7.11.2 What Is the Difference Between Multiplexing and Multiple Access?
- What Is Multiplexing?
- What Is Multiple Access?
- What Is the Fundamental Difference?
- Can One Exist Without the Other?
- How Are Multiplexing and Multiple Access Related?
- What Are Common Multiplexing Techniques?
- What Are Common Multiple-Access Techniques?
- Why Do the Names Sound Similar?
- How Does a Cellular Network Use Both?
- How Does a Satellite System Use Both?
- How Does Wi-Fi Use Both?
- Why Is This Distinction Important?
- Do Modern Networks Combine Many Techniques?
- Which Concept Came First?
- Why Is It Helpful to Learn Both Concepts Together?
Description
Understand the distinction between combining multiple signals and allowing multiple users to share a communication channel. Learn why multiplexing and multiple-access techniques solve different engineering problems and how they complement one another.
Introduction
The terms multiplexing and multiple access are frequently encountered in communications engineering, and they are often used interchangeably. Although the two concepts are closely related and frequently employed together, they are not the same. Confusing them can make it difficult to understand how modern communication systems operate.
Both techniques are concerned with sharing communication resources efficiently. However, they address different parts of the communication process. Multiplexing combines multiple information streams onto a common transmission medium, while multiple access allows multiple independent users to share that medium. In other words, multiplexing focuses on signals, whereas multiple access focuses on users.
Modern communication systems almost always employ both techniques simultaneously. For example, a cellular base station allows hundreds of subscribers to share the available radio spectrum using multiple-access techniques. At the same time, the resulting traffic is multiplexed onto high-capacity microwave or optical-fiber links for transport through the wider network.
Understanding the distinction between multiplexing and multiple access provides the foundation for understanding not only communication networks but also the technologies that make today's Internet, mobile-phone systems, satellite networks, and wireless communications possible.
What Is Multiplexing?
Multiplexing is the process of combining multiple independent information streams so they can be transmitted simultaneously over a single communication channel.
The information streams may originate from:
- different users;
- different applications;
- different sensors; or
- different communication systems.
The objective is to make efficient use of the available transmission medium.
At the receiving end, a demultiplexer separates the composite signal back into its original information streams.
What Is Multiple Access?
Multiple access refers to the techniques that allow multiple independent users or devices to share the same communication system.
Instead of combining signals for transmission, multiple-access methods determine how users obtain access to the shared communication resource.
Examples include:
- mobile phones communicating with a cellular base station;
- aircraft sharing an air-traffic-control frequency;
- satellite terminals sharing a transponder; and
- computers communicating through a Wi-Fi access point.
The objective is to allow many users to communicate efficiently without causing unacceptable interference.
What Is the Fundamental Difference?
The simplest way to distinguish the two concepts is to consider what is being shared.
Multiplexing shares transmission capacity. Multiple access shares communication access. A multiplexer combines several signals into one transmission. A multiple-access system determines how different users are allowed to transmit and receive information over the shared medium.
Although the two concepts often operate together, they solve different engineering problems.
An Analogy: A Multi-Lane Highway
A useful analogy is a multi-lane highway.
Multiplexing is comparable to organising many individual vehicles into several orderly lanes so they can travel efficiently along the highway. Multiple access is comparable to determining how vehicles enter the highway, merge with traffic, and leave again without collisions. The highway itself represents the communication channel.
Both organisation and access control are necessary for efficient operation.
Can One Exist Without the Other?
Yes.
A multiplexing system may combine several signals generated within a single piece of equipment even though only one user is involved. Conversely, a simple radio network may employ multiple access among several users without performing any significant multiplexing.
However, most practical communication systems employ both techniques together.
How Are Multiplexing and Multiple Access Related?
Multiple-access techniques often generate many independent information streams.
These streams must then be transported efficiently through the communication network. Multiplexing provides this transport. For example, a cellular base station receives traffic from hundreds of mobile users. The base station then combines these individual communication streams for transmission across high-capacity microwave or optical-fiber links.
Thus:
- multiple access connects the users; and
- multiplexing transports their information.
What Are Common Multiplexing Techniques?
Several multiplexing methods have been developed.
