Real-time fax over IP operates in a similar way of a regular fax transmission. The fax machines involved in the transmission synch up and then the fax data is sent between them over the intervening IP Network.
There are 2 methods of transporting fax in real time across the network:
When using fax-relay, the T30 fax signal from a connected fax machine is demodulated by the sending fax gateway and sent over the IP Network to a remote fax gateway. The remote fax gateway then recontructs the T30 fax signal and send it to the fax.
There are 2 types of fax-relay mechanisms:
Cisco fax-relay is an older method. So a fax gateway terminates T30 fax tones from a local fax machine and then sends the fax data across an IP network by breaking the tones into HDLC frames and then transmitting them using RTP.
T38 fax-relay is the ITU standard T30 fax signal, it is demodulated at the local gateway and encapsulated into IP packets for transport over a network to a remote fax gateway which will then reconstruct the signal and play it to the fax. T38 includes also a mechanism by which a fax gateway can inform the remote gateway of its desire to change the media type from voice to data. T38 can also use TCP or UDP connections but will use more UDP.
For fax pass-through, modulated fax data is sent in-band across the IP network by a fax gateway using a voice codec (like G711 without any VAD or echo-cancellation). Also with fax pass-through, T30 fax calls are not distinguished from regular voice calls, they are simply sent in-band over the IP Network. With the fax detection tones, the gateway must be able then to switch to high-bandwidth codec. Fax pass-through is relatively bandwidth hungry and is sensitive to delay,jitter and packet loss