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SS7 Protocol
SS7 History
• CCITT developed a digital signaling standard called Signaling System 6
• SS6 was based on Packet-Switched, proprietary data network.
– Uses 2.4 Kbps data links to send packets of data to distant switches to request service.
• SS7 began deployment in 1983, was initially used for inter office network, but now it is deployed in local central offices.
• Provide a global standard for call setup, routing, control and database access.
• SS7 Signaling Points Diagram
• Signaling Architecture Components
• SS7 Signaling Links
• Signaling links
Access links
– are used between SSP and STP or SCP and STP
– always two A links, one to each of the home STP pairs
– Maximum of 16 A links int one STP
• Bridge links
– connect mated STPs to other mated STPs at the same hierarchical level
• Cross links
– Used to connect STP to its mate STP. STPs are always deployed in pairs to maintain redundancy in the network
– Not used for routing, up to 8 C links between paired STPs
• Diagonal links
– used to connect mated STP pairs from one hierarchical level to another mated STP pair at a higher level
– Max. of 8 D links between mated STP pairs
• Extended links
– used to connect SSPs to remote STP pairs
– used to diversify and create redundancy
• Fully associated links
– used to route large amount of traffic between two SSPs also used when an SSP cannot be connected directly to an STP
• SS7 Protocol Stack
• Protocols
Message Transfer Part (MTP Level 1) Physical
• Provides an interface to the actual physical channel over which communication takes place
• CCITT recommends 64Kbps transmission whereas ANSI recommends 56 Kbps
• Protocols
MTP Level 2 (Data Link)
• Ensures accurate end-to-end transmission of a message across a signaling link
• Variable Length Packet Messages are defined here
• Implements flow control, message sequence validation, error checking and message retransmission
• Monitor links and reports their status
• Test links before allowing their use
• Provides sequence numbers for outgoing messages
• Protocols
MTP Level 3 (Network)
• Message routing between signaling points in the SS7 network
• Signaling network management that provides traffic, links and routing management, as well as congestion (flow) control
• Re-routes traffic away from failed links and signaling points, controls traffic when congestion occurs
• Protocols
Signaling Connection Control Part (SCCP)
• Provides connectionless and connection-oriented network services
• Provides global title translation (GTT) capabilities above MTP level 3; translates numbers to DPCs and subsystem numbers
• Provides more detailed addressing information than MTPs
• Used as transport layer for TCAP (Transaction capabilities applications part) based services
• Protocols
Transaction Capabilities Applications Part (TCAP)
• Exchange of non-circuit related data
– Between applications across the SS#7 network
– Using the SCCP service
• Queries and responses sent between Signaling Switching Point (SSPs) and Signaling Control Point (SCPs)
• Sends and receives database information
– Credit card validation
– Routing information
• Protocols
Telephone User Part (TUP)
• Basic call setup and tear down
• In many countries, ISUP has replaced TUP for call management
ISDN User Part (ISUP)
• Necessary messaging for setup and tear down of all circuits (voice and digital)
• Messages are sent from a switch, to the switch where the next circuit connection is required
• Call circuits are identified using circuit identification code (CIC)