[Wi-Fi] IEEE802.11是什麼?

IEEE 802.11 Overview

IEEE 802.11是針對Wireless Local Area Network (WLAN) MAC and PHY而制定的標準

802.11的演進是 原始802.11-> b -> a -> g -> n (a比b早提出但較晚才上市)

  • 802.11: First Standard for 2.4GHz, RF with 1, and 2 Mbps.(FHSS/DS)
  • 802.11a: For 5GHz at up to 54Mbps. (OFDM)
  • 802.11b: For 2.4GHz at up to 11Mbps. (HR/DSSS)
  • 802.11g: For 2.4GHz at up to 54Mbps. 它藉由展頻及調變方式的改變,使得傳輸速度提昇至54Mbps
  • 802.11n: MIMO
  • 802.11d: International Roaming.
  • 802.11e: Quality of Service (QoS)
  • 802.11f: Inter-Access Point Protocol (IAPP)
  • 802.11i: Security.


IEEE 802.11 PHY

Spread Spectrum

  1. FHSS, Frequency Hopping Spread Spectrum
    • Only used in 802.11, Tx Rate only 1, 2 Mbps.
  2. DSSS, Direct Sequence Spread Spectrum
    • Original signal is spread out by a PN (pseudo number) code to a wider bandwidth signal.
    • Used in 802.11, 802.11b
    • Security is higher than FHSS
  3. OFDM, Orthogonal Frequency Division Multiplexing
    • Used in 802.11a, 802.11g
    • To solve single carrier multi-path problem, fading and phase shift
    • Multi carrier is used, where data are divided into multiple *orthogonal sub-carriers. With higher data rate.


  • Channel ID is identified by the center frequency
  • Band: 2.4GHz 5GHz 6GHz

For 2.4GHz
14 channels are designated in the 2.4 GHz range, spaced 5 MHz apart from each other except for a 12 MHz space before channel 14. Divided into channels of width 22MHz
Overlapped Channel: The difference of Channel IDs < 5.
Non-Overlapped Channel: The difference of Channel IDs >= 5.

For 5GHz
Spaced 5 MHz apart from each other and divided into channels of width 20MHz.

IEEE 802.11 MAC

Packet Intro

  • 3 Types frame
    • Control Frame: Ack, RTS, CTS, PS-Poll, etc.
    • Mangement Frame: Auth, Assoc, Probe, Beacon, etc
    • Data Frame: PS-Null Frame


  • Active Scan
    • Probe request (STA, broadcast)
    • Probe response (AP)
  • Passive Scan
    • Beacon (AP, 100ms, broadcast)

STA Connect with AP

  • Need Authentication and Association with AP before STA can access WLAN.

CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance)

  • Listen before transmit
  • On the DCF, Distributed Coordination Function mode use DIFS + random backoffs to avoid collision.
  • On the PCF, Point Coordination Function mode is introduced  for AP centralized control, with contention free.
  • ACK is required for reliability. If ACK is not received at sender side, the sender must retransmit the frame.
    • IFS: Inter-frame space
    • SIFS: Short IFS

Power Management

  • STA will inform AP that it is Awake or Sleep by setting the PS bit in packet header. (Data frame with null data, Null-frame)
  • AP buffers the packets intended for a power saving STA, and deliver to the STA when it becomes awake. (PS-Poll or Null Frame)
  • Another improvement method to reduce STA to transmit data. When STA leaves power-save mode, AP can send all the queued frame to STA. After STA receive all the queued frame, it can enter power-save mode again.
  • AP broadcasts Beacon frame contains the buffered information, which is called TIM, Traffic Indication Map.
  • DTIM used in Infrastructure mode.
  • ATIM used in IBSS.


  1. List_of_WLAN_channels
  2. WiFi Insider – Power Save
  3. https://ithelp.ithome.com.tw/articles/10077706

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