An Enhanced Expanding Ring Search for Route Discovery in MANETs Ngoc Duy Pham and Hyunseung Choo School of Information and Communication Engineering Sungkyunkwan University [email protected] Abstract A mobile ad-hoc network is a self-configuring network of mobile routers that are connected by wireless links in infrastructure-less situation. These kinds of networks have many challenges such as unreliable links, lack of scalability, etc. and energy consumption is a major issue in designing network protocols because of battery constraints. The route discovery process in reactive routing protocols uses Expanding Ring Search heuristic algorithm for reducing broadcast overhead and saving energy consumption. However, based on our observation we see that there are still some redundant broadcasts in this process. So, in this paper we propose a method which solves the redundancy of route discovery based on expanding ring search. The performance evaluation results show that our proposal helps to reduce the overhead of expanding ring search based route discovery up to about 20%, and therefore the total energy consumption of is decreased by 15%.

1. Introduction

overhead of the route discovery process,

Mobile Ad hoc Networks (MANETs) [1] provide

the

Expanding Ring Search [3], [4] is applied. This

a flexible, infrastructure-less communication which

heuristic

is an alternative for conventional wired networks.

non-destination

Because of the infrastructure-less characteristic,

request that they have cached information about

MANETs have many challenges and the success

destination.

of

Expanding Ring Search (ERS)

the

MANETs

nodes

a

replying

method to

the

for route

on

of

greatest

making the route discovery process be more

limitations of mobile computing environment is

efficient. However, it still has some disadvantages

the

mobile

that need to be alleviated in order to get better

devices. For extending the lifetime of mobile ad

results. Historically, there have been some ideas

hoc, many energy efficient protocols have been

proposed for improving ERS such as Optimising

designed

by

ERS [5] and Blocking ERS [6]. In our study, we

proposing an energy efficient scheme to improve

focus on the flooding mechanism of ERS. Because

the sustainability of MANETs.

ERS uses pure flooding for broadcasting the

In reactive multi-hop routing protocols [2], when

query message to all nodes, we propose a new

having

method to improve the flooding of ERS, and

sustainability.

finite

power

[2].

a

We

packet

One

supplies

inherent

continue

to

send

the

those

the

in

works

source

the

provides

depends

network

paradigm

algorithm

node

is applied for

broadcasts a request message to query the route

hence making it become more efficient.

to the destination, and then all nodes in the

In this paper, we describe a method which makes

network cooperate to re-transmit this message.

the ERS based route discovery of traditional

This process is called route discovery and the

reactive routing protocols like AODV and DSR [8]

re-transmitting

become

message

of

all

nodes

in

the

more

energy

efficient.

This

scheme

network in some cases causes a large overhead

reduces the number of re-transmitting request

with energy consumption. In order to reduce the

messages in the route discovery process which

85

means we can reduce the network overhead and save

energy

consumption.

The

performance

results show that our proposal can make ERS based

route

discovery

of

AODV

reduce

the

number of re-transmitting route request messages by 20%; therefore, up to 15% of the total energy consumption of this protocol can be saved. 2. Related Work

radius of the search ring and start searching

The Time To Live Sequence based Expanding

again. At the second searching, the nodes in the

Ring Search (ERS) [4] algorithm is a technique

first ring have to relay RREQ messages twice:

that can avoid network-wide broadcasting by

once in the first search and once for the second

searching a larger area around the source of

search. Similarly, if the third search occurs, nodes

broadcast. The goal of this algorithm is to find

in the first ring have to relay a RREQ message

nodes that have information needed about the

three times; nodes in the second ring have to

destination in their route caches. ERS is widely

relay RREQ twice. From those analyses, in this

used especially in multi-hop wireless networks.

section,

The source node is the center of the search ring;

optimizing ERS.

we

introduce

a

new

method

for

ERS successively searches a larger area until the node having needed information being searched is

