Multiplexing in Computer Network

So far we have learned how transmission media serves a greater purpose in physical layer. In this guide, we will learn a popular technique which is used for effective transmission of multiple signals over a single transmission medium. Multiplexing is a technique used to combine multiple signals and transmit the combined signal over a transmission medium.

Multiplexer and Demultiplexer

Multiplexer (MUX): Multiplexer is a combinational circuit that has 2ⁿ input signal lines and a single output line. For example 4 X 1 multiplexer has 4 input data lines and 1 output line and 8 X 1 multiplexer has 8 input data lines and 1 output line. Multiplexer is used for multiplexing technique as it is combining multiple signals and producing single output signal.

Demultiplexer (DEMUX): Demultiplexer does exactly opposite of Multiplexing, it receives a single input signal and generates 2ⁿ output signals. Multiplexer is used at the source to combine the multiple input signals while Demultiplexer is used at the receiving side to generate the multiple signals from the received signal. For example 1 X 4 Demultiplexer is used to generate 4 output signals from a single signal.

Why we use multiplexing?

1. Multiplexing reduces the number of physical connections or wireless channels to transmit the multiple signals as it combines the signals and send it over a single transmission medium.
2. Reduces the cost of transmission services.
3. Bandwidth is utilized in effective manner.

Real world example of multiplexing are: Cable tv receives all the channels using a single cable only because of multiplexing only.

Types of Multiplexing Techniques

Multiplexing techniques are divided in three major categories:

1. Frequency division multiplexing (FDM)
2. Wavelength division multiplexing (WDM)
3. Time Division Multiplexing (TDM) – It is further divided in two types: Synchronous TDM and Asynchronous TDM.

Frequency division multiplexing (FDM)

In frequency division multiplexing technique, the total bandwidth of transmission medium is divided into non overlapping frequency ranges to carry separate signals. This technique allows a single transmission medium to carry multiple signals.

Advantages of Frequency division multiplexing (FDM)

1. It doesn’t require synchronization between sender and receiver.
2. It is simple, easy to implement and less expensive technique.
3. FDM is a reliable system compared to other multiplexing systems.
4. This technique is mainly used for analog signals.
5. This technique allows large number of signals to be transmitted simultaneously over a single transmission medium.

Disadvantages of Frequency division multiplexing (FDM)

1. FDM suffers from interference caused by electromagnetic signals, generally referred as cross talk.
2. FDM provides less throughput (throughput is the rate of successful message delivery over a communication channel)
3. FDM doesn’t utilize the full bandwidth of the transmission medium.
4. FDM requires significantly large number of modulator and filter to transmit multiple signals.
5. Unlike TDM, FDM needs a carrier signal to transmit multiple signals.

Wavelength Division Multiplexing (WDM)

Wavelength division multiplexing, commonly referred a WDM is mainly used for fiber optic communications. In this technology, multiple signals are transmitted over a single fiber optic cable by using different wavelengths of laser light.

Advantages of Wavelength Division Multiplexing (WDM)

1. Easy to implement.
2. full duplex transmission is possible in WDM.
3. Provides better security of data.
4. WDM provides higher bandiwidth.

Disadvantages of Wavelength Division Multiplexing (WDM)

1. Transmitted signals cannot be very close to each other.
2. WDM is more expensive compared to FDM.
3. It is difficulty to fine tune wavelength in WDM technique.

Time Division Multiplexing (TDM)

Unlike frequency division multiplexing where signals are transmitted over a different frequency ranges at the same time, In TDM multiple signals are transmitted at the same frequency but different time intervals. In TDM signals appear on the line only a fraction of time in an alternating pattern.

Types of time division multiplexing

1. Synchronous TDM
2. Asynchronous TDM

Synchronous TDM

1. In synchronous TDM, a time slot is assigned to each device irrespective of whether it is sending signal or not.
2. If the connected device is not sending any signal, the time slot assigned to that device will remain empty and cannot be re-assigned to the other devices on the network.
3. In synchronous TDM, signals are sent in form of frames.

Drawbacks of synchronous TDM:

The full capacity of the transmission medium is not utilized as the time slot remains empty in case there is no signals from the assigned devices.

Asynchronous TDM

1. In Asynchronous TDM, the time slots are not preassigned. Time slots are assigned to only those devices which have data to be sent.
2. The time slots are dynamically assigned to the devices.
3. Unlike synchronous TDM, the Asynchronous TDM technique fully utilized the capacity of transmission medium.
4. Since slots are not preassigned, to identify the source of the data, an address part is associated with the slots.