The principles of analogue transmission

Analogue data transmission consists of sending information over a physical transmission medium in the form of a wave. Data is transmitted via a carrier wave, a simple wave whose only purpose is to transport data by modification of one of its characteristics (amplitude, frequency or phase), and for this reason analogue transmission is generally called carrier wave modulation transmission. Three types of analogue transmission are defined depending on which parameter of the carrier wave is being varied:

• Transmission by amplitude modulation of the carrier wave
• Transmission by frequency modulation of the carrier wave
• Transmission by phase modulation of the carrier wave

Analogue transmission of analogue data

This type of transmission refers to a scheme in which the data to be transmitted are already in analogue form. So, to transmit this signal, the DCTE must continuously convolve the signal to be transmitted and the carrier wave, so that the wave it will transmit will be a combination of the carrier wave and the signal to be transmitted. In the case of transmission by amplitude modulation, for example, transmission occurs as follows:

Analogue transmission of digital data

When digital data appeared on the scene, the transmission systems were still analogue, so it was necessary to find a means of transmitting digital data in an analogue manner.

The solution to this problem was the modem. Its role is:

• When transmitting: to convert digital data (a sequence of 0s and 1s) into analogue signals (continuous variation of a physical phenomenon). This process is called modulation.
• When receiving: convert the analogue signal into digital data. This process is called demodulation.

In fact, the word modem is an acronym for MOdulator/DEModulator...

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