Monday, August 5, 2013

DTMF or Telephony event support on SDP and in RTP in Microsoft lync audio conference

DTMF (Dual Tone Multi Frequency) is a type of signaling used primarily in voice telephony systems. It features audible tones in the frequency range of the human voice which are typically used when dialing a call (on analog lines) or when operating an IVR menu. There are many other applications for this signaling. For more information, see the wikipedia article on DTMF (


A dtmf packet is 16 bytes.

DTMF data = 12 bytes (RTP header) + 4 bytes (RTP payload)

4 bytes (RTP payload) has mapping of key press value as event. After parsing this 4 bytes the receiver end can get instruction or event what to do.

       dtmf payload is 101 . It is passed on audio level of lync SDP.
m=audio 5476 RTP/AVP 101
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16

 a=fmtp:101 0-16  indicates the key value that is supported.

  1. Start the Lync Server Management Shell: Click Start, click All Programs, click Microsoft Lync Server 2013, and then click Lync Server Management Shell.
  2. Run the following at the command prompt:
    This cmdlet returns the DTMF settings used for dial-in conferencing.

Default Mapping:

Identity                                                       :  Global
CommandCharacter                                    :  *
MuteUnmuteCharacter                                :  6
AudienceMuteCommand                             :  4
LockUnlockConferenceCommand               :  7
HelpCommand                                            :  1
PrivateRollCallCommand                             :  3
EnableDisableAnnouncementsCommand      :  9
AdmitAll                                                      :  8

If you want to mute a call in conference , then in lync key pad just type *6 .

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