epJO Introduction ------------ Edinburgh University is spread accross many parts of Edinburgh with two main campus areas and a number of other scattered departments. This situation has lead to a very high dependance on Networking being built up over fifteen years in order to ensure that students and staff can communicate with either the main University Services or local department machines (see fig 1). The current network is based on three GEC packet switches supporting 33 hosts and 100 synchronous links. The nature of the University has also lead to a seperate complex speach network (see fig 2). The University telehone exchanges now need to be replaced and the opportunity granted by the current range of PBXs lead to an extensive examination of the possibilities of using the speach network for carrying some, or all, of the University data traffic. The University was also able to take the opportunity of early exposure to one of the latest technology telehone exchanges in the form of a joint project with ICL, partly funded by grants from the DTI under a 'Focus' and Preproduction Order scheme. This project is to connect a Mitel PBX to an ICL OSLAN (Open Systems Local Area Network) and to examine the consequences of carrying data traffic accross the PBX onto the LAN. A project diagram is shown in Fig 3. The project is now one quarter through the evaluation year but a number of lessons have already become clear and are discussed below. Advantages ---------- There are a number of advantages gained by using a PABX for data traffic, they are:- in3 Common Wiring ------------- The biggest attraction of using a PABX for data traffic is that the wiring is already in place. The penetration of telephones is greater than that of terminals. In Edinburgh University we have ???? telephone extensions and 1520 asynchronous connections. The number of telephone extensions is static but the number of asynchronous connections has risen by about 20% per year for the last few years. So an alternative to laying yet more copper is very attractive. Unfortuantly, this situation is far more complex than that and because the existing telephone wiring is owned by BT we will actually be rewiring the entire university when the new exchanges are installed. This new wiring will be capable of fully supporting both the speach and data requirements for most users. Direct connection of terminals ------------------------------ One disadvantage of the existing University netowork for connection to certain hosts is that the terminal image for users is not the same as that of a directlu connected terminal. The PABX gives a direct terminal image even accross multiple exchanges by the nature of circuit switching. Handling multiple terminal protocols ------------------------------------ A PABX can be effectively used to switch a variety of terminal types using both different types of protocol and synchronous or asynchronous signalling in a manner that cannot be done with an X25 packet switch. This can be of use to users who are running mixtures of differering protocols such as IBM 3270, ICL CO3 or specialised asynchronous block mode terminals such as those used by the Unversities GEAC Library machine. Connection of a large number of infrequently used terminals ------------------------------------------------------------ Terminals whose use match the same sort of profile as that of the average telephone call can be effectively handled by a PABX. With a large number of terminals per host port the overall package becomes more economic. Single exchange management -------------------------- A conventional PACX such as a Gandalf or a Micom also provides the last three advantages. However, both a PABX and PACX must be purchased and maintained. The advantage of using the PABX for data traffic is that there is only one system to purchase and maintain. Carrying synchronous X25 traffic -------------------------------- One method of operation would be to use the exchange to carry a synchronous X25 call from a PAD to a centrally located X25 switch. This has the advantage of only needing one 64kbaud circuit at maximum, but carrying up to 16 or 32 simultaneous user calls. Unfortuantly we have not yet been able to try this out as the Data Interface Unit (DIU) originally supplied by ICL (The Dataset 1) is not capable of handling synchronous data, we have only just received the Dataset 2 which can. in0 Disadvantages ------------- There are a number of disadvantages in using a PABX:- in3 No system is truly non-blocking ------------------------------- The new generation of exchanges have been heralded as non-blocking. Or in other words all extensions on the exchange can be active at one time. Although this may be true in theory, in practice it is very expensive and it is not generally the case if satellite exchanges are taken into consideration. Consider the following: a satellite exchange is connected via a 2Mb trunk to the main exchange. The 2Mb trunk can carry at maximum 30 calls at 64Kb, the remaining two channels being used for signalling. If more than 30 extensions are installed on the satellite exchange then the SYSTEM is not non-blocking. Although multiple