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New Member

T1-CAS vs T1-PRI

Hi All,

I think, i know whats a T1-CAS and T1-PRI circuit is meant for..but however i didn't find any document which explain why would i choose one over the other except for notes on differences between CAS vs CCS , 23 channels vs 24 , robbed-signalling vs dedicated D channel etc.

Is that some PBXs support only T1-CAS or T1-PRI ?

If your PBX supports both what advantages would you get by choosing one over the other ?

Does a T1-CAS cost you less than T1-PRI or vice-versa ?

What other factors should be looked for before choosing one over the other.

Can someone please help me

Hall of Fame Super Gold

Re: T1-CAS vs T1-PRI

CAS is a technology with roots in the 60's and before. It is static multiplexing with basic signaling features, often one way only.

PRI is a an 80's technology that allows a much richer sets of features and service, at the cost of loosing 1 channel on T1. To alleviate this a bit on larger installations, in north america there is NFAS PRI where one signaling channel is shared among more T1s.

Some PBX can support only CAS often in the sense that the specific unit lacks the appropriate HW, SW, license or the management people doesn't quite know how to configure it.

The advantages of using PRI whenever possible are very major. You can easily diagnose and easily and fix all issues. A variation of PRI, called Q.SIG is the basis for sophisticated PBX networking in legacy PBX systems.

As far the cost for PRI circuits, this is a matter of markets and may vary based on location and offering. In Europe and other countries, telcos have standardized on PRI (and BRI) only since more than 10 years.

Hope this helps, please rate all useful posts!

New Member

Re: T1-CAS vs T1-PRI

p.bevilacqua , Thanks for clarifying my doubts. I have few more questions ..

Can i connect PBX-PBX within the enterprise using T1-PRI or T1-CAS ? is connecting a PBX to CO technically anything different than connecting a PBX-PBX ?

Do we use QSIG protocol for signalling between CO and the PBX/Voice-gateway ? or just between PBX-PBX ? or just between CO-CO ? or can we use in all the above thress scenarios ?

I am kind of confused with this signalling protocols and just wondering if i can get hold of a good document !!

Hall of Fame Super Gold

Re: T1-CAS vs T1-PRI

There is plenty of good documents on CCO website. Too many to list. Also good books, from Cisco press.

Basically, you can connect anything with anything, but each case is different. The router can do (almost) anything, all the rest depends on the connected device.

Q.SIG being a protocol for private use is never used with COs.

The key for a successful integration with PBX is to try to have them do PRI or QSIG, and if not possible, live with the less features.

Hall of Fame Super Red

Re: T1-CAS vs T1-PRI

Hi Praveen,

Just to add a note to Paolo's excellent info, here are some fairly descriptive Cisco docs.

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. These signals can include supervision, addressing, and tones such as busy tone or dial tone.

You can use T1 CAS and E1 CAS digital trunk interfaces to connect a Cisco CallManager call to a CO, a PBX, or other analog device.


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.

T1 Primary Rate Interface (T1 PRI)

T1 Primary rate interface (PRI) is used for corporate communications links in North America and Japan. T1 PRI provides 23 B-channels for voice and data and one D-channel for common channel signalling. T1 PRI uses a communication rate of 1.544Mbps.

E1 Primary Rate Interface (E1 PRI)

E1 PRI Primary rate interface (PRI) is used for corporate communications in Europe. E1 PRI provides 30 B-channels for voice and data , one D-channel for common signalling, and one framing channel. E1 PRI uses a rate of 2.048 Mbps

Q.Signaling (QSIG)

The QSIG protocol is a series of international standards that define services and signalling protocols for Private Integrated Services Networks (PISNs). These standards use ISDN concepts and conform to the framework of International Standards for Open Systems Interconnection as defined by ISO/IEC. The QSIG protocol is a variant of ISDN D-channel voice signalling. It is based on the ISDN Q.921 and Q.931 standards and is a worldwide standard for PBX interconnection.

The integration of QSIG protocol support with voice over IP (VoIP) enables Cisco voice switching services to connect to PBX's, key systems, and central office (CO) switches that communicate by using QSIG protocol. Cisco devices can route incoming voice calls from a private integrated services network exchange (PINX) device across a WAN to a peer Cisco device that can transport the signalling and voice packets to a second PINX device. PINX devices can be PBXs, key systems, or Cisco CallManager nodes that support QSIG protocol.

From this good doc;

Understanding IP Telephony Protocols

Here is some additional info;

Cisco Voice Gateways

Cisco CallManager supports several types of Cisco IP telephony gateways. Gateways use call control protocols to communicate with the PSTN and other non-IP telecommunications devices, such as private branch exchanges (PBXs).

Trunk interfaces specify how the gateway communicates with the PSTN or other external devices by using time-division multiplexing (TDM) signalling. Cisco CallManager and Cisco gateways use a variety of TDM interfaces, but supported TDM interfaces vary by gateway model. The following list gives available interfaces that Cisco CallManager supports:

Foreign Exchange Office (FXO)

Foreign Exchange Station (FXS)

T1 Channel Associated Signalling (CAS)

T1 PRI North American ISDN Primary Rate Interface (PRI)

E1 PRI European ISDN Primary Rate Interface (PRI)

QSIG Q signalling protocol that is based on ISDN standards

Cisco CallManager can use H.323 gateways that support E1 CAS, but you must configure the E1 CAS interface on the gateway.

From this doc;

Hope this helps!