CLAYTON@XRT.UPENN.EDU ("Clayton, Paul D.") (06/15/88)
Derek Haining has asked the question as to the trade offs of LAT terminal servers versus the DMF/DMZ type terminal interface. During my past employement, the use of terminal servers came to show a large benifit in MANY areas, over the single line (DMF/DMZ type) interface. 1. No longer were you tied to a single machine, which in the event the machine failed you did other things or just sat. 2. Placement of the terminal relative to the machine became a mute point. I still chuckle about all the 16 and 32 line muxes I had (on both ends) that enabled users outside the building to get to the machine(s). Now only a single T1 (only part of it in reality) is used with a Enet bridge to get as many users on the machines. The limiting factor is terminal servers AND you only have to put them on one end of the 'cable'. In other words, you do not have to have matching muxes. 3. The terminal servers are performing 'packetizing' of character input from a terminal based on the number of available bytes from all the various ports in the terminal server, currently being used at the time. Note that this packetizing is on a BY SERVER basis. The larger the terminal server, ie. DS500 with up to 128 users, the more bytes (characeters) that can be placed together in the same packet. The item to note here is that while this packetizing process is on a by server basis, it is only grouping bytes (characters) that are going to the same CPU. Hence the biggest boost if all the port users are going to the same CPU (cluster alias targets do not count as the same CPU!!). This is a management problem when wiring the servers up. 4. Both LAT and DMF interfaces provide for DMA input/output so the trade off on this issue is minor. The only item to note is that the 'depacketizing' done by the CPU can be considered 'overhead' to this process. I consider this to be 'equal' between the two types of interfaces when you consider the work the CPU has to do for each I/O completion on a DMF interface. 5. As for the busses, consider that having a UNIBUS on a 8700 now means that to get to the memory you now have to go the following path. UNIBUS ----> BI Bus ----> NMI Bus ---> Memory Bus That is a LOT of bus, and each has its own speed, rate of bus grant to the next higher partner and so forth. The Digital Technical Journals that are available indicate the relative peformance of these busses and how they interact. VERY interesting reading for those interested. The moral here, is that more busses, and slower ones at that, that can be eliminated the better. 6. In summation, I had to 158 DS100 and DS200 and 2 full DS500's and the performance imapct to the Enet was very small, as measured by Lan Traffic Monitor (LTM) that is sold by DEC. The terminals were set to 9600 baud, and the screens painted very quickly. 7. There is a higher initial cost associated with terminal servers when compared to the DMF/DMZ type interfaces, as these are readily available from MANY sources at rock bottom prices. 8. In terms of flexability, Enet is FAR FAR superior to DMF/DMZ interfaces when it comes time to plan for expansion of the user community, or the computers that are being used. For the above reasons, I would stay with the terminal servers and not go back to the DMF/DMZ type interfaces unless the specific application called for it. Hope this helps in generating an implementation plan. :-) pdc Paul D. Clayton Address - CLAYTON%XRT@CIS.UPENN.EDU Disclaimer: All thoughts and statements here are my own and NOT those of my employer, and are also not based on, or contain, restricted information.