Performance Improvement of

Optimized Link State Routing (OLSR) Protocol

Navaid Akhter1, Ammar Masood

2, Irfan Laone

3

Institute of Avionics and Aeronautics/Department of Avionics, Air University, Islamabad, Pakistan 1,2,3

Abstract- OLSR, a leading proactive protocol of MANET

maintains consistent and up to date network topology at

all the times and has emerged as the choice for MANETs

due to low latency for route determination. Hence, OLSR

generate a large amount of control overhead in order to

maintain an up-to-date routing table which consumes

bandwidth that should have been employed by user data

traffic instead. This paper addresses this issue by

optimizing OLSR under specific network and mobility

conditions which are actually more of practical interest

and thereby, our work does have a valuable contribution

to provide guidelines for large number of cases of general

interest. The proposal has shown to consistently

outperform the default implementation by reducing the

routing overhead under specific network and mobility

conditions considered at no extra cost. Other parameters

like data traffic and end-to-end delay also improved with

the approach presented in this study which shows the

efficiency of the scheme selected.

Keywords: MANETs, Routing Protocols, OLSR,

Improvement in Control Messages’ Intervals, Optimality in

performance

1 Introduction

Research concerning MANETs is currently of great

interest. The performance of MANET is related to the

efficiency of the routing protocols in adapting to frequently

changing network topology and link status [1]. Because of

the importance of routing protocols in the dynamic multi

hop networks, a number of routing protocols have been

proposed in the last few years; concurrently, a great deal of

research work is being undertaken by researchers to

improve their performances. In OLSR (a leading MANET

routing protocol), maintaining an up-to-date routing table

for the entire network calls for excessive communication

between the nodes as periodic control messages updates

are flooded throughout the network. Hence OLSR generate

a large amount of control overhead which consumes

valuable bandwidth that should have been employed by

user data traffic instead. Therefore, excessive control

overhead in OLSR is detrimental to its overall performance

in data forwarding, which has been analyzed for

improvement in our research work. Other parameters like

data traffic and end-to-end delay also improved with the

approach presented in this study.

2 MANET Routing Protocols

Mobile Adhoc Network (MANET) is an

autonomous system of mobile nodes connected by wireless

links [2]. MANET routing protocols are based on how

routing information is acquired and maintained by the

mobile nodes and thus, can be divided into proactive,

reactive and hybrid routing protocols [3]. With proactive

routing protocol, nodes in a MANET continuously evaluate

routes to all the reachable nodes and attempt to maintain

consistent, up-to-date routing information. On the other

hand, in reactive routing protocols for MANETs (also

called “on-demand” routing protocols), routing paths are

explored only when needed. Hybrid routing protocols are

proposed to combine the merits of both proactive and

reactive routing protocols and to overcome their

shortcomings.

3 Optimized Link State Routing

(OLSR) Protocol

The Optimized Link State Routing Protocol (OLSR)

[4] is developed for MANETs and does not need central

administrative system to handle its routing process.

Because of its proactive characteristic, the protocol

provides all the routing information to all participating

hosts in the network at all times. However, as a drawback,

OLSR protocol needs that each host periodically send the

updated topology information throughout the entire

network by flooding. This increases the protocol’s

bandwidth usage as the routing overhead is high. Although,

flooding in OLSR is minimized by the Multi Point Relays

(MPRs), which are the only nodes allowed to forward the

topological messages [5,6], still the routing overhead is

high as compared to reactive routing protocols.

3.1 Control Messages Intervals

OLSR employs two types of control messages:

Hello messages and TC messages.

3.1.1 Hello Interval

This parameter represents the frequency of

generating a Hello message. Hello Interval determines the

time between successive Hello messages, which is set to 2

seconds by default. Hello messages are never forwarded.

3.1.2 TC Interval

This parameter represents the frequency of

generating a TC message. In OLSR, the rate of the

topological state updates is the sending rate of TC

messages. TC messages are broadcasted periodically

within the TC interval, to other MPRs, which can further

relay the information to further MPRs. TC messages are

broadcasted once per refreshing period and the default

value is 5 seconds. TC messages are one of the major

sources of overhead in OLSR, as they are flooded

throughout the network, but they are essential to maintain

consistent connectivity knowledge of complete network.

