Data Communications Networking Devices - 4th Ed.pdf

132 ______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSXMODEM-1K protocolThe XMODEM-1K protocol is a derivative of the XMODEM standard. TheXMODEM-1K protocol follows the previously described XMODEM protocol,substituting 1024-byte blocks in place of byte data blocks. The XMODEM-1K isnot compatible with the YMODEM nor the YMODEM BATCH protocols,as theformer does not send or accept a block 0,which contains ®le information. Since theblock size of this protocol is signi®cantly longer than that of the XMODEMprotocol,you can expect a higher level of throughput when transmitting on goodquality circuits using XMODEM-1K.YMODEM-G and YMODEM-G BATCH protocolsThe development of error correction and detection modems essentially made theuse of CRC checking within a protocol redundant. In recognition of this,a `G'option was originally added to the YMODEM protocol which changed it into a`streaming' protocol in which all data blocks are transmitted one after another,withthe receiver then acknowledging the entire transmission. This acknowledgementsimply acknowledges the entire transmission without the use of error detection andcorrection. In fact,the two-byte CRC ®eld is set to zero during a YMODEM-Gtransmission. Thus,this protocol should only be used with error correctingmodems that provide data integrity. The use of error correcting modems isdescribed in detail in Chapter 5.Although some software programs enable users to initiate YMODEM-G byentering the character G as an optional parameter,most programs considerYMODEM-G as a separate protocol selected from a pull-down menu or via acommand line entry.Like the YMODEM BATCH protocol,YMODEM-G BATCH protocolpermits multiple ®les to be transmitted and sends the ®rst 128 data characterblock with ®le information in the same manner as carried by the YMODEMBATCH protocol. Typically,the multiple ®le transfer capability is selected by theuse of a YMODEM-G BATCH option available with many communicationsprograms. To differentiate YMODEM-G BATCH from YMODEM-G,thereceiver will initiate the batch transfer by sending the ASCII G instead of theASCII C. When the sender recognizes the ASCII G,it bypasses the wait for anACK to each transmitted block and sends succeeding blocks one after another,subject to any ¯ow control signals issued by an attached modem or by a packetnetwork if that network is used to obtain a transmission path. When the transmissionis completed,the sender transmits an EOT character and the receiverreturns an ACK which serves to acknowledge the entire ®le transmission. TheACK is then followed by the receiver transmitting another ASCII G to initiate thetransmission of the next ®le. If no additional ®les are to be transmitted,the senderthen transmits a block of 128 characters with each character set to an ASCII 00H orNUL character.Figure 1.75 illustrates the transmission of the previously described STOCK.-DAT ®le using the YMODEM-G protocol. In comparing Figure 1.75 with Figure