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Data Link Control

Data Link Control. For effective data communication. Frame synchronization Flow control Error control Addressing Control and data on same link Link Management. Flow Control. Necessary when data is being sent faster than it can be processed by receiver

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Data Link Control

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  1. Data Link Control

  2. For effective data communication Frame synchronization Flow control Error control Addressing Control and data on same link Link Management

  3. Flow Control • Necessary when data is being sent faster than it can be processed by receiver • Computer to printer is typical setting • Can also be from computer to computer, when a processing program is limited in capacity

  4. Stop-and-Wait Flow Control • Simplest form • Source may not send new frame until receiver acknowledges the frame already sent • Very inefficient, especially when a single message is broken into separate frames, or when the data link is long enough for significant delays to be introduced

  5. Sliding-Window Flow Control • Allows multiple frames to be in transit • Receiver sends acknowledgement with sequence number of anticipated frame • Sender maintains list of sequence numbers it can send, receiver maintains list of sequence numbers it can receive • ACK (acknowledgement) supplemented with RNR (receiver not ready)

  6. Error Control Process • All transmission media have potential for introduction of errors • All data link layer protocols must provide method for controlling errors • Error control process has two components • Error detection • Error correction

  7. Error Detection: Parity Bits • Bit added to each character to make all bits add up to an even number (even parity) or odd number (odd parity) • Good for detecting single-bit errors only • High overhead (one extra bit per 7-bit character=12.5%)

  8. Error Detection: Cyclic Redundancy Check (CRC) • Data in frame treated as a single binary number, divided by a unique prime binary, and remainder is attached to frame • 17-bit divisor leaves 16-bit remainder, 33-bit divisor leaves 32-bit remainder • For a CRC of length N, errors undetected are 2-N • Overhead is low (1-3%)

  9. Error Correction • Two types of errors • Lost frame • Damaged frame • Automatic Repeat reQuest (ARQ) • Error detection • Positive acknowledgment • Retransmission after time-out • Negative acknowledgment and retransmission

  10. Stop-and-Wait ARQ • One frame received and handled at a time • If frame is damaged, receiver discards it and sends no acknowledgment • Sender uses timer to determine whether or not to retransmit • Sender must keep a copy of transmitted frame until acknowledgment is received • If acknowledgment is damaged, sender will know it because of numbering

  11. Go-Back-N ARQ • Uses sliding-window flow control • When receiver detects error, it sends negative acknowledgment (REJ) • Sender must begin transmitting again from rejected frame • Transmitter must keep a copy of all transmitted frames

  12. Data Link Control • Specified flow and error control for synchronous communication • Data link module arranges data into frames, supplemented by control bits • Receiver checks control bits, if data is intact, it strips them

  13. Automatic Repeat Request • Mechanism used in data link control and transport protocols • Relies on use of an error detection code (such as CRC) • Flow Control • Error Control

  14. Go-back-N ARQ • Acknowledgments • RR = receive ready (no errors occur) • REJ = reject (error detected) • Contingencies • Damaged PDU • Damaged RR • Damaged REJ

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