PSTN Bidirectional DS 0 circuit DS 0 lines

PSTN Bidirectional DS 0 circuit DS 0 lines T-S-T Switch Trunks T 1 • • T-S-T Switch A T 1 (1. 5 Kbps) is a time-multiplexed signal of 24 DS 0 (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 SS 7 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 SS 7 network. The packet switches in the SS 7 network are called STPs, which perform MTP L 3 datagram forwarding STPs are comparable to IP routers; the network layer protocol they operate on is MTP L 3 instead of IP; the addresses they use are called Point Codes. The SS 7 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 SS 7 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

SS 7 network architecture Databases (e. g. 800) T-S-T Switch STP T-S-T Switch 11/21/99 T-S-T Switch STP: Signal Transfer Point: datagram router that forwards MTP L 3 datagrams Databases (e. g. calling card) 3

SS 7 protocol stack Switch I S U P TCAP SCCP 11/21/99 STP SCCP MTP L 3 MTP L 2 MTP L 1 MTP L 3 -L 1 Message Transfer Part Level 3 to Level 1 TCAP Transaction Capabilities Application Part SCCP Signaling Connection Control Part ISUP ISDN User Part 4

MTP L 3 • Connectionless (datagram) NL protocol • Addressing scheme: point code (14 or 24 bits) MTP L 3 Header MTP L 2 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 SSP M N M C A MTP L 3 packet forwarding 11/21/99 IAM SSP E NS O SP M IA STP NG RI RE IAM AN M AC M A DTMF Signaling 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 STP -44 -55 5 800 STP Long Distance 1278 SCP Switch 555 - 00 Switch Local Carrier 516 - Long distance carrier STP ISUP IAM STP SCP 0 40 -4 5 5 -5 0 0 Switch Signaling links Voice trunks 8 800 -555 -4400 11/21/99 STP: Signal Transfer Point SCP: Service Control Point Switch ring 516 -555 -1278 7