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S5720HI V200R010C00 Configuration Guide - WLAN-AC

This document describes native AC (hereinafter referred to as WLAN AC) configuration procedures and provides configuration examples.

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802.11 Standards

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.

Figure 4-4  Role of 802.11 standards within the IEEE 802 standard family
  • 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 standards
    802.11 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 and 2 Incompatible Released in 1997, supported by most products
    802.11b DSSS 2.4 1, 2, 5.5, and 11 Incompatible Released in 1999, supported by most products
    802.11a OFDM 5 6, 9, 12, 18, 24, 36, 48, and 54 Incompatible Released in 1999, rarely used
    802.11g DSSS/OFDM 2.4 6, 9, 12, 18, 24, 36, 48, and 54 Compatible with 802.11b Released in 2003, 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 Released in 2009, widely used
    802.11ac OFDM/MIMO 5 A maximum of 1300 Mbit/s in theory, depending on the MCS, spatial flow quantity, channel bandwidth, and guard interval (GI) length Compatible with 802.11a and 802.11n Released in 2013, widely used
    802.11ac wave2 OFDM/MU-MIMO 5 A maximum of 6.9 Gbit/s in theory, depending on the MCS, spatial flow quantity, channel bandwidth, and guard interval (GI) length Compatible with 802.11a, 802.11n and 802.11ac Released in 2016, 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).


    For details about CSMA/CA, see WMM.

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.
Figure 4-5  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.
    • 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.
    Figure 4-6  WLAN networking
  • 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.
Updated: 2019-12-28

Document ID: EDOC1000141952

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