Enhancing Delay in MANET Using OLSR Protocol

Abstract

The performance of a Mobile Ad hoc Network (MANET) is closely related to the capability of the implemented routing protocol to adapt itself to unpredictable changes of topology network and link status. The Optimized Link State Routing (OLSR) protocol is a one key of the proactive routing protocols for MANETs. It is based on the multi-point relays (MPRs) technique to reach all nodes in the network with a limited number of broadcasts. In this paper, we propose new versions of the original OLSR protocol based on a new mobility parameter, in the goal to enhance and adapt it in the presence of the mobility. For this objective we define new three criterions for MPRs selection. The first criteria take for selection, just the mobility of nodes at one-hop. The two others criterions are based on both mobility of nodes at one-hop and two-hops.

Full text

Int. J. Communications, Network and System Sciences, 2009, 5, 392-399
doi:10.4236/ijcns.2009.25044 Published Online August 2009 (http://www.SciRP.org/journal/ijcns/).
Copyright © 2009 SciRes. Int. J. Communications, Network and System Sciences
Enhancing Delay in MANET Using OLSR Protocol
N. ENNEYA, K. OUDIDI, M. ELKOUTBI
E.N.S.I.A.S, Laboratory SI2M, University Mohammed V-Souissi, Rabat, Morocco
Email: {enneya,oudidi,elkoutbi}@gmail.com
Received October 30, 2008; revised January 22, 2009; accepted March 31, 2009
ABSTRACT
The performance of a Mobile Ad hoc Network (MANET) is closely related to the capability of the imple-
mented routing protocol to adapt itself to unpredictable changes of topology network and link status. The
Optimized Link State Routing (OLSR) protocol is a one key of the proactive routing protocols for MANETs.
It is based on the multi-point relays (MPRs) technique to reach all nodes in the network with a limited num-
ber of broadcasts. In this paper, we propose new versions of the original OLSR protocol based on a new mo-
bility parameter, in the goal to enhance and adapt it in the presence of the mobility. For this objective we de-
fine new three criterions for MPRs selection. The first criteria take for selection, just the mobility of nodes at
one-hop. The two others criterions are based on both mobility of nodes at one-hop and two-hops.
Keywords: Ad Hoc Networks, OLSR Protocol, Multipoint Relays, Node Mobility Degree,
Mobility Quantification
1. Introduction
A Mobile Ad hoc Network (MANET) is a collection of
mobile nodes (MNs) that cooperatively communicate
with each other without any pre-established infrastruc-
tures such as a centralized access point. These nodes may
be computers or Devices such as laptops, PDAs, mobile
phones, pocket pc with wireless connectivity are com-
monly used. Due to the fact that MNs change their
physical location by moving around, the network topol-
ogy may change unpredictably. This causes changes of
link status between each MN and its neighboring. Thus,
MNs which join and/or leave the communication range of
MN in the network will surely change its relationship
with its neighbors by detection of a new link breakages
and/or link additions. In the same way, the change of the
all routes printed by this MN is also based on the rela-
tionship. This change of routes is made with an overhead
traffic in the process of maintenance routes assured by
the implemented routing protocol in a MANET. For re-
sume, the performance of a MANET is closely related to
the capability of the routing protocols to adapt them-
selves to unpredictable changes of topology network and
link status [23,24].
One of the most important aspects of the communica-
tion process is the design of routing protocols used to
establish and maintain multi-hop routes to allow data
communication between nodes. Several researches have
been done in this area, and many multi-hop routing pro-
tocols have been developed. The Optimized Link State
Routing (OLSR) protocol [1,2], Dynamic Source Rout-
ing protocol (DSR) [5], Ad Hoc on Demand Distance
Vector protocol [6], Temporally Ordered Routing Proto-
col (TORA) [12], and others protocols that establish and
maintain routes on a best-effort basis. There are three
main categories of MANET routing protocols: Proactive
(table-driven), Reactive (on-demand) and Hybrid. Proac-
tive protocols build their routing tables continuously by
broadcasting periodic routing updates through the net-
work; reactive protocols build their routing tables on
demand and have no prior knowledge of the route they
will take to get to a particular node. Hybrid protocols
create reactive routing zones interconnected by proactive
routing links and usually adapt their routing strategy to
the amount of mobility in the network.
In this paper, we present a new quantitative measure
of node mobility reflecting the mobility degree at each
node in the MANET. This node mobility degree is re-

