To understand the functioning of a modem at a conceptual level is very easy. Modulation basically uses a coding scheme or a convention. This coding can be achieved using the three properties of a signal viz., Amplitude, Frequency and Phase. Depending upon the technique used, it is called Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) or Phase Shift Keying (PSK).
In all these techniques, we choose a carrier signal with given amplitude, frequency and phase, and then choose a form of encoding. Another variation of the basic techniques is the Quadrate Amplitude Modulation (QAM), which is a combination technique.
Amplitude Shift Keying (ASK)
In amplitude shift keying or amplitude modulation, we do not alter the frequency or phase of the carrier signal. However, we specify different amplitudes, i.e. we shift the amplitude values to represent a binary 1 and a binary 0. It illustrates this coding scheme. Note that only the amplitudes of the signal change as per the values 0 and 1, but the phase and the frequency of the signals for both 0 and 1 are same and the frequency is between 0 and 4000 Hz.
Therefore, the signal can be sent over the telephone lines.
At the receiving end, the modem, working on this principle, measures amplitudes at regular intervals to decode them as 0s and 1s and then generates a digital signal. The binary bits then can be stored at the destination node. This technique is normally used for transmitting data over optical fibre Noise affects the amplitude of a signal and ASK is highly susceptible to noise.
Frequency Shift Keying (FSK)
In Frequency Shift Keying (FSK) or frequency modulation technique, you keep the amplitude and the phase of the carrier signals unaltered.
You assign a certain frequency f1 to denote 1 and f2 to denote 0. The frequency of the carrier signal is varied to represent binary 1 (using f1) and binary 0 (f2). Both f1 and f2 must be in the bandwidth of the channel i.e. between 0 and 4000 Hz, which can be easily carried by the telephone wires.
The signal component with slower cycle is f1, and the signal component that shows the rapid cycle portions is f2. The two represent 1 and 0 of the input digital signal respectively. It is obvious that the modem at the destination decodes these signals into 0s and 1s by measuring the frequencies of the received signals at regular predefined time intervals. This scheme is less prone to error than Amplitude Shift Keying. Noise is not an issue with FSK.
Phase Shift Keying (PSK)
In Phase Shift Keying (PSK), we keep the amplitude and the frequency of the carrier signal unchanged and only change the phase to denote 0s and 1s.
For instance, we can start with a phase of 0 degrees to represent binary 0 and then change the phase to 180 degrees to represent binary 1. In PSK, we change the timing of the carrier wave abruptly to encode data. After a phase shift happens, the carrier wave still continues to oscillate, but it immediately jumps to a new point in its cycle. The phase of the signal during each bit duration is constant and its value depends on whether it is 0 or 1. This technique is more noise-resistant. It is also more efficient than Frequency Shift Keying.
Quadrate Amplitude Modulation (QAM)
The main limitation of PSK is the inability of the hardware equipment to distinguish small differences in terms of phase changes. This puts a limitation on its data rate. In ASK, FSK or PSK, we alter only one characteristic (amplitude, frequency and phase respectively) of the carrier wave. What if we alter two of these together? Since the bandwidth of the transmission medium is a major limitation, we cannot combine FSK with anything else. Therefore, the only possibility we have is to combine ASK and PSK. Thus, we can have x variations in phase and y variations in amplitude. QAM does just that. QAM makes higher data rates possible.
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