trunks could be used, in real systems this can cause shortages of ports as well as being extremely expensive compared to a system just carrying speach traffic. The duration of the average speach call is much shorter than our experience of the average data call (2-3 mins vs 15-25 mins). This means that a data call using the exchange uses up far more than would be expected of the total resource (remember that as it is based on circuit switching a terminal running at 9.6Kb will still use an entire 64Kb circuit). A group of thirty terminals used simultaneously will use up an entire 2Mb trunk, whereas if packet switching is used, 38 or 48Kb is quite sufficient. It is therefore clear that if an exchange is being used heavily for data traffic, its configuration will need to be much greater than one which is just carrying speach. Achieving 64K baud for data traffic still causes problems -------------------------------------------------------- The cost is greater than conventional solutions ---------------------------------------------- Consider Fig 4, which shows the typical manner of connecting a data terminal (or micro). Two DIUs are required; in the Mitel case, the one-off cost of a Dataset id $450; in addition to this a data port is required in the exchange at a cost of about $1000 (?). The equivalent cost, excluding wiring, in an X25 network (see Fig 5) is one sixteenth of a cost of a PAD and its network connection. In our case this comes to less than $500 on average. The cost of a Gandalf port is about $???. Limitations of 64K baud ----------------------- A much quoted limitation of a PABX is that any one user is limited to a speed of 64Kb. In reality this is not as great a limitation as appears at first sight. Most terminals currently only run at 9.6Kb and it will be a long time before 64Kb becomes an embarassment. A more serious situation is file transfer between micros or multi-user machines. The comparison here is with the scale of bandwidth that an Ethernet or similar fast LAN can provide. In our experience this is still rather a myth. An Ethernet's bandwidth is best utilised when it is supporting the interconnection of a number of such machines and our measurements with both Ethernets and Cambridge Rings, both running at 10Mb, show that hosts interchanging real data using other than the most light weight protocols generally have difficulty in even achieving a true point-to-point speed of 64Kb. Multiple connection speeds/protocol problems ------------------------------------------ in0 From the above it is quite clear that at this time it is not economic for Edinburgh University to use a new PABX for extensive data traffic. There are other circumstances where it is economic such as a 'green field' site where all wiring is being done from scratch or where a large nuber of terminals whose characteristics are nearer those of telephones are being handled. Even in these cases there are other solutions which could be cheaper. The Future ---------- The question which now arises is what needs to be done to make the use of a PABX more attractive in a general data environment. Considering Fig 4 again, each of the elements need to be examined. a) Data Portin3 A Data Port, at least in the Mitel case, is more complex than the equivalent speach port. There are two ways by which a user can establish a call from one data port to another. The first is to use the telephone handset to dial the number of the other port, this procedure can be made very simple by using the 'speed call' feature although there is an interesting problem in that this can't be done when there is an active speach call. The second method, which is only applicable to RS232 type devices, is to enter into a conversation with software within the exchange. This software announces itself and then prompts the user for a command. The user may then call a port by name. Unfortuantly, this has not yet been extended to allow the user access to the directory to find out what the valid names are. Since data ports are at least as complex and are used in smaller numbers than speach ports one can assume that its cost will allways be greater than the speach equivalent. The general cost will fall over time but the overall cost is going to be heavily dependant on what the PABX manufacturers can charge for their products in the voice market. in0b) Data Interface Unit in3 The current cost is extremely high. There are developments in hand to reduce this cost and in timeshould not be so overwhelming. Three examples of such developments are:- 1) Combined micro/telephone The ICL product called 'One Per Desk' is an interesting example of the combination of a micro computer and a telehone, particuarily in an office environment where the 'footprint' of the device counts. Its price is still quite expensive ($1200 - $1800) and has the disadvantage that the user is locked-in to the actual hardware used. 2) Data Network Gateways The use of Gateways should be cheaper than multiple DIUs and are discussed in detail below. 3) Reimplementation of the DIU The function of the DIU is quite simple and as dema`•…Í•ÍÝ¥±±either be implemented within a telephone or even as part of a cable. in0