SS7 signaling links are characterized according to their use in the signaling network. Virtually all links are identical in that they are 56–kbps or 64–kbps bidirectional data links that support the same lower layers of the protocol; what is different is their use within a signaling network. The defined link types are shown in Figure 5 and defined as follows:

Figure 5. Link Types

A Links

A links interconnect an STP and either an SSP or an SCP, which are collectively referred to as signaling end points ("A" stands for access). A links are used for the sole purpose of delivering signaling to or from the signaling end points (they could just as well be referred to as signaling beginning points). Examples of A links are 2–8, 3–7, and 5–12 in Figure 5.

Signaling that an SSP or SCP wishes to send to any other node is sent on either of its A links to its home STP, which, in turn, processes or routes the messages. Similarly, messages intended for an SSP or SCP will be routed to one of its home STPs, which will forward them to the addressed node over its A links.

C Links

C links are links that interconnect mated STPs. As will be seen later, they are used to enhance the reliability of the signaling network in instances where one or several links are unavailable. "C" stands for cross (7–8, 9–10, and 11–12 are C links). B links, D links, and B/D links interconnecting two mated pairs of STPs are referred to as either B links, D links, or B/D links. Regardless of their name, their function is to carry signaling messages beyond their initial point of entry to the signaling network towards their intended destination. The "B" stands for bridge and describes the quad of links interconnecting peer pairs of STPs. The "D" denotes diagonal and describes the quad of links interconnecting mated pairs of STPs at different hierarchical levels. Because there is no clear hierarchy associated with a connection between networks, interconnecting links are referred to as either B, D, or B/D links (7–11 and 7–12 are examples of B links; 8–9 and 7–10 are examples of D links; 10–13 and 9–14 are examples of interconnecting links and can be referred to as B, D, or B/D links).

E Links

While an SSP is connected to its home STP pair by a set of A links, enhanced reliability can be provided by deploying an additional set of links to a second STP pair. These links, called E (extended) links provide backup connectivity to the SS7 network in the event that the home STPs cannot be reached via the A links. While all SS7 networks include A, B/D, and C links, E links may or may not be deployed at the discretion of the network provider. The decision of whether or not to deploy E links can be made by comparing the cost of deployment with the improvement in reliability. (1–11 and 1–12 are E links.)

F Links

F (fully associated) links are links which directly connect two signaling end points. F links allow associated signaling only. Because they bypass the security features provided by an STP, F links are not generally deployed between networks. Their use within an individual network is at the discretion of the network provider. (1–2 is an F link.)