Multiplexing in Computer Networks

In this tutorial, we will be covering the concept of Multiplexing in Computer Networks.

The set of techniques that allows the simultaneous transmission of multiple signals across a single data link is commonly referred to as Multiplexing. Multiplexing is done by using the hardware that is called as Multiplexer(MUX).

The Multiplexer(MUX) mainly combines 'n' input lines in order to generate '1' output line(this is simply many-to-one) on the sender side. And on the receiver side, this stream is fed into the demultiplexer(DEMUX), which then separates the stream back to its component transmission (this is one-to-many) and then directs them to their corresponding lines.

The main aim of the multiplexing technique is to share scarce resources.

Let us understand with the help of a diagram given below to divide 1 link into n channels:

In the above diagram, the word link refers to the physical path, and the word channel simply refers to the portion of the link that carries a transmission between a given pair of lines. Thus 1 link can have many channels.

History of the Multiplexing

In telecommunications, several telephone calls may be carried by using a single wire. Also, Multiplexing was originated in telegraphy in the 1870s. Now, this technique is widely applied in communications. George Owen Squier in telephony was credited with the development of telephone carrier multiplexing in the year 1910.

Need for the Multiplexing

As we have already told you that multiplexing is basically a set of techniques that mainly allows the transmission of multiple signals simultaneously across a signals data link.

At the time when there is a need to transmit many signals from the sender side that sends simultaneously then multiplexer is used to convert many signals into one so that on the receiving end we can get them simultaneously.

As it is very expensive to send many signals differently and it also requires more wires to send. Thus there is a need for multiplexing. Let us take an example of T.V cable distributor who sends many channels through a single wire.

Advantages of Multiplexing

Given below are some advantages of using Multiplexing:

• With the help of multiplexing, more than one signal can be sent easily over a single medium or link.

• Multiplexing helps in the effective utilization of the bandwidth of the medium.

Let us take a look at the given below figure to understand multiplexing vs no-multiplexing:

Categories of Multiplexing

Let us take a look at the different categories of Multiplexing:

• Frequency-division multiplexing

• Wavelength-division multiplexing

• Time-division multiplexing

Let us discuss all the above-given categories one by one in the following sections.

1.Frequency-Division Multiplexing

Frequency-Divison Multiplexing i.e FDM is an analog technique.

• With this technique, signals having different frequencies are combined in a composite signal and then transmitted on the link

• It is mainly applied at the time when the bandwidth of the link is greater than the combined bandwidths of the signal to be transmitted.

• In this, each signal is of a different frequency.

• The channel is usually separated by the strips of unused bandwidth that is the guard bands in order to prevent the signals from overlapping.

• In the case of frequency division multiplexing, suppose the input signal is in the digital form then it must be converted to analog before giving it as the input to the modulator.

Frequency Division Multiplexing

From the above diagram, in FDM the transmission path is divided into three parts and each part mainly represents a channel that carries one transmission.

Advantages

Given below are some advantages of using FDM:

• The Simultaneous transmission of a large number of signals is done easily.

• The demodulation of FDM multiplexing is easy.

• There is no need for synchronization between the transmitter and receiver for proper operation.

• In the case of slow narrowband fading, there is only one single channel that gets affected.

Disadvantages

There are some drawbacks of using FDM:

• Communication channels must have a very large bandwidth.

• There occurs the problem of crosstalk while using FDM.

• In the case of wideband fading, all channels in the FDM gets affected.

• There is a need for a large number of filters and modulators.

Applications

The main applications of FDM are as follows:

• One of the main applications of FDM is that it is AM and FM radio broadcasting.

• Another application of FDM is that it is used in television broadcasting.

• FDM is also used by first-generation cellular telephones.

2.Wavelength-Division Multiplexing

Wavelength-Divison Multiplexing i.e WDM is an analog technique.

• This technique is similar to FDM.

• With the help of Wavelength Divison multiplexing different signals that include: optical or light signals are transmitted through the Optical fiber.

