Cisco IOS Software Release 12.0(5)T supports four out-of-band DTMF transport methods:
Cisco RTP sends tones in the RTP stream that are coded differently from voice.
RTP NTE sends tones in the RTP stream as NSEs.
H.245 alphanumeric sends tones as ASCII text characters.
H.245 signal includes tone duration with the tone.
To send digits out-of-band, do the following:
Enable the chosen method of DTMF relay under dial-peer configuration.
The peer must indicate during call establishment that it is capable of receiving the DTMF option that is used.
There are four methods for DTMF relay with H.323 gateways. One is an in-band, Cisco proprietary method, one is an in-band method using named signaling events, and the other two, available since Cisco IOS Release 12.0(5)T, are out-of-band methods. All four of these techniques use the H.245 capabilities exchange, which is part of H.323v2, to signal to the remote VoIP gateway that a DTMF tone has been received and that the remote VoIP gateway should regenerate it. These are the four methods:
Cisco RTP with the dtmf-relay cisco-rtp command
RTP Named Telephony Event (NTE) with the dtmf-relay rtp-nte command H.245 alphanumeric with the dtmf-relay h245-alphanumeric command H.245 signal with the dtmf-relay h245-signal command
Note H.245-alphanumeric is most typically used because it is required by the standard. H245 signal is optional and therefore may not be supported by other devices.
The ability of a gateway to receive DTMF digits in a particular format and the ability to send digits in that format are independent functions. The Cisco H.323 version 2 gateway is capable of receiving DTMF tones transported by any of these methods at all times. However, to send digits out-of-band using one of these methods, two conditions must be met.
You must enable the chosen method of DTMF relay under dial-peer configuration using the dtmf-relay command.
The peer (the other endpoint of the call) must indicate during call establishment that it is capable of receiving DTMF in that format.
Cisco Proprietary Mode
This method of DTMF relay provides a way to transport DTMF digits in an RTP voice stream by encoding them differently from the voice samples. The RTP digit events are encoded using a proprietary format similar to Frame Relay as described in the FRF.11 specification. The events are transmitted in the same RTP stream as non-digit voice samples, using payload type 121. To use the Cisco proprietary mode, Cisco gateways are required at both the originating and terminating endpoints of the call.
To enable the Cisco proprietary DTMF-relay mode, use the dtmf-relay cisco-rtp command in dial-peer configuration mode.
RTP NTE Mode
Using NTE to relay DTMF tones provides a standardized means of transporting DTMF tones in RTP packets according to section 3 of RFC 2833, RTP Payload for DTMF Digits, Telephony Tones and Telephony Signals. RFC 2833 defines formats of RTP NTE packets used to transport DTMF digits, hookflash, 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.
To enable the Cisco proprietary DTMF-relay mode, use the dtmf-relay rtp-nte command in dial-peer configuration mode.
H.245 Alphanumeric and H.245 Signal
Thedtmf-relay h245-alphanumeric anddtmf-relay h245-signal commands 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 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. However, a reliable protocol generates overhead, which along with network congestion conditions, can cause DTMF tones to be slightly delayed. This delay is not known to cause problems with existing applications.
Thedtmf-relay h245-alphanumeric command 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. When the Cisco H.323 gateway receives a DTMF tone using this method, the Cisco H.323 gateway generates the tone on the PSTN interface of the call using a fixed duration of 500 ms. All H.323 version 2-compliant systems are required to support the alphanumeric method, while support of the h245-signal method is optional.
The H.245 Signal DTMF Relay feature supports keypad-initiated applications. The dtmf-relay h245-signal command relays a more accurate representation of a DTMF digit than does the dtmf-relay h245-alphanumeric command because tone duration information is included along with the digit value. This information is important for applications requiring that a key be pressed for a particular length of time. For example, one popular calling card feature allows the caller to terminate an existing call by pressing the # key for more than two seconds and then making a second call without having to hang up in between. This feature is beneficial because the caller does not have to dial the access number and personal identification number (PIN) code again. Outside-line access charges, which are common at hotels, may also be avoided.
DTMF relay can be turned off, and DTMF tones can be sent in-band by using the no dtmf-relay version of the command in Cisco IOS in dial-peer configuration mode.
Note If none of these options is selected, DTMF tones are transported in-band. Likewise, if the peer is not capable of receiving DTMF in any of the modes that were enabled, DTMF tones are sent in-band. When the Cisco H.323 version 2 gateway is involved in a call to a Cisco gateway that is running a version of Cisco IOS software prior to Release 12.0(5)T, DTMF tones are sent in-band because those systems do not support DTMF relay.
© 2005 Cisco Systems, Inc. All rights reserved. GWGK v1.01-20