Fast Resilient JumboFast Resilient JumboFrames in Wireless LANsFrames in Wireless LANs

Apurv Bhartia Apurv Bhartia University of Texas at AustinUniversity of Texas at Austin

apurvb@cs.utexas.eduapurvb@cs.utexas.edu

Joint work with Joint work with Anand Padmanabha Iyer, Gaurav Deshpande, Anand Padmanabha Iyer, Gaurav Deshpande,

Eric Rozner and Lili QiuEric Rozner and Lili Qiu

IWQoS 2009IWQoS 2009July 15, 2009July 15, 2009

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MotivationMotivation• Lossy wireless medium• Novel techniques have been proposed …

… but each of them alone is insufficient

Partial Recovery

Jumbo Frames Rate Adaptation

Our goal: identify the synergy between these techniques and exploit it

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State of the ArtState of the Art• Jumbo Frames

– Proprietary solutions for frame aggregations [Atheros Super G, TI frame concatenation]

– 802.11n frame aggregation standard• Require specific hardware support• Entire packet needs to be retransmitted

• Partial Packet Recovery– Require specific hardware support [MRD, SOFT, PPR]– Leverage PHY layer information [SOFT, PPR]

• if PHY layer information is available, FRJ can benefit to provide higher gain

• Rate Adaptation – SampleRate, ONOE (madwifi), RRAA– Over-estimates the actual loss rate

• Adapt rate according to frame loss rate• Over-estimates the actual loss rate

Holistic Approach is missing !

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Our ContributionsOur Contributions• Identify interactions between the three

techniques– Exploit the synergy between the schemes – Works for both single and multi-hop

topologies

• Develop resilient jumbo frames– Achieve high throughput under both low and

high loss conditions

• Develop partial recovery aware rate adaptation

• Develop a prototype implementation

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Synergy Between Design SpaceSynergy Between Design Space

Partial Recovery

Jumbo Frames Rate Adaptation

Constant MAC overhead

Reduces relative cost of RTS/CTS

Loss Increases with frame size

Increases effectiveness of jumbo frames

Less collisions – effective recovery

Higher tx rates!

Increased tx rates reduces contention losses

Reduces effective data loss rate

Better partial recovery

Higher tx rates – increases relative MAC overhead

More data for constant overhead

Benefit increases with increased tx rates

Partial Recovery Aware Rate AdaptationPartial Recovery Aware Rate AdaptationPartial Recovery Aware Rate AdaptationPartial Recovery Aware Rate AdaptationPartial Recovery Aware Rate AdaptationPartial Recovery Aware Rate Adaptation

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Resilient Jumbo FramesResilient Jumbo Frames

S R

• Use jumbo frames– High throughput in good conditions– In bad conditions …

• … re-transmit only corrupted segments– Saves the overhead of retransmitting complete

frames

2.5 ACK

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Resilient Jumbo FrameResilient Jumbo Frame

• Data Frames

• Core Components– Resilient Jumbo Frames which applies partial

recovery to jumbo frames – Partial recovery ‘aware’ rate adaptation

Header

4 4 4

4 441 1 2 2

Segment 1 CRC Segment 2 CRC Segment N CRC

Frame ID Type Rate Bitmap SSHeader

CRCLength

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Resilient Jumbo Frame (Cont.)Resilient Jumbo Frame (Cont.)• Receiver Feedback

– Combination of MAC-layer and 2.5-layer ACKs– MAC-layer ACKs

• Adjustment of back-off window in IEEE 802.11• Increased reliability and efficiency than 2.5 ACKs

– 2.5-layer ACKs• To support partial recovery• Unicast for improved reliability and cumulative

Frame Offset

Segment Bitmap 1

Frame CRC

HeaderFrame Offset N

Segment Bitmap N

Start FrameSeg No

Type RateFrameBitmap

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ApproachApproach• Retransmission

– Disable MAC layer retransmissions• set MAC retry count = 0• Retransmit the frames at the 2.5-layer

– Triggered by • 2.5-layer ACKs

– If 1st Retx: frames with higher seq nos or some segments in this frame are ACKed [first data transmissions is in-order]

– If 2nd or higher: some new segments in this frame are ACKed• Retransmission Timeout

– Standard approach as in TCP

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Partial Recovery Aware Rate AdaptationPartial Recovery Aware Rate Adaptation– Traditional schemes identify optimal rate using frame loss rate

• Overestimates the loss rate• Lower data transmissions rates are selected

– Challenges for the ‘new’ scheme• Accurate estimation of channel condition at various data rates• Selecting rate that maximizes throughput under partial recovery

Estimate throughput based on loss statistics !

