ZyXEL Communications Comet 3356, User Manual

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22 Advanced Data Communications
Data Compression
The Comet supports both V.42bis and MNP5 data compression protocols. Data
compression works by representing the original data information in less bits and
transmitting the reduced data bits through the data link. The receiver recovers the
original information by reversing the representing process. The process of representing
original data in less bits is called redundancy removal.
Data compression needs an error-free data link to work correctly. MNP5 is used with
MNP4 error control and V.42bis is used with V.42 error control.
MNP5 data compression utilizes the run-length encoding and adaptive frequency
encoding techniques. V.42bis uses a string coding algorithm.
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. Comet 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.
AT&K4 enables data compression negotiation. For some applications, like BBSs,
where transmission of compressed files is common, the MNP5 data compression
process actually produces more redundant data and slows down transmission.