Data Communications Networking Devices - 4th Ed.pdf

136 ______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSThe information contents of the packet are included in the data ®eld. Aspreviously mentioned,control characters and binary data are pre®xed prior to theirplacement in this ®eld.The check ®eld can be one,two or three characters in length depending uponwhich error detection method is used since the protocol supports three options. Asingle character is used when a checksum method is used for error detection. Whenthis occurs,the checksum is formed by the addition of the ASCII values of allcharacters after the Header character through the last data character and the loworder 7 bits are then used as the checksum. The other two error detection methodssupported by Kermit include a two-character checksum and a three-character 16-bit CRC. The two-character checksum is formed similar to the one-characterchecksum; however,the low order 12 bits of the arithmetic sums are used andbroken into two 7-bit printable characters. The 16-bit CRC is formed using theCCITT standard polynomial,with the high order 4 bits going into the ®rstcharacter while the middle 6 and low order 6 bits are placed into the second andthird characters,respectively.By providing the capability to transfer both the ®lename and contents of ®les,Kermit provides a more comprehensive capability for ®le transfers thanXMODEM. In addition,Kermit permits multiple ®les to be transferred in comparisonto XMODEM,which requires the user to initiate ®le transfers on anindividual basis.Bisynchronous protocolsDuring the 1970s IBM's BISYNC binary synchronous communications) protocolwas one of the most frequently used for synchronous transmission. This particularprotocol is actually a set of very similar protocols that provides a set of rules whicheffect the synchronous transmission of binary-coded data.Although there are numerous versions of the bisynchronous protocol inexistence,three versions account for the vast majority of devices operating in abisynchronous environment. These three versions of the bisynchronous protocolare known as 2780,3780 and 3270. The 2780 and 3780 bisynchronous protocols areused for remote job entry communications into a mainframe computer,with themajor difference between these versions the fact that the 3780 version performsspace compression while the 2780 version does not incorporate this feature. Incomparison to the 2780 and 3780 protocols that are designed for point to pointcommunications,the 3270 protocol is designed for operation with devices connectedto a mainframe on a multidrop circuit or devices connected to a clustercontroller which,in turn,is connected to the mainframe. Thus,3270 is a poll andselect software protocol.Originally,2780 and 3780 workstations were large devices that controlled suchperipherals as card readers and line printers. Today,an IBM PC or compatiblecomputer can obtain a bisynchronous communications capability through theinstallation of a bisynchronous communications adapter card into the PC's systemunit. This card is designed to operate in conjunction with a bisynchronouscommunications software program which with the adapter card enables the PC to