[802.11] Wi-Fi Basic concepts

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[802.11] Wi-Fi Basic concepts

[802.11] Wi-Fi Basic concepts

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.

 

Wi-Fi Standard basic terms

  • Station (STA): Stations comprise of all devices that are connected to the wireless LAN. Station is any device that contains 802.11-compliant MAC and PHY interface to the wireless medium. A station may be a laptop, desktop PC, Access Point (AP) or smartphone. A station may be fixed, mobile or portable.
  • Access Point (AP): An access point is a device that creates a wireless local area network. It has station functionality and provides access to the distribution services via the wireless medium.
    An access point is a device that allows Wi-Fi clients and Wi-Fi enabled routers to connect to a wired network. Access point connects to a wired router, switch or hub via an Ethernet cable and projects Wi-Fi signal to the defined area.
    An access point receives data by wired Ethernet, and converts to a 2.4GHz or 5GHz wireless signal. It communicates with nearby wireless clients.
    In a Wi-Fi network, wireless client communicate to other wireless clients via the AP.
  • Client: A device that connects to a Wi-Fi (wireless) network. Any device that transmits and receives Wi-Fi signals, such as a laptop, printer, smartphone is a Wi-Fi client.
  • Basic Service Set (BSS): A group of stations that are successfully synchronized for 802.11 communications. BSS contains one AP and one or more client stations. In BSS, stations have layer 2 connection with AP and are known as associated.
  • Basic Service Set Identifier (BSSID): All basic service sets can be identified by a 48-bit (6-octet) MAC address known as the Basic Service Set Identifier (BSSID). The BSSID address is the layer 2 identifier of each individual basic service set. Most often the BSSID address is the MAC address of the access point.
  • Distribution System (DS): A system that interconnects a set of basic service sets and integrated Local Area Networks (LANs) to create an Extended Service Set (ESS). It is used to extend wireless network coverage.
  • Extended Service Set (ESS): In extended service set, one or more basic service sets are connected. An extended service set is a collection of multiple access points and their associated clients.
  • Independent Basic Service Set (IBSS): An IBSS consists only of client stations that do peer-to-peer communications. An IBSS is a self-contained network that does not have an access point.
  • SSID/ESSID: The logical network name of an Extended Service Set (ESS) is often called a Service Set Identifier (SSID). This name allows stations to connect to the desired network when multiple independent networks operate in the same physical area.
  • Roaming: It is a process of a client moving from one access point to another access point within the same Extended Service Set (ESS) without losing connection. It is described in detail in 802.11 connection disconnection process post: [802.11] Wi-Fi Connection/Disconnection process .

 

Below figure shows DS, AP, Station, BSS, SSID, BSSID and ESS.

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Figure 1. Overview of Distribution system

 

802.11 Standards / Wi-Fi Generations

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.

Details of Wi-Fi generations with 802.11 specifications

 

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

 

  • 802.11b: This technology is focused on achieving higher data rates within the 2.4GHz ISM band and that is achieved by using a different spreading/coding technique called Complementary Code Keying (CCK) and modulation methods using the phase properties of the RF signal.
    802.11b devices support data rates of 1, 2, 5.5 and 11 Mbps.
  • 802.11a: This technology uses 5GHz frequency band. It supports data rate up to 54Mbps with the use of a spread spectrum technology called Orthogonal Frequency Division Multiplexing (OFDM).
    802.11a can coexist in the same physical space with 802.11b and 802.11g devices as these devices are using different frequency ranges (5GHz and 2.4GHz respectively).

