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BOCA PRO16 Reference Guide
Data Compression
Two forms of data compression are supported by your modem; V.42bis
and MNP 5.
V.42bis is based on the Lempel-Ziv compression technique (similar to
the techniques used by PC compression programs) and can work with
both V.42 and MNP. V.42bis is very good at compressing data that has
repetitions of sequences of characters. For example, in an English
sentence there are usually repetitions of ‘ions’ or ‘ings’ or ‘ere’; V.42bis
works by creating a library of these repetitions and substituting small
symbols for them. V.42bis is also able to switch off data compression if
the type of data being sent is unsuitable for compression.
MNP 5 is MNP’s stable-mate; it uses ‘run length encoding’ and a
variation of the Huffman compression technique. It can double your
data throughput on some types of data and MNP 5 works best when
there are lots of characters repeated in sequence or a particular character
repeated. For example, if you have a file which contained 50 zero
characters in sequence MNP 5 will do a good job of compressing them
(in some instances, better than V.42bis) – this is run length encoding. Or,
if you had a text file where every second character was an ‘i’ and the other
characters were random, MNP 5 would again outperform V.42bis (in
this instance, Huffman coding provides superior compression). How-
ever, MNP 5 does not employ V.42bis’ automatic switching techniques.
If the data you are sending is not suitable for compression (for example,
an application file or a graphics file), it can actually take longer to send
than if data compression was not being used.
For this reason, it is best to use V.42bis whenever possible. If the modem
you connect to does not support V.42bis, it is best not to use MNP 5
unless you are only transferring plain text information. If possible, use
one of the many popular data compression programs available for
computers to compress applications, graphics and other non-text files.
The %C command is used to select the type of data correction used by
your modem.
Error Correction
Error
Correction
pro16ref.pm6 6/14/96, 2:36 PM28
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