A survey of Energy Efficient Network Protocols for Wireless Networks

1
A survey of Energy Efficient Network Protocols
for Wireless Networks

• Christine E. Jones
• Krishna M. Sivalingam
• Prathima Agrawal
• Jyh-Cheng Chen

2
Issue 1/2

• Rapid expansion of wireless services, mobile data
  and wireless LANs
• Greatest limitation finite power supplies

3
Issue 2/2

• Typical example of power consumption from a
  mobile computer (Toshiba 410 CDT)
• 36 Display
• 21 CPU/memory
• 18 Wireless interface
• 18 Hard drive
• Goal
• Incorporate energy conservation at all layers of
  protocol stack

4
Energy Efficiency Research in Protocol Stack
5
Physical Layer

• Two different perspectives
• Increase battery capacity
• Increase capacity while restricting weight
• However battery technology hasnt experienced
  significant advancement in the past 30 years
• Decrease of energy consumed
• Variable clock speed CPUs
• Flash memory
• Disk spindown

6
Sources of Power Consumption

• Two types
• Communication related
• Computation related
• Tradeoff between them

7
Communication related

• Three modes
• Transmit
• Receive
• Standby
• Example
• Proxim RangeLAN2 2.4 GHz 1.6 Mbps PCMCIA card
  1.5W transmit, 0.75W receive, 0.01W standby
• Turnaround between transmit and receive typically
  takes 6 to 30 microseconds
• Optimize the transceiver usage

8
Computation related

• Usage of CPU, main memory and disk
• Data compression techniques for reduction of
  packet length increase power consumption

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General Guidelines and Mechanisms 1/5

• Reduce collisions in MAC
• Retransmissions lead to power consumption and
  delays
• Cannot be completely eliminated due to user
  mobility and varying set of mobiles
• Change typical broadcast mechanism
• 802.11 Receiver keeps track of channel status
  through constant monitoring

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General Guidelines and Mechanisms 2/5

• Turnaround between transmit and receive mode
  spends time and power
• Allocate contiguous slots for transmission or
  reception
• Request multiple transmission slots with a single
  reservation packet
• Computation of transmission schedule should be
  relegated to base station

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General Guidelines and Mechanisms 3/5

• Scheduling algorithm may additionally consider
  battery power level
• Allow mobile to re-arrange allocated slots under
  low-power conditions
• At link layer
• Avoid transmissions when channel conditions are
  poor
• Combine ARQ and FEC mechanisms

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General Guidelines and Mechanisms 4/5

• Energy efficient routing protocols
• Ensure all nodes equally deplete their power
  level
• Avoid routing through nodes with lower battery
  power
• Requires mechanism for dissemination of node
  battery power
• Periodicity of routing updates can be reduced
• May result in inefficient routes

13
General Guidelines and Mechanisms 5/5

• OS level
• Suspend of specific sub-unit (disk, memory,
  display etc.) when detect prolonged inactivity

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MAC Sublayer

• Three specific MAC protocols
• IEEE 802.11
• EC-MAC
• PAMAS

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IEEE 802.11 Standard 1/2

• A mobile that wishes to conserve power may switch
  to sleep mode and inform the base station
• The base station
• Buffers packets that are destined for the
  sleeping mobile
• Periodically transmits a beacon that contains
  information about such buffered packets
• When the mobile wakes up, it listens for this
  beacon, and responds to the base station which
  then forwards the packets

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IEEE 802.11 Standard 2/2

• Conserves power but results in additional delays
  and may affect the QoS
• Experimental measurements of per packet energy
  consumption
• Same incremental costs for both unicast and
  broadcast traffic
• Higher fixed costs for unicast transmission
  because of MAC coordination and CTS and ACK
  messages

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EC-MAC Protocol 1/7

• Energy Conserving-Medium Access Control
• Developed with the issue of energy efficiency as
  a primary goal
• Defined for infrastructure network but can be
  extended to ad-hoc by allowing mobiles to elect a
  coordinator
• It is based on reservation and scheduling and
  supports QoS

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EC-MAC Protocol 2/7
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EC-MAC Protocol 3/7

