Networking in Edinburgh University

Introduction

For more than 15 years Edinburgh University has depended on a large data 
network. This has grown up under the pressure from Departments, scattered 
widely across Edinburgh, to communicate first with the central mainframes and 
latterly also with other departments and services both within the University 
and other Universities connected to JANET (The Joint Academic Network). A 
diagram of the Edinburgh University network is shown in Fig. 1.

   The dispersed nature of the University has also resulted in the development 
of a separate complex speech network which is now due to be replaced. There was 
wide optimism that when the telephone exchanges  were replaced the opportunity 
could be taken to use the new equipment as a basis for the integration of the 
speech and data networks.

   The University was able to take up the opportunity of government grants to 
set up a pilot project. The project's function was to connect an ICL DNX2000 
PABX to an ICL Open Systems Local Area Network (OSLAN) and to investigate the 
consequences of carrying data traffic across the PABX to the LAN. A project 
diagram is shown in Fig. 2.

   The project is now complete and has given the University the opportunity to 
study the data carrying characterists of a DNX2000 in a depth which has not 
been possible with any of the other manufacturers.

Comparison of networking alternatives

   It is worth comparing the PABX's data capabilities against the more 
conventional methods of terminal networking.

Use of the speech network

The use of the speech network can provide advantages over the alternative 
techniques, the main ones of which are a direct, fast terminal image and a 
simplicity of wiring.
 The direct terminal image advantage arises from the number of mainframe and 
multi-user mini programes and packages that expect to have a very tight control 
over what the user sees on his terminal. In particular the control, and speed of 
response, of  characters echoed can be very important. A PABX by operating a 
fixed circuit from the user's terminal to the host provides a similar image to 
that of a  directly connected terminal.  A further use of such a connection, but 
one which will only be realised in the future, is one utilising an ISDN 
connection across the public network.
  The other main advantage of using the speech network is the ability to use 
common wiring for terminals and telephones. This particular feature is probably 
most relevant when a new building is being wired. In an existing building, even 
if new wiring is being installed for telephones, most of the terminals will 
already be wired up.  A solution already in use at some institutions is to use 
common wiring for terminals and telephones but then to split the wiring again 
at the distribution racks, wiring the telephones to the PABX and the data to 
one of the alternative methods.









   The main disadvantage of using a PABX for data traffic is the cost. The 
current method of attaching a terminal, shown in Fig. 3, is by a Data Interface 
Unit (DIU), this box tends to be given a different name by each of the PABX 
manufactures but the general purpose is the same. In addition to the DIU at the 
terminal end, a module capable of handling data must also be used within the 
PABX. There are various differences for an exchange between a terminal and a 
telephone call, the more obvious ones being that the terminal user may have to 
interact with the exchange, using his terminal, to determine the distination of 
the call and that the terminal user requires a number of additional signals, 
such as RTS and DTR, to be faithfully relayed to the remote end of the call.  
The current incremental cost for a data terminal is about #800 to #1000. 
Although this figure could even half in the next year, the cost is still far 
greater than the alternatives.

   Data ports are connected to a host using a further DIU, see Fig. 4, when the 
host is fairly small this is not a serious problem, but in the case of a large 
host is physically extremely messy.  The manufactures have been announcing high 
speed host interconnects for their PABXs, but as there are no agreed standards,  
for a given make of PABX only a limited number of machines will be capable of 
interconnecting.

   When  a multiple exchange network is utilised then a full 2 Mb/s megastream 
connection is required for each 30 simultaneous calls between the exchanges.

   A far less obvious problem is in the quality of service offered to the 
terminal user. None of the exchanges currently on the U.K. market offer features 
that are taken for granted on longer standing data-only equipment. The main 
features are the lack of a directory service at call set-up time, ie, the user 
must always remember the exact name or number of the desired service, there are 
virtually no management facilities available, eg, call logging only operates on 
external exchange lines, and finally there are no facilities for the control, and 
prioritisation of contention ports such as found on the data only circuit 
switches.

Data-Only Circuit Switches

   The Data-only circuit switch (also called a PACX) gives the same direct 
connection as a PABX and is also very much cheaper. The total incremental cost 
of both a terminal port and a proportional share of a host port is about #105. 
The entry cost of such a switch is also low. The manufacturers of such switches 
have been in the market for a considerable period and a consequence is that  
the facilities offered are much more sophisticated than those offered by the 
PABX manufacturer. For example, none of the exchanges currently on the UK 
market offer a directory service to the data user at call set-up time and the 
'hunt group' features are primative compared to the priority and extensive 
grouping features of modern PACXs.

   Until very recently the PACX was only viable as a solution on a single site. 
Although it was possible to interlink between sites it was necessary to make 
compromises such as fixing the direction of use of a port or forcing the user 
to interact with each PACX that was traversed. Although it is now possible to 
interlink PACXs over BT circuits using only a single piece of hardware at each 
end (rather than multiple single ports) the flexibility is not as extensive as 
that of a packet switched network.









Packet Switched Network

 The main advantages of using terminal concentrators in a packet switched 
network are the low cost and the flexibility. The incremental cost of attaching 
a terminal, as shown in Fig. 5, is about #200. The major advantage is that only 
a single connection is needed to a host machine which, due to standardisation, 
is relatively cheap. Very small, cheap, X.25 switches are now available on the 
market which means that networks can be constructed, using BT circuits, in an 
extremely flexible arrangement, particularly for organisations that are spread 
over a number of sites.

The main disadvantage is the slower response that individual users experience, 
the severity of which is inversely dependant on the excess bandwidth that has 
been built into the network. The consequence of this is that the terminal image 
can suffer especially for programs that expect a very direct interaction with a 
connected terminal.

Conclusions

The networking solution for any particular organisation is determined is very 
dependant on the precise layout of buildings and sites. Each of the techniques 
described can have their place but the cost factor alone of using a PABX has 
lead Edinburgh University to decide not to attempt to unify the speech and data 
networks at this time.