MTBA and PSMP in 802.11n

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MTBA and PSMP in 802.11n. Abhay Annaswamy 39977539. Introduction to 802.11n. 802.11n is the evolution of 802.11 a, b and g IEEE 802.11 a, b and g provided speeds up to 54 Mbps where as IEEE 802.11n theoretically could provide speeds up to 600Mbps (nearly 10 times)
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MTBA and PSMP in 802.11nAbhay Annaswamy39977539Introduction to 802.11n
  • 802.11n is the evolution of 802.11 a, b and g
  • IEEE 802.11 a, b and g provided speeds up to 54 Mbps where as IEEE 802.11n theoretically could provide speeds up to 600Mbps (nearly 10 times)
  • This meant that it could be used as a suitable alternative to wired internet as it accounted for bandwidth-hungry applications and also supported larger number of simultaneous users.
  • 802.11n (contd….)
  • The reason for increased data rate was the usage of 4 independent spatial streams of data multiplexed using SDM.
  • MIMO(Multiple Input Multiple Output) antenna configuration is used.
  • 40 MHz wide channels and frame aggregation are couple of other enhancements.
  • Advantages
  • It provided for increased range, increased capacity and higher data rates compared to previous 802.11 technologies like a, b and g.
  • Backward compatible with earlier technologies.
  • Drawbacks/Technical challenges of earlier 802.11 versions
  • User data throughput could not be increased beyond a point due to 802.11 protocol overheads, inter-frame spacing, Physical Layer headers, contention process and acknowledgement frames.
  • Power save methods had to be improved upon to efficiently save and conserve power expended.
  • Solution
  • Frame aggregation – packing multiple Data units/Frames together to reduce overall overheads.
  • Realised by Block Acknowledgement – Instead of transmitting ACK for every frame, multiple frames can be acknowledged together using a single BlockACK frame.
  • Polling and PSMP can also be used to save power consumed.
  • Powersave in 802.11
  • In traditional systems , power save polling was done by
  • Waking up, Station sends a PS Poll to AP
  • For every successful PS Poll, AP sends one packet of data(MPDU)
  • Before that Station has to DTIM beacon to find out if AP has data queued for it
  • The AP sends data with the More Data bit set to 1, if more data is buffered for this STA.
  • Upon receiving the data frame with the More Data bit set to 1, the STA sends another PS-Poll. After downloading all the buffered frames, the STA switches to sleep mode.
  • Resulted in high overhead and also inefficient power save method as stations just doze moderately.
  • Solution
  • Legacy Power Save Poll -> U-APSD/S-APSD -> PSMP
  • U-APSD : Unscheduled – Automatic Power Save Delivery
  • Service Period begins with Trigger and ends with a packet with EOSP bit set
  • Triggers frame is a QoS+Data or QoS+Null frame
  • No pre-set schedules of when to wake up
  • Regular Powersave & U-APSDLegacy power saveU-APSDPSMP - Power Save Multi Poll
  • Was developed as an enhancement of an existing 802.11e standard (APSD).
  • Was developed due to concerns of power consumption of MIMO based products thereby affecting battery life.
  • Working
  • AP will send a schedule during its own TXOP as to when to be awake to receive data and also schedule as to when to transmit.
  • Since this schedule is known Stations can sleep more and also at the same time not miss any frames
  • MTBA – Multi TID Block ACK
  • Evolution
  • Single Data-Ack -> Block Ack -> Compressed Block-Ack for A-MSDU -> MTBA
  • ACK – Each data packet is acknowledged with an ACK packet -> results in more traffic and more time.
  • Enhancement :- Block ACK - Enables multiple frames to be transmitted and then acknowledged with a single ACK frame
  • 2nd enhancement :- Compressed BlockACK
  • To aggregate multiple data units with a single Timing ID(TID) so that overhead can be reduced.
  • It is an enhanced version of BA. In compressed BA, Fragmented MSDUs cannot be transmitted and hence the bitmap size is reduced from 1024 (64*16) bits to 64 (64*1) bits.
  • All the above BA methods can handle only single TIDs and in next slide we will look at MTBA which handles multiple TIDs
  • MTBA
  • Allows for single frame to respond to (implicit) BAR for multiple TIDs.
  • Are used in conjunction with Power Save operation like PSMP sequences instead of BlockACK.
  • PSMP in conjunction with MTBA
  • Frames of different TIDs are transmitted within a PSMP-DTT(Downlink Transmission Time) or PSMP-UTT(Uplink) allocation of a (Scheduled or Unscheduled) PSMP sequence without regard to Access Category.
  • PSMP schedules when a STA receives and when it may transmit.
  • DL Acknowledgement is scheduled in the uplink & vice versa
  • UL data acknowledged by following PSMP sequence
  • Summary
  • Both PSMP and MTBA were introduced in 802.11e and have been optimized in 802.11n.
  • Both methods have been very effective in reducing power consumption and also increasing the data rate compared to earlier version.
  • References
  • Questions?Thank You!
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