Energy Efficient Network Protocols for Wireless Networks

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Energy Efficient Network Protocols for Wireless
Networks

• Kiran Muthabatulla

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Introduction

• Wireless Networking exploding in Mobile and
  personal communications
• Cell Phones, Personal Computing Systems, Wireless
  LAN's
• As a result
• Energy Efficiency is an important design
  consideration due to limited battery life of
  Mobile terminals.

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Introduction

• Network Interface significant consumer of
  power.
• Power consumption On an Average system CPU-21,
  36 LCD. 18 Wireless Interface 18 Hard Disk,
  Power consumption.
• Hence..
• Low Power design of the Entire Network Stack is
  needed to enhance energy efficiency.

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Infrastructure Network
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Base Stations in BSS Mode

• Coordinate Access to one or more transmission
  channels for mobiles within the coverage cell.
• Wireless Access to and from the wired host occurs
  in the last hop between base stations and mobile
  hosts that share the bandwidth of the wireless
  channel.

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Ad hoc Network
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Ad-Hoc

• Multi Hop wireless Networks, in which a set of
  mobiles cooperatively maintain network
  connectivity.
• On-demand network, dynamic unpredictable, random,
  multi hop topologies which has no infrastructure
  support.
• Mobiles must periodically update topology
  information for routing.

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Protocol Stack
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Responsibilities Data Link

• Data Link reliable and secure Logical Layer over
  unreliable Wireless Link.
• Security Encryption and Decryption
• Network layer packet conversion into frames
• Packet re-transmissions.
• MAC Allocates the time-frequency or code space
  modulations among mobiles sharing wireless
  channels in a region.

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Low Power at PHY Layer

• Increase Battery capacity
• Slow progress. Developments in Battery technology
  is very slow
• Decrease amount of energy consumed at the
  Wireless terminal.
• Hardware means
• Variable Clock Speed CPUs, flash memory, disk
  spin down.

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Sources of Power Consumption

• Communication related.
• Transceiver at the source, intermediate and
  destination nodes.
• Transmitter is used for sending control, route
  request and response, as well as data packets
  originating at the sender.
• Receiver is used to receive data and control
  packets destined for the node.
• Computation related Usage of CPU, Main Memory,
  disk, data compression techniques.

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Mobile Radio Power Modes

• Proxim Range LAN2 2.4 GHz
• Transmit
• 1.5 W in transmit
• Receive
• 0.75 W in receive
• Standby
• 0.01W in Standby

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General Guidelines

• Collisions must be eliminated as they result in
  re-transmissions.
• Re-transmissions lead to unnecessary power
  consumption and to possibly unbounded delays.
• Broadcast environment, receiver remains on at all
  times.
• Node Receives the packets and forwards only if it
  is meant to the receiving node.
• Channel status monitoring.

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Scheduling

• Broadcast scheduling that contains the data
  transmission starting times for each mobile.
• This enables the mobiles to switch to a standby
  mode until the receive start time.
• Turn off the transceiver whenever the node
  determines that it will not be receiving data for
  a period of time.

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Scheduling..

• Mobile Radio Switching between TX and RX.
• A protocol that allocates permissions on a slot
  by slot basis suffers substantial over head.
• Mobile should be allocated contiguous slots for
  TX or Rx to reduce turnaround.
• Mobiles may request multiple transmission slots
  with a single reservation packet.

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Scheduling..

• Mobiles transmit data transmission requests to
  the BS. The BS computes the system transmission
  schedule and broadcasts each mobiles transmission
  schedule.
• Mobiles Wake up when its their schedule time.
• Considerations
• Battery power at stations
• QOS
• Avoid TX when channel conditions are poor.

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General Guidelines

• Load Balancing of the Battery Power
• Avoid routing through nodes with lower battery
  power.
• May want not to update routing information but
  may suffer performance.
• Taking advantage of Broadcast Multicast packets.

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MAC Sub Layer

• IEEE 802.11 Standard.
• CSMA/CA
• Back off timers/Slots
• Positive ACKs
• RTS/CTS.
• PSP, NULL Data Frame.
• Base Stations Store the Packets
• Beacon updates

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EC-MAC

• Based on Reservation and Scheduling
• Transmission organized by the BS into Frames, and
  each slot equals the basic unit of Wireless data
  transmission.
• At the Start of Each Frame, The BS transmits the
  FSM which contains sync info and the uplink
  transmission order for the subsequent information
  and the uplink information order for the
  subsequent reservation phase.

