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PSTN

Bidirectional DS0 circuit. PSTN. A T1 (1.5 Kbps) is a time-multiplexed signal of 24 DS0 (64 kbps) A T-S-T (Time-Space-Time) division multiplexed switch maps bits arriving in one time slot to another time slot on an output port. DS0 lines. T-S-T Switch. T-S-T Switch. T-S-T Switch. T-S-T

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PSTN

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  1. Bidirectional DS0 circuit PSTN • A T1 (1.5 Kbps) is a time-multiplexed signal of 24 DS0 (64 kbps) • A T-S-T (Time-Space-Time) division multiplexed switch maps bits arriving in one time slot to another time slot on an output port DS0 lines T-S-T Switch T-S-T Switch T-S-T Switch T-S-T Switch Trunks T1

  2. Role of SS7 network • To set up a phone call, messages are sent from one T-S-T switch to the next until the called party is reached. • These messages are called ISUP messages (see protocol slide) and sent through a packet-switched connectionless network called the SS7 network. • The packet switches in the SS7 network are called STPs, which perform MTP L3 datagram forwarding • STPs are comparable to IP routers; the network layer protocol they operate on is MTP L3 instead of IP; the addresses they use are called Point Codes. • The SS7 network is also used to carry TCAP messages (see protocol slide) to provide value-added services, such as 800-number, calling card calls, etc. These messages are sent from the switches to SCPs. • With the help of the SS7 network, the PSTN is now considered an “Intelligent Network,” allowing the network to offer value-added services beyond the basic phone call service (called Plain Old Telephone Service or POTS).

  3. Databases (e.g. calling card) SS7 network architecture Databases (e.g. 800) T-S-T Switch T-S-T Switch STP STP T-S-T Switch T-S-T Switch T-S-T Switch T-S-T Switch STP STP T-S-T Switch T-S-T Switch STP: Signal Transfer Point: datagram router that forwards MTP L3 datagrams

  4. Switch STP IS U P MTP L3 MTP L2 MTP L1 SS7 protocol stack TCAP SCCP SCCP MTP L3 MTP L2 MTP L1 MTP L3-L1 Message Transfer Part Level 3 to Level 1 TCAP Transaction Capabilities Application Part SCCP Signaling Connection Control Part ISUP ISDN User Part

  5. MTP L3 Header SLS OPC DPC SLS Signaling link Selection DPC Destination Point Code Network Identifier Network Cluster Network Cluster Member OPC Originating Point Code 8 8 8 MTP L3 • Connectionless (datagram) NL protocol • Addressing scheme: point code (14 or 24 bits) MTP L2 Header ISUP or TCAP/SCCP message 8 24 24

  6. SSP SSP MTP L3 packet forwarding Basic call setup (using ISUP) DTMF Signaling SSP ANM ANM RING ACM IAM ACM RESPONSE IAM STP IAM Initial Address Message ACM Address Complete Message ANM Answer Message SSP IAM

  7. 516-555-1278 ISUP IAM Long distance carrier 516-555-1278 800-555-4400 800-555-4400 800-555-4400 Call setup with advanced services (using TCAP) SCP STP STP Local Carrier Local Carrier Switch Switch Long Distance SCP STP SCP STP Switch Switch ring Signaling links Voicetrunks 516-555-1278 STP: Signal Transfer Point SCP: Service Control Point

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