The bandwidth is a measure of the amount of information that is be carried through a communications channel. 

In the early days of telecommunications, we had to rely in a pair of copper wires to carry the signal. This has a very low bandwidth compared with modern fibre-optic cables or microwave links. This switch from copper to glass-fibre has been intimately linked to the change from analogue to digital networks.

The phone network was designed for a low quality, 2-way phone conversation. In the early days this took the voice pattern of the sender along the wires, with suitable amplification, to the receiver.  Most of us still rely, to some extent, on this old technology.

Digital networks do it differently. Instead of sending the whole signal, a regular sample is sent and the sound recreated at the other end. Ironically, this can produce a much more accurate image of the signal because a digit is either on or off, so there is no need for hi-fi amplifiers. 

Once all communications are reduced to digital signals, it makes sense to use links that have very high carrying capacity. It is easy to package the digits and fill a cable. These are then unpacked at the other end. 

High bandwidth has been likened to a multi-lane highway. This is a poor physical analogy because the carrying capacity is increased by packing the digits nose to tail rather than side by side, but it will do. Very high frequencies are employed as they can pack the digits millions of times closer than is possible with audio signals. Because light signals do not travel along copper, glass is the conductor of choice for high-bandwidth cables. 

Modems

The current 28 or 56k modems are often limited by the speed of their local exchange. Crude rates give a poor guide to the actual transfer rate. Those in the country will have much lower transfer speeds than those in the city.  So sending and receiving times are often independent of the alleged speed of the modems! A rate of 2K per second is useful for planning. At these speeds the text of a typical book will take about 3 minutes to send.

Once you go digital, you don't need a traditional modem but you still need some technology to convert the signal and extract the data from it.

Full circle

Life became complicated for the digital age when engineers of the early phone system realised they could chop the high and low frequencies and people would still recognise the voice. Next they found they could compress several phone calls into one pair of wires. They found they could take a sample of the call and the listener at the other end would not notice that there were bits missing because the bandwidth of the wires was being shared. (This is just like a movie file when the eye is fooled by lots of photos into thinking that the image is moving).

Then the digital age dawned and the phone companies found they had to send data. It was no good sending a sample of the data. So a way had to be found to allow digital information to use the phone wires. Enter the modem.

A modem imposed the pattern of data digits on the frequencies the phone lines could cope with. This is called modulation. The long beeps made by a modem are to alert the phone network and allow it to change over and set the right filters for a data call. When the data arrived the digits were extracted in a process called demodulation. So we arrive at the MoDem or  MoDulator, DEModulator. 

Then the phone work went digital and things got really complicated for the phone engineers. There was a difficult decade when data calls were turned into audio signals, then into digital signals to pass through the network, then back from digital to audio, before reaching the destination where the modem extracted the data. It was not brilliant but it's amazing that it worked at all.  

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