facsimile (FAX)
Telecopying (FAX) is an internationally standardized telematic service for text and image transmission. In the case of fax, the paper originals are scanned pixel by pixel or line by line on the sending fax machine using a line ofCCD cells or a Contact ImageSensor( CIS), converted into a serial data signal, transmitted via the public switched telephone network and printed out on the receiving fax machine.
Telecopying is divided into groups according to various performance characteristics and analog or digital transmission type. For analog transmission, there were fax groups 1 and 2, but these are no longer used and have been replaced by fax groups 3 and 4. Fax groups 1 and 2 work with 1,728 pixels per line and 3.85 lines per millimeter. Group 1 takes six minutes to transmit a page of text, while Group 2 is twice as fast at 3 minutes.
Group 3 fax has been around since 1980 and is still the current method that can be used in analog telephone networks and also in ISDN. It is based on the ITU recommendations T.4 for processing the fax template and T.30 for the transmission process.
In fax group 3, documents are transmitted with resolutions of 204 x 98 dpi (normal) or 204 x 196 dpi (high resolution). The data transmission rateis 9,600 bit/s or 14,400 bit/s. The transmission time for a DIN A4 page is about 30 seconds. To ensure that the devices, modems and fax cards in fax group 3 operate smoothly, the International Telecommunication Union (ITU) has developed command sets based on the AT commands, which are grouped into fax classes.
In fax group 4, the international standard for digital fax transmission in ISDN, scanners and printers operate at 400 dpi and a data transmission rate of 64 kbit/s, which reduces transmission time to about 10 seconds.
Fax as an IP service
In addition to fax transmission via analog telephone networks and ISDN according to the ITU recommendation T.30, there is also the possibility of transmitting faxes via the Internet. The technology - Fax over IP( FoIP) - is specified in recommendation T.38 and works with the TCP/IP protocol. On the one hand, transmission can take place in accordance with recommendation T.37 via Internet telephony( VoIP) using the store-and-forward method, and on the other hand in real time in accordance with T.38 as fax over IP (FoIP).