3.2 Problem in OLSR

One advantage of OLSR is that it provides lower

route discovery latency than on-demand protocols because

of its proactive nature. But the flip side is that OLSR

generates a large amount of control overhead which

consumes precious bandwidth. Since the resources in

wireless networks are severely constrained, the increased

channel contention could lead to network congestion

resulting in significant lowering of network performance.

Further, scalability issues arise in OLSR due to the

excessive routing message overhead caused by the

increased network population. The size of routing table

grows non-linearly with the increase in number of nodes

and the control messages can block the actual data packets.

Hence, excessive control overhead in OLSR is detrimental

to its overall performance in data forwarding and poses a

research challenge that need to be addressed.

3.3 Proposed solution

Optimization of local and global topology

dissemination intervals (i.e. Hello and TC intervals

respectively) is proposed under specific network and

mobility conditions which are actually more of practical

interest and thereby, our work does have a valuable

contribution to provide guidelines for large number of

cases of general interest that result in low routing overhead

(as compared to the default settings) and thus beneficial for

OLSR performance. This study targets on reduction in

control overhead with improvement in performance of

OLSR by optimizing control messages intervals.

3.3.1 Logical Reasoning of proposed solution

Hello messages are broadcasted periodically for

link sensing and neighbor detection. This is also required

to complete the MPR selection process. After the MPR

selection process is completed, TC messages are generated

and are disseminated throughout the network. Subsequent

to the receipt of these TC messages, the nodes calculate the

routing table and the links are available for data

communication. Further, MPRs broadcast the TC messages

in the network to maintain a consistent and up-to-date view

of complete network topology. In case of any topology

changes, the MPR selection process is re-initiated and the

routing table is re-computed by the nodes [8]. MANETs

require minimum control overhead to reduce channel

contention and battery consumption problems. TC

messages share a large amount of overhead in OLSR

because of its global dissemination nature. Decreasing the

broadcast frequency of TC messages reduces the overall

routing traffic sent while not incurring any degradation in

throughput / end-to-end delay under specific network and

mobility conditions as shown in this study. Further, due to

frequent topology changes caused by high mobility, the

routing information needs to be updated more frequently so

as to update the topology changes and guarantee the

correctness of route selection. This requires that nodes of

OLSR employed MANET detect link changes more

quickly and broadcast topology updates with lesser delay.

This can be achieved by increasing the Hello messages

sending rate for faster response to the link and neighbor

changes (especially in case of high mobility scenarios);

hence, providing better throughput as compared to the

default Hello interval. The increase in routing overhead

because of the increase in Hello sending rate above is

compensated by the reduction in routing overhead due to

the decrease in TC messages sending rate as mentioned

earlier.

4 Related Work

The authors of OLSR in RFC 3626 (OLSR) [4]

pointed out that the nodes may send control messages at

different rates, if beneficial for specific deployment. Many

strategies have been proposed by OLSR researchers using

different performance metrics to improve the performance

of OLSR by varying control messages intervals [7, 8, 9, 10,

11, 12, and 13]. However, these works usually target to

reduce control overhead while having certain deficiencies

and implementation complexities. Our work, however,

does not include any added complexity or depends upon

any measurement of network parameters and provides

improved performance of OLSR under specific network

and mobility conditions by just modifying the OLSR

control messages intervals. With our approach, now

network is able to achieve an increase in data traffic

received (vis-à-vis the payload with default control

messages intervals) while routing traffic and end-to-end

delay are both reduced. We have constructed fairly robust

scenarios for experiments to investigate the effect of

control messages intervals on the routing overhead of

OLSR.

5 Performance Evaluation

Because of the unavailability of wide range of real

MANETs, the performance analysis of wireless

applications or protocols in the context of MANETs often

require to be evaluated through simulation studies [14].

The performance analysis on a real network (if available)

can be rather tedious if large networks are considered

(typically hundreds of nodes). This is why simulation is an

important tool in the sense that it can often help to improve

or validate protocols [15]. OPNet Modeler 14.5 network

simulator was used for analysing the performance of OLSR

in this study.

5.1 Choice of Network and Mobility

Conditions

The MANET routing protocols perform

differently under different network & environmental

factors like node mobility, number of nodes, number of

source-destination pairs, traffic type, traffic intensity,

propagation models etc. For the purpose of performance

analysis in this study, we selected two factors: Node

mobility and Number of nodes, because of their major

impact on the mechanics of the protocol vis-à-vis routing

overhead. We started with a carefully designed network

scenario for all the experiments and varied one parameter

at a time and thus stressed the network in different axis as

shown in Table 1.

