H.323 DTMF Support
Cisco gateways currently support four methods of DTMF relay using H.323:
■ Cisco-proprietary: DTMF tones are sent in the same RTP channel as voice data.
However, the DTMF tones are encoded differently from the voice samples and are
identified as payload type 121, which enables the receiver to identify them as DTMF
tones. This method requires the use of Cisco gateways at both the originating and
terminating endpoints of the H.323 call.
■ H.245 Alphanumeric: Separates the DTMF digits from the voice stream and sends
them through the H.245 signaling channel instead of through the RTP channel. The
tones are transported in H.245 User Input Indication messages. The H.245 signaling
channel is a reliable channel, so the packets that transport the DTMF tones are guaranteed
to be delivered. This method does not send tone length information.
■ H.245 Signal: This method does pass along tone length information, thereby
addressing a potential problem with the alphanumeric method. This method is
optional on H.323 gateways.
Chapter 2: Considering VoIP Design Elements 83
Note All H.323 Version 2 compliant systems are required to support the “h245-
alphanumeric” method, whereas support of the “h245-signal” method is optional.
■ NTEs: Transports DTMF tones in RTP packets according to section 3 of RFC 2833.
RFC 2833 defines formats of NTE RTP packets used to transport DTMF digits,
hook flash, and other telephony events between two peer endpoints. With the NTE
method, the endpoints perform per-call negotiation of the DTMF relay method.
They also negotiate to determine the payload type value for the NTE RTP packets.
As a result, DTMF tones are communicated via RTP packets, using an RTP payload
type that prevents the tones from being compressed via the codec being used to
encode the voice traffic.