Hi Steven,
I'd have to say that you have a nice grip on these concepts :) Here is a little additional info;
T1 Signaling: CCS and CAS
Signaling in the Telephony world provides functions such as supervising and advertising line status, alerting devices when a call tries to connect, and routing and addressing information.
There are two different types of signaling information within the T1 world:
Common channel signaling (CCS)
CAS
CCS
CCS is the transmission of signaling information out of the information band. The most notable and widely used form of this signaling type is ISDN. One disadvantage of using an ISDN primary rate interface (PRI) is the removal of one DS0, or voice channel, in this case for signaling use. Therefore, one T1 has twenty-three DS0s, or B-channels for user data, and one DS0, or D-channel for signaling. It is possible to control multiple PRIs with a single D-channel each using Non-Facility Associated Signaling (NFAS)., Therefore, you are able to configure the other PRI's in the NFAS group to use all twenty-four DS0s as B-channels. Using PRI signaling ensures the maximum possible connection rates, especially with the advent of 56 K modems. This illustrates the clear channel capability of ISDN.
Another disadvantage of using CCS in the topology above is that the private branch exchange (PBX) needs a digital T1 PRI card. This is more expensive than a recEive and transMit (E&M) signaling card. An E&M signaling card is used in the same topology above if you run CAS between the AS5300 and the PBX.
CAS
CAS is the transmission of signaling information within the information band, or in-band signaling. This means that voice signals travel on the same circuits as line status, address, and alerting signals. As there are twenty-four channels on a full T1 line, CAS interleaves signaling packets within voice packets. Therefore, there are a full twenty-four channels to use for voice.
Various types of CAS signaling are available in the T1 world. The most common forms of CAS signaling are loopstart, groundstart, and E&M signaling. The biggest disadvantage of CAS signaling is that the network uses bits from information IP packets, such as voice packets, to perform signaling functions. CAS signaling is often referred to as robbed-bit signaling.
From this excellent doc;
http://www.cisco.com/en/US/tech/tk652/tk653/technologies_configuration_example09186a00800fa115.shtml#ccs
Channel Associated Signalling (CAS)
Channel associated signalling (CAS) sends the on hook and off hook signals as bits within the frames on the same channel as the audio transmission. CAS is often referred to as robbed bit signalling because CAS takes bits from the voice channel for signalling.
T1 CAS
The T1 CAS trunk interface uses in-band E&M signalling to carry up to 24 connections on a link. Both ends of the T1 link must specify T1 CAS signalling. Cisco CallManager provides the T1 CAS signalling option when you configure ports on some MGCP and H.323 voice gateways and network modules.
From this good doc;
Understanding IP Telephony Protocols
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00801ec5cc.html#95344
Here is some additional info;
From this doc;
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00801ec5cb.html#33733
Hope this helps!
Rob
PS: Here is a nice explanation from Paolo that makes it quite clear!
http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Unified%20Communications%20and%20Video&topic=General&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.1dde024b/0#selected_message
