nks to an STP. Figure 14.2 SS7 flow diagram with hierarchy. Downloaded from Digital Engineering Library @ McGraw-Hi (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 Key Telephony Concepts One End Other Figure 14.3 How we usually visualize SS7 networks. The transport used on these A through F links is entirely at the discretion of the net- work designer; SS7 does not care how its packet messages get from source to destination. In terms of the OSI model, we would say that SS7 specifies levels 4, 5, and 6 and leaves the implementation of levels 1 through 3 open. The links do have to be dedicated, that is, available for SS7 traffic at all times, and not used for anything else. When a link fails, the other links in the same set have to take over the traffic load. If an STP (or some other piece of SS7 equipment) should fail, then the remaining STP of the pair has to do the work of both of them. SS7 links and equipment are designed to use no more than 40 percent of their capacity so that the network will perform well in the event of a single failure. Because the telephone engineers did their job so well, we usually can think of SS7 as shown in Fig. 14.3. 14.3 SS7 Protocol The SS7 protocol suite is an evolutionary product, and over time, many changes and additions have been made to it. The purpose of this section is to give a brief overview of its major features and functions. The SS7 protocol suite predates the seven-layer OSI model, but this model is very useful in describing the functions performed by SS7. Let us review the OSI reference model outlined in Table 14.1. The physical layer of the SS7 protocol is the 64-kbps links described in Sec. 14.2. Rid- ing on this physical layer is the SS7 message transfer part (MTP), which can be looked at as layers 2 and 3 of the OSI model. The functionality of the MTP that most closely corresponds to layer 2 is referred to as MTP-2, and correspond