Data Communications Networking Devices - 4th Ed.pdf

144 ______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSBoth the response and sequence ®elds are used to transmit message numbers.DDCMP stations assign a sequence number to each message they transmit,placingthe number in the sequence ®eld. If message sequencing is lost,the control stationcan request the number of the last message previously transmitted by anotherstation. When this request is received,the answering station will place the lastaccepted sequence number in the response ®eld of the message it transmits back tothe control station.The address ®eld is used in a multipoint line con®guration to denote stationsdestined to receive a speci®c message. The following CRC1 ®eld provides amechanism for the detection of errors in the header portion of the message. ThisCRC ®eld is required since error-free transmission depends upon the count ®eldbeing detected correctly. The actual data is placed in the information ®eld and,aspreviously mentioned,can include special control characters. Finally the CRC2®eld provides an error detection and correction mechanism for the data in theinformation ®eld.OperationUnlike IBM's bisynchronous protocol,DDCMP does not require the transmissionof an acknowledgement to each received message. Only when a transmissionoccurs or if traf®c is light in the opposite direction,a condition where no datamessages are to be sent,is it necessary to transmit a special NAK or ACK.The number in the response ®eld of a normal header or in either a special NAKor ACK message is used to specify the sequence number of the last good messagereceived. To illustrate this,assume messages 3,4,5 and 6 were received since thelast time an acknowledgement was sent and message 7 contains an error. Then,theheader in the NAK message would have a response ®eld value of 6,indicating thatmessages 3,4,5 and 6 were received correctly and message 7 was receivedincorrectly. Under the DDCMP protocol up to 255 messages can be outstandingdue to the use of an 8-bit response ®eld.Another advantage of DDCMP over IBM's bisynchronous protocols is theability of DDCMP to operate in a full-duplex mode. This eliminates the necessityof line turnarounds and results in an improved level of throughput. Another functionof the response ®eld is to inform a transmitting station of the occurrence of asequence error. This is accomplished by the transmitting station examining thecontents of the response ®eld. For example,if the next message the receiver expectsis 4 and it receives 5,it will not change of the response ®eld of its data messageswhich contains a 3. In effect,this tells the transmitting station that the receivingstation has accepted all messages up through message 3 and is still awaitingmessage 4.Bit-oriented protocolsA number of bit-oriented line control procedures were implemented by computervendors that are based upon the International Organization for StandardizationISO) procedure known as high-level data link control HDLC). Various names