Signaling Links and Link Sets

Signaling links are physical connections between SS7 nodes and are responsible for the reliable transmission of SS7 signaling messages between those nodes, as shown in Figure 2-9. Signaling links can be configured so that they load share traffic over any or all of the links in a designated link set between signaling points. Even distribution of traffic is only achieved if there are several signaling links of the power 2 (2, 4, 8, and so on).

Figure 2-9 Signaling Links Between SS7 Network Nodes

Signaling links are generally deployed over T1s at 56 kbps per digital signal level 0 (DS-0) in North America, and E1s at 64 kbps per DS-0 in the international community. There is a new movement for using an entire digital signal level 1 (DS-1) to transport SS7 signaling traffic. If a DS-1 is used, 1.536 kbps is used for the signaling traffic and 8 kbps is used for framing and maintenance.

Japan specifies the use of a 48-kbps signaling link and a 64-kbps signaling link. The 64-kbps signaling link is left in the Japanese specification because it was the recommended speed set forth by the ITU. A low bandwidth link of 4.8 kbps is also available for signaling transport, but it is not discussed in this book. Signaling links can be deployed on satellite links if SS7 connectivity is required to remote locations.

Some areas of North America offer 64-kbps SS7 links, so it is important that you verify what the speed of your SS7 link is from your provider. When a call setup message is generated by a SSP, a Signaling Link Selection (SLS) occurs. This selection process identifies which signaling link the call is transferred over. Because this operation occurs in each direction, independent of each other, it is possible for the messages in opposite directions to take different paths.

Signaling links are deployed in logical groupings called link sets. Link sets are collections of up to 16 signaling links, and each link is uniquely identified by a Signaling Link Code (SLC) from 0 to 15. Figure 2-10 shows a link set between a SSP and a mated pair of STPs.

SLCs are important identifiers because they notify the SS7 network devices which links are connected to which SS7 network nodes. SLCs are given to you by your provider, and if you don't match what is set in their network (wrong SLC set), you are not able to complete the alignment process.

Figure 2-10 Links Within a Link Set

Link Set A

Figure 2-10 Links Within a Link Set

Link Set A

SLC 8

The traffic load of signaling links also differs between the international and North American communities—the ITU and ANSI. ITU signaling links can handle a maximum of 4096 circuits, and ANSI signaling links can handle about 16,384 circuits. Just because the technology supports thousands of Circuit Indentification Codes (CICs) per link does not mean that you ever have that many CICs for any single signaling link. The reason for this is simple: most countries don't allow it.

Hypothetically, if you had 16,384 circuits worth of signaling between SSPs on one link and that link was lost, that would be a disaster for your location. You would lose them all at one time. The number of CICs you are allowed to have per signaling link differs from country to country, so you need to inquire about each country's guidelines when requesting a link.

When loading links, it is generally recommended that you do not design any one link with more than 40 percent traffic capacity, or .40 erlang. The reason for this is simple, if you have a link failure you want to be able to reroute the signaling traffic to a redundant link or a load-shared link. In the case of 40 percent load, even with the traffic reroute you only have 80 percent load on the redundant or load-shared link. Any more than 40 percent causes a link to carry 100 percent capacity or more during a failure, and that certainly can cause problems. This design principle varies from customer to customer, but this is the rule of thumb.

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  • nathaniel
    Why maximum Signalling link no are 16 in telecommunications?
    6 months ago
  • Sarah
    What is signalling link in telecom?
    3 months ago

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