MTBA and PSMP in 802.11n

Abhay Annaswamy39977539

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)

• 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….)

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.

• 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.

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.

SolutionLegacy 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-APSD

Legacy power save

U-APSD

PSMP - 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

– 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

EvolutionSingle Data-Ack -> Block Ack -> Compressed Block-Ack for A-MSDU -> MTBA

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.

References1) http://en.wikipedia.org/wiki/IEEE_802.11n-20092) http://www.vocal.com/networking/ieee-802-11n/3) http://en.wikipedia.org/wiki/Block_acknowledgement4) http://ieeexplore.ieee.org.proxy.libraries.smu.edu/stamp/stamp.jsp?tp=&arnum

ber=41097515) http://www.cwnp.com/power-save-multi-poll-psmp/6) http://wiki.mwnl.snu.ac.kr/twiki/pub/Main/WirelessNetworking20112/07-IEEE-8

02.11e-for-QoS-part1-and-part2-v2008.pdf

Questions?

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