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Signalling Systems

Signalling Systems

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Signalling Systems

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  1. Signalling Systems • System which allows various network components to exchange information • In particular, it supports • call / connection control • network management

  2. Signalling Environments • Subscriber Signalling • Applied on Access Links • Tends to be only Call Control • Relatively Simple • e.g. DSS1 in ISDN • Network Signalling • In the network • Call Control and Network Control • Complex • e.g Signalling System 7 (SS7)

  3. Signalling • Types of signalling • in-band signalling • signalling messages carried in same channel as user information • e.g. • TCP/IP • analogue access links in telephone networks • out-band signalling • Separate signalling channel for signalling messages • e.g. • ISDN access links (Frame Relay) • SS7

  4. Signalling • Within out-band signalling another distinction can be made • Channel Associated Signalling (CAS) • Direct relationship between the signalling channel and information channel • Common channel signalling (CCS) • Signalling channel is shared on a demand basis

  5. Signalling • Modern signalling systems use out-band CCS • More secure (main reason why out-band signalling developed) • Signalling can be carried out at the same time as user information is exchanged • More efficient use of resources - allocated on a demand basis • Separate signalling network means less processing on messages - no need to separate signalling from user information • Signalling protocol can evolve independently, without effecting user application communication stack, can also be more complex. • At call set-up, signalling messages can be transmitted in parallel with the establishment of information channel (Not possible with CAS) • set-up times tend to be shorter

  6. Signalling • Signalling network is a separate network • Two modes of operation are possible with CCS • Associated mode • Same topology as information network • Signalling switches located with information switches • Non Associated mode • Combined signalling / information switches • Also, separate signalling switches called Signalling Transfer Points (STPs) • Result is a separate signalling network topology, with links and interfaces into underlying information network

  7. Signalling • Two signalling standards which operate • Subscriber signalling • e.g. Digital subscriber system 1 • Network signalling • Signalling System 7 (SS7)

  8. Signalling

  9. Signalling • SS7 • Concerned with connection set-up and network management • Defined in Q700 series • Network components are • Signalling Points • Process signalling messages • Signalling Transfer Points • SPs with switching facilities • Signalling links

  10. Signalling • SS7 Protocol architecture • Message Transfer Part • 3 layers, which provide a reliable, but connectionless service • similar in functionality to IP • i.e. provides carrier for higher layers • Signalling Data Link • typical physical layer functionality • Signalling Link Layer • provides typical layer 2 functionality, i.e. error free link • Signalling Network Layer • Signalling Message Functions • Switching of signalling messages across the network • Signalling Network Management Functions • traffic, link and route management functions

  11. Signalling • Signalling Connection Control Part (SCCP) • Equivalent to upper layer 3 • (so concerned with routing) • Augments the basic service provided by MTP • A range of services • 2 connectionless services • 2 connection-oriented services

  12. Signalling • User Parts • Equivalent to layer 4 • These typically relate to the control of specific types of calls • e.g. ISDN User Part • Used to set-up ISDN calls

  13. Signalling • Transaction Capabilities Part (TCAP) • Used to • execute remote processes • provide means for transfer of data between nodes • Used extensively in Intelligent Networks (INs) • Essentially application layers

  14. Signalling • Consider how SS7 supports setting up a call • Consider how SS7 supports the 0800 service