File Name: multiplexing and its types .zip
In any communication system that is either digital or analog, we need a communication channel for transmission.
- Frequency-division multiplexing
- MULTIPLEXING FDM, TDM AND TCM.pdf
- Introduction to Communication systems Chapter 5-Multiplexing
In any communication system that is either digital or analog, we need a communication channel for transmission. This channel can be a wired or a wireless link. It is not practical to allocate individual channels for each user. Therefore a group of signals are combined together and sent over a common channel.
For this we use multiplexers. A multiplexer is a device that allows digital information from several sources to be routed onto a single line for transmission to a single destination. A demultiplexer does the reverse operation of multiplexing. It takes digital information from a single line and distributes it to a given number of output lines. Multiplexing is the process of transmission of information from more than one source into a single signal over a shared medium.
We can be able to multiplex analog or digital signal. If analog signals are multiplexed, then this type of multiplexer is called analog multiplexer. If digital signals are multiplexed, then this type of multiplexer is called digital multiplexer. The advantage of multiplexing is that we can transmit a large number of signals to a single medium. This channel can be a physical medium like a coaxial, metallic conductor or a wireless link and will have to handle multiple signals at a time.
Thus the cost of transmission can be reduced. Even though the transmission occurs on the same channel, they do not necessarily occur at the same instant. In general multiplexing is a technique in which several message signals are combined into a composite signal so that these can be transmitted over a common channel. In order to transmit various signals over the same channel, it is essential to keep the signals apart to avoid the interference between them, and then it can be easily separated at the receiving end.
Domains in which multiplexing can be accomplished are time, phase, frequency wavelength, etc. Multiplexing circuits are called multiplexer or MUX. Multiplexers are mainly classified as shown in Figure 1.
Analog multiplexing and digital multiplexing are the major classification. The most commonly used analog multiplexing techniques are frequency division multiplexing FDM and wavelength division multiplexing. Frequency division multiplexing [ 1 , 2 , 3 , 4 ] is a networking technique which combines many signals into a single one and then transmitted the combined signal through a common communication channel.
In the receiver side, the opposite process is carried out which is known as demultiplexing which extracts the individual channel signals. Here the transmitter side performs multiplexing, and the receiver side performs demultiplexing. In FDM the total bandwidth available in a communication medium is divided into a series of nonoverlapping frequency bands. Each of these bands is used to carry a separate signal. In FDM all users use the same common channel at full time.
But each of the users is allocated with different frequencies for transmission for avoiding the signal interference. Sometimes there is a possibility of cross talk because all the users use the transmission medium at the same time.
FDM is used for analog signal transmission. It does not need synchronization between the transmitter and receiver. Here a large number of signals can be transmitted simultaneously. It suffers the problem of cross talk, and intermodulation distortion may take place.
The allocation of frequency bands to different users is shown in Figure 2. Fiber-optic communications require a different kind of multiplexer called a wavelength division multiplexer WAD [ 2 , 4 ]. It is an analog multiplexing technique. It is designed for high data rate capability fiber cable. In this technique the bandwidth of the communication channel should be greater than the combined bandwidth of the individual channels.
Here signals are converted to light signals; each light which has different wavelengths is transmitted through the same fiber cable. WDM transmission system divides the optical fiber bandwidth into a number of nonoverlapping optical wavelengths; these are referred to as WDM channels. WDM mixes all incoming signals having different wavelengths and are transmitted over a common channel.
A demultiplexer does the reverse operation and separates the wavelengths. This multiplexing mechanism provides a much higher available transmission capacity. Figure 3 shows the representation of WDM system that consists of both multiplexer and demultiplexer. WDM multiplexing and demultiplexing are similar to the refraction of light through a prism as shown in Figure 4.
In unidirectional WDM, the data is sent only from one side and received on the other side. Multiplexing of the wavelength occurs on the sender side, and demultiplexing of the wavelengths takes place on the receiver side. In bi-directional WDM the data can be sent from both sides which means that both sides can do multiplexing and demultiplexing as shown in Figure 5. Scalability is a concern as optical line termination OLT ; optical line termination has to have transmitter array with one transmitter for each optical network unit ONU.
Adding a new ONU could be a problem unless transmitters were provisioned in advance. Each ONU must have a wavelength-specific laser. Inefficiency in BW utilization, difficulty in wavelength tuning, and difficulty in cascaded topology.
FDM [ 1 , 2 , 3 , 4 ] is based on sharing of the available bandwidth of a communication channel among the signals to be transmitted. It is an analog multiplexing technique that uses a single transmission medium which is divided into several frequency channels. Here the total bandwidth of the channel must be higher than the sum of the individual bandwidth. If the channels are closer to each other, then cross talk may occur; thus, it is necessary to implement channel synchronization.
