Signalling System No.7(SS7)

What is SS7?

SS7(Signalling System No. 7) is a global standard for telecommunications defined by the ITU (International Telecommunication Union) and Telecommunication Standardization Sector (ITU-T). The standard defines the procedures and protocol by which network elements in the public switched telephone network (PSTN) exchange information over a digital signalling network to effect wireless (cellular) and wireline call setup, routing and control.

The SS7 network and protocol are used for:

  • Basic call setup, management and tear down
  • Wireless services, wireless roaming and mobile subscriber authentication
  • Local number portability (LNP)
  • Toll-free and toll wireline services
  • Enhanced call features such as call forwarding, calling party name/number display and three-way calling

 Signalling Links:

 SS7 messages are exchanged between network elements over 56 or 64 kilobit per second (kbps) bidirectional channels called signalling links. Signalling occurs out-of-band on dedicated channels rather than in-band on voice channels. Compared to in-band signalling, out-of-band signalling provides:

  • Faster call setup times (compared to in-band signalling using multi-frequency (MF) signalling tones)
  • More efficient use of voice circuits
  • Support for Intelligent Network (IN) services, which require signalling to network elements without
  • voice trunks (e.g., database systems)
  • Improved control over fraudulent network usage

Signalling Points:

 Each signalling point in the SS7 network is uniquely identified by a numeric point code. Point codes are carried in signalling messages exchanged between signalling points to identify the source and destination of each message. Each signalling point uses a routing table to select the appropriate signalling path for each message.

  • SSP (Service Switching Point)
  • STP (Signal Transfer Point)
  • SCP (Service Control Point)

SSP (Service Switching Point):

SSPs are switches like (MSC) that originate, terminate or tandem calls. An SSP sends signalling messages to other SSPs to setup, manage and release voice circuits required to complete a call. An SSP may also send a query message to a SCP (HLR) to determine how to route a call. An SCP sends a response to the originating SSP containing the routing Number associated with the dialed number.

STP (Signal Transfer Point):

STP used for exchanging or forwarding signalling traffic. An STP routes each incoming message to an outgoing signalling link based on routing information contained in the SS7 message. Because it acts as a network hub, an STP provides improved utilization of the SS7 network by eliminating the need for direct links between signalling points. An STP may perform global title translation, a procedure by which the destination signalling point is determined from digits present in the signalling message.

SCP (Service Control Point):

SCP is a centralized database which contains all the subscriber information like profile (prepaid/post-paid), call facility services, supplementary service etc.During a call setup SSP send query to SCP to get the subscriber details and based on that call is proceed to next step


SS7 Protocol Stack:

SS7 protocol stack is mapped with Open Systems Interconnect (OSI) 7-layer model defined by the International Standards Organization (ISO).

Message Transfer Part (MTP):

The Message Transfer Part (MTP) layer of the SS7 protocol provides the routing and network interface capabilities that support SCCP, TCAP, and ISUP. Message Transfer part (MTP) is divided into three levels.

MTP Level 1 (Physical layer):

It defines the physical, electrical, and functional characteristics of the digital signalling link. Physical interfaces defined include E-1 (2048 kb/s; 32 64 kb/s channels), DS-1 (1544 kb/s; 24 64 kb/s channels), V.35 (64 kb/s), DS-0 (64 kb/s), and DS-0A (56 kb/s).

MTP Level 2(Data Link):

It provides the reliability aspects of MTP including error monitoring and recovery. (MTP-2) is a signalling link which together with MTP-3 provides reliable transfer of signalling messages between two directly connected signalling points.

MTP Level 3(Network Layer):

It provides the link, route, and traffic management aspects of MTP. MTP 3, thus ensures reliable transfer of the signalling messages, even in the case of the failure of the signalling links and signalling transfer points. The protocol therefore includes the appropriate functions and procedures necessary both to inform the remote parts of the signalling network of the consequences of a fault, and appropriately reconfigure the routing of messages through the signalling network.

MAP (Mobile Application Part):

Used to control queries to the different databases in the mobile radio network (HLR, VLR and EIR). MAP responsibilities include access and location management (e.g. where is the called subscriber location?), MSC-MSC handover, security functions, O&M, SMS and supplementary services.

TCAP (Transaction Capabilities Application Part):

Provides universal calls and functions for handling requests to distributed application processes.

ISUP (ISDN User Part):

Controls interworking (e.g. call setup/takedown) between PLMNs and other networks, and provides the same basic functionalities as TUP.

INAP (Intelligent Network Application Part):

Implements intelligent supplementary services (e.g. free call, time-dependent routing functions in a central service centre).

TUP (Telephone User Part):

Implements inter working between PLMNs and other networks. TUP is normally used to provide international connections and is slowly being replaced by ISUP.

Signalling Connection Control Part (SCCP):

The Signalling Connection Control Part (SCCP) layer of the SS7 stack provides 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.

For GT translation, STP (Signalling Transfer Point) is used in GSM network as an external entity. In performing GTT, an STP does not need to know the exact final destination DPC(Destination Point Code) of a message. It can, instead, perform intermediate GTT, in which it uses its tables to find another STP further along the route to the destination. That STP, in turn, can perform final GTT, routing the message to its actual destination.


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