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Go-Back-N ARQ

Learn about Go-Back-N and Selective Repeat ARQ protocols, their differences, throughput comparison, error detection methods, and Forward Error Correction (FEC) in data transmission.

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Go-Back-N ARQ

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  1. Go-Back-N ARQ • packets transmitted continuously (when available) without waiting for ACK, up to N outstanding, unACK’ed packets • a logically different sender timer association with each unACK’ed packet: extension of AB protocol Receiver: • ACK packet if correctly received and in-order, pass to higher layer • NACK or ignore (possibly discard) corrupt or out-of-order packet Sender : if NACK received, or timeout, for packet n, begin resending from n all over again • cumulative ACK: ACK of n implicitly acknowledges up through n

  2. Go-Back-N continued • no receiver transport buffering with discard • saves resources at receiver • avoids large bursts of packet delivery to higher layers • simplicity in buffering and protocol processing at sender and receiver • tradeoff between buffering/processing complexity and bandwidth

  3. Go Back N: Example

  4. Selective Repeat ARQ As in go back-N: • packet transmitted when available, up to limit timer associated with each unACK’ed packet • receiver NACK’s or ignores corrupted packets Unlike Go-Back-N: • out-of-order (but otherwise correct) packet is ACK’ed • receiver: buffers out-of-order packets • sender: on timeout or NACK of packet n, just retransmit n

  5. Selective Repeat ARQ (cont) Notes: • more receiver buffering than Go back-N • more complicated buffer management by both sides • saves bandwidth: no need to retransmit correctly received packets

  6. Selective Repeat ARQ: example

  7. How Big Can a Window Be? Suppose sequence number space size is N Q: How big can window be and have SR still work? Partial answer: N-1 won’t work: Fundamental problem: sender and receiver • do not have synchronized info • cannot know exact same information

  8. Throughput Comparison p - loss probability t_trans - pkt transmission time rtt - round trip time tput_SW = (1-p)/(rtt+t_trans) tput_GB = (1-p)/(p rtt + t_trans) tput_SR = (1-p)/t_trans

  9. Throughput Comparison 1GB/sec link 1KB pkt -> t_trans = 8ms rtt =1ms

  10. Throughput Comparison 1GB/sec link 1KB pkt -> t_trans = 8ms rtt =30ms

  11. Detecting Errors: checksums Need to detect errors: bits in packet may be flipped while in transit or stored at intermediate notes. Approach: add extra bits to packets that will allow us to detect (possibly correct) bit errors

  12. Simple example: Parity Given n-1 bit packet, add n-th bit, choosing value so that total number of 1 • bits in packets (including nth bit) is odd (odd parity). • example packet: At receiver: • count # 1’s in packet, if even, then error! • what if even number of bit flips? • what if odd number of bit flips? error detection seq ack data (parity bit) 0111 0001 10101011 0

  13. Simple example: Parity Note: • many codes with more powerful error detection capabilities • CRC-16: 16 bit code, detects 2 random bit errors, 16 errors in sequence. • packet header itself often separately checksummed • checksumming also done at data link layer • hardware support for transport-level checksum: SGI

  14. Forward Error Correction: FEC ARQ protocols operate by detecting errors and retransmitting • retransmission needs round-trip delay to recover • may be too long for deep space, or high-speed, real-time applications • FEC: key idea is to transmit enough redundant data to allow receiver to recover from errors itself! (no sender transmission required)

  15. row parity d1,1 … d1,J d1,J+1 d2,1 … d2,J d2,J+1 dI,1 … dI,J dI,J+1 dI+1,1 … dI+1,J d1,j+1 seq # ACK# data EDF column parity (b) packet format (a) 2-dimensional parity 1 0 1 0 1 0 1 0 1 1 0 1 0 1 1 1 0 0 1 1 0 1 0 0 1 0 1 0 1 0 1 1 1 1 0 1 0 1 1 1 0 0 1 1 0 1 0 0 (d) example: single bit error (c) example: no errors

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