Data Communications Networking Devices - 4th Ed.pdf

458 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTsingle column representing each character ordered by their frequency ofoccurrence. Under MNP Class 7, run-length encoding is combined with the useof a ®rst-order Markov model. This model is used to predict the probability of theoccurrence of a character based upon the value of the previous character, with anadaptive table of 256 character columns used to represent the ordered frequency ofoccurrence of each `succedding' character.Markov modelTo encode a character the compressor selects a code which depends on the immediatelypreceding encoded character. The selected code is based on thefrequency with which a character follows the previous character. For example,the probability of a `U' following a `Q' is very high and generally the `U' will beencoded as 1 bit. Likewise, an `H' following a `C' has a different probability than an`R' following a `C', and will be coded according to its frequency of occurrence.As noted in Table 4.27, the compressor keeps up to 256 coding tables, one foreach possible 8-bit character or pattern). To code a character for transmission, ituses the previous character to select the appropriate coding table. For example,when an `A' is transmitted the model looks under the `A' pointer for the nextcharacter, which is ordered according to its frequency of occurrence. If a `C' is thenext character, it is compressed based upon its location in the table. The modellooks next under the `C' pointer to ®nd the following character, and so on. Eachtable contains the codes for characters following the previous character, and isorganized according to the rules of Huffman coding.Table 4.27MNP Class 7 enhanced data compressionPointers A B C D ECharacters coded T L H O D Up toaccording to their H E O A R maximumfrequency of following C U R E Sof 256the previous character, M . . N charactersi.e., the pointer B . . P eachHuffman codingIn Table 4.27, each column of characters under the pointer character is compressedaccording to the rules of Huffman coding. Huffman coding changes the number ofbits representing a character when the character's frequency of occurrence changessuf®ciently. Huffman can adapt to various alphabets e.g., ASCII, EBCDIC, andall uppercase) and languages natural language, compiler code, and spreadsheets)without being pre-informed of the data used.Unlike Class 5, Huffman coding can represent a character with only one bit, if itoccurs often enough. In general, if one character occurs twice as often as another,its code is half as long.MNP Class 7 is adaptive, meaning that it changes the coding of the data when thefrequency of character occurrence changes. The compressor starts off with no