Table 1

Network & Mobility Factors

As mobility has the most significant impact on

MANET routing protocols [16], scenarios have been

constructed to evaluate the proposed solution against

varying nodes speed (from 5 mps to 40 mps) while keeping

the number of nodes fixed. The speed range has been

selected with keeping in view the speed selected for

extreme practical scenarios (low to high) and most of the

other research work in this area. Table 2 depicts the

parameters selected for the Scenarios.

Table 2

SIMULATION PARAMETERS

5.2 Performance Metrics

The performance metrics investigated during this

study were the data traffic received and routing overhead in

OLSR protocol vis-à-vis its improvement by optimizing the

OLSR control messages interval under various network

and mobility conditions. However, end-to-end packets

delay was also kept under-check so as to ensure that the

optimization of control messages for improvement in

routing overhead do not degrade this parameter. The

definition of improved performance is that the routing

protocol must provide applications with high data traffic

received, minimal routing overhead and low end-to-end

delay.

6 Results

6.1 Experiments

The simulation study adopts a step-by-step

performance optimization approach. Firstly, the impact of

each control messages’ interval of OLSR has been

analyzed distinctly on the data traffic received, routing

traffic overhead and the end-to-end packets delay by

stressing the mobility factor. The outcome of the results

has been analyzed further to see how these control

messages’ interval can be optimized simultaneously so as

to efficiently maximize the data traffic received (payload)

while minimizing the routing overhead and end-to-end

packets delay. The steps are depicted in Figure 1.

Figure 1: Simulation study steps

6.2 Experiment No 1

In experiment No 1, the Hello message update

rate has been increased from default value (2 seconds) to

various values like 1.9, 1.8, 1.7, 1.6, 1.5 seconds etc in

order to facilitate the routing protocol to speed up the

adaptation to neighbor changes while keeping the TC

interval at 5 seconds (default value). The value of the state

holding timer interval (neighbor hold time) was adjusted

accordingly. After various iterations, it was found that

Hello interval of 1.8 seconds (TC at default value)

provides the best balance between data traffic and routing

traffic and the results are shown in Figure 2.

Figure 2: Results of data traffic and routing traffic vs speed with change

in Hello interval

In Figure 2, the data traffic and routing traffic are

plotted on Y-axis and variation in speed is plotted on X-

axis. In all the simulations across specific range of nodes

speed while changing the Hello interval and keeping the

TC interval fixed, it can be appreciated that increase in

Hello sending rate (i.e. Hello 1.8 secs) from the default

value (Hello 2 secs) improves the data traffic received as it

helps the routing protocol to quickly adapts the changes in

neighbors and update the routing tables accordingly. On

the other hand, the routing traffic overhead also increases

with the increase in Hello sending rate which clearly depict

that although fast Hello messages improves the protocol

reactivity to link failures; however this is at the cost of

increased routing overhead.

Hence, from Experiment No 1, it is concluded that an

improvement in data traffic received by increasing the

Hello messages sending rate is at the expense of increased

routing traffic overhead.

6.3 Experiment No 2

In experiment No 2, the TC interval has been

decreased from default value (5 seconds) to various values

like 6, 7, 7.5 seconds etc in order to reduce the routing

overhead while keeping the Hello interval at 2 seconds

(default value). The value of state holding timer interval

(topology hold time) was adjusted accordingly. Since the

Hello interval is kept constant, the reduction in overall

routing overhead is the result of decrease in TC messages

overhead. The TC interval of 7.5 seconds was found to be

the optimized value under the considered mobility

conditions that provide a decrease in routing traffic

overhead while having almost no effect on data traffic and

the results are as shown in Figure 3.

