14.4.3 Medium Access In WLANs

IEEE 802.11 systems operate in unlicensed frequency bands allocated for industrial, scientific, and medical (ISM) or unlicensed national information infrastructure (U-NII) use. The principal bands used by modern Wi-Fi systems are:

2.4 GHz band (approximately 2.4–2.5 GHz). The 2.4 GHz ISM band was used by the earliest 802.11 standards and remains widely supported. It offers relatively good propagation characteristics and wall penetration compared with higher frequencies. However, it provides limited spectrum and only a small number of non-overlapping 20 MHz channels (typically three in many regulatory domains). The band is also shared with other devices such as Bluetooth equipment, microwave ovens, and various industrial transmitters, making it susceptible to interference and congestion.
5 GHz band (approximately 5.1–5.8 GHz). The 5 GHz band provides significantly more spectrum than 2.4 GHz and supports a greater number of non-overlapping channels. It was introduced with 802.11a and later adopted by 802.11ac and 802.11ax. Wider channels (40, 80, and 160 MHz) are more practical in the 5 GHz band due to the greater spectral availability. Propagation range is generally shorter than at 2.4 GHz because higher frequencies experience greater path loss and reduced wall penetration. Certain portions of the 5 GHz band require Dynamic Frequency Selection (DFS) to avoid interference with radar systems, as mandated by regulatory authorities.
6 GHz band (approximately 5.925–7.125 GHz, region dependent). Wi-Fi 6E and Wi-Fi 7 extend operation into the 6 GHz band, significantly increasing the amount of available spectrum. The 6 GHz band supports: a large number of non-overlapping 20 MHz channels, wide 80, 160, and 320 MHz channel configurations, and reduced interference compared with legacy bands. Because legacy devices do not operate in the 6 GHz band, deployments can achieve improved spectral efficiency and reduced congestion. However, propagation range at 6 GHz is typically shorter than at 2.4 GHz due to increased free-space path loss.
Channelization and Bandwidth

802.11 channels are defined with nominal bandwidths of: 20 MHz (baseline), 40 MHz, 80 MHz, 160 MHz, and 320 MHz (Wi-Fi 7). Wider channels provide higher peak data rates but consume more spectrum and reduce the number of non-overlapping channels available for spatial reuse. In dense deployments, particularly in the 2.4 GHz band, narrower channels may provide better aggregate network performance.

Shared Medium and Interference

Because the wireless channel is shared, all stations within range compete for access and interference from neighboring WLANs may degrade performance. Co-channel interference and adjacent-channel interference must therefore be considered in network planning These characteristics make medium access control mechanisms such as CSMA/CA essential for fair and efficient use of the wireless medium.

The shared and interference-prone nature of the wireless medium makes coordinated access control essential. We therefore next examine the medium access mechanisms defined in IEEE 802.11.

Having considered the physical characteristics of the shared wireless medium, we now examine how IEEE 802.11 coordinates access to that medium through its Distributed Coordination Function.

14.4.3.1 Distributed Coordination Function (DCF)

The primary access mechanism defined in IEEE 802.11 is the Distributed Coordination Function (DCF). DCF employs a carrier-sense methodology similar in principle to early Ethernet; stations first listen to determine whether the wireless medium is idle before attempting transmission. Unlike Ethernet CSMA/CD, however, collision detection is not feasible in wireless systems. Instead, IEEE 802.11 uses collision avoidance (CSMA/CA).

Under DCF:

A station senses the channel to determine whether it is idle.
If idle, the station waits for a defined interframe space.
The station selects a random backoff interval within a contention window.
The backoff timer counts down while the medium remains idle.
When the timer reaches zero, transmission begins.

Rather than transmitting and detecting collisions after they occur, stations avoid collisions by waiting a random time after the medium becomes free. The first station whose backoff timer expires gains access to the channel.

After transmitting a frame, the sender expects a positive acknowledgement (ACK) from the receiver. If no ACK is received, the transmission is assumed to have failed. The station then increases the size of its contention window, implementing a binary exponential backoff algorithm.

Following the basic DCF mechanism, later enhancements were introduced to improve performance, particularly for multimedia traffic and high-throughput operation. IEEE 802.11e defined Enhanced Distributed Channel Access (EDCA), which extends DCF by assigning different traffic categories (e.g., voice, video, best effort, background) distinct contention parameters. Each category is given its own contention window limits and interframe spacing, allowing delay-sensitive traffic such as voice and video to obtain faster access to the medium than lower-priority traffic.

To reduce protocol overhead and improve efficiency, modern WLAN standards also employ frame aggregation and block acknowledgements. Because each transmission under DCF requires contention, interframe spacing, and acknowledgement, sending many small frames individually results in significant overhead. Aggregation allows multiple data frames to be transmitted within a single channel access opportunity.

Two principal aggregation methods are defined:

Aggregated MAC Service Data Unit (A-MSDU): multiple higher-layer payloads are combined into a single MAC Protocol Data Unit before transmission. The entire aggregated frame is protected by a single MAC header and frame check sequence.
Aggregated MAC Protocol Data Unit (A-MPDU): multiple complete MAC frames are transmitted together within one transmission burst. Each subframe retains its own MAC header and error detection field.

A-MSDU provides lower overhead but requires the entire aggregate to be retransmitted if any error occurs. A-MPDU, by contrast, allows selective retransmission of only those subframes that were not correctly received.

To complement aggregation, 802.11 employs block acknowledgements, whereby the receiver acknowledges multiple frames collectively rather than individually. This further reduces control overhead and increases effective throughput.

Together, EDCA prioritization, frame aggregation, and block acknowledgement significantly improve the spectral efficiency of modern WLANs compared with early DCF implementations.