Communications Fundamentals FAQ / Chapter 8

8.15.7 What Is Carrier Sense Multiple Access (CSMA)?

1. What Is Carrier Sense Multiple Access?
2. Why Is It Called Carrier Sense Multiple Access?
3. How Does CSMA Work?
4. Why Does Listening Help?
5. Can Collisions Still Occur?
6. What Happens After a Collision?
7. What Is Persistence?
8. What Is 1-Persistent CSMA?
9. What Is Non-Persistent CSMA?
10. What Is p-Persistent CSMA?
11. What Is CSMA/CD?
12. How Does Collision Detection Work?
13. What Happens After Collision Detection?
14. What Is Binary Exponential Backoff?
15. Why Doesn't Wi-Fi Use Collision Detection?
16. What Is CSMA/CA?
17. What Are RTS and CTS?
18. What Is the Hidden Terminal Problem?
19. Where Is CSMA Used?
20. What Are the Advantages of CSMA?
21. What Are the Disadvantages?
22. Why Is CSMA Important?

Description

Understand how CSMA reduces collisions by listening before transmitting. Explore CSMA, CSMA/CD, and CSMA/CA, and learn why different versions are used in Ethernet and Wi-Fi networks.

Introduction

The ALOHA protocol demonstrated that communication networks could operate successfully even when users transmitted whenever they had data available. Its simplicity made it attractive for many applications, but it also revealed an important limitation. Because users transmitted without checking whether the communication channel was already in use, collisions became increasingly common as network traffic increased. Every collision wasted transmission capacity and required the affected packets to be sent again.

Engineers soon realised that many of these collisions could be avoided by adding one simple step before transmission. Instead of transmitting immediately, a device could first listen to determine whether another user was already using the channel. If the channel was busy, the device could simply wait until it became free before transmitting.

This idea became known as Carrier Sense Multiple Access (CSMA). Although conceptually simple, it represented a major improvement over ALOHA and became the basis of Ethernet, Wi-Fi, and numerous other communication systems. Several variations of CSMA were subsequently developed, including Carrier Sense Multiple Access with Collision Detection (CSMA/CD) for wired Ethernet and Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) for wireless networks.

Today, billions of communication devices rely upon some form of CSMA every day.

What Is Carrier Sense Multiple Access?

Carrier Sense Multiple Access (CSMA) is a random-access protocol in which a device listens to the communication channel before transmitting.

If the channel is idle, transmission begins immediately. If the channel is already busy, the device waits until the channel becomes available.

By avoiding many unnecessary collisions, CSMA provides much higher efficiency than ALOHA.

Why Is It Called Carrier Sense Multiple Access?

The name describes exactly how the protocol operates.

Carrier Sense means that the transmitter first checks whether another signal (or carrier) is already present on the communication channel. Multiple Access indicates that many independent users share the same communication medium.

Only after confirming that the channel appears idle does transmission begin.

How Does CSMA Work?

The basic sequence is straightforward.

A transmitting device:

• listens to the communication channel;
• determines whether another user is transmitting;
• transmits immediately if the channel is idle; or
• waits if the channel is busy.

This simple procedure significantly reduces the probability of collisions.

Why Does Listening Help?

Many collisions occur because two users begin transmitting while another transmission is already in progress.

If every transmitter first checks whether the channel is occupied:

• many collisions never occur;
• retransmissions become less frequent;
• network throughput increases; and
• communication delays decrease.

Although collisions cannot be eliminated completely, they occur far less often than with ALOHA.

Can Collisions Still Occur?

Yes.

Two users may both detect an idle channel and begin transmitting almost simultaneously. Since each user makes its decision independently, neither initially knows that the other has started transmitting. Their packets therefore collide.

Although CSMA reduces collisions, it cannot eliminate them entirely.

What Happens After a Collision?

When a collision occurs:

• the transmission is interrupted;
• the corrupted packets are discarded;
• each device waits for a random period; and
• retransmission is attempted later.

The use of random waiting intervals greatly reduces the likelihood of repeated collisions.

What Is Persistence?

Different CSMA systems adopt different strategies for deciding when to transmit after the channel becomes idle.

These strategies are known as persistence algorithms. The three most common approaches are:

• 1-persistent CSMA;
• non-persistent CSMA; and
• p-persistent CSMA.

Each offers different trade-offs between delay and collision probability.

What Is 1-Persistent CSMA?

In 1-persistent CSMA, a device continuously monitors the channel.

As soon as the channel becomes idle, it transmits immediately.

This minimizes waiting time but increases the probability that several users begin transmitting simultaneously when the channel becomes free.

What Is Non-Persistent CSMA?

In non-persistent CSMA, a busy channel causes the transmitter to wait for a random period before listening again.

This approach reduces the likelihood of several users transmitting simultaneously.

However, the additional waiting time may increase average communication delay.

What Is p-Persistent CSMA?

p-persistent CSMA is commonly used with slotted communication systems.

