Data Communications Networking Devices - 4th Ed.pdf

4.3 INTELLIGENT MODEMS ________________________________________________________ 455is downward negotiable for compatibility, a modem that supports MNP Class 7will communicate with a modem that is MNP Class 5 compatible using MNP Class5 data compression.MNP Class 5 speci®es that the sending modem apply two modi®cations to thetransmitted data stream in an attempt to reduce the number of bits actually sent.The ®rst manipulation or data compression method is referred to as run-lengthencoding, while the second method of compression is known as adaptive frequencyencoding.MNP Class 5 uses run-length encoding to avoid sending long sequences ofrepeated data octets, where each octet represents eight bits that can de®ne acharacter in a particular character set or any binary value from 0 to 255 representedby the individual bit settings within the octet. The MNP Class 5 version of runlengthencoding results in a repetition count being inserted into the data stream torepresent the number of repeated data octets which follow the ®rst threeoccurrences of a sequence. The ®rst three repeated data octets, which are actuallysent, signal the beginning of a run-length encoded sequence. The next octet isalways a repetition count which has a maximum value of 250. If the repeatedsequence is only three octets in length, a repetition count of 0 is used. Four octetsconsisting of three repeated data octets and a count octet are thus used to compressany repeating sequence from 3 to 250 octets. Figure 4.55 illustrates the format ofMNP Class 5 run-length encoding as well as a few examples of its operation.MNP Class 5 run-length encoding can be considered as the ®rst level of a twoleveldata compression scheme. The second level of compression is known asadaptive frequency encoding and is applied to the data stream after any repeateddata octets are removed by the use of run-length encoding.In adaptive frequency encoding a compression token is substituted in the datastream for the actually-occurring data octet in an attempt to transmit fewer thaneight bits for each data octet. The token used changes with the frequency ofFigure 4.55MNP class 5 run length encoding O 1 =O 2 =O 3 = repeating octets)