From the early 1970's it was realised that the current analogue telephone system, which had been established internationally over several decades, was now required to carry digital signals.
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In fact, much of the digital traffic at this time was associated with the control of the telephone system itself. It had enabled subscribers to directly dial national and eventually international calls. The problem encountered at this time was that the essentially DC signals generated by the digital control circuits could not be passed over their own telephone network since this had been designed for speech frequencies in the range 300 to 3000Hz.
It was not a viable option to modify every telephone circuit in the world to include a DC component just to handle these control signals and so the alternative option of converting the control signals into an analogue format within the range 300 to 3000Hz. The method used was to select pairs of frequencies, one from a high group and one from a low group, such that there was no harmonic relationship between them. Various combinations of these tones could be used to control the system with little to no chance that speech signals could be confused with control elements. This was known as "dual tone multi-frequency" or DTMF. You may have heard these signals when dialling on a tone phone and used it to control a remote computer terminal such as an automated answering service. This system got over the early problem of sending digital signals over an analogue network but it was not a cure.
The same technology that was being used to modernise the telephone network was also being used in other areas to improve performance. Generically, this was known as the computerisation of an industry or organisation and was to revolutionise the way things were done. It soon became obvious that information available on one machine was required on another and that passing this by copying to a floppy disc to affect the transfer was not efficient. The individual performance of the person working with the computer was increased but the interaction with other individuals was no better than when paper documents were produced and passed between participants. The solution is clearly to "link" the computers so that information may be transferred directly between them. For a simple "network" where the machines are in close proximity, a direct cable connection can be used. A facility of this kind is still provided with most operating systems. By adding a basic switching system, several computers can be connected together to facilitate information exchange. However, if it is required to connect machines, which are at any distance from each other, a privately operated cable must be employed or the existing PSTN must be used. Only very large organisations would be in a position to own and operate a private network, whereas small enterprises and even individuals could afford to buy time on the public systems. Once again, the same problem encountered by the telephone system engineers stood in the way of the network engineers. The simplest solution was to use the new existing DTMF method and combinations of DTMF codes were established to match the computer ASCII codes. The speed of data transfer using this system was very low and soon became unacceptable.
Work then started to create a special unit to MODulate an analogue tone in the range 300 to 3000Hz in such a way that it can represent the digital codes and then DEModulate these to recreate the original digital codes. The name given to such units is MODEM.
If a single frequency in the range 300 to 3000 Hz is selected as the basic information carrier, then there are three essential characteristics that it is possible to manipulate.
- Change the amplitude (size).
- Change the frequency (increase or decrease).
- Change the phase.
If a single tone is used with a single change of amplitude, or frequency shift, or phase change, then this will give two states and can, therefore, represent the digital "0" and "1". This provides a higher transfer speed than DTMF but is still limited by the bandwidth of the telephone circuit.
Nyquist derived a formula that applies to this two-state signal in terms of bandwidth "B" and data rate "C".
C = 2B
For a telephone circuit where B = 2700 Hz results in a data rate C = 2 x 2700, or 5400 bps.
However, it is possible to use more than one tone, more than one amplitude change, more than one frequency shift, or more than one phase change. In fact, a combination of several characteristics can be used which will result in a number of signalling states "M" and therefore increase the data rate.
The previous formula can now be modified to include this new feature.
C = 2B Log(M) (for binary code)
Using this new formula, for a telephone circuit and modem using two tones each with two amplitudes and two phases resulting in eight signal states (2 x 2 x 2) gives.
C = 2 x 2700 x 3, or 16200 bps.
Further signal states can be incorporated to obtain even higher rates and the bandwidth can be pushed slightly wider than that used for speech, but seriously high speeds are just not possible.
The solution now becomes that which was rejected in the early stages when only the system digital control signals needed to be transferred.
In 2000 the digital traffic exceeded the analogue traffic on the PSTN as had been predicted several years earlier. Conversion of the world's telephone system to an all-digital format is underway but may take a decade or more before most users have access to a digital line. This reverses the requirements of the user equipment. Computers will no longer need modems to connect into the new PSTN but telephone signals will need to be converted into a digital form in order to travel across the network. In fact, all analogue signals, no matter what their origin, will need to be digitised for transmission on the digital networks, which are envisaged in the future.
Last updated on: Saturday 17th June 2017
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