CAS exists in many varieties that operate over various analog and digital facilities. The analog facilities are either two- or four-wire and the digital facilities are either North American T1 or European E1. This section discusses Bell System MF, CCITT No. 5, R1, and R2 CAS systems.

The main areas of discussion for each CAS system are supervision signaling and address signaling over analog and digital facilities. Bell System uses in-band MF for address signaling. For supervision signaling it uses Single Frequency for analog and a/b bits for digital trunks. CCITT No. 5 was designed for analog trunks and uses different MF signals for supervision and address signaling. In-band tone detection is used to detect and interpret the MF signals.

It is important to cover a few points before proceeding with a discussion of CAS systems. When a call is placed from Exchange A toward Exchange B, Exchange A is considered the outgoing exchange and Exchange B the incoming exchange.

One-way trunks are trunks on which only Exchange A or Exchange B can initiate a call. Exchanges A and B can initiate a call over two-way trunks. Double seizures can occur over two-way trunks when both exchanges try to seize the trunk at the same time, however. When this occurs, mechanisms such as timers are used to detect and resolve such events.

Three groups of signals are present in channel-associated interexchange signaling systems:

• Supervision Signals—These signals represent events that occur on a trunk and can be specific to the CAS variant. Signals include seizure, wink, and answer; they also are referred to as line signals.

• Address Signals—These signals typically represent the digits dialed or called party number and, in some instances, other information. In this chapter, address signals are based on MF signaling and can be system- or variant-specific.

• Tones and Announcements—These include tones such as ringing and busy tones and announcements such as, "The number you have dialed is no longer in service."

One more concept to cover before moving forward is that of service circuits. Service circuits are used in most exchanges to send and receive address signals and tones, as well as to play announcements. These circuits are typically system-specific; the processor connects a path from the trunk to the appropriate service circuit inside the switch. The pools of service circuits are temporarily used to send and receive tones or to play announcements.

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