This relies on the construction of a dictionary, which is continually modified as the data is
transferred between the two modems. The dictionary consists of a set of trees in which
each root corresponds to a character in the alphabet. When communications is established
each tree comprises a root node with a unique code word assigned to each node. The
sequence of characters received by the modem (from its attached terminal) is compared
and then matched against the dictionary.
The maximum string length can vary from 6 to 250 characters and is defined by the
two connecting modems. The number of code words has a minimum of 512 (but any
value above this default minimum value can be agreed to between the two connecting
modems).
V.42 bis data compression, in substituting a code word for a string, is between 20 and
30% more efficient than MNP class 5 compression. V.42 bis is effective for large file
transfers, but not for short strings of data
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