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However, in a reliable link or a direct link the baud rate adjustment
capability may be disabled, so that the serial port speed may be
different to the on-line speed. If data comes in faster than it goes
out, the faster incoming data is stored in the data buffer.
Therefore, when flow control capability is enabled, it is advisable
that the on-line (DCE) speed be at a rate equal to or higher than
the serial (DTE) port speed. Otherwise, data may be lost after the
buffer is full.
2.7 Error-Correction and Data Compression (ECDC)
Many modems support error-correction and data compression
(ECDC) protocols. Error-correction assures error-free data
transmission because the modem re-transmits the incorrect block
of data once a transmission error is detected. Data compression
can increase data throughput which results in savings of
transmission time and telephone bill costs. Common ECDC
protocols include CCITT V.42, V.42bis, and MNP 2-5.
The CCITT V.42 recommendation offers an error-correction
protocol referred to as LAPM (Link Access Procedure for Modems),
which is the modem protocol for point-to-point communications.
The CCITT V.42bis provides both error-correction and data
compression features. In addition to its compatibility to V.42 in
error-correction, V.42bis offers a 4:1 data compression rate so that
data can be transferred up to 4 times faster than a non-MNP
modem. For example, a 14,400bps modem can reach a data
throughput of 57,600bps when the compression rate is 4:1 on a
V.42bis ECDC link. However, the true compression rate depends
on the pattern of the transmission file.
As an alternative to CCITT recommendations, MNP Class 2 to 4
offers error-correction capability, while MNP Class 5 offers a 2:1
data compression. The MNP protocol was popular before CCITT
ECDC protocols were released and there are still some modems
using this protocol.
Note that V.42/V.42bis and MNP are different both in format and
operation. Two modems must support the same ECDC protocol in
order to connect in ECDC mode. For example, a V.42bis modem
connects in V.42bis mode only when the remote modem also
supports the V.42bis protocol. For example, if your modem
supports all three ECDC protocols, but the remote modem only