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
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| 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
F Links:
Link between 2 SSPs “F” stands for
Fully Associated- used to connect SSPs when
significant traffic flows between them
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| SS7 Links |