When the channel becomes idle:

• transmission occurs with probability p;
• waiting occurs with probability 1 − p; and
• the decision is repeated during the next slot.

By adjusting the value of p, engineers can balance network throughput against collision probability.

What Is CSMA/CD?

Carrier Sense Multiple Access with Collision Detection (CSMA/CD) extends the basic CSMA protocol.

While transmitting, a device continues monitoring the communication channel. If it detects that another transmission has begun simultaneously, it immediately recognizes that a collision has occurred. Rather than continuing to transmit a corrupted packet, the device stops transmitting immediately.

This prevents further waste of channel capacity.

How Does Collision Detection Work?

In a wired communication system such as traditional Ethernet, the transmitting station can compare:

• the signal it is transmitting; and
• the signal actually present on the cable.

If the two differ significantly, another station must also be transmitting.

The collision is detected almost immediately.

What Happens After Collision Detection?

Once a collision has been detected:

• transmission stops immediately;
• a short jamming signal may be transmitted so that all stations recognise the collision;
• each station waits for a randomly selected interval; and
• retransmission is attempted later.

The waiting interval usually increases after repeated collisions, reducing congestion during periods of heavy traffic.

What Is Binary Exponential Backoff?

Ethernet employs a technique known as binary exponential backoff.

Following each collision, the range of possible waiting times doubles. This approach:

• reduces repeated collisions;
• adapts automatically to network traffic;
• stabilizes heavily loaded networks; and
• improves overall efficiency.

Binary exponential backoff became one of the defining features of Ethernet.

Why Doesn't Wi-Fi Use Collision Detection?

Wireless systems face a different challenge.

A transmitter cannot reliably receive weak incoming signals while simultaneously transmitting a much stronger outgoing signal. Consequently, a Wi-Fi transmitter cannot usually detect collisions directly.

Instead, Wi-Fi attempts to prevent collisions before they occur.

What Is CSMA/CA?

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is the access method used by Wi-Fi.

Instead of detecting collisions after they occur, CSMA/CA attempts to minimise their occurrence. Typical techniques include:

• listening before transmitting;
• random backoff intervals;
• acknowledgement packets;
• interframe spacing; and
• optional RTS/CTS handshaking.

These mechanisms significantly reduce the probability of collisions in wireless networks.

What Are RTS and CTS?

Some Wi-Fi networks employ:

• Request to Send (RTS); and
• Clear to Send (CTS)

control messages.

Before transmitting a large packet, a station requests permission. The receiving station replies with a CTS message. Other nearby devices hearing the CTS defer their own transmissions, reducing the likelihood of collisions.

RTS/CTS is particularly useful when hidden terminals are present.

What Is the Hidden Terminal Problem?

Suppose two wireless devices are both within range of an access point but too far apart to hear one another.

Each believes the channel is idle. Both begin transmitting simultaneously. Although neither detects the other transmitter, their signals collide at the access point. This situation is known as the hidden terminal problem.

It is one of the principal challenges addressed by CSMA/CA.

Where Is CSMA Used?

Carrier Sense Multiple Access appears in many communication systems.

Applications include:

• Ethernet;
• Wi-Fi;
• industrial communication networks;
• wireless sensor networks; and
• numerous local-area communication systems.

Its decentralized operation makes it particularly attractive wherever many devices share a common communication medium.

What Are the Advantages of CSMA?

Carrier Sense Multiple Access offers several important benefits.

These include:

• improved throughput compared with ALOHA;
• relatively simple implementation;
• decentralized operation;
• efficient support for bursty traffic; and
• excellent scalability for moderate network loads.

These characteristics contributed greatly to its widespread adoption.

What Are the Disadvantages?

Like all random-access protocols, CSMA has limitations.

These include:

• collisions may still occur;
• performance decreases under heavy traffic;
• communication delays are unpredictable;
• hidden terminals affect wireless systems; and
• additional protocol complexity is required.

For these reasons, many modern communication systems combine CSMA with other access techniques.

Why Is CSMA Important?

Carrier Sense Multiple Access represented a major improvement over ALOHA by demonstrating that simply listening before transmitting could dramatically reduce collisions and improve network performance. Variants such as CSMA/CD made Ethernet one of the world's most successful networking technologies, while CSMA/CA enabled reliable operation of modern Wi-Fi networks.

Although many communication systems now employ scheduled or centrally managed access methods, CSMA remains one of the most influential protocols in the history of computer networking and continues to underpin billions of wired and wireless devices worldwide.

Summary

Carrier Sense Multiple Access improves upon ALOHA by requiring devices to listen before transmitting. This simple addition significantly reduces collisions and increases network throughput. Different versions have been developed for different communication environments, including CSMA/CD for traditional wired Ethernet and CSMA/CA for Wi-Fi.

By combining carrier sensing, random backoff algorithms, and collision management, CSMA provides an efficient and decentralized method for sharing communication channels. Its influence extends throughout modern computer networking and wireless communications, making it one of the most important random-access techniques ever developed.

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