Fast Resilient Jumbo Frames in Wireless LANs

Transcript

1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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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21. Thank you!
    apurvb@cs.utexas.edu
    

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22. 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
    

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