V.44 Modem is a lossless data compression procedure for use with V-series data circuit-terminating equipment (DCEs).The principal characteristics are:
-Compression procedure based on an algorithm that encodes strings of characters input Terminal equipment (DTE) as binary codes of variable length;
-Decoding procedure that recovers the strings of characters from received binary codes of variable length;
-A string-building mechanism that rapidly extends existing strings;
-An automatically invoked transparent mode of operation when uncompressible data are detected.
V.44 Modem also describes the application of this data compression procedure to packet networks.
- Characters from the DTE input to the encoder are matched against any previously identified strings of characters. If a string is matched, the codeword representing the string is transferred, and then an attempt is made to extend the matched string to encode additional characters and create a new and longer string. If no string match is found, the ordinal corresponding to the first character is transferred
- The V.44 modem encoder continually adds to its set of strings available for matching, by placing input characters into the history and by adding nodes to the node-tree. When either of these becomes full, the dictionary is reinitialized, and the encoding operation continues as before
- The decoder creates strings that replicate the strings created by its peer encoder, using the same assigned codewords. By using these strings, it decompresses the received compressed data stream. The decoder dictionary is reinitialized upon receipt of a REINIT control code.