Monday, August 24, 2015

802.11 Legacy Power Save

Key Points:

  • AP buffers frames for the stations. 
  • After buffering frames, stations are notified using TIM IE present inside the beacon frames. Stations do not poll for checking buffered frames as that would involve transmitting by stations which is not power efficient. 
  • Stations inform AP that they are going is sleep mode by sending a NULL data frame with "Power Management Bit" inside the frame control flags to 1.
  • During association, station specifies Listen Interval (inside association request) which is the number of beacons which station will not listen while sleeping. It means AP should be capable of buffering frames for that much time. However, AP is free to discard frames beyond listen interval. 
  • No limit on listen interval, observed it 0xe1 with one of wireless adapter. 
  • Station goes to sleep mode again if it find that during the expiry of listen interval AP does not have any buffered frame for it.  
  • Each station gets an association ID (1-2007) after association. Association ID of 0 is reserved for indication multicast and broadcast frames.
  • Virtual bitmap is 2008 bits long value with each bit representing corresponding association. 
  • Since, all clients do not to power save mode simultaneously the Virtual bitmap is send partially (to reduce length of Beacon frame) and is called "Partial Virtual Bitmap". PVM picks window of certain association IDs from Virtual Bitmap and drops leading and trailing zeros of that window.
  • Any frame sent by station in up-link direction WILL inform AP that station is in active state (no NULL frame required). e.g when a ping request is send by the station, AP assumes that station is in active state, hence the ping reply will not be buffered and will be send as it comes. After this frame exchange, the station remains in active state. Station needs to send new NULL frame to AP if it wants to enter in Power Save state.
  • Station waits for certain time(~200 msec) for data frames and if there is no data frame during that period it goes to sleep state and informs AP by sending a NULL frame with "Power Management Bit" set.
  • Collecting Uni-cast Traffic:
    • PS-Poll Method (less efficient): To retrieve each frame from AP, station send a PS-Poll frame (acknowledged by AP). It keeps on doing it until there is no other frame left on AP (which is indicated by setting more data flag to zero in the data frame). 
    • Active State Method (more efficient): Station sends a null frame with "Power Management Bit" set as 0 which indicates that station has got into active state. After this frame, AP sends all the buffered frames to the station.
  • Indication of Legacy Power Save Support: No special fields, lacks of WMM power save is indication of legacy power save.
  • Format of TIM IE

  • Finding association IDs present in "Partial Virtual Bitmap" needs length field, bitmap offset field & "Partial Virtual Bitmapcheck here
  • Collecting Multicast & Broadcast Traffic [DTIM Period & DTIM Interval]
    • Every beacon contains DTIM period and DTIM Count. 
    • DTIM count is important to synchronize stations to collect multicast / broadcast traffic.
    • Beacon frame with DTIM count as zero can be referred to special DTIM beacon. After this DTIM beacon, AP will transmit broadcast and multicast traffic. 
    • DTIM period is decided by AP. A value of 2 indicates every second beacon is DTIM beacon and a value of 3 indicates that beacon after two normal beacons is DTIM beacon. DTIM period remains fixed in all beacons. 
    • DTIM count starts from DTIM period - 1 and decremented every beacon.
    • DTIM interval is decided by AP, if a station with large  listen interval (> DTIM Interval) connects, will it increase the latency in the delivery of multicast frames?
      • Ans: No as delivery of multicast & broadcast is linked to DTIM interval not with listen interval. However, because of the low DTIM interval, even stations with high listen interval needs to wake up regularly for multicast & broadcast which negates the effect of power save. To avoid this some features are implemented:
        • Proxy-ARP: As most frequent Broadcast traffic is ARP requests to clients which can be avoided with the usage of proxy-arp feature where AP responds to ARP requests from the distributed network. Supported by Cambium APs.
        • Another thing which can be done is converting Multicast to Unicast packets something which Aruba calls it "Battery Boost".

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