Get documents about "Tora-Adaptive Clustering for Mobile Wireless networks
Description
mobile computing
Document Sample
Adaptive Clustering for Mobile
Wireless networks
Dr. R. B. Patel
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Mobile Ad hoc Network (MANET)?
• A Mobile Ad hoc Network (MANET) is an autonomous
system of nodes called mobile stations (MSs) connected by
wireless links.
• A MANET does not necessarily need support from any
existing network infrastructure like an Internet gateway or
other fixed stations.
• The network’s wireless topology may dynamically change in
an unpredictable manner since nodes are free to move.
• Information is transmitted in a store-and forward manner
using multi hop routing.
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Continued…
• Each node is equipped with a wireless transmitter and a
receiver with an appropriate antenna.
• We assume that it is not possible to have all nodes within
each other’s radio range.
• When the nodes are close-by i.e., within radio range, there
are no routing issues to be addressed.
• At a given point in time, wireless connectivity in the form of
a random multi-hop graph exists between the nodes.
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Continued…
MS2 MS2
MS4
MS3 Asymmetric link
MS5
Symmetric link MS7
MS1 MS6
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Characteristics
• Dynamic topologies: Network topology may change
dynamically as the nodes are free to move.
• Bandwidth-constrained, variable capacity links: Realized
throughput of wireless communication is less than the
radio’s maximum transmission rate. Collision occurs
frequently.
• Energy-constrained operation: Some nodes in the ad hoc
network may rely on batteries or other exhaustible means
for their energy.
• Limited physical security: More prone to physical security
threats than fixed cable networks.
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Applications
• Virtual navigation: Data from a remote database is transmitted
periodically in small relevant blocks using links present in the
path of the automobile. This database may contain the
graphical representation of streets, buildings, maps and the
latest traffic information, which may be used by the driver to
decide on a route.
• Tele-medicine: Conference assistance from a surgeon for an
emergency intervention.
• Tele-Geo processing: Queries regarding location information
of the users.
• Crisis-management: Natural disasters, where the entire
communication infrastructure is in disarray.
• Education via the internet
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Routing in MANETS - Goals
• Provide the maximum possible reliability - use alternative routes
if an intermediate node fails.
• Choose a route with the least cost metric.
• Give the nodes the best possible response time and throughput.
• Route computation must be distributed. Centralized routing in a
dynamic network is usually very expensive.
• Routing computation should not involve the maintenance of
global state.
• Every node must have quick access to routes on demand.
• Each node must be only concerned about the routes to its
destination.
• Broadcasts should be avoided (highly unreliable)
• It is desirable to have a backup route when the primary route has
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become stale.
Reactive On-demand Source-based Routing
Schemes
• Routes are created when necessary
• Initiates a route discovery process
• Route will be maintained until it is no longer needed.
• Examples:
– Ad Hoc On Demand Distance Vector (AODV) routing
protocol
– Dynamic Source Routing (DSR)
– Temporary Ordered Routing Algorithm (TORA)
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A Highly Adaptive Distributed Routing
Algorithm for Mobile Wireless Networks
• Problems of routing in a mobile wireless
– Nodes move about arbitrarily
– Potentially rapid and unpredictable changing
topology
– Wireless links inherently have significantly
lower capacity than hardwired links
– More prone to congestion
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Temporally-Ordered Routing
Algorithm (TORA)
• Highly adaptive, loop-free, distributed routing
algorithm based on the concept of link reversal
• Proposed to operate in a highly dynamic mobile
networking environment
• It is source initiated and provides multiple routes for
any desired source/ destination pair
• This algorithm requires the need for synchronized
clocks
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Continued…
• TORA is a highly adaptive loop-free distributed routing
algorithm based on the concept of link reversal.
• TORA decouples the generation of potentially far-reaching
control messages from the rate of topological changes.
• The height metric is used to model the routing state of the
network.
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Continued…
Source
H=3
H=2
H=1
H=0
Destination
Illustration of Tora height metric
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Continued…
• The protocol can be separated into three
functions:
– Creating routes
establishment of a sequence of directed links
leading from node to dest.
