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A New ACK Policy for Coexisting IEEE 802.11/802.11e Devices

A New ACK Policy for Coexisting IEEE 802.11/802.11e Devices. Haithem A Al- Mefleh , J. Morris Chang Electrical and Computer Eng. Dept. Iowa State University U. S. A. March 2008. Outline. Introduction IEEE 802.11 Problem Statement Related Work Proposed Solution - NZ-ACK Evaluation

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A New ACK Policy for Coexisting IEEE 802.11/802.11e Devices

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  1. A New ACK Policy for Coexisting IEEE 802.11/802.11e Devices Haithem A Al-Mefleh, J. Morris Chang Electrical and Computer Eng. Dept. Iowa State University U. S. A. March 2008

  2. Outline • Introduction • IEEE 802.11 • Problem Statement • Related Work • Proposed Solution - NZ-ACK • Evaluation • Conclusions

  3. Introduction • IEEE 802.11 Std for Wireless LANS • 802.11b,g,a • DCF • PCF • 802.11e • backward compatible • QoS • EDCA • HCCA

  4. Introduction • EDCA: • AIFS[AC] • CWmin[AC], CWmax[AC] • TXOP[AC] • Both: • Duration of frames used for NAV • DCF: • DIFS • CWmin, CWmax

  5. Problem Statement Coexisting EDCA and legacy DCF users • EDCA contention parameters are MAC-Level • DCF contention parameters are PHY-Level No Control over legacy users

  6. Problem Statement • Smallest AIFS is equivalent to DIFS • DCF has no TXOP • Smaller CW for EDCA  Higher Collisions • Higher CW for EDCA  Lower Priority EDCA users may get lower priority and performance

  7. Problem Statement • Simple example: • A WLAN of EDCA and DCF users • EDCA users: VoIP, CWmin 8, AIFS=50us • DCF users: CWmin 32, DIFS=50us • An increase in number of EDCA users  The QAP broadcasts new CWmin 32 • AIFS cannot be smaller, DCF users keep their CWmin DCF and EDCA users are now having same priority, and therefore QoS of EDCA could be affected

  8. Problem Statement There is a need for mechanisms to: • Mitigate the impact of legacy DCF • Maintain priority of service for EDCA users - QoS

  9. Related Work • Swaminathan and Martin 2006 • Simulation Analysis on coexisting 802.11e/802.11b • Conclusions: • AIFS is best for delay, but would result in throughput starvation for DCF • to achieve fairness, both CWmin and AIFS should be adapted • G. Bianchi, I. Tinnirello, and L. Scalia 2005 • Conclusions in addition to those above • the increase of collisions due to small CW reduces the difference between EDCA and DCF • J. Majkowski and F. C. Palacio 2006 • Suggests a scheme to improve DCF performance when they have multimedia traffic • HTB (Hierarchal Token Bucket) discipline between IP layer and Layer 2 at legacy DCF users to classify, police, and schedule and shape incoming traffic. • Requires modifications to DCF users • Does not show how to solve coexistence effects

  10. Related Work • ACKS 2005 • A solution in which the QAP skips sending an ACK to a DCF user with probability S • Waste of time needed for transmitting successful packet and its ACK, which is still reserved because of the use of NAV by all others • Not good for wireless medium which is noisy • Proposed for a saturated network by fixing AIFS to DIFS for all ACs, CWmax=CWmin, and adapting CWmin to achieve weighted throughput ratios • A. Banchs, A. Azcorra, C. Garcia, and R. Cuevas 2005 • even if weighted throughputs are met, EDCA users may be affected • DCF users do not deploy TXOP limits

  11. Related Work • J. Majkowski and F. C. Palacio 2006 • A mechanism to prevent DCF users from starting a data transmission if such transmission would overlap with TBTT (Target Beacon Transmission Time). • Requires modification to DCF users • QAP broadcasts a parameter which is used by DCF users to determine when not to transmit • Divides beacon interval into • First period: all are contending. During this period, no solution to coexistence effects. Contention of DCF is accumulated • Second period: only EDCA are contending. What if no EDCA traffic is available?

