Improving Quality of Service for VoIP Traffic in IEEE 802.11 Wireless Networks

1. Improving Quality of Service for VoIP Traffic in IEEE 802.11 Wireless Networks Sangho Shin Henning Schulzrinne

2. Introduction • Increased Usage of VoIP service over wireless networks • Widely deployed WLANs • Shopping Malls • Coffee shops • Streets • Parks • Wi-Fi phones • New service plans • Sprint – cellular and Wi-Fi • Limited QoS support in 802.11 WLANs • IEEE 802.11e ? [skyhook] Map of APs in Manhattan

3. Outline • Overview of QoS problems • My earlier work for the problems • Fair resource distribution b/w uplink and downlink using APC • Call Admission Control with QP-CAT • Conclusion • Future work

4. Introduction Framework of VoIP over WLANs Internet Router Router PBX 160.38.x.x 128.59.x.x AP AP Wireless client

5. Introduction QoS problems of VoIP service • The network disruption during L2/L3 handoff • Fast L2/L3 handoff • Limited VoIP capacity • Dynamic PCF (DPCF) • Unbalanced uplink and downlink delay • Adaptive Priority Control (APC) • Significant deterioration of QoS in the case of channel congestion • Call admission control using QP-CAT

6. Outline • Overview of QoS problems • My earlier work for the problems • Fair resource distribution b/w uplink and downlink (APC) • Call Admission Control with QP-CAT. • Conclusion • Future work • Seamless layer-2 handoff • Fast layer-3 handoff • Dynamic PCF

7. Earlier work L2 handoff trigger Probe request (broadcast) Probe responses Probe delay Authentication request Authentication delay Authentication response Association request Association delay Association response Fast layer-2 handoff [26](1/2) Wireless client All APs Router 300m ~ 1s AP AP New AP 2 ms 2 ms [26] Sangho Shin, Andrea G. Forte, Anshuman Singh Rawat, and Henning Schulzrinne. ReducingMAC layer handofflatency in IEEE 802.11 wireless LANs. In ACM MobiWac '04, pages 19~26, New York, NY

8. Earlier work ms Fast layer-2 handoff (2/2) • Selective Scanning • Scan non-overlapping channels first • Channel 1, 6, and 11 in 802.11b • Reduces the scanning time to 1/3 • Caching • Motivated by locality • Store the scanned AP information in a cache • Use it in the future handoffs • Handoff without scanning • Reduces the total handoff time to 4 ms • No changes in the infrastructure  Practical solution! Experiments in 802.11b WLANs

9. Earlier work DHCP Discover DHCP Offer DHCP Request DHCP procedure DHCP ACK DAD Fast layer-3 handoff [9](1/2) Router Router • Subnet change detection • A few minutes • New IP acquisition- DHCP • 1 second 160.38.x.x 128.59.x.x AP AP L3 handoff L2 handoff DAD: Duplicate Address Detection [9] Andrea Forte, Sangho Shin, and Henning Schulzrinne. Improving layer 3 handoff delay in IEEE 802.11 wirelessnetworks. In WICON, Aug 2006.

10. Earlier work L2handoff DHCP Request DHCP NACK Subnet change DHCP Discover ARP timeout Update sessions DHCP Offer Update sessions DHCP Request DHCP Ack ms Total handoff time Experiments in 802.11b Fast layer-3 handoff (2/2) • Fast subnet change detection • Broadcast a bogus DHCP request • The DHCP server responds with DHCP NACK • Check the IP address of the DHCP server • Temporary IP address • Scan potentially unused IP addresses in the new subnet • Transmit multiple ARP packets • Pick a non-responded IP address as a temporary IP address • Use it until a new IP address is assigned by the DHCP server • Revisits the previous subnets • IP address lease not expired  Use the old IP address! 180 30

11. Earlier work Contention Free Repetition Interval Contention Period (CP) Contention Free Period (CFP) Beacon CF-End DCF poll poll Poll+data poll poll data data data Null Null Dynamic PCF (DPCF) [18] (1/2) • PCF (Point Coordination Function) • Polling based media access • No contention, no collision • Polling overhead • No data to transmit  Unnecessary polls waste bandwidth • Big overhead, considering the small VoIP packet size. Polling overhead [18] Takehiro Kawata, Sangho Shin, and Andrea G. Forte. Using dynamic PCF to improve the capacity for VoIP traffic in IEEE 802.11 networks. In WCNC, IEEE, vol 3,pages 13~17, Mar 2005.

