11/21/991 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 T-S-T Switch DS0 lines T-S-T Switch T-S-T Switch T-S-T Switch Trunks T1 Bidirectional DS0 circuit

11/21/992 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).

11/21/993 SS7 network architecture STP T-S-T Switch Databases (e.g. calling card) STP: Signal Transfer Point: datagram router that forwards MTP L3 datagrams Databases (e.g. 800)

11/21/994 STP Switch SS7 protocol stack MTP L1 MTP L2 MTP L3 SCCP TCAP 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 SCCP ISUPISUP MTP L1 MTP L2 MTP L3

11/21/995 MTP L Network Identifier Network Cluster Network Cluster Member SLS Signaling link Selection DPC Destination Point Code OPC Originating Point Code Connectionless (datagram) NL protocol Addressing scheme: point code (14 or 24 bits) MTP L2 Header SLS OPC DPC MTP L3 Header ISUP or TCAP/SCCP message

11/21/996 Basic call setup (using ISUP) SSP DTMF Signaling IAM ACM RING RESPONSE ANM IAM Initial Address Message ACM Address Complete Message ANM Answer Message SSP STP IAM MTP L3 packet forwarding

11/21/997 Call setup with advanced services (using TCAP) Switch STP SCP Switch STP SCP STP SCP STP Local Carrier Long Distance ring STP: Signal Transfer Point SCP: Service Control Point Voice trunks Signaling links Long distance carrier ISUP IAM