Data Communications Networking Devices - 4th Ed.pdf

4.3 INTELLIGENT MODEMS ________________________________________________________ 447Table 4.25 continued )ProtocolclassDescriptionClass 7 Uses an advanced form of Huffman encoding in conjunction with a predictiveMarkov algorithm to represent user data in the shortest possible Huffman codes.This compression technique is called Enhanced Data Compression. EnhancedData Compression has all of the characteristics of Class 5 compression, but, inaddition, predicts the probability of repetitive characters in the data stream. Class7 compression, on the average, reduces data by 42%. In addition to Class 5 andClass 7 data compression, modern implementation of MNP now supports theITU-T V.42 bis data compression standardClass 8 Adds ITU V.29 fast-train modem technology to Class 7 Enhanced DataCompression, enabling half-duplex devices to emulate full-duplex transmission.Class 9 Combines V.32 modem modulation technology with Class 7 Enhanced DataCompression, resulting in full-duplex throughput that can exceed that obtainablewith a V.32 modem by 300%. Class 9 also employs selective retransmission, inwhich error packets are retransmitted, and piggybacking, in which acknowledgementinformation is added to the data. Retransmission is facilitated byindicating in the error or Negative Acknowledgement NAK) packet the ordersequence of each of the failed messages. Instead of having to retransmit allmessages from the point of error to include the good messages, only the failedmessages are resent.Class 10 Adds Adverse Channel Enhancement ACE), which optimizes modem performancein environments with poor or varying conditions, such as cellularcommunications, rural telephone service, and some international connections.Adverse Channel Enhancements fall into ®ve categories: Negotiated Speed Upshift: modem handshake begins at lowest possiblemodulation speed, and when line conditions permit, the modem upshifts to thehighest possible speed. Robust Auto-Reliable Mode: enables MNP10 modems to establish a reliableconnection during noisy call setups by making multiple attempts to overcomecircuit interference. In comparison, other MNP classes make only one callsetup attempt. Dynamic Speed Shift: causes an MNP10 modem to adjust its operating ratecontinuously throughout a session in response to current line conditions. Thisadjustment includes both downshifting and upshifting across modulationschemes, which enables modems to use the highest possible operating rate agiven line connection can support. Aggressive Adaptive Packet Assembly: results in packet sizes varying from 8to 256 bytes in length. Small data packets are used during the establishment ofa link, and there is an aggressive increase in the size of packets as conditionspermit. The computations required under Aggressive Adaptive PacketAssembly to determine the optimum packet size are performed during idletime. Dynamic Transmit Level Adjustment DTLA): designed for cellular operations,DTLA results in the sampling of the modem's transmit level and its automaticadjustment to optimize data throughput. Once a cellular connection isestablished, transmit-level statistics are continually sampled using linkmanagement packets. When necessary, transmit levels are adjusted basedon the existing conditions to optimize data throughput.