12. Earlier work Delay (90th%tile) and throughput of 28 VoIP sources and data traffic 312 Capacity for 64kb/s VBR VoIP traffic Queue 45 183 32% 40 DPCF 37 Polling List 1 2 3 4 5 6 7 8 7 26 29 3000 600 22 11 Data throughput 35 6 67 30 PCF 28 CFP CP VoIP throughput 18 30 5 2500 500 545 24 28 2 3 4 7 8 1 DCF 25 PCF 487 PCF PCF PCF 5 6 7 23 Number of VoIP Flows 2000 400 20 Throughput (kb/s) 1 3 8 400 14 15 End-to-End Delay (ms) 1500 300 13 Null Null ACK ACK ACK 7 10 7 DCF Dynamic Polling List 1 3 8 1000 200 PCF 5 DPCF CFP CP 0 500 100 0 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 Transmission Rate (Mb/s) 5 6 7 0 0 25 8 1 3 0 1 2 3 DCF DCF DCF DCF DPCF DPCF DPCF DPCF Number of Data Sessions ACK ACK ACK Dynamic PCF (DPCF) (2/2) • Dynamic Polling List • Store only “active” nodes • Higher priority for VoIP Simulation results in 802.11b

13. Outline • Overview of QoS problems • My earlier work for the problems • Fair resource distribution b/w uplink and downlink using APC in DCF • Call Admission Control with QP-CAT • Conclusion • Future work • Motivation • Adaptive Priority Control • Simulation results • Related work • Conclusion

14. APC Solution ? Give a higher priority to the AP Motivation • Big gap between uplink and downlink delay • Why? - DCF • The same chance to transmit frames among nodes • In VoIP traffic, the AP has more packets to transmit than each client 64kb/s VBR VoIP traffic 802.11b 11 Mb/s via simulations DCF = Distributed Coordination Function

15. APC Tx in DCF DIFS BO frame Channel DIFS = DCF Inter-Frame Spacing Node A Node B backoff Node A Node B How to control the priority? (1/2) • Control Contention Window (CW) • Shorter CW  smaller backoff time  higher priority • Hard to control • Backoff = random (0, CW) • Higher collision rate • Control Inter-Frame Spacing (IFS) • Increase IFS  lower priority • Not accurate because of BO • Increase the delay due to the increased IFS

16. APC How to control the priority? (2/2) • Contention Free Transmission (CFT) • Transmit frames w/o backoff • Control the number of frames to be sent contention free • Accurate priority control • No overhead • Reduce the overall transmission time  Improve the capacity Priority of node A is 3 times of node B Node A Node B

17. APC Optimal priority of the AP (P) • Intuitive method • P the number of wireless clients (N) • Adapts to change in the number of VoIP sources • Not adapts to change in the traffic volume • APC • P QAP/QNodes • QAP is the number of packets in the queue of the AP • QNodes is the average number of packets in the queue all nodes • Adapts to instant change of uplink and downlink traffic load

18. APC APC – example 1 queue DS QAP = 4 QNode = 1 P = 4/1 = 4 AP queue queue queue queue

19. APC APC – example 2 queue DS QAP = 4 QNode = 2 P = 4/2 = 2 AP queue queue queue queue

20. APC APC – example 2 queue DS QAP = 2 QNode = 1 P = 2/1 = 2 AP queue queue queue queue

21. APC DCF APC Threshold Threshold Capacity Capacity Simulation results of APC (1/3) 802.11b 11Mb/s 64kb/s VBR traffic 20ms pkt intvl 0.39 activity ratio 28 Calls  35 calls (25%)

22. APC Simulation results of APC (2/3) 90th%tile delay of VoIP traffic (35 calls)

23. APC 10ms + 20ms Packetization Interval 20ms + 40ms Packetization Interval Simulation results of APC (3/3)

24. APC Related work • Many papers about fairness in throughput ([Deng et. al. IEICE Trans.Comm.], [Barry et. al. Infocom ’01], [Aad Infocom ’01], [Tickoo et. al. Infocom ’04]) • In VoIP traffic, Jain’s Fairness Index is almost 1 regardless of the big gap • Wang et al. [AINA ’04] - New Fair MAC • Fairness between clients • Clients transmit all packets consecutively at most for Maximum Transmission Time (MTT) • Improved only a few ms  low uplink delay • Casetti et al. [PIMRC ’04] • Based on EDCA • Found a fixed optimal CW by simulations • Improved the capacity by 15% • Not a global solution for all traffic types

