- ters. Called the datagram service (once again using the telephone system anal- ogy), this operates pretty much as a pac- ket switch system. It should be noted, however, that smart devices can also use the virtual circuit system as well.

Again it is worth looking at what IBM is doing for comparisons. The major shift in the last few years within IBM has been towards the use of the Virtual Machine concept for communications. IBM's internal network connects 400 machines of different types (although of similar manufacture, of course) using a component of VM called Vnet. The use of logical rather than physical paramet- ers for controlling the system parallels that used in N e t / O n e - although it is not possible to make direct comparisons. But a similar philosophy appears to permeate the thinking of both sets of designers.

As Zilog's Dr Geisler points out: " N o one appears to be really looking at the idea of complete systems based on local area n e t w o r k s - and they should be".

Perhaps the reason that no one has done so is that it is an extremely complex area and there are more pressing (and simpler) problems to address.

There can be no doubt that once the problems of protocols and standards are resolved, the more complex problems of network control will have to be addres- sed for local area networks to live up to their promise as a means of linking dif- fering devices together effectively.

The key to this lies in the software which is needed to provide that control. It is to be hoped that there are enough clever folk around to address those problems. •

36 DATA PROCESSING