Data Communications Networking Devices - 4th Ed.pdf

1.15 THE DATA LINK LAYER ______________________________________________________ 139Figure 1.79BSC, error control methodsresults in the `internally' generated BCC being different from the BCC that wastransmitted with the second block. This causes the terminal device to transmit aNAK to the mainframe,which results in the retransmissions of the second block.In the example on the right-hand part of Figure 1.79,let us assume that theterminal received block 2 and sent an acknowledgement which was lost or garbled.After a prede®ned timeout period occurs,the master station transmits an ENQcommunications control character to check the status of the terminal. Upon receiptof the ENQ,the terminal will transmit the alternating acknowledgement,currentlyDLE1; however,the mainframe was expecting DLEO. Thus,the mainframe isinformed by this that block 2 was never acknowledged and as a result retransmitsthat block.Other control codesThree additional transmission codes commonly used in a bisynchronous protocolare the WACK,RVI and TTD characters.