– Maintaining routes
re-establish routes
– Erasing routes
upon detection of network partition, all links
must be undirected to erase invalid routes
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Continued…
• It accomplishes three functions through
use of control packets:
– Query (QRY) – used for creating routes
– Update (UPD) – for both creating and
maintaining routes
– Clear (CLR) – for erasing routes
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Continued…
• Creating Routes
– Requires use of the QRY and UPD packets
– QRY packet – dest-ID (did)
– UPD – did and height of node I broadcasting
the packet
– Each node maintains a route-required (RR)
flag, initially un-set
– Node with no directed links and an un-set RR
flag requires route to dest, it broadcasts a
QRY packet, set RR flag
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Continued…
• During the route creation and maintenance phases nodes
use a height metric to establish a Directed Acyclic Graph
(DAG) rooted at the destination.
• Thereafter links are assigned a direction based on the
relative heights
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Continued…
2 7
(-,-) 5 (-,-)
(-,-)
Source 1 3 8 Destination
(-,-)
(-,-) (0,0)
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4 (-,-)
(-,-)
Figure 13.6(a) – Propagation of the query message
2 7
(0,3) 5 (0,1)
(0,2)
Source 1 3 8 Destination
(0,3)
(0,3) (0,0)
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4 (0,1)
(0,2)
Node’s height updated as a result of the update message
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Continued…
• Provides multiple routes
• Minimizes algorithm’s reaction
– Localization of control messages (close to topological
change)
• Uses “height” metric to establish DAG
• If node other than destination is local minimum
– full / partial reversal method
S
D
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Continued…
• Provides multiple routes
• Minimizes algorithm’s reaction
– Localization of control messages (close to topological
change)
• Uses “height” metric to establish DAG
• If node other than destination is local minimum
– full / partial reversal method
S
D
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Continued…
• Provides multiple routes
• Minimizes algorithm’s reaction
– Localization of control messages (close to topological
change)
• Uses “height” metric to establish DAG
• If node other than destination is local minimum
– full / partial reversal method
S
D
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Continued…
• Provides multiple routes
• Minimizes algorithm’s reaction
– Localization of control messages (close to topological
change)
• Uses “height” metric to establish DAG
• If node other than destination is local minimum
– full / partial reversal method
S
D
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Creating Routes
QRY
UPD UPD
QRY
A B
UPD
QRY UPD E
C
UPD
QRY
D G (DEST)
F
H
UPD
QRY UPD
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Continued…
Route-required flag set
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Route maintenance
• Full reversal
– At each iteration each node other than the destination that
has no outgoing link reverses the directions of all its
incoming links.
• Partial reversal
– Every node u other than the destination keeps a list of its
neighboring nodes v that have reversed the direction of the
corresponding link (u, v)
– At each iteration each node u that has no outgoing link
reverses the directions of the links (u; v) for all v which do
not appear on its list, and empties the list. If no such v
exists, node u reverses the directions of all incoming links
and empties the list.
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Continued…
UPD
A B
UPD E
C
UPD
D G (DEST)
X
F
H
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Continued…
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Continued…
• Maintaining Routes performed only for nodes height > 0
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Erasing Routing
• Following detection of a partition, node I
sets its height and the height of neighbor
to NULL
• Update all entries in link-state array
• Broadcast a CLR packet
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Continued…
Link (D,H) marked as failing
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Continued…
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Performance
• TORA is able to localize its reaction to
topological changes
• Best suited for relatively dense networks,
only several nearby nodes involved in a
reaction
• Effect of localization – increased of
scalability
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Comparative Study
Protocols DSDV DSR AODV TORA
Attributes
Loop-free Y Y Y No, short lived loops
Multiple routes N Y N Y
Unidirectional links
N Y N N
supported
Periodic Broadcast Y N Y Y
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Advantages and Drawbacks:
• Advantages: provides loop free paths at all instants
and multiple routes so that if one path is not available,
other is readily available. It establishes routes quickly
so that they may be used before the topology changes.
• Drawbacks: exhibits instability behavior similar to
"count-to-infinity" problem in distance vector routing
protocols.
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Concluding remarks
• TORA not designed to find shortest path
• TORA uses paths close to optimum when node mobility is low
• Proposed a highly adaptive distributed routing algorithm
that well-suited in mobile wireless networks
• Decouple the generation of far-reaching control message
propagation from the dynamics of the network topology
• Possible enhancement would be to periodically propagate
refresh packets outwards from the dest.
• The refresh process permits intro of far-reaching control
message propagation into the protocol independent of the
network topology
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