Data Communications Networking Devices - 4th Ed.pdf

454 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTto communicate using the MNP method of error detection and correction you caneither consider purchasing a modem that is MNP Class 4 compatible or which isV.42 compatible and supports the alternative error-correction procedure.Fortunately, most modem vendors that have adopted V.42 support also supportthe alternative error-correction procedure more commonly referred to as MNPClass 4.Data compressionSimilar to error detection and correction, modem manufacturers have incorporateda variety of data compression algorithms into their products. Until 1989, mostcompression methods, while representing a proprietary scheme of one vendor, werelicensed by that vendor to other manufacturers. This resulted in several de factodata compression standards being incorporated into modems manufactured bydifferent vendors.In 1990 the CCITT promulgated the V.42 bis recommendation which de®nes anew data compression method known as Lempel-Ziv as an international standard.Unlike V.42, which concerns error detection and correction and speci®es MNPClass 4 as an alternative, V.42 bis does not specify an alternative method of datacompression.RationaleA data compression performing modem results in each modulated bit conveyingmore information than an equivalent modem that does not compress data. Toillustrate this concept assume that over a period of time the ratio of characters¯owing into a modem's compression circuitry to the number of compressedcharacters output and modulated by the modem is 2:1. This 2:1 compression ratioindicates that, on average, a compression performing modem can transfer informationtwice as fast as a similar modem that does not compress data.Modem vendors have incorporated data compression into their products toprovide users with two key advantages over non-compression performing modemsÐa reduction in transmission time and an alternative to purchasing higher-speedand higher-cost modems. The reduction in transmission time is probably the moreobvious of the two advantages as compacted data takes less time to be transmitted.Concerning the latter advantage, consider a data compression modem that operatesat 14 400 bps and has a 2:1 compression ratio. This modem would provide you withan effective throughput of 28 800 bps and would probably be less expensive than a28 800 bps modem since the modulation scheme required to operate at the higherdata rate is more complex and more costly than the modulation method used by 14400 bps modems.MNP Class 5 compressionOf the two methods of data compression supported by the Microcom NetworkingProtocol, Class 5 is the more popular as it predates Class 7. Since each MNP Class