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5.7 Error Correction and Data Compression
In synchronous operation, the data being transmitted/received is protected
from the effects of line errors by the data transmission protocol used.
As asynchronous working does not have this built-in protection (although
it may include a simple parity check), it is advisable to use error-correction
for long distance fast transmission.
In addition to error correction, the modem also provides data compression.
The commands are:
–E
Error correction format
%C Data compression
Error Correction
This modem includes error correction options for use when in V.22bis or
V.22 modes. The error correction provided is ITU-T V.42 using the new
LAPM protocol. It will also support the widely-adopted MNP standard up
to Class 4.
LAPM protocol is preferred to MNP because it has the following
advantages:
– faster to connect,
– faster response in interactive communications,
– more flexible block sizes,
– an international standard.
ITU-T V.42 error correction, using LAPM protocol, is especially
recommended for:
– transmission over long distances or bad lines,
– bulk data transmissions requiring greater throughput.
Data Compression
Data compression enables certain types of data to be compressed by a ratio
of up to 4:1. V.42bis will operate with all types of asynchronous data
transmission. It intelligently monitors the compression performance, and
if it detects a situation where data compression is not effective (for instance
a file which has already been compressed), it will automatically
X460-306851 Issue 1 5-9 Rev.1