数据与计算机通信07-DataLinkControl.ppt

Shandong University Go Back N - Lost Frame (1) Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits Shandong University Go Back N - Lost Frame (2) Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor rejection Transmitter times out and sends acknowledgement frame with P bit set to 1 Receiver interprets this as command which it acknowledges with the number of the next frame it expects (frame i ) Transmitter then retransmits frame i Shandong University Go Back N - Damaged Acknowledgement Receiver gets frame i and send acknowledgement (i+1) which is lost Acknowledgements are cumulative, so next acknowledgement (i+n) may arrive before transmitter times out on frame i If transmitter times out, it sends acknowledgement with P bit set as before This can be repeated a number of times before a reset procedure is initiated Shandong University Go Back N - Damaged Rejection As for lost frame (2) Shandong University Go Back N - Diagram Shandong University Selective Reject Also called selective retransmission Only rejected frames are retransmitted Subsequent frames are accepted by the receiver and buffered Minimizes retransmission Receiver must maintain large enough buffer More complex login in transmitter Shandong University Selective Reject -Diagram Shandong University High Level Data Link Control HDLC ISO 33009, ISO 4335 Shandong University HDLC Station Types Primary station Controls operation of link Frames issued are called commands Maintains separate logical link to each secondary station Secondary station Under control of primary station Frames issued called responses Combined station May issue commands and responses Shandong University HDLC Link Configurations Unbalanced One primary and one or more secondary stations Supports full duplex and half duplex Balanced Two combined stations Supports full duplex and half duplex