802.11 Frames

1. Introduction

In this course, we’ll take you through the essential components of 802.11 wireless frames and the process of connecting to a 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!

2. The 802.11 Frame Format

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).

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.

⚠️ Note: 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.

🔍 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.

✅ 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.

💡 Tip: 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.

3. 802.11 Frame Types

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.

Understanding Frame Types

🔍 Management Frames

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

🔍 Control Frames

Control frames manage network operations and avoid data transmission conflicts. Examples include:

• Request to Send (RTS)/Clear to Send (CTS): Prevents collisions in busy networks.
• Acknowledgment (ACK): Confirms the successful reception of data.

🔍 Data Frames

Data frames carry actual user data between devices.

📢 Next Steps: Now that we know how frames are structured, let’s explore the 802.11 Association Process, which describes how devices join a Wi-Fi network.

4. 802.11 Association Process

To connect to a Wi-Fi network, a client follows these steps:

1. Discovery: The client scans for available access points (APs).
   
2. Authentication: The client verifies its identity with the chosen AP.
   
3. Association: The client establishes a connection to the AP and can now send and receive data.

🔍 Let’s break down each step in detail:

Step 1: Discovery of Available APs

The AP announces its presence by broadcasting beacon frames at regular intervals. These frames are sent to all devices and include:

• SSID: The network name.
• Supported data rates: The speeds the AP can handle.
• Supported standards: Wireless standards (e.g., 802.11n/ac).
• Security settings: Encryption types used by the network (e.g., WPA2).

The client can also send probe requests to actively search for networks:

• Probe Request: Sent by the client to ask for more details or confirm the presence of a specific network.
• Probe Response: Sent by the AP directly to the client (unicast) with the same details as the beacon frame.

💡 Example: Imagine you’re sitting at your computer and opening the Wi-Fi settings. The list of networks you see corresponds to the beacon frames sent by the nearby APs. When you click on a network name (SSID), your device sends a probe request to confirm that the network is still available and ready for connection.

Step 2: Authentication with the AP

Once the client finds a network, it sends an authentication request to the AP:

🔹 Authentication Request: The client requests to authenticate with the AP.

🔹 Authentication Response: The AP replies with an acceptance or rejection.

✅ If accepted, the client is authenticated but not yet associated with the AP.

💬 We will cover the different types of authentication methods (e.g., WPA2, WPA3) in the upcoming lessons.

📢 Example: Think about when you select a Wi-Fi network and type in the password. This is where the authentication happens. If the password is correct, the AP validates your request, and you can proceed to the next step. If the password is wrong, the AP denies access.

Step 3: Association with the AP

After successful authentication, the client sends an association request to join the network:

🔹 Association Request: Contains the SSID and the client’s supported features (e.g., data rates and security preferences).

🔹 Association Response: The AP responds:

• ✅ Accepted: The client is assigned an association ID (AID) and gains network access.
• ⚠️ Rejected: The client cannot join the network.

📢 Example: You’ve been there—you type in the right password, and after a brief moment, you see the “connected” status. This is the association phase, where your device and the Wi-Fi network finalize their communication parameters.

✅ Summary of the 802.11 Association Process

1. Discovery: The client sends a probe request and receives a probe response to confirm network details (like seeing the list of available Wi-Fi networks and selecting one).
   
2. Authentication: The client sends an authentication request and receives an authentication response (like entering the Wi-Fi password).
   
3. Association: The client sends an association request and receives an association response to finalize the connection (like seeing the “connected” status).

By completing these steps, the client becomes part of the Wi-Fi network and can transfer data.

5. Active vs Passive Discovery Modes

When a wireless client searches for networks, it can do so in two ways: passive or active mode. Let’s break down how each mode works:

🔍 Passive Mode

In passive mode, the AP announces its presence by automatically sending beacon frames at regular intervals. These frames are broadcast to all devices and include:

• SSID: The network name.
  
• Supported data rates: The speeds the AP can handle, which helps the client know if it is compatible with the network.
  
• Supported standards: The wireless protocols the AP can operate on (e.g., 802.11n/ac).
  
• Security settings: Encryption types used by the network (e.g., WPA2, WPA3).

✅ The client does not need to send any request—it passively listens to the beacon frames to detect the available networks. Once the network appears in the list, the client can decide whether to connect to it.

💡 Tip: In passive mode, it’s the AP that advertises its presence to clients.

🔍 Active Mode

In active mode, the AP does not broadcast its SSID in beacon frames, meaning it does not openly show its presence. The client must know the network name (SSID) in advance to connect.

Here’s how it works:

• The client initiates the process by broadcasting probe request frames, asking for a specific network by name (SSID).
• If the AP has the same SSID, it sends a probe response directly to the client, containing:
  • SSID: The network name.
  • Supported data rates: The speeds and data transfer rates the AP supports.
  • Security settings: The encryption protocols in use.

⚠️ Warning: In active mode, the client must actively search and know the SSID of the network to connect.

6. Conclusion

In this course, we explored the fundamentals of 802.11 wireless frames and the Wi-Fi connection process essential for CCNA. Understanding these concepts is key to grasping how devices communicate over wireless networks.

✅ Key Takeaways:

• 802.11 Frame Format: Includes essential fields like Frame Control (protocol version, frame type) and multiple address fields that enable wireless communication.
  
• Frame Types:
  • Management Frames: Handle network discovery, authentication, and association.
  • Control Frames: Regulate data flow (e.g., RTS/CTS, ACK).
  • Data Frames: Transport user data across the network.
    
• Association Process:
  • Discovery: Clients identify available APs through beacon frames or probe requests.
  • Authentication: Clients verify their identity before accessing the network.
  • Association: Clients finalize their connection and gain full access to the Wi-Fi network.