• FSM
• transmitted at the start of each frame by the
  base station
• contains synchronization information and uplink
  transmission order for subsequent reservation
  phase
• Request/Update Phase
• Each registered mobile transmits new connection
  requests and status of established queues
• Collisions avoided

20
EC-MAC Protocol 4/7

• New User Phase (Aloha)
• Registration of new users
• Collisions occur
• Provides time for BS to compute the data phase
  transmission schedule
• Schedule Message
• Broadcasted by the base station
• Contains the slot permissions for the subsequent
  data phase

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EC-MAC Protocol 5/7

• Data phase (Downlink)
• Transmission from base station to mobiles
• Scheduled considering QoS requirements
• Data phase (Uplink)
• Slots allocated using a suitable scheduling
  algorithm

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EC-MAC Protocol 6/7

• Collisions are avoided and this reduces the
  number of retransmissions
• Mobile receivers are not required to monitor the
  channel because of schedules
• Centralized scheduler can optimize schedule so
  that mobiles transmit and receive within
  contiguous slots

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EC-MAC Protocol 7/7

• Scheduling algorithms may consider also battery
  power level in addition to packet priority
• Frames may be fixed or variable length
• Fixed are desirable from energy efficient
  perspective since a mobile will know when to wake
  up to receive FSM
• Variable are better for meeting the demands of
  bursty traffic

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PAMAS Protocol 1/3

• Designed for ad hoc network, with energy
  efficiency as primary goal
• Provides separate channels for RTS/CTS control
  packets and data packets

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PAMAS Protocol 2/3

• A mobile with a packet to transmit sends a RTS
  over the control channel, and awaits the CTS
• If no CTS arrives the mobile enters a backoff
  state
• However, if CTS is received, then the mobile
  transmits the packet over the data channel
• The receiving mobile transmits a busy tone over
  the control channel for the others to determine
  that the data channel is busy

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PAMAS Protocol 3/3

• The use of control channel allows mobiles to
  determine when and for how long to power off
• A mobile can power off when
• It has no packets to transmit and a neighbor
  begins transmitting a packet not destined for it
• It does have packets to transmit but at least one
  neighbor-pair is communicating
• The length of power off time is determined
  through the use of a probe protocol (Singh and
  Raghavendra, 1998)

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LLC Sublayer

• Is responsible for the error control
• The two most common techniques for the error
  control are Automatic Repeat Request (ARQ) and
  Forward Error Correction (FEC)
• Both waste network bandwidth and power resources
  due to retransmissions and greater overhead

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LLC Sublayer

• Recent research has addressed low-power error
  control and several energy efficient link layer
  protocols have been proposed
• Adaptive Error Control with ARQ
• Adaptive Error Control with ARQ/FEC Combination
• Adaptive Power Control and Coding Scheme

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Adaptive Error Control with ARQ 1/3

• Three guidelines
• Avoid persistence in retransmitting data
• Trade off number of retransmission attempts for
  probability of successful transmission
• Inhibit transmission when channel conditions are
  poor

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Adaptive Error Control with ARQ 2/3

• Works as normal until the transmitter detects an
  error due to the lack of a received ACK.
• Then the protocol enters a probing mode in which
  a probing packet is transmitted every t slots.
  Probe packet contains only header
• This mode continues until an ACK is received.
  Then the protocol returns to normal mode and
  continues transmission from where it was
  interrupted

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Adaptive Error Control with ARQ 3/3

• Analysis results show that under slow fading
  channel conditions it is superior to standard ARQ
  in terms of energy efficiency
• There is an optimal transmission power in respect
  to energy efficiency
• Decreasing the transmission power results in an
  increased number of transmission attempts but may
  be more efficient than attempting to maximize the
  throughput

32
Adaptive Error Control withARQ/FEC Combination

• Each packet stream
• is associated with service quality parameters
  (packet size, QoS requirements)
• maintains its own time-adaptive customized error
  control scheme
• Error control scheme
• is a combination of
• an ARQ scheme (Go-Back-N, CACK, SACK, etc.) and
• a FEC scheme
• modifies as channel conditions change over time

