DTMF on VoIP
When carrying them on the SIP Network you could probably see the following methods of conveying these tones across:
DTMF are sent using the same RTP stream as the media is using, and can be heard by carries in a session. Compression Codecs such as G.729 and G.723 may make tones unintelligible so it really works on better codecs like G.711
2.- RCF 2833:
(config)#dial-peer voice 100 voip
rtp-nte RTP Named Telephone Event RFC 2833
this is an out of band method that takes DTMF out of the RTP Stream, this means that the DTMF codes works even if the voice stream is compressed. This packets travelling out of band of RTP, hold events that can be understood by UA and regenerated, DTMF-related named events within the telephone-event payload format. http://www.ietf.org/rfc/rfc2833.txt
(config-dial-peer)#voice-class sip dtmf-relay force rtp-nte
A hidden command that forces the “voice-class sip dtmf-relay force rtp-nte” DTMF relay negotiation to rtp-nte and It’s only necessary if the other side doesn’t advertise rtp-nte.
output from deb ccsip media -
000373: Dec 4 02:11:45.727: //55/18EBD6C48068/SIP/Media/sipSPIUpdCallWithSdpInfo: Stream type : voice+dtmf Media line : 1 State : STREAM_ADDING (2) Callid : -1 Negotiated Codec : g711ulaw, bytes :160 Nego. Codec payload : 0 (tx), 0 (rx) Negotiated DTMF relay : rtp-nte Negotiated NTE payload : 101 (tx), 101 (rx) Negotiated CN payload : 0 Media Srce Addr/Port : 10.0.1.5:0 Media Dest Addr/Port : 192.168.21.31:20424 000374: Dec 4 02:11:45.727: //55/18EBD6C48068/SIP/Info/sipSPIHandleInviteMedia: Negotiated Codec : g711ulaw, bytes :160 Preferred Codec : g711ulaw, bytes :160 Preferred DTMF relay 1 : 6 Preferred DTMF relay 2 : 0 Negotiated DTMF relay : 6 Preferred and Negotiated NTE payloads: 101 101 Preferred and Negotiated NSE payloads: 100 100 Preferred and Negotiated Modem Relay: 0 0 Preferred and Negotiated Modem Relay GwXid: 1 0
output from debug voip rtp session named-event will show digit 5 sent in 7 packets -
Feb 06 10:03:00.910: Pt:101 Evt:5 Pkt:04 00 00 <Snd>>> Feb 06 10:03:00.910: Pt:101 Evt:5 Pkt:04 00 00 <Snd>>> Feb 06 10:03:00.910: Pt:101 Evt:5 Pkt:04 00 00 <Snd>>> Feb 06 10:03:00.910: Pt:101 Evt:5 Pkt:04 01 90 <Snd>>> Feb 06 10:03:00.910: Pt:101 Evt:5 Pkt:84 03 20 <Snd>>> Feb 06 10:03:00.910: Pt:101 Evt:5 Pkt:84 03 20 <Snd>>> Feb 06 10:03:00.910: Pt:101 Evt:5 Pkt:84 03 20 <Snd>>>
The first packet says that it is the start of a new NTE digit because it does not have the endbit set .
The second and third packets are repeats of the first packet for redundancy.
The fourth packet is a refresh packet with a duration of 50ms (0×0190 = 400 samples * 1sec / 8000 samples).
The fifth packet is the endbit packet (84) with a duration of 100ms (0×0320 = 800 samples * 1sec / 8000 samples).
The sixth and seventh packets are redundant packets for packet five.
in this RFC more events are defined, like for example: Fax related tones, Standard subscriber and Country Specific line tones and Trunk Events
This http://www.rfc-editor.org/rfc/rfc4733.txt supersedes RFC 2833, since devices do not have to support every tone and event there is, they just simply advertise what they DO support when setting up a a connection
3.- SIP INFO: http://www.faqs.org/rfcs/rfc2976.html
This method is used to carry session control information along the SIP Signaling path during an existing session. SIP info can carry the digits you type without changing the characteristics of the SIP Session.
(config)#dial-peer voice 100 voip (config-dial-peer)#session proto sip (config-dial-peer)#dtmf-relay ? cisco-rtp Cisco Proprietary RTP h245-alphanumeric DTMF Relay via H245 Alphanumeric IE h245-signal DTMF Relay via H245 Signal IE rtp-nte RTP Named Telephone Event RFC 2833 sip-kpml DTMF Relay via KPML over SIP SUBCRIBE/NOTIFY sip-notify DTMF Relay via SIP NOTIFY messages
you can not configure Cisco SIP-INFO to generate requests for DTMF tones, since this method is Considered Harmful based on http://tools.ietf.org/html/draft-rosenberg-sip-info-harmful-00
The SIP INFO Method for DTMF Tone Generation feature is always enabled, and is invoked when a SIP INFO message is received with DTMF relay content. This feature is related to the SIP NOTIFY-Basec Out-of-Band DTMF Relay Support feature, which provides the ability for an application to be notified about DTMF events using SIP NOTIFY messages. Together, the two features provide a mechanism to both send and receive DTMF digits along the signaling path.