802.11 Frames

In this course, we’ll explore the essential components of 802.11 frames and the process of connecting to a wireless network.

Here’s the plan: we’ll break down each part step by step, using clear examples and diagrams to help you visualize everything. Don’t worry we’ll keep it simple!

Figure 1 – Laptop initiating connection to an Access Point (AP)

802.11 frames are used to transmit information wirelessly between devices. These frames are similar to Ethernet frames but contain more fields to support additional requirements such as retransmissions, power-saving features, and security.

Layer 2 Frame Structure

Below is a diagram showing the structure of a Layer 2 Frame, simplified to include only the header, payload, and Frame Check Sequence (FCS).

Figure 2 – Simplified 802.11 Layer 2 Frame Structure

All 802.11 wireless frames include these core fields. Don’t worry if it feels a bit technical—we’ll break down the purpose of each field so you can understand how they contribute to reliable wireless communication.

Key Fields in an 802.11 Frame

Frame Control

This field is like the "command center" of the frame. It contains key information such as:

• Protocol Version: Indicates the version of 802.11 being used.

• Frame Type and Subtype: Identifies whether the frame is for management, control, or data.

• Power Management: Indicates if the sending device is in active mode or power-save mode.

• Security: Specifies whether encryption and authentication are used.

We will dive deeper into these subfields later!

Duration

This field shows how long the channel will be reserved for the transmission.

Address Fields

Depending on the type of frame, an 802.11 frame can have up to four address fields:

• Address 1 (Receiver Address - RA): The device that directly receives the frame.

• Address 2 (Transmitter Address - TA): The device sending the frame.

• Address 3 (Destination Address - DA): The final destination of the data.

• Address 4 (Source Address - SA): The original sender of the data, used in ad-hoc or relayed communications.

Figure 3 – Detailed Breakdown of 802.11 Frame Header Fields

QoS Control

This field prioritizes certain types of traffic. For example, voice calls might be prioritized over regular data to avoid delays.

HT Control

This was introduced in 802.11n (Wi-Fi 4) to support High Throughput (HT) operations. In Wi-Fi 5 (802.11ac), it evolved to Very High Throughput (VHT).

Frame Control Field

Here we will go a little deeper into the Frame Control Field because it contains key information that affects how wireless communication works.

Figure 4 – Frame Control Field Structure in 802.11 Frames

Key Components of the Frame Control Field:

🔹 Protocol Version: Indicates the version of the 802.11 standard being used to ensure compatibility.

🔹 Frame Type: Specifies the purpose of the frame—whether it’s for management, control, or data. This helps devices understand how to process the frame.

🔹 Frame Subtype: Provides more detail about the frame's role. For example, a management frame subtype could specify whether it’s a beacon, authentication request, or disassociation.

You don’t need to memorize every subfield, but understanding protocol version, frame type, and frame subtype will help you analyze how wireless devices communicate.

802.11 wireless networks use three main types of frames to manage communication:

• Management Frames: Handle network discovery, authentication, and association between the client and the AP.

• Control Frames: Manage the flow of data and help avoid collisions.

• Data Frames: Carry the payload (user data) between the client and the AP.

Figure 5 – Frame Type Field in 802.11 Frame Control Structure

Understanding Frame Types

Management Frames

Management frames handle network discovery, authentication, and association. These frames help clients find and join Wi-Fi networks.