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Loss Distinguishing and Performance Improvement of WLAN

Loss Distinguishing and Performance Improvement of WLAN. IE Department, CUHK Qixiang Pang (龐其祥) April 15, 2004. Outline. Existing Backoff and AutoRate algorithms and Problems Loss Detection – Basic access mode New Backoff and AutoRate algorithms Performance comparison

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Loss Distinguishing and Performance Improvement of WLAN

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  1. Loss Distinguishing and Performance Improvement of WLAN IE Department, CUHK Qixiang Pang(龐其祥) April 15, 2004

  2. Outline • Existing Backoff and AutoRate algorithms and Problems • Loss Detection – Basic access mode • New Backoff and AutoRate algorithms • Performance comparison • Reliability improvement of the loss detection method • Loss Detection – RTS/CTS access mode • Possible future work

  3. Existing Backoff and AutoRate algorithms Backoff algorithm in Standard • If ACK is received, the transmission is successful; The sender resets contention window to CWmin, transmits the next frame; • If ACK is not received, a collision is assumed; cw = cw*2; The sender retransmits. AutoRate from Lucent(WaveLAN) • If an ACK is received, the transmission is successful; rate_up_counter++; If (rate_up_counter > 10) phy_rate is increased. • If ACK is not received, a link error is assumed; rate_down_counter ++; If (rate_down_counter > 2) phy_rate is reduced.

  4. Problems • Doubled Backoff – assumes all losses are due to collision. • Auto-Rate – assumes all losses are due to link error. • Both types of losses could happen in reality. • Conflict!

  5. Loss detection – basic access mode RECEIVER • If receiver receives correct MAC header, • If MAC body is correct, an ACK is sent back; • If MAC body is wrong, a NAK is sent back; • If wrong MAC header is received, the receiver sends nothing. SENDER • If an ACK is received, the transmission is successful; • If a NAK is received, a linkerror is assumed; • Nothing is received, a collision is assumed. • Remarks: • Separate Header and Body • Header has much lower error probability than Body • ACK, NAK have much lower error probability.

  6. New Backoff algorithm at Sender New Backoff • If a link error is detected, DATA is sent immediately without backoff; • If a collision is detected, doubled backoff; • If transmission is successful, CWnew = max {CWmin, CWold/2} Always doubled backoff in standard Always CWmin in standard

  7. New AutoRate algorithm at Sender New AutoRate • If a link error is detected, rate_down_counter ++; If (rate_down_counter > 2), phy_rate is reduced; • If a transmission is successful, rate_up_counter++; If (rate_up_counter > 10), phy_rate is increased.

  8. CWmin 16 CWmax 1024 SlotTime 9 s SIFSTime 16 s PHY overhead 20 s BasicDataRate 6 Mbps Performance Comparison • 802.11a – 8 PHY modes/rates

  9. Bit Error Rate vs SNR and Data Rate SNR: Signal to Noise Ratio

  10. Throughput vs SNR and Data Rate # of contending stations = 1

  11. Thru vs Data Rate and SNR # of contending stations = 10

  12. Comparison – new vs standard backoff (fixed BER - simulation) BER= 1e-5 BER= 5e-5 BER= 1e-4 phy_rate=54Mbps

  13. Comparison – new vs standard backoff (fixed BER - analytical) • 1 station; phy_rate=54Mbps

  14. Comparison - variable link condition Gilbert Elliot Channel model–A two-state discrete time Markov chain to model the wireless channel variation SNR=(15,30) SNR=(0,15) • Exponential distributed “good” and “bad” time; • Uniformly distributed SNR values during “good” or “bad” states

  15. Comparison – new vs standard backoff (variable BER) # of station = 1 # of station = 5 # of station = 10 # of station = 50

  16. Comparison – new AutoRate vs WaveLAN AutoRate Conclusion: In case of many stations, new AutoRate outperforms the existing AutoRate algorithm.

  17. Reliability Improvement • Multiple repetitive headers: MAC Header, MAC Header, …, • Lower rate for header • Separate CRC

  18. RTS/CTS access mode (hidden terminal) • 4-way handshaking • Useful to overcome “hidden terminal” problem

  19. Loss Detection – RTS/CTS access mode RECEIVER • If RTS is received, CTS is sent; • If DATA is received, ACK is sent. SENDER • If both CTS and then ACK are received, the transmission is successful; • If CTS is received but ACK is missed, a link error is assumed; • If CTS is missed, a collision is assumed. • Remarks: • RTS/CTS is sent at lower data rate. • RTS/CTS is much shorter

  20. New backoff and AutoRate algorithms at Sender – same New Backoff • If a link error is detected, DATA is sent immediately after IFS; • If a collision is detected, doubled backoff; • If transmission is successful, CWnew = max {CWmin, CWold/2} New AutoRate • If a link error is detected, rate_down_counter ++; If (rate_down_counter > 2) phy_rate is reduced; • If a transmission is successful, rate_up_counter++; If (rate_up_counter > 10) phy_rate is increased.

  21. Possible future work • More simulation scenarios • More applications over WLAN, e.g. TCP • Mixed heterogeneous channel conditions • Theoretical analysis • Measurement based (collision rate, error rate => proper scheme)

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