CAS vs CCS: Channel Associated Signaling vs Common Channel Signaling

This page describes CAS (Channel Associated Signaling) and CCS (Common Channel Signaling) and highlights the key differences between these two signaling techniques.

CAS: Channel Associated Signaling

In CAS, signaling information is transmitted over the same channel that carries the voice calls. A common CAS technique is PCM (Pulse Code Modulation) signaling, where signaling frame samples are transmitted within the PCM frame’s dedicated channel.

CCS: Common Channel Signaling

In contrast, CCS sends signaling over a separate, dedicated channel, distinct from the channels carrying voice calls. CCS is widely used globally in telephone exchanges and between PBXs (Private Branch Exchanges).

CCS typically requires just one signaling channel to handle up to approximately 1000 traffic channels. While PCM signaling can also be used for CCS, in a 30-channel system, time slot number 16 is often designated as the common signaling channel, operating at a rate of about 64 kbps.

In a 24-channel system, the 193rd bit can be used alternately in the PCM frames to transmit signaling information at a rate of approximately 4 kbps.

PCM T1 frame

Key Differences Summarized

The fundamental difference lies in how signaling is handled:

• CAS: Signaling is embedded within the voice channel.
• CCS: Signaling is sent on a dedicated channel, separate from the voice channels.

Understanding PCM (Pulse Code Modulation) and its frame structure is crucial for grasping the differences between CAS and CCS, especially for those involved in switching technology.