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Abstract
The fatal weakness of wireless sensor is the
problem of energy consumption. The AODV protocol is
based on the minimum number of hops. However, it may
not be the optimal routing. Considering from the
perspective of remaining energy of nodes, we found
this protocol is more likely to result in the death of
nodes. In order to solve this problem, we divided the
nodes into three stages according to the residual
energy of them. Then we improved each phase of the
protocol in order to ensure the maximize life of each
node, so that the survival time of entire network
would be improved obviously.
Keywords: wireless sensor; energy;
optimal routing; nodes; survival time.
1 Introduction
Mobile Ad—hoc Network is a multi-hop temporary
autonomous system that consisted with a group of
mobile nodes with wireless transceiver devices. Since
each node can be mobile, as a result, it may cause
changes in network topology.
If each network node is required to have a
router function at the same time, the corresponding
routing protocol must be able to adapt to the changing
link between the nodes, that is, fast-switching.
However, because each node does not rely on fixed
infrastructure, nodes can only rely on batteries to
obtain energy. So the energy is limited.
The energy conservation is essential for nodes
rely on battery-powered. If energy of one node is run
out, the entire network will be affected. If many
routings run through the same node or two nodes use
the same routing path, the energy of nodes will be
depleted rapidly. Therefore, the routing mechanism
decides the length of life of the Mobile Ad—hoc
Network. This paper began with the study on efficiency
of the energy usage, carried out some simple
modifications on the AODV routing protocol to ensure
that the energy consumption of each node is as fair as
possible in order to improve the life of the entire
network.
2 Introduction of AODV protocol
2.1 Routing Discovery
AODV routing protocol is an on-demand
routing protocol. Only when two nodes need for
communication and the source node hasn't reach the
destination node of the routing, the routing discovery
process occurs[2]. The discovery mechanism of AODV is
broadcasting routing. If one node would like to link
with another node when the routing table has not
reached the destination node of the routing entry, it
will broadcast a RREQ message. When relay nodes
received the message, if it is the destination node or
has routing leading to the destination node, it will
send a RREP message to source node. When the RREP
message arrives at the source node, the source node
can establish routing to the destination node.
2.2Routing maintenance
When movements of nodes in the MANET cause the
network topology changed, it resulting in part of the
routing entries in the routing table can not be
reused. If is caused by the movement of source node,
we can only initiate routing discovery process from
the source node again; While if relay node, we send an
RRER message to upstream node when the failure routing
node is detected. Related routing entries will be
updated when the node has received RRER message. Then
it will try to repair the routing. If the repair is
unsuccessful, a routing discovery process will be
initiated from the source node again.
2.3 Defects of AODV protocol on the
balance of energy
When traditional AODV
routing protocol establish routing from the source
node to the destination node, it chooses routings that
newer or with less routing hops. It will cause a high
frequency of use of certain nodes in the network if we
don't consider the current residual energy of nodes.
If many routings run through the same node, the speed
of the energy consumption in one node will be greatly
increased, so that the surplus energy in the node will
be soon run out of. On the other hand, as a result of
this unfair routing mechanism, many nodes died while
some other nodes consume almost no energy. If we work
in certain circumstances that we've defined the entire
network can not work any more when 1/3 of the nodes
found dead, the life of the network is short.
3. The improvement of the protocol
3.1 Definition of the work stage of
nodes
Each node in network has an energy value. In
this paper, we divide it into three stages according
to the energy value of the node including full of
energy stage, energy preservation stage and energy
weak phase stage. When the residual energy of nodes is
greater than 80% of the initial energy, we call it
full of energy stage; when the residual energy of
nodes is less than 5% of the initial energy, we call
it energy weak phase stage; other situation is called
energy preservation stage. If nodes are in the full of
energy stage, they work according to the original
protocol; while if the energy weak phase stage, nodes
don't transmit any information-only to receive and
send information; while if the energy preservation
stage, they work according to the following protocol.
3.2 Routing protocol when nodes in
the energy preservation stage
Since the AODV protocol is based on the minimum
number of hops and the routing has protection
mechanisms, once the routing table is determined, the
routing path will not be changed almost.
This will lead to a number of relay nodes die of
excessive energy consumption.
We discuss the following two cases.
1.Because the routing path will hardly be changed when
the routing table is determined, when the source node
sends data to the destination node frequently, it will
lead to a number of relay nodes die of excessive
energy consumption. In order to avoid this, once relay
nodes forwarding a message, automatic counting table
will cumulate a time. When the cumulate value overrun
a certain value will prompted to the source node
re-select the routing, and node itself is selected
automatically reduce the priority level. If the source
node to select this node for the relay node again,
Then their priority was to select a lower level again,
when it request re-select routing next time. Should be
noted here, if the two nodes of equal priority when
the relay competition, following the principle of
minimal hops. Source node in accordance with the
priority of the surrounding nodes considered for
selection relay node. Figure 1,if N1 sending data to
N2 lead excessive energy consumption in N0, N1 may
send data to N2 through N3, N4 or N5,N6, which prolong
the life span of N0.
2.A number of list routing are use the same relay
node, This situation is likely to result in the node's
energy drain easily. In order to avoid this situation,
Similarly the establishment of a table. If a node
elected to three different routing table at the same
time, then automatically reduces its priority level,
After this node to be selected more than one routing
table each time, and its priority level will be lower.
Of course, for some reason, this node have been kicked
out by one of routing tables, then its priority will
be a corresponding increase in level. If the two nodes
of equal priority when the relay competition, the same
following the principle of minimal hops. Figure 2, if
N1 send data to N6, N2 send data to N5, N3 send data
to N4, they will select N0 as routing, but after a
while, N3 may select N7 as the routing to send data to
N4, N1 will send data to the N6 through N8 and N9,
which reduce the N0 energy consumption due to heavy
load.
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