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Quality of Service Support in Wireless Networks. Hongqiang Zhai http://www.ecel.ufl.edu/~zhai Wireless Networks Laboratory Department of Electrical and Computer Engineering University of Florida In Collaboration with Dr. Xiang Chen and my advisor Professor Yuguang ``Michale’’ Fang. Outline.

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Quality of Service Support in Wireless Networks


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    1. Wireless Networks Laboratory (WINET) Quality of Service Support in Wireless Networks Hongqiang Zhai http://www.ecel.ufl.edu/~zhai Wireless Networks Laboratory Department of Electrical and Computer Engineering University of Florida In Collaboration with Dr. Xiang Chen and my advisor Professor Yuguang ``Michale’’ Fang

    2. Wireless Networks Laboratory (WINET) Outline • Introduction • Performance analysis of the IEEE 802.11 MAC protocol • A call admission and rate control scheme • Conclusion and future research issues

    3. Wireless Networks Laboratory (WINET) Wireless Landscape

    4. Wireless Networks Laboratory (WINET) Wireless Local Area Networks/ Wi-Fi Hot Spots • Web traffic • Email • Streaming video • Instant messaging • Gaming over IP • Voice over IP over Wi-Fi Next Call May Come from a Wireless Hot Spot

    5. Wireless Networks Laboratory (WINET) Mobile Ad Hoc Networks and Wireless Mesh Networks

    6. Wireless Networks Laboratory (WINET) Quality of Service (QoS) Requirements • Bandwidth • Delay and delay jitter • Packet loss rate

    7. Wireless Networks Laboratory (WINET) Challenges • Unreliable physical channel • Time-varying propagation characteristics • Interference • Limited bandwidth • Limited processing power and battery life • Distributed control • Mobility

    8. Wireless Networks Laboratory (WINET) Medium Access Control • Coordinate channel access • Reduce collision • Efficiently utilize the limited wireless bandwidth B D C A

    9. DIFS SIFS SIFS SIFS Backoff DIFS … NAV(RTS) RTS NAV(CTS) Backoff Transmitter A Receiver B … ACK Others Wireless Networks Laboratory (WINET) IEEE 802.11 Distributed Coordinate Function (DCF) MAC Protocol • Carrier sense multiple access with collision avoidance (CSMA/CA) • Carrier sensing • Physical Carrier Sensing • Virtual Carrier Sensing • Interframe Spacing (IFS) • Short IFS (SIFS) < DCF IFS (DIFS) • Binary Exponential Backoff • Randomly chosen from [0, CW] • CW doubles in case of collision Contention based MAC Can it support QoS requirements of various applications? Request to send DATA RTS DATA CTS ACK Acknowledge Clear to send

    10. Wireless Networks Laboratory (WINET) Previous Work on Performance Analysis of the IEEE 802.11 MAC Standard • Previous studies focus on saturated case • Each device always has packets in the system and keeps contending for the shared channel. • Collision probability is very high • Delay performance is very bad • Only throughput and average delay have been derived. • Related work • Bianchi, JSAC March 2000 • Cali et al., IEEE/ACM Tran. Networking, Dec. 2000 QoS requirements of real-time services can not be guaranteed if there are many contending users?

    11. Wireless Networks Laboratory (WINET) Previous Work on Supporting QoS in WLANs • Service differentiation • Provide different channel access priorities for different services by differentiating • Contention window • Interframe spacing (IFS) • IEEE 802.11e draft (based on 802.11b) • Related work • Ada and Castelluccia, Infocom’01 (CW, IFS) • Veres et al., JSAC Oct. 2001 (real-time measurement in virtual MAC) • S.T. Sheu and T.F. Sheu, JSAC Oct. 2001 (real-time traffic periods) • S. Mangold et al., Wireless Communications Dec. 2003 (802.11e) Service differentiation is still not enough to meet the strict QoS requirements Can the IEEE 802.11 MAC protocol do better than service differentiation? • Research issues • Performance in both non-saturated and saturated case • Probability distribution of medium access delay

    12. 3 3 2 2 1 Wireless Networks Laboratory (WINET) MAC Service Time Packet arrival • Probability Generating Function (PGF) • Pr{Ts=tsi}=pi (0 ≤ i < ∞) • MAC service time is discrete in value • SIFS, DIFS, EIFS • Backoff time is measured in time slots • Packet to be transmitted is also discrete in length Transmit queue 3 MAC 1 2 3

    13. Wireless Networks Laboratory (WINET) MAC Service Time • Generalized state transition diagram (GSTD) • Mark the PGF of the transition time on each branch along with the transition probability • PGF of the transition time between two states is the corresponding system transfer function start end • Widely used method • Calculate the average # of retransmissions NR = p/(1-p) • Average transition time is NR × τ1 + τ2=

