thinQmaster Virtual testing partner Adaptive practice exams that find your weak spots and drill them. Every answer explained. From $69
Enroll

thinQmaster

Find your exam

Five questions free, before you decide.

Step 1 · choose a vendor

In one of our camps? Your practice exams come with the tuition — they are on your study page already.

Collaboration, Voice & AV · Intermediate

Fax Passthrough

Sending fax tones as uncompressed G.711 audio within RTP, with VAD and echo cancellation disabled.

Definition

With passthrough the gateway detects fax tones and upspeeds the call to G.711 if needed, turning off features that would distort modem signals. It is simple and widely interoperable but sensitive to packet loss and jitter because any lost packet corrupts the fax modulation. Modem passthrough works the same way for analog modems.

Example

modem passthrough nse codec g711ulaw lets an alarm panel modem call through a SIP gateway.

Exam tip

Passthrough requires a lossless, low-jitter path; T.38 relay is more resilient.

Related terms