Editing Voice over IP (section)

==Fax support==
Sending [[fax]]es over VoIP networks is sometimes referred to as Fax over IP (FoIP). Transmission of fax documents was problematic in early VoIP implementations, as most voice digitization and compression [[codec]]s are optimized for the representation of the human voice and the proper timing of the modem signals cannot be guaranteed in a packet-based, connectionless network.

A standards-based solution for reliably delivering fax-over-IP is the [[T.38]] protocol. The T.38 protocol is designed to compensate for the differences between traditional packet-less communications over analog lines and packet-based transmissions which are the basis for IP communications. The fax machine may be a standard device connected to an [[analog telephone adapter]] (ATA), or it may be a software application or dedicated network device operating via an Ethernet interface.<ref>{{cite web |url=http://soft-switch.org/foip.html |website=Soft-Switch.org |title=Faxing over IP networks |url-status=dead |archive-url=https://web.archive.org/web/20231218003952/https://www.soft-switch.org/foip.html |archive-date= Dec 18, 2023 }}</ref> Originally, T.38 was designed to use UDP or TCP transmission methods across an IP network.

Some newer high-end fax machines have built-in T.38 capabilities which are connected directly to a network switch or router. In T.38 each packet contains a portion of the data stream sent in the previous packet. Two successive packets have to be lost to actually lose [[data integrity]].