33
Adaptive Power Control andCoding Scheme

• Each transmitter operates at a power-code pair
• Power level lies between a specified minimum and
  maximum
• The error code is chosen from a finite set
• At each iteration (timeframe)
• Receiver checks the word error rate (WER)
• If the WER lies within an acceptable range,
  power-code is retained, otherwise a new
  power-code pair is computed by the transmitter
• Variations of algorithm include average WER

34
Network Layer

• Energy efficient routing algorithms for ad hoc
  networks
• Does not apply to infrastructure networks because
  all traffic is routed through BS
• Two different approaches
• Frequent topology updates
• Improved routing
• Consumes bandwidth
• Infrequent topology updates
• Decreased update messages
• Inefficient routing and occasional missed packets

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Network Layer

• Typical metrics for ad hoc routing protocols
• Shortest-hop
• Shortest-delay
• Locality-stability
• However they may result in the overuse of energy
  resources of a small set of mobiles decreasing
  mobile and network life

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Network Layer example

• Using shortest-hop routing, traffic from A to D
  will always be routed through E
• Es energy reserves will be drained faster and
  then F will be disconnected from network
• A to D traffic should also use the B-C path
  extending networks life

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Network Layer Unicast Traffic 1/6

• Five different metrics
• Energy consumed per packet
• Time to network partition
• Given a network topology, a minimal set of
  mobiles exist such that their removal will cause
  the network to partition
• The traffic in that mobiles should be divided in
  such a way that they drain their power at equal
  rates

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Network Layer Unicast Traffic 2/6

• Variance in power level across mobiles
• All mobiles are equal and remain powered-on
  together for as long as possible
• Cost per packet
• Routes should be created such that mobiles with
  depleted energy reserves do not lie on many
  routes
• Maximum mobile cost
• By minimizing the cost experienced by a mobile
  when routing a packet through it significant
  reductions in the maximum mobile cost result

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Network Layer Unicast Traffic 3/6

• The goal is to minimize all the metrics except
  for the second which should be maximized
• Shortest-cost routing protocol is more
  appropriate instead of shortest-hop
• So although packets may be routed through longer
  paths, the paths contain mobiles that have
  greater amounts of energy reserves
• Also routing traffic through lightly loaded
  mobiles conserves energy because it minimizes
  contention and retransmission

40
Network Layer Unicast Traffic 4/6

• Simulation results showed no extra delay over the
  traditional shortest-hop metric
• This is true because congested paths are often
  avoided
• However this approach requires that every mobile
  have knowledge of every other mobile and the
  links between them
• This creates significant communication overhead
  and increased delay

41
Network Layer Unicast Traffic 5/6

• Stojmenovic and Lin proposed localized routing
  algorithms
• These algorithms depend only on information about
  the source location, the location of neighbors
  and location of the destination
• This information is collected through GPS
  receivers which are included in every mobile

42
Network Layer Unicast Traffic 6/6

• They proposed a new power-cost metric
• Incorporates both a mobiles lifetime and
  distance based power metrics
• Three power-aware localized routing algorithms
  were developed
• Power
• Minimize total amount of power utilized when
  transmitting a packet
• Cost
• Avoid mobiles with low battery reserves
• Power-cost
• Combination of the other two

43
Network Layer Broadcast Traffic 1/4

• Each mobile needs to receive a packet only once
• Intermediate mobiles are required to retransmit
  the packet
• Key idea allow each mobiles radio to turn off
  after receiving a packet if its neighbors have
  already received a copy of the packet

44
Network Layer Broadcast Traffic 2/4

• In traditional networks broadcast technique is a
  simple flooding algorithm
• No global information topology gathered
• Requires little control overhead
• Completes with minimum number of hops
• Not suitable for wireless networks because many
  intermediate nodes must retransmit packets
  needlessly
• It is more beneficial to spend some energy in
  gathering topology information in order to
  determine the most efficient broadcast tree

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Network Layer Broadcast Traffic 3/4

