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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). However, 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.
Reliable Mode
Reliable mode allows the modem to use error correction to connect to another modem. If the
modem you attempt to connect to does not support a compatible method of error correction,
your modem will hang up and return a NO CARRIER message.
Use \N6 to select V.42/MNP reliable mode, \N4 to select V.42-only reliable mode or \N2 to select
MNP-only reliable mode.
Auto-Reliable Mode
Auto-reliable mode allows your modem to use V.42 or MNP if the remote modem also supports
these. If the remote modem does not support a compatible type of error correction, your modem
will fallback to either constant speed or variable speed mode.
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The \J command determines whether your modem falls back to constant speed or