dtmf-relay rtp-nte vs. no dtmf-relay

Answered Question
Sep 19th, 2009

I have read that Cisco gateways will do in-band DTMF if 'no dtmf-relay' is used. What is the difference between that and 'dtmf-relay rtp-nte'?

Correct Answer by Nicholas Matthews about 7 years 5 months ago

It's a confusion on terms. When you talk about DTMF relay, there are two primary types : inband, and out of band. The out of band messaging is passed between the signaling elements (SIP,H323,MGCP), and inband is RFC 2833. This is important because sometimes the signaling elements need to have information about the DTMF, but if you're using RFC 2833 they will not.

But that's DTMF relay. When you're not doing DTMF at all, many people still call this in-band. This is when the tones physically exist in the audio stream.

I find that most of the time people say in-band they're referring to RFC 2833 (rtp-nte). I like to clarify anyways.

Hope this clarifies.

-nick

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Paolo Bevilacqua Sat, 09/19/2009 - 13:12

In-band, the GW leaves the received tones into the RTP stream.

Relay, tones are removed (replaced with silence) and signaling is used to tell the other GW, which reproduces them again.

Relay method conveys DTMF reliably no matter the codec used and any network impairment.

Correct Answer
Nicholas Matthews Mon, 09/21/2009 - 05:32

It's a confusion on terms. When you talk about DTMF relay, there are two primary types : inband, and out of band. The out of band messaging is passed between the signaling elements (SIP,H323,MGCP), and inband is RFC 2833. This is important because sometimes the signaling elements need to have information about the DTMF, but if you're using RFC 2833 they will not.

But that's DTMF relay. When you're not doing DTMF at all, many people still call this in-band. This is when the tones physically exist in the audio stream.

I find that most of the time people say in-band they're referring to RFC 2833 (rtp-nte). I like to clarify anyways.

Hope this clarifies.

-nick

snickered Mon, 09/21/2009 - 05:42

Thanks for the clarification. So, when I do 'no dtmf-relay' it's not RFC 2833? That's how I have it configured towards my carrier and it's still working fine. But it also works when I do 'dtmf-relay rtp-nte'. My carrier is supposedly configured to RFC 2833 which is what started making me wonder why both work. They are both in-band but as far as I can tell the only difference is the payload. But if the payload is different then why do both work?

Nicholas Matthews Mon, 09/21/2009 - 06:01

This depends on your endpoint. It's quite likely that

1) The RFC 2833 packets are getting there even though you didn't configure it

2) You're using an unexpected configuration

3) The endpoint you're using recognizes both rtp-nte and in-band tones.

-nick

toormehdi Mon, 07/19/2010 - 13:16

Hi Nick,

I have SIP 3911 phone with UCM 7.0 and MGCP gateway to call out. . In the release note it mentions to use RFC2833 on the GW end to ensure that DTMF tones work.

question is whether all I need is

mgcp dtmf-relay voip codec all mode  nte-gw | nte-ca}

What is the difference between nte-gw and nte-ca

which one shall I use to make it work?

Thanks for your assistance.

ayush.gupta1 Sun, 02/15/2015 - 11:36

Out-of-band DTMF-Relay passes DTMF digits using a signaling protocol (SIP or H.323) instead of using the RTP media stream.

DTMF relay prevents loss of integrity of DTMF digits caused by VoIP compressed codecs. The relayed DTMF is then regenerated transparently on the peer side.

Figure 1. DTMF Relay Mechanism

DTMF relay mechanisms supported on VoIP dial-peers are listed below based on the keywords used to configure them. The DTMF relay mechanism can be either out-of-band (H.323 or SIP) or inband (RTP).

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