Aggregate Block-ACK definition July 2010 doc.: IEEE 802.11-10/0788r1 July 2010 Aggregate Block-ACK definition Date: 2010-07-12 Authors: Jochen Miroll Jochen Miroll, Saarland University

July 2010 doc.: IEEE 802.11-10/0788r1 July 2010 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 Jochen Miroll Jochen Miroll, Saarland University

Feedback aggregation in the same time slot July 2010 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 data frame: it responds with an ACK If STA j did not receive the data frame: it responds with NACK at the same time ? AP1 ACK STA 1 NACK STA 2 STA 3 NACK STA 4 Jochen Miroll

Leader-based feedback cancellation July 2010 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 Jochen Miroll

Recap: Results in the LBP-worst-case July 2010 Recap: Results in the LBP-worst-case Worst case results where leader-selection would not be able to reliably determine SNR difference between receivers Ns2 – 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 Measured: 3 rcvrs, 1 leader ~89% feedback cancellation success Measurement parameter Rate Software ACK loss avg. 0.894134 Software NACK loss avg. 0.753818 Hardware ACK-6 loss avg. 0.893892 Hardware ACK-12 loss avg. 0.854081 Jochen Miroll

Recap: Hybrid LBP (HLBP)* cf. doc.: IEEE 802.11-09/0290r1 July 2010 Recap: Hybrid LBP (HLBP)* cf. doc.: IEEE 802.11-09/0290r1 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 Jochen Miroll

Motivation for (H)LBP with aggregation through cancellation July 2010 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 Jochen Miroll

Motivation for (H)LBP with aggregation through cancellation cont‘d July 2010 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 Jochen Miroll

Aggregate Block-Ack Request July 2010 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 Bits: 8 1 7 Variable MRG BAR Information Length reserved MRG BAR Bitmap Offset MRG BAR Partial Bitmap Bits: 8 1 7 8 Variable MRG BAR Information Length ABAR Flag (set to 1) MRG BAR Bitmap Offset (ABAR leader) ABAR minimum MRG BAR Partial Bitmap Jochen Miroll

Aggregate Block-Ack Request Minimum July 2010 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 Jochen Miroll

Frame exchange sequences (1) July 2010 Frame exchange sequences (1) „Did you get 5 since BAR start seq#?“ BAR start seq# Aggregate BA is cancelled frame exc. end … D D D D D ABAR P P D AP1 D D D D D ABA P Leader D D D N-ABA P P STA 2 D D D D D P STA 3 D D D D N-ABA P P STA 4 D D D D D P P D Packets avaliable in AP buffer for currently serviced MRG group: Jochen Miroll

Frame exchange sequences (2) July 2010 Frame exchange sequences (2) „Did you get 5 since BAR start seq#?“ (note that 6 have been sent) „Did you get 5 since BAR start seq#?“ BAR start seq# N-ABA is received frame exc. end D D D D D P ABAR P ABAR AP1 D D D D P ABA Leader D D D P N-ABA P STA 2 D D D D D P STA 3 D D D D P P N-AB STA 4 AP can decide whether to add redundancy/parity a-priori Reception of N-ABA and lack of ABA are treated as the same case Jochen Miroll

Frame exchange sequences (3) July 2010 Frame exchange sequences (3) „Did you get 6 since BAR start seq#?“ sent: 7 Would ask „got 5 now?“ sent: 6 = 5 data + 1 parity „Did you get 5 since BAR start seq#?“ interrupted by beacon BAR start seq# Aggregate BA is cancelled frame exc. end D D D D D ABAR P beacon P ABAR AP1 D D D D D ABA P ABA Leader D D D N-ABA P P STA 2 D D D D D Assume everyone received the beacon Increases our seq# counter by one P STA 3 D D D D N-ABA P P STA 4 Jochen Miroll

Frame exchange sequences (4) July 2010 Frame exchange sequences (4) „Did you get 5 since BAR start seq#?“ BAR start seq# frame exc. end D D D D D ABAR P ABAR P ABAR AP1 D D D P Leader D D D N-ABA N-ABA P N-ABA STA 2 D D D N-ABA P N-ABA N-ABA STA 3 D D N-ABA P N-ABA P N-ABA N-AB N-AB N-AB STA 4 AP is limited in parity delivery as provided by the application layer Application layer can implicitly control MAC error correction delay Jochen Miroll

Frame exchange sequences (5) July 2010 Frame exchange sequences (5) „Did you get 6 since BAR start seq#?“ „data“ packet carries parity „Did you get 6 since BAR start seq#?“ BAR start seq# frame exc. end D D D D D P ABAR P ABAR AP1 D D D D P ABA Leader D D D P N-ABA P STA 2 D D D D D P STA 3 D D D D P P N-AB STA 4 Application layer can enforce a-priori packet FEC Jochen Miroll

July 2010 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 Jochen Miroll

Questions and discussion July 2010 Questions and discussion Jochen Miroll