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Energy Conservation Techniques

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Conserving radio transmission and reception is more effective than conserving CPU cycles ... Coordinator role is rotated in order to conserve and balance energy usage ... – PowerPoint PPT presentation

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Title: Energy Conservation Techniques


1
Energy Conservation Techniques
  • Prof. Sunggu Lee
  • EE Dept., POSTECH

2
Overview
  • Most of the material in this talk is based on
    material used in previous MANET course and
    Basagni 2004
  • Material in Chaps. 4 and 14 of text is lacking
    and does not provide a comprehensive overview of
    energy conservation techniques
  • Chap. 4 Sensor and Sensor-Actuator Networks
  • Sensor-actuator networks are heterogeneous
    networks that comprise networked sensor and
    actuator nodes that communicate among each other
    using wireless links
  • actuator node ? sink node with actuator
    capability
  • Also emphases the need to use localized
    algorithms in order to provide scalable,
    distributed energy-conserving solutions
  • Use only information from 1-3 hops away
  • Chap. 14 mostly discusses the parts of the IEEE
    802.11 protocol used for energy conservation

3
Measuring Energy Consumption
  • It is impossible to make all-encompassing general
    statements about energy consumption in all
    possible portable devices
  • Due to the variety of devices available,
    operating modes, energy management techniques
    used and usage scenarios
  • However, measurements on example ZigBee
    radio-based WSN devices demonstrates several
    relationships about the energy normally required
    for various types of operations
  • Refer to Table 3.1 of text
  • Reception and transmission require the most
    energy
  • Idle state (CPU idle, radio idle) saves a LOT of
    energy
  • Conclusions
  • Conserving radio transmission and reception is
    more effective than conserving CPU cycles
  • Thus, send and receive packets sparingly (as
    little as possible)
  • Make nodes sleep (both radio and CPU idle) as
    much as possible to save energy

4
Energy Usage in FireFly Node
Table 3.1 of text
5
Approaches
  • Approaches for energy-efficient communication
  • Power-save protocols
  • Attempt to maximize the amount of time nodes
    spend in the sleep state
  • Power-control protocols
  • Nodes transmit using minimum transmit-power
    levels required for network connectivity while
    attempting to forward traffic with least energy
    usage
  • Maximum-lifetime routing
  • Select paths with aim of maximizing network
    lifetime
  • Try to balance the energy consumption across the
    nodes of the network
  • Each node wants to avoid packet forwarding as
    much as possible
  • However, if all nodes avoid packet forwarding,
    then multi-hop paths will become ineffective

6
Power-Save Protocols
  • Network-Layer Power-Save Protocols
  • Network-layer traffic is buffered at MAC layer
    for sleeping neighbors or routed using only
    nonsleeping neighbors
  • Three basic strategies
  • Synchronized power-save mechanism
  • Nodes wake up periodically in a synchronized
    manner
  • Network topology-based approach
  • A dominating set of nodes is found to route
    packets
  • Fully asynchronous operation
  • Neighboring nodes all operate asynchronously, but
    use a schedule that allows them to meet eventually

7
Synchronized Power-Saving
  • Nodes periodically wake up and exchange info
    about pending traffic
  • Nodes required for the pending traffic remain
    awake while others go back to sleep
  • Representative example method used in the IEEE
    802.11 standard
  • Using an IBSS (independent basic service set) or
    BSS, synchronized beacon intervals are used
  • At beginning of beacon interval, nodes contend to
    send the beacon frame each node sends its
    beacon after a random backoff interval
  • First beacon received is used for the
    synchronization
  • Each node that needs to send a packet then sends
    an ATIM (ad hoc traffic indication message)
    refer to Figs. 14.1 and 14.3-14.5
  • Determines nodes that need to remain awake
  • Simulation results
  • Short beacon intervals result in higher energy
    savings, but at the cost of substantially reduced
    throughput
  • The choice of the ATIM interval also effects the
    throughput
  • Throughput maximized when ATIM window is 25 of
    beacon interval

8
Overlapping Beacon Intervals
Fig. 14.1 of text
Figs. 14.4-14.5 of text
9
Topology-Based Power-Saving
  • Want to select a subset of nodes that cover all
    of the nodes in the network
  • Dominating set
  • A dominating set of a network is a subset of
    nodes, such that each node is either in the
    dominating set or is a neighbor of a node in the
    dominating set. Basagni 2004
  • Also refer to general graph theory defn
    wikipedia.org
  • Synonymous with maximal independent set
    NP-complete problem
  • A connected dominating set can be used as routing
    backbone
  • Nodes in the connected dominating set remain
    awake, while other nodes can go to sleep for long
    periods of time
  • Finding a minimal connected dominating set
    requires a lot of computation NP-hard problem
  • In general, we use a heuristic algorithm that
    finds a good connected dominating set
  • What is a possible algorithm? Describe it.

