# A Distributed Mechanism for Power Saving in IEEE 802.11 Wireless LANs

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A Distributed A Distributed MechanismMechanism for Power for Power Saving in IEEE 802.11 Wireless Saving in IEEE 802.11 Wireless

LANsLANs

LUCIANO BONONILUCIANO BONONIMARCO CONTIMARCO CONTI

LORENZO DONATIELLOLORENZO DONATIELLO

ΠΑΡΟΥΣΙΑΣΗ :ΜΑΝΙΑΔΑΚΗΣ ΑΠΟΛΛΩΝΠΑΡΟΥΣΙΑΣΗ :ΜΑΝΙΑΔΑΚΗΣ ΑΠΟΛΛΩΝ

Introduction(1/1)Introduction(1/1) Power Save-Distributed Contention Control (PS-Power Save-Distributed Contention Control (PS-

DCC) used on top of the IEEE 802.11 WLANs DCC) used on top of the IEEE 802.11 WLANs Power Saving Strategy at the MAC levelPower Saving Strategy at the MAC level Wireless ad hoc networksWireless ad hoc networks Carrier Sense Multiple Access with Collision Carrier Sense Multiple Access with Collision

Avoidance (CSMA/CA) Avoidance (CSMA/CA) Based on Distributed Coordination Function (DCF)Based on Distributed Coordination Function (DCF) Maximize channel utilization and QoSMaximize channel utilization and QoS No power wasted due to the collisions and Carrier No power wasted due to the collisions and Carrier

SensingSensing Balancing the power consumed by the NI in the Balancing the power consumed by the NI in the

transmission and reception phasestransmission and reception phases Adaptive to the congestion variationsAdaptive to the congestion variations

IEEE 802.11 Standard DCF for WLANs(1/2)IEEE 802.11 Standard DCF for WLANs(1/2)

Active stations perform Carrier Sensing activityActive stations perform Carrier Sensing activity DIFS-Basic Access mechanismDIFS-Basic Access mechanism Collision Avoidance-Binary Exponential Backoff Collision Avoidance-Binary Exponential Backoff

schemescheme

Backoff_Counter:number of empty slots station Backoff_Counter:number of empty slots station must observe the channelmust observe the channel

Rnd(): function returning pseudo-random Rnd(): function returning pseudo-random numbers uniformly distributed in [0,1]numbers uniformly distributed in [0,1]

IEEE 802.11 Standard DCF for WLANs(2/2)IEEE 802.11 Standard DCF for WLANs(2/2)

Backoff_Counter=0 – successful transmissionBackoff_Counter=0 – successful transmission ACK after a SIFSACK after a SIFS If the transmission generates a collision,the If the transmission generates a collision,the

CW_Size is doubledCW_Size is doubled

Num_Att :number of transmission attemptsNum_Att :number of transmission attempts Low utilization channel Low utilization channel Congested systems-High collision probabilityCongested systems-High collision probability

The DCC mechanism(1/3)The DCC mechanism(1/3)

Every active station counts (Num_Busy_Slots) and Every active station counts (Num_Busy_Slots) and (Num_Available_Slots)(Num_Available_Slots)

Normalized lower bound for the actual contention Normalized lower bound for the actual contention level of the channellevel of the channel

The DCC mechanism(2/3)The DCC mechanism(2/3) Each station controls its transmission via Probability of Each station controls its transmission via Probability of

Transmission(P_T(…))Transmission(P_T(…))

Privilege old transmissionPrivilege old transmission requests (queue-emptyingrequests (queue-emptying behavior)behavior) When the congestion levelWhen the congestion level grows, the P_T(…) reduces to 0grows, the P_T(…) reduces to 0

The DCC mechanism(3/3)The DCC mechanism(3/3) If slot utilization=1, it means no accesses in the If slot utilization=1, it means no accesses in the

next slotnext slot Modifying the P_T(…)Modifying the P_T(…)

SU_limit :arbitrary upper limit to the slot SU_limit :arbitrary upper limit to the slot utilizationutilization

DCC mechanism reduces all the P_T(…)DCC mechanism reduces all the P_T(…)

Power consumption analysis(1/10)Power consumption analysis(1/10)

M active stationsM active stations

PTX:power consumed (mW) by the Network Interface (NI) PTX:power consumed (mW) by the Network Interface (NI) during transmissionduring transmission

PRX:power consumed (mW) by the (NI) during receptionPRX:power consumed (mW) by the (NI) during reception

