Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
PSTN Bidirectional DS0 circuit DS0 lines T-S-T Switch T-S-T Switch Trunks T1 • • T-S-T Switch T-S-T Switch 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 11/21/99 1 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). 11/21/99 2 SS7 network architecture Databases (e.g. 800) T-S-T Switch T-S-T Switch STP T-S-T Switch STP T-S-T Switch T-S-T Switch T-S-T Switch STP STP T-S-T Switch 11/21/99 T-S-T Switch STP: Signal Transfer Point: datagram router that forwards MTP L3 datagrams Databases (e.g. calling card) 3 SS7 protocol stack Switch I S U P TCAP SCCP 11/21/99 STP SCCP MTP L3 MTP L3 MTP L2 MTP L2 MTP L1 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 4 MTP L3 • Connectionless (datagram) NL protocol • Addressing scheme: point code (14 or 24 bits) MTP L3 Header MTP L2 SLS Header 8 11/21/99 OPC 24 DPC 24 Network Identifier Network Cluster Network 8 8 8 Cluster Member ISUP or TCAP/SCCP message SLS Signaling link Selection DPC Destination Point Code OPC Originating Point Code 5 Basic call setup (using ISUP) SSP DTMF Signaling SSP SSP STP MTP L3 packet forwarding 11/21/99 SSP IAM Initial Address Message ACM Address Complete Message ANM Answer Message 6 Call setup with advanced services (using TCAP) 516-555-1278 Local Carrier SCP Long distance carrier STP STP ISUP IAM Switch Local Carrier Switch Long Distance SCP STP STP Switch Switch Signaling links Voice trunks 800-555-4400 11/21/99 STP: Signal Transfer Point SCP: Service Control Point SCP ring 516-555-1278 7