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Signaling System 7. Chapter 4. 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. SS7.
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Signaling System 7 Chapter 4
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
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
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
STP • STP route or switch all Signaling messages in the network, based on routing info • STP’s are configured in pairs for redundancy
SCP • Access databases • 800 database • Line Info Database
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
SS7 Protocol Stack • p89
SS7 Protocol Stack • ISUP – Level 4 • SCCP – Level 4 • MTP L3 – Level 3 • MTP L2 – Level 2 • MTP L1 – Level 1
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
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
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
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.
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.
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
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
SCCP • Provides network services on top of MTP3 • Connectionless services – provides transport layer connectionless services for TCAP • 800, calling card and mobile applications
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
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.
SS7 Examples • Book