What is the concept behind fax machine?

Question

in terms of transmitting and receiving.

Answer 1

the basic concept (not getting into the highly technical aspect of baud rate, compression, etc - since they are fairly technical & actually very similar to old style computer modems communications) is very simple & has been around for a very long time.  essentially, the idea is the same as what you have for television broadcast, though much simpler in terms of reslolution & of course color depth (assuming you're high tech faxing in color).  if you have a scanner & a printer, you have the main pieces of a fax machine.  same as with a copier, it just lacks the ability to call & send the scanned image.  it works like this, it starts at the top of the page & scans it one line at a time (for whatever resolution it's capable of & what you select at the time of faxing - for this lets say you're sending 100 dpi - really not great looking but they fax really fast). the fax will call the receiving machine, & assuming it connects, handshakes & says, "ok, send your fax", the sending fax machine depending on the imager,  will scan several groups of lines (perhaps 100) at a time per pass.  it will then move down the page & scan again - and again - and again - until it's complete.   as it's doing this, it's sending the data for each completed pass into a buffer, then from that buffer to the receiving machine.  the reason for the buffer is, if the receiver says it has received an error, the sending machine wont have to re-scan the page in order to send, it will just take the image from the buffer.  at the receiving end, it is printing out the lines in the same contrast & brightness as the data that's been sent from the sending fax.   many faxes aren't even done with a fax machine though.  if you've ever called an automated fax system for a manual or some piece of technical data, it's all done by a computer that has a library of previously scanned in documents & it uses a standard modem to call your fax machine (or computer) to send the fax in the same fashion.  

the whole process in its simplest terms could be thought of as being essentially the same as a single frame of video being sent from video camera, to television screen.  image is scanned in, & then it is "printed" out on the screen -- the fax machine just does a single picture though instead of a rapid succession of pictures.  hope this helps