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William Stallings Data and Computer Communications

William Stallings Data and Computer Communications. Chapter 9 Circuit Switching. Switching Networks. Long distance transmission is typically done over a network of switched nodes Nodes not concerned with content of data End devices are stations Computer, terminal, phone, etc.

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William Stallings Data and Computer Communications

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  1. William StallingsData and Computer Communications Chapter 9 Circuit Switching

  2. Switching Networks • Long distance transmission is typically done over a network of switched nodes • Nodes not concerned with content of data • End devices are stations • Computer, terminal, phone, etc. • A collection of nodes and connections is a communications network • Data routed by being switched from node to node

  3. Nodes • Nodes may connect to other nodes only, or to stations and other nodes • Node to node links usually multiplexed • used for several connections • Some redundant connections are desirable for reliability • Two different switching technologies • Circuit switching • Packet switching

  4. Simple Switched Network

  5. Circuit Switching • Dedicated communication path between two stations • Three phases • Establish • Transfer • Disconnect • Must have switching capacity and channel capacity to establish connection • Must have intelligence to work out routing

  6. Circuit Switching • Inefficient • Channel capacity dedicated for duration of connection • If no data, capacity wasted • Set up (connection) takes time • Once connected, transfer is at fixed rate with almost no delay • Developed for voice traffic (phone)

  7. Public Circuit Switched Network

  8. Blocking or Non-blocking • Blocking • A network is unable to connect stations because all paths are in use • Used on voice systems • Short duration calls • Non-blocking • Grants all connection requests at once • Used for data connections

  9. Space Division Switching • Initially developed for analog environment • is also used for digital • Separate physical paths • Crossbar switch • connection requires establishment of a dedicated path through a switch • Number of crosspoints = square of number of stations • Loss of crosspoint prevents connection • Inefficient use of crosspoints • All stations connected, only a few crosspoints in use • Non-blocking

  10. Crossbar Matrix

  11. Multistage Switch • Interconnect several crossbar switches • Reduced number of crosspoints • More than one path through network • Increased reliability • More complex control • multiple gates are controlled • May be blocking

  12. Three Stage Switch

  13. Time Division Switching • low speed bit streams share higher speed stream • e.g. TDM bus switching • based on synchronous time division multiplexing • Each station connects through controlled gates to high speed bus • Time slot allows small amount of data onto bus • Another line’s gate is enabled for output at the same time • Control Unit • can be blocking or non-blocking

  14. Routing • Many connections will need paths through more than one switch • Need to find a route • cost-effective • resilient • trade-off between them • Public telephone switches are a tree structure • Static routing uses the same approach all the time • Dynamic routing allows for changes in routing depending on traffic • Uses a peer structure for nodes

  15. Alternate Routing • Possible routes between end offices predefined • Routes listed in preference order • Different sets of routes may be used at different times

  16. Alternate Routing Diagram

  17. Control Signaling Functions • Audible communication with subscriber (dial, busy tone, etc.) • Transmission of dialed number • Call can not be completed indication • Call ended indication • Signal to ring phone • Billing info • Equipment and trunk status info • etc.

  18. In Channel Signaling • Use same channel for signaling and call • Requires no additional transmission facilities • Inband • Uses same frequencies as voice signal • Can go anywhere a voice signal can • Out of band • Voice signals do not use full 4kHz bandwidth • Narrow signal band within 4kHz used for control • Can be sent whether or not voice signals are present • continuous supervision of call • Need extra electronics • Slower signal rate (only simple control messages)

  19. Drawbacks of In Channel Signaling • Can support only simple signaling • Delay between entering address (dialing) and connection • long setup time • Interference between subscriber and control signals

  20. Common Channel Signaling • Control signals carried over paths independent of voice channel • Associated Mode • Common channel closely tracks • Disassociated Mode • Additional nodes (signal transfer points) • Effectively two separate networks • Advantages • less interference between subscriber and control signal • reduced call setup time • control messages do not need to wait circuit establishment • dissassociated mode: central STPs allow centralized control

  21. Signaling Modes

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