Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared.
Definitions of Multiplexing
Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link.
As data and telecommunications use increases, so does traffic. We can accommodate this increase by continuing to add individual links each time a new channel is needed; or we can install higher-bandwidth links and use each to carry multiple signals.
In a multiplexed system, n lines share the bandwidth of one link.
The lines on the left direct their transmission streams to a multiplexer (MUX), which combines them into a single stream (many-to-one).
The word channel refers to the portion of a link that carries a transmission between a given pair of lines. One link can have many (n) channels.
There are three basic multiplexing techniques: frequency-division multiplexing, wavelengthdivision multiplexing, and time-division multiplexing. The first two are techniques designed for analog signals, the third, for digital signals.
Frequency-Division Multiplexing (FDM)
Frequency-division multiplexing (FDM) is an analog technique that can be applied when the bandwidth of a link (in hertz) is greater than the combined bandwidths of the signals to be transmitted.
In FDM, signals generated by each sending device modulate different carrier frequencies. These modulated signals are then combined into a single composite signal that can be transported by the link.
Carrier frequencies are separated by sufficient bandwidth to accommodate the modulated signal.
These bandwidth ranges are the channels through which the various signals travel. Channels can be separated by strips of unused bandwidth-guard bands-to prevent signals from overlapping.
In addition, carrier frequencies must not interfere with the original data frequencies.
Each source generates a signal of a similar frequency range. Inside the multiplexer, these similar signals
modulate different carrier frequencies f1, f2, and f3).
The resulting modulated signals are then combined into a single composite signal that is sent out over a media link that has enough bandwidth to accommodate it.
The de-multiplexer uses a series of filters to decompose the multiplexed signal into its constituent component signals.
The individual signals are then passed to a demodulator that separates them from their carriers and passes them to the output lines.
Wavelength-Division Multiplexing (WDM)
Wavelength-division multiplexing (WDM) is designed to use the high-data-rate capability of fiber-optic cable.
The optical fiber data rate is higher than the data rate of metallic transmission cable.
Using a fiber-optic cable for one single line wastes the available bandwidth. Multiplexing allows us to combine several lines into one.
WDM is conceptually the same as FDM, except that the multiplexing and de-multiplexing involve optical signals transmitted through fiber-optic channels.
The idea is the same: We are combining different signals of different frequencies. The difference is that the frequencies are very high.
Time-Division Multiplexing (TDM)
Time-division multiplexing (TDM) is a digital process that allows several connections to share the high bandwidth of a link Instead of sharing a portion of the bandwidth as in FDM, time is shared.
Each connection occupies a portion of time in the link.