Facsimile, as an application, is a means of transmission of fixed images, with or without half-tones. It is not a general graphics format, but intended to transfer standard-sized paper documents. It can be transmitted in several ways, both analog or digital, which, with appropriate converters, can use the Internet.
Also part of facsimile is the process by which a physical document is scanned into a form compatible with standard facsimile transmission, which are made up of raster lines made up of black-and-white, greyscale, or color pixels.
The International Telecommunication Union defines four groups of facsimile, differing principally in the amount of bandwidth required for transmission. For a given document scanned at a particular resolution, the less bandwidth required, the faster the entire document can be sent. Reducing the bandwidth requirement also allows the document to be scanned at a higher resolution, and still be transmitted in a reasonable time.
Older analog transmission on the PSTN
Anyone who has used an analog fax knows there are a series of tones at the start of the call. This tone sequence has two purposes:
- Disables, for the duration of the call, some telephone line features (e.g., echo cancellation) that improve analog voice transmission but interfere with facsimile or data modems
- Establish when the facsimile will start sending the actual fax information, starting from the top left corner.
To transmit a nominal 216x279mm/8.5x11 inch document takes 6 minutes with a G1 facsimile and 3 minute with a G2 link. Transmission time can be reduced by reducing the resolution, and thus the amount of information to be transmitted.
Digital public network transmission
Group 3  and Group 4  both transmit digital encoding for the values of pixels. G3 expects to use a facsimile modem, more tolerant to error than a data modem, over a voice channel, while G4 expects to run over a 64 Kbps digital channel. Such a digital channel can be provided directly with a connection on the public Integrated Services Digital Network (ISDN), or through any compatible digital multiplexer channel.
These two groups will transmit the reference document, used as the example of transfer speed for G1 and G2, in 1 minute or less.
New PSTN-compatible fax modes
Current fax machines use the V.34 specification, which is not only faster, but has more robust fallback modulation than that of the V.17 (14.4 Kbps) machines. Robust fallback is especially useful on telephone lines of poor quality. Even if a V.34 fax machine cannot operate at its full speed of 33 Kbps, it may be able to transmit at 15 or 19 Kbps during fallback, where a corresponding V.17 fax machine would fallback to 9600 or 4800 bps.
Color fax has been a solution in search of a problem. Graphics programs, of course, can send their native files. Color fax would be useful in the case where one has a hard copy of a color document to send, but how often is this a requirement? Most machines sold as color faxes are principally used as color printers.
An alternative to fax, addressing the delivery confirmation issue, is the Internet Printing Protocol (IPP) by the IETF 
Facsimile over Internet Protocol
Facsimile over Internet Protocol (FoIP) involves taking information in an appropriate facsimile-compatible format encapsulating it in packets and sent over the network. With appropriate software, a computer can print to what the application software thinks is a physical printer, but actually is a program that creates facsimile-compatible files. The information goes through the Internet as a MIME attachment to a mail message. The MIME attachment is in the format defined by ITU T.37 specification to be a TIFF-S (Tagged Image File Format profile S) formatted document.
There are three modes in which FoIP works. The first two need a "fax gateway" device, which is usually a commercial router with both data (e.g., Ethernet/IEEE 802.3) and analog PSTN interfaces, and appropriate conversion software.
- On-ramp, where the source is a computer and the destination is a Group 3 analog facsimile. The Internet is way to get "on" the FoIP highway. Somewhere along that highway, there must be a packet-to-analog device that converts the packets to an analog telephony format.
- Off-ramp, where the destination is a computer and the source is an analog fax; the gateway receives from the analog network and produces packets
- Faxmail, where both ends are computers, but using a facsimile format is a very simple way to send images between the two machines, making no assumptions about graphic software.
Real-time versus store-and-forward
There are two ways the information can be transferred:
- T.38—Real-time Group3 Fax over IP networks
- T.37—Store and Forward (S&F) fax on the Internet
Off-ramp can be limited to things done in a router and a mail server, making it transparent to end users of computers and fax machines. On-ramp may require additional software in the end computer, such as a desktop PC.
Fax and VoIP
High-efficiency codecs, such as G.729, encode using quite a few assumptions about the information flow having the characteristics of speech, which means fax will not work over this type of encoding. Such encoding is normally use for VoIP. To pass fax over codecs, you must use a high-bit rate codec such as 64 Kbps G.711.
While you can hear the bleepity-beep tones of analog fax, its statistical characteristics are not at all the same as of speech. High-efficiency codecs make assumptions that the analog tones carried have the characteristics of speech. Simply connecting a fax machine to an analog interface to a VoIP system does not mean the fax can transmit. Most VoIP service providers, which provide gateways that use an Ethernet connection to the broadband network, have one or more analog connectors, but they must set designated analog connectors to be fax-compatible. There is usually a small extra charge, because the digitized FoIP consumes more bandwidth than than that of speech.
There are two methods and protocols to packetize fax, the older store-and-forward ITU T.37 and the newer real-time T.38. T.38 is the method called for in the multimedia H.323 architecture. H.323 is one of several protocol families used in the broad sense of VoIP (including FoIP), the others being Session Initiation Protocol (SIP) and Media Gateway Control Protocol (MGCP).
Some older fax protocols still are a part of FoIP. ITU T.30 deals with negotiating the setup between fax machines, and T.4 defines the actual formatting of the page being digitized and sent. Even with Group 4, digital fax requires guaranteed Quality of Service.
Both T.37 and T.38 have advantages and disadvantages. Store-and-forward T.37 is inherently more efficient, in that it can be programmed to send faxes only when no voice calls are in progress. T.37 integrates cleanly with electronic mail. You are able to configure email servers to retry continuously until successful and offer never busy fax service. The use of email aliases and distribution lists allow a single fax to be sent to multiple email addresses and conversely, for a single email to be sent to multiple fax machines. Some real-world facsimile applications, as in medical or legal areas, require that receipt of the fax must be confirmed. Only T.38 can provide such confirmation in real time.
Internet Printing Protocol (IPP)
IPP provides a standard way to print to a remote printer, and receive a status report from that printer. IPP has the potential to replace even T.37 as the way an IP fax is delivered. There are issues around finding the address of the destination printer and getting rights to print on that printer, but once those issues are resolved, IPP could be deployed fairly rapidly. No new hardware is required, only new software drivers. T.37 offers no advantages over IPP.
- CCITT/ITU Recommendation T.2
- CCITT/ITU Recommendation T.3
- While these are obsolete specifications, the North American and international frequencies for black and for white are different.
- CCITT/ITU Recommendation T.3
- CCITT/ITU Recommendation T.4
- Butler S, Moore P, Turner R, Wenn J (September 2000), Herriot R, ed., Internet Printing Protocol/1.1: Encoding and Transport, RFC2910
- Hastings T, Herriot R, Kugler C, Lewis H (September 2000), Internet Printing Protocol (IPP): Job and Printer Set Operations, RFC3380