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

20. 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

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

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

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

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

25. 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)

26. Time Division Switch Time-slot interchange Fall 2009

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

28. Packet Switch Components Fall 2009

30. A Banyan Switch Fall 2009

31. Example of Routing in a Banyan Switch