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Chapter 11

Chapter 11. Data Link Control and Protocols. 11.1 Flow and Error Control. Flow Control. Error Control. Note :. Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment. Note :.

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Chapter 11

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  1. Chapter 11 Data LinkControland Protocols

  2. 11.1 Flow and Error Control Flow Control Error Control

  3. Note: Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment.

  4. Note: Error control in the data link layer is based on automatic repeat request, which is the retransmission of data.

  5. 11.2 Stop-and-Wait ARQ Operation Bidirectional Transmission

  6. 11.1Normal operation

  7. 11.2Stop-and-Wait ARQ, lost frame

  8. 11.3Stop-and-Wait ARQ, lost ACK frame

  9. Note: In Stop-and-Wait ARQ, numbering frames prevents the retaining of duplicate frames.

  10. 11.4Stop-and-Wait ARQ, delayed ACK

  11. Note: Numbered acknowledgments are needed if an acknowledgment is delayed and the next frame is lost.

  12. 11.5Piggybacking

  13. 11.3 Go-Back-N ARQ Sequence Number Sender and Receiver Sliding Window Control Variables and Timers Acknowledgment Resending Frames Operation

  14. 11.6Sender sliding window

  15. 11.7Receiver sliding window

  16. 11.8Control variables

  17. 11.9Go-Back-N ARQ, normal operation

  18. 11.10Go-Back-N ARQ, lost frame

  19. 11.11Go-Back-N ARQ: sender window size

  20. Note: In Go-Back-N ARQ, the size of the sender window must be less than 2m; the size of the receiver window is always 1.

  21. 11.4 Selective-Repeat ARQ Sender and Receiver Windows Operation Sender Window Size Bidirectional Transmission Pipelining

  22. 11.12Selective Repeat ARQ, sender and receiver windows

  23. 11.13Selective Repeat ARQ, lost frame

  24. Note: In Selective Repeat ARQ, the size of the sender and receiver window must be at most one-half of 2m.

  25. 11.14Selective Repeat ARQ, sender window size

  26. 11.5 HDLC Configurations and Transfer Modes Frames Frame Format Examples Data Transparency

  27. 11.15NRM

  28. 11.16ABM

  29. 11.17HDLC frame

  30. 11.18HDLC frame types

  31. 11.19I-frame Flag:8bit field just to identify the start and end of frame. Address: Generally 8 bit Control:1 or 2 byte for flow control and error control Information: Contains user data FCS: Frame Check Sequence is error detection field Three bits of the control field decide the sequence number of frame. P/F: Poll or Final. Poll if sent from primary to secondary and final if sent from secondary to primary. The last three bits correspond to the value of ACK when piggybacking is used

  32. 11.20S-frame control field in HDLC The two bit are for RR,RNR,REJ,SREJ

  33. 11.21U-frame control field in HDLC

  34. Table 11.1 U-frame control command and response

  35. Example 3 Figure 11.22 shows an exchange using piggybacking where is no error. Station A begins the exchange of information with an I-frame numbered 0 followed by another I-frame numbered 1. Station B piggybacks its acknowledgment of both frames onto an I-frame of its own. Station B’s first I-frame is also numbered 0 [N(S) field] and contains a 2 in its N(R) field, acknowledging the receipt of A’s frames 1 and 0 and indicating that it expects frame 2 to arrive next. Station B transmits its second and third I-frames (numbered 1 and 2) before accepting further frames from station A. Its N(R) information, therefore, has not changed: B frames 1 and 2 indicate that station B is still expecting A frame 2 to arrive next.

  36. 11.22Example 3

  37. Example 4 In Example 3, suppose frame 1 sent from station B to station A has an error. Station A informs station B to resend frames 1 and 2 (the system is using the Go-Back-N mechanism). Station A sends a reject supervisory frame to announce the error in frame 1. Figure 11.23 shows the exchange.

  38. 11.23Example 4

  39. Note: Bit stuffing is the process of adding one extra 0 whenever there are five consecutive 1s in the data so that the receiver does not mistake the data for a flag.

  40. 11.24Bit stuffing and removal

  41. 11.25Bit stuffing in HDLC

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