Doc.: IEEE 802.11-10/0788r3 Submission Aggregate Block-ACK definition Date: 2010-07-15 July 2010 Jochen MirollSlide 1 Authors:

doc.: IEEE 802.11-10/0788r3

Submission

Aggregate Block-ACK definition

Date: 2010-07-15

July 2010

Jochen MirollSlide 1

Authors:Name Affiliations Address Phone email Jochen Miroll [email protected]

Zhao Li

Saarland University

Campus C6 3, 66123 Saarbruecken, Germany

+49 681 302 6546 [email protected]

doc.: IEEE 802.11-10/0788r3

Submission

Abstract

• This presentation explains how a Leader-Based Protocol (LBP) that aggregates feedback within the same time slot should be incorporated into TGaa

• Normative text will follow based on discussions

• Revisions of this document– Rev. 1 provides additional examples for frame exchange sequences– Rev. 2 has errors on slide 15: Frame Exchange Sequences (5) fixed– Rev. 3 finalizes this proposal by elaborating in detail a possible way of

incorporating this into 11aa/D1.0

July 2010


doc.: IEEE 802.11-10/0788r3

Submission

Feedback aggregation in the same time slot

• How to get rid of the dependency on n?• All receivers provide feedback, but this feedback from k≤n

STAs is aggregated in a single time slot

overhead(k) = overhead(1)

• Introduction of NACK– AP transmits a data frame– Then, AP asks for ACK/NACK

• If STA i has received the dataframe: it responds with an ACK

• If STA j did not receive the dataframe: it responds with NACKat the same time

July 2010

Jochen Miroll3

AP1

STA 1

STA 2

STA 3

AC

KN

AC

K

STA 4 NA

CK

?

doc.: IEEE 802.11-10/0788r3

Submission

Leader-based feedback cancellation

• Groupcast becomes unicast to leader– non-leaders transmit a negative ACK if a frame is lost after being

asked to do so – introduce the question “did you receive the frame?”• Target: Larger groups (than 11aa MRG-BA should

practically handle)– Overhead for asking about frame reception – same thing for 11aa

MRG block-ACK request– In this presentation: Show how this overhead is reduced by data frame

aggregation similar to 11aa MRG-BA• Leader selection: choose the „weakest“ receiver (as seen by

the AP)– So this STA’s ACKs can be cancelled with very high probability– But: No error correction guarantees in this scheme– “More reliable”, as compared to Multicast

July 2010

Jochen Miroll4

doc.: IEEE 802.11-10/0788r3

Submission

5 10 15 20 250.75

0.8

0.85

0.9

0.95

1

Receivers' distance to AP (m)

Jam

min

g pr

obab

ility

R=2 SimulationR=2 TheoreticalR=3 TheoreticalR=4 TheoreticalR=5 Theoretical

Recap: Results in the LBP-worst-case

• Worst case resultswhere leader-selection would not be

able to reliably determine SNR difference between receivers

• Ns2 – scenario:Rayleigh fading channel, equal AP-

STAs distancefeedback cancellation rate is about 76%

for 2,more than 90% for more than 2, andalready 99% for 5 receivers

• Measured: 3 rcvrs, 1 leader~89% feedback cancellation success

July 2010

Jochen Miroll5

Measurement parameter RateSoftware ACK loss avg. 0.894134Software NACK loss avg. 0.753818Hardware ACK-6 loss avg. 0.893892Hardware ACK-12 loss avg. 0.854081

doc.: IEEE 802.11-10/0788r3

Submission

Recap: Hybrid LBP (HLBP)* cf. doc.: IEEE 802.11-09/0290r1

July 2010


* Assume e.g. DVB-IPDC or Raptor code on upper layer, MAC somehow knows which packets are systematic (DATA) or parity

DIFS RTS

SIFS CTS

SIFS

SIFS DATA k

DATA k

DATA k

SIFS

SIFS

ACK

NACK

SIFS SIFS Parity 1

Parity 1

SIFS ACKERROR

ERROR

ERRORAP

R1(Leader)

