Fast Resilient JumboFast Resilient JumboFrames in Wireless LANsFrames in Wireless LANs
Apurv Bhartia Apurv Bhartia University of Texas at AustinUniversity of Texas at Austin
[email protected]@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 AdaptationConstant MAC overheadReduces relative cost of RTS/CTS
Loss Increases with frame size
Increases effectiveness of jumbo framesLess collisions – effective recovery
Higher tx rates!Increased tx rates reduces contention losses
Reduces effective data loss rateBetter partial recovery
Higher tx rates – increases relative MAC overhead
More data for constant overheadBenefit 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 FramesS 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 SS Header 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 CRCHeader Frame
Offset NSegment Bitmap N
Start FrameSeg No Type Rate Frame
Bitmap
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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 Rate HeaderCRC
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 Rate1 Frame 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 estimationT = ∑ 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 = 24Mbps• 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)
Cum
ulat
ive
Frac
tion
SR/1500: 0.68 MbpsSR/3000: 0.68 MbpsFRJ: 1.1 Mbps
SR/1500: 14.17 MbpsSR/3000: 16.93 MbpsFRJ: 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
-51 2 4 6 8
# Flows
Aver
age
Tota
l Thr
ough
put
(M
bps)
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)
Cum
ulat
ive
Frac
tion
Average ThroughputSR/1500: 0.84 Mbps FRJ: 1.68MbpsSR/3000: 1.05 Mbps
SR/1500: 0.30 MbpsSR/3000: 0.38 MbpsFRJ: 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
Fairn
ess I
ndex
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 [email protected]@cs.utexas.edu
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0
5
10
15
20
25
-51 2 4 6 8
# Flows
Aver
age
Tota
l Thr
ough
put
(
Mbp
s)FRJ SR/1500 bytes SR/3000 bytes FRJ w/ RTS SR/1500 bytes w/ RTS SR/3000 bytes w/ RTS
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