Signaling System 7

1. Signaling System 7 Chapter 4

2. SS7 • Used to perform out of band signaling on the PSTN • SS7 handles call establishment, exchange of info, routing, operations, billing • Used by VoIP • Provides link of VoIP to old voice infrastructure

3. SS7 • SSP – Service Switching Point – End office switches that connect voice services STP – Service Transfer Point – routes all signaling messages in the SS7 network • SCP – Service Control Point – provides access to databases for additional routing info

4. SSP • Telephone switches for SS7 • End office SSP’s originate and terminate calls • Core network switches provide transit calls • Provide circuit based signaling messages to other SSP’s for the purpose of connecting, disconnecting and managing calls

5. STP • STP route or switch all Signaling messages in the network, based on routing info • STP’s are configured in pairs for redundancy

6. SCP • Access databases • 800 database • Line Info Database

7. Signaling Links • Associated signaling – same path for signaling and voice. Seldom used in North America • Nonassociated signaling – separate path for signaling and voice. Most common • Quasi-associated signaling – separate path for signaling through the minimal number of transfer points to reach the final destination. Minimizes delay

8. SS7 Protocol Stack • p89

9. SS7 Protocol Stack • ISUP – Level 4 • SCCP – Level 4 • MTP L3 – Level 3 • MTP L2 – Level 2 • MTP L1 – Level 1

10. Physical layer MTP L1 • Defines the physical and electrical characteristics of the signaling link • T1 is the standard in North America • 1.544 Mbps • 24 full duplex channels • DS-0 – standard speed for digitizing voice conversations using pulse code modulation

11. Data Layer MTP L2 • Error Detection and Correction Uses CRC –Cyclical Redundancy Chk • FISU – Fill in Signal Unit packets fill in when no traffic is being sent on the network. Monitors the link even without traffic • This layer is used to create reliable point to point links between the endpoints in a network Message Signal Unit – Structure to carry information messages on the SS7 network

12. LSSU • Link Status Signal Unit • Provide link status information over the signaling links between two adjacent signaling endpoints • Identifies status of remote endpoint link interfaces and processors • Maintain link alignment and identify processor outage at an endpoint

13. Network Layer MTP3 • MTP3 protocol routes SS7 messages and relies on delivery of messages from MTP2 • SMH – Signal Message Handling – Routes SS7 messages during normal conditions. • SNM – Reroutes link traffic during failure conditions.

14. Signal Message Handling • SMH routes SS7 messages during normal conditions • SMH identifies whether the destination address is the receiving endpoint or whether the message needs to be routed.

15. MSU • MSU provides the structure for transmitting circuit and non-circuit based messages • Use circuit based messages set up manage and release telephone calls • Non-circuit based messages refer to queries for additional routing info • Transfer ISUP messages

16. Network Layer – MTP3 • Routes the SS7 messages and relies on the delivery of messages from MTP2. • SMH – routes SS7 signaling messages during normal conditions • SNM – reroutes link traffic during network failure

17. SCCP • Provides network services on top of MTP3 • Connectionless services – provides transport layer connectionless services for TCAP • 800, calling card and mobile applications

18. ISUP • ISUP connects, manages and disconnects all voice and data calls in the PSTN • ISUP sets up and tears down the circuits used to connect PSTN voice and data subscribers • End to End Signaling procedures establish, maintain and release connections

19. TCAP • Access remote databases • First used for 800 number translation • TCAP messages carry the instructions SCP’s use to query databases • TCAP routes messages from end to end • ISUP passes messages from switch to switch.

20. SS7 Examples • Book