MODEM

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TCM Trellis-Coded Modulation (with Viterbi coding) is anoptional error-correction method included in the V.32 standard. TCM allows modems to check for transmission errors with aredundancy bit, which results in fewer errors on noisy lines. Trellis Encoding works WITH other error correction methods suchas V.42. Unlike other methods, TCM is true error correction, inthat for small errors, it does not force a retransmit, itactually fixes them. MNP - Microcom Network Protocol is a set of standards developedby Microcom, and made available by them to other manufacturers.

MNP Class 1 is referred to as Block Mode. It uses asynchronous,byte-oriented, Half-duplex transmission. This method providesonly about 70% efficiency, and is rarely used today. MNP Class 2 is called Stream Mode, and uses asynchronous, byte-oriented, Full-duplex transmission. Because of protocol overhead(the time it takes to establish the protocol and operate it),throughput at Class 2 is actually only about 84% of that for aconnection without MNP, delivering about 202 cps (characters persecond) at 2400 bps and is rarely used today.

MNP Class 3 incorporates Class 2, and is more efficient. It usesa synchronous, bit-oriented, Full-duplex method. This procedureyields throughput about 108% of that of a modem without MNP,delivering about 254 cps at 2400 bps. MNP Class 4 uses Adaptive Packet Assembly and Optimized DataPhase techniques, it improves throughput and performance by about5%, although actual increases depend on the type of call (noisyor clean), and can be as high as 25% to 50% on some links. V.42: is a CCITT error-correction standard that's similar to MNPClass 4, In fact, because the V.42 standard includes MNPcompatibility through Class 4, all MNP 4- compatible modems canestablish error-controlled connections with V.42 modems. Thisstandard, however, prefers to use its own better performingprotocol .

LAPM Link Access Procedure for Modems, like MNP, copes withphone line impairments by automatically re-transmitting data thatis corrupted during transmission assuring that only error freedata passes through the modems. LAPB : Link Access Procedure Binary, like MNP or LAPM, copeswith phone line impairments by automatically re-transmitting datathat is corrupted during transmission assuring that only errorfree data passes through the modems.