Wireless 802.11 Frame

All Layer 2 frames contain a header, payload, and FCS section. The 802.11 frame format is similar to the Ethernet frame format, with the exception that it contains more fields. The figure below illustrates the wireless 802.11 frame formats and fields.

Wireless 802.11 Frame

Frame Control – It is 2 bytes long field which determines the type of wireless frames.  it is an important field of wireless 802.11 frame containing subfields including control information. The subfields are protocol version, frame type, address type, power management, and security settings etc.

Duration – The 4-byte long field usually used to indicate the remaining duration needed to receive the next frame transmission.

Address 1 to 4 –  Each address field is 6 bytes long. The fields contain standard IEEE 802 MAC addresses (48 bit each). The meaning of each address depends on the DS bits in the frame control field. The address 1 is usually the MAC address of the receiving wireless device or AP. The address 2 usually contains the MAC address of the transmitting wireless device or AP. The address 3 sometimes contains the MAC address of the destination, such as the router interface to which the AP is attached. Address 4 is usually missing because it is used only in ad hoc mode.

Sequence Control – The 16 bits long field contains the Sequence Number (12 bits) and the Fragment Number (4 bits) subfields. The Sequence Number indicates the sequence number of all frames. The Fragment Number indicates the number of each frame sent of a fragmented frame.