  12. NZ-ACK • DCF/EDCA: duration of last ACK is 0 • New ACK Policy – NonZero-ACK • Only last ACK of ongoing transmission

  13. NZ-ACK • Implementation • Backward Compatibility • QAP issues NZ-ACK frames • EDCA users are only required to distinguish NZ-ACK from ACK frames • Transparent to DCF users • Frame Control Field of control frames (RTS, CTS, ACK) • All bits B8 – B15 except B12 are always 0

  14. NZ-ACK • Implementation • Last ACK: • recognize time to send last ACK • EDCA: duration is not enough for more frames • DCF: More Fragment bit (0; last one) of Frame Control Field of data frame fragment 14

  15. NZ-ACK • Main challenges: • When to issue NZ-ACK frames • What value should be used for the duration of an NZ-ACK

  16. NZ-ACK • Virtual Queues (VQ) at QAP • EDCA flow i: • Peak data rate • Average data rate • nominal data size (li) ri = average data rate for VBR Flow Utilization: ui = riTs / li Ts = AIFS+SIFS+Tdata+TACK QAP estimates active EDCA users

  17. Virtual Queue Management • Add one Virtual Packet to the VQ when • The packet arrived QAP (from QSTA) with piggybacked information to indicate more data in QSTA and the VQ is empty • Every ri • Drop one Virtual Packet when: • Received one packet from QSTA • a maximum delay is met – 100ms for voice • When used to issue an NZ-ACK frame • Empty the VQ when • piggybacked info indicates no more data • Queues ordered with smallest uifirst QAP estimates active EDCA users

  18. NZ-ACK • Issue NZ-ACK frames: • When there are non-empty virtual queues, and, • When UDCF_Measured>= UDCF , and, • With probability p • Duration of NZ-ACK: • dc = ucT • uc = utilization of first virtual frame found, which is also the smallest available. • The frame used to calculate dc is dropped. • T is a design parameter – beacon interval(s)

  19. NZ-ACK • Features: • No change to legacy users • Backward Compatibility • Adaptively provides control over legacy users • Minimal overhead • No change to IEEE 802.11 frames formats • All processing is at the QAP • Works with contention-based operations • EDCA, DCF

  20. Evaluation • Simulation • Opnet Modeler 11.5.A • Implement NZ-ACK, and ACKS by modifying 802.11e model • Compare to 802.11e EDCA, and ACKS • Performance measures: • Throughput • Total data bits transmitted successfully • Fairness Index (FI) [12,13] • Si : Throuput per user i • 0 <= FI <=1 • The closer FI to 1, the higher the fairness • We used FI to measure how fair among DCF users • Delay • delay of a data frame is measured from frame arrival at MAC until it is successfully received (until its ACK received correctly) • Retransmission Attempts • Number of retransmission attempts per data frame • Used as an indication of collision rates

  21. Evaluation • Saturation • Stations always have frames to transmit • 802.11g PHY, 54Mbps/24Mbps • EDCA: 50 users, Voice • DCF: 50 users • Compare to: • EDCA with 2 settings of CWmin/max • ACKS • OneSlot: an NZ-ACK frame is always issued with a duration of 1 slot

  22. Evaluation

  23. Evaluation OneSlot: • The best performance for EDCA • lowest delays, highest throughputs, highest ratio of EDCA throughputs to that DCF • High degradation of DCF users’ performance • Throughput is 20% lower than achieved with any other scenario • Lowest FI value

  24. Evaluation • NZ-ACK vs. ACKS • Average delay, and delay per EDCA are lower • e.g. with NZ-ACK2 by 6.7%, and 8.8% respectively • ACKS is about weighted throughput • Throughput ratio are 3/4 with both variants of NZ-ACK, and 3.4 with ACKS (3 is the goal) • Higher fairness with NZ-ACK variants • Retransmissions lower than with ACKS by 11%, 17.5%. • ACKS adds to collisions – skipping ACK • NZ-ACK reduces number of contending users

  25. Evaluation • NZ-ACK vs. EDCA • Higher EDCA throughput (6.67%, 7.99%) • Lower EDCA delay (10.9%, 13.2%) • Lower retransmissions (at least 14%) • NZ-ACK reduces number of contending users • Higher throughput ratios (46.7%, 42%)

