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CS 5565 Network Architecture and Protocols

CS 5565 Network Architecture and Protocols. Godmar Back. Lecture 4. Announcements. Created Resources Page Assignment: Do Ethereal Lab 1 (pg 69 in book) Don’t need to hand it in. Restored Spring 2005 webpage Created CS5565 Forum Use this to find a project partner

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CS 5565 Network Architecture and Protocols

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  1. CS 5565Network Architecture and Protocols Godmar Back Lecture 4

  2. Announcements • Created Resources Page • Assignment: • Do Ethereal Lab 1 (pg 69 in book) • Don’t need to hand it in. • Restored Spring 2005 webpage • Created CS5565 Forum • Use this to find a project partner • All 4 projects will be done in groups of up to 2. CS 5565 Spring 2006

  3. Today • Delay in packet-switched networks • Nodal processing delay • Queuing delay • Transmission delay • Propagation delay • End-to-end Delay • traceroute • Structure of Internet CS 5565 Spring 2006

  4. 1. Nodal processing delay: check bit errors determine output link transmission A propagation B nodal processing queueing Four sources of packet delay • 2. Queuing delay • time waiting at output link for transmission • depends on congestion level of router CS 5565 Spring 2006

  5. R=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate Queueing Delay traffic intensity = La/R • La/R ~ 0: average queueing delay small • La/R  1: delays become large • La/R > 1: more “work” arriving than can be serviced, average delay infinite! CS 5565 Spring 2006

  6. 3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R 4. Propagation delay: d = length of physical link s = propagation speed in medium (~2x108 m/sec) propagation delay = d/s transmission A propagation B nodal processing queueing Delay in packet-switched networks Note: s and R are very different quantities! CS 5565 Spring 2006

  7. Transmission vs. Propagation Delay • Show Applet here • http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/transmission/delay.html • Transmission delay depends on speed of link (10Mbps, 1000Mbps, …) • Propagation delay depends on distance (and speed of light in medium) CS 5565 Spring 2006

  8. Nodal delay • dproc = processing delay • typically a few microsecs or less • dqueue = queuing delay • depends on congestion • dtrans = transmission delay • = L/R, significant for low-speed links • dprop = propagation delay • a few microsecs to hundreds of msecs CS 5565 Spring 2006

  9. Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps Entire packet must arrive at router before it can be transmitted on next link: store and forward Example: L = 7.5 Mbits R = 1.5 Mbps delay = ? (assume no propagation/processing/queuing delay) Packet-switching: store-and-forward L R R R 15 seconds CS 5565 Spring 2006

  10. End-to-end vs. nodal delay • Question: in store-and-forward model, does end-to-end delay for a message of length L depend on the number/size of packets the message is split into? • Let’s look at: • http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/message/messagesegmentation.html CS 5565 Spring 2006

  11. Circuit Switching Dedicated link bandwidth Dedicated switch capacity Low link utilization Low overall utilization Bounded delay variance Packet Switching Better Link utilization Better overall utilization Need for congestion control Need to identify to which “call” a packet belongs Smaller average delay than TDM High variance in delay Circuit vs. Packet Switching (2) CS 5565 Spring 2006

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