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PN 88300150
For all practical purposes, the result of the V.42 error correction protocol is error-free transmission. Using
the 16-bit redundancy check, it will detect every error which is 16 bits or smaller, with 100% probability.
As a result, the chances of an error occurring are actually so small that you can, in practice, ignore them.
5.3
Data Compression
The MT2834MR has both V.42bis and MNP class 5 data compression. ITU-T V.42bis is an international
data compression standard which can provide data compression of up to four to one in certain types of
data. MNP class 5 is a proprietary technique for data compression that provides a data compression
capability of two-to-one.
The MT2834MR must be in error correction mode before it can compress data (LAP-M error correction for
V.42bis data compression or MNP error correction for MNP 5 data compression). By using the #L
command, you can select which error correction to use.
The type of file transfer protocol used to send and receive data will have a big effect on the speed gain
due to compression. In general, a protocol which uses large data blocks transfers files quicker. For
example, YMODEM sends 1000 characters per block. It will also help to have the serial port of the
receiving modem set to the highest possible speed (115,200 bps), if the sending modem is set to a lower
speed.
To achieve a data rate which is higher than the modem baud rate, use the modem's speed conversion
features by turning Baud Adjust off ($BA0) and operate the serial port at a higher speed than the modem
baud rate. For example, the serial port must be set to 4800, 9600, 19200, 38400, 57600, or 115,200 bps
($SB115200).
To use data compression, the modem must to be driven at full capacity. In other words, the data needs to
be present at enough volume (file transfers or batch operations) and speed to get maximum compression
benefits. The modem speed conversion features must be used to utilize the port at a higher speed than
the modem connection speed. Data compression works by locating repeated strings of characters and
repeating these strings using shorter codewords.
When operating the serial port at a higher speed than the modem baud rate, some type of flow control
should be used, otherwise data can be lost. (See &E4 and &E5 commands.)
To enable data compression, type AT&E15 and hit ENTER; to disable data compression, type AT&E14
and hit ENTER.
Note:
For data compression to take place, both the answer and the originate modems must have data
compression and error correction enabled.
5.4
MNP Classes
The MNP protocol is divided into several levels, or "Classes". The classes you would encounter today in
full duplex dial-up in 1200, 2400 and 9600 bps modems are Classes 3 through 5. When modems
equipped with MNP start communicating, they will negotiate operation to the highest common MNP class
between them and operate at that level.
4/3/01
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