25. APC Conclusion • Uplink and downlink delay of VoIP traffic in DCF are significantly unbalanced  Unfair resource distribution b/w uplink and downlink • APC balances the uplink and downlink delay very well by allowing the AP to transmit QAP/QNodes packets using Contention Free Transmission (CFT) • APC improves the capacity for VoIP traffic from 28 calls to 35 calls, by 25%

26. APC Future work • Implement APC in 802.11e and evaluate it with various background traffic • Implement APC using the Mad-Wifi driver and evaluate the performance in the OBRIT test-bed • Improve APC so that it does not require client information

27. Outline • Overview of QoS problems • My earlier work for the problems • Fair resource distribution b/w uplink and downlink using APC • Call Admission Control with QP-CAT • Conclusion • Future work • Motivation and requirements • Metric for admission control • Queue size Prediction using CAT • Simulation results • Related work • Conclusion

28. Motivation • When channel is congested, delay of all VoIP flows significantly increases Experimental results in the ORBIT test-bed • 64kb/s CBR VoIP traffic • 802.11b • 11Mb/s • 20ms packet interval

29. Admission Control using QP-CAT Requirements for CAC • Accurate (Guarantee QoS of existing flows) • Need a good metric for QoS • Fast • Users should not wait for a long time to call • Efficient • Minimum waste of bandwidth • Flexible (extensible) • Many types of VoIP traffic need to be supported

30. Admission Control using QP-CAT 802.11b 11 Mb/s 64 kb/s CBR 20 ms Pkt Intvl Experimental results in the ORBIT test-bed Metric (1/3) • Queue size of the AP (The number of packets in the queue of the AP) • Correlation b/w the queue size and downlink

31. Admission Control using QP-CAT Downlink delay Queuing delay Transmission delay Processing time of the AP Queue size of the AP According to computation using 802.11b parameters Metric (2/3) • Theoretical model

32. Admission Control using QP-CAT Errors according to the delay Cumulative Distribution Function of errors Metric (3/3) • Errors of the model

33. Admission Control using QP-CAT Queue size Prediction (QP) • How to use the metric ? • When the queue size goes beyond a threshold, then reject the future calls ? • Too late  QoS already deteriorated • Cannot disconnect already admitted flows • See the future! • Need to predict the queue size in advance

34. Admission Control using QP-CAT Emulate VoIP traffic Compute Additional Transmission Decrease the queue size Predict the future queue size Additional transmission + Packets from a new flow channel Actual packets additional packets current packets Computation of Additional Transmission (CAT) (1/5) • Basic concepts • Emulate the additional VoIP flows Predict the queue size

35. Admission Control using QP-CAT CAT (2/5) • Emulation of VoIP flows • Two counters: DnCounter, UpCounter • Follow the same behavior of VoIP flows • Increase the counters every packetization interval of the flows • Decrement the counters alternatively Example : 20ms packetization interval time 20ms 20ms DnCounter++ UpCounter++ DnCounter++ UpCounter++ DnCounter++ UpCounter++

36. Admission Control using QP-CAT CAT (3/5) • Computation of transmission time of a VoIP frame (Tt) Tt = DIFS + Tb + Tv + SIFS + TACK VoIP packet SIFS DIFS backoff ACK frame TACK Tb Tv Tt PLCP MAC IP UDP RTP Voice data

37. Admission Control using QP-CAT Tc= t2 - t1 Busy medium Tr Tt Tt DnCounter-- UpCounter-- CAT (4/5) • Computing Additional Transmission (np) t2 t1

38. CAT (5/5) • Additional considerations • Collision • Deferral • Serialization of downlink packets

39. 17 Calls 18 Calls 17 calls + 1 call 16 calls + 1 call 16 calls 17 calls Another call is added Simulation results (1/3) 32 kb/s VoIP traffic with 20ms packetization interval

40. 14 calls + 1 call 15 calls + 1 call 16 calls 15 calls 14 calls 15 calls Simulation results (2/3) 64 kb/s VoIP traffic with 20ms packetization interval

