Questions about signaling on E1/T1

Unanswered Question
Jun 5th, 2007

I'm reading through CVOICE study guide and just trying to get my head around some of the concepts. Can anyone tell me if I've got this right?

1) If you configure a T1 for or E1 controller with the command "ds0-group" does this inherently imply that you are using CAS signaling on the digital circuit?

2) I'm noticing that all the options for the "signal-type" parameter in the "ds0-group" command (listed below) seem to be signalling types that I would normally associate with analog interfaces. Does this imply that all CAS signaling methods on a T1/E1 are just digital emulations of analog signaling methods or am I thinking about this the wrong way?

e&m-fgb E & M Type II FGB

e&m-fgd E & M Type II FGD

e&m-immediate-start E & M Immediate Start

fgd-eana FGD Exchange Access North American

fgd-os FGD Operator Services

fxs-ground-start FXS Ground Start

fxs-loop-start FXS Loop Start

none Null Signaling for External Call Control

r1-itu R1 ITU

sas-ground-start SAS Ground Start

sas-loop-start SAS Loop Start

3) From my ref book examples of CCS signaling include:

- Proprietary Implementations

- ISDN PRI or BRI (Has its own call control protocol called Q.931)

- QSIG (Only implemented on ISDN PRI interfaces)

- DPNSS (Operates over standard ISDN physical interfaces)

- SS7

So... The examples I've seen seem to suggest that if you want CCS signaling on a T1/E1 (and I understand it's the modern prefered method) it impies the use of ISDN service from your provider and you configure your T1/E1 as a PRI to match. If I'm undertanding the option right with ISDN PRI you can then choose from several differnet call control protocols including Q.931, QSIG, or in rare circumstances DPNSS??? I presume it would depend on what you're connecting too?

To sum up... a T1 configured as ISDN PRI implies use of CCS and allows you to choose from various digital signaling protocols and lack of ISDN on a T1/E1 implies use of CAS with a choice of older analog protocol emulated digitally. Is that an accurate statement?

Thanks much for any assistance and yes I will rate responses :)

I have this problem too.
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rob.huffman Tue, 06/05/2007 - 05:31

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)



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 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;

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.


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

Here is some additional info;

From this doc;

Hope this helps!


PS: Here is a nice explanation from Paolo that makes it quite clear!

flash2200 Tue, 06/05/2007 - 06:55

Hi Rob,

Many thanks for the detailed response on this!



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