The document discusses the Public Switched Telephone Network (PSTN) architecture and components. It describes how PSTN uses digital trunks between central office switches and analog lines from phones to the central office. It also discusses digitization of voice signals, the major components of PSTN including local loops, trunks, and switching offices.

1. Public Switched Telephone Network (PSTN) BY M C KOLADIYA

6. Switching in public networks CSPDN: Circuit switched public data net* PSPDN: Packet switched public data net** DQDB: Distributed queue dual bus * Used by European Telecom’s that use X.21 in circuit switched nets **Used by British Telecom’s Packet-switched Service (PSS), Data Pac (Canada) ... Cell switching - resembles packet switching - difference: cells (packets) have a fixed size : offers bounded delay guarantees (QoS compatible, long packets won’t stuck cells)

7. Circuit switching Time switch - Makes switching between time slots - In the figure incoming slot 3 is moved to outgoing slot 3 for one voice direction - Each coming timeslot stored in Speech Store (SS) - Control store (CS) determines the order the slot are read from SS - The info in CS is determined during setup phase of the call Space switch - makes switching between PCM lines - works with electronic gates controlled by CS Cross-point controlled by CS Circuit switching - dedicated path - constant delay/bandwidth - voice/data - paid by time - examples: PSTN, VPN TDMA

9. Packet switching Node structure Packet structure Seq: sequence number Op code: message/control identifier CRC: Cyclic Redundancy Code Note: - source address required for retransmission in ARQ - byte count could be also an end flag

10. Connection-oriented and connectionless switching Connection oriented - Applies same route - QoS well defined - Phases - Connection setup - Data transmission - Release - Packets received in same order - Example: ATM, frame relay, PCM Connectionless - Use of different routes for each packet possible - Each packet have address fields - QoS not guaranteed - Packets may come in different order - Example: IP (Internet Protocol), TCP takes care of cleaning the mess

11. Transfer modes & connections summarized Circuit switching Packet switching Connection oriented Connectionless - hand-shaking - strict error requirements - for fast data transfer - broadcasting - modest error rates often accepted - fast data in good channels Transfer modes Connection types - developed for data - nowadays also for voice - Statistical multiplexing - variable delays - developed for voice - nowadays also for data - well-specified delays - echo problems PSTN ISDN PCM IP, Frame-relay ATM X.25 ATM Frame-relay IP, UDP* *User Datagram Protocol

14. Integrated Services Digital Network( ISDN ) Telephone network Digital data network Facsimile network Telex network Services and network integrations ISDN Digital technologies Computer control technologies

15. ISDN Basic Rate Interface (BRI) B B D 64 kbit/s 64 kbit/s 16 kbit/s Basic rate interface (2B+D) 144 kbit/s B ch for information transfer D ch for signaling transfer 2B+D

16. Primary Rate Interface (PRI) D B B B 30B+D 64 kbit/s 64 kbit/s 64 kbit/s 64 kbit/s Primary Rate Interface (2.048 Mbit/s) 30 B channels

17. Broadband ISDN Today’s N-ISDN Circuit switched network Packet switched network

18. To be integrated by B-ISDN ATM and Optical Fiber Network Broadband ISDN (continued)

19. ATM (Asynchronous Transfer Mode) Information (48 Bytes) Header (5Bytes) ATM Cell Structure Incoming data is chopped every 48 bytes, added a header then an ATM cell is formed. Transmission and switching are conducted by cell basis.

20. STM/ATM STM MUX # 1 # 4 # 3 # 2 STM MUX STM: Synchronous Transfer Mode # 1 # 4 # 3 # 2 ATM MUX

21. Example of cell switching: Distributed queue dual buss (DQDB) Function - transport units constant length - access units access info in the ring: - know subscribers in access unit’s subnets and route packets for them - access protocol applies token ring Properties - distributed switching (Compare to FDDI*) - ATM compatible - rates: 64 kb/s ... 45 Mb/s - geographical limit up to 200 km *FDDI: Fiber Distributed Data Interface see: http://www.iol.unh.edu/training/fddi/htmls/ Transport Unit (same as in ATM)

22. Connecting into PSTN exchange: Equipment in the access network ISDN connection example: 30B+D (2.048 Mb/s) Twisted pair - connection Private Branch Exchange Multiplexer On-line subscriber with several telephones Business subscriber Wireless access (or radio access point) Distribution point Cross connection point ISDN 2B+D 144 kb/s Q.512 specifies exchange interfaces

23. Local exchange Switch ETC: Exchange terminal circuit IN: Intelligent network conference calls, call waiting, broadcasting ... Signaling (SS7) with users and other exchanges Traffic concentration - Operation & maintenance support (Q.513) - Charging - Supplementary (IN) services - Subscriber data, switch control Control

24. Subscriber stage To ETC ETC: Exchange terminal circuit Speech store: shift registers storing bits for time switching Control store: gates guiding speech store switches Connects to: digit receivers, info tones, test equipment Control System: subscriber authentication, routing, billing, O & M, ... Concentrator internet access (DSLAM) centrex service MUX

30. PSTN ISDN exchange interfaces Q.512 ) NT:Network T. (in ISDN) T:Terminal V4 Peek to Q-recommendations

34. Connecting the local loop: Line interface circuit (LIC) Used for signaling in certain coin-operated pay-phones and PBX

37. The hybrid-circuit If the impedance Z b equals the line impedance no incoming voice (down right) leaks to outgoing voice (up right) but the signal goes via the two wire connection on the left To exchange From exchange Local loop

42. Modern PSTN hierarchy

44. Subscriber signaling for local calls

47. Example of inter-exchange signaling

48. Inter-exchange signaling (cont.)