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SS7 Introduction

    1. SS7 Overview
    2. SS7 References

     

    SS7 Overview

    The Common Channel Signaling System No. 7 (SS7 or C7) is a global standard that defines the procedures and protocols used to setup most of the world's public switched telephone network (PSTN) calls.The ITU definition of SS7 allows multiple national variants such as North America's American National Standards Institute (ANSI) and Europe's European Telecommunications Standards Institute (ETSI) standard.

    Each time you place and release a telephone call that extends beyond the local exchange, SS7/C7 signaling takes place.

    The SS7 network and protocol are used for:

    • To set up and tear down telephone calls
    • Number translation (LNP)
    • Toll-free (800) wireline services
    • Wireless services such as SMS

    SS7 messages are exchanged between two endpoints called signaling points on a 64 kpbs bi-directional channels (DS-0) known as signaling links. Each signaling point is uniquely identified by a numeric point code used to indentify the source and destination of each message.

    There are three types of signaling points in the SS7 network:

    • Service Switching Point (SSP)
      • Terminate signaling links
      • Start, end, and switch calls
    • Service Transfer Point (STP)
      • Main routing switches
    • Service Control Point (SCP)
      • Switches attached to databases

    SS7_sig_points.png

    Signaling links are categorized by link type ranging from A to F.

    SS7_link_types.png

    A Link (acces)

    Link between an SSP or SCP to an STP. Its purpose is to deliver signaling messages
    B Link (brigde)

    Link between 2 mated STPs
    C Link (cross) Link between 2 STPs making them a mated pair
    D Link (diagonal) Link between 2 mated STPs (different hierarchical levels)
    E Link (external) Link between an SSP and a secondary mated STP
    F Link (fully associated) Link between 2 SSPs

     

    SS7_stack.png

    The Message Transfer Part (MTP) consist of 3 layers

    MTP1 is the physical layers protocol that can be E-1 (2048 kbps, 32 x 64kbps), DS-1 (1544kbps, 24 x 64kbps), V.35 (64kbps), DS-0 (64kbps) and DS-0A (56kbps).

    MTP2 is the data link layer protocol that ensures reliable communications on a signaling link via error checking, flow control and sequence checking.

    MTP3 is the network layer protocol that ensures reliable communications to other nodes in the network via addressing, routing and congestion control.

    ISND User Part (ISUP) defines the protocols used to set-up, manage and release trunk circuits that carry voice and data on the PSTN.

    Signaling Connection Control Part (SCCP) provides connectionless and connection-oriented network services and global title translation (GTT) capabilities above MTP level 3. SCCP is used as the transport layer for TCAP-based services.

    Transaction Capabilities Applications Part (TCAP) supports the exchange of non-circuit related data between applications across the SS7 network using the SCCP connectionless service.  

     

    SS7 References

    1. Signaling System No 7
    2. Signaling System No 7:  MTP Stacks