# Signaling

Two types of information are carried on a trunk:

- Voice information

- Signaling information (indicating, for example, a channel is on-hook or off-hook)

Signaling information can be conveyed using either channel associated signaling (CAS) or common channel signaling (CCS).

Note: The following information is provided for informational use only. Your board's hardware performs all the operations necessary to support the framing system used on the trunk. The TCPs perform all necessary signaling operations.

## Channel associated signaling (CAS)

With channel associated signaling (CAS), signaling information is sent for all channels at regular intervals, regardless of whether each channel's state changes. The information for each channel consists of a set of bits called the ABCD bits. Whenever a channel's state changes, the ABCD bit pattern for that channel changes to convey the signaling bits.

On T1 trunks using a CAS protocol, the signaling information for each channel is transmitted using a method called robbed-bit signaling. With this method, one of the bits in the voice information in each channel is changed at regular intervals to indicate the state of the channel. Since the intervals are widely spaced, sound quality in the channel is not compromised.

On E1 trunks using a CAS protocol, channel 16 carries the ABCD bits for all of the other channels. No robbed-bit signaling is used.

Different CAS protocols use the ABCD bits in different ways. For example, some protocols use only two bits to signal four separate states; the other bits are not used. Other protocols convey signaling using one bit only, by setting and resetting the bit at specific intervals to signal different states. The specific patterns of bits used to indicate signaling states differ from country to country. Refer to the appropriate protocol reference manual for more information.

## Common channel signaling (CCS)

With common channel signaling (CCS), packets of signaling information for a channel are sent when the channel's state changes, instead of signaling bits. CCS information is sent in a dedicated channel, the data channel or D channel. Voice information is carried in bearer channels (B channels).

Different CCS protocols reserve different channels for transmitting data. For example, GR303 occupies channels 24 and 12 on T1 trunks to run TMC and EOC protocols. GR303 also uses robbed-bit signaling for call control on established connections. On E1 trunks using the V5 protocol, signalling takes place on channels 15, 16, and 31.