Frame Structure 

As you may noticed from other technology that I posted, the way I study about a communication technology is always same. Study and understand the details of frame structure and then understand how these frames are exchanged at each step of communication process (protocol).

• PHY/MAC Frame
• PLCP (Physical Layer Convergence Protocol) Structure
• MAC Header Structure
• Frame Control Field Structure
• Duration ID Field Structure
• Sequence Control Field Structure
• Example 1 > MAC Header / Beacon Frame
• RTS Frame
• CTS Frame
• ACK Frame
• Example 1 > MAC Header / ACK Frame
• Beacon Frame
• Example 1 > MAC Header / Beacon Frame
• Trigger Frame
• Example : MU-RTS

Overview of WLAN Frame

Followings are some of the bullelts for WLAN Frame. (The list would get longer as I learn more)

• WLAN doesn't use 802.3 Ethernet frames
• There three different types of WLAN frame named Control, Management and Data frame.
• Max Frame size is 2346 bytes and they are typically fragmented at 1516 bytes.
• Preamble is always sent at 1 Mbps

You see four different locations allocated for Address. What kind of address is assigned to which address field is determined by 'To DS' and 'From DS' field. The mapping between DS field and Address field are specified as follows.

Regardless of the contents in the frame, the structure of MAC header is same. Then how do we (the WLAN device) knows what kind of the information (data) is contained in the frame. 'Type' and 'Sub Type' field determines the characteristics of the frame.

Type field (2 bits) determines the major characteristics of the contents carried by the frame and 'Sub type' defines the details of the information.

The 'Type'/'Sub Type' and characteristics of the contents are mapped as shown in the following table. This table is mostly for 802.11 a,b,g and there is some changes (additions) in recent specification (e.g, 802.11ac, 802.11ad). Regarding the changes in recent specification, I would not list in this table and I will list those changes in separate pages dealing with 802.11ac or 802.11ad.

Frame Type Table

Duration ID Field Structure

The value in the duration field has different meaning (interpretation) depending on the one or two bits at Most Significant Bits (MSB) as shown below.

Sequence Control Field Structure

When a packet comes into the MAC layer from higher layer, a sequence number is assigned at 'Sequence Number' field. If the incoming packet is too big for a single MAC frame, it be splitted into multiple fragment. In this case, a fragment number is assigned at 'Fragment No' field. When a packet gets into multiple MAC frame, those fragmented frame gets the same value at 'Sequence Number' field and different values at 'Fragment No' field.

802.11 can transmit the max 2304 bytes of higher layer packet. Considering WEP overhead and 8 bytes LLC header, the maximum MAC frame size should be 2296 bytes.

Example 1 > MAC Header / Beacon Frame

RTS Frame

• Duration : Time in microseconds. This is the time required for "Data/Management Frame + CTS + ACK + 3 SIFS"
• RA : Reciever Address
• TA : Transmitter Address

CTS Frame

• Duration : Time in microseconds.
• RA : Reciever Address
• TA : Transmitter Address

ACK Frame

Example 1 > MAC Header / ACK Frame

• Duration : Time in microseconds.
• RA : Reciever Address
• TA : Transmitter Address

Beacon Frame

The contents of the Beacon Frame (Beacon Body) is a huge structure, so I created a separate page for Beacon and it's contents.

Trigger Frame

Example : MU-RTS

    NOTE :  Log file is from Gjermund Raaen who kindly allowed me to share the log clips.

Reference

• Packets never lie: An in-depth overview of 802.11 frames
• 802.11ac Analysis Webinar