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V.25bis Command Set
V.25bis is a set of auto-dialing commands defined by the ITU-T. V.25bis supports
both synchronous and asynchronous data interfaces and is mainly used in Europe
and in IBM mainframe or mini-environments which use synchronous communica-
tion.
Error Correction
In synchronous data communication, data is checked and corrected in the host by
the so-called "link layer protocol" to ensure data integrity. Normally, no data check-
ing is provided in the host for asynchronous data communication. However, file
transfer protocols which include error checking are available with asynchronous
hosts.
Nowadays, intelligent modems incorporate error correction protocols inside the mo-
dem for asynchronous data communication and it is transparent to the host. The
modem packs the asynchronous data characters into blocks. The data is transmitted
as synchronous data between the modems. The data blocks are error checked at the
remote end, and an erroneous block is retransmitted.
The commonly used error correction protocols are MNP
and V.42 protocols.
Data Compression
Intelligent modems use redundancy removing methods to reduce the number of
data bits actually sent for asynchronous character transmission. The full character
data is recovered at the other end, thus the total throughput of the modem data
transmission is increased.
The data compression protocols commonly used by modems are MNP
class 5 and
V.42bis. (See definitions below.)
Data compression works on redundancy removing and its efficiency is dependent on
both the compression algorithm and data statistics.
MNP
Protocols
MNP
(Microcom Network Protocol) is a set of protocols first introduced by Mi-
crocom, Inc., and later used by many modem manufacturers. It consists of many
classes. Classes 1 to 4 are for error correction, and class 5 is for data compression.
MNP class 1 and 2 are obsolete. Normally class 4 is used for error correction and it
uses class 3 internally.
MNP class 5 is a data compression protocol with a maximum efficiency of 2 to 1.
MNP-5 is used with MNP-4 for error correction.