Signaling Links and Linksets

The signaling links in the SS7 network are identified by the function provided to the corresponding endpoints, as illustrated in Figure 4-6.

Figure 4-6. Signaling Links

Figure 4-6. Signaling Links

The following list outlines the six types of links present in the SS7 network:

• A-links are interconnects between signaling endpoints and STPs, as illustrated in Figure 4-6. The signaling endpoints are SSPs or SCPs, and each has at least two A-links that connect to the "home" STP pair. It is possible to have only one A-link to an STP; however, this is not common practice. These links provide access to the network for the purposes of transmitting and receiving signaling messages. The STP routes the A-link signaling messages received from the originating SSP or SCP toward the destination.

• Bridge Links (B-links) are interconnects between two mated pairs of STPs, as illustrated in Figure 46. These mated STPs are peers operating at the same hierarchical level and are interconnected through a quad of B-links. B-links carry signaling messages from the origin to the intended destination.

• Cross Links (C-links) interconnect an STP with its mate, as illustrated in Figure 4-6. The STP pairs perform identical functions and are mated to provide redundancy in the network. C-links are used only when failure or congestion occurs, causing these links to become the only available path to the network. Under normal conditions, these links carry only management traffic.

• Diagonal Links (D-links) are used to interconnect mated STP pairs of one hierarchical level to mated STP pairs of another hierarchical level, as illustrated in Figure 4-7. The D-links are connected in a quad-like fashion similar to B-links. These links provide the same function as B-links; the distinction between B- and D-links is arbitrary.

Figure 4-7. D-Links Interconnect Mated STPs on Different Hierarchical Levels

Figure 4-7. D-Links Interconnect Mated STPs on Different Hierarchical Levels

• Extended Links (E-links) are used to interconnect an SSP to an alternate STP, as illustrated in Figure 4-6. The SSP also is connected to the home STP pair through A-links; however, if more reliability is required, you can implement E-links. This is not common practice, however, because the SSPs have dual A-links to redundant mated STP pairs. These links are used only if failure or congestion occurs in the home STPs.

• F-links are used to directly interconnect two signaling endpoints, as illustrated in Figure 4-6. These links are used when STPs are not available or when high traffic volumes exist. This is the only link type whose signaling traffic is allowed to follow the same path as the voice circuits. The signaling messages between the two signaling endpoints are associated only with the voice circuits directly connected between the two signaling endpoints. This method is not commonly used in North America; it is common in Europe, however.

Signaling links are grouped together into linksetswhen the links are connected to the same endpoint. Signaling endpoints provide load sharing across all the links in a linkset. Combined linksets are used when connecting to mated STP pairs with different point code addresses. In this case, links are assigned to different linksets and are configured as a single combined linkset.

Load sharing across combined linksets occurs when signaling endpoints re-address the messages to adjacent point codes. You can configure alternate linksets to provide redundant paths, increasing reliability over other signaling links such as E- and F-links, as described later in this section.

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