Figure 3: Results of data traffic and routing traffic vs Speed with change

in TC interval

In Figure 3, the routing traffic and data traffic are

plotted on Y-axis and variation in speed is plotted on X-

axis. Firstly observing the routing traffic behavior with

default values of OLSR, it is revealed that as the speed

increases, the routing traffic decreases. This is because of

the reason that as mobility increases, link breakages

increases and therefore TC messages are either not

generated or if they are generated than they are not

forwarded to the entire network. Now with the modified

settings, it is evident that as TC messages sending rate is

reduced from 5 seconds (default) to 7.5 seconds, the

routing traffic is less as compared to the routing traffic at

default TC interval. Further, it is observed that decreasing

the TC sending rates from default value to 7.5 secs

although reduces large routing overhead; brings no

significant change in data traffic received. This is because

of the fact that repetitive TC messages are broadcasted

throughout the network to maintain the network topology.

Lowering down the sending rate of these TC messages

under specific mobility conditions although reduces large

routing overhead, brings no significant change in data

traffic received at the nodes which is more sensitive to the

change in Hello interval than the TC interval.

6.4 Outcome of Experiment No 1 and

Experiment No 2

Through simulations it has been explored that TC

messages generate more overhead than Hello messages

because TC messages are forwarded globally to each node

in the network while Hello messages are only exchanged

locally between neighboring nodes. Increasing the Hello

messages sending rate helps the routing protocol to quickly

adapt the changes in neighbors and update the routing

tables accordingly. Hello interval rate has been increased

from default value (2 secs) to 1.8 secs in order to speed up

the adaptation to neighbor changes and thus achieving

higher data traffic received than what it is achieved at the

default value. This is particularly to cater the declined

performance of OLSR under high mobility scenarios.

Decreasing TC messages sending rate leads to significant

reduction in control overhead but do not downgrade the

data traffic received under specific mobility conditions

considered in the experiments.

6.5 Experiment No 3

Experiments 1 and 2 provides a comprehensive

understanding of OLSR’s control timers’ behavior vis-à-vis

performance metrics and gives insightful guidance in

optimizing these timers for an improved performance in

data traffic received while introducing low routing

overhead as compared to the default values. The results

have been exploited further in experiment 3 to formulate

that how these two timers can be optimized simultaneously

under considered network and mobility conditions so as to

efficiently minimize the routing overhead while achieving

maximum data traffic received without compromising on

end-to-end delay. The OLSR control messages intervals

Hello 1.8 secs and TC 7.5 secs (as discussed in

experiments 1 and 2) were selected and compared against

the default intervals to see if there is any improvement as

stated above.

6.5.1 Routing traffic vs Speed:

In Figure 4, the routing traffic is plotted on Y-

axis and variation in speed is plotted on X-axis.

Figure 4: Result of Routing traffic vs Speed with change in HELLO and

TC intervals

First observing the routing traffic behavior with

default values of OLSR (i.e. Hello def_TC def), it is

revealed that as speed increases the routing traffic

decreases. This is because of the reason that as mobility

increases, link breakages increases and therefore TC

messages are either not generated or if they are generated

than they are not forwarded to the entire network. This

results into decrease in routing traffic as the speed

increases and vice versa. Now with the modified intervals

(i.e. Hello 1.8_TC 7.5), the similar behavior of decrease in

routing overhead with increase in mobility is observed as

stated above. Further, as the TC messages sending rate is

reduced from 5 seconds (default) to 7.5 seconds, the

routing traffic is noticeably reduced as compared to the

routing traffic at default TC interval. Also due to the

increase in Hello sending rate from 2 seconds (default) to

1.8 seconds, the routing traffic would have increased (as

observed in experiment 1). However, this has been

compensated with the reduction of large routing overhead

due to the decrease in TC interval. Hence, the overall result

is the reduction of routing overhead as compared to the

routing overhead with default Hello and TC values. The

average reduction in routing overhead achieved with the

modified Hello and TC intervals is 14.06 %.

6.5.2 Data traffic vs Speed

In Figure 5, the data traffic is plotted on Y-axis

and variation in speed is plotted on X-axis.

Figure 5: Result of Data traffic vs Speed with change in HELLO and

TC intervals

Firstly, observing the data traffic behavior with default

values of OLSR (i.e. Hello def_TC def); it is revealed that

as speed increases the data traffic decreases. This is

because of the reason that as mobility increases, link

breakages increases and therefore the nodes are unable to

forward the data to the required destination which resulted

into dropping of data packets before reaching to the

destinations. Now with the modified interval i.e. Hello

1.8_TC 7.5, similar behavior of decrease in data traffic is

observed with the increase in node’s mobility (due to the

same reason as mentioned above). However the data traffic

is now improved than what it is achieved with the default

OLSR intervals (i.e. Hello def_TC def). This is because of

the increase in Hello messages sending rate which speed up

the routing protocol’s adaptation to neighbor changes and

route maintenance and thus resulting into less data drop

and increase in data traffic received at the destinations.

Further, it is observed that both the curves are tending to

converge at very low speeds and at very high speeds. It is

because of the reason that at very low speeds, there are no

significant changes in neighbors so the default interval as

well as modified Hello interval works almost the same

manner and the change in Hello interval does not make any

difference. Similarly at very high speeds, the topology

changes might be too dynamic to be captured by the

periodic updates of OLSR with default as well as with the

modified settings so the change in Hello interval does not

make any significant impact in this regime also. The

average increase in data traffic achieved with the modified

values of Hello and TC intervals is 6.19%

6.5.3 End to End packets delay vs Speed:

In Figure 6, the delay is plotted on Y-axis and

variation in speed is plotted on X-axis. Firstly, observing

the end to end packets delay behavior with default values

of OLSR (i.e. Hello def_TC def); it is revealed that as

speed increases, the end to end packets delay decreases.

Figure 6: Result of End to End packets delay vs Speed with change in

HELLO and TC intervals

This is because of the reason that as mobility increases,

link breakages increases and therefore less number of

source-destination pairs are now available at high speeds as

compared to the scenarios at low speeds. This results into

increase in channel capacity because of the occupation of

same number of available channels now with less number

of source destination pairs. Hence, the packets reach to the

destination with lesser problems of channel contention and

therefore end to end packets delay decreases. The similar

behavior is observed with the modified intervals of OLSR

because of the same reason as mentioned above. However,

now with the modified intervals, the end to end packets

delay is less as compared to the default settings. This is

because of the increase in hello sending rate which

increases the routing protocol’s adaptation to neighbor

changes and route maintenance that decreases the overall

end to end packets delay.

6.6 Summary of Experiment No 3

The simulation results demonstrated that by

optimizing the Hello and TC intervals, optimality in the

routing protocol performance is achieved under specific

mobility factors considered. Through simulations it has

been explored that increasing rate of hello update leads to

improvement in link establishment and node status

maintenance. Further, decreasing rate of TC updates leads

to significant reduction in control overhead but do not

downgrade the data traffic received under specific mobility

conditions considered. Hello interval has been slightly

decreased from default value (2 secs) to 1.8 secs in order to

alleviate the degraded performance of OLSR under high mobility scenarios thus achieving higher data traffic

received than the default value. Increase in routing traffic

due to increase in Hello interval has been compensated by

decreasing the TC sending rate from 5 to 7.5 secs which

drastically reduced the overall routing overhead while not

posing any significant impact on data traffic received. This

also resolves the problem of high routing overhead of

OLSR (generated due to its proactive nature) under the

specific mobility conditions considered. In the proposed

Hello and TC intervals, OLSR is now able to sustain an

increased data traffic received compared to the default

values of Hello and TC intervals and at the same time, both

the routing traffic and end-to-end packet delay are also

reduced.

7. Conclusion

MANET is an autonomous system of mobile

nodes connected by wireless links. The performance of

MANET is related to the efficiency of the routing

protocols. OLSR, a well known proactive protocol has

emerged as the choice for MANETs (especially for delay

sensitive applications) due to low latency for route

determination. But at the same, time associated high

routing overhead (due to proactive nature) has emerged as

a major performance issue in OLSR. In this study, we have

addressed this issue by optimizing the OLSR under specific

mobility conditions which are actually more of practical

interest and thereby, our work does have a valuable

contribution to provide guidelines for large number of

cases of general interest. The default parameters of Hello

and TC intervals of OLSR are selected such that the

network performance is improved. The behavior of the

routing protocol is tested based on the influence of node

mobility using various performance metrics. From the

results of simulations, it is concluded that the optimization

of OLSR control messages intervals has shown to

consistently outperform the default implementation of

OLSR under specific mobility conditions considered

during this study. We envisage undertaking research to

analyze the scalability of OLSR protocol vis-à-vis control

messages intervals with number of nodes and to set the

boundary limits through detailed simulation studies.

Furthermore, the performance analysis of OLSR protocol

must be analyzed with realistic mobility models [17] so as

to finalize realistic protocol performance.

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