R2

R3

Phas

e II

DATA 1AP

R1(Leader)

R2

R3

Phas

e I

DATA 2

DATA 1

DATA 1

DATA 2

DATA 2

DATA 2

ERROR

DATA k

SEQ SEQ

SIFS DATA k-1

DATA k-1

DATA k-1

DATA k-1

SIFS

Phase I Transmit a block of frames, as in MRG BA. Here: systematic FEC partPhase II Parity phase. Instead of BAR/BA, do AggregateAckRequest/AggregateAck

doc.: IEEE 802.11-10/0788r3

Submission

Motivation for (H)LBP with aggregation through cancellation

• Due to aggregation: Scales with the number of receivers– Only the number of retransmissions may increase with increasing

group size, but not the protocol overhead• Due to additional FEC: Degraded channel at one STA does

less harm to overall performance– both in terms of errors and delay

• Hybrid LBP FEC– Enable cross-layer error correction through systematic packet level

FEC controlled by upper layers (e.g. the application or a transport protocol other than TCP/UDP)

– The remaining MAC mechanism is simple and easily implementable• Predictable delay in error correction

– due to aggregation of feedback in the same time slot– application layer is able to control the error correction delay

July 2010

Jochen Miroll7

doc.: IEEE 802.11-10/0788r3

Submission

Motivation for (H)LBP with aggregation through cancellation cont‘d

• About the error floor of feedback cancellation:– If leader is the weakest receiver, it probably looses packets at higher

probability than the rest– If the weakest receiver is significantly weaker: cancellation success increases

and the error floor becomes negligible (now also due to the capture effect!)– With increasing block length, independent errors at all stations within block

become more likely• How to choose the leader?

– Incorporate aggregate Block-ACK in a way that AP can switch from ABA to BA without overhead. MRG BA feedback used to measure signal strength (or any other traffic from resp. STAs)

• Leader selection has further benefits– Rate adaptation: for the leader (since it’s the weakest STA in the group)– If AP detects that the stream rate is higher than what is served to the current

leader, it can just select another STA in the group as the leader for the next block

July 2010

Jochen Miroll8

doc.: IEEE 802.11-10/0788r3

Submission

Aggregate Block-Ack Request

• Define an aggregate-BAR: “ABAR”• MRG BAR information field: (Figure 7-13aa)

• Use reserved bit to indicate the request for aggregate feedback to MRG group members

• Bitmap Offset becomes ABAR leader indicator• Bitmap preceded by ABAR minimum

July 2010


Bits: 8 1 7 VariableMRG BAR Information

Length

reserved MRG BAR Bitmap Offset

MRG BAR Partial Bitmap

Bits: 8 1 7 8 VariableMRG BAR Information

Length

ABARFlag

(set to 1)

MRG BAR Bitmap Offset(ABAR leader)

ABAR minimum


doc.: IEEE 802.11-10/0788r3

Submission

Aggregate Block-Ack Request Minimum

• Define an aggregate-BAR minimum (m):“m is the minimum number of frames a station has to have received within the last k frames, such that the upper layers can decode the current FEC block”

• The AP may, after transmitting k frames, ask the group members to (Negatively-)AggregateBlockACK m < k frames

July 2010


doc.: IEEE 802.11-10/0788r3

Submission

Frame exchange sequences (1)

July 2010


AP1

Leader

STA 2

STA 3

STA 4

D P

AB

AR

ABA

N-A

BA

N-A

BA

D

D

D

P

P

P

P

P

P

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

„Did you get 5 since BAR start seq#?“

Aggregate BA is cancelledBAR start seq#

P

frame exc. end

D P PPackets avaliable in AP buffer for currently serviced MRG group: DD D D D

D …

doc.: IEEE 802.11-10/0788r3

Submission


July 2010


AP1

Leader

STA 2

STA 3

STA 4

D P

AB

AR

N-A

BA

N-A

B

D

D

D

P

P

P

P

P

P

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

„Did you get 5 since BAR start seq#?“(note that 6 have been sent)

