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William Stallings Data and Computer Communications

Learn about circuit switching technology, phases, space division switching, time division switching, routing strategies, alternate routing, control signaling functions, and different signaling modes in switching networks.

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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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