  26. Evaluation • Overall network performance with NZ-ACK is higher than that with EDCA and ACKS • Highest total throughput • Lowest average delay • Lowest retransmission attempts • NZ-ACK reduces number of contending users

  27. Evaluation • Non-Saturation • 802.11b PHY, 11Mbps/1Mbps • DCF: • CWmin/CWmax 32/1024, DIFS 50us • starting with 1 users, another is added every 3 seconds • max 50 users • Traffic generator: Exponential(40ms), 1000 bytes per packet • EDCA: • CWmin/CWmax 32/64, DIFS 50us • 18 users with one voice flow per user • ON/OFF model • Both ON/OFF periods are Exponential(0.352 seconds) • G.711 (silence) encodes, 64kbps, 160 bytes per packet. • Simulation period: 170 seconds • T = Beacon Interval • Virtual packets dropped after a delay of 0.1 second

  28. Evaluation • Throughput • EDCA: slight enhancement starts at 40s, i.e. when there are about 14 DCF user – Almost the same • DCF users not affected • Delay, Delay Variation • 40s – 14 DCF users • With NZ-ACK, kept small

  29. Evaluation • Retransmissions • 40s -14 DCF users • Reduction of number of contending users • Delay (DCF) • EDCA: • up to 0.2s • Prob[delay>0.1s] > 0.2 • NZ-ACK: • less than 0.026s

  30. Conclusions • There is a need for mechanisms that address the coexistence of EDCA and DCF users in future IEEE 802.11 WLANs • NZ-ACK adaptively controls DCF users, and maintains priority of service of EDCA users while providing acceptable throughput performance for DCF users • NZ-ACK does not require any modification to legacy DCF users

  31. References [1] IEEE Std 802.11b-1999, “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Higher-Speed Physical Layer Extension in the 2.4 GHz Band.” [2] IEEE Std 802.11g-2003, “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications Amendment 4: Further Higher Data Rate Extension in the 2.4 GHz Band.” [3] IIEEE Std 802.11a-1999, “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications Amendment 1: High-speed Physical Layer in the 5 GHz band.” [4] IEEE Std 802.11e-2005, “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications. Amendment 8: Medium Access Control (MAC) Quality of Service Enhancements.” [5] A. Swaminathan and J. Martin, “Fairness issues in hybrid 802.11b/e networks,” in Consumer Communications and Networking Conference, 3rd IEEE CCNC, vol. 1, 8-10 Jan 2006, pp. 50–54. [6] G. Bianchi, I. Tinnirello, and L. Scalia, “Understanding 802.11e contention-based prioritization mechanisms and their coexistence with legacy 802.11 stations.” IEEE Network, vol. 19, no. 4, pp. 28–34, 2005. [7] J. Majkowski and F. C. Palacio, “Coexistence of ieee 802.11b and ieee 802.11e stations in qos enabled wireless local area network.” in Wireless and Optical Communications, A. O. Fapojuwo and B. Kaminska, Eds. IASTED/ACTA Press, 2006, pp. 102–106. [8] L. Vollero, A. Banchs, and G. Iannello, “Acks: a technique to reduce the impact of legacy stations in 802.11e edcawlans,” in Communications Letters, IEEE, vol. 9, no. 4, April 2005, pp. 346–348. [9] A. Banchs, A. Azcorra, C. Garcia, and R. Cuevas, “Applications and challenges of the 802.11e edca mechanism: an experimental study.” IEEE Network, vol. 19, no. 4, pp. 52–58, 2005. [10] J. Majkowski and F. C. Palacio, “Qos protection for ieee 802.11e in wlan with shared edca and dcf access.” in Communication Systems and Networks, C. E. P. Salvador, Ed. IASTED/ACTA Press, 2006, pp. 43–48. [11] Opnet, “Opnet Modeler.” www.opnet.org. [12] D. R. Jain, Chiu, and W. Hawe, “A Quantitative Measure of Fairness and Discrimination for Resource Allocation in Shared Computer Systems,” DEC Research Report TR-301, September 1984. [13] C. Koksal, H. Kassab, and H. Balakrishnan, “An Analysis of Short- Term Fairness in Wireless Media Access Protocols,” In Proc. of ACM SIGMETRICS, 2000.

  32. Thank You Questions?

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