Showing posts from February, 2012


The current implementations of DTMF relay for MGCP are Cisco proprietary, Named Signaling Event (NSE), NTE-CA, NTE-GW, and out-of-band. Keep in mind that NSEs are the Cisco-Proprietary version of NTEs. They use different PT value than NTEs which is 100 (fixed value).
MGCP RTP-NTE has two implementations which are Gateway (GW) Controlled and Call Agent (CA) Controlled. In GW-controlled mode, GWs negotiate DTMF transmission and RTP PT values by exchanging capability information in SDP messages. That transmission is transparent to the CA. Both GWs are running MGCP and connected to same CUCM. In CA-controlled mode, CA will negotiate DTMF relay capabilities on behalf of the gateways (SDP messages are sent to CA). This is a must when the other GW/Endpoint is non-MGCP. After negotiation, CA will instruct MGCP gateway to use the agreed RTP-NTE values.
Note: In MGCP, RTP-NTE PT values range from 98-119 which is different that RFC2833 (96-127).
Similar to H323, in case negotiation fails, in-band v…

H323 DTMF Relay

H323 endpoints support four types of DTMF relay methods which are:
1. Cisco Proprietary In this method DTMF signals are carried in RTP stream with PT 121. This PT value won't be negotiated during call setup and thus both ends should be configured to use same method. OGW will send RTP PT 121 without verifying if TGW supports this or not.
RTR#debug voice rtp session named-event s=DSP d=VoIP payload 0x79 ssrc 0x1F1E sequence 0xE93 timestamp 0xB437938A Pt:121Evt:1Pkt:01 03 20>> s=DSP d=VoIP payload 0x79 ssrc 0x1F1E sequence 0xE94 timestamp 0xB437942A Pt:121Evt:1Pkt:02 03 20>>
*** Note: DTMF digit pressed is '1'

2. H.245 Alphanumeric
In this method DTMF signals are carried as H.245 messages (OOB). However, this method can't provide the duration of DTMF digit, i.e. for how long this digit was pressed. It always assume that the duration is 500 msec.
RTR#debug h245 asn1 value MultimediaSystemControlMessage ::= indication : userInput : alphanumeric : "1"
During call …