SS7 Network Architecture and Protocols

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 , and a similar overview of SS7 by ANSI can be found in T1.110.

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.

Pre-SS7 Systems

The following are the main systems that preceded SS7:

·         CCITT R1 (regional 1) was deployed only on a national level. R1 is a Channel Associated Signaling (CAS) system that was employed in the U.S. and Japan. It uses multifrequency (MF) tones for signaling. It is no longer in general operation, although some remnants might remain in the network.

·         CCITT R2 (regional 2) was deployed only on a national level. R2 is a CAS system that was employed in Europe and most other countries. It used Multifrequency Compelled (MFC) for signaling; it compelled the receiver to acknowledge a pair of tones before sending the next pair. It is no longer in general operation, although some remnants might remain in the network.

·         Signaling systems that have been deployed for both national and international (between international switches) signaling have progressed from CCITT #5 (C5) to CCITT #6 (C6) and finally to CCITT #7 (C7):

- C5 (CCITT Signaling System No. 5) is a CAS system standardized in 1964 that has found widespread use in international signaling. It is still in use today on a number of international interfaces. National implementations are now scarce, except in less-developed regions of the world, such as Africa, which makes extensive use of the protocol. C5 can be used in both analog and digital environments. In an analog setting, it uses tones for signaling. In a digital setting, a digital representation of the tone is sent instead (a pulse code modulation [PCM] sample).

- C6 (CCITT Signaling System No. 6), also called SS6, was the first system to employ Common Channel Signaling (CCS). It was standardized in 1972. (CAS and CCS are explained in Chapter 1, "The Evolution of Signaling.") C6 was a pre-OSI model and as such had a monolithic structure as opposed to a layered one. C6 was a precursor to C7 and included the use of data links to carry signaling in the form of packets. It had error correction/detection mechanisms. It employed a common signaling channel to control a large number of speech circuits, and it had self-governing network management procedures. C6 had a number of advantages over C5, including improvements in post-dial delay and the ability to reject calls with a cause code. The use of locally mapped cause codes allowed international callers to hear announcements in their own language. Although C6 was designed for the international network, it was not as widely deployed as C5. However, it was nationalized for the U.S. network and was deployed quite extensively under the name Common Channel Interoffice Signaling System 6 (CCIS6) in the AT&T network. C6 was introduced into the Bell system in the U.S. in 1976, and soon after, Canada. All deployments have now been replaced by SS7.

The next section provides a brief history of SS7.