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10–3
Run-length Encoding
Run-length encoding is applied in an attempt to avoid sending long sequences of
repeated characters (data). When three or more repeated characters appear in suc-
cession, only the first three tokens (representing the compressed format of that
character) and a repetition count will be sent.
Adaptive Frequency Encoding
Adaptive frequency encoding is applied after removing repeated characters (data).
In adaptive frequency encoding, a token is substituted in the data stream for the
actually occurring character in an attempt to send fewer than 8 bits for each charac-
ter. The token is generated from a dynamic tabulation of character appearing fre-
quency. The total number of available tokens is 256, of which only the first 32
tokens are smaller than 8 bits, so random data will gain no advantage from this
technique.
String Coding
Instead of sending each data character individually, a token for a character string is
sent. The modem adaptively builds a dictionary of string tokens according to data
that appears. 2864 series modems support a dictionary size up to 2K string tokens.
The input data characters are combined and checked for a matching string in the
dictionary. The token is sent for the longest matched string. Compressibility is high
if there are some regularities of character pattern in the data.
For the 2864 series of modems, the error control and data compression option can
be enabled either from the front panel or the terminal.
&K4 is the default. For some applications, like BBSs, where transmission of already
compressed files is usual, the MNP5 data compression process actually produces
more redundant data and slows down transmission. Setting S38b5 will disable
MNP5 protocol negotiation regardless of the error control setting.
*. default
Control Level
AT Command Error Control Data Compression
NONE AT&K0 none none
MNP4 (MNP3) AT&K1 MNP4 none
MNP4 + MNP5 AT&K2 MNP4 MNP5
V.42 (MNP4) AT&K3 V.42 / MNP4 none
V.42(b) + MNP4 (5)
*
AT&K4 V.42 / MNP4 V.42bis / MNP5