Chapter 4. SS7 Network Architecture and Protocols Introduction

The International Telecommunication Union (ITU) is the international governing body for Signaling System No. 7. More specifically, it is governed by the Telecommunication Standardization Sector of the ITU (ITU-TS or ITU-T for short). Formerly it was governed by the ITU's Consultative Committee for International Telegraph and Telephone (CCITT) subcommittee until that was disbanded in 1992 as part of a process to speed up the production of recommendations (as well as other organization changes). See Chapter 2, "Standards," for more information on standards-making bodies.

Signaling System No. 7 is more commonly known by the acronyms SS7 and C7. Strictly speaking, the term C7 (or, less commonly, CCS7) refers to the international Signaling System No. 7 network protocols specified by the ITU-T recommendations as well as national or regional variants defined within the framework provided by the ITU-T. The term C7 originates from the former title found on the specifications—CCITT Signaling System No. 7. The term SS7 tends to specifically refer to the North American regional standards produced by Telcordia (formerly known as Bell Communications Research or Bellcore) and the American National Standards Institute (ANSI). The North American standards themselves are based on the ITU-T recommendations but have been tailored outside the provided framework. The differences between ITU and Telcordia/ANSI are largely subtle at the lower layers. Interaction between ANSI and ITU-T networks is made challenging by different implementations of higher-layer protocols and procedures.

For the purpose of this book, we will use the term SS7 to refer generically to any Signaling System No. 7 protocol, regardless of its origin or demographics. An overview of SS7 by the ITU-T can be found in recommendation Q.700 [111], and a similar overview of SS7 by ANSI can be found in T1.110 [112].

Chapter 3, "The Role of SS7," provides a comprehensive list of the functions and services afforded by SS7. These can be summarized as follows:

• Setting up and tearing down circuit-switched connections, such as telephone calls made over both cellular and fixed-line.

• Advanced network features such as those offered by supplementary services (calling name/number presentation, Automatic Callback, and so on).

• Mobility management in cellular networks, which permits subscribers to move geographically while remaining attached to the network, even while an active call is in place. This is the central function of a cellular network.

• Short Message Service (SMS) and Enhanced Messaging Service (EMS), where SS7 is used not only for signaling but also for content transport of alphanumeric text.

• Support for Intelligent Network (IN) services such as toll-free (800) calling.

• Support for ISDN.

• Local Number Portability (LNP) to allow subscribers to change their service, service provider, and location without needing to change their telephone number.

After reading the preceding chapters, you know that signaling serves the requirements of the telecommunications service being delivered; it is not an end in itself. Signaling enables services within the network.

This chapter makes you familiar with the SS7 network, protocols, fundamental concepts, and terminology so that the topics covered in the rest of the book will be more accessible if you're unfamiliar with the subject. This chapter begins with a brief description of pre-SS7 systems and SS7 history. The chapter then presents the protocol stack, showing how SS7 protocols fit together. It concludes with a discussion of the relevant protocols.