10
ASCENT Protocol
  • Adaptive self-configuring sensor network
    topologies (ASCENT) protocol is designed to help
    a node find an equilibrium that saves power while
    preserving connectivity
  • Nodes using the ASCENT protocol are in one of
    four state (active, test, passive, sleep)
  • Active nodes form a communication backbone
  • When node is in sleep state, it periodically
    wakes up and enters passive state
  • When node is in passive state, it listens to
    neighboring nodes and transitions to sleep or
    test state based on number of active neighbor
    nodes and loss rate
  • A node in test state transitions to active state
    (after a timeout) if it determines that it can
    improve the network connectivity

11
Figure 14.13 of text
12
Figure 14.14 of text
13
Span Power-Save Protocol
  • Coordinators are defined as those nodes in a
    connected dominating set
  • Coordinators form a low-latency routing backbone
    for the network
  • A node is eligible to be a coordinator if it
    finds two neighbors that cannot communicate with
    each other
  • Nodes with greater effectiveness at connecting
    pairs of neighbors are given higher priority
  • Nodes with higher energy reserves are given
    higher priority
  • A higher priority node uses a smaller backoff
    interval before sending out a coordinator
    announcement
  • After spending some time as a coordinator, a node
    withdraws itself from the coordinator role
  • Coordinator role is rotated in order to conserve
    and balance energy usage
  • Span is a synchronous power-save protocol
  • Nodes must be awake at the same time to exchange
    traffic in order to participate in coordinator
    election
  • Underlying buffering and traffic announcement
    based on IEEE 802.11

14
Geometric Adaptive Fidelity (GAF)
  • Power-save protocol that selects its covering
    set based on geographic position information
  • GAF partitions the network into a grid such
    that each node in a grid square can communicate
    with any node in an adjacent grid square
  • Implies grid size of R/sqrt(5), where R is the
    node transmission range
  • Selecting the active node in a grid square does
    not require exchange of connectivity information
  • Nodes periodically wake up and transition to
    discovery state
  • One node in a grid square is chosen as an active
    node based on its remaining energy in a random
    manner
  • Higher remaining energy ? shorter random backoff
    interval

15
Asynchronous Power-Saving
  • Nodes wake up periodically in an asynchronous
    manner, but with a period that eventually
    overlaps with adjacent nodes
  • BECA/AFECA protocol
  • Like GAF, intended for use in sensor networks
  • In basic energy conservation algorithm (BECA),
    nodes independently transition between sleep and
    listening/active states
  • Once a node transmits or receives traffic, it
    transitions to active state
  • Then transition back to sleep state if no traffic
    for a while
  • Due to independent behavior patterns, there is no
    guarantee that there is a live path to the
    destination
  • In order to ensure that route request (RREQ)
    reaches its destination, sleep interval is set to
    a multiple k of the listening/retry interval
  • Once route is established, only nodes that
    forward traffic need to remain active

16
  • Adaptive fidelity energy conservation algorithm
    (AFECA)
  • Extension of BECA in which nodes adapt their
    sleeping interval depending on the estimated
    network density
  • Higher network density ? ?
  • Simulation evaluation of BECA/AFECA using ns-2
    with AODV as the routing protocol
  • With a minimum sleep interval of 10 seconds,
    overall energy savings was about 35-45
  • There was a significant increase in route latency
  • Under one second for unmodified AODV
  • Between 6 and 10 seconds using AFECA
  • Network half-life increases by as much as 50,
    but there is almost no increase in time to first
    node failure

17
Power Control Techniques
  • Nodes modify their transmit power to increase
    network capacity and reduce energy usage
  • Topology control
  • Assign per-node transmit powers to minimize the
    maximum transmit power used in the network while
    maintaining network connectivity
  • Refer to Fig. 14.12 of text
  • Also has effect of increasing throughput since
    interference is reduced

