Physical layer - modulation
Modulation is the process of varying one or more properties of a carrier wave for the purpose of encoding information onto that waveform for transmission.
Modulation is used with both a wireless link as well as copper cabling. A carrier wave is modulated by the sender, and once it reaches the other end of the link, the information is demodulated, that is, retrieved from the waveform.
The properties of a waveform that are modulated to encode information are the amplitude, the frequency, and the phase.
A typical example of a device that performs modulation and demodulation is a modem. Dialup modems used in the 1990s and early 2000s are typical examples of such devices, as are xDSL modems that used today.
Modulation can be analog or digital. Analog modulation is applied continuously in response to an analog information signal. Amplitude Modulation (AM) and Frequency Modulation (FM) are two common analog modulation techniques that are used for broadcast radio.
Digital modulation is applied discretely (digitally) in response to a digital information signal, typically in the form of bits. There are many digital modulation techniques that can be used to encode information onto a carrier signal. Some of the most common are:
- PSK (phase-shift keying): a finite number of phases are used.
- FSK (frequency-shift keying): a finite number of frequencies are used.
- ASK (amplitude-shift keying): a finite number of amplitudes are used.
- QAM (quadrature amplitude modulation): a finite number of at least two phases and at least two amplitudes are used.
Note that modulation can also be used to transmit multiple channels of information through a single communication medium, which is called multiplexing. Some such multiplexing techniques include:
- Frequency Division Multiplexing (FDM)
- Space Division Multiplexing (SDM)
- Time Division Multiplexing (TDM)
- Code Division Multiplexing (CDM)