G.711 is an ITU-T standard for sending voice calls over the Internet on 64 kbs. links. G.711 uses pulse-code modulation (PCM) to take samples of voice frequencies at a rate of 8,000 samples per second. G.711 also can be used to support fax over IP. A technique has been used, called G.711 fax passthrough, which makes it possible to send IP fax through G.711-compliant networks.
G.711 fax passthrough does not distinguish between a G.711 voice call and a fax call, treating both the same way and not doing anything in particular to address the problems of packet loss, jitter and delay. The fax message is carried in its entirety in-band over the voice call. This is a technique that is tried by many businesses to send faxes over the Internet since all they need is the same G.711 standard support that they already have for VoIP calls. G.711 passthrough works over LANs or networks that do not suffer packet losses or excessive delays. It does not work reliably over the open Internet. Some customers mistake G.711 passthrough for all FoIP techniques, which often has them staying away from Internet fax due to lower reliability. G.711 is a voice standard, but G.711 passthrough is not a fax standard.
Faxing is a real-time synchronous technology, while the Internet uses asynchronous packet-switching. Fax requires tight timing tolerances or sessions fail. Latency and jitter, which occur frequently in IP networks, pose a challenge for fax technology engineers. What may work for voice conversations will not necessarily work for fax devices. T.38, an ITU standard, solves this problem with techniques like spoofing, which keeps the fax connection alive while fax data in transit catches up with the process. That way, the transmission does not fail because the handshaking process runs into time-outs. There are times in the fax handshaking process when a timeout can occur because of delays of just a matter of seconds. In packet-switched networks, fax needs to be able to spoof the receiving terminal to eliminate time outs. For companies to achieve the PSTN fax reliability (about a 98% success rate) that they have become accustomed to, T.38, not G.711 passthrough, is a necessary requirement for mission-critical fax applications.
Nonetheless, some companies have had success using G.711 because the IP networks they use have excellent transmission characteristics including not suffering any packet losses, keeping jitter below 300 ms., and keeping delays to less than 150 ms. If a network can maintain such a high-level of performance, then G.711 will work reasonably well. Users still have to worry about IP fax transmissions that go off the main network and have to travel across additional network links.