• With the help of the WDM technique, the high data rate capability of optical fiber cable gets utilized.

• With this technique, various light waves from different sources are combined into a composite light signal and this signal is transmitted across the channel to the receiver.

• On the receiver side, this composite light signal gets broken down into different light waves with the help of Demultiplexer.

• The process of combining and splitting the light waves is done with the help of Prism.

• This Prism helps to bend the beam of light on the basis of the angle of incidence and frequency of light.

• In the WDM technique mainly the role of the multiplexer is played by the Prism and it then combines the various optical signals in the order to form a composite signal after that this composite signal is transmitted through an Optical fiber cable.

The above Figure indicates Wavelength Divison Multiplexing

Let us take a look at the diagram given below where we will use prism for wavelength-division multiplexing and demultiplexing.

Advantages

Given below are some advantages of using WDM:

• With the help of WDM, the full-duplex transmission is possible.

• WDM is easy to reconfigure.

• Various Signals can be transmitted simultaneously with the help of WDM.

• This technique is less expensive and the expansion of the system is easy.

• This technique provides high security.

• As we are using an optical fiber in WDM; also Optical components are more reliable and they also provide high bandwidth.

Disadvantages

There are some drawbacks of using WDM:

• There is the use of optical equipment so cost increases.

• Utilization of bandwidth can be inefficient which causes difficulty in wavelength tuning.

• The main concern in this technique is scalability.

3.Time-Division Multiplexing

Time-Divison multiplexing is a digital technique for multiplexing.

• In this technique, the channel/link is divided on the basis of time instead of frequency.

• The total available time on the channel is divided between the different users on the channel.

• A particular time interval is allotted to each user on the channel and it is known as time slot/slice.

• In the time-division multiplexing, the data rate capacity should be much greater than the data rate that is required by the sending and receiving device.

TDM is further categorized into two:

• Synchronous Time-Divison Multiplexing

• Asynchronous Time-Divison Multiplexing

1.Synchronous Time-Divison Multiplexing

In Synchronous TDM, each of the Input connection has an allotment in the output even if it is not sending the data.

• In this multiplexing, each device is given the same time slot in order to transmit data over the link whether it has to send data to the receiver or not.

• Each device places data on the link whenever its time slot arrives Thus control is given to each device turn by turn.

• In case if any devices do not have any data to send then in that case the time slot for that device remains empty.

• In this multiplexing, if there are 'n' sending devices then simultaneously there will be 'n' time slots which means one time slot for each device.

• Also, time slots are organized in the form of frames, where each frame consists of one or more time slots.

Advantages

• This technique is easy to implement.

• The performance is guaranteed in using this technique.

Disadvantages

• If a user has no data to transmit in that case time slots will get wasted.

• In this multiplexing, the capacity of the transmission link must be always higher than the total capacity of the input lines.

2.Asynchronous Time-Divison Multiplexing

Another name of Asynchronous TDM is Statical Time Divison Multiplexing. In this time slots are not fixed, rather time slots are allocated dynamically in order to improve the efficiency of bandwidth.

• The total speed of all the Input lines can be greater than the capacity of the path.

• In this Multiplexing, there are n input lines and m slots; thus always (m<n).

• There is no concept of predefined slots rather than slots are allocated dynamically on demand.

• In this multiplexing, the multiplexor mainly accepts the incoming input data and then it creates a frame that contains only data without any empty slots.

• Each slot mainly contains the address part that is used to identify the source of the input data.

• The number of frames in this multiplexing depends upon the statical analysis of the number of input lines.

Thus in the above diagram, out of 6 only 4 devices are sending data that are 1,2,4,6. In the above diagram, you can see that the data part contains the address in order to determine the source of the data. Like A1(data along with its source).

Advantages

• In this multiplexing, there is an efficient use of the capacity of transmission.

Disadvantages

• In this Multiplexing, frames are of different sizes.

• There is a need for the buffer address information is also needed because there are no separate slots assigned for each user.

• This technique does not provide a fixed waiting time guarantee.