How CUCM knows that there is DTMF pressed in RTP-NTE

[nawaz.pasha]

I want to know how the CUCM will come to know DTMF is pressed in in-Band DTMF like RTP-NTE is used and when it needs to introduce MTP to extract the DTMF to send it in signal channel. Since, RTP is end to end phone communication rather CUCM doesn't come into picture. Does the other end of the phone says I need to invoke the MTP as I don't under the DTMF that came to RTP...Please explain in detail... if possible

I know out of band always sends the DTMF in signalling so, just want to make it clear...

[Vivek Batra]

CUCM knows it because only CUCM decides when to invoke MTP (during call setup) depends on DTMF capabilities of both end points involved in a call.

Following is an excerpt from SRND (same is described for gateways as well in other chapters);

DTMF Relay Between Endpoints in the Same Unified CM Cluster

The following rules apply to endpoints registered to Unified CM servers in the same cluster:

• Calls between two non-SIP endpoints do not require MTPs.

All Cisco Unified Communications endpoints other than SIP send DTMF to Unified CM via various
signaling paths, and Unified CM forwards the DTMF between dissimilar endpoints. For example,
an IP phone may use SCCP messages to Unified CM to send DTMF, which then gets sent to an
H.323 gateway via H.245 signaling events. Unified CM provides the DTMF forwarding between
different signaling types.

• Calls between two Cisco SIP endpoints do not require MTPs.

All Cisco SIP endpoints support NTE, so DTMF is sent directly between endpoints and no
conversion is required.

• A combination of a SIP endpoint and a non-SIP endpoint might require MTPs.

To determine the support for NTE in your devices, refer to the product documentation for those
devices. Support of NTE is not limited to SIP and can be supported in devices with other call control
protocols. Unified CM has the ability to allocate MTPs dynamically on a call-by-call basis, based
on the capabilities of the pair of endpoints.

- Vivek

[nawaz.pasha]

I agree that CUCM decides but the question is not about the explanation how the negotiation happens...my question...is in band DTMF is part of RTP and how come CUCM will come to know if there is digit sent by the endpoint does the other party informs this...A--->B does B informs this to invoke the MTP that I don't know the DTMF that came in RTP or send me the outof band DTMF to me...will this be asked by phone B

[Vivek Batra]

Hi Nawaz,

As you agree on that CUCM decides when to invoke MTP in case of DTMF mismatch, hence media will not flow end to end any more if CUCM has been invoked MTP. RTP will now flow from MTP (either CUCM or IOS).

Inband DTMF (RFC 2833) is a part of RTP stream but having different 'Payload Type' from voice RTP. You can simply see the wireshark traces and will found different payload type for voice RTP and inband DTMF.

Let say one end point negotiated inband (RFC 2833) and other end point agreed on KPML, since media is flowing through CUCM/MTP, MTP will detect the DTMF in inband (RFC2833) and convert it into outband (KPML) before sending to other end point. On the other side, since signaling always happens through CUCM, ouband (KPML) DTMF when received by CUCM will be converted to inband (RFC 2833) before sending it to other end point. 

Does it answers your question?

- Vivek

PS: Edited: not -> now

[nawaz.pasha]

Thanks for explaining. In other words CUCM still can see the RTP as MTP is mapped to each device through MRG/MRGL......MTP is like transparent RTP filter which filters and extracts to signal channel when the other end requires the out of band...So, basically the RTP is still watched by CUCM by means of MTP...please correct.

[abdul dawabsheh]

This is very old, but I came across this while searching for another thing, anyway the provided answer is inaccurate, the MTP will send a SCCP message to the call manager once a digit is pressed, even if the MTP is the call manager it self, you will see a line in the logs indicating that a digit was pressed. if we are talking about an external MTP/Xcoder you will see a Stationinit (SCCP message) coming to the call manager.