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EC-MAC
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PAMAS Protocol

• Power Aware Multiple Access
• For Ad hoc Networks.
• Separate Channels for RTS/CTS and data packets.
• Mobile sends RTS, Waits CTS, if received, the
  mobile sends over Data Channel.
• Sets the Control Channel to busy.

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PAMAS Protocol

• Mobiles not receiving and sending packets turn
  off the wireless interface.
• Data transmissions need not be overheard by all
  the neighbors of the transmitter.
• Separate control channel determines when and how
  long to power off.
• Mobile should power itself off
• No packets to TX and neighbor begins to send
  packet not for it.
• Has packets to send, but some other pair already
  started to communicate.
• Probe protocol to see how long to sleep.

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LLC Error Control.

• Automatic Repeat Request (ARQ)
• A type of communications link where the receiver
  asks the transmitter to re-send a block of data
  when errors are detected
• Forward Error Correction (FEC)
• A method of error control where the receiving
  node automatically corrects as many channel
  errors as it can without referring to the sending
  node.

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Disadvantages

• Both of them waste network bandwidth and consume
  power resources due to retransmissions of data
  packets
• Greater overhead necessary in error correction.

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Adaptive Error Control With ARQ.

• New Design Metric Energy efficiency of a
  protocol which is defined as the ratio between
  total amount of data delivered and total amount
  of energy consumed.
• If more data is transmitted for a given amount of
  energy consumption, the energy efficiency
  increases.

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Guidelines

• Avoid Persistence in Retransmitting Data.
• Trade off number of re-transmission attempts for
  probability of successful transmission
• Inhibit transmissions when channel conditions are
  poor.

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Guidelines..

• Probing Protocol that slows data transmission
  when degraded channel conditions are encountered.
  
• ARQ works normal until the Transmitter detects an
  error in either the data or control channel due
  to lack of an ACK.
• At this time, the protocol enters a probing mode,
  in which the probing packet is transmitted every
  t-slots.
• This mode is continued until ACKs start to show
  up.
• Then protocol returns to normal mode.

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Other Schemes proposed.

• Adaptive Error control with ARQ/FEC combination
• Adaptive Power control and coding scheme

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Network Layer

• Routing Packets
• Mobiles cooperate to maintain Topology
  information
• Use Multi hop packet routing
• Usual Approaches
• Frequent topology results in improved routing
• Infrequent topology results in decreased update
  messages.
• Congestion Control

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Network Layer

• Typical Metrics
• Shortest Hop
• Shortest Delay
• Locality stability

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Unicast Traffic Metrics

• Energy Consumed per packet Min
• Time to network Partition Max
• Critical Nodes drain their power at equal rates.
• Variance in power levels across mobiles Min
• All nodes are at the same priority level
• All mobiles are equal and no one mobile is
  penalized or privileged over the other.
• Cost per packet Min
• Routes with depleted energy reserves dont lie in
  many routes.
• Maximum Mobile cost Min
• Minimize cost to move the packet through this
  node.

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Broadcast Traffic

• Single transmission, a mobile is able to
  broadcast a packet to all immediate neighbors.
• Intermediate Nodes re-transmit the packet to all
  its immediate.
• Turn off after receiving the packet if the
  neighbor already received the packet.

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Broadcast Traffic

• Traditional Flooding
• No Global topology information requiring little
  control overhead and completes the broadcast with
  minimum number of hops.
• Solution
• Collect topology information.
• Ensure that transmission reaches as many new
  nodes as possible.
• Construct trees with lowest cost per outgoing
  degree.

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Transport Layer

• TCP degrades over Wireless Link
• Large number of Re-transmissions
• Frequently invoke congestion control measures.
• Confusing Wireless Link Errors
• Loss due to handoff as channel congestion.
• Significantly reduces throughput and introduce
  unacceptable delays.

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New Schemes

• TO reduce Retransmissions
• Split connection protocols
• Hide the wireless link from the wired network by
  terminating the Tcp connection at the Base
  station.
• Link Layer protocols
• Hide link related Losses from the TCP source by
  using a combination of local retransmissions and
  FEC.
• End to End Protocols.
• Modified versions of TCP sensitive to Wireless
  environment

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Split Connection
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Link Layer Protocols
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References

• This presentation is a understanding based on the
  research paper.
• A survey of energy efficient network protocols
  for wireless networks. Wireless Networks,
  7(4)343--358, July 2001. by C. E. Jones, K. M.
  Sivalingam, P. Agrawal, and J. C. Chen.