N-ABA is receivedBAR start seq#

AB

AR

AB

A

frame exc. end

P


AP can decide whether to add redundancy/parity a-prioriReception of N-ABA and lack of ABA are treated as the same case

doc.: IEEE 802.11-10/0788r3

Submission


July 2010


AP1

Leader

STA 2

STA 3

STA 4

D P

AB

AR

ABA

N-A

BA

N-A

BA

beacon

D

D

D

P

P

P

P

P

P

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D


Aggregate BAis cancelled

BAR start seq#

Would ask „got 5 now?“sent: 6 = 5 data + 1 parity

interrupted by beacon

ABA

R

ABA

frame exc. end

P

„Did you get 6 since BAR start seq#?“sent: 7

Assume everyone received the beacon


Increases our seq# counter by one


doc.: IEEE 802.11-10/0788r3

Submission


July 2010


AP1

Leader

STA 2

STA 3

STA 4

D

AB

AR

N-A

BA

N-A

B

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D


BAR start seq# frame exc. end

AP is limited in parity delivery as provided by the application layerApplication layer can implicitly control MAC error correction delay

N-A

BA

N-A

BA

P

P

P

P

AB

AR

N-A

BA

N-A

BN

-AB

AN

-AB

A

P

P

P

AB

AR

N-A

BA

N-A

BN

-AB

AN

-AB

A

doc.: IEEE 802.11-10/0788r3

Submission


July 2010


AP1

Leader

STA 2

STA 3

STA 4

D

AB

AR

N-A

BA

N-A

B

D

D

D

P

P

P

P

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D


BAR start seq#

AB

AR

AB

A

frame exc. end

P


Application layer can enforce a-priori packet FEC

P

P

P

„data“ packet carries parity

N-A

BA

N-A

BA

doc.: IEEE 802.11-10/0788r3

Submission

Conclusion

• Reception of N-ABA and lack of ABA are treated as the same case– It is not always necessary that feedback from the leader is cancelled

• If packets are (somehow) marked or the AP (somehow) determines that IP packets carry systematic FEC data or respectivly parity and it has some of them buffered

– Application layer block coding and MAC Block-ACK can work more effectively together

– AP can decide whether to add redundancy/parity a-priori– How this is implemented can be left to the manufacturer– Marking packets needs additional mechanisms, but this is out of scope of 11aa

• If application layer is not satisfied by MAC layer performance– it can conceal parity as data (purposely false marking) to enforce packet FEC

July 2010


doc.: IEEE 802.11-10/0788r3

Submission

Revised Terminology

• Avoid the term „aggregate“ to avoid confusion with e.g. A-MSDU, i.e. frame aggregation

• Instead, since feedback from non-AP stations shall be transmitted at the same time, denote „simultaneous“

• MRG Simultaneous-Block-ACK mode (SBA mode)• MRG Simulaneous-Block-ACK Request (SBAR)

– The request for a simultaneously transmitted Block ACKs

• MRG Simultaneous-Block-ACK (SBA)• DEI is the Drop Eligibility Indicator present in 11aa

Slide 17 Jochen Miroll, Saarland University

July 2010

doc.: IEEE 802.11-10/0788r3

Submission

SBAR definition

• MRG BAR information field: (Figure 7-13aa)

• Use reserved bit to indicate the request for simultaneous feedback to MRG group members

• SBAR leader is encoded in MRG BAR Partial Bitmap• Bitmap optionally preceded by SBAR Minimum* if MRG

SBAR Mode is set to 1

– *Note: when MRG SBAR Mode is set to 1: 2 Bytes, 0: 1 Byte

July 2010


MRG BAR Information

Length

Reserved MRG BAR Bitmap Offset


Bits: 8 1 7 Variable

MRG BAR Information

Length

MRG SBARMode

SBAR Minimum

MRG BAR Bitmap Offset


Bits: 8 1 8 7 Variable

doc.: IEEE 802.11-10/0788r3

Submission

SBAR leader AID and SBAR Minimum

July 2010


doc.: IEEE 802.11-10/0788r3

Submission

SBA definition

• BA Control field with MRG bit (P802.11aa/D1.0)