41. 29 calls + 1 call 30 calls + 1 call 30 calls 31 calls 29 calls 30 calls Simulation results (3/3) 32 kb/s VoIP traffic with 40ms packetization interval

42. Related work (1/3) • Numerical or theoretical approaches • Yang Xiao et al. [Communication Magazine ‘04 ] • 802.11e EDCA based access control • Compute available bandwidth using TXOPs and announce it to clients • Guarantee bandwidth, but not delay Applicable Video traffic only • Pong et al. [Globecom’03] • Estimate available bandwidth using an analytical model • Check if the requested BW available by changing CW/TXOP • The assumption of the analytical model is far from real environment • Kuo et al [Globecom’03] • Pure analytical model based • Expected bandwidth and delay are computed using an analytical model

43. Related work (2/3) • CBR/CUE - Sachin et al. [Globecom’03] and Zhai et al. [QShine04 ] • New metric : Channel Utilization Estimate (CUE)/Channel Business Ratio (CBR) = fraction of time per time unit needed to transmit the flow • CUE per flow is computed using the average Tx rate of each flow • Clients compute the CBR from their Tx rate and send it to the AP regularly. • Compare the remaining CUE and the requested CUE • Assume 15% of wasted bandwidth due to collision or fluctuation  0.85 max total CUE • Actual probing • Metric: delay and packet loss • Used for wired networks • Very accurate and simple • Waste a certain amount of bandwidth

44. Related work (3/3) • Comparison

45. Conclusion • The queue size of the AP is a good indicator for QoS of all existing calls • QP-CAT can predict the future queue size of the AP very well • We can perform call admission control using QP-CAT accurately and efficiently

46. Future work • Evaluate the QP-CAT using various types of background traffic • Implement call admission control framework using QP-CAT • Evaluate the efficiency of CAC with QP-CAT using actual call arrival rate and call duration

47. Contributions • APC • Introduced a New dynamic priority control method, CFT and showed it is better than others (CW, IFS). • Verified that the optimal priority of the AP for balancing uplink and downlink delay is QAP/QNode, using CFT. • Increased the VoIP capacity by 25% • QP-CAT • Verified the linear correlation between the number of packets in the queue of the AP and downlink delay of VoIP traffic via experiments • Enabled the AP to predict the future downlink delay accurately using CAT • Protect the QoS of the existing VoIP traffic in fluctuating channel conditions, minimizing the waste of bandwidth

48. Timetable

49. Thank you

50. References (1/3) [1] W. Arbaugh A. Mishra, M. Shin. An Empirical Analysis of the IEEE 802.11MAC Layer Handoff Process. ACMSIGCOMM Computer Communication Review, 33(2):93~102, April 2003. [2] I Aad and C Castelluccia. Differentiation mechanism for IEEE 802.11. In IEEE INFOCOM, pages 209~218,Apr 2001. [3] N. Akhtar, M. Georgiades, C. Politis, and R. Tafazolli. SIP-based end system mobility solution for all-IP infrastructures.In IST Mobile & Wireless Comm. Summit 2003, June 2003. [4] M Barry, A T Campbell, and A Veres. Distributed control algorithms for service differentiation in wirelesspacket networks. In IEEE INFOCOM, pages 582~590, Apr 2001. [5] G. Camarillo, W. Marshall, and J. Rosenberg. Integration of Resource Management and Session InitiationProtocol (SIP). RFC 3312, IETF, Oct 2002. [6] Casetti, C. Chiasserini, and C.F. Improving fairness and throughput for voice traffic in 802.11e EDCA. PIMRC 2004. vol 1, pages 525~530, 2004. [7] D J Deng, R S Chang, and A Veres. A priority scheme for IEEE 802.11 DCF access method. IEICE Trans.Commun., E82-B(1):96 ~ 102, Oct 1999. [8] R. E. Droms. Dynamic Host Conguration Protocol (DHCP). RFC 2131, Internet Engineering Task Force,March 1997. [9] Andrea Forte, Sangho Shin, and Henning Schulzrinne. Improving layer 3 handoff delay in IEEE 802.11 wirelessnetworks. In WICON 2006, Aug 2006. [10] Sachin Garg and M. Kappes. Admission control for VoIP trafc in IEEE 802.11 networks. In GLOBECOM,pages 3514~3518, Dec 2003.