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
- FHSS, Frequency Hopping Spread Spectrum
- Only used in 802.11, Tx Rate only 1, 2 Mbps.
- 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
- 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.
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