The 802.11 Frame Structure

The IEEE 802.11 standard defines the architecture and specifications of wireless local area networks (WLANs). WLAN or WiFi uses high frequency radio waves instead of cables for connecting devices in a LAN. Users connected by WLANs can move around within the area of network coverage.

The 802.11 MAC sublayer provides an abstraction of the physical layer to the logical link control sublayer and upper layers of the OSI network. It is responsible for encapsulating frames and describing frame formats.

MAC Sublayer Frame Structure of IEEE 802.11

The main fields of a frame in WLANs as laid down by IEEE 802.11 are depicted in the following diagram:

Frame Control Field Components

The Frame Control field is a 2-byte starting field composed of 11 subfields containing control information:

• Protocol Version − A 2-bit field set to 00, allowing future versions of IEEE 802.11 to operate simultaneously.

• Type − A 2-bit subfield specifying whether the frame is a data frame, control frame, or management frame.

• Subtype − A 4-bit subfield indicating frame subtypes like Request to Send (RTS) or Clear to Send (CTS). For regular data frames, the value is 0000.

• To DS − A single bit indicating whether the frame is going to the access point (AP), which coordinates communications in centralized wireless systems.

• From DS − A single bit indicating whether the frame is coming from the AP.

• More Fragments − A single bit that when set to 1 indicates more fragments will follow.

• Retry − A single bit that when set to 1 specifies a retransmission of a previous frame.

• Power Management − A single bit indicating the sender is adopting power-save mode.

• More Data − A single bit showing the sender has additional data frames for the receiver.

• Protected Frame − A single bit indicating this is an encrypted frame.

• Order − A single bit informing the receiver that frames should be delivered to higher layers in ordered sequence.

Other Frame Fields

• Duration − A 2-byte field specifying the time period for which the frame and its acknowledgement occupy the channel.

• Address Fields − Three 6-byte address fields containing addresses of source, immediate destination, and final endpoint respectively.

• Sequence − A 2-byte field storing frame numbers to detect duplicate frames and determine frame order for higher layers. The first 4 bits identify fragments, while the remaining 12 bits contain the sequence number that increments with each transmission.

• Data − A variable-sized field carrying the payload from upper layers. The maximum size is 2312 bytes.

• Frame Check Sequence (FCS) − A 4-byte field containing error detection information for frame integrity verification.

Conclusion

The IEEE 802.11 frame structure provides a robust framework for wireless communication with detailed control fields, addressing mechanisms, and error detection. Understanding this frame format is essential for wireless network troubleshooting and optimization.