1. 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. 2 Outline Introduction IEEE 802.11 IEEE 802.11 Problem Statement Related Work Proposed Solution - NZ-ACK EvaluationConclusions

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

4. 4 Introduction DCF: DIFS DIFS CW min, CW min, CW max CW maxEDCA: AIFS[AC] AIFS[AC] CW min [AC], CW max [AC] CW min [AC], CW max [AC] TXOP[AC] TXOP[AC]Both: Duration of frames used for NAV Duration of frames used for NAV

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

6. 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. 7 Problem Statement Simple example: A WLAN of EDCA and DCF users A WLAN of EDCA and DCF users EDCA users: VoIP, CW min 8, AIFS=50us EDCA users: VoIP, CW min 8, AIFS=50us DCF users: CW min 32, DIFS=50us DCF users: CW min 32, DIFS=50us An increase in number of EDCA users  The QAP broadcasts new CW min 32 An increase in number of EDCA users  The QAP broadcasts new CW min 32 AIFS cannot be smaller, DCF users keep their CW min AIFS cannot be smaller, DCF users keep their CW min DCF and EDCA users are now having same priority, and therefore QoS of EDCA could be affected

8. 8 Problem Statement There is a need for mechanisms to: 1.Mitigate the impact of legacy DCF 2.Maintain priority of service for EDCA users - QoS

9. 9 Related Work Swaminathan and Martin 2006 Simulation Analysis on coexisting 802.11e/802.11b Simulation Analysis on coexisting 802.11e/802.11b Conclusions: Conclusions: AIFS is best for delay, but would result in throughput starvation for DCF to achieve fairness, both CW min and AIFS should be adapted G. Bianchi, I. Tinnirello, and L. Scalia 2005 Conclusions in addition to those above 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 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. 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 Requires modifications to DCF users Does not show how to solve coexistence effects Does not show how to solve coexistence effects

10. 10 Related Work ACKS 2005 A solution in which the QAP skips sending an ACK to a DCF user with probability S 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 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 Not good for wireless medium which is noisy Proposed for a saturated network by fixing AIFS to DIFS for all ACs, CW max =CW min, and adapting CW min to achieve weighted throughput ratios Proposed for a saturated network by fixing AIFS to DIFS for all ACs, CW max =CW min, and adapting CW min to achieve weighted throughput ratios 2005 A. Banchs, A. Azcorra, C. Garcia, and R. Cuevas 2005 even if weighted throughputs are met, EDCA users may be affected even if weighted throughputs are met, EDCA users may be affected DCF users do not deploy TXOP limits DCF users do not deploy TXOP limits

11. 11 Related Work 2006 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). 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 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 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. 12 NZ-ACK DCF/EDCA: duration of last ACK is 0 New ACK Policy – NonZero-ACK Only last ACK of ongoing transmission Only last ACK of ongoing transmission

13. 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 All bits B8 – B15 except B12 are always 0 except B12 are always 0

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

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

16. 16 NZ-ACK Virtual Queues (VQ) at QAP EDCA flow i: Peak data rate Peak data rate Average data rate Average data rate nominal data size (l i ) nominal data size (l i ) r i = average data rate for VBR Flow Utilization: u i = r i T s / l i T s = AIFS+SIFS+T data +T ACK QAP estimates active EDCA users

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

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

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

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

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

22. 22 Evaluation

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

24. 24 Evaluation NZ-ACK vs. ACKS Average delay, and delay per EDCA are lower Average delay, and delay per EDCA are lower e.g. with NZ-ACK 2 by 6.7%, and 8.8% respectively e.g. with NZ-ACK 2 by 6.7%, and 8.8% respectively ACKS is about weighted throughput 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) 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 Higher fairness with NZ-ACK variants Retransmissions lower than with ACKS by 11%, 17.5%. Retransmissions lower than with ACKS by 11%, 17.5%. ACKS adds to collisions – skipping ACK ACKS adds to collisions – skipping ACK NZ-ACK reduces number of contending users NZ-ACK reduces number of contending users

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

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

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

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

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

30. 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. 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 edca wlans,” 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.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. 32 Thank You Questions?