A Modified Routing Algorithm for Reducing Congestion in Wireless Sensor Networks

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European Journal of Scientific Research
ISSN 1450-216X Vol.35 No.4 (2009), pp.529-536
© EuroJournals Publishing, Inc. 2009

A Modified Routing Algorithm for Reducing Congestion in
Wireless Sensor Networks

N. Sengottaiyan
Research Scholar, Department of Computer Science and Engineering
Nandha Engineering College, Erode – 63 80 52, India
E-mail: nsriram3999@yahoo.co.in
Tel: +91 9942771217

Dean & Research Supervisor
Nandha Engineering College, Erode – 63 80 52, India

S. Arumugam
CEO, Nandha Engineering College, Erode – 63 80 52, India


With access to information type becoming complex, this forces demands in
increased machine load, data availability. The wireless communication lacks infrastructure
and hence data loss and overlapping is highly possible. The data being sent has varied
levels of priority. Congestion leads to indiscriminate dropping of data. Data loss may occur
in some deployments when sensors in one area of interest are requested to gather and
transmit data at a higher rate than others and hence a differentiated service must be
provided to these data and a congestion avoidance algorithm must be implemented to
overcome this problem. In this paper, CAR (Congestion Aware Routing) dynamically
discovers the congestion zone (conzone) and enforces differentiated routing based on
conzone and data priority. The proposed routing algorithm is implemented by building a
conzone and then the traffic is analyzed. The proposed approach also builds a minimal
spanning tree structure which reduces the path length in communication.

Keywords: Zone, Delay, Sensor nodes, Congestion, Spanning tree

1. Introduction
A wireless sensor network (WSN) is a wireless network consisting of spatially distributed
autonomous devices using sensors to cooperatively monitor physical or environmental conditions, such
as temperature, sound, vibration, pressure, motion or pollutants, at different locations. Wireless sensor
networks are now used in many civilian application areas, including environment and habitat
monitoring, healthcare applications, home automation, and traffic control. Each node in a sensor
network is typically equipped with a radio transceiver or other wireless communications device, a
small microcontroller, and an energy source, usually a battery. Limited power, ability to withstand
harsh environmental conditions, dynamic network topology is some of its characteristics. A sensor
A Modified Routing Algorithm for Reducing Congestion in Wireless Sensor Networks 530
network normally constitutes a wireless ad-hoc network, meaning that each sensor supports a multi-
hop routing algorithm. Congestion in network will lead to the following problems:
• It can lead to indiscriminate dropping of data. Some packets of high priority might be dropped
while others of less priority are delivered.
• Congestion can cause an increase in energy consumption as links become saturated.

2. Existing Protocols
2.1. AODV Protocol
The AODV routing protocol is a reactive routing protocol; therefore, routes are determined only when
needed. Hello messages may be used to detect and monitor links to neighbours. If Hello messages are
used, each active node periodically broadcasts a Hello message that all its neighbours receive. Because
nodes periodically send Hello messages, if a node fails to receive several Hello messages from a
neighbour, a link break is detected [1]. When a source has data to transmit to an unknown destination,
it broadcasts a Route Request (RREQ) for that destination.
At each intermediate node, when a RREQ is received a route to the source is created. If the
receiving node has not received this RREQ before, is not the destination and does not have a current
route to the destination, it rebroadcasts the RREQ [11]. If the receiving node is the destination or has a
current route to the destination, it generates a Route Reply (RREP). The RREP is unicast in a hop-by-
hop fashion to the source. Control messages are route request route reply and Hello message.

2.2. Dynamic Source Routing (DSR)
Dynamic Source Routing (DSR) also belongs to the class of reactive protocols and allows nodes to
dynamically discover a route across multiple network hops to any destination [1]. Source routing
means that each packet in its header carries the complete ordered list of nodes through which the
packet must pass. DSR uses no periodic routing messages (e.g. no router advertisements), thereby
reducing network bandwidth overhead, conserving battery power and avoiding large routing updates
throughout the ad-hoc network [1]. Instead DSR relies on support from the MAC layer (the MAC layer
should inform the routing protocol about link failures). The two basic modes of operation in DSR are
route discovery and route maintenance.

2.3. Performance Comparison of Routing Protocols
First, by virtue of source routing, DSR has access to a significantly greater amount of routing
information than AODV. Second, to make use of route caching aggressively, DSR replies to all
requests reaching a destination from a single request cycle. The main difference between both
protocols is that in DSR a source routing option is used; i.e. when a node wants to send something to a
destination it sets the whole route for that packet, indicating the addresses of the terminals it has to pass
through. In this sense all packets have a DSR header included, and it is needed that all nodes within the
ad hoc network know the whole network topology.
On the other hand, AODV does not perform source routing at all; when a terminal wants to
send something to a destination, it checks its routing table, looking for the next hop towards that
destination, and sends the packet to it, and so on[1]. In this sense, data packets "travel" through the ad
hoc network without any AODV specific information. AODV, however, outperforms DSR in more
stressful situations, with widening performance gaps with increasing stress [8].

3. Architectural Design
Routing protocol implementation constitutes the third OSI layer which is the Network Layer. In
wireless sensor networks, the protocol which is considered and modified is – AODV(Ad-hoc On-
531 N. Sengottaiyan, Rm.Somasundaram and S. Arumugam

demand Distance Vector Routing) [2]. In this present work, routing management is handled by two
protocols, AODV and Congestion Aware Routing.

