Fax pass-through takes place when incoming T.30 fax data is not demodulated or compressed for its transit through the packet network. The two endpoints (fax machines or modems) communicate directly to each other over a transparent IP connection. The gateway does not distinguish fax calls from voice calls.
On detection of a fax tone on an established VoIP call, the gateways switch into fax pass-through mode by suspending the voice codec and loading the pass-through parameters for the duration of the fax session. This process, called upspeeding, changes the bandwidth needed for the call to the equivalent of G.711.
With pass-through, the fax traffic is carried between the two gateways in RTP packets using an uncompressed format resembling the G.711 codec. This method of transporting fax traffic takes a constant 64-kbps (payload) stream plus its IP overhead end-to-end for the duration of the call. IP overhead is 16 kbps for normal voice traffic, but when switching to pass-through, the packetization period is reduced from 20 ms to 10 ms, which means that half as much data can be put into each frame. The result is that you need twice as many frames and twice as much IP overhead. For pass-through, the total bandwidth is 64 plus 32 kbps, for a total of 96 kbps. For normal voice traffic, total bandwidth is 64 plus 16 kbps, for a total of 80 kbps.
Packet redundancy may be used to mitigate the effects of packet loss in the IP network. Even so, fax pass-through remains susceptible to packet loss, jitter, and latency in the IP network. The two endpoints must be clocked synchronously for this type of transport to work predictably.
Performance may become an issue. To attempt to mitigate packet loss in the network, redundant encoding (1X or one repeat of the original packet) is used, which doubles the amount of data transferred in each packet. The doubling of packets imposes a limitation on the total number of ports that can run fax pass-through at one time. You can calculate that two voice sessions with no VAD equate to one fax pass-through session with redundancy.