Chapter 4 - Advanced Data Communications
4-7
The compression efficiency of V.42bis is generally higher than that of MNP5. In
some cases it can be 50% to 100% higher and in other cases it is just slightly
higher. In general, it is about 50% more efficient.

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
succession, 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
character. The token is generated from a dynamic tabulation of character
appearance frequency. 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. Omni 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.
The error control and data compression option can be enabled either from the 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.
Hardware or Software Flow Control
Flow control determines how data will be transferred between your computer and
modem. Your modem supports two flow control methods. But regardless of
which flow control method you use, make sure your communications software is
configured to use the same flow control method that your modem is using.
Otherwise, you may experience erratic data transfer where portions of received
data are lost, or you will receive frequent errors during data transfers.
The flow control option is set by AT&Hn as follows: