July 2010
doc.: IEEE 802.11-10/0768r1
Cancellation of aggregate Multicast
feedback – measurement results
Date: 2010-07-12
Authors:
Name
Jochen Miroll
Zhao Li
Submission
Affiliations Address
Saarland
University
Phone
Campus C6 3,
+49 681 302
66123 Saarbruecken, 6546
Germany
Slide 1
email
[email protected]
[email protected]
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Abstract
• This presentation is an update on the Leader-based
aggregate feedback Protocol (LBP) proposal previously
made to TGaa by the authors and provides
measurement results obtained on a consumer 802.11
hardware test bed
• The feedback cancellation probability in the worst case
of LBP is measured and compared to previous
theoretical / simulation results
• These results have also been published and presented
at the IEEE ISCE 2010 conference in June 2010
Submission
Slide 2
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Motivation
• 11aa is standardizing Multicast ARQ: MRG
– Gathering per-receiver feedback, the overhead due to the positive
ACKs grows linearly with the number n of receivers
• How does 11aa MRG compensate for this increased
overhead?
– Aggregation of multiple frames: single-TID, uncompressed BlockACK (802.11n) for MRG
– Per-frame ACK becomes multi-frame Block-ACK bitmap for the last
k frames
– Still: overhead increases linearly with receivers n
• How to get rid of the dependency on n?
– We have previously proposed a leader-based Multicast retransmission
scheme to 11aa
Submission
3
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Feedback aggregation in the same time slot
• All receivers provide feedback, but this feedback from k≤n
STAs is aggregated in a single time slot
overhead(k) = overhead(1)
?
– AP transmits a data frame
– Then, AP asks for ACK/NACK
• If STA i has received the data
frame: it responds with an ACK
• If STA j did not receive the data
frame: it responds with NACK
at the same time
Submission
4
NACK
STA 1
STA 2
STA
3
STA 4
NACK
• Introduction of NACK
ACK
AP1
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Feedback cancellation premise
• If ACK and NACK are approx. equally „strong“
– Is it possible to cancel an ACK by a simultaneous NACK and thus
enforce a retransmission?
• The „capture effect“:
– Describes the phenomenon that a frame (e.g. ACK) may be received
correctly in the presence of another, similarly strong (e.g. NACK)
• Main reasons for this „imperfect collision“
– Locking the preamble and then Viterbi decoding the locked-onto
frame is a very robust mechanism.
– E.g.: ACK is BPSK, rate ½ and only 14 Bytes in length.
It is the most robust 802.11 frame
(OFDM: few dB difference between ACK and NACK may suffice to
„capture“)
Submission
5
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Earlier comments from TGaa (resolved)
• Will feedback cancellation actually work?
1.
2.
answer: Yes, collisions are happening all of the time
answer: No, due to the capture effect
• We have consequently provided Matlab and ns-2 results for
feedback cancellation to Tgaa
– cf. doc.: IEEE 802.11-09/1150r2
• Provided in this document: measurement results using real
and cheap 802.11 hardware
Submission
6
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Leader-based feedback cancellation
• Idea: Multicast is essentially handled as a unicast connection
to a „leader receiver“
– „Non-leaders“ transmit a NACK if a frame is lost
• Target: Larger Multicast groups (large n)
– If ACK survives the somewhat weaker NACK, does it survive many?
Does it survive many equally strong, many somewhat stronger?
• Intuitive leader selection: choose the „weakest“ receiver (as
seen by the AP, no power control, just due to path loss)
– If no loss: Leader’s ACKs can be received (ACKs are most robust)
– Else:
Expect a good chance that whenever several somewhat
stronger NACKs are transmitted at the same time, the Leader’s ACK
will be cancelled
Submission
7
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Aggregation through Leader-Based
feedback cancellation Protocol (LBP)
cf. doc.: IEEE 802.11-09/0290r1
First transmission
AP
Retransmission
optional
AP
DIFS
RTS
R1(Leader)
SIFS
SIFS
SEQ
SIFS
DATA
CTS
DATA
R2
ERROR
R3
SIFS
ACK
SIFS
NACK
SIFS
ACK
DATA
DIFS
R1(Leader)
RTS
SIFS
SIFS
SEQ
CTS
SIFS
DATA
ERROR
R2
DATA
R3
ERROR
SEQ# indicator and NAV updater
Submission
ERROR
Slide 8
to synchronize
aggregate feedback
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Feedback cancellation constraint
• Failure of feedback cancellation results in uncorrectable
packet loss at non-leaders
– (i.e. capture of ACK happens, no collision)
• Question that arises:
– What is the error floor in the worst case?
• What is the worst case for the leader-based feedback
cancellation approach?
