Let’s continue our fax machine invention article. Now we will take a look at fax standards. The first time fax data transmission attempted unification was in 1966 by Electionic Industries Alliance (EIA) with the so called “Group 1″ standard. It is used only for analog lines and it was the first step of making compatibility between fax machines from different manufacturers. In these days the speed was not very high. It was taking between 4 and 6 minutes to transmit a single fax page! The resolution was 96 vertical scan lines per inch. The standard scan speed of the “Group 1″ machines was 180 lines per minute. The frequency of the signal was between 1300Hz (white color) up to 1900Hz (black color).
The biggest manufacturers in the USA did not comply well with the “Group 1″ standard inthe beginning. Some of them even used different schemes for modulation of the signal. That way the transmission of fax data between USA and the rest of the world (where the standard was adopted better at this time) was practically impossible. The “Group 1″ fax standard is now outdated and these machines have not been manufactured for some time.
In 1978 the international organization “Comite Consultatif International de Telegraphique et Telephonique” (CCITT), which is now known as ITU, defined the “Group 2″ standard which was immediately adopted by all fax machine manufacturers. The analog transmission speed was doubled at almost the same resolution as “Group 1″ – 100 scan lines per inch. These machines were used for a long time, however in the last two decades their usage is decreasing significantly since the digital phone lines are actively replacing analog ones.
The modern fax machines era came with the ITU-T Recommendation T.4 in 1980 with the digital phone line standard called “Group 3″. The speed of transmission can be from 9600 up to 33600 bits per second with which the required time for transmission of one fax page is decreased significantly. There are different variants available for resolutions; however most of the fax machines are using two settings:
- Standard mode: 203×98 dots per inch;
- Fine mode: 209×196 dots per inch.
There is also a “superfine” or “ultrafine” mode available on some machines. Most of the fax machines today use the “Group 3″ standard and usually use the fine mode as default resolution.
The “Group 4″ standard came in 1984. This standard is primarily designed for digital ISDN lines but today it is also very popular for VoIP lines. It provides capabilities for much higher transfer rate of up to 64 kilobits per second. In “Group 4″ the available resolutions can be higher – up to 400×600 dots per inch (this is comparable to the quality of regular printers). If a “Group 4″ machine transmits to a “Group 3″ machine (or vice versa), the “Group 4″ machine will “fall back” to a “Group 3 compatibility mode” and of course both the resolution and the transfer rate will be lower.
How are the different fax machines “understanding” each other? During the fax-modem “handshake” (this happens before the transmission of the actual fax letter) the fax machines are exchanging information between each other. First they tell exchange information about the standards that they are following. If one of the machines is using a higher Group standard, it should be able to “fall” to “compatibility mode” in order to conform with the machine which is using the older (lower) standard. After that the machines must define what transfer rate will be used. The rule is that they will use “the highest transfer rate of the slower machine”. That way the transfer rate will be the maximum possible in which both machines can work properly. If there are some issues (like distorted phone signal or low bandwidth, the connection may fail to continue properly. If that happens the sender or the recipient of the fax letter should retry with lowering the transfer rate of the fax machine. That way he will simulate a slower machine and this will give more time for the machines to “clear” the distortions from the phone signal.
For “Group 3″ machines today the speed shouldn’t be slower than 14400 bits per second. Since the v34bis protocol modems are the most popular today, usually the transfer rate is always at the maximum of 33600 bits per second. It should be lowered manually on one of the negotiating machines only if there are issues with the phone line. The same applies for “Group 4″ machines which suffer from insufficient bandwidth.
With the Internet era many alternative variants started to be available like the “Fax over IP” for example. Probably the most popular method for transmitting faxes today is the “Internet fax” and the “fax to e-mail” services in particular. The Internet fax services are fully compatible with the Group 2, 3 and 4 standards; however the faxes are transmitted through a single server with many lines and fax modems in rack and that way the users do not have to buy and maintain the expensive fax machine and dedicated phone/ISDN line. To use a “fax to e-mail” service all you need to have is an Internet connection and an e-mail account. Additional comfor is obtained by knowing that no paper is used – all messages are transferred electronically. That way the user has an option to choose which of the received fax letters are important and which are not and they can print “hard copies” only for the documents they need. Therefore the Internet faxing is much more “environment friendly” than regular fax machines.
References:
Some information is rephrased from the Wikipedia “Fax article”










