IT3111: Telecom and Computing

1. IT3111: Telecom and Computing Lecture 7: The PSTN - Part 3:

2. Contents • Telephone Exchanges • The structure • Tasks • Switching fabric • SS#7 • Signaling • SS#7 elements Telecom and Computing

3. Telephone exchanges • Subscriber lines are connected to the telephone exchanges – Originally each customer had a direct connection (electrical pair) to the exchange • Current exchanges are complex and expensive – A need to reduce their number • Physical cabling becomes an obstacle – Concentrators are used to reduce the number of cables • As the number of telephone exchanges is • decreasing, the hierarchy of telephone exchanges • is flattening Telephone exchanges Telecom and Computing

4. The structure of a telephone exchange a) Control unit b) I/O c) Switching fabric Telecom and Computing

5. The most important tasks fortelephone exchanges • Connecting to subscriber terminal equipment and other exchanges • Telephone number analysis and translation • Collecting accounting data for billing • Logging data for the use of authorities • Switching calls Telecom and Computing

6. I/O • I/O circuitry connects the telephone exchange to the outside world. The I/O is arranged in hotswappable I/O cards. • There are several types of I/O cards (part of which are also used in concentrators) – subscriber line cards (which contains circuits for connecting several subscribers) – PDH/SDH/ATM cards Telecom and Computing

7. Subscriber line cards haveseveral tasks: • [B] Giving power to the terminal equipment (battery) • [O] Over voltage protection • [R] Ringing • [S] Supervising the status of the terminal equipment (e.g. if the handle is on or off hook in an old style telephone) • [C] Coder and Decoder (A/D or D/A converters, A-law/m-law (de)coding ) • [H] Hybrid - a transformer between two half-duplex and one full-duplex connection • [T] Testing the status of the subscriber line • Memory rule; BORSCHT, the red soup. Telecom and Computing

8. Switching fabric • The task of the switching fabric is to transmit the PCM streams between two I/O ports. • Both Time and Space Division multiplexing are used. • One measure of the size of the exchange is the capacity of its switching fabric. • It is easy to find books describing design of switching fabrics – in a sense it is one of the "easy'' but interesting parts of the telephone exchanges. We will cover the structure of the switching fabrics only briefly; • Two main types of switching fabric are used: the crossbar • switch and the interconnection network. Telecom and Computing

9. Crossbar switches • Often used in small scale exchanges • Usually has an electronic switch for each line – To connect n lines, n^2 switches are needed – A typical space division switch – Uses physical space for connections – Solid state technology Telecom and Computing

10. Interconnection Network • • Used in larger switches • – Different types available • • Interconnection networks are made from a number of smaller switching elements. • • The idea in interconnection networks is to save in the number of crossbars using special topologies.

11. Time Division Switching • Time Division Multiplexing is used in combination with the above mentioned structures. There are different structures: Time-Space, Space-Time, Time-Space-Time etc. • The idea in Time division Multiplexing is to rearrange the time slots in a PCM frame (E1). • Time slot interchanger (TSI):

12. Time-Space-Time switching • • In Time-Space-Time switching, the internal crossbar works at a higher rate than the input line rate • – Any time slot can be switched to any time slot • – The crossbar configuration changes for each time slot

13. Signaling • • The network made of exchanges and links must be co- ordinated in order to make calls possible • • The purpose of signaling is just to do this: it • – establishes calls, • – monitors the calls, and • – tears down the calls. • • In-band signaling was used earlier • – MF tones waste of resources • – Security problems • • A new signaling system was created: • – Common Channel Signaling Subsystem #7 Telecom and Computing

14. Common Channel SignalingSubsystem #7 • • SS#7 is a packet switched network on top of the telephone • network • • It is used to transport signaling information • – It controls the trunk connections • – SS#7 does not transport user data • • SS#7 packets use either a dedicated 56 or 64kbps channel • (such as E1 TS16) or dedicated links. • • If signaling does not work, the whole telephone network is • "dead''. Thus, stringent reliability requirements. • – There is a lot of redundancy; every part of the signaling system has at least one spare Telecom and Computing

15. SS#7 elements SS#7 overlay network: Operator A Operator B • The actual telephone exchanges switching the actual telephone traffic are called Signal Switching Points (SSP) (circles). • The packet switches (routers) of the SS#7 network are called Signal Transfer Points (STP) (squares with a diagonal). • The databases for advanced call-processing capabilities are called Signal Control Points (SCP) (cylinders).

16. SS#7 Protocol Stack • • SS#7 uses a packet switched protocol set to carry its mission. • • The protocol stack generates a packet switched network over a circuit switched one • • MTP-x protocols (Message Transfer Part) protocols provide transport of SS#7 messages • – MTP-2 is the link level protocol • – MTP-3 provides network level functionality • – Routing to specific elements Telecom and Computing

17. SS#7 Protocol Stack Telecom and Computing

18. SS#7 Protocol Stack • The upper parts (layers) provide functions for call management – Telephone User Part (TUP) for traditional telephone signaling – Data User Part (DUP) for datacom users – ISDN User Part (ISUP) and Broadband ISDN User Part (BISUP) for ISDN and ATM users. Telecom and Computing

19. Signaling ConnectionControl Part (SCCP) • Provides connections for the parts above it • Incremental routing – Global Title Translation, GTT (compare to DNS) • Sub-addressing for applications within an element – SSN = Subsystem Number (compare to ports in UDP and TCP). • Four classes of service available: – [0] basic connectionless class, – [1] sequenced connectionless class (messages arrive in order), – [2] basic connection oriented class, and – [3] flow control connection oriented class. Telecom and Computing

20. SS#7 Protocol StackAbove SCCP • Operations, Maintenance and Administration Part (OMAP) provides services for the operator – Include updating routing tables, and diagnostics • Transactions Capabilities Application Part (TCAP) provides database transaction service for the upper level protocols (such as INAP). Telecom and Computing

21. SS#7 Environment Adaptation • SS#7 is used for the core network • For the access network, there are different signaling stacks: • For ISDN: LAPB/LAPD corresponding MTP-2 on top of which Q.931 corresponding to ISUP. • For B-ISDN: – MTP layers 1-2 can be replaced by a specific Signaling AAL protocol (SAAL) – Three layer protocol: bottom: AAL5, then SSCOP (Service Specific Connection Oriented Peer-to-Peer Protocol), then SSCF (Service Specific Coordination Function). SSCOP+SSCF = Service Specific Convergence Sublayer. – BISUP can be replaced with ATM Forum UNI or ITU-T Q.2931. Telecom and Computing