Data Communications Networking Devices - 4th Ed.pdf

134 ______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSfeatures of the ZMODEM protocol include its streaming ®le transfer operation,anextended error detection capability,automatic ®le transfer capability,the use ofdata compression,and downward compatibility with the XMODEM-1K andYMODEM protocols.The streaming ®le transfer capability of ZMODEM is similar to thatincorporated into YMODEM-GÐthat is,the sender will not receive an acknowledgementuntil the ®le transfer operation is completed. In addition to thestreaming capability,ZMODEM supports the transmission of conventional 128-and 1024-byte block lengths of XMODEM-based protocols. In fact,ZMODEM isbackward-compatible with XMODEM-1K and YMODEM.The extended error detection capability of ZMODEM is based upon the abilityof the protocol to support both 16- and 32-bit CRCs. According to the protocoldeveloper,the use of a 32-bit CRC reduces the probability of an undetected errorby at least ®ve orders of magnitude below that obtainable from the use of a 16-bitCRC. In fact,32-bit CRCs are commonly used with local area network protocolsto reduce the probability of undetected errors occurring on LANs.The automatic ®le transfer capability of ZMODEM enables a sending orreceiving computer to trigger ®le transfer operations. In comparison,XMODEMand YMODEM protocols and their derivatives are receiver driven. Concerning the®le transfer startup process,a ®le transfer begins immediately under ZMODEMwhile XMODEM and YMODEM protocols and their derivatives have a 10-seconddelay as the receiver transmits NAKs or another character during protocol startupoperations.An additional signi®cant feature associated with the ZMODEM protocol is itssupport of data compression. When transmitting data between Unix systems,ZMODEM compresses data using a 12-bit modi®ed Lempel±Ziv compressiontechnique,similar to the modi®ed Lempel±Ziv technique incorporated into theITU-T V.42 bis modem recommendation. When ZMODEM is used betweennon-Unix systems,compression occurs through the use of Run Length Encodingsimilar to MNP Class 5.KermitKermit was developed at Columbia University in New York City primarily as amechanism for downloading ®les from mainframes to microcomputers. Since itsoriginal development this protocol has evolved into a comprehensive communicationssystem which can be employed for transferring data between most types ofintelligent devices. Although the name might imply some type of acronym,inactuality,this protocol was named after Kermit the Frog,the star of the wellknownMuppet television show.Kermit is a half-duplex communications protocol which transfers data invariable sized packets,with a maximum packet size of 96 characters. Packets aretransmitted in alternate directions since each packet must be acknowledged in amanner similar to the XMODEM protocol.In comparison to the XMODEM protocol and its derivatives which permit 7-and 8-level ASCII as well as binary data transfers in their original data