Technical Note: SRX/SPL Multidrop Multiplexer

Load Balancing for Multi-host systems

The SRX multidrop polling multiplexer host can be attached to a line using modem sharing units. The modem sharing unit (MSU-4D) can increase the number of host ports above 32 ports (the maximum size of an SRX multiplexer) by adding more SRX's at the host.

When more than one SRX-32 is used at the host, it is important to balance out the number of drops, so each SRX host has approximately the same number of remotes. This allows the sharing unit to evenly switch the transmit from one SRX host to the other. Ubalanced drops cause excessive delays for some users.

The following is an example of not balancing the load, where on a 5 drop system, one SRX host is polling 4 drops (drops 1,2 3 and 4), the other SRX host is polling just a single drop (drop 5). The polling sequence will be as follows:

	Drop 1. Drop 5, Drop 2, Drop 5, Drop 3, Drop 5, Drop 4, Drop 5, Drop 1...
Notice that drop 5 gets polled 4 times, while drops 1, 2, 3 and 4 each get polled once. Drop 5 gets polled 4 times as often because the two SRX hosts are both contending for the line. The SRX's contend by asserting Request to Send (RTS) each time the SRX is ready to transmit a poll (or poll plus data) to a drop. Polls from the SRX, and the related RTS, alternate by drop number. The line sharing resolves this SRX contention for the line by scanning the RTS inputs sequentially.

Suppose in the above example, the time for a poll and response was 30 milliseconds. Users at drop 5 could then see a response to a key press, or character echo, in 90 to 120 milliseconds. Drop 1, on the other hand, would take 240 to 480 milliseconds to echo a character from the remote to the host and back to the terminal.

If the load between the 2 SRX hosts was better balanced, all users would see more even response times. Consider the following polling sequence, where 2 drops (#'s 1 and 2) are on one SRX host, 3 drops (#'s 3, 4 and 5) are on the second SRX host.

  Drop 1, Drop3, Drop 2, Drop 4, Drop 1, Drop 5, Drop 2, Drop 3, Drop 1, Drop 4... 
With this change in mapping, the drops that get polled the least often, #'s 3, 4 and 5, are polled 1 out of 6 poll times, rather than one out of 8. The most often polled drops are no longer polled every other poll, but are polled one out of every 4 polls. This mapping balances the polling much better, resulting in more even response times across all the drops sharing the phone line.

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