Understanding 802.11 Frame Types

• 傳統802.3 Frame

Client associate AP, 然後收接frame都經AP

或client p2p 都靠AP, 用802.11z Direct Link Setup DLS

• 802.11 Frame

※ Distributed Coordination Function (DCF) ※

大家wireless device合作, 不要collision.

• Carrier Sense
  • Physical carrier sense (Clear Channel Assessment (CCA))
    • Overhear channel + 2nd channel
    • 如自己是收方, 便收frame
  • Virtual carrier sense
    • Duration = whole frame + interframe gap + return ACK
    • Client 根據overhear的duration, 來增加自己NAV, network allocation vector. NAV timer倒數到才可開始發frame
• Collision Avoidance
  • 802.3 因可以測realtime wire, 所以用CSMA/CD (Collision Detection)
  • 802.11 用half-dulplex, 防止client transmitting signal 同時又receiving signal, 所以client transmitting detect不到collision, 所以用CSMA/CA (Collision Avoidance)
  • Collision Avoidance
    • By Backoff timeslot (Random inside contention window)
    • By SIFS/RIFS/DIFS/EIFS

• Interframe space
  • Reduced interframe space (RIFS)
    • 最短timer, 用於802.11n大frame之間
  • Short interframe space (SIFS)
    • Frame同ack之間timer
  • Distributed interframe space (DIFS)
    • Default standard frame timer
  • Extended interframe space (EIFS)
    • 最長timer, collision後用, before retransmitted frame.
• 如Collision Avoidance 做好, 但要retransmition
  • Backoff time double
• 802.11 Frame Types
  • Management frames
    • Beacon – data rate allowed, SSID, per 100ms, per SSID
    • Probe – client問AP拿Beacon
    • Authentication and deauthentication – Open System authentication (無) or WEP (AP及client對key)
    • Association, disassociation, and reassociation(只能SSID) – client 拿 AID, association ID.
    • Action – 802.11k可拿neighbor ap list, 802.11v可幫power savings, 802.11y可提client就快更換channel/channel width
  • Control frames (only frame header, no data payload)
    • ACK
    • Block ACK – acknowledgment of a burst of frame
    • PS-Poll (Power Save Poll) – client要慳電mode, 提ap 先buffer frame
    • RTS/CTS – reserve a channel, frame有duration
  • Data frames
• Client active scan

• Client passive scan

• Client Joins a BSS

• Client leaves a BSS
  • Or AID ages out

• Roaming

• Power save mode (default由ap主導)
  • TIM 是sleeping client 名單 (buffer unicast frame)
  • DTIM 名單 (buffer multicast/broadcast frame)
  • 802.11e由client主導power save mode

• Power save mode (U-APSD)
  • Unscheduled automatic power save delivery
  • QoS