ECEN4533 Data Communications Lecture #244 March 2013 Dr. George Scheets n Read 4.6 n Problems Web n Corrected Exams u Due 6 March (Live) u One week after return (DL) n Design #2 u Due 15 March (Live) u Due 22 March (DL) n Quiz #2 on 27 March

ECEN4533 Data Communications Lecture #256 March 2013 Dr. George Scheets n Corrected Exams u Due Today (Live) u One week after return (DL) n Design #2 u Due 15 March (Live) u Due 22 March (DL) n Quiz #2 on 27 March

ECEN4533 Data Communications Lecture #268 March 2013 Dr. George Scheets n Read 8.1 – 8.3 n Problems 8.1, 8.2, Web 19 n Corrected Exams u One week after return (DL) n Design #2 u Due 15 March (Live) u Due 22 March (DL) n Quiz #2 on 27 March

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

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

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

Layer 4 Flow Control Efficiencies n Stop & Wait u Error Free 1/(K + 2*NPD) K = 1 + T ack /E[T s ] u Errors (1 - Pe)/(K + 2*NPD) K = 1 + (T ack /E[T s ]) (1 - Pe)

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

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

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

ISO OSI Seven Layer Model n Layer 7 Application Software n Layer 6 Presentation Windows API n Layer 5 Session TCP, Windows n Layer 4 TransportTCP, Windows n Layer 3 Network IP, Windows n Layer 2 Data LinkPC NIC n Layer 1 Physical PC NIC

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

TCP Header Source PortDestination Port Sequence Number ACK Number Window Checksum

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