• Proposed BA Control field with MRG bit and SBA bit

July 2010

Jochen Miroll, Saarland UniversitySlide 20

BA AckPolicy

Multi-TID

CompressedBitmap MRG Reserved TID_INFO

B0 B1 B2 B3

Bits: 1 1 1 1 4

B4 B11 B12 B15

6

Figure 7-16—BA Control field

BA AckPolicy

Multi-TID

CompressedBitmap MRG Reserved TID_INFO

B0 B1 B2 B3

Bits: 1 1 1 1 4

B4 B11 B12 B15

6

B5

SBA

1

doc.: IEEE 802.11-10/0788r3

Submission

SBA bit definition

• STAs receiving an MRG BAR with SBAR bit set to 1 shall immediately compare their own AID to the one that is found in the BAR

• Then, they shall eithera) set the SBA bit in the corresponding MRG BA to 0 if own AID is

not equal to the single BAR AIDb) set the SBA bit in the corresponding MRG BA to 1 if own AID is

equal to the single BAR AID• If the SBA bit is set to 1, the BA Information field in the

Block Ack frame shall be a single octet equal to 0– Note that this ensures the BA with SBA bit equal to 1 to be shorter

than the otherwise unmodified BA with SBA bit equal to 0

July 2010

Jochen Miroll, Saarland UniversitySlide 21

doc.: IEEE 802.11-10/0788r3

Submission

MRG SBA mode frame exchange

1. Preconditions:a) In MRG SBA mode, the AP may not transmit group addressed DEI frames together with non-

DEI frames in the same blockb) In MRG SBA mode, the AP may not transmit frames with different DA within the same block

2. In MRG SBA mode, the AP shall transmit (subject to the TXOP limit)a) all k non-DEI group addressed frames it has consecutively buffered, immediately followed by

an SBAR with SBARmin=kb) an arbitrary subset p of DEI group addressed frames it has consecutively buffered,

immediately followed by an SBAR with SBARmin=k (i.e. the previous k)c) any group addressed frame but shall then leave MRG SBA mode

3. The AP shall then flush all k or p consecutive, just transmitted frames from its buffers (all receiving STAs may do the same)

4. If interrupted before the SBAR, and management frames are transmitted during the interruption, the AP shall increase k by the number of management frames

5. If the AP, after having transmitted the SBAR, receivesa) an SBA with SBA (negation) bit set to 0, goto 2.a) or 2.c)b) no SBA or an SBA with SBA (negation) bit set to 1, goto 2.b) or 2.c)


July 2010

doc.: IEEE 802.11-10/0788r3

Submission

MRG SBA mode frame exchange sequence

July 2010


AP1

Leader

STA 2

STA 3

STA 4

nDEI

DEI

SBA

R

N-S

BA

N-S

BA

beacon

nDEI

nDEI

nDEI

DEI

DEI

nDEI

nDEI

nDEI

nDEI

nDEI

nDEI

nDEI

nDEI

nDEI

nDEI

nDEI

nDEI

nDEI

nDEI

nDEI

nDEI

nDEI


BAR start seq#

Would ask „got 5 now?“sent: 6 = 5 nDEI+ 1 DEI

interrupted by beacon

SBA

R

SBA

frame exc. end






DEI

doc.: IEEE 802.11-10/0788r3

Submission

Motion

• Move to incorporate the text changes in doc.: IEEE 802.11-10/0904r0 into the TGaa draft 1.0

• Moved:• Seconded:• Result:


July 2010

Documents

Doc.: IEEE 802.11-10/0788r3 Submission Aggregate Block-ACK definition Date: 2010-07-15 July 2010 Jochen MirollSlide 1 Authors: