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Fair and Efficient Resource Allocation with QoS Support over ...
 

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    Fair and Efficient Resource Allocation with QoS Support over ... Fair and Efficient Resource Allocation with QoS Support over ... Presentation Transcript

    • 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
      • Introduction
      • Performance analysis of the IEEE 802.11 MAC protocol
      • A call admission and rate control scheme
      • Conclusion and future research issues
    • Wireless Landscape
    • Wireless Local Area Networks/ Wi-Fi Hot Spots
              • Instant messaging
              • Gaming over IP
              • Voice over IP over Wi-Fi
          • Web traffic
          • Email
          • Streaming video
      Next Call May Come from a Wireless Hot Spot
    • Mobile Ad Hoc Networks and Wireless Mesh Networks
    • Quality of Service (QoS) Requirements
      • Bandwidth
      • Delay and delay jitter
      • Packet loss rate
    • Challenges
      • Unreliable physical channel
        • Time-varying propagation characteristics
        • Interference
      • Limited bandwidth
      • Limited processing power and battery life
      • Distributed control
      • Mobility
    • Medium Access Control
      • Coordinate channel access
        • Reduce collision
        • Efficiently utilize the limited wireless bandwidth
      A B C D
    • 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)
      RTS CTS DATA ACK
        • Binary Exponential Backoff
          • Randomly chosen from [0, CW]
          • CW doubles in case of collision
      Request to send Clear to send Acknowledge DATA Contention based MAC Can it support QoS requirements of various applications? DIFS Backoff SIFS SIFS SIFS NAV(RTS) NAV(CTS) DIFS RTS … Backoff Transmitter Receiver Others B A ACK …
    • 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?
    • 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
    • MAC Service Time
      • Probability Generating Function (PGF)
        • Pr{Ts=t si }=p i (0 ≤ i < ∞)
      1 2 3 3 Transmit queue MAC
      • 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
      Packet arrival 1 2 3 2 3
    • 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 N R = p/(1-p)
        • Average transition time is N R × τ 1 + τ 2 =
    • MAC Service Time of IEEE 802.11 State variable (j, k): j is the backoff stage, k is the backoff timer W j : the contention window at backoff stage j p: collision probability perceived by a node  : maximum # of retransmissions
    • MAC Service Time of IEEE 802.11 Observation: When p is small, both the mean and standard deviation of MAC service time are small. MAC service time (ms) PDF Collision probability p payload size = 8000 bits, with RTS/CTS MAC service time (ms)
    • Delay and Delay Variation Transmit queue MAC Packet arrival T W 1 2 3 T S T R
    • Network Throughput ( T idl p i ) ( T col p c ) ( T suc p s ) Channel utilization : Normalized throughput : Channel busyness ratio : With RTS/CTS Without RTS/CTS n: # of nodes  : the prob. that a node transmits in any slot
    • Network Throughput Collision Probability p Channel Busyness Ratio is an accurate, robust, and easily obtained sign of network status. Maximum throughput with good delay performance
    • 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
    • Model Validation Channel Busyness Ratio The optimal operating point denoted by U max
      • Simulation settings
            • 50 nodes, RTS/CTS mechanism is used
            • Each node has the same traffic rate.
            • We monitor the performance at different traffic rates .
    • Call Admission and Rate Control (CARC)
    • 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 U rt (= γ U max , 0< γ < 1 ) can be assigned to real-time traffic
    • Rate control
      • Notation:
        • r : Channel resource r allocated to each node
          • Allowable channel time occupation ratio
        • t p: 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
          • br th = U max
    • Rate control
      • Initialization Procedure: r=r start
      • Three-Phase Resource Allocation Mechanism:
        • multiplicative-increase if underloaded, i.e., br < B M = α ×br th
        • Additive-increase if moderately loaded, i.e., B M ≤ br < br th
        • Multiplicative-decrease if heavily loaded, i.e., br ≥ br th
    • Theoretical Results of CARC Convergence of Multiplicative-Increase Phase
    • Theoretical Results of CARC Convergence to Fairness Equilibrium
    • 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.
    • Throughput and MAC delay 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 Each node is a source of greedy traffic
    • Fairness Higher aggregate throughput Short term fairness Fairness convergence speed: 0-2 s A new greedy node joins the network every other 10 seconds Throughput Time (s) Throughput Time (s)
    • Quality of Service for Voice Traffic 50 greedy nodes A new voice node joins the network every other 10 seconds. 0.0811 s 0.0406 s 99%ile 97%ile
    • 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.