Wednesday, February 23, 2011

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)