    14. Wireless Networks Laboratory (WINET) MAC Service Time of IEEE 802.11 State variable (j, k): j is the backoff stage, k is the backoff timer Wj: the contention window at backoff stage j p: collision probability perceived by a node : maximum # of retransmissions

    15. MAC service time (ms) PDF Collision probability p payload size = 8000 bits, with RTS/CTS MAC service time (ms) Wireless Networks Laboratory (WINET) MAC Service Time of IEEE 802.11 Observation: When p is small, both the mean and standard deviation of MAC service timeare small.

    16. TS 3 2 TR 1 Wireless Networks Laboratory (WINET) Delay and Delay Variation Packet arrival Transmit queue TW MAC

    17. Channel utilization: Normalized throughput: Channel busyness ratio: With RTS/CTS n: # of nodes : the prob. that a node transmits in any slot Without RTS/CTS Wireless Networks Laboratory (WINET) Network Throughput (Tidl pi )(Tcol pc)(Tsuc ps )

    18. Wireless Networks Laboratory (WINET) Network Throughput Maximum throughput with good delay performance Collision Probability p Channel Busyness Ratio is an accurate, robust, and easily obtained sign of network status.

    19. Wireless Networks Laboratory (WINET) Packet Loss Rate • Given the collision probability p, the MAC layer may drop the packet with the probability Avg. queue length Pkt loss rate Channel busyness ratio

    20. Wireless Networks Laboratory (WINET) Model Validation Channel Busyness Ratio The optimal operating point denoted by Umax • Simulation settings • 50 nodes, RTS/CTS mechanism is used • Each node has the same traffic rate. • We monitor the performance at different traffic rates.

    21. Wireless Networks Laboratory (WINET) Call Admission and Rate Control (CARC)

    22. Wireless Networks Laboratory (WINET) Call Admission Control • Channel utilization/channel busyness ratio for a flow • Admission control test R: flow data rate (bps) L: average packet length (bits) Up to Urt (= γUmax, 0<γ<1) can be assigned to real-time traffic

    23. Wireless Networks Laboratory (WINET) Rate control • Notation: • r: Channel resource r allocated to each node • Allowable channel time occupation ratio • tp: channel time for packet p • Time that a successful transmission of packet p will last over the channel. • ∆: scheduled interval • Time between two consecutive packets that DRA passes to the MAC layer • br: channel busyness ratio • brth = Umax

    24. Wireless Networks Laboratory (WINET) Rate control • Initialization Procedure: r=rstart • Three-Phase Resource Allocation Mechanism: • multiplicative-increase if underloaded, i.e., br < BM=α×brth • Additive-increase if moderately loaded, i.e., BM ≤ br < brth • Multiplicative-decrease if heavily loaded, i.e., br ≥ brth

    25. Wireless Networks Laboratory (WINET) Theoretical Results of CARC Convergence of Multiplicative-Increase Phase

    26. Wireless Networks Laboratory (WINET) Theoretical Results of CARC Convergence to Fairness Equilibrium

    27. Wireless Networks Laboratory (WINET) Simulation Studies • Simulation settings in ns2 • Channel rate = 11 Mbps • Voice traffic with an on-off model • The on and off periods are exponentially distributed with an average value of 300 ms each. • During on periods, traffic rate is 32kb/s with a packet size of 160 bytes. • Greedy best effort traffic • Saturated CBR traffic with a packet size of 1000bytes.

    28. Wireless Networks Laboratory (WINET) Throughput and MAC delay Each node is a source of greedy traffic CARC improves the throughput by up to 71.62% with RTS/CTS, and by up to 157.32% without RTS/CTS CARC achieves up to 95.5% of maximum throughput with and without RTS/CTS

    29. Throughput Throughput Time (s) Time (s) Wireless Networks Laboratory (WINET) Fairness A new greedy node joins the network every other 10 seconds Higher aggregate throughput Fairness convergence speed: 0-2 s Short term fairness

    30. Wireless Networks Laboratory (WINET) Quality of Service for Voice Traffic 50 greedy nodes A new voice node joins the network every other 10 seconds.

    31. Wireless Networks Laboratory (WINET) Conclusion • The IEEE 802.11 MAC protocol can support strict QoS requirements of real-time services while achieving maximum throughput. • Channel busyness ratio is a good network status indicator of the IEEE 802.11 systems. • An efficient call admission and rate control framework is proposed to provide QoS for real-time service and also to approach the maximum throughput.