 V.42bis is an the Microcom Networking Protocol MNP (TM). CCITT recom- expensive proposition for vendors. Three companies, BT, IBM and mendations specify a negotiation process that attempts error correc- the Unisys Corporation all patented their contributions to it and tion using the V.42 standard or Link Access Protocol for Modems charge licence fees. (LAPM). If LAPM (Clark, 1991) is not available in both devices attempting 39.3.1 communication, MNP class 4 is attempted. MNP up to level 4 is The V42.bis algorithm included as an addendum to this international standard owing to the large installed base of modems using it, but higher levels will not be In order to compress data, the V.42bis algorithm must first identify included in further developments. If neither is acceptable, simple recurring strings of characters in the data stream. These code words asynchronous operation is used. are used to compile dictionaries which are duplicated at both ends Once error correction is established, modems operating CCITT of the transmission link and are accessed by a common syn- standards will attempt to instigate compression using V.42bis. Fail- chronised index system. ing this, the fallback is to MNP level 5 (2:1 compression) and from This process is dynamic and the dictionaries are constantly being there to uncompressed operation. V.42bis uses a technique called updated and modified. As data is transferred to the modem for the Lempel-Ziv algorithm (Section 39.3.1). This like other tech- transmission, a complex string matching or 'parsing' process must niques, builds dictionaries of code words for recurring characters in take place requiring an extensive dictionary search. It is easy to see the data stream. The code words are abbreviations, SO that it takes how less efficient compression techniques could consume large less bits to transmit, thereby increasing throughput and lowering amounts of memory and why it is necessary to disable the algorithm cost over dial up lines. when presented with incompressible or encrypted dat