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.

There is lots of Linux, Mac OS X and general Unix info elsewhere on this site: Search this site is the best way to find anything.

Serial Communications and UUCP

My data-compressing modem doesn't work much faster than my old modem

There are a couple of possible reasons for this. Firstly, you may be transferring data that has already been compressed and is therefore not compressible (compressed news batches, for example). If your modem connects to the remote site using V.42bis data compression, this will not adversely affect throughput. If, however, you are using an MNP level 5 connection, you will actually lose throughput when sending compressed data. If the majority of your transfers are already compressed, you should disable MNP level 5 data compression. Note that using MNP level 3 or 4 error correction, or V.42 error correction, without any compression will increase throughput regardless of the type of data being transmitted.

Also, due to the way data compression and error correction in modems work, there are delays involved while the two modems agree on whether or not each packet of data was received correctly. If possible, you and the remote site should try to increase uucico's window size; this will cause more data to be transmitted at once, which will not only improve the efficiency of your modem's data compression, but will also often mean that the response to a previous packet arrives before the window has expired. I'm afraid a complete discussion of packetization and sliding window protocols is beyond the scope of this FAQ; just trust me on this one if you don't quite follow me. Further discussion can sometimes be found in the newsgroup comp.mail.uucp.

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