Figure 3.1: Architectural design of routing protocol

4. Congestion Zone Design
CAR protocol is designed to provide differentiated routing for packets based on their priority. High
priority packets are routed inside a zone known as- conzone using CAR protocol and the low priority
packets are routed using the existing AODV protocol [6][7]. The CAR protocol design has two phases
– design of conzone and routing packets inside the conzone.

Figure 4.1: Working Model for the proposed routing approach

A Modified Routing Algorithm for Reducing Congestion in Wireless Sensor Networks 532
The present design comprises of the following steps:
• Access the routing table
• Include priority to the critical nodes
• Access the neighbour table
• Build Conzone using the RREQ broadcast
CAR provides differentiated routing based on priority of the nodes. High priority packets that
are generated from the critical nodes are routed inside the conzone while the low priority packets are
routed outside the conzone using the AODV protocol [10]. After the conzone is formed the routes from
the critical nodes to the sink are updated in the routing table and the high priority packets will use this
new route to the sink. The intermediate nodes which are on the conzone also updates its route to the
sink. The nodes are routed and handled using a spanning tree approach.

5. Implementation Details
The present routing algorithm is implemented using NS2. The proposed routing approach is
implemented with changes in the scripts as follows:
(i) Routing Table
Every node has its own routing table which stores the information about the various routes. For
each destination in the table the corresponding next hop, total hops, and the expiry time are
specified. As the expiry time elapses the route gets deleted or updated[4]. In ns2 the routing
table can be accessed at any time from the tcl script and the routing updation can be studied.
(ii) A new parameter called “priority” has been added to the routing table in aodv_rtable file to
specify the priority to critical nodes. The critical nodes are assigned a priority of 1 while the
other nodes are assigned 0.
(iii) Neighbour Table
Every node has its own neighbour table which stores all its neighbour ids. The neighbour table
is updated using the HELLO broadcast.
(iv) Conzone formation
The conzone is built in recvRequest(..) function in aodv.cc file. The critical node ids are
checked and the function to build conzone is called. When all parents for a level is entered, the
value in id_no array is changed to the new set of values. When the level is 3, the parents of
current level nodes are statically assigned the id of the sink node.
(v) Conzone Routing
The high priority packets are routed inside the conzone. First, the sendReply(..) function in
aodv.cc file is modified inorder to block the reply to the critical nodes. After the formation of
conzone the routing table of each node is accessed from tcl file and checked if its on conzone
(vi) If the node is on conzone then another function is called in rtable.cc which has index,
corresponding parent and level as its parameters. This function updates the route entries for the
conzone nodes.

6. Results and Discussions
The CAR protocol was analyzed in ns-2 and simulated in Network Animator (NAM) for an 8 * 5 grid.
The following snapshots describe the scenario and packet transmission from various sources to their
respective sink. Here the sink is node 35 and critical nodes are node 2, node 3 and node 4.
533 N. Sengottaiyan, Rm.Somasundaram and S. Arumugam

Figure 6: Node 4 sending packets to node 11

Figure 6.1: Node 3 sending to node 12 and node 12 sending to node 19

Figure 6.2: Congestion Zone formation

A Modified Routing Algorithm for Reducing Congestion in Wireless Sensor Networks 534
Figure 6.3
Routing Table using AODV

Figure 6.4: Routing Table using CAR

From the Fig 6.3 and 6.4 it is clear that the number of hops to reach to destination for node
number 35 is reduced.
535 N. Sengottaiyan, Rm.Somasundaram and S. Arumugam

Figure 6.5: Packet delivery ratio

Figure 6.6
Overall delay comparison

The nodes requests are handled as they arrive and leave the zone. The node relationships are
connected in form of a spanning tree. The path length for reaching the destination node using the
spanning tree structure. The performance is improved in proposed design [11].

7. Conclusion
In the present work data delivery issues in the presence of congestion in wireless sensor networks is
addressed. Congestion Aware Routing (CAR) is a routing protocol that uses data prioritization and
treats packets according to their priorities. We defined a conzone as the set of sensors that will be
required to route high priority packets from the data sources to the sink. Our solution does not require
active queue management, maintenance of multiple queues or scheduling algorithms, or the use of
specialized MAC protocols. Our extensive simulations show that as compared to AODV, CAR
increases the fraction of high priority data delivery, decreases delay and jitter for such delivery while
using energy uniformly in the deployment. By discovering the required conzone and using
differentiated routing we can free the conzone from most of the low priority traffic traveling through
the network. This will help nodes on the conzone to provide better service to high priority data.
A Modified Routing Algorithm for Reducing Congestion in Wireless Sensor Networks 536
8. Future Enhancement
The same approach can be extended by applying ant colony approach for identifying the traffic
behavior of the network. A routing mechanism verification unit using FPGA (Field Programmable
Gate Array) which will verify the proposed routing mechanism under worst conditions could be
designed and tested. The routing protocol proposed can be extended and tested under extreme traffic in
the network. CAR can work in a dynamic mode. If the rate of low priority data is not affecting the
service provided to high priority data then plain CAR is used. This allows more low priority data to be
delivered. As the rate of low priority increases and service provided to high priority data degrades,
CAR+ can be enabled to ease the congestion inside the conzone.
Low priority data that is generated outside the conzone stays outside; however, while the low
priority data generated inside the conzone is being routed out, it requires the conzone nodes to dedicate
some of their resources. This will degrade the service for high priority data. To better serve high
priority data, we disable any low priority message generation by on-conzone nodes.

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