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1
Department of Information EngineeringUniversity
of Padova, ITALY
Mathematical Analysis of IEEE 802.11 Energy
Efficiency.
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2
Department of Information EngineeringUniversity
of Padova, ITALY
Mathematical Analysis of IEEE 802.11 Energy
Efficiency
Andrea Zanella, Francesco De Pellegrini
andrea.zanella, depe_at_dei.unipd.it
Special Interest Group on NEtworking
Telecommunications
WPMC 2004, 12-15 September 2004
3
Motivations

• Wireless ad-hoc networks are becoming more and
  more popular
• Self-organization
• Mobility
• Portability
• IEEE 802.11 offers native support for ad-hoc
  networking
• Single cell managed by means of Distributed
  Coordination Function (DCF)?
• Terminals are battery-powered energy consumption
  is a primary issue!
• Energy consumption in transmission and reception
  is of the same order of magnitude Feeney 01
• The carrier-sense mechanism (CSMA/CA) reduces
  collision probability but draws energy Stemm 97
• Cost of sensing is exacerbated by transmissions
  occurring during the backoff
• Also collisions and alien traffic involve an
  energetic cost

4
Aim of the study

• Goal
• Providing a complete statistical description of
  the energy spent
• Characterize the impact of RTS/CTS on energy
  consumption
• Provide a mathematical tool for the design of
  energy-aware algorithms
• Case study
• Reference scenario Bianchi2000
• Ad hoc network with n saturated IEEE 802.11
  terminals
• Single-hop network
• No hidden or exposed node problem
• Heavy traffic conditions (saturation)?
• All terminals have always a packet ready for
  transmission

5
Energy Model

• Linear energetic model

A
B

• Energy is drawn proportionally to the time spent
  in each mode Feeney

C

• Each operating mode is associated to a different
  energetic coefficient

Transmitting (?)?
Receiving (?R)?
Sensing (?S)?
Virtual Sensing (?0)?

• Energy spent during SIFS periods is neglected

6
Detailing the Energy Consumption
Energy spent during backoff
Energy spent in non-colliding transmission
Energy spent in colliding transmissions
Overall energy spent for successful packet
delivery

• Hypothesis
• are i.i.d. and independent of ET
• Probability of collision p independent of the
  system state Bianchi01

Energy spent in each collision
Number of collisions before success
7
Detailing ET ETc,j

• ET Energy required for transmitting a packet
  with success

• ETc,j Energy spent during packet collision

Basic Access
RTS/CTS
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Detailing EB

• EB Energy spent in backoff

• Wr total number of tick periods spent in backoff
• Tick Period
• time between two successive decrements (tick) in
  the backoff countdown process
• Idle channel countdown 1 per time slot
• Busy channel freeze until the channel returns
  idle for a DIFS, then resume countdown
• ?j energy spent in each tick period

9
Detailing ?j

• During a tick period a node can be
• sensing the radio channel
• receiving a valid packet intended for that node
• discarding a valid packet for other destinations
• listening collided transmission on the channel

Idle Channel
Busy Channel
10
Putting al pieces together...

• Moment generating function for the energy spent
  by each node in the network

11
Case Study

• Lucent WaveLAN 11 Mbps Feeney2001
• Transmitting ? 1 (normalized)?
• Receiving ?R 2/3
• Sensing ?S 0.82?R
• Possible power saving policy
• Case 1
• Energy spent during NAV phase is negligible
  (?00)
• Case 2
• Energy spent during NAV phase is not negligible
  (?00.5?S)
• Case 3
• Regular sensing is performed during NAV phase
  (?0?S)

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Results node lifetime

• Normalized Lifetime
• Minimum theoretical energy per pck over Average
  energy per pck
• RTS/CTS outperforms Basic Access mode
• ?0 0 leads to large gain in nodes lifetime
• Gain rapidly fades for ?0 ? 1/2?S

0.6
?0 0
Basic Access
RTS/CTS
Normalized Lifetime
0.4
?0 1/2?S
0.2
?0 ?S
0 10 20 30 40
50 60 70 80 90 100
Number of stations
13
Basic Access-RTS/CTS threshold

• Energy vs Throughput perspective
• With ?0 ?1/2?S payload threshold is lower than in
  Throughput-base case
• Threshold shows less sensitivity to the number of
  nodes in the network
• With more than 20 nodes, the threshold remains
  almost const
• Threshold increases as ?0 gets close to ?S

Payload threshold after which RTS/CTS outperforms
Basic Access
1 Bianchi2000
14
Conclusions

• Complete statistical description for energy
  consumption
• Ad-hoc network with saturated IEEE 802.11 nodes
• Model allows for some interesting insights
• Channel sensing during backoff has a relevant
  energetic cost
• Switching to low-power mode during NAV can
  potentially save energy, but only for ?0 ltlt ?S
• Payload length after which RTS/CTS outperforms
  Basic Access is lower for Energy-base than for
  Throughput-base perspective
• Energy-based Threshold is less sensitive to the
  number of nodes in the network than
  Throughput-based Threshold

15
Department of Information EngineeringUniversity
of Padova, ITALY
Mathematical Analysis of IEEE 802.11 Energy
Efficiency
andrea.zanella, depe_at_dei.unipd.it
Andrea Zanella, Francesco De Pellegrini
Questions?
WPMC 2004, 12-15 September 2004
16
Extra Slides

• Spare Slides

17
Medium Access Control (MAC)?

• CSMA Carrier Sensing Multiple Access
• (Exponential) Backoff stage
• Choose a random number in the backoff window
• If the channel is sensed idle, then countdown by
  1 for each slot
• If the channel is busy then freeze the countdown
  until the channel becomes idle again for at least
  a DIFS
• When the countdown is over transmit the packet
• If no ACK is returned within a SIFS, a collision
  has occurred
• Double backoff window and re-enter the backoff
  stage
• Otherwise the transmission was successfull
• Reset the backoff window and enter the backoff
  stage for the next packet

18
Collision Avoidance

• Basic Access
• Transmit data packet
• RTS/CTS access
• Try to reserve the channel before transmission
• Send a very short Request To Send (RTS) packet
• Receiver replies with a very short Clear To Send
  (CTS) packet
• Stations that get RTS or CTS packets avoid
  transmissions in the successive time interval
  (setting the NAV)?

19
Detailing EB backoff strategy

• EB Energy spent in backoff
• Backoff strategy
• S(i) backoff stage after i successive
  collisions
• S(i) min(i,m)?
• CWi i-th backoff window
• CWiCW0 2S(i)-1
• xi i-th backoff counter
• xirandom0,1,...,CWi
• Countdown xi tick periods then retransmit the
  packet

20
Tick period (1/2)?

• Tick Period
• time between two successive decrements (tick) of
  the backoff countdown process
• Idle channel
• countdown 1 per time slot
• Busy channel (valid or collided packet on the
  air)?
• freeze until the channel returns idle for a DIFS,
  then resume countdown
• during a tick period a node can
• wait (idle channel)?
• receive valid packet intended for that node
• discard valid packet for other destinations
• listen collided transmission on the channel

Idle Channel
Busy Channel
21
Results complementary cdf of E

• Energy actually spent for a packet transmission
  is many times the theoretical minimum
• Jointly using RTS/CTS and smart sensing strategy
  drastically reduces energy costs

100
Basic Access ?0 ?S
RTS/CTS ?0 ?S
10-1
P E gt e
10-2
Basic Access ?0 0
RTS/CTS ?0 0
10-3
10-4
0 20 40 60 80
100 120 140 160 180 2000
Normalized energy eE / minE