Short and sweet for this lovely Wednesday morning:
Fax passthrough encodes fax traffic with in a G.711 voice codec and sends it across the VoIP network as a voice call. The call may use any codec (G.711, G.729, G.723) etc. initially and once a 2100 Hz CED tone is detected, the device (ATA, for example) tells the far end gateway to switch over to G.711 using a peer-to-peer message. This message is called a NSE message (Named Signalling Event) with in the RTP stream.
Here are the important things to know about NSE-based passthrough for fax faxes and modems:
- The 2100 Hz CED tone played from the terminating fax is the stimuli tone for initiating NSE-based passthrough for normal G3 fax devices. If the DSP on the TGW never detects this tone then the switchover to passthrough will never occur.
- Upon detecting the 2100 Hz CED tone, the TGW initiates the passthrough switchover using an NSE-192 message. The OGW responds with an NSE-192 message.
- The NSE-192 message upspeeds the codec to G.711, disables VAD, and sets the jitter buffer appropriately.
- The NSE-193 is triggered by an ANSam tone that is only found with high speed modem and SG3 fax calls. The NSE-193 signals that the echo cancellers on the voice gateways need to be disabled.