The best practice is to use NSE mode.
Configure selected mode in Cisco IOS global configuration mode.
Cisco supports MGCP DTMF transport in three ways:
Cisco proprietary mode
Named Signaling Event (NSE) mode:
MGCP-based DTMF relay: Gateway-controlled mode
MGCP-based DTMF relay: Call agent-controlled mode MGCP out-of-band DTMF relay
Cisco Proprietary Mode
The Cisco proprietary method has already been discussed with H.323. However since dial peers are normally not used on an MGCP gateway, the configuration of this method happens in the global configuration mode with the mgcp dtmf-relay codec {all | low-bit-rate} mode cisco command. To disable this process for uncompressed codecs, use the no mgcp dtmf-relay voip form of this command.
NSE Mode
Similar to the Cisco proprietary mode, MGCP-based DTMF relay sends digits in an RTP stream. However, for this mode, support is added for RFC 2833, RTP Payload for DTMF Digits, Telephony Tones and Telephony Signals. Support of RFC 2833 is standards-based and allows greater interoperability with other gateways and call agents.
In this method, each DTMF digit is encoded as a RTP NTEs packet, which is transmitted and received, whereas digit samples usually are carried in voice packets. The named events are sent as RTP packets over UDP. The packets are encoded with a payload type that is negotiated during connection establishment between the gateways using SDP.
MGCP-based DTMF relay provides two modes of implementation:
In gateway-controlled mode, gateways negotiate DTMF transmission by exchanging capability information in SDP messages. That transmission is transparent to the call agent.
Gateway-controlled mode allows use of the DTMF-relay feature without upgrading the call agent software to support the feature.
In call agent-controlled mode, call agents use MGCP messaging to instruct gateways to process DTMF traffic.
To configure this method of DTMF relay, use the mgcp dtmf-relay codec {all | low-bit-rate}
mode {nte-gw | nte-ca} command in global configuration mode where nte-gw and nte-ca are the triggers for gateway-controlled mode or call agent-controlled mode respectively.
The following lists the benefits of MGCP-based DTMF:
There is MGCP support for RFC 2833, RTP payload for DTMF digits, telephony tones, and telephony signals.
DTMF relay is more reliable.
There is a greater interoperability with third-party equipment.
Gateway-controlled and call agent-controlled modes allow for phased network upgrades.
Note The only restriction on MGCP NSE-mode DTMF relay is that DTMF relay supports the dynamic RTP payload range of 98 to 119 .
If DTMF relay is not configured, the DSPs on the gateways send and receive DTMF digits in-band in the voice codec.
MGCP Out-of-band DTMF Relay
Within the MGCP protocol is the concept of packages. The MGCP gateway loads the DTMF package on start-up. The MGCP gateway sends symbols over the control channel to represent any DTMF tones it receives. Cisco CallManager then interprets these signals and passes on the DTMF signals, out-of-band, to the signaling endpoint. MGCP digit events are sent using NTFY messages to the call agent, which plays them on the remote gateway using RQNT messages with signal playout request.
If either the Cisco proprietary mode or the NSE mode is used, each gateway is unaware of the remote gateway DTMF-relay configuration because it is not indicated in the SDP. Each side determines whether to use DTMF relay based on its locally configured mode setting and a combination of voice codec and codec filter settings. Each side uses its locally configured RTP payload type when encoding RTP named event packets.
Interoperability of endpoints relies on matching command-line interface (CLI) configurations.
The value used for NSE mode is configured using the mgcp tse payload command. The payload type for Cisco proprietary mode is 121. This value is used regardless of the configured payload value.
In MGCP out-of-band mode, the call agent uses MGCP control commands to keep both gateways informed of DTMF requirements.
To configure MGCP out-of-band DTMF relay, use the mgcp dtmf-relay voip codec {all | low-bit-rate} mode out-of-band command in global configuration mode.
Note Cisco Catalyst 6000, Cisco Digital Gateway DE-30+, and Cisco Digital Gateway DT-24+ all support MGCP with Cisco CallManager Release 3.1 and later. DTMF relay is enabled by default and does not need additional configuration.
Summary of DTMF Relay Configuration Commands
Command Description
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all: Dual tone multifrequency (DTMF) relay is to be used with all voice codecs.
low-bit-rate: DTMF relay is to be used with only low-bit-rate voice codecs, such as G.729.
cisco: 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 nondigit voice samples, using payload type 121.
nse: RTP digit events are encoded using the format specified in RFC 2833, Section 3.0, and are transmitted in the same RTP stream as nondigit voice samples, using the payload type that is configured using the mgcp tse payload command.
out-of-band: MGCP digit events are sent using NTFY messages to the call agent, which plays them on the remote gateway using RQNT messages with S: (signal playout request).
nte-gw: RTP digit events are encoded using the NTE format specified in RFC 2833, Section 3.0, and are transmitted in the same RTP stream as nondigit voice samples. The payload type is negotiated by the gateways before use. The configured value for payload type is presented as the preferred choice at the beginning of the negotiation.
nte-ca: Identical to the nte-gw keyword behavior except that the call agent local connection options a: line is used to enable or disable DTMF relay.
Defaults
For the Cisco 7200 series router, the command is not enabled.
For all other platforms, noncompressed codecs are disabled.
Usage Guidelines
Use this command to access an announcement server or a voice-mail server that cannot decode RTP packets containing DTMF digits. When the mgcp dtmf-relay command is active, the DTMF digits are removed from the voice stream and carried so that the server can decode the digits.
Only VoIP supports the mode keyword for forwarding digits on codecs.
You must enter additional configuration parameters in the Cisco CallManager MGCP gateway configuration interface.
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