The most common include:
- Frequency-Division Multiplexing (FDM);
- Time-Division Multiplexing (TDM);
- Wavelength-Division Multiplexing (WDM);
- Orthogonal Frequency-Division Multiplexing (OFDM); and
- Spatial Division Multiplexing (SDM).
Each divides the available communication resource in a different way.
What Are Common Multiple-Access Techniques?
Modern communication systems employ several multiple-access methods.
Examples include:
- Frequency-Division Multiple Access (FDMA);
- Time-Division Multiple Access (TDMA);
- Code-Division Multiple Access (CDMA);
- Orthogonal Frequency-Division Multiple Access (OFDMA); and
- Non-Orthogonal Multiple Access (NOMA).
These techniques determine how multiple users gain access to the communication channel.
The following chapter examines these methods in detail.
Why Do the Names Sound Similar?
The terminology reflects the close relationship between the concepts.
Many multiple-access techniques are derived from multiplexing methods. For example:
- FDM leads naturally to FDMA;
- TDM forms the basis of TDMA; and
- OFDM provides the foundation for OFDMA.
Although the names are similar, the objectives differ.
Multiplexing combines signals. Multiple access allocates communication resources among users.
How Does a Cellular Network Use Both?
A cellular communication system provides an excellent illustration. Each mobile phone competes with many others for access to the radio spectrum. The network manages this process using multiple-access techniques. Once traffic reaches the base station, the information from many users is combined and transported across high-capacity backhaul links using multiplexing.
The two processes therefore occur at different stages of the communication system.
How Does a Satellite System Use Both?
Satellite communication systems also employ both concepts.
Multiple earth stations share access to the satellite using appropriate multiple-access techniques. Within the satellite payload, numerous communication channels may be combined and distributed across available transponders. The gateway then multiplexes large volumes of traffic onto terrestrial optical-fiber networks.
Again, multiple access and multiplexing work together but perform different functions.
How Does Wi-Fi Use Both?
Wi-Fi provides another familiar example.
Many devices share a single wireless access point. The access point controls channel access using multiple-access procedures to minimise collisions.
Once traffic enters the wired network, multiplexing combines information from many users onto Ethernet links, optical fibers, and Internet backbone networks.
To the user, these processes are largely invisible, yet both are essential for efficient communication.
Why Is This Distinction Important?
Understanding the distinction helps explain the architecture of communication networks.
A communication system must answer two separate questions:
- Who may transmit?
- How will all of the resulting information be transported efficiently?
Multiple access answers the first question. Multiplexing answers the second.
Recognising these separate functions makes the operation of complex communication networks much easier to understand.
Do Modern Networks Combine Many Techniques?
Very often.
A modern 5G network might employ:
- OFDMA to allocate radio resources among users;
- MIMO to increase spatial capacity;
- adaptive modulation to maximise throughput;
- TDM within network equipment; and
- WDM across the optical-fiber backbone.
Similarly, a broadband satellite system may combine:
- multiple-access techniques for user terminals;
- frequency multiplexing within satellite payloads;
- time multiplexing for digital traffic; and
- wavelength multiplexing across terrestrial fiber links.
Each technique contributes to improving the overall efficiency of the communication system.
Which Concept Came First?
Historically, multiplexing developed first.
Early telegraph and telephone systems used multiplexing to increase the capacity of expensive transmission lines. As wireless communications evolved during the twentieth century, engineers developed increasingly sophisticated multiple-access techniques to allow growing numbers of users to share the radio spectrum.
Today, advances in both fields continue to support the rapid growth of global communications.
Why Is It Helpful to Learn Both Concepts Together?
Because they are complementary technologies.
Multiplexing ensures that communication channels are used efficiently. Multiple access ensures that users obtain fair and orderly access to those channels. Together they enable billions of people and devices to communicate simultaneously across shared communication networks.
Understanding the distinction between these concepts provides an excellent foundation for studying both multiplexing and multiple-access techniques in greater detail.
Summary
Multiplexing and multiple access are closely related but serve different purposes. Multiplexing combines multiple information streams so they can share a common transmission channel efficiently, while multiple access determines how multiple users share that channel without causing unacceptable interference.
Modern communication systems employ both techniques simultaneously. Multiple access manages communication between users and the network, while multiplexing transports the resulting traffic efficiently across high-capacity communication links. Together they form two of the fundamental building blocks of contemporary telecommunications.
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