In Session Initiation Protocol (SIP), Dual-Tone Multi-Frequency (DTMF) tones are used for dialing, navigating automated phone systems, and other tasks. For instance, when you call a bank number and press a digit to select an option, the pressed digit is transported using DTMF. There are three methods for transmitting DTMF tones in SIP:
- In-band DTMF: In this method, DTMF tones are transmitted as part of the regular audio stream. This is the simplest method, but it can suffer from quality issues if the audio stream is compressed or degraded.
- Out-of-band DTMF via RFC 2833: This method sends DTMF tones separately from the audio stream, within RTP (Real-Time Protocol) packets. It’s typically more reliable than in-band DTMF, particularly for VoIP applications where the audio stream might be compressed. RFC 2833 is the standard that initially defined this method. It is now replaced by RFC 4733 but this method is still referred by RFC 2833.
- Out-of-band DTMF via SIP INFO messages: In this approach, DTMF tones are sent as separate SIP INFO messages. While this can be more reliable than in-band DTMF, it’s not as widely supported as the RFC 2833 method.
To understand more about DTMF via RFC 2833, lets check a call trace. In the call flow below we can see that the DTMF stream (telephony-event) is separate from the RTP stream. This is because it has a different payload type (101 in this case).
Since the DTMF has a separate stream it remains uncompressed and the receiving equipment is easily able to identify the digit.
In the above DTMF packet we can see that the digit pressed was a pound sign ‘#’ . If you would like to analyze this trace, it is available on my Github repo here https://github.com/VoIPNuggets-com/VoIPNuggets/blob/main/DTMF.pcap
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Akash Gupta
Senior VoIP Engineer and AI Enthusiast
AI and VoIP Blog
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