inks (B) connect mated STPs to other mated STPs. Each STP-to-STP pair connection requires four separate B links, often referred to as quad links. Some SS7 networks in Europe do not use all four B links as shown here. B links are used for STP Figure 14.1 SS7 flow diagram. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Signaling with SS7 Signaling with SS7 connections at the same hierarchical level in the SS7 network, whereas D links (see be- low) are used when the STPs being connected are at different levels. Cross links (C) connect the redundant STPs to each other. Even C links come in pairs to maintain redundancy in the SS7 network. These links are used for SS7 network management messages only except when the network is so congested that the C links are needed for SS7 traffic or when an equipment failure requires SS7 traffic to use the C links. Diagonal links (D) are used to connect STPs at different hierarchical levels in the SS7 network. SS7 networks are not required to have a hierarchy, but it can be useful when there are centralized functions, such as SCP database access, or large numbers of central offices that benefit from having concentrator STP nodes. D links come in quad arrange- ments just like B links. The full SS7 picture with hierarchy is shown in Fig. 14.2. Extended links (E) connect SSPs to remote STPs. This is normally used for traffic overflow when the home STPs become congested. E links are also a diversity backup in case of equipment failure. Fully associated links (F) are direct CCIS links between two SSPs. F links typically are used when there is a large amount of signaling traffic between two SSPs, enough to justify a dedicated signaling link. They also can be used when a remote SSP cannot be connected directly to an STP for some reason. The F links then allow the remote SSP to access SS7 databases without direct A li