– Intuitively: the „weakest“ receiver can not be distinguished
– All receivers on average experience the same SNR
– We assume that all are sending approx. equally strong feedback
Submission
9
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Feedback cancellation measurements
• Examine two different cases of how feedback
aggregation may be implemented
– In the WLAN card‘s real-time OS
– In the WLAN card‘s host OS (e.g. Linux)
• Implications
– Cards allow for strict timing constraints (similar to 802.11 ACK,
±900ns), so we can examine short feedback
– Host OS is less accurate in timing, thus we examine feedback
cancellation with frames of several tens of Bytes
Submission
10
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Feedback cancellation test setup (1)
• We have used real consumer 802.11 hardware
– Limited freedom in implementing MAC algorithms
• But: We can fix some parameters in cancellation
experiments
– Here: Non-leaders transmit different frames
• Examine different frame sizes and timings with what is
possible…
…out of the box: Let positive feedback be a 6 Mbps ACK and the
negative feedback be a 12 Mbps ACK
…own implementation: Driver level (software) ACK/NACK
implementation
Submission
11
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Feedback cancellation test setup (2)
SEQ frame triggers feedback, assume this is the question „did you get the data frame“
Submission
12
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Feedback cancellation test setup (3)
• To obtain independence from
the (fading) environment:
–
–
Submission
Move receivers slowly around the
AP, changing their positions in the
environment
Periodically change the roles
(leader, non-leader) of the
Leader
receivers
(always have exactly 1 leader)
13
Non-leader 1
AP
Non-leader n1
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Validation of test setup
• CDF of SNR at
receivers is very steep
~identical channel conditions
for all receivers on average
• Error free reception
rates of different frames
at the end of
measurement run yield
valid results
• SEQ (trigger) loss?
loss rate < 0.1%
Submission
14
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Test results (representative example)
Parameter
802.11 wireless channel
AP transmit power
Station transmit power
Number of stations/nonleaders
SEQ frequency
SEQs transmitted
Role switching interval
Measurement parameter
Value
40 (5.2 GHz)
17 dBm
8 dBm
– Assume large n but only few
losses among stations, including
the leader
4/3
10 Hz
~26000
100 s
Rate
LBP ACK loss avg.
0.894134
LBP NACK loss avg.
0.753818
Hardware ACK-6 loss avg.
0.893892
Hardware ACK-12 loss avg.
0.854081
LBP SEQ loss at station 1
0.000137
LBP SEQ loss at station 2
0.000168
LBP SEQ loss at station 3
0.000246
LBP SEQ loss at station 4
0.000138
Submission
• 1 leader, 3 non-leaders
• Virtually no SEQ loss
• ~89% feedback cancellation
success probability
– Result seems independent of
frame length and timing
Worst case results (where
leader-selection would not
work)
15
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Theoretical / Simulation results
• Compare with ns-2 results
Scenario: Rayleigh fading channel, equal AP-STAs distance
feedback cancellation rate is about 76% for 2,
more than 90% for more than 2, and
already 99% for 5 receivers
1
• Again: worst case where
leader selection fails
0.95
Jamming probability
–
–
–
–
R=2
R=2
R=3
R=4
R=5
0.9
0.85
Simulation
Theoretical
Theoretical
Theoretical
Theoretical
0.8
0.75
Submission
Slide 16
5
10
15
20
Receivers' distance to AP (m)
25
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Conclusion
• Scalable Multicast error correction can be achieved by
aggregation through cancellation
• Real test bed results are backed up by simulations
• Channel will not be arbitrarily reliable but limited by an
error floor
– not as bad as it may sound if done right, as explained in doc.: IEEE
802.11-10/0788
• Combined MAC-layer and “Application Layer” error
correction feasible
– Assume overlay packet erasure FEC
– Audio/Visual streams typically can tolerate errors
– Residual error requirement can be dealt with on layers above MAC
Submission
17
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Questions?
(a further presentation will propose how this
scheme should be incorporated into 11a)
Submission
Slide 18
Jochen Miroll
July 2010
doc.: IEEE 802.11-10/0768r1
Recap: Hybrid LBP (HLBP)*
cf. doc.: IEEE 802.11-09/0290r1
Phase I
AP
DATA 2
DATA k-1
DATA 1
DATA 2
DATA k-1
R2
ERROR
DATA 2
DATA k-1
R3
DATA 1
DATA 2
DATA k-1
RTS
R1(Leader)
AP
Phase II
DIFS
SIFS
SIFS
SIFS
SEQ
SIFS
CTS
DATA k
DATA 1
ERROR
R1(Leader)
DATA k
SIFS
ACK
R2
DATA k
SIFS
NACK
R3
DATA k
SIFS
SIFS
SEQ
SIFS
Parity 1
ERROR
SIFS
ACK
Parity 1
ERROR
Phase I Transmit a block of frames, as in MRG BA. Here: systematic FEC part
Phase II Parity phase. Instead of BAR/BA, do AggregateAckRequest/AggregateAck
* Assume e.g. DVB-IPDC or Raptor code on upper layer, MAC somehow knows which packets are systematic (DATA) or parity
Submission
Slide 19
Jochen Miroll