Gateway Signaling Protocols and Fax Pass-Through and Relay
Figure 2-9 illustrates a fax pass-through operation. When a terminating gateway (TGW)
detects a called terminal identification (CED) tone from a called fax machine, the TGW
exchanges the voice codec that was negotiated during the voice call setup for a G.711
codec and turns off echo cancellation and VAD. This switchover is communicated to the
originating gateway (OGW), which allows the fax machines to transfer modem signals as
though they were traversing the PSTN. If the voice codec that was configured and negotiated
for the VoIP call is G.711 when the CED tone is detected, there is no need to make
any changes to the session other than turning off echo cancellation and VAD.
Figure 2-9 Fax Pass-Through Operation
If pass-through is supported, these events occur:
1. For the duration of the call, the DSP listens for the 2100-Hz CED tone to detect a
fax or modem on the line.
2. If the CED tone is heard, an internal event is generated to alert the call control stack
that a fax or modem changeover is required.
3. The call control stack on the OGW instructs the DSP to send an NSE to the TGW,
informing the TGW of the request to carry out a codec change.
4. If the TGW supports NSEs, it responds to the OGW instruction and loads the new
codec. The fax machines are able to communicate on an end-to-end basis with no
further intervention by the voice gateways.
Control of fax pass-through is achieved through NSEs that are sent in the RTP stream.
NSEs are a Cisco-proprietary version of IETF-standard named telephony events (NTEs),
which are specially marked data packets used to digitally convey telephony signaling
tones and events. NSEs use different event values than NTEs and are generally sent with
RTP payload type 100, whereas NTEs use RTP payload type 101. NSEs and NTEs provide
a more reliable way to communicate tones and events using a single packet rather
than a series of in-band packets that can be corrupted or partially lost.
Fax pass-through and fax pass-through with up speed use peer-to-peer NSEs within the
RTP stream or bearer stream to coordinate codec switchover and the disabling of echo
cancellation and VAD. Redundant packets can be sent to improve reliability when the
probability of packet loss is high.
When a DSP is put into voice mode at the beginning of a VoIP call, the DSP is informed
by the call control stack whether or not the control protocol can support pass-through.