ENHANCING DELAY IN MANET USING OLSR PROTOCOL
Copyright © 2009 SciRes. Int. J. Communications, Network and System Sciences
393
lated to the link status change in the vicinity of the
communication range. Therefore, based on this mobility
quantification at each MN in the MANET, we have pro-
posed three versions of the original OLSR protocol to
enhance and adapt it in the presence of high mobility, i.e.
high topology and link status changes.
The rest of this paper is organized as follows. Section
2 gives an overview of the original OLSR protocol. Sec-
tion 3, presents our proposed node mobility degree. Sec-
tion 4 presents performance metrics for evaluating per-
formance of routing protocols. In Section 5, simulations
and results are given. The last section concludes and
presents some future works.
2. Optimized Link State Routing Protocol
2.1. Overview
The optimized link state routing (OLSR) protocol [1] is a
proactive routing protocol that employs an efficient link
state packet forwarding mechanism called multipoint
relaying. This protocol optimizes the pure link state
routing protocol. Optimizations are done in two ways: by
reducing the size of the control packets and by reducing
the number of links that are used for forwarding the link
state packets. The reduction in the size of link state pack-
ets is made by declaring only a subset of the links in the
link state updates. These subsets of links or neighbors
that are designated for link state updates and are assigned
the responsibility of packet forwarding are called multi-
point relays. The optimization by the use of multipoint
relaying facilitates periodic link state updates. The link
state update mechanism does not generate any other con-
trol packet when a link breaks or when a link is newly
added. The link state update optimization achieves
higher efficiency when operating in highly dense net-
works. The Figure 1(a) shows the number of message
transmissions required when the typical flooding-based
approach is employed. In this case, the number of mes-
sage transmissions is approximately equal to the number
of nodes that constitute the network. The set consisting
of nodes that are multipoint relays is referred to as
MPRset. Each given node in the network selects an
MPRset that processes and forwards every link state
packet that this node originates (see Figure 1(b)). The
neighbor nodes that do not belong to the MPRset process
the link state packets originated by node P but do not
forward them. Similarly, each node maintains a subset of
neighbors called MPR selectors, which is nothing but the
set of neighbors that have selected the node as a multi-
point relay. A node forwards packets that are received
from nodes belonging to its MPRSelector set. The mem-
bers of both MPRset and MPRSelectors keep changing
over time. The members of the MPRset of a node are
selected in such a manner that every node in the node’s
two-hop neighborhood has a bidirectional link with the
node.
Figure 1. Example of MPRs selection in OLSR protocol.
The selection of nodes that constitute the MPRset sig-
nificantly affects the performance of OLSR because a
node calculates routes to all destinations only through the
members of its MPRset. Every node periodically broad-
casts its MPRSelector set to nodes in its immediate
neighborhood. In order to decide on the membership of
the nodes in the MPRset, a node periodically sends Hello
messages that contain the list of neighbors with which
the node has bidirectional links and the list of neighbors
whose transmissions were received in the recent past but
with whom bidirectional links have not yet been con-
firmed. The nodes that receive this Hello packet update
their own two-hop topology tables. The selection of mul-
tipoint relays is also indicated in the Hello packet. A data
structure called neighbor table is used to store the list of
neighbors, the two-hop neighbors, and the status of
neighbor nodes. The neighbor nodes can be in one of the
three possible link status states, that is, unidirectional,
bidirectional, and multipoint relay. In order to remove
the stale entries from the neighbor table, every entry has
an associated timeout value, which, when expired, re-
moves the table entry. Similarly a sequence number is
attached with the MPRset which gets incremented with
every new MPRset.
The MPRset need not be optimal, and during initiali-
zation of the network it may be same as the neighbor set.
The smaller the number of nodes in the MPRset, the
higher the efficiency of protocol compared to link state
routing. Every node periodically originates topology
control (TC) packets that contain topology information
with which the routing table is updated. These TC pack-
ets contain the MPRSelector set of every node and are
flooded throughout the network using the multipoint re-
laying mechanism. Every node in the network receives
several such TC packets from different nodes, and by
using the information contained in the TC packets, the
topology table is built. A TC message may be originated
by a node earlier than its regular period if there is a

