Fax Over IP or FoIP (pronounced "Foyp") refers to the transport of faxes over IP networks, using the surrounding protocols (T.37, T.38, SIP, H.323), transmission methods (Real Time, Store & Forward), PSTN to IP gateways and/or VoIP phone systems, and other fundamental components that help in the delivery of faxes over the internet ("internet faxing") or within a corporate intranet.
While VoIP (Voice over IP) only needs to be audibly reproduced, IP faxing needs to be quite resilient to preserve data without information loss and at the same time smart enough to communicate with unsophisticated fax machines around the world. Fax machines do not understand IP, and are subject to the effects of network jitter and latency that have no effect on VoIP calls.
All fax devices support the TIFF (Tagged Image File Format) format. Issues arise over the compression methods used to reduce image size. Although every fax device uses “MH” (Modified Huffman) compression, other compression methods may not be supported by the sender and the receiver. The handshaking method defined in the T.30 standard determines the best mutually compatible compression method, fax resolution (e.g., 200 dpi x 200 dpi or 300 dpi x 300 dpi), and transmission speed (e.g., 14.4 kbps or 33.6 kbps). Some devices that failed to comply with the full breadth of the T.30 protocol.
The T.38 protocol, while based on T.30 protocol, supports IP networks, which use packet-switching rather than the circuit-switching used in PSTN (public switched telephone network). T.38 also solves the problem created when packets arrive in the wrong sequence. T.38 will interoperate with T.30 only if a gateway between the two nodes converts the IP stream to the pulse-code modulation (PCM) used in circuit-switched networks.