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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
Jochen MirollSlide 2
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
Jochen MirollSlide 6
* 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
Jochen MirollSlide 9
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
MRG BAR Partial Bitmap
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
Jochen MirollSlide 10
doc.: IEEE 802.11-10/0788r3
Submission
Frame exchange sequences (1)
July 2010
Jochen MirollSlide 11
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
Frame exchange sequences (2)
July 2010
Jochen MirollSlide 12
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
„Did you get 5 since BAR start seq#?“
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
Frame exchange sequences (3)
July 2010
Jochen MirollSlide 13
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
„Did you get 5 since BAR start seq#?“
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
Assume everyone received the beacon
Increases our seq# counter by one
Increases our seq# counter by one
doc.: IEEE 802.11-10/0788r3
Submission
Frame exchange sequences (4)
July 2010
Jochen MirollSlide 14
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
„Did you get 5 since BAR start seq#?“
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
Frame exchange sequences (5)
July 2010
Jochen MirollSlide 15
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
„Did you get 6 since BAR start seq#?“
BAR start seq#
AB
AR
AB
A
frame exc. end
P
„Did you get 6 since BAR start seq#?“
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
Jochen MirollSlide 16
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
Jochen MirollSlide 18
MRG BAR Information
Length
Reserved MRG BAR Bitmap Offset
MRG BAR Partial Bitmap
Bits: 8 1 7 Variable
MRG BAR Information
Length
MRG SBARMode
SBAR Minimum
MRG BAR Bitmap Offset
MRG BAR Partial Bitmap
Bits: 8 1 8 7 Variable
doc.: IEEE 802.11-10/0788r3
Submission
SBAR leader AID and SBAR Minimum
July 2010
Jochen MirollSlide 19
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)
Slide 22 Jochen Miroll, Saarland University
July 2010
doc.: IEEE 802.11-10/0788r3
Submission
MRG SBA mode frame exchange sequence
July 2010
Jochen MirollSlide 23
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
„Did you get 5 since BAR start seq#?“
BAR start seq#
Would ask „got 5 now?“sent: 6 = 5 nDEI+ 1 DEI
interrupted by beacon
SBA
R
SBA
frame exc. end
„Did you get 6 since BAR start seq#?“
Assume everyone received the beacon
Assume everyone received the beacon
Increases our seq# counter by one
Increases our seq# counter by one
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:
Slide 24 Jochen Miroll, Saarland University
July 2010