Terminology » CSMA vs OFDM : Difference between CSMA and OFDM in WLAN

CSMA vs OFDM : Difference between CSMA and OFDM in WLAN

Introduction : Wireless Local Area Networks (WLANs) have become the backbone of modern communication, enabling seamless connectivity for billions of devices worldwide. To ensure reliable and efficient data transmission over a shared wireless medium, WLANs employ various technologies at different layers of the communication model. Two key technologies are Carrier Sense Multiple Access (CSMA) at the MAC layer and Orthogonal Frequency Division Multiplexing (OFDM) at the physical layer. While CSMA focuses on how devices share the wireless medium to avoid collisions, OFDM ensures high speed and robust data transfer over the channel. Let us understand each and explore key differences between them.

CSMA

It is an access technique used in MAC layer of WLAN.
CSMA is the short form for “Carrier Sense Multiple Access”. It is often used as CSMA/CA, CA stands for collision avoidance
It is used to control how devices access the shared wireless channel.
Algorithm : Before transmitting, a device listens to check if the channel is free. If it’s busy, it waits for random time and tries again.

CSMA persistent and non-persistent

The figure-1 depicts CSMA types viz. persistent and non-persistent.
It can also be categorized into CSMA/CA and CSMA/CD.

OFDM

It is a modulation technique used in PHY layer of WLAN.
OFDM is the short form for “Orthogonal Frequency Division Multiplexing”.
Algorithm : In this technique, channel is divided into many orthogonal subcarriers. Data symbols are allocated to each of these subcarriers and are transmitted simultaneously.
It increases data rate of the system. Moreover it improves spectral efficiency and robustness against multipath fading and interference.
For example, in 802.11a version of WLAN, 20 MHz bandwidth is divided among 64 subcarriers for parallel data transmission.

OFDM transmitter with spectrum

The figure-2 depicts OFDM waveform. It also shows how 4 bits of data are mapped on each of the subcarriers in 16QAM modulation type. The modulated data waveform is passed through the IFFT block for conversion into time domain before transmission into the air.

Key differences between CSMA and OFDM

Parameter	CSMA	OFDM
Position in OSI Stack	MAC layer 2	PHY Layer 1
Function	Avoids collisions by sensing channel before transmission	Increases spectrum efficiency by dividing channel into multiple subcarriers
Purpose	Determines when a device can access the channel	Determines how data is transmitted over the channel
Technique	Random backoff, collision avoidance	Parallel transmission using orthogonal subcarriers
Used in	All WLAN standard IEEE 802.11 family	High speed WLAN versions

Conclusion: In WLAN communication, CSMA and OFDM serve complementary but distinct purposes. CSMA/CA ensures fair and collision free channel access, while OFDM maximizes data transmission efficiency and resilience against interference. Together, they form the foundation of modern Wi-Fi standards, enabling smooth, high speed wireless connectivity. By combining effective medium access control with advanced modulation techniques, WLANs continue to support the growing demand for faster and more reliable wireless communication.