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Multiplexing: FDM, WDM and TDM

Multiplexing is a scheme that sends multiple signals or stream of information in the forms of analog data or digital data over a single physical trunk (also called as transmission lines /circuits/ channels).
These multiplexing scheme can be divided into two basic categories: FDM ( Frequency Division Multiplexing) and TDM (Time Division Multiplexing ).
In FDM, the frequency spectrum is divided among the logical channels or sub-channels of different frequency widths /bands within the main channel. And TDM, the multiple signals are carried over the same channel and each signal periodically getting the entire bandwidth for a little burst of time.

FDM (Frequency Division multiplexing):

FDM concerns with analog signals. It allows transmitting multiple data stream simultaneously over the same channel. Figure shows how three voice grade telephone channels are multiplexed using FDM. 

FDM
FDM Frequency Division Multiplexing
When many channels are multiplexed together, first the voice channels are raised in frequency, each by a different amount then they can be combined with some gaps (guard bands) between the channels. Even though there are gaps between the channels, there is some overlapping between adjacent channels, because the filters do not have sharp edges. This overlapping means non thermal noise between the two channels.
Example: 1. Radio broad casting (AM and FM) provides multiple signals or stations of different frequencies with the inter channel separation to prevent interference.
2. Cable TV or Television transmission relay center transmits all channels simultaneously and at the receiving end the TV “tunes in” to select a particular channel for watch.

WDM (Wavelength Division Multiplexing): 

Frequency Division multiplexing can be applied to fiber optics called as Wavelength Division Multiplexing. This scheme is a variation of frequency division multiplexing.
Actually WDM is a scheme of combining two or multiple fibers at a prism and each of its energy in a different band or variety of signals passed through the prism or grating, and transmit it to a single shared fiber optic media, at the destination point they are split again with the help of prism.
WDM is more popular than FDM because the bandwidth of a single fiber is about 25,000 GHz or more than that, and also the great potential for multiplexing many channels together over long-haul routes.
Example: WDM is used in the FTTC (fiber to the curb). Initially, a telephone company used fibers between the end office to nearly junction box where it met up with twisted pairs from the houses.

Time Division Multiplexing:

Time division Multiplexing is a scheme or method of carrying multiple signal over the same channel and each signal periodically getting the entire bandwidth for a short duration. It can handle digital data, so it has become more widespread. The local loop for telephone circuit (see structure of telephone circuit ) produce analog signals, a conversion is takes place from analog to digital by using Codec (Coder Decoder ) device. The codec makes 8000 samples per second and this is sufficient to capture all the information from the 4-KHz telephone channel bandwidth. At the lower sampling rate, information would be lost; at a higher one, no information would be gain. This technique is called PCM (pulse rate modulation).
Example: TV channels and advertising , they use entire bandwidth for short time. first program then advertise; again program , advertise and so on.

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