Types of Multiplexing in Data Communications

Imagine you have several friends who all want to send letters to the same person at the same time. Instead of sending each letter individually, which would take a lot of time and effort, you put all the letters into one big envelope and send that. When the big envelope arrives, the letters are taken out and delivered to the person one by one.

Multiplexing in data communications works in a similar way. It’s a method that combines multiple signals or data streams into one signal over a shared medium. This process allows for efficient use of resources and can significantly increase the amount of data that can be sent over a network.

Multiplexing in Communication Systems

Multiplexing is the sharing of a medium or bandwidth. It is the process in which multiple signals coming from multiple sources are combined and transmitted over a single communication/physical line.

Uses of Multiplexing

Multiplexing is used for a variety of purposes in data communications to enhance the efficiency and capacity of networks. Here are some of the main uses:

• Efficient Utilization of Resources: Multiplexing allows multiple signals to share the same communication channel, making the most of the available bandwidth. This is especially important in environments where bandwidth is limited.
• Telecommunications: In telephone networks, multiplexing enables the simultaneous transmission of multiple telephone calls over a single line, enhancing the capacity of the network.
• Internet and Data Networks: Multiplexing is used in internet communications to transmit data from multiple users over a single network line, improving the efficiency and speed of data transfer.
• Satellite Communications: Multiplexing helps in efficiently utilizing the available bandwidth on satellite transponders, allowing multiple signals to be transmitted and received simultaneously.

Types of Multiplexing 

The below are the different types of multiplexing techniques, each designed to handle various types of data and communication needs.

Frequency Division Multiplexing

Frequency division multiplexing is defined as a type of multiplexing where the bandwidth of a single physical medium is divided into a number of smaller, independent frequency channels.

Frequency Division Multiplexing is used in radio and television transmission.

In FDM, we can observe a lot of inter-channel cross-talk because in this type of multiplexing the bandwidth is divided into frequency channels. In order to prevent the inter-channel cross talk, unused strips of bandwidth must be placed between each channel. These unused strips between each channel are known as guard bands.

Time Division Multiplexing

Time-division multiplexing is multiplexing wherein FDM, instead of sharing a portion of the bandwidth in the form of channels, in TDM, time is shared. Each connection occupies a portion of time in the link. In Time Division Multiplexing, all signals operate with the same frequency (bandwidth) at different times.

There are two types of Time Division Multiplexing :

• Synchronous Time Division Multiplexing
• Statistical (or Asynchronous) Time Division Multiplexing

Synchronous TDM : Synchronous TDM is a type of Time Division Multiplexing where the input frame already has a slot in the output frame. Time slots are grouped into frames. One frame consists of one cycle of time slots. Synchronous TDM is not efficient because if the input frame has no data to send, a slot remains empty in the output frame. In this, we need to mention the synchronous bit at the beginning of each frame.

Statistical TDM: Statistical TDM is a type of Time Division Multiplexing where the output frame collects data from the input frame till it is full not leaving an empty slot like in Synchronous TDM. In this, we need to include the address of each particular data in the slot that is being sent to the output frame.

Statistical TDM is a more efficient type of time-division multiplexing as the channel capacity is fully utilized and improves the bandwidth efficiency. 

Read about Differences between TDM and FDM

Wavelength Division Multiplexing

Wavelength Division Multiplexing (WDM) is a multiplexing technology used to increase the capacity of optical fiber by transmitting multiple optical signals simultaneously over a single optical fiber, each with a different wavelength. Each signal is carried on a different wavelength of light, and the resulting signals are combined onto a single optical fiber for transmission.

At the receiving end, the signals are separated by their wavelengths, demultiplexed and routed to their respective destinations. It is used in telecommunications, cable TV, ISPs, and data centers for high-speed, long-distance data transmission.

WDM can be divided into two categories:

• Dense Wavelength Division Multiplexing (DWDM) is used to multiplex a large number of optical signals onto a single fiber, typically up to 80 channels with a spacing of 0.8 nm or less between the channels. 
• Coarse Wavelength Division Multiplexing (CWDM) is used for lower-capacity applications, typically up to 18 channels with a spacing of 20 nm between the channels.

Read about Differences between DWDM and CWDM.

Advantages over Time Division Multiplexing (TDM):

• Higher data rates & capacity
• Lower power consumption
• Reduced equipment complexity
• Flexible & easily upgradable

Read about Differences between TDM, FDM and WDM.

Advantages of Multiplexing

• Efficient Use of Bandwidth: You can send more than one signal over a single channel. This way, you can use the channel's capacity more efficiently.
• Increased Data Transmission: Multiplexing can significantly boost the amount of data that can be sent over a network simultaneously, enhancing overall transmission capacity.
• Scalability: Multiplexing allows networks to easily expand and accommodate more data streams without requiring significant changes to the existing infrastructure.
• Flexibility: Different types of multiplexing (TDM, FDM, WDM, CDM) can be used based on the specific needs and characteristics of the communication system, providing flexibility in network design.

Disadvantages of Multiplexing

• Synchronization Issues: Ensuring that multiple data streams remain properly synchronized can be challenging, leading to potential data loss or errors if not managed correctly.
• Latency: Combining multiple signals into one can introduce delays, as each data stream needs to be processed, synchronized, and demultiplexed at the receiving end.
• Signal Degradation: Over long distances, multiplexed signals can experience degradation and interference, requiring additional measures such as signal boosters or repeaters to maintain quality.
• Resource Management: Allocating and managing resources for multiplexing can be complicated, requiring careful planning and real-time adjustments to avoid congestion and ensure efficient operation.