MTBA and PSMP in 802.11n

MTBA and PSMP in 802.11n
Abhay Annaswamy
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….)
• 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.
• 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.
• 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.
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
– No pre-set schedules of when to wake up
Regular Powersave & U-APSD
power save
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
• Since this schedule is known Stations can sleep more and
also at the same time not miss any frames
MTBA – Multi TID Block ACK
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
– 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
• 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
Thank You!

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