Backoff interval sampled from a geometric distribution with Backoff interval sampled from a geometric distribution with parameter p, p=1/(E[B]+1), E[B]:average backoff timeparameter p, p=1/(E[B]+1), E[B]:average backoff time

Payload length sampled from a geometric distribution with Payload length sampled from a geometric distribution with parameter q, parameter q,

Power consumption analysis(2/10)Power consumption analysis(2/10)

Power consumption analysis(3/10)Power consumption analysis(3/10)

Jth renewal period: time interval between jth and Jth renewal period: time interval between jth and (j+1)th successful transmission(j+1)th successful transmission

EnergyEnergy :Energy required to a station to perform a :Energy required to a station to perform a

successful transmissionsuccessful transmission

System behavior in System behavior in virtual transmission timevirtual transmission time

Power consumption analysis(4/10)Power consumption analysis(4/10)

Nc :number of collisions Nc :number of collisions experienced in a virtual experienced in a virtual transmission time transmission time

In each subinterval, there In each subinterval, there are a number of not used are a number of not used slots (random variables slots (random variables sampled from a geometric sampled from a geometric distribution)distribution)

Station transmits in aStation transmits in a slot with probability pslot with probability p

Power consumption analysis(5/10)Power consumption analysis(5/10)

N_nusk : number of consecutive not_used_slotsN_nusk : number of consecutive not_used_slots EnergyEnergynus_k :nus_k :power consumption during the N_nusk slots power consumption during the N_nusk slots EnergyEnergytagged_collision_ktagged_collision_k : : power consumption experienced power consumption experienced

by the tagged station in kth collisionby the tagged station in kth collision EnergyEnergytagged_success :tagged_success :power consumption experienced by power consumption experienced by

the tagged station in jth successful transmissionthe tagged station in jth successful transmission

Power consumption analysis(6/10)Power consumption analysis(6/10)

Power consumption analysis(7/10)Power consumption analysis(7/10)

backoff interval sampled backoff interval sampled from a geometric from a geometric distribution (p)distribution (p)

CollisionCollision: average length of : average length of a collisiona collision

ττ : :maximum propagation maximum propagation

delay between 2 WLANsdelay between 2 WLANs

E[CollE[Collnot_taggednot_tagged]: average ]: average length of a collision not length of a collision not involving the tagged station involving the tagged station

Power consumption analysis(8/10)Power consumption analysis(8/10)

S: average length of a S: average length of a successful transmissionsuccessful transmission

The tagged station The tagged station average power average power consumption during a consumption during a not_used slot isnot_used slot is

Power consumption analysis(9/10)Power consumption analysis(9/10)

The tagged station The tagged station average power average power consumption, when it consumption, when it performs a successful performs a successful transmission in a slottransmission in a slot

The tagged station power The tagged station power consumption, when it consumption, when it experiences a collision experiences a collision while transmittingwhile transmitting

Power consumption analysis(10/10)Power consumption analysis(10/10)

The average energy requirement (in mJ units) for The average energy requirement (in mJ units) for a frame transmissiona frame transmission

ppoptopt: the value of p which minimizes the energy : the value of p which minimizes the energy consumptionconsumption

M, q, PTX, PRX :fixed system’s parametersM, q, PTX, PRX :fixed system’s parameters

The PS-DCC mechanism(1/5)The PS-DCC mechanism(1/5) Used to enhance an IEEE 802.11 from the power Used to enhance an IEEE 802.11 from the power

consumption standpointconsumption standpoint Asymptotical Contention Limit (ACL) :optimal parameter Asymptotical Contention Limit (ACL) :optimal parameter

setting for power consumption in a boundary value for the setting for power consumption in a boundary value for the network slot utilizationnetwork slot utilization

Each of the M stations uses the optimal backoff value pEach of the M stations uses the optimal backoff value poptopt

Negative 2Negative 2ndnd order term order term

M M x x ppopt opt : tight upper bound of the Slot_Utilization : tight upper bound of the Slot_Utilization

The PS-DCC mechanism(2/5)The PS-DCC mechanism(2/5)

IEEE 802.11 does not depend on payload parameter value IEEE 802.11 does not depend on payload parameter value and Slot_Utilization greater than optimal valuesand Slot_Utilization greater than optimal values

DCC does not produce the optimal contention levelDCC does not produce the optimal contention level

