Fast Resilient Jumbo Frames in Wireless LANs

Transcript

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   Fast Resilient Jumbo Frames in Wireless LANs Apurv Bhartia University

   of Texas at Austin apurvb@cs.utexas.edu Joint work with Anand Padmanabha Iyer, Gaurav Deshpande, Eric Rozner and Lili Qiu IWQoS 2009 July 15, 2009
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   2 Motivation •  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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   3 State 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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   4 Our 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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   5 Synergy 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 Adaptation Partial Recovery Aware Rate Adaptation Partial Recovery Aware Rate Adaptation
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   6 Resilient 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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   7 Resilient 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 4 4 1 1 2 2 Segment 1 CRC Segment 2 CRC Segment N CRC Frame ID Type Rate Bitmap SS Header CRC Length
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   8 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 Header Frame Offset N Segment Bitmap N Start Frame Seg No Type Rate Frame Bitmap
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   9 Approach •  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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    10 Partial 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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    11 Partial Recovery Aware Rate Adaptation •  Estimating Channel Condition

    –  Sender periodically broadcasts probe packets –  Sent at different data rates •  CurrRate r [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 Payload Probe ID Rate Header CRC Per rate
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    12 Partial 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 Type Probe Response ID Rate1 Frame CRC BER1 HL1 Rate1 BER1 HL1
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    13 Partial 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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    14 Testbed 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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    15 Schemes 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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    16 Experimental Results: Single Flow Throughput (Mbps) Cumulative Fraction 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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    17 Experimental Results: Multiple Flows 0 5 10 15 20

    25 -5 1 2 4 6 8 # Flows Average Total Throughput (Mbps) 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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    18 Experimental Results : Multiple Flows Throughput (Mbps) Cumulative Fraction

    Average Throughput SR/1500: 0.84 Mbps FRJ: 1.68Mbps SR/3000: 1.05 Mbps SR/1500: 0.30 Mbps SR/3000: 0.38 Mbps FRJ: 0.57 Mbps
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    19 Experimental Results: Multiple Flows •  Fairness –  Difference is

    within 10% –  Most cases it is close to 0 # Flows Fairness Index FRJ’s performance gain does not come at the cost of compromising fairness!
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    20 Conclusion •  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
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    Thank you! apurvb@cs.utexas.edu

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    22 0 5 10 15 20 25 -5 1 2

    4 6 8 # Flows Average Total Throughput (Mbps) FRJ SR/ 1500 bytes SR/3000 bytes FRJ w/ RTS SR/1500 bytes w/ RTS SR/3000 bytes w/ RTS