27-06-2012, 05:04 PM
Fault-Tolerant Routing in Mobile Ad Hoc Networks
[attachment=25824]
Introduction
Mobile Ad Hoc Networks (MANETs) are characterized by the cooperative engagement of
mobile nodes that constitute networks possessing continuously-changing infrastructures,
the absence of centralized network managers, access points, fixed base stations, a backbone
network for controlling the network management functions, and the absence of designated
routers for making routing decisions. All the nodes in MANETs participate in the routing
process by acting as routers for one another. However, for the transmission of data from one
node to another, such networks normally require several hops because of the limited
wireless transmission range associated with the operation of the mobile nodes [2,7,9].
The above-mentioned characteristics of MANETs, particularly those arising due to the
mobility of the nodes, and the continuously-changing network infrastructure, pose several
challenges. Due to the continuously changing infrastructure, the routes that were once
considered to be the “best” may no longer remain as the “best” at a later time instant.
Therefore, one needs to continuously re-compute the routes, implying that in such
networks, there is no permanent convergence to a fixed set of routes. Thus, any routing
protocol that needs to operate in MANET network environments should take these issues
into consideration [2].
Routing protocols for MANETs
Routing in MANETs is currently a challenging and interesting problem studied by the
community primarily due to the dynamic nature of the infrastructure present in MANETs,
e.g., due to nodes joining and leaving the network. For routing, the transmission of data
from one node to another is direct, if the source and destination nodes are neighbors, i.e., if
they are within the wireless range of each other. On the other hand, the transmission is
indirect, if the source and destination nodes are not within their range of operation [7]. In
such a case, routing is achieved through a series of multiple hops, with intermediate nodes
between the source and the destination nodes serving the purpose of routers for relaying the
information in between. The dynamic nature of the topology of MANETs due to the
constant migration of nodes renders routing considerations difficult. The following
characteristics of MANETs make their routing further challenging [7]:
Unipath routing protocols
In unipath routing protocols, the transmission of messages between a source-destination
pair of nodes takes places through a unique path. All the unipath routing protocols may be
classified to be either table-based or on-demand [7]. Table-based protocols are characterized by
their ability to maintain routing tables that store information about routes from one node in
the network to the others. Obviously, this requires that the nodes in the network maintain the
table up-to-date by exchanging routing information between the participating nodes.
The AODV routing protocol
As the name suggests, AODV is classified as a unipath on-demand distance vector routing
protocol. It, therefore, functions by using both a route discovery phase and a route maintenance
phase by incorporating multihop routing in the intermediate nodes between the source and
destination. In the AODV, every mobile node functions as a specialized router. Routing
tables are maintained in the intermediate nodes, with routing information being obtained on
an “as-required” basis with no (or little) assumption on the presence of periodic
advertisements by the nodes [7,9]. The AODV has been shown to be scalable with the
increase in the number of mobile nodes in a MANET. It is characterized by its ability to
provide loop-free route information in which broken links are resolved by repairing existing
links or introducing new ones.
The DSR protocol
Like the AODV, the DSR is a unicast dynamic on-demand routing protocol. It is a source
routing protocol, where the source explicitly provides a packet with the complete
information of the route to follow, which is subsequently used by the intermediate nodes to
forward the packet to the correct destination node [7].
The DSR only routes packets between hosts that want to communicate with one another.
Like the AODV, the DSR also has a route discovery phase and a route maintenance phase.
When two nodes need to communicate with each other, the sender node determines a route.
This is done based on the information stored in its cache, or based on the results of a route
discovery phase, depending on whether or not the information about the destination node is
already available to the source node [5].
[attachment=25824]
Introduction
Mobile Ad Hoc Networks (MANETs) are characterized by the cooperative engagement of
mobile nodes that constitute networks possessing continuously-changing infrastructures,
the absence of centralized network managers, access points, fixed base stations, a backbone
network for controlling the network management functions, and the absence of designated
routers for making routing decisions. All the nodes in MANETs participate in the routing
process by acting as routers for one another. However, for the transmission of data from one
node to another, such networks normally require several hops because of the limited
wireless transmission range associated with the operation of the mobile nodes [2,7,9].
The above-mentioned characteristics of MANETs, particularly those arising due to the
mobility of the nodes, and the continuously-changing network infrastructure, pose several
challenges. Due to the continuously changing infrastructure, the routes that were once
considered to be the “best” may no longer remain as the “best” at a later time instant.
Therefore, one needs to continuously re-compute the routes, implying that in such
networks, there is no permanent convergence to a fixed set of routes. Thus, any routing
protocol that needs to operate in MANET network environments should take these issues
into consideration [2].
Routing protocols for MANETs
Routing in MANETs is currently a challenging and interesting problem studied by the
community primarily due to the dynamic nature of the infrastructure present in MANETs,
e.g., due to nodes joining and leaving the network. For routing, the transmission of data
from one node to another is direct, if the source and destination nodes are neighbors, i.e., if
they are within the wireless range of each other. On the other hand, the transmission is
indirect, if the source and destination nodes are not within their range of operation [7]. In
such a case, routing is achieved through a series of multiple hops, with intermediate nodes
between the source and the destination nodes serving the purpose of routers for relaying the
information in between. The dynamic nature of the topology of MANETs due to the
constant migration of nodes renders routing considerations difficult. The following
characteristics of MANETs make their routing further challenging [7]:
Unipath routing protocols
In unipath routing protocols, the transmission of messages between a source-destination
pair of nodes takes places through a unique path. All the unipath routing protocols may be
classified to be either table-based or on-demand [7]. Table-based protocols are characterized by
their ability to maintain routing tables that store information about routes from one node in
the network to the others. Obviously, this requires that the nodes in the network maintain the
table up-to-date by exchanging routing information between the participating nodes.
The AODV routing protocol
As the name suggests, AODV is classified as a unipath on-demand distance vector routing
protocol. It, therefore, functions by using both a route discovery phase and a route maintenance
phase by incorporating multihop routing in the intermediate nodes between the source and
destination. In the AODV, every mobile node functions as a specialized router. Routing
tables are maintained in the intermediate nodes, with routing information being obtained on
an “as-required” basis with no (or little) assumption on the presence of periodic
advertisements by the nodes [7,9]. The AODV has been shown to be scalable with the
increase in the number of mobile nodes in a MANET. It is characterized by its ability to
provide loop-free route information in which broken links are resolved by repairing existing
links or introducing new ones.
The DSR protocol
Like the AODV, the DSR is a unicast dynamic on-demand routing protocol. It is a source
routing protocol, where the source explicitly provides a packet with the complete
information of the route to follow, which is subsequently used by the intermediate nodes to
forward the packet to the correct destination node [7].
The DSR only routes packets between hosts that want to communicate with one another.
Like the AODV, the DSR also has a route discovery phase and a route maintenance phase.
When two nodes need to communicate with each other, the sender node determines a route.
This is done based on the information stored in its cache, or based on the results of a route
discovery phase, depending on whether or not the information about the destination node is
already available to the source node [5].