Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!texbell!vector!telecom-gateway From: motcid!segal%cell.mot.COM@uunet.uu.net (Gary Segal) Newsgroups: comp.dcom.telecom Subject: Re: FAX Modems Message-ID: Date: 13 Nov 89 20:08:18 GMT Sender: news@vector.Dallas.TX.US Organization: Motorola INC., Cellular Infrastructure Division Lines: 87 Approved: telecom-request@vector.dallas.tx.us X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@vector.dallas.tx.us X-TELECOM-Digest: volume 9, issue 508, message 7 of 7 piet@cs.ruu.nl (Piet van Oostrum) writes: >Does anyone know what kind of modulation technique is used for faxes. >As I understand, a modern fax does the following: >1. The document is scanned, giving a bitmap. >2. The bitmap is compressed. >3. The compressed bitmap is sent to a remote fax, using some kind of modem > and an appropriate protocol. >4. At the other side the reverse applies. >My question is about step 3. What kind of modem technology is used? >Are there normal modems that can be used to communicate with a fax? Is >a fax card in a computer more than just a modem? The most prevelant fax technology today is know as Group 3. Group 3 fax is specifed in CCITT recomandation T.4, while recomandation T.30 specifies the control link between fax machines (for Groups 1, 2, and 3). T.4 specifes the sizes of the scanning area, transmission times, coding (i.e. run length compression) scheme, line control commands, modulation and demodulation (modem type), and power levels. The modulation methods user are all simplex synchrounous data. They are V.29 9600bps and 7200bps, V.27ter 4800bps and 2400bps, and V.21 300bps. V.21 is used to transfer control information between machines, while the others are used for data transfer. Fax machines change the modulation used in accordance with the T.30 and T.4 recomandations. A typical fax call looks like this (typical means automatic machine at both ends, calling machine has doucment to send, no error correction): 1) Calling fax machine dials number, and generates calling tone, a 1100Hz tone for 0.5 sec repeated every 3 sec. Calling tone (CNG) is used to let a human who answeres the phone know that a fax machine is at the other end. A fax machine that answers ignores CNG. 2) The called fax machine answers, and generates Called Tone (CED), a continuous 2100Hz +- 15Hz tone for 2.6 to 4 seconds. 3) The called fax machine transmits information using V.21 at 300bps. Data transmitted includes the modulation methods the machine supports. Some machines may transmit and ID string or phone number that the operator has previously programmed. That is how the calling machine "knows" who it called (of course the information is only as valid at what was programmed into the called machine). 4) The calling machine responds with information using V.21 at 300bps. Most important, the calling machine tells the called machine what modulation method it is going to use, which is the highest common speed between the machines. The lastest and greates machines will use V.29 at 9600bps. 5) The calling machine transmits a training signal which the called machine uses to train its receiver to the modulation method selected. (ie, a V.29 training sequence). 6) The called machine responds at 300bps indicating if the trainning worked. The training may fail because of poor line conditions. If a failure to train message is sent to the calling machine, it will attempt to retrain at a lower speed (ie return to step 4). 7) The calling machine agains tramists a training signal, followed by a page of image data. This occurs at the data rate used in step 5. 8) The calling machine drops back to V.21 300bps and indicates if it is done or if it has another page to send. If it has another page to send, it waits for the called machine to respond that its ready, and then returns to step 7. If it's done, it disconnects from the line after sending the disconnect signal to the called machine. It is important to note that only one machine transmits at a time. Every time the direction of data flow reverses, each machine must reconfigure its modem to a different mode (ie recieve or transmit). So, to answer your question, a fax card or modem for a PC is nothing more than a modem that supports the above modulation standards. However, software must exist either on the card or on the PC to implement the T.4 and T.30 protocols. If you have a modem that includes the Group 3 fax modulation standards, you could write a T.4/T.30 driver for it, if you *really* wanted to. You could also use the modem for a data connection with any other such modem, but keep in mind that the connection would be simplex, unless you build your own half duplex protocol on top of it. Gary Segal @ Motorla C.I.D. 1501 W. Shure Drive ...!uunet!motcid!segal Arlington Heights, IL 60004 Disclaimer: The above is all my fault. +708 632-2354