WLAN - Frame Structure                                             Home : www.sharetechnote.com

 

 

 

 

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).

 

 

 

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

 

 

PHY/MAC Frame

 

This is the frame being exchanged between the mobile device and Access point. The overall frame is structed as shown below.

The 'Network Data' shown in Green is the part which are eventually eventually conveyed to wired backbone and all the other portion (PHY, DataLink, Packet Trailer) are used for communicating between the mobile client and access point. PHY and 'Data Link' part will be main subject of WLAN frame.

 

 

 

PLCP (Physical Layer Convergence Protocol) Structure

 

Now let's look into the details of PLCP. PLCP is a kind of header deing added at PHY layer. It consists of two main parts, preamble and Header as shown below.

 

 

The first part of PLCP is for 'Sync' (Synchronization). This is a part made of 80 bits of alternation 0 and 1s.

The next portion is SFD (Start Frame Delimiter). This is a kind of tag indicating the start of physical frame and it is a specifically determined 16 bit sequence (0000110010111101).

 

 

MAC Header Structure

 

MAC Header would be a most complicated structure of the frame. The most important information contained in the MAC header would be as follows.

  • What is the type of frame ?
  • What are the source and destination address for the frame.

 

< Frame Control Field Structure >

 

 

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 >

Type

Type Description

Sub Type

Sub Type Description

00

Management

0000

Association Request

00

Management

0001

Association Response

00

Management

0010

Reassociation Request

00

Management

0011

Reassociation Response

00

Management

0100

Probe Request

00

Management

0101

Probe Response

00

Management

0100-0111

Reserved

00

Management

1000

Beacon

00

Management

1001

ATIM

00

Management

1010

Dissociation

00

Management

1011

Authentication

00

Management

1100

Deauthentication

00

Management

1101-1111

Reserved

01

Control

0000-1001

Reserved

01

Control

1010

PS-Poll

01

Control

1011

RTS

01

Control

1100

CTS

01

Control

1101

ACK

01

Control

1110

CF End

01

Control

1111

CF End + CF ACK

01

Control

1010

PS-Poll

10

Data

0000

Data

10

Data

0001

Data + CF ACK

10

Data

0010

Data + CF Poll

10

Data

0011

Data + CF ACK + CF Poll

10

Data

0100

Null Function(No Data)

10

Data

0101

CF ACK(no Data)

10

Data

0110

CF Poll(no Data)

10

Data

0111

CF ACK + CF Poll(no Data)

10

Data

1000-1111

Reserved

11

Reserved

0000-1111

Reserved

 

 

< 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.

 

 

Reference