Glossary: DTMF Relay

Dual-tone Multi-frequency Relay

Dual-tone Multi-frequency Relay (DTMF) is the mechanism where an SBC listens for in-call tones, and relay them to the peer according to the negotiated method.

Relaying DTMF prevents loosing its signal integrity over VoIP compressed codecs. The relayed DTMF is then being regenerated transparently on the peer side.

There are 3 common ways of relaying DTMF:

1. RFC2833

2. SIP INFO

3. In band

RFC2833

RFC2833 (now RFC4733) is the preferred method for relaying, where the in-call DTMF are removed from the in band voice, and are sent to the peer over the RTP stream, as specially marked packet (RTP Events).

This method also offers a robustness against packet losses by using redundancy scheme.

SIP INFO

The SIP INFO method can be used by SIP network elements to transmit DTMF tones out-of-band as telephone-events in a reliable manner independent of the media stream.

In the DTMF relay method the body of the SIP message consists of signaling information and uses the content-type application/dtmf-relay

This method can be used where peer VoIP doesn't support RFC2833 and don't want to use in band DTMF.

In Band

In band is used when the two other methods aren't available. The DTMF is relay with in band voice, and is more likely to work on lossless codecs, like G.711.

DTMF-Relay processing

By default, the SBC will relay the incoming leg method to the outgoing leg. With the profiles, it can set a preference on the DTMF method and if both sides agree, this method will be used.

If it needs to convert from one method to another, a transcoding device is required and this will be handled automatically according to the preferences in the profiles.

If all negotiation fails, then in band DTMF will be the default option.

SIP Profiles

This default behavior can be overridden using SIP Profile. DTMF-Relay methods can be added/removed and ordered as needed.

It is possible to change the default mode of operation from one network to another by assigning a different profile to each Network Access Point (NAP). For example, if one network has in band DTMF and the other has RFC2833, it will capture the in band DTMF and create RTP events on the other leg and vice-versa. On ProSBC this requires a transcoding unit.