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CCNA Voice Quick Reference by Michael Valentine

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Introduction to Digital Circuits

Digital circuits have the chief advantage of allowing a much higher density of calls on a given physical connection; an analog circuit can handle only one call at a time, whereas a digital circuit can handle many.

There are two main types of digital circuits: Common Channel Signaling (CCS) and Channel Associated Signaling (CAS). CAS circuits are available in two speeds: T1 at 1.544Mbs supports 24 calls, and E1 at 2.048Mbs supports 30 calls. (For these values, we are assuming the calls are not compressed; more on this later). CCS circuits are designated as PRI T1, PRI E1, and BRI. A PRI T1 can support 23 calls, a PRI E1 30, and a BRI only 2.

The use of a digital circuit by definition implies that the voice signal ...

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