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802.11 Standards
Introduction to 802.11
Figure 4-4 illustrates the role of 802.11 standards within the IEEE 802 standard family, involving the physical layer and data link layer.
Physical Layer
The different 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 detailed in Table 4-1.
Table 4-1 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 guard interval (GI) length Compatible with 802.11a and 802.11n Occasionally used Data Link Layer
On a wired LAN, 802.3 standards use carrier sense multiple access with collision detection (CSMA/CD) to control the wired media access of different devices. CSMA/CD requires all terminals to detect each other's packets. However, CSMA/CD does not work for WLANs. WLANs provide only limited wireless signal coverage, so some terminals may fail to detect each other's packets.
To overcome the problems encountered with CSMA/CD, 802.11 standards use carrier sense multiple access with collision avoidance (CSMA/CA).
802.11 MAC Frame Format
An 802.11 MAC frame
consists of a MAC header, frame body, and frame check sequence (FCS).
The settings of attribute fields in the MAC header determine the frame
type. Figure 4-5 shows the
802.11 MAC frame format.
An 802.11 MAC frame has a maximum length of 2348 bytes. The
following describes the purpose of each field in an 802.11 MAC frame.
- 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, such as data, control, and management frames.
- Data frame: transmits data packets: includes a special type of
frame, the Null frame. A Null frame has a zero-length frame body.
A STA can send a Null frame to notify an AP of 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 being 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 to confirm the receipt.
- Request to Send (RTS) and Clear to Send (CTS) frames: These 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. Functions include 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, such as the SSID, 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 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 of the parameters specified in a Beacon frame.
- Data frame: transmits data packets: includes a special type of
frame, the Null frame. A Null frame has a zero-length frame body.
A STA can send a Null frame to notify an AP of 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 both 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 to retransmit a frame. This field helps eliminate duplicate frames.
- Pwr Mgmt: indicates the desired power management mode of a STA after the completion of a frame exchange, such as Active or Sleep mode.
- 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 for 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: transmits information about MAC addresses. An 802.11
frame can have up to four address fields. The four address fields
vary according to the values of 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 4-2 describes the scenarios
and rules for filling in the four address fields.
Table 4-2 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 4-6. 1 0 BSSID Source address Destination address Unused STA2 sends the frame to AP1 as shown in (2) in Figure 4-6. 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 4-6. - 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, it discards the frame if the 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.
