Data Communications Networking Devices - 4th Ed.pdf

1.15 THE DATA LINK LAYER ______________________________________________________ 135composition,all Kermit transmissions occur in 7-level ASCII. The reason for thisrestriction is the fact that Kermit was originally designed to support ®le transfers to7-level ASCII mainframes. Binary ®le transfers are supported by the protocolpre®xing each byte whose eighth bit is set by the ampersand &) character. Inaddition,all characters transmitted to include 7-level ASCII must be printable,resulting in Kermit transforming each ASCII control character with the pound £)character. This transformation is accomplished through the complementation ofthe seventh bit of the control character. Thus,64 modulo 64 is added or subtractedfrom each control character encountered in the input data stream. When an 8-bitbyte is encountered whose low order 7 bits represent a control character,Kermitappends a double pre®x to the character. Thus,the byte 100000001 would betransmitted as &£A.Although character pre®xing adds a considerable amount of overhead to theprotocol,Kermit includes a run length compression facility which may partiallyreduce the extra overhead associated with control character and binary datatransmission. Here,the tilde ) character is used as a pre®x character to indicaterun length compression. The character following the tilde is a repeat count,whilethe third character in the sequence is the character to be repeated. Thus,thesequence XA is used to indicate a series of 88 As,since the value of X is 1011000binary or decimal 88. Through the use of run length compression the requirementto transmit printable characters results in an approximate 25% overhead increase incomparison to the XMODEM protocol for users transmitting binary ®les. IfASCII data is transmitted,Kermit's ef®ciency can range from more ef®cient toless ef®cient in comparison to the XMODEM protocol,with the number ofcontrol characters in the ®le to be transferred and the susceptibility of the data torun length compression the governing factors in comparing the two protocols.Figure 1.76 illustrates the format of a Kermit packet. The header ®eld is theASCII start of header SOH) character. The length ®eld is a single characterwhose value ranges between 0 and 94. This one-character ®eld de®nes the packetlength in characters less two,since it indicates the number of characters to includethe checksum that follow this ®eld.Figure 1.76 The Kermit packet format. The ®rst three ®elds in the Kermit packet are onecharacter in length and the maximum total packet length is 96 or fewer charactersThe sequence ®eld is another one-character ®eld whose value varies between 0and 63. The value of this ®eld wraps around to 0 after each group of 64 packets istransmitted.The type ®eld is a single printable character which de®nes the activity the packetinitiates. Packet types include D data),Y acknowledgement),N negativeacknowledgement),B end of transmission or break),F ®le header),Z end of ®le)and E error).