A.P. Lawrence

This article is from a FAQ concerning SCO operating systems. While some of the information may be applicable to any OS, or any Unix or Linux OS, it may be specific to SCO Xenix, Open Desktop or Openserver.

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Serial Communications and UUCP

How do I maximize serial throughput?

This answer assumes that you have an error-correcting and/or data-compressing modem. The rule of thumb here is to feed data to the modem as fast as you can. The more data it has to work with, the more efficiently it can compress it. Not only that, but there is a more-or-less fixed overhead involved in bundling data into packets, which is how the modems transmit it, so when error correction and/or data compression is in use, you can reduce this overhead to a minimum by ensuring that the modem has as much data as possible to put into each packet. The usual rule of thumb is to feed the modem four times faster than the fastest connection rate it supports. If you have a V.32 modem, which supports connections of up to 9600 bps, you should communicate with it at 38 400 bps (assuming your hardware supports this rate). If you are using error correction but not data compression, the next speed up (19 200 bps, in this case) is generally sufficient.

The modem will have an internal data buffer of some size; it could be as large as a couple of kilobytes on better modems. If you are sending it data faster than it can transmit it (which you should be), this buffer will fill up over time. You will need to have handshaking between the modem and the computer so that the modem can signal to the computer to stop sending when the buffer is nearly full, and to start sending again once there is room in the buffer. This can be done in software (usually using the ASCII XON and XOFF characters), or in hardware (usually using the RS-232 CTS and RTS lines). The use of software handshaking requires less wires between the computer and the modem, but will interfere with the transmission of binary data. The use of hardware handshaking requires additional wiring, but will not interfere with binary data. There are advocates of both methods; I personally prefer hardware handshaking. Whichever method you choose, you must make sure that you configure both the modem and the computer (via stty settings) to use the same protocol.

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