SS7 entitiesFigure 9 illustrates a SS7 system and its relationship with the voice network.
Signal Switching Points (SSP) are hybrid devices that form part of the voice network as well as having an interface to the SS7 network. These are the central office switches which need to do a lookup to route a call. These run SS7 software. They refer a request for call routing to the SCP.Signal Control Points (SCP) maintain databases of routes, local or long distance, for the voice traffic to take. Therefore when the SSP needs to do a lookup on how to route the voice call, the SCP can provide the answer. In addition, the SCP's databases maintain records of 800/888/900 numbers, name and number pairs for call display and so on.
Signal Transfer Points (STP) are basically the routers used by the SS7 system. They forward requests from the SSPs to the SCPs. STPs make the SS7 network a packet switching network.