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Research in Wireless Ad-Hoc
Sensor Networks:
Routing and data transport
protocolsEdo Biagioni
presenting work done in collaboration with
Kim Bridges and Brian Chee, and
Shu Chen, Wei Chen, Lisa Fan, Dan Morton,
Ben Roy, Yihua Xie
April 23, 2004
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Wireless Ad-Hoc networks
● Each node has a radio transceiver● Each node generates and receives data● Each node must also transport data for other
senders● Node distribution is planned or random● Some issues: where to send data (routing)? how
to send data efficiently?● Could be mobile (MANet), fixed, or both
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Sensor Networks
● sensors are low-power (often battery
powered), deployed for long periods in
remote locations● data can be retrieved manually, but it is
better to do so automatically over a network● some sensor networks might be very large,
so scalability is an issue
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Ad-hoc Wireless Sensor
Networks● monitor remote sites over
extended periods at low cost:
science, agriculture, tourism,
military applications● e.g. study endangered plants
to find out why they are
endangered● e.g. monitor crops to determine when to water or apply
fertilizer● environmental monitoring, and also images, and
intrusion or herbivore detection
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Mobile Ad-Hoc Networks
● Nodes are assumed to be moving
continuously and at random● must minimize routing overhead● many protocols, including DSR, AODV● few applications: UAVs, vehicles● little study (so far) on performance e.g.
transmitting TCP content
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Challenges in Wireless
Ad-Hoc Sensor Networks● make sense of large amounts of data:
visualization● minimize the data transmitted: model
generation, distributed event detection
● conserve power, e.g. by “sleeping”● re-route around nodes that have failed and nodes that are
congested● deal sanely with disconnection● support encryption, heterogeneity
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Protocol Design
● efficient, reliable, high throughput, low delay
(not unusual)● low power requires: low delay to completion
(low packet delay and high throughput) and
high efficiency (do not transmit unnecessary
packets)● low power also requires synchronization● baseline is flooding broadcast
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Wireless Ad-Hoc Sensor
Network Protocol Examples
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WSNs and MANets
● Wireless sensor network nodes are even
lower power than MANet nodes● nodes may be simpler than MANet nodes● no motion (fixed) or limited motion (fixed-
mobile)● worth discovering and using good routes● nodes may know position
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building an environmental
sensor network: PODS● http://www.pods.hawaii.edu● high resolution images● sunlight, temperature, rain● V0: wired sensor boards● V1: PC-104, PC-compatibles running Linux,
802.11 for communications, BasicStamp power
control board● V2: Compaq Ipaq, Linux, 802.11● V3: in the planning stages
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Multipath On-Demand Routing
● Shu Chen● protocol to carry IP packets● establish routes on demand only: send a
broadcast, reply along the reverse path● may be multiple routes to a destination: using
all of them in turn provides load balancing,
redundancy, and reliability● put routes on probation if they fail: keep
trying them for a little while
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MOR performance
● faster transmission of fixed payload (over
TCP) than DSR, AODV● multipath with reliability layer effectively
routes around congestion● podr: long-term uninterrupted service in
deployed pods, also works under ns-2● hop-by-hop ack (e.g. 802.11) to decide
whether to retransmit on this hop
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Lusus Protocol
● Dan Morton● Really low power is only available on really
simple processors such as PIC and AVR, with
< 1KB RAM, small programs● Need a really simple protocol:● only send data (not IP)● only send to nearest base station● hop-by-hop, not end-to-end reliability● short packets, very low overhead
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Lusus Techniques
● base stations regularly broadcast
synchronization messages● each retransmission increases the distance● only send to nearest base station● data from different sources may be combined
enroute● messages are retransmitted if there is no ack
from the next hop● implementation is in progress
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Sensor Network Data
Transmission Protocol:
SNDT● Lisa Fan and Yihua Xie● IP over MOR, can use ssh, but:● ssh has high overhead for small transfers● want connectionless, secure, and efficient
protocol● use UDP, with explicit acks to minimize the
overhead● rate-limit transmission to avoid generating
congestion
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SNDT security issues
● initial secret is part of a node config● authentication to prevent bad data● encryption: data and commands should not be
visible without the key● initial secret is leveraged to exchange session
keys● nodes send data to base station● base station sends commands and
configuration to nodes
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SNDT open issues
● distributing time must be done quickly, but
authentication takes time● measuring RTT is hard if decryption is needed
before ack● each key must have an ID, which must be
present on packets: does this make the
attacker's life easier?● maintaining session keys, and recovering from
reboots, can be challenging
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Distributed Route Table (DiRT)
● Ben Roy● routing or forwarding to many hosts requires
large routing tables● distribute the routing tables so each node has
at most O(log(N)) routes, yet every node can
reach every other● node IDs are set to be 0..N-1● each node i has source routes for i±1, i±2,
i±4, i±8, etc.
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DiRT routing
● for node i, if the destination D is in the routing
table, send to it● otherwise, compute =D-i, the difference in
addresses● for each destination j in i's routing table, find the
one with the smallest , (will have at least one)
and send to that node● packets require at most O(log(N)) legs, each of
maximum length the diameter of the network
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Distributed Routing Tables:
Open Issues● Are there better ways to distribute large routing
tables?● for example, among neighbors● if DiRT is used, what is the likelyhood of
packets being rerouted along a better route
encountered along the way?● analysis: how much better are we really doing
than broadcast?● how does network geometry affect that?
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Geographic Routing
● each node knows its own position, e.g. via
GPS● if the destination is identified by position,
simply route to the neighbor nearest the
destination● can cause routing loops, e.g. at dead ends● many refinements, but no good solutions
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Geometric Routing: GEO
● geographic routing works fine as long as the
network is densely connected● only problems are at the edges of a
connected area● communicate the geometry of the connected
areas, and route around any “holes”● all the nodes on an edge must keep track of
the geometry of that edge
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GEO implementation
● Wei Chen● on the ns-2 simulator● complex topologies
simulated● many nodes simulated● scalability is good
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Related work
● Lots of protocols and ideas: Berkeley motes,
diffusion, Zigbee● The Capacity of Wireless Networks, by Gupta
and Kumar – there are limits to how much we
can send● SPINS: security protocols for sensor networks,
by Perrig et al. --practical secure transmission
on tiny processors● http://www2.ics.hawaii.edu/~esb/prof/pub/ijhpca02.html, Biagioni and
Bridges
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Interesting Issues
● new network, e.g. IP assumptions don't work,
connectivity may be intermittent, and it is OK to
design from scratch● low power operation is required, but there may
be different ways to achieve it: physical layer,
data link, network layer● should there ever be networks where data is
unencrypted? How can automatic encryption
be set up effectively?
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More issues
● Are there better ways of routing?● Are there reasonable “standard” benchmarks
for routing?● Many problems to be solved: power aware
routing, position determination, optimal node
placement, architecture, operating system
(e.g. TinyOS), scheduling, etc
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Summary
● Protocols to support goals of sensor
network deployment: MOR, Lusus,
SNDT, DiRT, GEO● building actual sensor networks to
evaluate and motivate the ideas