 call control, remote control, network database access and management, and maintenance signaling. Provides a reliable means of information transfer in correct sequence without loss or duplication (Ref. 1). CCITT SS No. 7, in the years since 1980, has become known as the signaling system for ISDN. This it is. Without the infrastructure of SS No. 7 embedded in the digital network, there will be no ISDN with ubiquitous access. One important point is to be made. CCITT SS No. 7, in itself, is the choice for signaling in the digital PSTN without ISDN. It can and does stand on its own in this capacity. As mentioned, SS No. 7 is a data communication system designed for only one purpose: signaling. It is not a general-purpose system. We then must look at CCITT SS No. 7 (1) a specialized data network and (2) a signaling system (Ref. 2). This would be called interoffice signaling in North America. Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman ISBN 0-471-71045-8 Copyright © 2005 by Roger L. Freeman CCITT SIGNALING SYSTEM NO. 7 14.2 OVERVIEW OF SS NO. 7 ARCHITECTURE The SS No. 7 network model consists of network nodes, termed signaling points (SPs), interconnected by point-to-point signaling links, with all the links between two SPs called a link set. When the model is applied to a physical network, most commonly there is a one-to-one correspondence between physical nodes and logical entities. But when there is a need (e.g., a physical gateway node needs to be a member of more than one network), a physical network node may be logically divided into more than one SP, or a logical SP may be distributed over more than one physical node. These artifices require careful administration to ensure that management procedures within the protocol work correctly. Messages between two SPs may be routed over a link set directly connecting the two points. This is referred to as the associated mode of signaling. Messages may also be routed via one or more intermediate SPs that relay messages at the network layer. Th