Fax Transmission

The picture above shows an Epson PX-8 with the CX-21 used to send text messages over phone lines. The CX-21 is an acoustic coupling device that allowed the PX-8 to communicate with another similar device or a fax machine.

In the following I explain how faxes are transmitted over a phone line.


First the fax machine will transform one line from the document to be sent in 0s (white) and 1s (black). Then it uses a mechanism to transform those 0s and 1s in an analog signal.

On a small scale, it is easy to have a electric signal that represents 0 or 1. For example, 0 may be represented by 0V and 1 using 3V. At times, this is inverted where 0 is represented by 3V and 1 is represented using 0V.

On long distances, however, the 3V would quickly become 0V. In other words, we would not be able to distinguish a 0 from a 1.

So instead we use what we call an analog signal. This is a signal going up and down like your voice. That works really well over our old copper lines.

Note that more and more, newer phone lines make use of digital data even if you have a copper line at your home, the phone companies transforms that signal to digital and transmit it long distance using digital lines. With Smart Phones, the digitalization is done by the phone. Today, we transmit digital data on long distances using light in fiber optic cables. Light can travel several times around the Earth in one second and it doesn’t lose the information.

Data Formats

Along the way, the format of the data has been changed to support better compression schemes and thus be able to send much more data per minute.

In the old days, we would send lines one at a time in a way similar to Television. We scanned one line and sent it. Sent a “End of Line”, then sent the next line.

This was one continuous stream of data and if it broke along one of the lines, it could synchronize the signal to the next “End of Line” signal it could detect. In other words, you would general only lose a few lines and the rest of the fax could still be captured.

USB Fax/Modem
Today’s Fax/Modem can be used on your USB port and cost $50 or less…

Later, we added compression such as Huffman and JPEG.

With the increase speed to send content and the apparition of color printers, fax also started to support color (note that support for color existed since 1924—however, in 1982, the cost of a black and white fax machine was still around USD $20,000).


Up until 1966, the transmission used various standards such as a television-like signal.

Xerox started in 1964 with a signal that was patented and started the scheme still in use today.

The signal itself, though, was always very similar. It used a machine that would generate noises to send and listen for noises to receive data. All communications would use noises in both direction:

  • At first, it emits signals to determine the speed of transmission.
  • Second, it determines the capabilities of the fax machines to select which mode of transmission to use.
  • Finally, once the fax machines agreed on a mode of transmission, the actual data would be transmitted.

That signal system is still in use in modems. If you live in a city, you may think that such things are gone… but the fact is that many people still need to use modems when living in a small village in the middle of nowhere because the cost of installing fast fiber optic lines in such small places is quite prohibitive.

How Does the Signal Work?


The speed of transmission increased greatly from the beginning.

The first commercially available fax machines would run at 110 baud. Today, we have fax/modem that run at 56,000 baud.

The amount of data transmitted is about 3× the baud speed. This is because we repeat the send additional data to make sure that what we are trying to transmit actually makes it on the other side without error. In other words, instead of just sending a 0 or a 1, we send a sequence of 0 1 0 or 1 0 1. That mechanism ensures proper transmission.

The number of bits sent, the parity, etc. is determined at the start when two modems connect to each other. The parity is used to verify that a byte was sent properly. We send a one first, then 8 bits, and then one more bit which is the XOR of the 8 bits sent (this is equivalent to doing a sum of the bits and only using the least significant bit, a.k.a. bit 0). Further, the fax and modem transmissions will make use of CRC codes to make sure no data was lost along the way.

The increase in speed is due to better and better quality in our devices. When I started, we already had modem capable of 1,200 bauds. It actually very quickly went to 28,800 once the Internet started. There was a very high demand for increased speeds.

High baud speeds are available on small distances between computers through their serial ports. The highest available rate is 921,600 bauds (which represents 115,200 bytes of data per seconds, not including the checksums, we get 92,160 bytes of data per seconds).

Note that an Ethernet card also transmits data via a serial port. However, it is much higher precision and this is why we can reach incredible speeds (at time of writing: 40Gb/s is already available and we are talking about getting 100Gb/s).

There are two main differences with Ethernet: cables remain relatively short in comparison to a fax/modem which can function from anywhere in the world and the number of cables is greater allowing a larger debit in both directions.

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