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.