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RFC 2833 and DTMF Relay

Aug. 12, 2009
Dennis Hartmann

Devices must send dual-tone-multi-frequency (DTMF) when a phone call is routed to an automated system.

Automated attendant (AA), voicemail (VM), or interactive voice response (IVR) systems are some examples of the types of automated systems that can pick up phone calls. Cisco Unity, Unity Connection, and Unity Express are all examples of systems that support both AA and VM functionality.

Cisco Unity Contact Center and Contact Center Express are examples of IVR systems supporting skill-based routing and full automatic call distribution (ACD) functionality. Voicemail systems normally prompt the calling party for a DTMF-based password used for authentication purposes, while AA and IVR systems use DTMF digits for various items including menu navigation and customer number.

DTMF digits can be sent in-band (IB) or out-of-band (OOB), but the most popular, standards-based approach used today is to send DTMF digits in band. In-band relates to the RTP media stream, while out-of-band relates to the signaling path.

After a call is set up by Cisco Unified Communications Manager (CUCM) or Cisco Unified Communications Manager Express (CUCME), the call setup signaling function has ended and the Cisco IP Phone has a direct media path (RTP) to the destination. Signaling is normally not used again until the call is ended or a supplementary service (hold, park, transfer, etc.) is initiated.

RFC2833 is the standards-based mechanism used to send DTMF digits in-band (RTP) that is supported by many vendors in the industry. Unfortunately, RFC2833 (in band) is not supported on older “Type A” Cisco IP phones (7905/7910/7940/7960).

Type B Cisco IP phones (7970/79x1, 79x2, 79x5, 7906) however, do support RFC2833. Media termination points (MTP) are required on CUCM when a device requiring RFC2833 is in a phone call with a device that only supports OOB mechanisms and DTMF digits are used. The CUCM MTP converts IB (RFC2833) signaling messages to OOB signaling messages. Cisco’s Type A phones converted to SIP use a SIP-Notify method for communicating DTMF digits OOB.

RFC2833 support is supported in Cisco IOS gateway configuration of voip dial-peers using the dtmf-relay rtp-nte command. The rtp-nte dtmf-relay option is specified RFC2833, but it can be hard to see the relation of NTE (named telephony events) to RFC2833. NSE (named signaling events - another acronym used in RFC2833) is sometimes referred to in documentation. Both NSE and NTE indicate that signaling events are being sent IB in accordance with RFC2833.

Media termination points (MTP) are dynamically assigned on a call-by-call basis by CUCM if required by the call setup parameters (SDP in SIP/MGCP and H.245 in H.323). MTPs can be supported in software (CUCM), hardware (DSPs), or router IOS (CPU of router) in Cisco Unified Communications. It is normally better to offload the MTP functionality burden from CUCM if possible.

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