N. ENNEYA ET AL.
Copyright © 2009 SciRes. Int. J. Communications, Network and System Sciences
394
)
change in the MPRSelector set after the previous trans-
mission and a minimal time has elapsed after that. An
entry in the topology table contains a destination node
which is the MPRSelector and a last-hop node to that
destination, which is the node that originates the TC
packet. Hence, the routing table maintains routes for all
other nodes in the network.
2.2. MPR Selection Algorithm
The computation of the MPR set with minimal size is a
NP-complet problem [16]. For this end, the standard
MPR selection algorithm currently used in the OLSR
protocol implementations is as follows:
For a node x, let N(x) be the neighborhood of x. N(x) is
the set of nodes which are in the range of x and share
with x a bidirectional link. We denote by N2(x) the
two-neighborhood of x, i.e, the set of nodes which are
neighbors of at least one node of N(x) but that do not
belong to N(x) (see Figure 2).
Based on the above notations, the standard algorithm
for MPR selection is defined as follows:
1)
2()UNx
2)
()MPR x 
3) :!():(while v v U w N x v N w do 
a)
()UUNw
b) () () {}
M
PR x MPR x w
4) ()while U do
a)
() : ( )choose w N x such as CRITERIA w
''
() max( : ())Nw U w U w Nx  
b)
()UUNw
c) () () {}
M
PR x MPR x w
5) ()return MPR x
3. Proposed Node Mobility Degree
Each node in a mobile ad-hoc network can be found in
four states with its neighbor nodes: the node moves and
its neighbors are static, the node is static and its
neighbors move, the node and its neighbors move, the
Figure 2. Example of MRRset calculation.
node and its neighbors are static. Consequently, these
four possible states result in a change of the link status of
the node with its neighbors. So, as the nodes move in the
mobile ad-hoc network, the link status changes in time.
Based on this observation, we define a mobility meas-
ure representing the degree of node mobility in the net-
work. This mobility measure has no unit and don’t de-
pend upon simulation artifacts such as mobility model
parameters or movement patterns. Moreover its evalua-
tion is done at discrete time intervals.
We define the mobility degree of a mobile node i at a
time t by the following formula:
1
iNodesOut( t ) NodesIn(t )
M(t) ( )
Nodes(t t ) Nodes(t )



 (1)
where:
NodesIn(t ) : The number of nodes that joined the
communication range of during the interval
i


tt,t .
NodesOut( t ) : The number of nodes that left the
communication range of during the interval
i


tt,t .
Nodes( t ) : The number of nodes in the communica-
tion range of at time t.
i

: The mobility coefficient between 0 and 1 defined
in advance.
This node mobility degree is quantified locally and
independently of the localization of a given node in the
network. We represent this local and relative quantifica-
tion by the change of the neighbors of each node. The
node mobility degree at a given time t for node in the
mobile ad-hoc network is defined as the change in its
neighbors compared to the previous (state) at time tt
i

.
Thus, mobile nodes that join and/or leave the neighbors
of node will surely have an impact on the evaluation
of its mobility degree. Moreover, we have chosen the
mobility coefficient
i

between 0 and 1in order to have
the node mobility degree at interval [0,1].
For illustration, let us take an example when node
is on the state shown in (Figure 3(a)) with 10 neighbors,
and during interval
i
t

, its neighbors will undergo the
state changes shown in (Figure 3(b)): four nodes (with
blue color) will leave the communication range, and two
nodes (with red color) will join it. Consequently the node
will be after t

(at time t) in the state (Figure 3(c)) with
six changes. At the end of each time interval, the node
will be able to make an evaluation of the change of its
neighbors represented by this relative mobility, which is
in our example equal to 13/40=32.5% (with 12/


).
Each node in the mobile ad-hoc network can make an
autonomous and automatic evaluation of its mobility at
regular time intervals (this evaluation can be periodically
done while exchanging the Hello messages). Mor- eover
the calculation and recalculation of the node mobility is

ENHANCING DELAY IN MANET USING OLSR PROTOCOL
Copyright © 2009 SciRes. Int. J. Communications, Network and System Sciences
395
Figure 3. Node mobility degree quantification.
fast, and does not require enough consumption of re-
sources (CPU and memory).
4. Our Improvement
Mobility is a crucial problem in MANETs, and until now,
the majority of routing protocols have shown some
weaknesses to face a high mobility in some parts of the
network. Our objective consists in positively using the
mobility, in order to adapt and improve the performance
of the OLSR protocol.
4.1. Link Mobility Estimation
Some OLSR experiments [4,13] show that links must be
more stable and less mobile to avoid fragile connections
which involves data loss and frequent route changes. The
OLSR protocol maintains constantly the shortest paths to
reach all possible destinations in the network. So, it is
more judicious to estimate the quality of links before
adding them in the topological information that serves to
calculate the best routes. The quality of a link can be
estimated based on the power of the received signal. This
information is provided by some wireless cards. If this
information is not available, OLSR protocol estimates
the link quality based on the number of control messages
lost. A link failure can be detected using the timer expiry
or by the link layer that informs upper layers of the fail-
ure with a neighbor node after reaching the maximal
number of retries.
With an aim to estimate the quality of links in terms of
mobility, we define the mobility of a link L between two
nodes A and B as the average mobility of the involved
nodes (see Figure 4), as showed in following equation:
(,)
() ()
2
AB
LAB
M
tMt
M



 (2)
Figure 4. The link mobility of the link L(A,B) is (40%+
50%)/2=45%.
This evaluation of the link mobility alone is not sig-
nificant because we can have a normal value of the link
mobility with a high mobility value of one of the in-
volved nodes. The dependence between the mobility of
nodes composing a link (in the network core) at the time
t can be seen as mobility dependence of link L(A,B) as
follows:
(,)
() | () ()|
LAB A B
PtMtMt



