Different 802.11 standards are used in Wi-Fi and they differ in terms of operating frequency and data rates. This post provides information about the different terms used in Wi-Fi, 802.11 standards and the three types of 802.11 MAC frames.
Below figure shows DS, AP, Station, BSS, SSID, BSSID and ESS.
Figure 1. Overview of Distribution system
802.11 standard defines an over the air communication interface between the wireless base station and clients. The 802.11 family has various specifications and it has been categorized in several versions as shown in table below.
Table 1. Wi-Fi Generation Overview
|Generation||Technology||Operating Frequency||Data rates|
|-||802.11b||2.4 GHz||1 - 11 Mbps|
|-||802.11a||5 GHz||Up to 54 Mbps|
|-||802.11g||2.4 GHz||Up to 54 Mbps|
|Wi-Fi 4||802.11n||2.4 and 5 GHz||Up to 600 Mbps|
|Wi-Fi 5||802.11ac||2.4 and 5 GHz||Up to 3.5 Gbps|
|Wi-Fi 6||802.11ax||2.4 and 5 GHz||Up to 9.6 Gbps|
802.11 frames are used for wireless communication and is much more involved because the wireless medium requires several management features and corresponding frame types that are not found in wired networks. There are three major frame types that are discussed below.
For details regarding 802.11 layer architecture, please refer to [802.x.x] IEEE 802.x.x and Wi-Fi basics.
Management frames are used by wireless stations to join and leave the basic service set. 802.11 management frame is also called Management MAC Protocol Data Unit (MMPDU). It has a MAC header, a frame body, and a trailer. It doesn’t carry any upper layer information.
There is no MAC Service Data Unit (MSDU) encapsulated in the MMPDU frame body, it carries only layer 2 information fields and information elements, it does not carry higher layer (Layer 3 to 7 of OSI model) data. A management frame must have fixed length information fields and it may have information elements that are variable in length.
Management/MMPDU frame body content depends on the sub type field, based on the sub type field it has payload like Status/Reason code, device capability information etc. Few of the management frames i.e. Beacon, Authentication, Association are described in the Connection setup process post [802.11] Wi-Fi Connection/Disconnection process.
Below figure shows management frame structure.
Figure 2. Management Frame structure
Type field available in frame control field, that is set to 00 for the management frame. Management frames have 24-bytes long MAC header and header contains three addresses.
Figure 3. Management Frame
Table 2. Management Frame description
[B7 B6 B5 B4]
|Announcement Traffic Indication Message (ATIM)||1001||Station (IBSS)|
|Action no ack||1110||AP/Station|
Control frames are associated with the delivery of data and management frames, it does not have a frame body. Control frames contain PHY, preamble, layer 2 header and trailer. Control frames can be transmitted at different data rates as they perform many different functions.
All control frames use the same Frame Control field that is shown in the figure below.
Figure 4. Control Frame structure
Figure 5. Control Frame
The type field value for the control frame is 01 and subtype fields identify the function of a frame. Table below shows the different types of control frames.
Table 3. Control Frame description
|Subtype description||Subtype value
[B7 B6 B5 B4]
|Reserved||0000 - 0110|
|Block ack request (BlockAckReq)||1000|
|Block ack (BlockAck)||1001|
|CF-End and CF-Ack||1111|
Data frames carry the higher level protocol data in the frame body. Data frames are categorized according to function. Total 15 sub types of data frames are defined in 802.11 standard. Type field value for the data frames is 10. One such distinction is between frame that carries data and frame that does not carry data (perform management function). Figure below shows data frame structure.
Figure 6. Data Frame structure
Figure 7. Data Frame
Each bit of the SubType field available in the frame control field has specific meaning as below.
Data frames that appear only in the contention-free period can never be used in an IBSS. Below is the list of data frames.
Table 4.Data Frame Details
B7 B6 B5 B4
|Consists Data||Contention Free Service|
|Data (simple data frame)||0||Yes||No|
|Data + CF-Ack||1||Yes||Yes|
|Data + CF-Poll||10||Yes||Yes(AP only)|
|Data + CF-Ack + CF-Poll||11||Yes||Yes(AP only)|
|Null||100||No||It can be contention based and free both|
|CF-Ack + CF-Poll||111||No||Yes(AP only)|
|QoS Data + CF-Ack||1001||Yes||Yes|
|QoS Data + CF-Poll||1010||Yes||Yes(AP only)|
|QoS Data + CF-Ack + CF-Poll||1011||Yes||Yes(AP only)|
|Qos Null||1100||No||It can be contention based and free both|
|QoS CF-Poll||1110||No||Yes(AP only)|
|QoS CF-Ack + CF-Poll||1111||No||Yes(AP only)|