802.11 Frame Structure Wireless LAN

802.11 Frame Structure Wireless LAN Operations 

This section is intended to describe the 802.11 frame structure. Describe the method of media access used by wireless technology. And finally, describe the management of channels on a WLAN.

802.11 Frame Structures

All Layer 2 frames consist of an FCS header, content and section, as shown in Image 1. The 802.11 frame format is similar to the Ethernet frame format, with the exception that it contains more fields .

802.11 wireless frame

All 802.11 wireless frames contain the following fields:

• Frame control : Identifies the type of wireless frame and contains subfields for the protocol version, frame type, address type, power management and security settings.
• Duration : in general, it is used to indicate the remaining duration necessary to receive the next frame transmission.
• Address 1 : Normally, it contains the MAC address of the wireless AP device or receiver.
• Address 2 : Normally, it contains the MAC address of the wireless transmitter device or AP.
• Address 3 : sometimes, it contains the MAC address of the destination, such as the router interface (default gateway) to which the AP connects.
• Sequence control : contains the subfields Sequence number and Fragment number. The Sequence Number indicates the sequence number of each frame. The Fragment Number indicates the number of each frame that was sent from a fragmented frame.
• Address 4 : usually missing, since it is used only in ad hoc mode.
• Content : contains the data for transmission.
• FCS : is the Frame Verification Sequence, used to control Layer 2 errors.

Frame control field

The Frame Control field contains several subfields, as shown in following figure.

Specifically, the Frame Control field contains the following subfields:

• Protocol version : provides the current version of the 802.11 protocol used. Receiving devices use this value to determine if the protocol version of the received frame is supported.
• Frame type and Frame subtype : determine the function of the frame. A wireless frame can be a control frame, a data frame or an administration frame.
• A DS and DS : indicate whether the frame enters or leaves the DS, and is only used in the data frames of the wireless clients associated with an AP.
• More fragments : indicates if there are more fragments of the frame to receive, whether of the type of data or administration.
• Retry : indicates whether the frame is transmitted again or not, whether data frames or administration frames.
• Power management : indicates whether the emitting device is in active mode or in energy saving mode.
• More data : indicates to a device in power saving mode that the AP has more frames to send. It is also used for APs to indicate that there are additional broadcast and multicast frames.
• Security : indicates whether encryption and authentication are used in the frame. It can be set for all data and administration frames that have the subtype set to authentication.
• Reserved : You can indicate that all received data frames must be processed in order.

Wireless frame type

The Frame Type and Frame Subtype fields are used to identify the type of wireless transmission. As shown in the illustration, a wireless frame can be one of three types of frame:

• Administration frame : it is used for the maintenance of communication, such as the detection of an AP, its authentication and the association with said AP.
• Control frame: used to facilitate the exchange of data frames between wireless clients.
• Data frame : used to transport content information, such as web pages and files.

Administration Frames

Administration frames are used exclusively to search for an AP, authenticate and associate with it.

In above figure, the field value of common administration frames is shown, including the following:

• Association request frame: (0x00) is sent from a wireless client, allows the AP to allocate resources and synchronize. The frame carries information about the wireless connection, including the supported data rates and the SSID of the network to which the wireless client is to be associated. If the request is accepted, the AP reserves memory and establishes an association ID for the device.
• Association response frame: (0x01) is sent from an AP to a wireless client, it contains the acceptance or rejection of the association request. If it is an acceptance, the frame contains information such as an association ID and the supported data rates.
• Re-association request frame: (0x02) a device sends a re-association request when it goes out of the scope of the AP to which it is currently associated and finds another AP with a stronger signal. The new AP coordinates the forwarding of all the information that the previous AP buffer may still contain.
• Re-association response frame: (0x03) is sent from an AP, it contains the acceptance or rejection of a re-association request frame of a device. The frame includes the information required for the association, such as the association ID and the supported data rates.
• Polling request frame: (0x04) is sent from a wireless client when it requires information from another wireless client.
• Polling response frame : (0x05) is sent from an AP after receiving a polling request frame and contains capacity information, such as supported data rates.
• Signal frame: (0x08) is sent periodically from an AP to announce its presence and provide the SSID and other parameters configured previously.
• Dissociation frame: (0x0A) is sent from a device that wishes to terminate a connection. Allows the AP to stop the memory allocation and remove the device from the association table.
• Authentication frame : (0x0B) the sending device sends an authentication frame containing its identity to the AP.
• Deauthentication frame: (0x0C) is sent from a wireless client that wishes to terminate the connection of another wireless client.

Note : Signals are the only administration frames that an AP can transmit on a regular basis. All other polling, authentication and association frames are used only during the association (or re-association) process.

Control frames

Control frames are used to manage the exchange of information between a wireless client and an AP. They help avoid collisions in a wireless medium.
The field value of common control frames, including the following:

• Send Request Frame (RTS) : RTS and CTS frames provide an optional collision reduction scheme for APs with hidden wireless clients. A wireless client sends an RTS frame as the first step in the two-way link, which is required before sending data frames.
• Ready to Send Frame (CTS) : A wireless AP responds to an RTS frame with a CTS frame. Provides authorization for the wireless client that made the request to send data frames. The CTS frame contributes to the management of collision control by including a time value. This delay minimizes the likelihood of other customers transmitting while the client who made the request does.
• Acknowledgment (ACK) frame: After receiving a data frame, the receiving wireless client sends an ACK frame to the sending client if no errors are found. If the sending client does not receive an ACK frame within a predetermined period, it resends the frame.
• Control frames are essential for wireless transmission and play an important role in the method of contention of the means used by wireless technologies, known as " multiple access by carrier detection and collision prevention " (CSMA / CA).

CSMA / CA Wireless network