The PS-DCC mechanism(3/5)The PS-DCC mechanism(3/5) PTX/PRX low, then M x pPTX/PRX low, then M x poptopt quasi-constant for M quasi-constant for M A quasi-optimal value for the M x pA quasi-optimal value for the M x poptopt as a function of the as a function of the

payload parameterpayload parameter

Represents the optimal level of slot utilization, to Represents the optimal level of slot utilization, to guarantee power consumption optimalityguarantee power consumption optimality

PTX/PRX high, M x pPTX/PRX high, M x poptopt significantly affected by M significantly affected by M

Not possible given the influence of M, thus considering Not possible given the influence of M, thus considering only the high values of Monly the high values of M

The PS-DCC mechanism(4/5)The PS-DCC mechanism(4/5) DCC mechanism limits the slot utilization by its DCC mechanism limits the slot utilization by its

optimal upper bound asymptotic contention limit optimal upper bound asymptotic contention limit (ACL)(ACL)

New probability of transmission (P_T)New probability of transmission (P_T)

PS-DCC mechanism requires payload and the PS-DCC mechanism requires payload and the Slot_Utilization estimations to determine the Slot_Utilization estimations to determine the value of the P_Tvalue of the P_T

The PS-DCC mechanism(5/5)The PS-DCC mechanism(5/5)

M=100M=100

Simulation results(1/9)Simulation results(1/9)

Average power consumption for a frame Average power consumption for a frame transmissiontransmission

Channel utilization level when varying the Channel utilization level when varying the contention level on the transmission channelcontention level on the transmission channel

Number of stations 2 to 200Number of stations 2 to 200 PTX/PRX=2 and 100PTX/PRX=2 and 100 Average payload length 2.5 and 100 slot unitsAverage payload length 2.5 and 100 slot units Random access schemes with respect to the Random access schemes with respect to the

contention level influencecontention level influence Confidence level 95%Confidence level 95%

Simulation results(2/9)Simulation results(2/9)

Simulation results(3/9)Simulation results(3/9)

Simulation results(4/9)Simulation results(4/9)

Results show that:Results show that: Power consumption in the Standard 802.11 DCF is Power consumption in the Standard 802.11 DCF is

negatively affected by the congestion levelnegatively affected by the congestion level

PS-DCC mechanism counterbalances the PS-DCC mechanism counterbalances the congestion growth by maintaining the optimality congestion growth by maintaining the optimality in the power consumptionin the power consumption

Energy saving achieved by PS-DCC is significant Energy saving achieved by PS-DCC is significant and increases with the average frame sizeand increases with the average frame size

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

Simulation results(6/9)Simulation results(6/9)

Simulation results(7/9)Simulation results(7/9) MAC access delay: time MAC access delay: time

between the first transmission between the first transmission and the completion of its and the completion of its successful transmissionsuccessful transmission

PS-DCC mechanism :PS-DCC mechanism :

leads to a reduction of the leads to a reduction of the mean access delaymean access delay

Stations with high Num_Att -Stations with high Num_Att -High probability of successHigh probability of success

FairnessFairness Queue-emptying behavior of Queue-emptying behavior of

the systemthe system

Simulation results(8/9)Simulation results(8/9)

Simulation traces of the average energy required Simulation traces of the average energy required for a frame transmission with and without the PS-for a frame transmission with and without the PS-DCC mechanismDCC mechanism

100 stations initially active100 stations initially active A burst of additional 100 station activates, A burst of additional 100 station activates,

causing the congestion level to grow up (twice)causing the congestion level to grow up (twice) PS-DCC mechanism obtains a lower energy PS-DCC mechanism obtains a lower energy

requirement (close to the optimal value) for the requirement (close to the optimal value) for the frame transmissions-fast to adopt new contention frame transmissions-fast to adopt new contention scenariosscenarios

Simulation results(9/9)Simulation results(9/9)

Conclusions and future researchConclusions and future research

PS-DCC:PS-DCC: Effective in implementing a distributed and adaptive Effective in implementing a distributed and adaptive

contention controlcontention control Guarantee the optimal power consumption of a random-Guarantee the optimal power consumption of a random-

access MAC protocolaccess MAC protocol No additional hardwareNo additional hardware FlexibilityFlexibility Stable behaviorStable behavior Fair reduction of contentionFair reduction of contention Queue-emptying behavior of the systemQueue-emptying behavior of the system Quasi-optimum channel utilization and power consumption, Quasi-optimum channel utilization and power consumption,

without affection of the contention levelwithout affection of the contention level