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Chapter 8 Switching

Chapter 8 Switching. Switched Network. Reason for switched networks Not cost effective to have all nodes DIRECTLY connected to all other nodes Unlike LANS, the nodes could be spread out and vast in number (can’t broadcast the signal) The signal can be switched from Tx to Rx.

ralph-osborne
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Chapter 8 Switching

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  1. Chapter 8Switching Fall 2009

  2. Switched Network • Reason for switched networks • Not cost effective to have all nodes DIRECTLY connected to all other nodes • Unlike LANS, the nodes could be spread out and vast in number (can’t broadcast the signal) • The signal can be switched from Tx to Rx Fall 2009

  3. Switched Network Fall 2009

  4. Taxonomy of Switched Networks Route the full message from Tx to Rx – not all widely used today Dedicated circuit from Tx to Rx (continuous signals) – widely used today Route packets from Tx to Rx – non-continuous signals - widely used today Temporary dedicated connection from Tx to Rx – up-and-coming Fall 2009

  5. Circuit-switched Network A circuit-switched network is made of a set of switches connected by physical links, in which each link is divided into n channels. • A circuit-switched network consists of a set of switches connected by physical links. • A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. • Each link is normally divided into n channels by using FDM or TDM. Fall 2009

  6. Circuit-switched Network Fall 2009

  7. Procedure • Phase 1: SETUP PHASE: before communication can occur, a dedicated circuit or connection must occur; Tx sends a setup request that includes the Rx address; each pair of switches between the Tx and Rx determines a channel it can devote to the overall route; then the Rx sends a ACK • Phase 2: DATA TRANSFER: After the Tx receives the ACK from the Rx, data transfer begins; the resources remain dedicated for the entire duration of data transfer until the teardown phase. • Phase 3: TEARDOWN: After the duration of data transfer, a signal is sent to each switch to release the resources. Fall 2009

  8. Efficiency Issue What happens when computers use a circuit-switch network ? Lost bandwidth (signals are bursty, not continuous) Delay in a circuit-switched network Although efficiency is an problem, delay at the switches is minimum due to the dedicated resources • Total Delay • Delay due to Tx request and Rx ACK • Delay due data transfer • Delay due connection tear down Fall 2009

  9. Delays in Circuit-Switch Network

  10. DATAGRAM NETWORKS • In data communications, we need to send messages from one end system to another. • If the message is going to pass through a packet-switched network, it needs to be divided into packets of fixed or variable size. • The size of the packet is determined by the network and the governing protocol. In a packet-switched network, there is no resource reservation; resources are allocated on demand. Fall 2009

  11. DATAGRAM NETWORKS Fall 2009

  12. A Datagram Network with Five Switches (routers) A switch in a datagram network uses a routing table that is based on the destination address. The destination address in the header of a packet in a datagram network remains the same during the entire journey of the packet. Fall 2009

  13. Delay in a Datagram Network Three transmission times plus three propagation delays Wait at each switch before being forwarded (wait times) Greater delay in the datagram type network versus the circuit-switch or virtual-circuit types Fall 2009

  14. VIRTUAL-CIRCUIT NETWORKS A virtual-circuit network is a cross between a circuit-switched network and a datagram network. It has some characteristics of both. • Like circuit-switch, there is a setup process between Tx and Rx • Instead of a continuous signal, data are packetized • Unlike datagram network, all packets follow the same path Fall 2009

  15. VIRTUAL-CIRCUIT NETWORKS

  16. Virtual-Circuit Identifier Both global and local addresses (VCI) are needed One VCI coming into switch, different VCI coming out Fall 2009

  17. Fall 2009

  18. Source-to-destination Data Transfer • In virtual-circuit switching, all packets belonging to the same source and destination travel the same path • However, the packets may arrive at the destination with different delays if resource allocation is on demand. Fall 2009

  19. Delay in a Virtual-circuit Network In addition to the regular datagram network delays, incur the setup and teardown delays like the circuit-switch case Fall 2009

  20. Switch Technologies • Space-Division Switch • Time-Division Switch Fall 2009

  21. Crossbar Switch • Uses micro-switches at each crosspoint in an N × M grid • Too many crosspoints are normally required Fall 2009

  22. STRUCTURE OF A Crossbar SWITCH Crossbar switch with three inputs and four outputs Only one row and column active for a connection Fall 2009

  23. Multistage Switch By having multiple paths in a switch, it decreases the number of crosspoints Fall 2009

  24. Three-stage Switch • In a three-stage switch, the total number of crosspoints is 2kN + k(N/n)2 • which is much smaller than the number of crosspoints in a single-stage switch (N2). • Where N – input lines K - # crossbars in middle stage Each group has n lines (divide N lines into groups)

  25. Time Division Switch Time-slot interchange Fall 2009

  26. Time-space-time switch (combo switch) Space-division switching (instantaneous) Time-division switching (no crosspoints) Fall 2009

  27. Packet Switch Components Fall 2009

  28. A Banyan Switch Fall 2009

  29. Example of Routing in a Banyan Switch

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