Frequency Division and Time Division Multiplexing

What is a Frequency-Division Multiplexing (FDM)?

Before learning about Frequency division and Time division multiplexing, we should first get a good understanding of multiplexing and the need for multiplexing.

If we want to combine and send multiple streams of data over a single communication medium or channel then we use the concept of multiplexing. This process of combination of multiple data streams is known as multiplexing. Now, the device that is used in multiplexing is known as multiplexer. Refer to the next section to learn more about multiplexers and demultiplexers.

Let us take a scenario in which there are 20 signals to be transmitted from the sender to the receiver and the bandwidth of the communication medium is 100 units then each of the signals will use 5 units of bandwidth (100/20). Now as there are multiple signals to be shared from the same common medium the possibility of collision is increased and as we know transmission is an expensive thing so we must avoid the collision. Hence, we need multiplexing so that multiple signals can be transmitted over a single medium without the collision of data.

Now, the signal transmission over various channels can be performed in two different ways namely - Frequency division and Time division multiplexing.

In the frequency division multiplexing or FDM, we transfer a large number of signals at the same time and each source transmits its signals in its allocated range of frequency. In simpler terms, we can say that frequency division multiplexing divides the overall available bandwidth of the communication channel into various non-overlapping frequency bands so that each band can carry distinct signals.

To avoid the overlapping of signal ranges, we provide a suitable gap of frequency between two adjacent signals. We divide the frequency spectrum into several logical channels so that every user can feel that he/she is getting a particular or unique bandwidth.

This gap between transmission signals decreases the collision probability. The band that is used between the two successive channels is termed as Guard band. Frequency division multiplexing is widely used in the files of TV and radio transmission.

Some of the important points related to the frequency division multiplexing are:

• The frequency division multiplexing uses an analog system.
• The frequency division multiplexing was first introduced by George Owen Squier in 1910.

Refer to the image provided below for more clarity.

In the frequency division multiplexing, we use MUX and DEMUX, which stands for Multiplexer and De-multiplexer. In the next section, let us learn about them and their use in frequency division multiplexing.

What are Multiplexers and Demultiplexers in Frequency-Division Multiplexing?

In frequency division multiplexing, we use a device that can combine all the different frequencies into a single signal (composite signal) so that transmission can take place easily. Similarly, at the receiver's end, we use a device that can do the reverse work which is to extract the individual frequency.

Now, the device that combines the various frequencies into a single composite signal at the sender's end is termed a multiplexer (MUX), and the device that extracts the various frequencies from the composite signal at the receiver's end is termed a de-multiplexer (DEMUX).

Frequency-Division Multiplexing Advantages and Disadvantages

So far we have got a good grasp on frequency-division multiplexing, let us now discuss some of the advantages and disadvantages of frequency-division multiplexing.

Advantages:

• We use multiplexers and demultiplexers in the frequency division multiplexing so there is no requirement of synchronization between the sender and the receiver.
• Several signals share a single channel of transmission in frequency division multiplexing. Some examples of such channels are cable or optical fiber.
• It is a very easy and simple modulation technique and it allows multiple signals to transmit simultaneously.
• We can send several signals at a single time with the help of frequency-division multiplexing.
• We should use the frequency-division multiplexing technique when we have the requirement of low-speed channels.

Disadvantages:

• The spectrum efficiency of the frequency division multiplexing is low, so we can say that it is inefficient.
• The guard bands separate the frequencies but these gaps result in a wastage of some bandwidths.
• The circuit orientation of the frequency-division multiplexing is quite complex.
• There exists cross-talk in frequency-division multiplexing.
• The arrangement and working cost of frequency-division multiplexing is quite high as compared to other multiplexing techniques.
• There is a need for a large number of modulators in frequency-division multiplexing.

Example of Frequency-Division Multiplexing

Let us take an example of frequency-division multiplexing for more clarity.

Suppose we have three transmitters and the transmission of the signals is as follows:

• transmitter 1: 30 KHz
• transmitter 2: 40 KHz
• transmitter 3: 50 KHz

As the signals are of varied frequencies, they are combined into one combined or multiplex into one combined signal. We can now transmit the combined signal over the medium from the sender's end.

