Modulation vs Multiplexing

Modulation and multiplexing are two concepts used in communication in order to enable networking. Modulation is varying the properties of a career signal to send information, whereas multiplexing is a way of combining multiple signals. Both functionalities are essential for successful networking.

Modulation

Modulation is known as varying the properties of a periodic waveform, known as ‘carrier’, according to the signal that carries the information we need to send. Let’s say, we need to send a bit sequence (10100) through a wireless communication channel. In order to send this bit sequence, we can use a high frequency signal (let’s say 40MHz), with varying amplitude of the signal at two levels. We can use the notation, that is ‘1’ to represent high amplitude and ‘0’ to represent low amplitude. This type of modulation is known as the ‘amplitude modulation’ (AM). On the other hand, we can slightly vary the frequency. For example, we can send a 40MHz for ‘1’ and 41MHz for ‘0’. Here, we are varying the frequency according to the original signal, and this type of modulation is known as ‘frequency modulation’ (FM). Other variable is the phase of the signal. It is known as the ‘phase modulation’ (PM).

In some cases, two parameters are varied. For example, in QAM (Quadrature Amplitude Modulation), both the amplitude and phase are varied in order to achieve a higher number of levels to represent the signal. Getting the original signal from a modulated signal is known as demodulation. Signals are modulated at the transmitter and demodulated at the receiver.

Multiplexing

Multiplexing is needed when we have to combine and send multiple signals that carrying information over a shared medium. For example, a number of telephones are connected to a single line and managed using multiplexing.

Let’s say senders A1, A2, A3, A4 need to send four bit streams (say 100, 111, 101, and 110) simultaneously to receivers B1, B2, B3, B4 through a single channel. In order to send this, we can mix their bit stream into a single stream by taking each sender’s first, second and third bits respectively. First we can take each sender’s first bit as 1111 (at the order of A1, A2, A3, A4), then second bits (0101) and finally third bits (0110). Hence we can create a combined stream 1111 0101 0110. This process is known as multiplexing. At the receiver, this stream can be split into four streams and sent to B1, B2, B3 and B4 as the order is known. This process is called de-multiplexing.

There are many types of parameters that can be shared. In Time Division Multiplexing (TDM), time axis is shared, whereas in Frequency Division Multiplexing (FDM), frequency band is shared.

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