Explain SS7 Protocols Stack ?

SS7 Protocols Stack

The standard SS7 protocol has 4 levels (layers) as defined in the OSI 7 Layer Model. The levels 1 to 3 constitute the Message Transfer Part (MTP) and level 4 is the User Part (Transport Layer in OSI).

• MTP1 = Message Transfer Part 1
• MTP2 = Message Transfer Part 2
• MTP3 = Message Transfer Part 3
• SCCP = Signaling Connection Control Part
• TCAP = Transaction Capabilities Application Part
• MAP = Mobile Application Part
• INAP = Intelligent Network Application Part
• ISUP = ISDN User Part

1.) 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

2.) 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

3.) 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

4.) 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

5.) 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

6.) Telephone User Part (TUP)

• Basic call setup and tear down
• In many countries, ISUP has replaced TUP for call management

7.) 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)