CCS7 network architecture and terminology

CCS7 network architecture

Common Channel Signaling No. 7 (CCS7 / SS7) uses a separate packet-switched signaling network to carry call-control and database messages between exchanges. The speech network and signaling network are separate but linked through the exchanges.

       Signaling plane
  [SCP]                    [SCP]
    | A                      | A
  [STP]--------B/D--------[STP]
    | \ C                  / |
    |  [STP]--------[STP]    |
   A|                        |A
  [SSP]=======trunks=======[SSP]
       Speech (transport) plane

• Exchanges (SSPs) are connected to a mated pair of STPs by A-links.
• STP pairs are joined to each other by B- or D-links and internally by C-links.
• SCPs (databases) hang off STPs.
• Heavy routes may have direct F-links between SSPs.
• Everything is duplicated so that a single link or node failure does not stop signaling.

CCS7 terminologies

• Signaling Point (SP): any node that sends or receives SS7 messages; identified by a point code (14 bits in ITU).
• SSP (Service Switching Point): an exchange with SS7 capability; originates and terminates calls.
• STP (Signal Transfer Point): routes messages between links without being their origin or destination.
• SCP (Service Control Point): database answering queries (toll-free, prepaid, HLR).
• Signaling link: a 64 kbps bidirectional data link between two SPs.
• Link set: group of links between the same two SPs (up to 16).
• Route / route set: the sequence of link sets used to reach a destination.
• Link types: A (access), B (bridge), C (cross), D (diagonal), E (extended), F (fully associated).
• Signaling modes: associated (direct link alongside trunks), quasi-associated (via STPs on fixed route), non-associated (via STPs, route may vary).
• OPC / DPC / SLS: originating point code, destination point code and signaling link selection, forming the routing label.
• CIC (Circuit Identification Code): identifies the speech trunk to which an ISUP message refers.
• Signal units: MSU (message signal unit, carries user data), LSSU (link status), FISU (fill-in, keeps link alive).
• MTP (Message Transfer Part): levels 1–3 giving reliable transport and routing.
• User parts: ISUP (call control), TUP, SCCP (global title translation), TCAP (transactions), MAP, INAP.
• Global Title (GT): an address such as a dialled number that the STP translates into a point code and subsystem.