802.11 Standards
Introduction to 802.11
Figure 2-2 shows the role of 802.11 standards in the IEEE 802 standard family, involving the physical layer and data link layer.
Physical Layer
802.11 standards use different physical layer technologies, including frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), orthogonal frequency division multiplexing (OFDM), and multiple-input multiple-output (MIMO). These physical layer technologies support different frequency bands and transmission rates, as shown in Table 2-1.
Table 2-1 Comparisons between 802.11 standards802.11 Standard
Physical Layer Technology
Frequency Band (GHz)
Transmission Rate (Mbit/s)
Compatibility with Other 802.11 Standards
Commercial Use
802.11
FHSS/DSSS
2.4
1, 2
Incompatible
Earlier standard, supported by most products
802.11b
DSSS
2.4
1, 2, 5.5, 11
Incompatible
Earlier standard, supported by most products
802.11a
OFDM
5
6, 9, 12, 18, 24, 36, 48, 54
Incompatible
Rarely used
802.11g
DSSS/OFDM
2.4
1, 2, 5.5, 11, 6, 9, 12, 18, 24, 36, 48, 54
Compatible with 802.11b
Widely used
802.11n
OFDM/MIMO
2.4, 5
A maximum of 600 Mbit/s, depending on the modulation and coding scheme (MCS)
Compatible with 802.11a, 802.11b, and 802.11g
Widely used
802.11ac
OFDM/MIMO
5
A maximum of 1300 Mbit/s in theory, depending on the MCS, spacial flow quantity, channel bandwidth, and GI length
Do not support 802.11ac wave2.
Compatible with 802.11a and 802.11n
Not widely used
Data Link Layer
On a wired LAN, 802.3 standards use the carrier sense multiple access with collision detection (CSMA/CD) mechanism to control wired media access of different devices. The CSMA/CD mechanism requires that all terminals should detect packets of each other. However, WLANs provide only limited wireless signal coverage, so some terminals may fail to detect the packets of each other. 802.11 standards use the carrier sense multiple access with collision avoidance (CSMA/CA) mechanism to overcome the deficiency in the CSMA/CD mechanism.
802.11 MAC Frame Format
- Frame Control field: includes the following sub-fields:
- Protocol Version: indicates the MAC version of the frame. Currently, only MAC version 0 is supported.
Type/Subtype: identifies the frame type, including data, control, and management frames.
- Data frame: transmits data packets, including a special type of frame: Null frame. A Null frame has a zero-length frame body. A STA can send a Null frame to notify an AP of the changes in the power-saving state.
802.11 supports the power-saving mode, allowing STAs to shut down antennas to save power when no data is transmitted.
- Control frame: helps transmit data frames, releases and obtains channels, and acknowledges received data. Some common control frames include:
- Acknowledgement (ACK) frame: After receiving a data frame, the receiving STA will send an ACK frame to the sending STA.
- Request to Send (RTS) and Clear to Send (CTS) frames: provide a mechanism to reduce collisions for APs with hidden STAs. A STA sends an RTS frame before sending data frames. The STA that receives the RTS frame responds with a CTS frame. This mechanism is used to release a channel and enable a sending STA to obtain data transmission media.
- Management frame: manages WLANs, including notifying network information, adding or removing STAs, and managing radio. Some common management frames include:
- Beacon frame: is periodically sent by an AP to announce the WLAN presence and provide WLAN parameters (for example, the SSID, supported rate, and authentication type).
- Association Request/Response frame: A STA sends an Association Request frame to an AP to request to join a WLAN. After receiving the Association Request frame, the AP sends an Association Response frame to the STA to accept or reject the association request.
- Disassociation frame: is sent from a STA to terminate the association with an AP.
- Authentication Request/Response frame: is used in link authentication between a STA and an AP for identity authentication.
- Deauthentication frame: is sent from a STA to terminate link authentication with an AP.
- Probe Request/Response frame: A STA or an AP sends a Probe Request frame to detect available WLANs. After another STA or AP receives the Probe Request frame, it needs to reply with a Probe Response frame that carries all parameters specified in a Beacon frame.
- Data frame: transmits data packets, including a special type of frame: Null frame. A Null frame has a zero-length frame body. A STA can send a Null frame to notify an AP of the changes in the power-saving state.
- To DS and From DS: indicates whether a data frame is destined for a distribution system (or an AP). If the two fields are set to 1, the data frame is transmitted between APs.
- More Frag: indicates whether a packet is divided into multiple fragments for transmission.
- Retry: indicates whether a frame needs to be retransmitted. This field helps eliminate duplicate frames.
- Pwr Mgmt: indicates the power management mode of a STA after the completion of a frame exchange, including Active and Sleep modes.
- More Data: indicates that an AP transmits buffered packets to a STA in power-saving mode.
- Protected Frame: indicates whether a frame is encrypted.
- Order: indicates whether a frame is transmitted in order.
- Duration/ID field: provides the following functions according to its values.
- Indicates the duration in which a STA can occupy a channel. This field is used for CSMA/CA.
- Identifies an MAC frame transmitted during Contention-Free Period (CFP). The value of this field is fixed as 32768, indicating that a STA keeps occupying a channel and other STAs cannot use the channel.
- Specifies the Association ID (AID) of a PS-Poll frame, which identifies the BSS to which a STA belongs. A STA may work in active or sleep mode. When a STA works in sleep mode, an AP buffers data frames destined for the STA. When the STA transitions from the sleep mode to the active mode, the STA sends a PS-Poll frame to request the buffered data frames. After receiving the PS-Poll frame, the AP delivers the requested data frames to the STA based on the AID in the PS-Poll frame.
- Address field: indicates MAC addresses. An 802.11 frame can have up to four address fields. The four address fields vary according to the To DS/From DS sub-field in the Frame Control field. For example, the values of the four address fields are different when a frame is sent from a STA to an AP and when a frame is sent from an AP to a STA. Table 2-2 describes the rules for filling in the four address fields.
Table 2-2 Rules for filling in the four address fields
To DS
From DS
Address 1
Address 2
Address 3
Address 4
Description
0
0
Destination address
Source address
BSSID
Unused
The frame is a management or control frame, for example, a Beacon frame sent by an AP.
0
1
Destination address
BSSID
Source address
Unused
AP1 sends the frame to STA1 as shown in (1) in Figure 2-4.
1
0
BSSID
Source address
Destination address
Unused
STA2 sends the frame to AP1 as shown in (2) in Figure 2-4.
1
1
BSSID of the destination AP
BSSID of the source AP
Destination address
Source address
AP1 sends the frame to AP2 as shown in (3) in Figure 2-4.
- Sequence Control field: is used to eliminate duplicate frames and reassemble fragments. It includes two sub-fields:
- Fragment Number: is used to reassemble fragments.
- Sequence Number: is used to eliminate duplicate frames. When a device receives an 802.11 MAC frame, the device discards the frame if its Sequence Number field value is the same as a previous frame.
- QoS Control field: exists only in a data frame to implement 802.11e-compliant WLAN QoS.
- Frame Body field: transmits payload from higher layers. It is also called the data field. In 802.11 standards, the transmitted payload is also called a MAC service data unit (MSDU).
- Frame Check Sequence (FCS) field: checks the integrity of received frames. The FCS field is similar to the cyclic redundancy check (CRC) field in an Ethernet packet.
