Multiplexing is used in cases where the signals of lower bandwidth and the transmitting media is having higher bandwidth. In this case, the possibility of sending a number of signals is more. In this, the signals are combined into one and are sent over a link that has greater bandwidth of media than the communicating nodes.
1. Frequency Division Multiplexing (FDM):
In this, a number of signals are transmitted at the same time, and each source transfers its signals in the allotted frequency range. There is a suitable frequency gap between the 2 adjacent signals to avoid over-lapping. Since the signals are transmitted in the allotted frequencies so this decreases the probability of collision. The frequency spectrum is divided into several logical channels, in which every user feels that they possess a particular bandwidth. A number of signals are sent simultaneously at the same time allocating separate frequency bands or channels to each signal. It is used in radio and TV transmission. Therefore to avoid interference between two successive channels Guard bands are used.
Application of FDM:
- In the first generation of mobile phones, FDM was used.
- The use of FDM in television broadcasting
- FDM is used to broadcast FM and AM radio frequencies.
2. Time Division Multiplexing (TDM):
This happens when the data transmission rate of media is greater than that of the source, and each signal is allotted a definite amount of time. These slots are so small that all transmissions appear to be parallel. In frequency division multiplexing all the signals operate at the same time with different frequencies, but in time-division multiplexing, all the signals operate with the same frequency at different times.
It is of the following types:
1. Synchronous TDM:
The time slots are pre-assigned and fixed. This slot is even given if the source is not ready with data at this time. In this case, the slot is transmitted empty. It is used for multiplexing digitized voice streams.
2. Asynchronous (or statistical) TDM:
The slots are allocated dynamically depending on the speed of the source or their ready state. It dynamically allocates the time slots according to different input channels’ needs, thus saving the channel capacity.
Advantages of Frequency Division Multiplexing (FDM):
Efficient Use of Bandwidth: FDM allows multiple signals to be transmitted over a single communication channel, which can lead to more efficient use of available bandwidth.
No Time Synchronization Required: FDM does not require precise time synchronization between the transmitting and receiving devices, making it easier to implement.
Low Implementation Cost: FDM is a relatively simple technique that does not require sophisticated hardware or software, making it less expensive to implement.
Disadvantages of Frequency Division Multiplexing (FDM):
Limited Capacity: FDM is limited in terms of the number of signals that can be transmitted over a single communication channel, which can be a disadvantage in applications where a large number of signals need to be transmitted.
Interference: FDM can be susceptible to interference from other signals transmitted on nearby frequencies, which can degrade the quality of the transmitted signals.
Difficulty in Assigning Frequencies: FDM requires careful assignment of frequencies to different signals to avoid interference, which can be a complex and time-consuming process.
Advantages of Time Division Multiplexing (TDM):
High Capacity: TDM can support a large number of signals over a single communication channel, making it ideal for applications where many signals need to be transmitted.
Simple Implementation: TDM is a relatively simple technique that is easy to implement, making it a cost-effective solution for many applications.
Precise Time Synchronization: TDM requires precise time synchronization between the transmitting and receiving devices, which can help ensure accurate transmission of signals.
Disadvantages of Time Division Multiplexing (TDM):
Inefficient Use of Bandwidth: TDM may not make optimal use of available bandwidth, as time slots may be left unused if there are no signals to transmit during a particular time slot.
High Implementation Cost: TDM requires sophisticated hardware or software to ensure precise time synchronization between the transmitting and receiving devices, making it more expensive to implement than FDM.
Vulnerable to Timing Jitter: TDM can be vulnerable to timing jitter, which can occur when the timing of the transmitting and receiving devices drifts out of sync, leading to errors in the transmission of signals.
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