• A broadcast approach is presented in (Singh et
  al., 1999)
• The tree is constructed starting from the source
  and expanding to the neighbor that has the lowest
  cost per outgoing degree
• Mobile costs continuously change so broadcast
  transmissions may traverse different trees
• Simulations showed very little difference in
  delay but 20 or better in energy consumption

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Network Layer Broadcast Traffic 4/4

• In (Wieselthier et al., 2000) is presented an
  algorithm for determining the minimum-energy tree
• There exists an optimal point in the trade-off
  between reaching greater number of mobiles in a
  single hop by using higher transmission power
  versus reaching fewer mobiles but using lower
  power levels

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Transport Layer

• TCP was designed initially for wired networks
• Physical links are fairly reliable
• Packet loss is random in nature
• Over a wireless link it degrades significantly
• It resorts to a larger number of retransmissions
  and frequently invoke congestion control measures
  because it confuses link errors and loss as
  channel congestion
• The increased retransmissions consume battery
  energy and bandwidth

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Transport Layer

• Various schemes have been proposed
• Split connection protocols
• Link-layer protocols
• End-to-end protocols

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Split connection protocols 1/2
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Split connection protocols 2/2

• Completely hide the wireless link from the wired
  network by splitting each TCP connection into two
  separate connections at the BS
• The second one may use modified versions of TCP
  that enhance performance over the wireless channel

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Link-layer protocols 1/2
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Link-layer protocols 2/2

• Hides link related losses from the TCP source
• Uses a combination of local retransmissions and
  FEC over the wireless link
• Local retransmissions use techniques that are
  tuned to the characteristics of the wireless
  channel

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End-to-end protocols

• Include modified versions of TCP that are more
  sensitive to the wireless environment
• Uses mechanisms such as
• SACK
• allow the TCP source to recover from multiple
  packet losses
• ELN
• Aid the TCP source to distinguish between
  congestion and other forms of loss

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Energy Consumption Analysis of TCP 1/4

• The energy consumption of Tahoe, Reno and New
  Reno is analyzed in (Zorzi and Rao, 2000)
• Efficiency is defined as the average number of
  successful transmissions per energy unit
• Results demonstrate that
• error correlation affects the energy performance
• congestion control algorithms of TCP allow for
  greater energy savings by backing off and wait
  during error bursts
• energy efficiency is sensitive to the version of
  TCP

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Energy Consumption Analysis of TCP 2/4

• The same versions of TCP were studied in
  (Tsaoussidis et al., 2000a) in terms of
  energy/throughput tradeoffs
• Results showed that
• no single version is most appropriate within
  wired/wireless heterogeneous networks
• the key to balancing energy and throughput is
  through the error control mechanism
• They proposed a modified version of TCP, referred
  to as TCP-Probing in (Tsaoussidis and Badr, 2000)

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Energy Consumption Analysis of TCP 3/4

• In TCP-Probing when a segment is delayed or lost,
  instead of invoking congestion control,
  transmission is suspended and a probe cycle is
  initiated
• Probe cycle
• exchange of probe segments (TCP header with no
  payload) between sender and receiver
• terminates when two consecutive RTT are
  successfully measured

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Energy Consumption Analysis of TCP 4/4

• The sender invokes standard TCP congestion
  control if persistent error conditions are
  detected
• However, if conditions indicate transient random
  error, then the sender resume transmissions
  according to available network bandwidth

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OS/Middleware

• The main functions of an operating system is to
  manage access to physical resources like CPU,
  memory and disk space
• CPUs can be operated at lower speeds by scaling
  down the supply voltage (quadratic relationship
  between power and supply voltage)
• Predictive shutdown
• Different page placement algorithms exploit the
  new power management features of memory technology

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Application Layer 1/2

• Load Partitioning
• Applications may be selectively partitioned
  between the mobile and base station
• Most of the power intensive computations of an
  application are executed at the BS
• Proxies
• Middleware that automatically adapt the
  applications to changes in battery power and
  bandwidth
• Either on the mobile or BS side of wireless link

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Application Layer 2/2

• Databases
• Minimize power consumed per transaction through
  embedded indexing
• Video Processing
• Reduce effective bit rate of video
• Carefully discard video frames