Hello Ali,

in Channel Associated Signalling the supervisory signalling travels on channel 16 in a  single DS0 64 kbps channel.

However, signalling is sampled like the audio signalling.

In PRI a DS 0 channel is dedicated for framing and another DS0 for signalling (D channel) and it carries supervisory information for all B DS0 channels.

The difference is that signalling is carried within small packets in LAPD frames and this provides some advantages.

PRI is typically used with ISDN that requires a signalling plane that is implemented over the D channel to perform call setup and to provide additional services like hold, call transfer and so on.

PRI is an example of CCS = common channel signalling

see

http://www.asteriskguru.com/tutorials/e1t1.html

Hope to help

Giuseppe