Learn Telecom: SS7 and Its Links

SS7 and Its Links

The SS7 standard defines the procedures and protocol by which network elements in the public switched telephone network (PSTN) exchange information over a digital signaling network to enable wireless (cellular) and wireline call setup, routing, and control.

Signaling Points:

All nodes in the SS7 network are called Signaling Points (SPs). Each SP is identified by a unique address called a Point Code (PC). SPs have the ability to read a Point Code and determine if the message is for that node and the ability to route SS7 messages to another SP. Each signaling point in the SS7 network is uniquely identified by a numeric point code. Point codes are carried in signaling messages exchanged between signaling points to identify the source and destination of each message. Each signaling point uses a routing table to select the appropriate signaling path for each message. There are three kinds of signaling points in the SS7 network

• SSP (Service Switching Point or Signal Switching Point)

• STP (Signal Transfer Point)

• SCP (Service Control Point

SSP: SSPs are switches

SCP: SCP provides application access. It is an interface to applications such as databases

STP: STP is a router and/or a gateway in the SS7 network. Messages are not originated by an STP. STPs switch SS7 messages between signaling points. If an originating SSP does not know the address of a destination SSP, the STP must provide it using Global Title Translation. Gateway STPs serve as the interface into another network and they can provide protocol conversion. STPs also provide traffic and usage measurements

Functions of STP:-

Receives the MSU's & direct them to appropriate destination.
Network Management.
(ANSI to ITU) or (ITU to ANSI) protocol conversion.
Global Title Translation (GTT).
Measurement of Data.
Gateway Function.
Gateway Screening (GWS)
Local Number Portability (LNP).

Different Links used with STP :-

A Links:

It used to connect SSPs to STPs or STPs to SCPs. “A” stands for Access. Purpose is to deliver signaling messages. Connects an SSP to an STP (or) an SCP to an STP

B Links:

Link between 2 mated STP pairs. “B” stands for Bridge Link (used to connect paired STPs to other paired STPs

A-link and B-link

C Link:

Link between 2 STPs (making them a “mated” pair). “C” stands for Cross Link connects STPs performing identical functions into a mated pair

D Links:

Link between 2 mated STP pairs (different hierarchical levels). “D” stands for Diagonal Link- used to connect paired STPs at one in the hierarchy to paired STPs at another level in the hierarchy

Typically, a quad of "B" links interconnect peer (or primary) STPs (e.g., the STPs from one network to the STPs of another network). "D" (diagonal) link connects a secondary (e.g., local or regional) STP pair to a primary (e.g., inter-network gateway) STP pair in a quad-link configuration. Secondary STPs within the same network are connected via a quad of "D" links. The distinction between a "B" link and a "D" link is rather arbitrary. For this reason, such links may be referred to as "B/D" links.

E Links:

Link between an SSP and a secondary mated STP. “E” stands for Extended Link- It connects a SSP to other STP other than its Home STP & provides an alternate route for SS7 messages if congestion occurs at home STP