Data Communications Networking Devices - 4th Ed.pdf

4.3 INTELLIGENT MODEMS ________________________________________________________ 443Rationalebased upon the use of cyclic or polynomial code error detection schemes previouslydescribed in Chapter 1. After a modem computes a CRC it is appended to the blockof data to be transmitted. The receiving modem uses the same prede®ned generatingpolynomial to generate its own CRC based upon the received block and thencompares the locally generated CRC with the transmitted CRC. If the two match,the receiving modem transmits a positive acknowledgement to the transmittingmodem, which not only informs the distant modem that the data was receivedcorrectly but can also serve to inform the remote modem that if additional blocks ofdate remain to be transmitted the next block can be sent. If an error has occurred,the locally generated CRC will not match the transmitted CRC and the receivingmodem will transmit a negative acknowledgement which informs the remotemodem to retransmit the previously transmitted data block.Although most modem manufacturers used the CRC-16 polynomial,X 16 +X 15 +X 5 +1, which has the bit composition 1100000000010001, to operateagainst each data block, incompatibilities between the methods used to block dataand transmit negative and positive acknowledgements resulted in the method oferror detection and correction employed by one vendor being incompatible withthe method used by another modem manufacturer. The one major exception to thisincompatibility between vendor error detection and correction methods is theMicrocom Networking Protocol MNP), which has been licensed by Microcom toa large number of modem manufacturers. Until 1989, MNP was considered as a defacto standard due to a base of approximately one million modems supporting oneor more MNP classes. In 1989 the MNP method of error detection and correctionwas recognized by the ITU as one of two methods for performing this functionwhen the V.42 recommendation was promulgated.One of the issues that confuse many modem users is the rationale for using amodem's error detection and correction feature. After all, ®le transfer protocolssuch as XMODEM, YMODEM, ZMODEM, and their derivatives also provide anerror detection and correction capability.When a modem's error control feature is enabled and operates successfully inconjunction with a distant modem, error detection and correction is operatingduring the entire communications session. This means that regardless of the functionyou are performing, whether reading an electronic mail message, transferring a®le, or sending a message to SYSOP, your transmission is protected. In comparison,the error detection and correction function embedded into a ®le transferprotocol operates only during the ®le transfer. Thus, a logical question you mayhave is why you should use a ®le transfer protocol with error detection andcorrection when your modem performs that function.Although you need to use a ®le transfer protocol to transfer a ®le in an orderlymanner, the error detection and correction function of the ®le transfer protocol maybe redundant in most situations, because the ®le transfer protocol protects datafrom computer to computer, while the modem's error detection and correctionfeature protects data from modem to modem. Thus, the former protocol protectsdata leaving the serial port and ¯owing to the modem. However, since data errors