6.5 Using T.38 Fax

The IP media software supports sending fax information during a session using the T.38 protocol, as shown in Figure 6.

Note: Another method of transferring fax information is to use the G.711 protocol. In this case, the fax data is sent from a fax-capable board in the system across the TDM bus. The IP media software can then send the data outside the system using IP. However, this method uses more bandwidth than the T.38 method.

To set up a channel to handle T.38 fax, do the following:

1. Open a channel using ipm_Open("ipmB1C1",NULL,EV_SYNC)
2. Enable event reporting using ipm_EnableEvents(chDev, *pEvents) and the IPMEV_T38CALLSTATE and IPMEV_FAXTONE events.
3. Get local RTP information using ipm_GetLocalMediaInfo(chDev,&MediaInfo) and setting the eMediaType field to MEDIATYPE_LOCAL_RTP_INFO
4. Start an RTP call using ipm_StartMedia(chDev)
5. When the fax event IPMEV_FAXTONE is received, the application should first stop the call in progress using ipm_Stop(chDev,STOP_MEDIA), then retrieve the local T.38 fax information using ipm_GetLocalMediaInfo(chDev,&MediaInfo) and the eMediaType field MEDIATYPE_LOCAL_UDPTL_T38_INFO

   Note: It is the responsibility of the application to respond promptly when the fax event is received or latency errors may occur. Refer to the T.38 Fax specification and ITU-T T.30 specification for latency guidelines. (Details on fax timing are in the T.30 specification.)

   Note: CED and CNG tones must be exchanged before switching to T.38 mode. Also, CED and CNG tones will not transmit reliably over coders other than G.711 and G.726.

6. Once the remote fax information is available, the application then starts a fax session by calling ipm_StartMedia(chDev)
7. When the fax event IPMEV_T38CALLSTATE is received, with the reason code T38CALLSTATE_DISCONNECT, the application can stop the fax session using ipm_Stop(chDev,STOP_MEDIA)

Figure 6. Using T.38 Fax Scenario Diagram

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