  • 802.11g: This Technology is an enhancement of 802.11b Physical layer to achieve the greater bandwidth yet remain compatible with 802.11 MAC. The technology that was originally defined by the 802.11g amendment is called Extended Rate Physical (ERP), So the term ERP can be used in the place of 802.11g.
    Data rate differs with different 802.11g PHY technology, there are two mandatory ERP PHYs and two optional ERP PHYs.
    The First mandatory PHY technology called Extended Rate Physical-OFDM (ERP-OFDM) is used to achieve data rate up to 54Mbps.
    Second mandatory PHY technology called Extended Rate Physical DSSS (ERP-DSSS/CCK) is used to maintain backward compatibility and achieve data rate up to 11Mbps.
    ERP-PBCC and DSSS-OFDM are the two optional PHYs.
    ERP-PBCC PHY offers same data rates as the ERP-DSSS/CCK physical layer. It is used to provide higher performance in the range (the 5.5 and 11 Mbps rates) by using DSSS technology with Packet Binary Convolution Code (PBCC) scheme.
    DSSS-OFDM PHY is a hybrid combination of DSSS and OFDM. The transmission of packet physical header is done by DSSS, whereas the transmission of packet payload is performed by OFDM. Usage of this physical layer is to cover interoperability aspects.

  • 802.11n: This Technology is an improvement of the 802.11 standard to get the higher throughput. 802.11n has a new operation known as High Throughput (HT) which provides MAC and PHY enhancements to provide data rates up to 600Mbps.
    802.11n supports Multiple-Input Multiple-Output (MIMO) technology in unison with OFDM technology. MIMO uses multiple radios and transmitting and receiving antennas called radio chains. It capitalizes on the effects of multipath as opposed to compensating for or eliminating them. Transmit Beamforming can be used in MIMO system to steer beams & provide greater range & throughput.

  • 802.11ac: Wi-Fi certified 802.11ac devices are dual band, operating in both 2.4 GHz and 5 GHz. 802.11ac is built on the foundation of 802.11n. 802.11ac devices use the 5 GHz band, while 802.11n products use the 2.4 GHz frequency band, so 802.11b and 802.11g compatibility can be achieved with 802.11ac.
    802.11ac provides high-performance through Multi-User Multiple Input Multiple Output (multi-user MIMO), wider channels, and support for four spatial streams.

  • 802.11ax: Wi-Fi certified 802.11ax provides improved data rates, power efficiency and support for eight spatial streams. Target Wake Time (TWT) feature helps to improve battery performance.

 

802.11 Frame types

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

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.

 

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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.

  • DA field is the destination address of the frame, it can be broadcast or unicast depending upon frame subtype.
  • SA field is MAC address of the station transmitting the frame.
  • BSSID is MAC address of AP.
  • Frame body is variable size. Size and content of the body depend on the management frame subtype.

 

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Figure 3. Management Frame

 

Table 2. Management Frame description

Frame SubType

SubType Value

[B7 B6 B5 B4]

Initiator (AP/Station)
Association request 0 Station
Association response 1 AP
Reassociation request 10 Station
Reassociation response 11 AP
Probe request 100 Station
Probe response 101 AP/Station
Beacon 1000 AP
Announcement Traffic Indication Message (ATIM) 1001 Station (IBSS)
Disassociation 1010 AP
Authentication 1011 Station
Deauthentication 1100 AP/Station
Action 1101 AP/Station
Action no ack 1110 AP/Station

 

Control Frames

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.

 

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Figure 4. Control Frame structure

 

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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
Control wrapper 0111
Block ack request (BlockAckReq) 1000
Block ack (BlockAck) 1001
PS-Poll 1010
RTS 1011
CTS 1100
ACK 1101
CF-End 1110
CF-End and CF-Ack 1111

 

Data Frames

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.

 

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Figure 6. Data Frame structure

 

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Figure 7. Data Frame

 

Each bit of the SubType field available in the frame control field has specific meaning as below.

  • Bit 4 (B4): Changing it from 0 to 1 indicates the data subtype includes +CF-Ack.
  • Bit 5 (B5): Changing it from 0 to 1 indicates the data sub type include +CF-Poll.
  • Bit 6 (B6): Changing it from 0 to 1 indicates that the frame contains no data, specifically, that it contains no Frame Body field.
  • Bit 7 (B7): Changing it from 0 to 1 indicates Quality of Service (QoS) data frame.

 

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

Frame SubType

SubType Value

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 101 No Yes
CF-Poll 110 No Yes(AP only)
CF-Ack + CF-Poll 111 No Yes(AP only)
QoS Data 1000 Yes No
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)

 

References

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