DTMF Relay on Cisco routers

Dual tone multi-frequency (DTMF) signaling is used on traditional analog plain old telephony service (PoTS) phones to send digits to the switching center for call routing or some other specialized function. DTMF digits are also used to specialized functions like voice mail login, automated attendant (AA), and interactive voice response (IVR) systems. DTMF digits are frequency combinations that enable tone dialing on the PSTN. Pulse dialing was historically used with rotary phones. Voice over IP systems have many methods to leverage DTMF digits depending on the signaling mechanism used. All traditional time division multiplexing (TDM) interfaces on Cisco routers support passing DTMF digits by default. Cisco VoIP (Voice over IP) dial-peers do not support DTMF digits by default. Multiple dtmf-relay capabilities can be configured on a VoIP dial-peer depending on the signaling protocol in use. The VoIP dial-peer will then pass the DTMF digits in-band (IB) or out of band (OOB). In band DTMF-Relay will use a special payload type identifier (PTI) in the RTP header of the voice media stream to distinguish digits from the DTMF pad from actual voice communication. An out of band mechanism will pass DTMF digits in the signaling protocol (SCCP, SIP, MGCP, or H.323) instead of using the RTP media stream. There are four DTMF relay mechanisms that are supported on VoIP dial-peers: • H245-alphanumeric • H245-signal • SIP-Notify • Rtp-nte H.245 mechanisms are only available on H.323 dial-peers. H.245 is part of the H.323 protocol suite. is the default protocol used on VoIP dial-peers. SIP Notify is only used in the SIP signaling protocol. RTP NTE stands for Real-Time Protocol (RTP) Named Telephone Events (NTE). NTE are mechanisms used in the IETF (Internet Engineering Task Force) RFC (request for comments) 2833. RFC2833 is the de facto standard used in the industry standard for multi-vendor interoperability in SIP environments. The next blog entry will focus on the method in which DTMF digits are passed from Cisco phones.

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