6.1 Introduction to DTMF Handling

When a session is started on an IPM device, the IPM device receives data from its IP interface and transmits data towards the TDM bus. A DTI device receives data from its PSTN interface and transmits towards the TDM bus as well. In a gateway configuration, the DTI and IPM devices will be configured, via gc_Listen( ) and ipm_Listen( ) respectively, to listen to each other and thus create a full duplex communication path. The IPM device will forward DTMF that it receives on one interface to the other interface. Figure 1, "IP Media Architecture" shows the data flow between the IP media library, the IP network, and the PSTN network.

When an IPM device receives DTMF from the TDM bus, there are several ways to forward it towards the IP interface. These include: forwarding it in the RTP stream (also called in-band), sending via RFC 2833, and using an application-controlled/defined method (also called out-of-band).

The IPM device can automatically forward the DTMF when either the in-band or RFC 2833 DTMF transfer mode has been selected. DTMF is not automatically forwarded when the application controlled/defined method, also known as out-of-band mode, has been selected. In the out-of-band case, the application must call ipm_ReceiveDigits( ) and have an IPM_DIGITS_RECEIVED event handler in place. Upon receiving the IPM_DIGITS_RECEIVED event, the DTMF information is contained in the IPM_DIGIT_INFO structure delivered with the event. The application has the responsibility to forward the DTMF via whatever mechanism, open or proprietary, it desires.

When using RFC 2833 mode, the DTMF could optionally be sent in both RFC 2833 packets and in-band. The default is that the DTMF is only sent in the RFC 2833 packet and the audio is muted. Setting the mute audio parameter (IPM_RFC2833MUTE_AUDIO) to RFC2833MUTE_AUDIO_OFF will cause the DTMF to be sent in-band as well as via RFC 2833.

Note: Use caution when using both RFC 2833 and in-band DTMF, because an endpoint device may recognize two separate digits instead of one.

When using out-of-band mode, the DTMF is never transmitted in-band. As mentioned earlier, the application has the responsibility to forward the digits.

The setting for DTMF transfer mode also affects the handling of DTMF that is received from the IP interface. When the mode is set to in-band, the DTMF is automatically forwarded to the TDM bus.

If out-of-band mode has been selected, then the application will use its own mechanism to be notified that a DTMF digit has been received. Then, ipm_SendDigits( ) is used when necessary to transmit a DTMF digit towards the TDM bus.

When the mode is set to RFC 2833, DTMF is automatically forwarded to the TDM bus as PCM data.

Note: For Intel® NetStructure™ DM/IP Series boards only: if you wish to be notified of RFC 2833 packets as they arrive at the IP port, the application must enable the EVT_RFC2833 event via a call to ipm_EnableEvents( ). Upon receiving the IPMEV_RFC2833SIGNALRECEIVED event, the DTMF information is contained in the IPM_RFC2833_SIGNALID_INFO structure. The application must use ipm_SendDigits( ) to forward the digit towards the DTMF bus.

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