![]() The DSP also triggers an internal event to notify the call control stack that fax switchover is required. The DSP in the Cisco IOS gateway attached to the fax machine that generated the DIS message (normally the TGW) detects the HDLC flag sequence at the start of the DIS message and initiates fax relay switchover. A normal fax machine, after generating a CED or hearing a CNG, sends a DIS message with the capabilities of the fax machine. The ANSam or CED tone causes a switch to modem pass-through, if enabled, to allow the tone to pass cleanly to the remote fax.ģ. At anytime during the life of the call, if a fax answer or calling tone (ANSam or CED) is heard, the DSP does not interfere with the speech processing. ![]() Call control procedures are followed, and the DSP is put into voice mode, after which human speech is expected to be received and processed.Ģ. Initially, a VoIP call is established as if it were a normal speech call. The SIP T.38 fax relay call flow is as follows:ġ. Then, during the call, when the DSP detects fax HDLC flags, it signals the detection of the flags to the call control layer, and the call control layer initiates a SIP INVITE message mid-call to signal the desire to change the media stream. The DSP needs to be informed that it can support T.38 mode while it is put into voice mode. ![]() Initially, a normal VoIP call is established using SIP INVITE messages. When the call control protocol is SIP, T.38 Annex D procedures are used for the changeover from VoIP to fax mode during a call. Figure 2-12 illustrates a SIP T.38 relay operation.
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