Generally, a communication channel such as an optical fiber or coaxial cable can carry only one signal at any moment in time. This results in wastage of bandwidth. However, we can overcome this drawback by using a technique called multiplexing. By using the multiplexing technique, we can easily send multiple signals simultaneously over a communication channel (medium).

Multiplexing Definition

Multiplexing is a technique which combines multiple signals into one signal, suitable for transmission over a communication channel such as coaxial cable or optical fiber. Multiplexing is also sometimes referred to as muxing.

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The multiplexing technique divides the communication channel into several logical sub-channels. Each logical sub-channel is dedicated to an individual signal.

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Thus, the multiple signals are sent simultaneously over a shared communication channel (medium). Multiplexing has-been used for many years in long-distance telephony.

Multiplexing is done by using a device called Multiplexer or MUX. The multiplexer combines n input lines to generate one output line.

Without Multiplexing vs With Multiplexing

The below figure shows the communication system without multiplexing.

The communication system without multiplexing carries only one signal at any moment in time. Thus, it uses three communication channels to carry three signals. In this technique, a large amount of bandwidth is wasted.

The below figure shows the communication system with multiplexing. It carries three signals simultaneously. Thus, it uses only one communication channel to carry 3 signals (multiple signals). In this technique, the bandwidth is effectively used.

Types of Multiplexing

Multiplexing is mainly classified into two types:

• Analog multiplexing
• Digital multiplexing

Analog multiplexing is again classified into two types:

• Frequency Division Multiplexing
• Wavelength Division Multiplexing

In digital multiplexing, the Time Division Multiplexing is the most popular technique. The time division multiplexing is again classified into two types:

• Synchronous TDM
• Asynchronous TDM

Analog Multiplexing

The process of combining multiple analog signals into one signal is called analog multiplexing. It multiplexes the analog signals according to their frequency or wavelength.

Multiplexing requires that the multiple signals be kept apart so that they do not overlap with each other and thus can be separated at the receiving end. This can be achieved by separating the signal in frequency.

There are two types of analog multiplexing:

• Frequency division multiplexing
• Wavelength division multiplexing

Frequency Division Multiplexing

Frequency division multiplexing is an analog technique. It is the most popular multiplexing technique. We use this technique extensively in TV and radio transmission. This technique combines multiple signals into one signal and transmitted over the communication channel. Frequency division multiplexing is also known as FDM.

In this technique, the bandwidth of the communication channel should be greater than the combined bandwidth of individual signals.

The frequency division multiplexing divides the bandwidth of a channel into several logical sub-channels. Each logical sub-channel is allotted for a different signal frequency. The individual signals are filtered and then modulated (frequency is shifted), in order to fit exactly into logical sub-channels.

In this technique, each logical sub-channel (individual signal frequency) is allotted to each user. In other words, each user owns a sub-channel.

Each logical sub-channel is separated by an unused bandwidth called Guard Band to prevent overlapping of signals. In other words, there exists a frequency gap between two adjacent signals to prevent signal overlapping. A guard band is a narrow frequency range that separates two signal frequencies.

How FDM system works

The below figure shows the schematic diagram of an FDM system. The transmitter end contains multiple transmitters and the receiver end contains multiple receivers. The communication channel is present between the transmitter and receiver.

At transmitter end, each transmitter sends a signal of different frequency. In the below figure, the transmitter 1 sends a signal of 30 kHz, transmitter 2 sends a signal of 40 kHz, and transmitter 3 sends a signal of 50 kHz. These signals of different frequencies are then multiplexed or combined by using a device called multiplexer. It then transmits the multiplexed signals over a communication channel.

At the receiver end, the multiplexed signals are separated by using a device called demultiplexer. It then sends the separated signals to the respective receivers. In the above figure, the receiver 1 receives signal of 30 kHz, receiver 2 receives signal of 40 kHz, and receiver 3 receives signal of 50 kHz.

Advantages and Disadvantages of Frequency Division Multiplexing (FDM)

Advantages of Frequency Division Multiplexing (FDM)

1. It transmits multiple signals simultaneously.

2. In frequency division multiplexing, the demodulation process is easy.

3. It does not need Synchronization between transmitter and receiver.

Disadvantages of Frequency Division Multiplexing (FDM)

It needs a large bandwidth communication channel.

Applications of Frequency Division Multiplexing (FDM)

1. Frequency division multiplexing is used for FM and AM radio broadcasting.

2. It is used in first generation cellular telephone.

3. It is used in television broadcasting.

Wavelength Division Multiplexing

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Wavelength division multiplexing is an analog technique. It is the most important and most popular method to increase the capacity of an optical fiber. We know that wavelength and frequency are inversely proportional to each other (I.e. longer wavelength means low frequency and shorter wavelength means high frequency). Therefore, the working principle of wavelength division multiplexing is similar to frequency division multiplexing. The only difference is in wavelength division multiplexing optical signals are used instead of electrical signals. In wavelength division multiplexing, optical signals are transmitted through fiber optic cables.

