Two types of information are carried on a trunk:
Signaling information (indicating, for example, if a channel is on-hook or off-hook)
Signaling information can be conveyed using either channel associated signaling (CAS) or common channel signaling (CCS).
With 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 CAS protocol, such as Wink Start, 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, such as Wink Start, 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, MFC-R2 protocols use only two bits to signal four separate states; the other bits are not used. Pulsed E&M 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 NMS protocol reference manual for more information.
To interpret the signaling bits properly in a given country, your board must run a trunk control program (TCP) compatible with that country's protocol.
With 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).
On T1 trunks using a CCS protocol (such as ISDN), channel 24 is used as the D channel. It transmits packets of signaling information whenever the status of any of the other channels changes. No robbed-bit signaling is used. On E1 trunks using ISDN, the packets are sent in channel 16.