3. 1 Basic Idea

found. The mechanism is: the searching process

When a node sends a message to the entire

begins by sending out a query with a small Time

network, it first broadcasts the message to its

To Live (TTL) value. Each time the query is

neighbors. In pure flooding, every neighbor has to

relayed by an immediate node, the TTL value is

relay the message by continuously re-transmitting

decreased by 1. If the TTL value is greater than

the receiving message to its own neighbors if it

0, the query will be forwarded; otherwise it is not

receives the message the first time. If the node

forwarded any further. After sending out a query

already has received this message before, it will

with a given TTL value, the source waits for a

drop the message immediately. However, if the

period of a timeout to receive the reply. If there

message has some data regarding the sender then

is no reply within the timeout period, the source

dropping the duplicate message will waste the

node increases the TTL by adding an incremental

neighbor’s information.

value and starts searching again. The searching

From the idea described above, in order to make

process, as above, continues until the information

duplicate messages more useful, we modify the

needed is found or the TTL value reaches a

pure flooding by a node before sending a message

threshold T. At this time, if no reply is received,

out and that will add the predecessor address into

the

entire

the message data. The predecessor of one node is

network (TTL value is set to a very large

the node from which it receives the message; in

number). Fig. 1 illustrates an example of an ERS.

this case, the predecessor is also called the

source

starts

a

broadcast

to

the

previous node or the sender. When relaying a 3. The Proposed Scheme

message,

the

node

also

adds

a

predecessor

Fig. 1(b) is an example of ERS that source

address into the message data before sending the

node S expands the search ring one time. In the

message out to neighbors. On receiving that

first ring, there is no node has information about

message, neighbors can collect the address of its

the destination so the node S has to increase the

sender and also the predecessor address of the

86

sender by extracting this information from the

address of the sender in the message data

incoming message data. This data is seen as the

before

local topology information of 2-hop neighbors.

predecessor address has been extracted, the

3. 2 Efficient Flooding Based on Overhearing

node

dropping compares

the the

message. address

When

with

its

the own

We now discuss an overhearing based efficient

address. If the two addresses have the same

flooding scheme for ad hoc networks. With the

value, the node sets the value of the variable

assumption that there are at least two floodings

‘relay’ to true. The example in Fig. 3.3 shows

happening

how each node works in the first step.

continuously,

the

proposed

scheme

reduces the overhead of flooding based on the

Fig. 2(a) shows the network topology. Each node

neighbor information collected in the previous

has a variable name ‘relay’ whose initial value is

pure flooding. The proposed scheme is divided

false. Node A wants to send a message to an

into

entire network; it broadcasts the message to its

two

parts:

the

first

is

collecting

local

topology information and the second is reducing

neighbors.

the overhead of pure flooding.

transmission

The collecting local topology information step

message. A becomes the predecessor of B and C

needs a process of pure flooding. When pure

in the message path. This is the first time B and

flooding occurs, every node collects neighbors’

C receive a flooding message so they participate

information by overhearing sending messages out

in relaying the message. But before relaying the

of its neighbors. Each node has a variable named

message, B and C add their predecessor addresses

‘relay’ and its initial value is false. The process at

into the message data and send them out.

each node occurs as in these steps:

Assume that B first relays the message received

z

When receiving a flooding message, the node

from A as in Fig. 2(b). A, C, and D receive the

relays the message if it has not received this

message of B. A receives a duplicate message; it

message

before.

the

extracts the predecessor address in the message

message

out,

sender

data and compares this to its address. They are

(predecessor) address into the message data

same so A sets its variable ‘relay’ to true. C also

and then broadcasts.

receives a duplicate message but the predecessor

But

the

before

node

adds

sending the

z If a node has already received this flooding

Because

B

and

range

of

A,

C they

are

in

the

receive

the

address in the message is different to its address

message before, it extracts the predecessor

so

87

C

drops

the

duplicate

message

without

changing anything.

node determines the value of the variable

Likewise, O and C relay the message received

‘relay’ by overhearing messages sent out by

from A (Fig. 2(c)). Each node receiving messages

neighbors.

from O and C completes the same process as

z The second search: Some nodes participated in

done in A and C above. Finally, D relays the

the first search can be silent by applying

message received from B (Fig. 2(d)). B overhears

reducing the overhead of pure flooding. The

the

the

silent node has its variable ‘relay’ set to false.

this

Nodes that are not in the overlapping area of

address with its own one. They are same so B

the first search and the second search have to

sets its variable ‘relay’ to true. C does nothing

use pure flooding while collecting the local

because the predecessor address is different to its

topology information for changing the value of

address.

