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Data Link Control

Data Link Control. Flow control Coordinates the amount of data that can be sent before having to stop sending and wait for the acknowledgement. Error control

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Data Link Control

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  1. Data Link Control • Flow control • Coordinates the amount of data that can be sent before having to stop sending and wait for the acknowledgement. • Error control • Allows the receiver to inform the sender of any frames lost or damaged (error detection) and coordinates the retransmission of those frames (error correction). • Achieved by means of an automatic repeat request (ARQ). Data Link Layer

  2. ARQ Mechanism • Stop-and-wait ARQ • The sending device keeps a copy of the last frame transmitted until an acknowledgement (ACK) for that frame is received. • Both data frames and ACK frame are numbered alternately 0 and 1 for identification purpose. • A damaged or lost frame is equally by the receiver. • The sender has a control variable called S that holds the number of the recently sent frame (0 or 1). Data Link Layer

  3. ARQ Mechanism (continued…) • The receiver has a control variable called R that holds the number of the next frame expected (0 or 1). • The sender starts a timer when it sends a frame. • The receiver sends only positive ACK for frames received safe and sound; it is silent about the damaged or lost frames. Data Link Layer

  4. 11.1Normal operation

  5. 11.2Stop-and-Wait ARQ, lost frame

  6. 11.3Stop-and-Wait ARQ, lost ACK frame

  7. Note: In Stop-and-Wait ARQ, numbering frames prevents the retaining of duplicate frames. Data Link Layer

  8. 11.4Stop-and-Wait ARQ, delayed ACK

  9. Note: Numbered acknowledgments are needed if an acknowledgment is delayed and the next frame is lost. Data Link Layer

  10. 11.5Piggybacking

  11. ARQ Mechanism (continued…) • Stop-and-wait ARQ disadvantage • Inefficient use of the transmission medium because at any one time for a sender, there is only one frame that is sent and waiting for acknowledgement. Data Link Layer

  12. ARQ Mechanism (continued…) • Go-back-N ARQ • Up to W frames can be sent before having to worry about the acknowledgement (ACK). • A copy of these frames will still be kept until the ACK arrives. • To achieve this, the following are required: • Sequence numbers • Sender and receiver sliding windows • Control variables Data Link Layer

  13. 11.6Sender sliding window

  14. 11.7Receiver sliding window

  15. 11.8Control variables

  16. 11.9Go-Back-N ARQ, normal operation

  17. 11.10Go-Back-N ARQ, lost frame

  18. 11.11Go-Back-N ARQ: sender window size

  19. 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. Data Link Layer

  20. ARQ Mechanism (Continued…) • Selective repeat ARQ • Similar to Go-back-N ARQ except that when a frame is damaged, only the damaged frame will be retransmitted • For this, the following are needed: • Sender and receiver sliding windows • Control variables • Negative acknowledgement Data Link Layer

  21. 11.12Selective Repeat ARQ, sender and receiver windows

  22. 11.13Selective Repeat ARQ, lost frame

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

  24. 11.14Selective Repeat ARQ, sender window size

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