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Mobile and Wireless Computing
Lecture 1: Preliminaries
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Lecture 1.1 : Introduction to mobile ad hoc
networks
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Lecture 1.2 : Introduction to networking and
wireless communication
&
A brief overview of the course
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
The Importance of Mobile and Wireless Computing
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There is tremendous technological advance in
producing small and smart devices.
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It is a natural consequence that such devices
work in a collaborative way.
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However, users carry around many such smart
devices and they are not fixed in the sense of a
desktop computer.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Networking without an Infrastructure
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Hence, there is a need for networking such
mobile devices without any infrastructural
support.
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Cellular phones work with infrastructural support
like mobile phone towers and satellite
communication.
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However, such support comes at a cost like preregistration with a mobile service provider etc.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Networking Anywhere and Anytime
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On the other hand, there is a growing demand of
using networks of mobile devices anywhere and
anytime.
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In many situations, the Internet may not be an
efficient solution.
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For example, a collection of people trying to
communicate in a hotel or conference hall.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Smart Devices
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Another important application of wireless
communication is the use of small smart
devices.
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The number of embedded devices in appliances
and vehicles is increasing at a rapid rate.
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There is a growing need of networking such
embedded devices without infrastructural
support.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Sensor Dust
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Sensor dust is a collection of small and
inexpensive devices which work through
collaboration.
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Thousands of such devices can be used for
applications like : environmental data collection,
weather forecasting, measuring toxicity levels at
hazardous sites etc.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Sensor Dust
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NASA has planned a sensor dust deployment for
its 2007 Mars mission for monitoring Martian
climate.
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The purpose of a sensor dust is to collect data
through inexpensive sensor devices and gather
this data in a base station for analysis.
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Sensor dust is typically deployed in places
where there is no infrastructural support.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Ad hoc Networks
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This course is about ways of information
exchange in a network of mobile and wireless
nodes without any infrastructural support.
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Such networks are often called ad hoc networks
to emphasize that they do not depend on
infrastructural support.
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The purpose of an ad hoc network is to set up
(possibly) a short-lived network for a collection of
nodes.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Ad hoc Networks
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Most network operations involve exchange of
information among the computers participating
in a network.
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There are numerous protocols for networks with
infrastructural support , starting from LANs,
Ethernets and global networks like the Internet.
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However, most of these protocols do not work
well for ad hoc networks.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Routing in an Ad hoc Network
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If all the wireless nodes are within the
transmission range of each other, routing is
easy. Every node can listen to all transmissions.
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However, this is not true in most situations, due
to short transmission range. Hence, most ad hoc
neworks are multi-hop.
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A message from a source node must go through
intermediate nodes to reach its destination.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Routing in multi-hop networks
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All nodes cooperate in delivering messages
across the network.
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The nodes must collect local neighbourhood
information in order to make global routing
decisions.
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This situation is quite different from wired
networks where the routing decisions are made
based on the infrastructure.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
An Example
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Personal Area Network (PAN)
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The aim of a PAN is to create a very localized
network for devices associated with a single
person.
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Such devices include PDAs, virtual reality
devices and laptop computers.
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Such devices usually need not have connection
to the wider internet, but they should be able to
network among themselves.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Bluetooth
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Mobility becomes much more important when
there is a need to network between several
PANs.
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Bluetooth is a short-range radio technology for
wireless connectivity of PDAs and other similar
devices.
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If each device is equipped with a Bluetooth
radio, it is possible to connect upto 8 such
devices into a piconet.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Scatternets
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When there are more than eight devices,
Bluetooth can be used to form multiple piconets.
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These piconets can be connected together into
a scatternet.
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Mobility is again an issue if individual piconets
are not static.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Factors Affecting Ad hoc Networking
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There are several factors specifically important
for ad hoc networking :
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Address assignment
Scalability
Power budget versus latency
Incompatible standards
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Addressing Problems
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Nodes in an ad hoc network are assumed to
have addresses that are preassigned.
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These addresses are not directly related to the
current positions of the nodes relative to the
overall network.
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This is significantly different from the way
addresses are assigned within the Internet.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Addressing within the Internet
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Routing within the Internet depends on the
ability to collect reachability information to IP
nodes.
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All the nodes within the same network share the
same routing prefix.
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Nearby networks have similar routing prefixes.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Hierarchy of Addresses
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Smaller prefixes sit at the higher levels of the
hierarchy.
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Reachability to all nodes within the hierarchy
can be described by advertising a single,
smallest routing prefix.
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The key to the scalability of the internet is the
collection of routing information in hierarchies.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Addressing in Ad hoc Networks
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Such a hierarchy is typically not available for ad
hoc networks.
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It is possible to introduce this hierarchy by
controlling the IP addresses of the mobile nodes.
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However, this requires that the IP addresses
change depending on the movement of the
nodes.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Addressing in Ad hoc Networks
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However, changing IP addresses introduces
extensive changes in the routing tables.
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It is not clear whether maintaining a hierarchy is
useful since it causes major revisions of the
routing tables.
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There may be major limitations for ad hoc
network protocols for large collections of nodes.
With increased mobility, route maintenance will
consume a major part of the bandwidth.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Scalability
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Ad hoc networks are not as easily scalable as
the Internet. Loss of hierarchy introduces larger
routing tables.
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Hence, we have to look for other means of
introducing hierarchy by not using IP addresses.
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Hence, IP based protocols must maintain
additional routing information.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Other Scalability Issues
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Since routing tables change as the nodes move,
control messages have to be sent around the
network to maintain the current connectivity
information.
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The control messages have to be sent more
often if mobility is high.
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If the network has a large number of highly
mobile nodes, the control messages take up a
lot of bandwidth.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Role of Control Messages
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Efficient routing protocols should try to send less
number of control messages for maintaining
connectivity information.
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Otherwise, there will be no bandwidth left for
sending useful messages.
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Also, processing a large number of control
messages put a heavy load on individual nodes.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Evaluating Protocols for Ad hoc Networks
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If the nodes are slow in processing control
messages, they may generate more control
messages and hence flood the network.
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Routing protocols must be evaluated carefully
with increased number of nodes and increased
mobility to see their effect on control mesages.
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The network should settle down fast (or
converge) when a node moves.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Power Budget versus Latency
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One of the key issues in ad hoc networks is
power management.
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Most nodes in an ad hoc network are powered
by batteries. Batteries cannot be recharged
easily in many cases.
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Each node participates in two kinds of activities,
sending and receiving messages useful for itself
and forwarding messages for other nodes.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Power Management
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The second activity is essential for maintaining
an ad hoc network, since nodes should forward
messages in a multi-hop network.
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However, the battery-power of a node will
deplete fast if there are too many control
messages to forward.
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A node may refuse to forward messages if its
power level is too low.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Energy-efficient Protocols
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Hence, routing protocols should be designed to
be energy-efficient.
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However, good routing decisions can only be
taken if the nodes have good knowledge of the
network topology.
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On the other hand, the nodes need to send
more control messages for maintaining topology
information.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing
Incompatible Standards
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There are many different proocols and
engineering solutions existing for ad hoc
networks.
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IETF (Internet Engineering Task Force) is
currently evaluating several protocols and
standards within its MANET (Mobile Ad hoc
Network) working group.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)