18
Power Control for Less Communication Interference
Fig. 14.12 of text
19
  • Minimum energy routing
  • Aims to minimize the total energy consumed in
    forwarding a packet from source to destination
  • Sequence of low power transmissions may use less
    energy than a single direct transmission
  • Another factor at a given transmit-power level,
    effective transmission ranges may differ in
    different directions due to terrain and obstacles
  • Example methods
  • Methods based on relay regions
  • Given transmitter i and relay node r, relay
    region Ri(r) contains the set of receivers for
    which transmitting via relay node r reduces the
    total energy consumption. Basagni 2004, p. 317
  • Power-aware routing optimization protocol (PARO)
  • Nodes can nominate themselves as relay nodes
    based on observed traffic behavior

20
Power-Controlled MAC (1/2)
  • Antennas can be designed to transmit/receive at
    different power levels
  • Spatial reuse using power control
  • Each node can be assigned a different maximum
    power
  • Each packet (from each node) can be sent with
    different power levels
  • More powerful method
  • Basic method
  • Send RTS and CTS at maximum power
  • Based on received signal strengths of RTS/CTS,
    reduce the power of DATA and ACK packets
  • Does this help spatial reuse a lot? Why or why
    not?

21
Power-Controlled MAC (2/2)
  • RTS/CTS exchange does not prevent hidden-terminal
    problems when heterogeneous power levels are used
  • Proposed Solutions
  • Use busy tones for virtual carrier sensing
  • Each active receiver advertises its noise
    tolerance using pulses in the busy tone channel
    (a separate channel)
  • Then use Request-Power-to-Send (RTPS) and
    Acceptable-Power-to-Send (APTS) handshake in the
    normal data channel
  • Implies use of a second transceiver
  • Dont address the collisions simply send anyway
  • Use backoffs and retransmissions to solve the
    problem

22
Minimum-Energy Broadcasting and Multicasting
  • Power consumption for communication between two
    nodes at distance r is proportional to p(r) r a
    c, where a gt 2 and c is an overhead constant
  • Local shortest path tree (LPST) algorithm Ref
    7 of text, Ch. 4 for broadcasting
  • Each node u applies Dijkstras algorithm to find
    the shortest weighted path (weight p(r)) to
    each of its neighboring nodes using only 1-hop
    info
  • Each node adjusts its transmission radius
    according to the links in a pruned logical tree
  • Each node transmits with a radius equal to the
    longest adjacent link
  • Refer to Sec. 4.3 of text for details

23
Maximum Lifetime Routing
  • Maximum-lifetime routing, given a set of constant
    rate flows and a fixed network topology, can be
    formulated as a linear programming problem
  • Analogous to maximizing minimum lifetime subject
    to multicommodity flow conservation
  • The general maximum-lifetime routing problem is a
    much more difficult problem
  • Flow-augmentation algorithm
  • Hierarchical zone-based method
  • Using game theory to determine if a node should
    forward another nodes packet

24
Alternative Approaches
  • Battery-energy-efficient (BEE) routing
  • Electrochemical behaviors of batteries
  • Recovery effect
  • Bursty discharge pattern is more efficient than a
    constant-current discharge
  • Rate-capacity effect
  • Drawing even a small percentage of current
    impulses that exceed the rated current capacity
    of the battery significantly degrades battery
    performance. text, p. 323
  • BEE routing uses cost function with conventional
    max-min factor and penalty factor based on the
    above
  • Reliable energy-aware routes
  • Bit error rates vary depending on the transmit
    power used
  • Cost function should reflect the expected cost of
    retransmissions at each link

25
Discussion
  • Energy-efficient communication is a multi-faceted
    problem
  • Trying to optimize a single facet may lead to
    suboptimal performance with respect to other
    facets, thereby leading to worse node and
    network lifetimes
  • Energy consumption and wireless propagation
    models play a central role in the design and
    evaluation of energy-efficient systems
  • Direct measurement studies of wireless systems
    are useful in designing and evaluating energy
    management techniques
  • Better and more realistic simulation techniques
    and testbed environments are necessary in order
    to effectively evaluate and compare different
    protocols
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