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Week 8 Flow Control Error Control

Week 8 Flow Control Error Control. Where are we?. Up until now: Physical Layer Now: Data Link Layer Flow Control Error Control Later… Synchronisation Multiplexing. “sending signals over a transmission link”. “sending data over a data communications link”. Flow Control.

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Week 8 Flow Control Error Control

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  1. Week 8 • Flow Control • Error Control

  2. Where are we? • Up until now: Physical Layer • Now: Data Link Layer • Flow Control • Error Control • Later… • Synchronisation • Multiplexing “sending signals over a transmission link” “sending data over a data communications link”

  3. Flow Control • Ensuring the sending entity does not overwhelm the receiving entity • Preventing buffer overflow • Transmission time • Time taken to emit all bits into medium • Propagation time • Time for a bit to traverse the link

  4. Stop and Wait • Source transmits frame • Destination receives frame and replies with acknowledgement (ACK) • Source waits for ACK before sending next frame • Destination can stop flow by not send ACK • Works well for a few large frames

  5. Fragmentation • Large block of data may be split into small frames • Limited buffer size • Errors detected sooner (when whole frame received) • On error, retransmission of smaller frames is needed • Prevents one station occupying medium for long periods • Stop and wait becomes inadequate

  6. Question • If the data rate is 10 Mbps and transmission is over a distance of 100 m, how long will it take to send 1 Kb in eight 128 byte chunks with stop and wait FC? [assume v = 2×108 ms-1].

  7. Stop and Wait Link Utilization

  8. Sliding Window Flow Control • Allow multiple frames to be in transit • Receiver has buffer W long • Transmitter can send up to W frames without ACK • Each frame is numbered • ACK includes number of next frame expected • Sequence number bounded by size of field (k) • Frames are numbered modulo 2k • W= 2k-1

  9. Sliding Window Enhancements • Receiver can acknowledge frames without permitting further transmission (Receive Not Ready) • Must send a normal acknowledge to resume • If duplex, use piggybacking • If no data to send, use acknowledgement frame • If data but no acknowledgement to send, send last acknowledgement number again, or have ACK valid flag (TCP)

  10. Error Detection • Additional bits added by transmitter for error detection code • Parity • Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones • Even number of bit errors goes undetected

  11. Cyclic Redundancy Check • For a block of k bits transmitter generates n bit sequence • Transmit k+n bits which is exactly divisible by some number • Receive divides frame by that number • If no remainder, assume no error

  12. Error Control • Detection and correction of errors • Lost frames • Damaged frames • Automatic repeat request • Error detection • Positive acknowledgment • Retransmission after timeout • Negative acknowledgement and retransmission

  13. Automatic Repeat Request (ARQ) • Stop and wait • Go back N • Selective reject (selective retransmission)

  14. Stop and Wait • Source transmits single frame • Wait for ACK • If received frame damaged, discard it • Transmitter has timeout • If no ACK within timeout, retransmit • If ACK damaged,transmitter will not recognize it • Transmitter will retransmit • Receive gets two copies of frame • Use ACK0 and ACK1

  15. Stop and Wait ARQ

  16. Stop and Wait - Pros and Cons Simple Inefficient

  17. Go Back N • Based on sliding window • If no error, ACK as usual with next frame expected • Use window to control number of outstanding frames • If error, reply with rejection • Discard that frame and all future frames until error frame received correctly • Transmitter must go back and retransmit that frame and all subsequent frames

  18. Go Back N - Damaged Frame • Receiver detects error in frame i • Receiver sends rejection-i (REJ-i) • Transmitter gets rejection-i • Transmitter retransmits frame i and all subsequent

  19. Go Back N - Lost Frame (1) • Frame i lost and transmitter sends i+1 • Receiver gets frame i+1 out of sequence • Receiver sends reject i • Transmitter goes back to frame i and retransmits

  20. 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

  21. 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

  22. Go Back N - Damaged Rejection • Receiver sends rejection which is lost • As for lost frame (2) • (Transmitter times out and sends acknowledgement frame with P bit set to 1)

  23. Selective Reject • Also called selective retransmission • Only rejected frames are retransmitted • Subsequent frames are accepted by the receiver and buffered • Minimises retransmission • Receiver must maintain large enough buffer • More complex log at transmitter

  24. Which is best ??? Performance Issues • What is the line utilisation for various flow control methods? • Stop and Wait • Sliding Window • What is the line utilisation for various ARQ schemes? • Stop and Wait • Selective Reject • Go Back N

  25. Stop and Wait Flow Control • Assume stations A and B are communicating frames F1,F2,F3,…,Fn • Total transmission time T = nTF where TF is the time to transmit one frame and receive an acknowledgement • TF = tprop+tframe+tproc+tprop+tack+tproc • tprop: propagation delay • tframe: time spent transmitting a frame • tproc: processing time • tack:time spent transmitting an acknowledgement

  26. Stop and Wait Flow Control • TF = tprop+tframe+tproc+tprop+tack+tproc = 2tprop+tframeT = n(2tprop+tframe) • T is the total time to transmit, the actual time spent transmitting is only ntframe • Line utilisation U = ntframe/T

  27. Line utilisation • Define line utilisation U as the ratio of transmission time to the time taken to transmit data • For Stop and Wait define a = tprop/tframe, then U = 1/(1+2a)

  28. The parameter a • We have defined a = tprop/tframe • Alternatively, define • V: propagation speed (ms-1) • d: transmission distance (m) • R: data rate (bps) • L: frame size (bits)

  29. Spread Spectrum • Analog or digital data • Analog signal • Objective: Spread data over wide bandwidth • Makes jamming and interception harder • Frequency hoping • Signal broadcast over seemingly random series of frequencies • Direct Sequence • Each bit is represented by multiple bits in transmitted signal • Chipping code

  30. General model for SS

  31. Spread Spectrum • Frequency hoping • Signal broadcast over seemingly random series of frequencies • Direct Sequence • Each bit is represented by multiple bits in transmitted signal • Chipping code

  32. Direct Sequence

  33. Question • What is the bandwidth of a digital data stream encoded with direct sequence spreading?

  34. Generating noise • Need to generate same “noise” at source and destination • Computers are deterministic – generating true noise is not possible (i.e. no truly random numbers) • May use • pseudo-random algorithm • predetermined sequences (e.g. Gold sequences) • chaos

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