How did I Decipher 802.11 Frames! #CWAP-2

Main Objective: To successfully transfer every bit of information(data) from one device to another.

802.11 MAC HEADER

Let us now go through the basics of the frame header and the components. I have captured a simple beacon (management) frame using Wireshark.

I will briefly explain each of the fields. Notice the number in the bracket refers to the bytes. For memory 1 Byte = 8 bits. 🙂

802.11 Beacon frame capture
Frame Control Field dissection

Frame Control > 16 bits | 2 Bytes – contains 11 subfields as displayed in the above examples. Considering the amount of valuable information contained in 802.11 Frame Control sub-fields is mind-boggling

Protocol Version (2 bits): For now, always set to 0 by default. Changes in the version are expected in the future.

Type: Management (0,0), Data(1,0), Control(0,1), Extension Frame(1,1)*only available with 802.11D

Sub Type (4 bits): There are different kinds of management, control and data frames. Therefore the 4-bit Subtype field is required to differentiate. The above examples have Beacon & ACK subtypes.

To DSif set to “1” – Frame going from STA > Distribution System (DS)
From DSif set to “1” – Frame going from DS > STA


To DS = 0, From DS = 0  > Management or Control frames where it does not go to DS, Can be STA to STA communication in an ADHOC/IBSS setup.
To DS =0, From DS = 1 > Downstream traffic from AP to the STA.
To DS =1, From DS = 0 > Upstream traffic from STA to AP
To DS =1, From DS = 1 > Data frame using 4 MAC header format, usually occurs in WDS or Mesh Network
.

More Fragments – If set to “1” it is usually preceded by another fragment of current MSDU or MMPDU to follow.

Retry – 0 or 1. 1 is for retransmissions. Lot of 1’s may indicate a network with a lot of retry rate due to some issue. The issues can impact the performance by increased application/network latency thereby degrading user experience.

Power Management – if set to “1”, STA is using power save mode.

More Data: if set to “1” it indicates that the AP or STA is holding more frames for the STA to which the current frame is targeted.

Protected Frame – if set to “1” it indicates payload is encrypted.

Order – If set to “1” in any non-QoS data frame when a higher layer has requested that the data be sent using strictly ordered CoS, which tells the receiving STA to process the frames in order.

Duration/ID > 2 Bytes | 16 bits – May be used for 2 purposes, it may contain the duration of the frame. Secondly, it may contain association identifier (AID) of the STA that transmitted the frame.

Address 1,2,3 and 4: Each address contains 6bytes/48 bits of data.

SA > Source Address
DA > Destination Address
TA > Transmitting Address
RA > Receiving Address
BSSID >

Sequence Control Field (2 Bytes/16 bits): Divided into 4-bit fragment number and a 12-bit sequence number. Used when MSDUs are fragmented. 802.11-2016 allows for fragmentation of frames.

QoS Control Field: (2 Bytes/16 bits): Only used in MAC header of QoS frames. Sometimes referred to as WMM (Wi-Fi Multimedia) which provides traffic prioritization.

HT Control Field (4 bytes/32 bits): Parameters related to HT & VHT operations. Only used in Management + QoS control frames.

Frame Body: Contains the actual MSDU payload to be transmitted.

FCS: (Frame check sequence field 4Bytes/32 Bits) – Final field on the frame header. Also known as Trailer as the word says. Used to detect errors in communication.