MGCP DTMF Support
The four current implementations of MGCP-based DTMF relay include
■ Cisco proprietary: DSPs on the gateways send and receive DTMF digits in-band in
the voice RTP stream but code them differently so they can be identified by the
receiver as DTMF tones.
■ NSE: Conforms to RFC 2833 to provide a standardized method of DTMF transport
using NTEs in RTP packets. RFC 2833 support is standards-based and allows greater
interoperability with other gateways and call agents.
■ NTE: Provides for two modes of implementation:
■ Gateway-controlled mode: In gateway-controlled mode, the gateways negotiate
DTMF transmission by exchanging capability information in SDP messages. That
transmission is transparent to the call agent. Gateway-controlled mode allows the
use of the DTMF relay feature without upgrading the call agent software to support
the feature.
■ Call agent-controlled mode: In call-agent-controlled mode, call agents use
MGCP messaging to instruct gateways to process DTMF traffic.
■ Out-of-band: Sends the tones as signals to Cisco Unified Communications Manager
out-of-band over the control channel. Cisco Unified Communications Manager interprets
the signals and passes them on.