Wavelength division multiplexing is a technology in which multiple optical signals (laser light) of different wavelengths or colors are combined into one signal and is transmitted over the communication channel. Thus multiple signals are transmitted simultaneously over a single communication channel.

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Wavelength division multiplexing is a technology that increases the bandwidth of a communication channel (optical fiber) by simultaneously allowing multiple optical signals through it.

In wavelength division multiplexing, the communication channel is a fiber optic cable. Wavelength division multiplexing is also known as WDM. A demultiplexer at the receiver end separates the optical signal wavelengths or colors.

In this technique, the bandwidth of the communication channel should be greater than the combined bandwidth of individual signals.

The wavelength division multiplexing divides the bandwidth of a channel into several logical sub-channels according to its wavelength. It allots each logical sub-channel for a different light color or optical signal wavelength. The individual signals are filtered and then modulated (wavelength is shifted), to fit exactly into logical sub-channels.

In this technique, each logical sub-channel (individual signal wavelength) is allotted to each user. In other words, each user owns a sub-channel.

The main advantage of WDM system is that you only need to upgrade the multiplexer and demultiplexer at each end; you no need to buy more fibers which are more expensive.

Wavelength division multiplexing enables bi-directional communication and multiplication of optical signal capacity.

How WDM system works

The schematic diagram of a WDM system is shown in the below figure. The transmitter end contains multiple optical transmitters and the receiver end contains multiple optical receivers. The communication channel (optical fiber) is present between the transmitter and receiver.

At transmitter end, each transmitter sends an optical signal of different wavelength or color. These optical signals of different wavelengths or colors are then multiplexed or combined by using a device called multiplexer.

The multiplexed signals are then transmitted over a single communication channel (optical fiber). In between the transmitter and receiver, optical amplifiers are used to compensate the optical signal loss caused during the transmission.

At the receiver end, the multiplexed signals are separated by using a device called demultiplexer. The separated signals are then sent to the respective receivers.

Types of Wavelength Division Multiplexing

WDM techniques are of two types:

• Dense Wavelength Division Multiplexing
• Coarse Wavelength Division Multiplexing

Dense Wavelength Division Multiplexing

Dense Wavelength Division Multiplexing is also simply referred to as DWDM. It is a technology in which a large number of optical signals (laser light) of different wavelengths or colors are combined into one signal and is transmitted over the communication channel to a long distance.

When the optical signal transmission distance becomes hundreds of kilometers, some signal loss will occur. In order to compensate this signal loss, optical fiber amplifiers are used in the DWDM communication system. The optical amplifiers used in DWDM technology are Optical Line Amplifier (OLA), Optical Boost Amplifier (OBA), and Optical Pre Amplifier (OPA). However, these amplifiers will not work effectively in the entire bandwidth of the communication channel. They work effectively in a small wavelength range. This small wavelength range is called gain bandwidth.

Therefore, very small wavelength spacing is needed between two optical signals to put a large number of optical signals into the gain bandwidth. The DWDM technology uses this gain bandwidth to effectively transmit a large number of optical signals simultaneously. DWDM has the ability to transmit up to 80 channels (80 optical signals) with 100 GHz (0.8 nm) spacing.

Coarse Wavelength Division Multiplexing

Coarse wavelength division multiplexing is a technology in which multiple optical signals (laser light) of different wavelengths or colors are combined into one signal and is transmitted over the communication channel for a short distance. Coarse wavelength division multiplexing is also simply referred to as CWDM

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The DWDM technology is mainly useful for long distance communication (>100 km). However, if the transmission distance is less than 100 kilometers, we no need to use optical amplifiers. Hence, gain bandwidth is not required. We can send optical signals in the entire bandwidth of the communication channel. This allows the wider channel spacing between the optical signals. A typical CWDM system has a channel spacing of 20 nm between the optical signals. This allows the use of low-cost components such as uncooled lasers, MUX and DEMUX.

Coarse Wavelength Division Multiplexing has the ability to transport up to 18 optical signals per optical fiber. It is designed for short distance.

Advantages of Wavelength Division Multiplexing (WDM)

1. WDM allows transmission of data in two directions simultaneously

2. Low cost

3. Greater transmission capacity

4. High security

5. Long distance communication with low signal loss

Digital Multiplexing

The process of combining multiple digital signals into one signal is called digital multiplexing.

Time Division Multiplexing

Time Division Multiplexing is a technique in which multiple signals are combined and transmitted one after another on the same communication channel.