For that purpose some bandwidth is allocated as guard band; these are unused channels placed between the successive transmission channels to avoid cross talk. For frequency division multiplexing if the input signal is digital, it must be converted to analog before giving it as the input to the modulator.
In FDM signals are generated by sending devices and there are multiple input lines. From the block diagram Figure 6 , channel 1 to channel n are taken as the input channels. These signals reach at the input of the corresponding modulator where it receives another signal from a crystal oscillator known as carrier signal, which is a high-frequency high-amplitude signal. The carrier signal is modulated with the input signal.
Different modulators use different carrier signals for modulation. It should be noted that the frequency band of one modulator will not make any interference to the frequency band of other modulators. Each of the modulator produces the corresponding modulated signal at their output.
All the output of the modulators will be given to an adder or mixer circuit; from there it is given to another modulator for further shift of total bandwidth. Finally, this higher-frequency signal will be transmitted over the channel.
The following block diagram Figure 7 shows the concept of demodulation of FDM signal at the receiving side. The antenna receives the multiplexed modulated signal from the transmitter. This signal will be weak at the receiver. Therefore it is necessary to amplify the signal.
This is done at the initial stage of the receiver. The amplified signal is then forwarded to the demodulator. The output of the demodulator will be given to the band-pass filters which are well designed with the central frequencies of the carriers as used individually at the transmitting side. Thus the output of each BPF will be the same as that of the originally modulated output of the corresponding modulator.
A large number of signals channels can be transmitted simultaneously. FDM does not need synchronization between its transmitter and receiver for proper operation. In time division multiplexing TDM [ 1 , 2 , 3 , 4 ], all signals operate with the same frequency at different times, i. Here the signal transmitted can occupy the total bandwidth of the channel, and each signal will be transmitted in its specified time period only.
In TDM all signal operates at same frequency at different time slots. Figure 8 shows the schematic diagram of implementation of TDM system. From this it is clear that a circular ring has been split into eight equal segments and is completely separated from one another. It is also noted that there is a movable arm attached to the inner ring, and it slides over the eight segments over the ring. The eight segments are eight inputs, and the selector moves in clockwise direction from A to H; after completing one revolution, it starts again.
The output is taken from the inner ring that contains the signal from only one slot at a time. The same arrangement is also made at the receiving side.
The two inner rings of the transmitting and receiving stations are rotated at the same speed, and they are synchronized. Thus the signal at segment A of the transmitter will reach segment A of the receiver in the period the arm is contacting the segment A.
The same is in the case of other segments. Time division multiplexing is used when data transmission rate of media is greater than the total transmission rate of the sources.
Here each signal is allotted to a definite amount of time. These slots are too small so that the transmission appears to be parallel. In TDM all the signals to be transmitted are not transmitted simultaneously. Instead, they are transmitted one by one. When all the signals are transmitted once on the transmission channel, it is said to be one cycle of completion.
Synchronization between the multiplexer and demultiplexer is a major issue in TDM. If the multiplexer and the demultiplexer are not properly synchronized, a bit belonging to one channel may be received by another channel.
MULTIPLEXING FDM, TDM AND TCM.pdf
A Multiplexer is a device that allows one of several analog or digital input signals which are to be selected and transmits the input that is selected into a single medium. Multiplexer is also known as Data Selector. A multiplexer of 2n inputs has n select lines that will be used to select input line to send to the output. Multiplexer is abbreviated as Mux. MUX sends digital or analog signals at higher speed on a single line in one shared device.
Download full-text PDF and digital multiplexing are the major classification.. Analog In synchronous TDM, if a device does not have data to send, then its.
Introduction to Communication systems Chapter 5-Multiplexing
This technique is applicable in telecommunications as well as computer networks. For instance, in telecommunications, one cable is used for carrying different telephone calls. In the year , the multiplexing technique is invented first in telegraphy, and at present, it is extensively used in communications.
Multiplexing is the process of combining multiple signals into one signal, over a shared medium. If analog signals are multiplexed, it is Analog Multiplexing and if digital signals are multiplexed, that process is Digital Multiplexing. The process of multiplexing divides a communication channel into several number of logical channels, allotting each one for a different message signal or a data stream to be transferred.
In telecommunications and computer networks , multiplexing sometimes contracted to muxing is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource. For example, in telecommunications, several telephone calls may be carried using one wire. Multiplexing originated in telegraphy in the s, and is now widely applied in communications. In telephony , George Owen Squier is credited with the development of telephone carrier multiplexing in The multiplexed signal is transmitted over a communication channel such as a cable. The multiplexing divides the capacity of the communication channel into several logical channels, one for each message signal or data stream to be transferred.
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