An SS7 network is constructed from signaling points (SP) interconnected by point-to-point signaling links. Hardware reliability is provided by replicating STPs and SCPs. Reliability is also provided by using a mesh network to connect the SPs.
SPs are connected by access links (A-links) with the STPs. A-links are typically provided to both STPs of a mated pair. STPs in turn are interconnected by bridge (B) links and cross (C) links. Each mated pair of STPs is connected to one another by C-links and to other STPs with B-links. SCPs also use pairs of A-links to access a mated pair of STPs.
When four STPs are fully interconnected with B- and C-links, they create what is called the North American Quad. This structure is the predominant deployment of the STPs in major SS7 networks. This deployment option provides maximum reliability for the SS7 transport.
Other Link Designations
In addition to A-, B-, and C-links, there are three other link designators defined for the Signaling System 7 infrastructure.
The first is the D-link. These diagonal links are used to interconnect separate quads of STPs within a single hierarchical network. A large signaling network would be comprised of multiple levels of STP quads. The D-links would provide the connectivity between these levels.
E-links are extended links used to connect a service switching point or a service control point to a remote STP pair. These links provide additional reliability to an SP.
The third designator is the F-link. These fully extended links are used as a direct connection between SPs. F-links allow an SP to serve as a concentrator for other SPs and as a backup link for reliability purposes.