Download

158 INTRODUCTION TO SIGNALING SYSTEM NO. 77.1 SS7 STRUCTURESS7 is defined in terms of messages and functions; hardware architecture issues areleft to equipment manufacturers. The signaling system is divided into a number ofprotocols (or parts), each of which handles a group of related functions. The interfacesbetween those parts were defined in the early stages of the specification work.The various parts could then be specified simultaneously and independently of eachother. That has made the overall specification effort more manageable.7.1.1 SS7 HierarchyThe parts of SS7 are organized in a four-level hierarchy [1–4]. We say that a higherlevelpart is a user of the services provided by a lower-level part. This arrangement issimilar to the seven-layer structure of the Open Systems Interconnection (OSI) usedfor data communication protocols (Chapter 20). Several efforts have been made toalign the SS7 levels and OSI layers. The efforts, however, have been only partiallysuccessful. OSI layers 1 and 2 correspond to SS7 levels 1 and 2, but things start todiffer in the higher parts of the hierarchies. In what follows the four-level SS7 hierarchywill be used. It works well for trunk-related applications, and less well forother applications.7.1.2 SS7 ProtocolsThe protocols of SS7 and their levels are shown in Fig. 7.1-1.Figure 7.1-1. Structure of SS7. (From Rec. Q.700. Courtesy of ITU-T.)