At the receiver side, the signals are separated and received. Each signal is received by a user at a different time.

Time Division Multiplexing is also simply referred to as TDM. It is the digital multiplexing technique.

In frequency division multiplexing, all signals of different frequencies are transmitted simultaneously. But in time division multiplexing, all signals operate with the same frequency are transmitted at different times.

In frequency division multiplexing, the sharing of a channel is done on the basis of frequency. But in time division multiplexing, the sharing of a channel is done on the basis of time.

In time division multiplexing, each user is allotted a particular time interval called time slot during which data is transmitted. The time interval (time slot) allotted to each receiver (user) is so small that the receiver will not detect that some time was used to serve another receiver (user).

In time division multiplexing, all signals are not transmitted simultaneously; instead, they are transmitted one after another. For example, as shown in the above figure, at first, we send signal A. Then after second signal B and then after third signal C and finally, we send last signal D. Thus, each user occupies an entire bandwidth for a short period of time.

The time division multiplexing technique is used to multiplex analog signals or digital signals. However, the time division multiplexing is more suitable for digital signal multiplexing.

In time division multiplexing, the bandwidth capacity of the communication channel should be greater than the multiple input signals.

Types of TDM (Time Division Multiplexing)

Time Division Multiplexing is mainly classified into two types:

• Synchronous TDM (Time Division Multiplexing)
• Asynchronous TDM (Time Division Multiplexing)

Synchronous TDM (Time Division Multiplexing)

In synchronous time division multiplexing, each device (transmitter) is allotted with a fixed time slot, regardless of the fact that the device (transmitter) has any data to transmit or not. The device has to transmit data within this time slot. If the device (transmitter) does not have any data to send then its time slot remains empty.

As shown in the below figure, the various time slots are arranged into frames and each frame consists of one or more time slots dedicated to each device (transmitter). For example, if there are 3 devices, there will be 3 slots in each frame. Similarly, if there are 5 devices, there will be 5 slots in each frame.

The above figure shows 4 devices (transmitter A, transmitter B, transmitter C, and transmitter D) that have 4 dedicated time slots (time slot A, time slot B, time slot C and time slot D).

The transmitter A data is sent at time slot A, transmitter B data is sent at time slot B, transmitter C data is sent at time slot C and transmitter D data is sent at time slot D.

In the time frame 2, the transmitter B and C does not have any data to send so the time slot B and C remains empty.

The main drawback of synchronous time division multiplexing is that the channel capacity is not fully utilized. Hence, the bandwidth goes wasted.

Asynchronous TDM (Time Division Multiplexing)

In Asynchronous time division multiplexing, the time slots are not fixed (I.e. time slots are flexible). The asynchronous TDM is also known as statistical time division multiplexing.

In synchronous TDM, the number of time slots is equal to the number of devices (transmitters). But in Asynchronous TDM, the number of time slots is not equal to the number of devices (transmitters). The time slots in asynchronous TDM are always less than the number of devices (transmitter). For example, if we have X devices and Y time slots. Y should always be less than X (I.e. Y < X).

In asynchronous time division multiplexing, time slots are not fixed to a particular device; instead, they are allotted to any of the devices that have data to send.

In the above figure, it is shown that the number of devices are 4 and time slots are 3. The timeframe 1 (all slots) is completely filled with data from devices A, B, and C. The timeframe 1 has only 3 time-slots. So the data from device D is filled in the next timeframe (I.e. timeframe 2) in timeslot 1. The data from devices A and D will be filled in timeslots 2 and 3 in timeframe 2.

In asynchronous time division multiplexing, the multiplexer scans all the devices (transmitters) and accepts input only from the devices that have actual data to send and fills all the frames, and then sends it to the receiver.

If there is not enough data to fill all the slots in a frame, then the partially filled frames are transmitted. In most of the cases, all the time slots in frames are completely filled.

Advantages and Disadvantages of Time Division Multiplexing (TDM)

Advantages of Time Division Multiplexing (TDM)

1. Full bandwidth is utilized by a user at a particular time.

2. The time division multiplexing technique is more flexible than frequency division multiplexing.

3. In time division multiplexing, the problem of crosstalk is very less.

Disadvantages of Time Division Multiplexing (TDM)

In time division multiplexing, synchronization is required.

Advantages of multiplexing

1. Multiple signals can be sent simultaneously over a single communication channel.

2. Effective use of channel bandwidth

3. Multiplexing reduces cost

4. Multiplexing reduces circuit complexity

Applications of Multiplexing

1. Communication system

2. Computer memory

3. Telephone systems

4. TV broadcasting

5. Telemetry

6. Satellites

Compare FDM, WDM, and TDM