Data Compression and Error Control
Data compression results depend on the type of data being
compressed. Some types, such as ASCII files, can be compressed quite a bit.
Other types of data can compress only a little. Even though certain software
applications can be used to achieve data compression, normally it's better to
leave this operation up to the modem. This is because modem hardware compression
algorithms are faster than the ones used by host software.
Compression normally works with error-correction algorithms.
Error detection and correction techniques can be used to guarantee data
integrity at any transmission speed. Two examples of such techniques are
V.42bis and MNP5 are the compression algorithms that commonly
operate over LAP-M or MNP4 correction. The V.42 and V.42bis compression
algorithms can be implemented in V.32 and V.34 modems as well as in other
equipment with lower speed capability. In theory, V.42bis can provide the 4:1
compression ration. However, in practice, this is rarely accomplished.
V.42bis compression is achieved when both communicating modems
agree to use it. In such instances, the software compression option should be
turned off. If hardware compression is used, the data transfer between the DTE
and DCE can occur at a higher speed.
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