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Lecture 12

Lecture 12. Packet Switching: A Network Layer Function. Limitations of shared media networks-Ethernet or FDDI. Scalability Number of users Ethernet-collision problem FDDI-access limitations Distance Ethernet Collision recovery FDDI Ring latency. Star-topology provided by a switch.

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Lecture 12

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  1. Lecture 12 Packet Switching: A Network Layer Function

  2. Limitations of shared media networks-Ethernet or FDDI • Scalability • Number of users • Ethernet-collision problem • FDDI-access limitations • Distance • Ethernet • Collision recovery • FDDI • Ring latency

  3. Star-topology provided by a switch

  4. How does a switch solve these problems? • A switch is selective • Adding other networks will not necessarily reduce the performance of any user’s network • More efficient (Network level) protocols are used to guarantee packet delivery across a net.

  5. Switches and Network Protocols

  6. Switching Strategies • Connectionless—datagram approach • Connection-oriented-virtual circuit approach

  7. Datagrams • Each packet contains a complete destination address • Switches contain forwarding (or routing) tables that allows packet to find it’s destination • The procedure for creating the forwarding tables is called routing—Chapter 4

  8. Datagram routing

  9. Virtual Circuit (VC) Switching • Stage 1: Connection Set-up • Permanent Virtual Circuits (PVCs) • Have a long lifetime-e.g. connection between campus network and ISP • Set up by administrator manually or through a signaling process • Switched Virtual Circuits (SVCs) • Typically are created to handle a single session • Created by a host through the signaling process • Stage 2: Data Transfer

  10. Connection States of a VC

  11. VC Switching Tables

  12. PVCs in the UH Campus ATM Network

  13. SVC setup

  14. Benefit of SVCs • Reduced address size in header-lower overhead—this is a big deal with small packets-e.g. ATM • Sender knows that receiver and route exist at cost of a RTT-or so. • Resources are allocated at set up time • VCs can be assigned a QoS

  15. X.25 Network example • Buffers are allocated during setup • Sliding window protocol+flow control runs between each pair of nodes • Circuit is rejected if these resources are not available • Highly reliable hop-by-hop flow control

  16. Source Routing

  17. Where we need routing so far • Datagram switching table set up • SVC signalling packet routing • Source route determination • Note that source routing can be used in SVC signaling

  18. PC as a switch—we will do this in the lab exercise

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