A Distributed Mechanism for Power
Saving in IEEE 802.11 Wireless
LANs
LUCIANO BONONI
MARCO CONTI
LORENZO DONATIELLO
ΠΑΡΟΥΣΙΑΣΗ :ΜΑΝΙΑΔΑΚΗΣ ΑΠΟΛΛΩΝ
Introduction(1/1)
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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
IEEE 802.11 Standard DCF for WLANs(1/2)
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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]
IEEE 802.11 Standard DCF for WLANs(2/2)
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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
The DCC mechanism(1/3)
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Every active station counts (Num_Busy_Slots)
and (Num_Available_Slots)
Normalized lower bound for the actual contention
level of the channel
The DCC mechanism(2/3)
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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
The DCC mechanism(3/3)
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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(…)
Power consumption analysis(1/10)
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M active stations
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PTX:power consumed (mW) by the Network Interface (NI)
during transmission
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PRX:power consumed (mW) by the (NI) during reception
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Backoff interval sampled from a geometric distribution with
parameter p, p=1/(E[B]+1), E[B]:average backoff time
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Payload length sampled from a geometric distribution with
parameter q,
Power consumption analysis(2/10)
Power consumption analysis(3/10)
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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
Power consumption analysis(4/10)
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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
Power consumption analysis(5/10)
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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
Power consumption analysis(6/10)
Power consumption analysis(7/10)
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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
Power consumption analysis(8/10)
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S: average length of a
successful transmission
The tagged station average
power consumption during
a not_used slot is
Power consumption analysis(9/10)
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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
Power consumption
analysis(10/10)
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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
The PS-DCC mechanism(1/5)
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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
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M x popt : tight upper bound of the Slot_Utilization
The PS-DCC mechanism(2/5)
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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
The PS-DCC mechanism(3/5)
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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
The PS-DCC mechanism(4/5)
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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
The PS-DCC mechanism(5/5)
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M=100
Simulation results(1/9)
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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%
Simulation results(2/9)
Simulation results(3/9)
Simulation results(4/9)
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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
Simulation results(5/9)
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Power consumption in the “worst case” for a frame transmission
Simulation results(6/9)
Simulation results(7/9)
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MAC access delay: time
between the first transmission
and the completion of its
successful transmission
PS-DCC mechanism :
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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
Simulation results(8/9)
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Simulation traces of the average energy required
for a frame transmission with and without the PSDCC 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
Simulation results(9/9)
Conclusions and future research
PS-DCC:
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Effective in implementing a distributed and adaptive
contention control
Guarantee the optimal power consumption of a randomaccess 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