Wireless Ad Hoc Internets and Sensor Nets: Progress and Challenges

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Wireless Ad Hoc Internets and Sensor Nets
Progress and Challenges

• Anurag Kumar
• Professor, ECE Department
• Indian Institute of Science, Bangalore
• anurag_at_ece.iisc.ernet.in

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A Historical Perspective

• Ideas of the 1970s,
• packet networking
• packetised voice
• packet radio networks
• facilitated by advances in technology,
• digital transmission, digital signal processing,
  microelectronics, high performance networking
• are deployed in the 1990s
• ATM Networks and Internets
• VoIP (Internet Telephony)
• Wireless Ad Hoc Networks

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Structured Wireless Networks

• Point-to-point links
• Terrestrial or satellite
• Point-to-multipoint networks
• Satellite TDM/TDMA
• Cellular mobile networks

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Unstructured Wireless Networks

• Each node has a wireless transceiver
• Every node can forward packets
• Nodes associate in an Ad Hoc manner to form a
  network
• need to self organise to form a network
• multiple access wireless communication
• Certain periphery nodes may be linked to the
  wired network

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Wireless LANs

• Usually extensions to an Ethernet LAN
• IEEE 802.11 MAC
• Several PHYs defined
• 2.4GHz ISM Band
• FHSS 1 Mbps and 2 Mbps
• DSSS 1, 2, 5.5, 11 Mbps
• OFDM up to 54 Mbps
• 5GHz Band
• OFDM 6 54 Mbps
• TCP/IP protocol stack
• Expected functionality is the same as from a LAN

access point
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Wireless Device Interconnection

• Bluetooth
• 2.4Ghz ISM Band
• Slow FH CDMA
• 79 hops, each 1Mhz band
• Bit rate 1Mbps
• Short range 10m
• Multiple access
• master slave polling
• self organising (ad hoc n/w)
• Voice and Data support
• IEEE 802.15 (WPAN)

piconet
piconet
piconet
Bluetooth scatternet
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Ad Hoc Sensor Networks

• Multifunction devices
• Sensing
• temperature, chemicals, light, body pulse rate
• Processing
• e.g., 8 bit, 4Mhz, 8KB flash, 512 B RAM
• Digital Radio
• Battery operated
• long idle life

The Berkeley Mote with a light temperature
sensor
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Sense, Compute and Communicate

• Sensor nodes pick up data
• e.g., temperature at (x,y), V(x,y)
• Process the data on the fly
• e.g., what is the maximum temperature?
• or, what are the locations of the maxima
• data compression and signal processing
• Communicate results to designated nodes
• Provides a rich class of problems in distributed
  computing and communications

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Sensor Nets Forest Resource Management

• Sensors embedded in trees and attached to wild
  animals
• smart nails and smart collars
• Tree inventory management
• Felled timber tracking
• Forest fire detection and management
• Wildlife conservation

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Sensor Nets Disaster Management

• Emergency management networks for disaster areas
• Earthquakes, tornadoes, floods
• Search and rescue
• Sensors embedded in walls, environment, wearable
  sensors (e.g., in watches)
• Form an ad-hoc network after building collapse
• Biosensors for detecting the injured but living
• Locating such people for directing rescue teams

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Sensor Nets Ad Hoc Instrumentation

• Possible applications
• in hazardous situations monitoring radioactive
  or chemical leaks
• factories or large machines
• locating people in large offices, hospitals
  (e.g., for forwarding calls)
• Ease of deployment and replacement
• Minimal disruption of ongoing operations

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Common Technical Aspects

• Physical digital wireless communication
• a common spectrum is shared
• Multiple access
• how nodes coordinate their transmissions
• Self organisation, routing and forwarding,
  scheduling
• determining neighbours
• which packet to send, and to which node?
• Traffic flow in the network
• depends on application

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Capacity A High Level Model
arriving pkts
multihop pkts
complex server physical layer and multiple access
arriving pkts
departing pkts
node pkt queues
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Is the Network Connected?

• Consider a given area
• Randomly located n nodes
• Transmission range is r(n)
• power path loss receiver noise
• Small r(n) gt more spatial reuse
• but could lose connectedness
• r(n) should shrink no faster than

r(n)
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What is the Capacity?

• New packet arrival rate per node ?
• r(n) scaled to keep network connected
• Avg number of hops that a packet is relayed
• Rate of packets to be transmitted by the ad hoc
  network n ? h(n)
• Service capacity of the network S(n)
• spatial reuse
• physical layer and multiple access
• routing and forwarding approach

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Capacity Scaling with n

• Clearly it is necessary that

• This yields a bound on ? as a function of n
• If we consider only spatial reuse

• Hence ? scales slower than

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Capacity Remarks

• Thus the per node capacity of ad hoc networks
  scales poorly with network size
• scaling is much worse in practice
• multiple access TCP effects
• Multihop packet relaying is the key problem
• Need to develop approaches and applications that
  minimise multihopping

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Performance of Multihop WLANS

• WLANS adopt the TCP/IP protocol suite
• TCP is well known to have poor performance over
  lossy links
• because of the inability to distinguish between
  congestion related loss and wireless link
  performance related loss
• Multihop ad hoc situation causes additional
  problems
• TCP max window needs to be optimally set
• unfairness between connections over paths that
  interfere

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Local Commn. Computation

• Sensor network needs to communicate

• Each node sends all values to the wired node
• no. of hops required
• Each node sends to neighbour that computes
  cumulative maximum
• no. of hops required n

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Routing in Ad Hoc Networks

• Multiple access and routing have been the most
  active areas
• Proactive routing
• periodic updates (as in the wired Internet)
• routes between all nodes maintained
• DSDV (1994)
• improvement to Bellman-Ford algorithm
• poor performance under high mobility
• since routes are often stale
• excessive routing overhead under low mobility

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(contd.) Routing

• On-demand routing protocols
• routes created only when desired by nodes
• such routes are cached, but timed out
• large overheads under high mobility
• under low mobility less overhead than DSDV
• DSR (1996)
• route entries contain entire sequence of nodes
• AODV (1999)
• route entries contain only next hop

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Routing in Sensor Networks

• On-demand routing preferred
• low mobility
• energy efficiency
• Among the protocols discussed AODV seems the most
  appropriate
• Data based routing
• instead of providing node address, provide a
  value that we are seeking
• all nodes with temperature reading above 30ºC?

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Locating Nodes

• What is the absolute position of a node?
• what is the location of the node with max (V) ?
• GPS may not be appropriate
• cost, size, energy needed, indoor applications
• Landmarks
• these could be fixed, with known locations or
  could have GPS receivers
• Relative location with respect to these landmarks
• inferring range with multipath propagation
• combining these range measurements to yield
  absolute location estimates

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Major Initiatives

• Wireless LANs
• IEEE 802.11, Bluetooth, HomeRF
• Sensor Networks
• UCLA WINS 1993-1998
• DARPA funded projects 1999 onwards
• DSN Project (USC, UCLA, Virginia Tech)
• Energy efficient routing, sensor management
• SCADDS Project (USC)
• Distributed coordination and information
  processing
• Smart Dust Project (UC Berkeley)
• Hardware (including MEMS)and operating systems
• Distributed signal processing

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Final Remarks

• Sensor networks will be a rich source of
  applications and problems
• Challenges
• Limited communication and computing resources
• Energy conscious networking
• Decentralised coordination
• Distributed information processing
• Compact and efficient electronics, sensors,
  operating systems and batteries
• Standardisation of physical and MAC for sensors