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How do you divide DTMF transmissions?

How do you divide DTMF transmissions, such as in-band or out-of-band? Can RFC2833 be considered in-band?

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In-band DTMF specifically refers to sending the DTMF digit frequencies along the same voice path as the actual speech in a conversation. This works well for an uncompressed channel using G.711, but does not work reliably when using a compressed channel such as G.729.

RFC2833 DTMF uses an out-of-band approach. The idea here is that special Real-Time Transport Protocol (RTP) even messages are sent instead of the raw tones. For example, a message is sent indicating a "DTMF digit 7" versus sending the two frequencies that make up a DTMF 7. This approach is more reliable.

This was first published in March 2006

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