Information Technology Reference
In-Depth Information
In the next paragraph we will describe the pro-
posed enhanced AODV algorithm named QBAR
(Queue Based Ad hoc Routing).
routers to discover neighboring routers. In our
algorithm during the Route Maintenance phase,
each node uses the HELLO packet to inform peri-
odically the neighbor nodes about its queue length
(Figure 1c). In this way each node can check if the
queue length of its next hop to a specific destina-
tion is becoming congested and in that case can
redirect flow traffic to an alternative next hop by
using the information stored in the routing table
during the Route Discovery phase.
In what follows we will describe the afore-
mentioned phases for the scenario depicted in
Figure 1a.
During the Route Discovery phase (Figures
1a-1b), node S
i1
wishes to communicate with the
sink node and so it sends a flood of RREQs to
the intermediate nodes. Let us suppose that the
intermediate node S
i1
receives first RREP1 from
the sink node through the path S
i2-
S
i5
(Figure 1b).
When node S
i1
receives RREP2 through the path
S
i3-
S
i6
, it uploads its routing table by selecting
S
i3
as an alternative next hop to the sink node.
Notice that, in contrast to the standard AODV
protocol, the information about the alternative
route to the sink destination from the intermediate
node S
i3
is stored in the routing table of node S
i1
as “alternative” next hop information. During the
Maintenance phase, node S
i1
uses the path S
i2-
S
i5
and periodically receives information about the
queue length of its neighbor nodes S
i2
and S
i3
by
the Hello packets (Figure 1c). In the same way
,
S
i2
has information about S
i5
's queue length. If the
queue lengths of nodes S
i5
and S
i2
are increasing
(Figure 1.d) then node S
i1
checks its routing table
observing that S
i3
is an “alternative” next hop.
Therefore, node S
i1
will send packets to the sink
node by the route S
i3-
S
i6
.
We maintain that the proposed QBAR is a hop
by hop strategy that can dynamically balance the
load avoiding congestion and QoS performance
degradations. In contrast, the AODV modifications
proposed in the literature mainly set a path in
the Discovery phase on the basis of an aggregate
performance index. Moreover, the modifications
A Queue Based Ad Hoc
Routing Algorithm
The AODV routing algorithm is composed of two
phases: Route Discovery and Route Maintenance.
In the following sections, we will describe the main
modifications introduced at the top level of AODV
to implement the proposed QBAR algorithm that
dynamically balances the load avoiding congestion
and QoS performance degradations.
The proposed modification enhances the
AODV algorithm and distributes the traffic among
nodes through a simple load balancing mechanism.
The protocol adopts the basic AODV procedure.
As stated above, in the AODV protocol each node
measures the number of packets queued up in its
interface. When a source node initiates a route
discovery procedure by sending out a flood of
RREQ messages, each node receiving an RREQ
will rebroadcast it adding its own interface queue
length. The destination node will select the best
route and reply with a RREP.
In the Route Discovery phase, the destination
node receives the RREQ and sends the RREP to
the intermediate nodes in the reverse paths. Due
to the flood of RREP messages, an intermediate
node can receive multiple RREPs. Usually the
routing table is uploaded, taking into account the
fresher RREP route, referred to as the shorter route
or the smaller aggregate path quality (i.e. energy,
aggregate queue). With the proposed algorithm
modification, we do not eliminate the previous
information of the RREP relating to the alternative
paths to a destination but store it in a routing table.
By so doing, at the end of the Route Discovery
phase, each node has in its routing table both the
current next hop and the alternative ones to use
to send data to a specific destination node.
In WSN a special packet “HELLO” is periodi-
cally sent from a node simultaneously to other
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