Advantages of T1 CAS over PRI ... There are many, but the most obvious ones are -
T1 CAS uses robbed bit signaling, meaning the least significant bit of the 6th, 12th, 18th and 24th timeslot is taken for passing signaling information (ABCD bits). As a result of this, the signal to noise ratio on a CAS trunk is reduced very slightly.
There is no 'standardised' form of passing calling numbers with CAS - all digit signalling is done with DTMF or MF tones, and even though many providers will pass an ANI to the end user, there is no official method to pass the calling number, so what works with one system may not work with another. There are workarounds of using TCL scripts to decode the called/calling digit string, but you still need to set up digit string delimeters between the called and calling numbers.
CAS can be very difficult to debug ...
CAS most often uses E&M wink start operation, but can use E&M immediate start, wink/wink feature group D, FXS loopstart or ground start, FXO loop start or ground start, or even completely custom variants. In many cases the telcos have no idea of what mode of operation they are using, when you ask them they simply say it is CAS and it's up to you to reverse engineer their switch ...
PRI uses a dedicated 64K channel on the interface to pass signaling traffic. The setup message contains called and calling numbers, and also includes other important details about the call such as bearer capability, call ID reference number, B channel being used and actual call state (Connected, alerting etc ...)
PRI has almost 100 specific disconnect reasons, so it is very easy to fault find. 'Debug ISDN Q931' often gives enough information to point to the exact reason and location of a fault.
If at all possible, make you life simple and use ISDN PRI.
Paul, thank you for the explaination. Now only if we can get our customer to "see the light" and accept that their old CAS circuit is not worth the last 3 weeks we have been messing with it and switch to PRI or SIP.
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