1. Feedback-jamming ARQ mechanisms
Mar 09
doc.: IEEE /0290r1
Mar 09
Feedback-jamming ARQ mechanisms
Date:
Authors:
Jochen Miroll et al., Saarland University
Jochen Miroll et al., Saarland University

2. Abstract NACK-jamming leader based Multicast
Mar 09
doc.: IEEE /0290r1
Mar 09
Abstract
NACK-jamming leader based Multicast
Comparison with ACK polling
Pros and Cons
HLBP: Hybrid leader based Multicast protocol
Evaluation criteria (cf. 08/1093r0)
Summary
Jochen Miroll et al., Saarland University
Jochen Miroll et al., Saarland University

3. Glossary ACK-polling AP collects indivudial ACK from each STA
Mar 09
Glossary
ACK-polling AP collects indivudial ACK from each STA
LBP Leader-Based Protocol
NACK-jamming enforced collision of leader-ACK with non-leader NACK
Synchronisation of Multicast STAs, event-driven or clocked
HLBP Hybrid (w.r.t. coding) LBP
Jochen Miroll et al., Saarland University

4. NACK jamming Leader Based Protocol
Mar 09
doc.: IEEE /0290r1
Mar 09
NACK jamming Leader Based Protocol
optional
SEQ# indicator and NAV updater to synchronize jamming
Jochen Miroll et al., Saarland University
Jochen Miroll et al., Saarland University

5. NACK-jamming vs. Block-ACK polling
Mar 09
NACK-jamming vs. Block-ACK polling
NACK-jamming and
ACK-polling
Jochen Miroll et al., Saarland University

6. NACK jamming LBP summary (1)
Pros
Scalability: Depending on SEQ# signalling overhead
Our simulation shows an example for 10-Block-ACK vs. 25 STAs
From a latency PoV: Maximum delay is constant
Determined by (pre-configured) number of retransmission rounds, independent of number of STAs
Event driven, distributed, optimal feedback
NACK+ACK at the same time
Requires less strict synchronisation as compared to ACK-polling
Purposely jamming the ACK is enforcing a collision – synchronisation need not be as strict as for TDMA, see next slide
Jochen Miroll et al., Saarland University

7. Synchronisation: jamming vs. ACK
Mar 09
Synchronisation: jamming vs. ACK
Jamming
Collision, Fail!
SIFS
SIFS
Block
ACK
Req
AP1
ACK
Block
ACK
STA 1 (leader)
NACK
Block
ACK
STA r-1
NACK
Block
ACK
STA r
NACK Jamming
Block Polling
Jochen Miroll et al., Saarland University

8. NACK jamming LBP summary (2)
doc.: IEEE /0290r1
Mar 09
NACK jamming LBP summary (2)
Cons
NACK-jamming still requires synchronisation of individual STAs
But: If a STA is e.g. “too slow” to jam, it does not affect the others
Scalability may not be an issue at home
But: Other use-cases exist: large audience video broadcast, e.g. at public places like airports, etc.
LBP: How to select the leader?
But: ACK polling also requires knowledge at STAs about ACK sequence => Either mechanism requires some additional signalling
Not applicable for blocks of frames
But: This may be compensated by increased scalability
Residual packet error rate (PER) (loss at each STA) unknown
ACK-polling could count #ACKs from each STA
Jochen Miroll et al., Saarland University
Jochen Miroll et al., Saarland University

9. Increasing NACK-jamming efficiency: HLBP
Mar 09
doc.: IEEE /0290r1
Mar 09
Increasing NACK-jamming efficiency: HLBP
NACK-jamming destroys „positive information“
Block-NACK (similar to Block-ACK) is thus not possible
Solution: Apply FEC to a block and
Transmit block of frames
Assume at least one STA did not receive at least one frame in block
=> ACK gets jammed
Transmit FEC parity frames
STAs try to recover the data from the frames + parity they received
Repeat 2. until ACK does not get jammed anymore
(or some retry limit reached)
Problem solved, Block-NACK-jamming possible:
The use of parity for retransmission eliminates the need for positive info
Jochen Miroll et al., Saarland University
Jochen Miroll et al., Saarland University

10. HLBP* Mar 09 Phase I Transmit a block of frames, similar to block-ACK
Phase II Replace actual block-ACK phase with parity-NACK phase
* A rateless FEC code is assumed in the figure, for e.g. for RS, parity would be parity OR data
Jochen Miroll et al., Saarland University

11. HLBP vs. Block-ACK polling
Mar 09
HLBP vs. Block-ACK polling
Jochen Miroll et al., Saarland University

12. Evaluation criteria (cf. 08/1093r0)
Mar 09
Evaluation criteria (cf. 08/1093r0)
Flexibility of number of STAs in a group
ACK-polling may be highly reliable (even the exact residual PER can be determined) and efficient for small audiences (including 5-10 STAs)
NACK-jamming scales better (no limit on the size of the audience) but is less reliable (residual PER unknown, may even be perfectly unreliable for some STAs)
Flexibility in the number of simultaneously active multicast streams
We propose to have both ACK and NACK scheme, choose one for each MC group individually depending on the requirements
Each STA receives a QoS comparable to a single stream unicast with limited retries
Delay: w.r.t audience size: Const. for NACK-jamming
Jitter: We believe it is not a major issue of the retransmission scheme (power-saving / time-slot management and the OS largely affect jitter)
Jochen Miroll et al., Saarland University

13. Evaluation criteria (cf. 08/1093r0)
Mar 09
Evaluation criteria (cf. 08/1093r0)
Should make efficient use of the wireless medium, as video bandwidths can be high
Probably good: to use a NACK scheme for „video“, ACK for „voice“
Flexible support for traffic shaping of video transmissions and power saving
Again, flexibility by switching/selection between ACK/NACK scheme
Compatible with legacy non-11aa STAs
HLBP not backwards-compatible, but assume plain NACK-jamming is
Provide support for duplicate detection
Proposed NACK schemes require SEQ# to work
Provide a mechanism to support rate adaptation
NACK-jamming could even use unicast rate adaptation mechanism
Provides a mode of operation that minimises the potential for requiring hardware modifications
Provides a mode of operation that supports collision avoidance
Jochen Miroll et al., Saarland University

14. Summary LBP with NACK jamming LBP with NACK jamming and power-saving
For low jitter, low latency applications
For error tolerant applications
Improved scalability as compared to ACK-polling
Represents the Broadcast paradigm (delay-constrained, quasi-reliable: QoS at a STA within a MC group could depend on distance from sender)
LBP with NACK jamming and power-saving
Block-ACK polling may be more efficient for small audiences
=> No Block-NACK possible, Block-ACK may have lower CHT
HLBP
It is a Block-NACK scheme, comparable to Block-ACK
Most efficient in any scenario
Most complex due to required FEC (could be done on application layer)
Jochen Miroll et al., Saarland University

15. Mar 09
Questions?
Jochen Miroll et al., Saarland University

16. Mar 09
References
Multicast MAC Extensions for high rate real-time traffic in Wireless LANs
Jochen Miroll et al., Saarland University