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What is concept of multiplexing in telephone system?

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MultiplexingMultiplexing is a form of data transmission in which one communication channel carries several transmissions at the same time. The telephone lines that carry our daily conversations can carry thousands or even more of conversations at a time using multiplexing concept. The exact number of simultaneous transmission depends on the type of communication channel and the data transmission rate.
Economics of scale play an important role in the telephone system. It costs essentially the same amount of money to install and maintain a high-bandwidth trunk as low-bandwidth trunk between two switching officers. Consequently, telephone companies have developed elaborate schemes for multiplexing many conversations over a single physical trunk.

Accordingly, the communication channel is shared in such a way as to maximum the utilization of the channel capacity. Thus the method of dividing a single channel into many channels so that a number of independent signals may be transmitted on it is known as Multiplexing.  Multiplexing schemes can be divided into two basic categories:
  1. Frequency Division Multiplexing FDM
  2. Time Division Multiplexing TDM

Frequency division multiplexing ( FDM) is the technique used to divide the bandwidth available in a physical medium into a number of smaller independent logical channels with each channel having a small bandwidth. The method of using a number of carrier frequencies each of which is modulated by an independent speech signal is in fact frequency division multiplexing.
The following figure depict how three voice-grade telephone channels are multiplexing using FDM. When many channels are multiplexed together, 400Hz is allocated to each channel to keep them well separated. First the voice channels are raised in frequency, each by a different amount. Then they can be combined, because no two channels how occupy the same portion of the spectrum. Notice that even though there are gaps(guard bands) between the channels, there is some overlap between adjacent channels, because the filters do not have sharp edges. This overlap means that a strong spike at the edge of one channel will be felt in the adjacent one as non-thermal noise.
Frequency-division multiplexing works best with low-speed devices. The frequency division multiplexing schemes used around the world are to some degree standardized. A wide spread standard is 12 400-Hz each voice channels ( 300Hz for user, plus two guard bands of 500Hz each) multiplexed into the 60 to 108 KHz band. Many carriers offer a 48 to 56 kbps leased line service to customers, based on the group. Other standards upto 230000 voice channels also exist.
Example:
The allocated spectrum is about IMHz, roughly 500 to 1500 KHz. Different (stations, each operating in a portion of the spectrum. With the interchannel separation great enough to prevent interference. This system is an example of frequency division multiplexing.
Advantages of FDM
  1. Here user can be added to the system by simply adding another pair of transmitter modulator and receiver domodulators.
  2. FDM system support full duplex information flow which is required by most of application.
  3. Noise problem for analog communication has lesser effect.

Disadvantages of FDM
  1. In FDM system, the initial cost is high. This may include the cable between the two ends and the associated connectors for the cable.
  2. In FDM system, a problem for one user can sometimes affect others.
  3. In FDM system, each user requires a precise carrier frequency.

Time Division Multiplexing ( TDM) is another popular method of utilizing the capacity of a physical channel effectively. Each user of the channel is allotted a small time interval during which is may transmit a message. Thus the total time available in the channel is divided and each user is allocated a time slice. In TDM, user send message sequentially one after another. Each user can, however, use the full channel bandwidth during the period he has control over the channel. The channel capacity is fully utilized in TDM by interleaving a number of messages belonging to different users into one long message. This message sent through the physical channel must be separated at the receiving end. Individual chunks of message sent by each user should be reassembled into a full message as shown
Unfortunately, TDM can only be used for digital data multiplexing. Since local loops produce analog signals, a conversion is needed from analog to digital in the end office. Where all the individual local loops come together to be combined onto outgoing trucks.
Example:
In some countries, the individual stations have two logical sub channels : music and advertising. These two alternate in time on the same frequency first a burst of music, then a burst of advertising, then more music and so on. This situation is time division multiplexing.
Advantages of TDM
  1. It uses a single links
  2. It does not require precise carrier matching at both end of the links.
  3. Use of capacity is high.
  4. Each to expand the number of users on a system at a low cost.
  5. There is no need to include identification of the traffic stream on each packet.

Disadvantages of TDM
  1. The sensitivity to other user problem is high
  2. Initial cost is high
  3. Technical complexity is more
  4. The noise problem for analog communication has greater effect.
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