1. MTBA and PSMP in n
Abhay Annaswamy

2. Introduction to 802.11n 802.11n is the evolution of 802.11 a, b and g
IEEE a, b and g provided speeds up to 54 Mbps where as IEEE n 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.

3. 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 technologies like a, b and g.
Backward compatible with earlier technologies.

4. Drawbacks/Technical challenges of earlier 802.11 versions
User data throughput could not be increased beyond a point due to 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.

5. Powersave in
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

6. 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

7. Regular Powersave & U-APSD
Legacy power save
U-APSD

8. PSMP - Power Save Multi Poll
Was developed as an enhancement of an existing e 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

9. The PSMP sequence starts with the transmission of a non-aggregated PSMP, and terminates when the last scheduled UL (uplink) transmission ends. The PSMP frame comprises of PSMP control header and one or more STA info fields, as shown in Figure 4. The STA info field carries timing details of scheduled UL and DL periods. The STA info includes two additional sub-fields for UL and DL offsets, which help in determining the exact time instant when the UL and DL durations would start. The More PSMP field, in the PSMP control header, when set to 1 indicates whether this PSMP sequence will be followed immediately by another PSMP sequence. When set to 0 it indicates that the current PSMP sequence is the last in the current service period. Figure 5 shows a typical PSMP sequence. It is possible to aggregate multiple streams such as MP3 audio, chat, VoIP, etc. for a single STA in a PSMP sequence. Some benefits of PSMP over other power saving mechanisms in the IEEE standard are:
- Erroneous packets can be retransmitted in the same service period.
- For a single STA, multiple data packets belonging to different applications can be aggregated together and transmitted in one DL period.
- Burst traffic such as real-time video can be supported using multiple PSMP frames in the current service period.

10. 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

11. PSMP in conjunction with 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

12. Summary
Both PSMP and MTBA were introduced in e and have been optimized in n.
Both methods have been very effective in reducing power consumption and also increasing the data rate compared to earlier version.

13. References http://en.wikipedia.org/wiki/IEEE_802.11n-2009

14. Questions?

15. Thank You!