Imagine you're a fax machine...
Suppose you have an urgent contract you want me to sign and you need to get it to me as quickly as possible. You could mail it, of course, but that will take at least a day to reach me and another day for me to return it.
You could use a courier—but, unless we live near one another, we're still talking about a turnaround time of hours. Or you could send the contract down the phone with a fax machine in a minute or so.
Let's imagine for a moment that fax machines haven't been invented, but you still want to use the phone. Suppose you need to transmit a one-page document to me. What can you do? Let's make the problem really easy.
Let's say the document can be either a totally black page or a totally white one. Now transmitting the document is really easy. You simply pick up your phone, dial my number, wait for me to answer, and then say either "black" or "white".
You can walk into nearly any office in the United States today, big or small, hi-tech or lo-tech, and you'll find a fax machine.
Connected to a normal phone line, a fax machine allows you to transmit pieces of paper to someone else instantly! Even with FedEx and e-mail, it is nearly impossible to do business without one of these machines today.
Basic Idea Behind Fax Machines:
Most of the early fax machine designs involved a rotating drum. To send a fax, you would attach the piece of paper to the drum, with the print facing outward. The rest of the machine worked something like this:
There was a small photo sensor with a lens and a light.
The photo sensor was attached to an arm and faced the sheet of paper.
The arm could move downward over the sheet of paper from one end to the other as the sheet rotated on the drum.
To transmit the information through a phone line, early fax machines used a very simple technique: If the spot of paper that the photo cell was looking at were white, the fax machine would send one tone; if it were black, it would send a different tone (see How Modems Work for details). For example, it might have sent an 800-Hertz tone for white and a 1,300-Hertz tone for black.
At the receiving end, there would be a similar rotating-drum mechanism, and some sort of pen to mark on the paper. When the receiving fax machine heard a 1,300-Hertz tone it would apply the pen to the paper, and when it heard an 800-Hertz tone it would take the pen off the paper.
Modern Fax Machines
A modern fax machine does not have the rotating drums and is a lot faster, but it uses the same basic mechanics to get the job done:
At the sending end, there is some sort of sensor to read the paper. Usually, a modern fax machine also has a paper-feed mechanism so that it is easy to send multi-page faxes.
There is some standard way to encode the white and black spots that the fax machine sees on the paper so that they can travel through a phone line.
At the receiving end, there is a mechanism that marks the paper with black dots.
The fax machine typically has a CCD or photo-diode sensing array. It contains 1,728 sensors (203 pixels per inch), so it can scan an entire line of the document at one time. The paper is lit by a small fluorescent tube so that the sensor has a clear view.
The image sensor looks for black or white. Therefore, a single line of the document can be represented in 1,728 bits.
Here are some basic instructions for sending and receiving a fax.
Sending a fax:
Make sure the fax machine is plugged into a power source and also plugged into a working phone jack.
Turn the fax machine on.
Obtain the fax number of the destination fax machine.
Gather the documents you want to send and put them in the order you want them to be received.
Fill out a separate piece of paper called a coversheet with the recipient's name, fax number/phone number, your name, your phone number, a short message and number of pages (including coversheet).
Lay the documents face-up in the fax machine feeder tray with the coversheet on top
Dial the recipient's fax number (dialing instructions for international calls)
Press the "fax" or "send" button, depending on the particular fax machine model
Now the fax machine will scan each of the document pages into its memory. After all of the pages have been scanned, you'll hear a series of fax tones. These tones signal the "handshake" between the sending and receiving fax machines, establishing a communications link.
Wait for a few minutes as the fax is sent. If the fax machine has a small display screen, look for a confirmation that the fax went through. Some fax machines will also print out a short confirmation report.
Here's how to receive a fax:
Make sure the fax machine is plugged in, powered on and connected to a working phone jack. This phone jack can either be your regular phone line or a dedicated fax line. The important thing is that the sender has the right number.
Make sure that the fax machine has enough ink in its toner cartridge. Toner cartridges usually have some sort of indicator when toner is low. Most modern fax machines will also alert you when toner is low.
Make sure that there's enough printer paper loaded in the fax machine's paper tray. Fan the paper (run your thumb along the bottom, separating the individual pages) to avoid paper jams in the machine.
If there's a phone on the fax machine, the phone will ring. Don't pick it up.
Wait for the "handshake" tones indicating that the fax machine is talking with the sender's machine.
The fax machine will automatically begin to print each page of the fax.
Check the coversheet to make sure you received as many pages as were sent.
If it's an important document, it's office etiquette to call or e-mail the sender to confirm that you received the fax.
You can see that there are really two separate machines in one: a fax-sender and a fax-receiver. When you use a fax machine to make quick "photocopies" of documents, the two machines link up together: instead of sending a fax down the phone line, the circuit reroutes the scanned data directly to the printer so you get a copy of your original document.