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ECEN4533 Data Communications Lecture #24 4 March 2013 Dr. George Scheets

ECEN4533 Data Communications Lecture #24 4 March 2013 Dr. George Scheets. Read 4.6 Problems Web 16 - 18 Corrected Exams Due 6 March (Live) One week after return (DL) Design #2 Due 15 March (Live) Due 22 March (DL) Quiz #2 on 27 March.

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ECEN4533 Data Communications Lecture #24 4 March 2013 Dr. George Scheets

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  1. ECEN4533 Data CommunicationsLecture #24 4 March 2013Dr. George Scheets • Read 4.6 • Problems Web 16 - 18 • Corrected Exams • Due 6 March (Live) • One week after return (DL) • Design #2 • Due 15 March (Live) • Due 22 March (DL) • Quiz #2 on 27 March

  2. ECEN4533 Data CommunicationsLecture #25 6 March 2013Dr. George Scheets • Corrected Exams • Due Today (Live) • One week after return (DL) • Design #2 • Due 15 March (Live) • Due 22 March (DL) • Quiz #2 on 27 March

  3. ECEN4533 Data CommunicationsLecture #26 8 March 2013Dr. George Scheets • Read 8.1 – 8.3 • Problems 8.1, 8.2, Web 19 • Corrected Exams • One week after return (DL) • Design #2 • Due 15 March (Live) • Due 22 March (DL) • Quiz #2 on 27 March

  4. Normalized Propagation Delay • NPD = End-to-End Propagation Delay Average time to inject a Packet • NPD > 1 1 or more packets can simultaneously be in transit • NPD < 1 Packet front end hits far side before back end transmitted NPD > 1NPD < 1 Transmitter Receiver

  5. Layer 4 Acknowledgement • Stop & WaitTransmit one packet. Wait until ACK. • Throughput Horrible as NPD increases • High Speed • Long Distances • Shorter Packets • Used on WiFi (wireless- layer 2) • Window Flow ControlAllow N unACKnowledged packets enroute • Used on Wired systems • On Wireless, TCP uses windows at Layer 4 & 802.11 uses form of Stop & Wait at Layer 2

  6. Layer 4 ARQ w/ Window Flow Control • How do you retransmit when a packet is mangled or lost? • Go Back N • Retransmit packet in error & all that follow • Selective Repeat • Retransmit packet in error

  7. Layer 4 Flow Control Efficiencies • Stop & Wait • Error Free1/(K + 2*NPD) K = 1 + Tack/E[Ts] • Errors(1 - Pe)/(K + 2*NPD) K = 1 + (Tack/E[Ts]) (1 - Pe)

  8. Layer 4 Flow Control Efficiencies • Windows • error free • 1 if W (in segments) > K + 2*NPD • W/(K + 2*NPD); otherwise K = 1 + Tack/E[Ts] • with errors using Selective Repeat • [1-Pe] if W (in segments) > K + 2*NPD • W(1 - Pe)/(K + 2*NPD); otherwise

  9. Layer 4 Flow Control Efficiencies • Windows • error free • 1 if W (in segments) > K + 2*NPD • W/(K + 2*NPD); otherwise K = 1 + Tack/E[Ts] • with errors using Go Back N • ≈ [1-Pe]/[1 + Pe*2*NPD] if W (in segments) > K + 2*NPD • ≈ W(1 - Pe)/[(1 - Pe + WPe)(K + 2*NPD)]; otherwise

  10. TCP Flow Control • Sliding Window • Allows multiple packets to be enroute • Positive Acknowledgements • Packet Dumped (Congestion, Bit Error(s))?ACK never sent from RCV to TRANS • Transmitter needs a Retrans Timer • Adaptive Timers are best • Can respond to network congestion • Requires Estimate of Round-Trip Time

  11. ISO OSI Seven Layer Model • Layer 7 Application Software • Layer 6 Presentation Windows API • Layer 5 Session TCP, Windows • Layer 4 Transport TCP, Windows • Layer 3 NetworkIP, Windows • Layer 2 Data Link PC NIC • Layer 1 Physical PC NIC

  12. TCP Header Layer 6 & 7 Traffic IP Header Layer 2 Header Typical Packet • Exception: UDP used for Interactive Voice or Video 20 B 20 B Layer 2 Trailer

  13. TCP Header Source Port Destination Port Sequence Number ACK Number Window Checksum

  14. Wireshark Capture showing Opening of TCP Logical Link 139.78.76.93 = office PC 139.78.132.6 = OSU server hosting ECEN5533 home page

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