Modem Technical Details
What are the methods of modulating digital data?
There are 4 techniques a modem may employ:
Frequency Shift Keying (FSK)
FSK modems modulate data by varying the frequency of a signal (Frequency Modulation). FSK modems can transmit binary information by using different frequencies to represent bit patterns. In a binary system, one frequency represents one binary value and a different frequency represents the other. These frequencies lie within the bandwidth of the transmission channel.
The figure below is an example of a modulated frequency using FSK. The signal is represented in binary as 0010110.
A V.21 low speed modem that transmits data at 300bps (bits per second) uses frequency shift keying. For rates over 1200bps other modulation methods are used.
Amplitude Shift Keying (ASK)
This varies the amplitude of a signal by modulating a higher frequency. Binary information can be transmitted by assigning discrete amplitudes to bit patterns.
If we look at the graph below, we can see that the strength of the signal, or the amplitude, is 1. ('P' stands for 'period', which is the amount of time before a wave repeats itself). The signal with amplitude 1 could be represented by a binary 0.
If we look at the next graph, this is the same signal as above as it has the same period 'p'. However, this signal has a higher amplitude of 2, which could be represented as a binary 1.
The graph below shows what Amplitude Shift Keying looks like. The period is the same for the entire signal and it is only the amplitude that varies. As we have said that amplitude 1 = binary 0 and amplitude 2 = binary 1, this signal can be represented as 0011010.
Phase Shift Keying (PSK)
Phase Modulation (PM) varies the phase of a signal. Information is contained within the change of phase. Phase shift keying is a technique which shifts the period of a signal. As with FM, binary information can be transmitted by assigning phases to bit patterns.
If we look at the graph below, we can see that the start of this signal's period 'p' is at 0.
However, if we look at the graph below, we can see that it's phase has been shifted. The period now starts at the wave's highest point which is '1'.
It has shifted by a quarter of the signal's full period. This could be shifted another quarter and another quarter so that it would be shifted three quarters of its original period.
We can see from the above graph that by having 4 separate signals, four different binary values could be applied to each, with each binary value signifying 2 bits (00,01,10,11). The applying of bit values to each signal is the principle of phase shift keying.
Quadrature Amplitude Modulation
This method of modulation combines amplitude shift keying (ASK) and phase shift keying (PSK) and is the technique most often used for modern modems. The idea is to 'pack' as many bits as possible into a baud. 16-QAM uses four amplitudes and four phase shifts, allowing the binary value to be represented four bits at a time. In this case, the baud rate will be a quarter of the bit rate.
The example below shows a signal that is transmitting at 3600bps, or 3 bits per baud.
This means that we can represent 8 binary combinations of 3 bits. The amplitude can be either 1 or 2 and there are also 4 possible phase shifts (one quarter each). A table can be generated to show which binary combinations correspond to each signal:
|Bit value||Amplitude||Phase shift|
This table relates to a signal transmitting at 3600bps. If we consider a modem that operates at 28800bps and uses 24 bits per baud, we can begin to realise how many different amplitudes and phase shifts must be required.
QAM is the limit of these techniques and is incorporated into modern high speed modems that operate at 56000bps.
References for this web page:
Asynchronous Serial Transmission - Modulation and Data Transmission
A Brief Introduction To Modem Technology - Amplitude Modulation
A Brief Introduction To Modem Technology - Phase Shift Keying
A Brief Introduction To Modem Technology - Quadrature Amplitude Modulation
Buchanan, W - Distributed Systems And Networks, Chapter 8.9 Modulation
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