[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.

Architecture

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.
     *凡兩個幾何量標度相同的物體，且彼此互相垂直。即稱具有正交性

Channels

• 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.
不同的國家，所支援的Channel也不同

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

Scan

• 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
  • DIFS: DFS IFS
  • 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.

References

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