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A Distributed Mechanism for Power Saving in IEEE 802.11 Wireless LANs

A Distributed Mechanism for Power Saving in IEEE 802.11 Wireless LANs. LUCIANO BONONI MARCO CONTI LORENZO DONATIELLO ΠΑΡΟΥΣΙΑΣΗ :ΜΑΝΙΑΔΑΚΗΣ ΑΠΟΛΛΩΝ. Introduction(1/1). Power Save-Distributed Contention Control (PS-DCC) used on top of the IEEE 802.11 WLANs

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A Distributed Mechanism for Power Saving in IEEE 802.11 Wireless LANs

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  1. A Distributed Mechanism for Power Saving in IEEE 802.11 Wireless LANs LUCIANO BONONI MARCO CONTI LORENZO DONATIELLO ΠΑΡΟΥΣΙΑΣΗ :ΜΑΝΙΑΔΑΚΗΣ ΑΠΟΛΛΩΝ

  2. Introduction(1/1) • Power Save-Distributed Contention Control (PS-DCC) used on top of the IEEE 802.11 WLANs • Power Saving Strategy at the MAC level • Wireless ad hoc networks • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) • Based on Distributed Coordination Function (DCF) • Maximize channel utilization and QoS • No power wasted due to the collisions and Carrier Sensing • Balancing the power consumed by the NI in the transmission and reception phases • Adaptive to the congestion variations

  3. IEEE 802.11 Standard DCF for WLANs(1/2) • Active stations perform Carrier Sensing activity • DIFS-Basic Access mechanism • Collision Avoidance-Binary Exponential Backoff scheme • Backoff_Counter:number of empty slots station must observe the channel • Rnd(): function returning pseudo-random numbers uniformly distributed in [0,1]

  4. IEEE 802.11 Standard DCF for WLANs(2/2) • Backoff_Counter=0 – successful transmission • ACK after a SIFS • If the transmission generates a collision,the CW_Size is doubled • Num_Att :number of transmission attempts • Low utilization channel • Congested systems-High collision probability

  5. The DCC mechanism(1/3) • Every active station counts (Num_Busy_Slots) and (Num_Available_Slots) • Normalized lower bound for the actual contention level of the channel

  6. The DCC mechanism(2/3) • Each station controls its transmission via Probability of Transmission(P_T(…)) • Privilege old transmission requests (queue-emptying behavior) • When the congestion level grows, the P_T(…) reduces to 0

  7. The DCC mechanism(3/3) • If slot utilization=1, it means no accesses in the next slot • Modifying the P_T(…) • SU_limit :arbitrary upper limit to the slot utilization • DCC mechanism reduces all the P_T(…)

  8. Power consumption analysis(1/10) • M active stations • PTX:power consumed (mW) by the Network Interface (NI) during transmission • PRX:power consumed (mW) by the (NI) during reception • Backoff interval sampled from a geometric distribution with parameter p, p=1/(E[B]+1), E[B]:average backoff time • Payload length sampled from a geometric distribution with parameter q,

  9. Power consumption analysis(2/10)

  10. Power consumption analysis(3/10) • Jth renewal period: time interval between jth and (j+1)th successful transmission • Energy :Energy required to a station to perform a successful transmission • System behavior in virtual transmission time

  11. Power consumption analysis(4/10) • Nc :number of collisions experienced in a virtual transmission time • In each subinterval, there are a number of not used slots (random variables sampled from a geometric distribution) • Station transmits in a slot with probability p

  12. Power consumption analysis(5/10) • N_nusk : number of consecutive not_used_slots • Energynus_k :power consumption during the N_nusk slots • Energytagged_collision_k : power consumption experienced by the tagged station in kth collision • Energytagged_success :power consumption experienced by the tagged station in jth successful transmission

  13. Power consumption analysis(6/10)

  14. Power consumption analysis(7/10) • backoff interval sampled from a geometric distribution (p) • Collision: average length of a collision • τ :maximum propagation delay between 2 WLANs • E[Collnot_tagged]: average length of a collision not involving the tagged station

  15. Power consumption analysis(8/10) • S: average length of a successful transmission • The tagged station average power consumption during a not_used slot is

  16. Power consumption analysis(9/10) • The tagged station average power consumption, when it performs a successful transmission in a slot • The tagged station power consumption, when it experiences a collision while transmitting

  17. Power consumption analysis(10/10) • The average energy requirement (in mJ units) for a frame transmission • popt: the value of p which minimizes the energy consumption • M, q, PTX, PRX :fixed system’s parameters

  18. The PS-DCC mechanism(1/5) • Used to enhance an IEEE 802.11 from the power consumption standpoint • Asymptotical Contention Limit (ACL) :optimal parameter setting for power consumption in a boundary value for the network slot utilization • Each of the M stations uses the optimal backoff value popt Negative 2nd order term • M x popt : tight upper bound of the Slot_Utilization

  19. The PS-DCC mechanism(2/5) • IEEE 802.11 does not depend on payload parameter value and Slot_Utilization greater than optimal values • DCC does not produce the optimal contention level

  20. The PS-DCC mechanism(3/5) • PTX/PRX low, then M x popt quasi-constant for M • A quasi-optimal value for the M x popt as a function of the payload parameter • Represents the optimal level of slot utilization, to guarantee power consumption optimality • PTX/PRX high, M x popt significantly affected by M • Not possible given the influence of M, thus considering only the high values of M

  21. The PS-DCC mechanism(4/5) • DCC mechanism limits the slot utilization by its optimal upper bound asymptotic contention limit (ACL) • New probability of transmission (P_T) • PS-DCC mechanism requires payload and the Slot_Utilization estimations to determine the value of the P_T

  22. The PS-DCC mechanism(5/5) • M=100

  23. Simulation results(1/9) • Average power consumption for a frame transmission • Channel utilization level when varying the contention level on the transmission channel • Number of stations 2 to 200 • PTX/PRX=2 and 100 • Average payload length 2.5 and 100 slot units • Random access schemes with respect to the contention level influence • Confidence level 95%

  24. Simulation results(2/9)

  25. Simulation results(3/9)

  26. Simulation results(4/9) Results show that: • Power consumption in the Standard 802.11 DCF is negatively affected by the congestion level • PS-DCC mechanism counterbalances the congestion growth by maintaining the optimality in the power consumption • Energy saving achieved by PS-DCC is significant and increases with the average frame size

  27. Simulation results(5/9) • Power consumption in the “worst case” for a frame transmission

  28. Simulation results(6/9)

  29. Simulation results(7/9) • MAC access delay: time between the first transmission and the completion of its successful transmission PS-DCC mechanism : • leads to a reduction of the mean access delay • Stations with high Num_Att -High probability of success • Fairness • Queue-emptying behavior of the system

  30. Simulation results(8/9) • Simulation traces of the average energy required for a frame transmission with and without the PS-DCC mechanism • 100 stations initially active • A burst of additional 100 station activates, causing the congestion level to grow up (twice) • PS-DCC mechanism obtains a lower energy requirement (close to the optimal value) for the frame transmissions-fast to adopt new contention scenarios

  31. Simulation results(9/9)

  32. Conclusions and future research PS-DCC: • Effective in implementing a distributed and adaptive contention control • Guarantee the optimal power consumption of a random-access MAC protocol • No additional hardware • Flexibility • Stable behavior • Fair reduction of contention • Queue-emptying behavior of the system • Quasi-optimum channel utilization and power consumption, without affection of the contention level

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