The most elementary signaling network consists of origination and destination signaling points (SP) connected by a signaling link (SL). One of the protocols used over the SL provides message error control (detection and correction) and correct message sequencing. SP functions include message routing and user functions if the SP is either an origination or destination point. If the origination and destination SPs are directly connected for all signaling relations, then the network operates in “associated mode.”
For technical or economic reasons, a simple associated network might not be suitable. Instead, a “quasi-associated” network could be implemented; information between the origination and destination SPs is transferred via a number of transit signaling points known as signaling transfer points (STP). STPs route messages from one SL to another.
An SP that originates or terminates signaling messages must also be capable of performing user functions. User functions can include deciding what type of signaling message to send, such as an integrated services digital network (ISDN) message or a transaction message associated with an enhanced service (e.g., 911E or 800 service).
The signaling links, signaling transfer points, and origination or destination signaling points can be combined in many different ways to form a signaling network.
Types of Signaling Points
The SSP is a telephone switch capable of connecting to the SS7 network and of understanding the SS7 protocol. It originates and terminates signaling messages for both call control and database query activities. The SSP function can be implemented in the telephone switch architecture or in an adjunct processor attached to the telephone switch. In either case, this signaling point is responsible for all interactions with the signaling network for this location.
An STP is an SP that routes signaling messages from one signaling link to another. It uses the lower layers of the SS7 protocol for routing and error control. Effectively, the STP is a packet switch that interfaces only with SS7 links, not voice or data trunks.
An SCP is a host computer and database that centralizes the call processing intelligence necessary to handle calls for advanced network services such as 800 service and alternate billing service. The database resides on a high performance transaction processing computer system. It is located in a telephone facility and can be collocated with an STP.