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Partial Recovery Aware Rate AdaptationPartial Recovery Aware Rate Adaptation• Estimating Channel Condition

– Sender periodically broadcasts probe packets– Sent at different data rates

• CurrRater [current data rate]• CurrRate-

r [one rate below the current data rate]• CurrRate+

r [one rate above the current data rate]– Sent at a frequency of 5 probes/second

• Limit the overhead

Type PayloadProbe ID RateHeader

CRC

Per rate

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Partial Recovery Aware Rate AdaptationPartial Recovery Aware Rate Adaptation• Probe Response

– Sent by the receiver– Estimates the channel condition using

• Header Loss Rate (HL) – header corruption• Segment Loss Rate (SL) – segment corruption• Communicates this info using probe response

– Transmitted via MAC-layer unicast• High reliability

– Default Probe response [HL = 1, SL = 1]• To account for lost probes

TypeProbe Response ID Rate1Frame

CRCBER1 HL1 Rate1 BER1 HL1

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Partial Recovery Aware Rate AdaptationPartial Recovery Aware Rate Adaptation• Sender selects the rate that gives the

best throughput estimation

T = ∑ Pi × (Backoff + DIFS +i=1..MaxRetries + 1

DATA + SIFS + ACK + useRTS + RTSOverhead )

preambleTime +(HS + NSi + segmentSize)

rate

Pi = 1 i = 1 Pi-1 × (HL + (1 – HL) × (1- (1 – SL) )) otherwise

NSi-1

Throughput = (NS1 – NSMaxRetries + 2) × SegmentSize/T

NSi = 30 i = 1 NSi-1 × (HL + (1 – HL) × SL ) otherwise

RTS + SIFS + CTS + SIFS

NSi

Probability of sending the ith

tx

Time for ith data tx

No of segments in ith

tx

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Testbed TopologyTestbed Topology• 24 machines• Madwifi driver and

CLICK toolkit• Initial rate = 24Mbp

s• Tx Power = 18 dBm

Total throughput Per flow throughput Jain’s Fairness Index

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Schemes ComparedSchemes Compared• Sample Rate using 1500 byte frames

[SR/1500-bytes]• Sample Rate using 3000 byte frames

[SR/3000-bytes]– Same as SR/1500, but uses jumbo frames– Similar to Atheros Super G Fast Frame feature

• FRJ using 3000 byte frames, 30 segments

With and without RTS/CTS

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Experimental Results: Single FlowExperimental Results: Single Flow

Throughput (Mbps)

Cu

mu

lati

ve F

ract

ion

SR/1500: 0.68 Mbps

SR/3000: 0.68 Mbps

FRJ: 1.1 Mbps

SR/1500: 14.17 Mbps

SR/3000: 16.93 Mbps

FRJ: 23.81 Mbps

Moderate Link Conditions: Partial Recovery is more effective

FRJ benefit is 40.6% - 68.0% under single flow

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Experimental Results: Multiple FlowsExperimental Results: Multiple Flows

0

5

10

15

20

25

-5

1 2 4 6 8

# Flows

Avera

ge T

ota

l Th

rou

gh

pu

t

(M

bp

s)

FRJ SR/1500 bytes SR/3000 bytes FRJ w/ RTS SR/1500 bytes w/ RTS SR/3000 bytes w/ RTS

Schemes w/o RTS/CTS perform well

Randomly chosen flows!

FRJ constantly outperforms

More collisions => increase in header losses

FRJ benefit ranges from 10% (1 flow) to 64% (6 flows)

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Experimental Results : Multiple FlowsExperimental Results : Multiple Flows

Throughput (Mbps)

Cu

mu

lati

ve F

ract

ion

Average ThroughputSR/1500: 0.84 Mbps FRJ: 1.68MbpsSR/3000: 1.05 Mbps

SR/1500: 0.30 Mbps

SR/3000: 0.38 Mbps

FRJ: 0.57 Mbps

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Experimental Results: Multiple FlowsExperimental Results: Multiple Flows

• Fairness– Difference is

within 10%– Most cases it is

close to 0

# Flows

Fair

ness

In

dex

FRJ’s performance gain does not come at the cost of compromising fairness!

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ConclusionConclusion• Main contributions

– Identify interplay between jumbo frames, PPR and rate adaptation• Jumbo frames with partial recovery• Partial recovery aware rate adaptation

– Demonstrate the effectiveness of this solution through testbed experiments

• Future work– More effective partial recovery schemes and

coding techniques– Dynamically configurable RTS/CTS– FRJ-aware route selection

Thank you!Thank you!apurvb@cs.utexas.eduapurvb@cs.utexas.edu

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0

5

10

15

20

25

-5

1 2 4 6 8# Flows

Avera

ge T

ota

l Th

rou

gh

pu

t

(M

bp

s)FRJ SR/1500 bytes SR/3000 bytes FRJ w/ RTS SR/1500 bytes w/ RTS SR/3000 bytes w/ RTS