DTMF (Dual Tone Multi Frequency) Relay

DTMF Relay

Dual-Tone Multifrequency (DTMF) is the tone generated on a touch-tone phone when you press keypad digits. During a call you might enter DTMF to access Interactive Voice Response (IVR) systems such as voicemail, automated banking services and so on.

In previous releases of Cisco IOS, DTMF is transported in the same way as voice.

This approach can result in problems accessing IVR systems.

While DTMF is usually transported accurately when using high-bitrate voice CODECs such as G.711.

low-bitrate CODECs such as G.729 and G.723.1 are highly optimized for voice patterns, and tend to distort DTMF tones. As a result, IVR systems may not correctly recognize the tones. 

DTMF relay solves the problem of DTMF distortion by transporting DTMF tones "out of band", or separate from the encoded voice stream.

Cisco H.323 Version 2 support introduces three options for sending DTMF tones out of band. These are:

• A Cisco proprietary RTP-based method ("dtmf-relay cisco-rtp").
• H.245 signal ("dtmf-relay h245-signal").
• H.245 alphanumeric ("dtmf-relay h245-alphanumeric").

If none of these options is selected, DTMF tones are transported inband, and encoded in the same way as voice traffic. 

The "cisco-rtp" option sends DTMF tones in the same RTP channel as voice. However, the DTMF tones are encoded differently from the voice samples and are identified by a different RTP payload type code. 

Use of this method accurately transports DTMF tones, but since it is proprietary it requires the use of Cisco gateways at both the originating and terminating endpoints of the H.323 call.

· The "h245-signal" and "h245-alphanumeric" options are modes of DTMF transport defined by the ITU H.245 standard.

· These methods separate DTMF digits from the voice stream and send them through the H.245 signalling channel instead of the RTP channel.

· The tones are transported in H.245 User Input Indication messages.

· The H.245 signalling channel is a reliable channel, so the packets transporting the DTMF tones are guaranteed to be delivered. However, because of the overhead of using a reliable protocol, and depending on network congestion conditions, the DTMF tones may be slightly delayed. This delay is not known to cause problems with existing applications. 

The "h245-signal" option relays a more accurate representation of a DTMF digit than the "h245-alphanumeric" option, in that tone duration information is included along with the digit value.

This information is important for applications that require you to press a key for a particular length of time. For example, one popular calling card feature allows you to break out of an existing call by pressing the (#) key for more than two seconds and then make a second call without having to hang up in between. This feature is beneficial because it allows you to avoid having to dial your access number and PIN code again, and it allows you to avoid access charges if you are charged for accessing an outside line as is common at hotels.

The "h245-alphanumeric" option simply relays DTMF tones as ASCII characters. For instance, the DTMF digit 1 is transported as the ASCII character "1". There is no duration information associated with tones in this mode.