If it is using CCS, multi-octet messages are sent on the signalling channel (out of band to the payload) that include identification of the channel being signalled. The separation of the signalling, and the use of a common channel allows the payload to have access to the full 64Kbs bandwidth. This is used with SS7, for example.
With CAS, each channel is associated with 4 bits (2Kbs) of signalling. Because the signalling is embedded (inband) with the payload, the payload speed is reduced to 56Kbs. You'll also see this referred to as robbed bit signalling, since you're stealing bits from the payload.