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CSCI 465 D ata Communications and Networks Lecture 11

CSCI 465 D ata Communications and Networks Lecture 11. Martin van Bommel. Error Control. Two types of errors Lost frame - never arrives or too error filled Damaged frame - error in bits but recognizable Techniques involve Error detection (e.g. CRC) Positive acknowledgement – if error free

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CSCI 465 D ata Communications and Networks Lecture 11

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  1. CSCI 465Data Communications and NetworksLecture 11 Martin van Bommel CSCI 465Data Communications & Networks

  2. Error Control • Two types of errors • Lost frame - never arrives or too error filled • Damaged frame - error in bits but recognizable • Techniques involve • Error detection (e.g. CRC) • Positive acknowledgement – if error free • Retransmission after timeout – no ACK received • Negative acknowledgement and retransmission CSCI 465Data Communications & Networks

  3. ARQ • Automatic Repeat reQuest (ARQ) • Collective name for error control techniques • Make potentially unreliable data link reliable • Three versions • Stop-and-wait ARQ • Go-back-N ARQ • Selective-reject ARQ CSCI 465Data Communications & Networks

  4. Stop-and-Wait ARQ • source transmits single frame • waits for ACK • no other data can be sent until destination’s reply arrives • if frame received is damaged, discard it • transmitter has timeout • if no ACK within timeout, retransmit • if ACK is damaged, transmitter will not recognize it • transmitter will retransmit after timeout • receiver will get two copies of same frame • use alternate frame numbering and ACK0 / ACK1 (one bit) CSCI 465Data Communications & Networks

  5. Stop and Wait ARQ • Pros • Simplistic • Cons • inefficient

  6. Go-Back-N ARQ • most commonly used error control • based on sliding-window • use window size to control number of outstanding frames • if no error, ACK as usual with frame number • if error, reply with rejection REJ • destination will discard that frame and all future frames until frame in error is received correctly • transmitter must go back and retransmit that frame and all subsequent frames CSCI 465Data Communications & Networks

  7. Go-Back-N Situations • Damaged frame i – already received (i – 1) • Receiver takes no action • Sender either • Sends next frame (i + 1) – send back REJ i • Times out on frame – Sender either • Retransmits frame i (and subsequent frames if any) • Sends a RR query (P bit = 1) – receiver sends RR i • Damaged RR – either • Lost RR (i ) – sender receives RR (i + 1) and continue • Timeout – send RR query and set new timer • Damaged REJ – same as time out above CSCI 465Data Communications & Networks

  8. Selective Reject ARQ • 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 logic in transmitter • less widely used • useful for satellite links with long propagation delays CSCI 465Data Communications & Networks

  9. COMPARE Go-Back-N vs Selective Reject

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