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Different Types of Signalling Points
There are three kinds of signaling points in the SS7 network (Fig. 1):
- SSP (Service Switching Point)
- STP (Signal Transfer Point)
- SCP (Service Control Point)
Service Switching Point(SSP)
SSPs are switches that originate, terminate, or tandem calls. An SSP sends signaling 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 centralized database (an SCP) to determine how to route a call (e.g., a toll-free 1-800/888 call in North America). An SCP sends a response to the originating SSP containing the routing number(s) associated with the dialed number.
An alternate routing number may be used by the SSP if the primary number is busy or the call is unanswered within a specified time. Actual call features vary from network to network and from service to service.
Because signaling messages will either originate or terminate within them, SSPs are considered "end points" on a network.
To Summarize, the core function of an SSP is to process calls that originate, tandem, or terminate at a specific office. For any given call, the SSP may generate signaling messages to transfer call-related information to other SSPs, or may send a query to an SCP database for instructions on routing the call.
Signal Transfer Point(STP)
Network traffic between signaling points may be routed via a packet switch called an STP. An
STP routes each incoming message to an outgoing signaling 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 signaling points.
An STP may perform global title translation, a procedure by which the destination signaling point is determined from digits present in the signaling message (e.g., the dialed 800 number, calling card number, or mobile subscriber identification number). An STP can also act as a "firewall" to screen SS7 messages exchanged with other networks.
An STP is a packet switch that acts as a hub to transfer messages to other STPs, SCP databases, or SSPs.
Since an STP's core function is to route signals, the need for direct connections between end points on a network is eliminated. The STP routes messages by checking routing information attached to each signaling message and sending it (via the proper link) to the desired signaling point.
Service Control Point(SCP)
SCP is signaling node with database facility & intelligence to control the signalling traffic.e.g. HLR & IN Node are SCPs.