At the receiver's end, the de-multiplexer separates the multiplex signal and sends the signals to the intended receiver.

Frequency-Division Multiplexing Applications

Let us now discuss some of the applications of frequency-division multiplexing.

• The frequency division multiplexing was widely used in first-generation mobile phones.
• We can use frequency division multiplexing in the broadcasting of FM and AM radio frequencies.
• The frequency division multiplexing is also used in television broadcasting.
• Since the frequency division multiplexing uses analog signals rather than digital ones, it is more prone to disruption of noises.

What is Time-Division Multiplexing (TDM) and Its Types?

Let us now discuss the second signal transmission way i.e. the Time division multiplexing.

There are some scenarios in which the rate of transmission of the source is lesser than the rate of transmission of the medium, so in such scenarios, we divide the transmission time, and each signal is allocated a definite time. This definite division of time for sharing signals is called time division multiplexing. We divide the time into smaller sections called slots and as these slots are very small, it appears that the transmission is happening in parallel.

As we have earlier discussed in frequency division multiplexing, the signals are divided according to the frequency and they operate at the same time. But in the time division multiplexing, the scene is just the opposite. In the time division multiplexing, the time of transmission is divided but all the signals operate with the same frequency. This time slot division provides control to the user for a certain fixed amount of time which lowers the probability of cross-talk (which was a problem of frequency division multiplexing). Apart from that, the data is transmitted one after the other, unlike frequency division multiplexing in which the signal is transmitted simultaneously.

We have another term associated with the time division multiplexing i.e. frames. So, what is a frame? A frame contains the signals and the cycle of time slots for the particular user. Please refer to the time image provided below for more clarity.

Time-division multiplexing can work with both analog and digital signals but it is mainly used to multiplex the digital signal. We have two types of time division multiplexing.

1. Synchronous Time Division Multiplexing

As the name suggests, in the synchronous time division multiplexing the time slot for transmission is pre-assigned to each sender device. Even if the sender device has the data to transmit or not, the time is allocated to the device. As we have discussed we send the data in the form of frames so in the case when there is no data to be sent, only the time slot is sent in the form of the frame.

2. Asynchronous Time-Division Multiplexing or Statistical Time-Division Multiplexing

In the asynchronous time division multiplexing, the allocated time slot is not fixed and pre-assigned but we only provide the time slot to those devices which have the data to be sent. So, we can conclude that in asynchronous time division multiplexing, the data is only sent from the active senders and hence the frame contains the data as well at the time slot.

Time Division Multiplexing Advantages and Disadvantages

So far we have got a good grasp on time division multiplexing, let us now discuss some of the advantages and disadvantages of time division multiplexing.

Advantages

• We divide the time slot only so the user gets full bandwidth to be utilized by the user.
• The user gets more flexible usage as the user is independent in his/her allocated time slot.
• There is very little chance of cross-talk due to independence in the allocated time slot.
• The modulation and demodulation (in particular) are easier as compared to the frequency division multiplexing.
• It is more efficient and the cost of implementation is also lesser than the frequency division multiplexing.
• It can support both analog and digital signals.

Disadvantages

• There is a need for synchronization in case of time division multiplexing.
• It is more complex to implement than the frequency division multiplexing.

Example of Time Division Multiplexing

An example of time division multiplexing can be television broadcasting. When we watch any television show, we are not provided with the serial for the entire time. After every certain interval, there is a fixed duration of the advertisement. Hence, at a certain duration, only a specific frequency is being broadcasted (at the time of serial, only serial is broadcasted and at the time of advertisement, only advertisements are broadcasted).

Time Division Multiplexing Applications

Let us now discuss some of the applications of frequency-division multiplexing.

• The time division multiplexing is widely used in ISDN (Indian Deep Space Network) telephone lines.
• It is also used in the PSTN (Public Switched Telephone Network) networks.

Difference between Frequency Division and Time Division Multiplexing

Let us know the differences between the Frequency division and Time-division multiplexing techniques.

Additional Resources

At Scaler Topics we have more such great articles, please refer to the links provided below to check out other articles on our platform.

• What is Bandwidth in Computer Network?
• IEEE Standards in Computer Networks
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• What is Transport Layer Protocol?
• Socket Programming in Computer Network
• Sub-netting in Computer Networks