variable ‘relay’.

sending

predecessor

message address,

out of and

D,

then

obtains

compares

In the second phase, reducing the overhead of

z The third search: Some nodes participating in

pure flooding step, if another pure flooding occurs

the second search can be silent by applying

then the overhead of this flooding can be reduced

reducing the overhead of pure flooding step as

by using the value of the variable ‘relay’ at each

in the second search. Nodes that are not in

node. Only nodes that have the variable ‘relay’ set

the overlapping area have to use pure flooding

to true relay the message received from its

while collecting the local topology information.

z The same process to the forth search, fifth

neighbor; nodes with ‘relay’ of false receive a message but do not relay. With the network in

search,

etc.

if

the

source

node

continues

Fig. 2(a), after a pure flooding of A, assume that

expanding the search ring for find information

O wants to send a message to the entire network

about the destination.

as in Fig. 2(e). At this time, only the nodes that 4. Simulation Results

have the variable ‘relay’ set to true (node A and

To

B) can relay a message for O. So, in this case,

analyze

the

efficiency

of

the

proposed

a

scheme, we compare the performance of ERS

message to the entire network and that message

with that of ERS using the proposed scheme. We

is propagated to all of the nodes.

use ERS based route discovery of AODV for an

there

are

only

3

broadcasts

for

sending

instance of ERS. The modified ERS based route discovery

3. 3 ERS based Route Discovery

of

AODV

proposed scheme.

In ERS based route discovery, the source node

is

an

instance

of

the

We also select pure route

uses continuous flooding with a larger searching

discovery of AODV

area to find information about the destination. The

comparisons.

mechanism of ERS satisfies the assumption of the

Qualnet simulator environment and perform the

proposed scheme, so in this part we present a

simulation

until

way how to apply our scheme to reduce the

converged;

then

overhead

of

the

i

th

searching th

based

on

the

Hundred

We

different

as a basic protocol

develop

the

performance we

obtain networks

simulation measures average are

for in are

values.

generated

searching. Assume that

randomly in an area, 1500 x 1500 m2. The

a source node invokes the route discovery process

transmission radius is set to 250 meter, each data

to find a route to a destination, the application of

packet with attached information has a constant

the

length of 512 bytes, and the bandwidth of a

information from the i-1

proposed

scheme

into

ERS

based

route

discovery is summarized as follow,

z The first search:

Collecting local

wireless channel is set to 2M b/s as the default. topology

In the simulation, we place a certain number of

information based on the pure flooding. Each

nodes from 50 to 200 randomly on the network

88

area. The network load is set to 50 active routes

process by reducing the number of nodes relaying

which means that during 30 seconds of simulation

the query messages. The results are energy is

time, at random moments, one random source

saved and collisions in the network are decreased.

sends a packet to a random destination and this

Our

occurs 50 times. Our observation from the figures

efficiency of route discovery using the proposed

of simulation results is:

scheme as compared with route discovery based

z The proposed idea makes AODV using ERS

on ERS only.

performance

evaluation

demonstrates

the

reduce by 20% of the number of RREQs forwarded. AODV using ERS is 20% better

Acknowledgement

than pure AODV in term of the number of

This research was supported by MIC, Korea

RREQs

under ITRC IITA-2007-(C1090-0701-0046).

forwarded;

therefore,

in

total

the

proposed scheme improves AODV up to about

Dr. Choo is the corresponding author.

40% for the reduced number of RREQs.

z Reducing the number of relaying messages has the

same

meaning

as

reducing

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energy

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Fig. 3(c), we see that the proposed scheme makes

AODV

using

ERS

reduce

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longer.

z Another important metric in the comparison is

Ring Search,” Proceedings of the IEEE AMS, 2007.

the successful route discovered. From the Figs.

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Networks,” Proceedings of the IEEE VTC, 2004.

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request process but does not affect the ratio of

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5. Conclusion

[6] I. Park, J. Kim, and I. Pu, “Blocking Expanding

In this paper, we proposed an efficient way to

Ring

Search

Algorithm

for

Efficient

Ad

Hoc

Energy

improve the route request process of AODV in

Consumption

particular and an efficient way to improve the

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in

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