(3)
Therefore, a reliable symmetric link in terms of mobil-
ity can be seen as a link satisfying the two following
conditions:
1) The average mobility of the link L(i,j) is lower than
a threshold THRESHOLD_Link which depends on the
characteristics of the wireless network (network density,
network mobility, network scalability, network dimen-
sion, ...):
(, )
( ) THRESHOLD_Link
Lij
Mt

 (4)
2) The mobility dependence of link L(i,j) is near to
zero :
(, )
() 0
Lij
Pt

 (5)
The choice of such a link satisfying these two condi-
tions ensures the link to have a low mobility, with a
strong dependence between the involved nodes.
4.2. Proposed Mobility Criterions
In this section, we propose three new criterions for the
operation of MPRs selection. The first criteria is direct
because it selects as MPRs set, neighbor nodes with less
mobility (Figure 5 (a)). Precisely the node selected as
MPR node is a node where its mobility is the smallest
(Equation 6). The two other criterions are based on the
estimation of links quality between neighbors at one-hop
and the neighbors at two-hop (Figure 5 (b)). The quality
of the link in terms of mobility is given by the two con-
ditions cited in the previous sub-section. So, the new
selection of the MPR set is a compromise between the
number of links towards the nodes at two-hops and its
reliability in terms of mobility. The selection of a
Figure 5. Criterions evaluation.

N. ENNEYA ET AL.
Copyright © 2009 SciRes. Int. J. Communications, Network and System Sciences
396
neighbor as a MPR node can be viewed as an operation
of maximization of the selection criteria. The second
criteria suggested is based on sum (Equation (7)) and the
third is based on the product (Equation (8)). The princi-
pal advantage of these three criterions is the facility on
calculation and doesn’t require enough of resources in
memory and CPU. Indeed, their evaluation is based on
data base of neighbor nodes at one-hop and two-hop used
by the OLSR protocol.
()
() min ()
w
wNx
DIR CRITERIA w M t



(6)
(,)
1()
() 1
N
Lwi
i
M
t
SUM CRITERIA w N



 (7)
(,)
1
() 1 ()
N
Lwi
i
PRD CRITERIA w M t



 (8)
5. Metrics of Performance
In this paper we have considered the most important
metrics for analyzing and evaluating performance of
MANET routing protocols during simulation. These con-
sidered metrics are:
Normalized Routing Overhead (NRL): It represents the
ratio of the control packets number propagated by every
node in the network to the data packets number received
by the destination nodes. This metric reflect the effi-
ciency of the implemented routing protocols in the net-
work.
Packet Delivery Fraction (PDF): This is a total num-
ber of delivered data packets divided by total number of
data packets transmitted by all nodes. This performance
metric will give us an idea of how well the protocol is
performing in terms of packet delivery by using different
traffic models.
Average End-to-End delay (Avg-End-to-End): This is
the average time delay for data packets from the source
node to the destination node. This metric is calculated by
subtracting ”time at which first packet was transmitted
by source” from ”time at which first data packet arrived
to destination”. This includes all possible delays caused
by buffering during route discovery latency, queuing at
the interface queue, retransmission delays at the MAC
layer, propagation and transfer times.
6. Simulations and Results
In this section we have compared the performance of the
original OLSR protocol based on the MPR selection
standard algorithm, and the two modified OLSR proto-
cols related to the direct and product criterions:
DIR-OLSR and PRD-OLSR protocols. In this study we
have eliminated the sum criteria for his hard cost in
terms of MPRs nodes number [18,19].
6.1. Simulation Environment
For simulating the original OLSR protocol and the modi-
fied OLSR protocols related to our proposed criterions,
we have used the OLSR protocol implementation [21]
which runs in version 2.9 of Network Simulator NS2 [20]
and uses the ad-hoc networking extensions provided by
CMU, with a radio range of 250m.
We use a network consisting of 50 mobile nodes to
simulate a high-density network. These nodes are ran-
domly moved in an area of 1000m by 1000m according
to the Random Waypoint (RWP) mobility model [22].
Moreover, to simulate a high dynamic environment (the
worst case), we have consider the RWP mobility model
with a pause time equal to 0. All simulations run for
300s.
A random distributed CBR (Constant Bit Rate) traffic
model is used which allows every node in the network to
be a potential traffic source and destination. The CBR
packet size is fixed at 512 bytes. The application agent is
sending at a rate of 10 packets per second whenever a
connection is made. All peer to peer connections are
started at times uniformly distributed between 5s and
290s seconds. The total number of connections and
simulation time are 10 and 500s, respectively.
For each presented sample point, 50 random mobility
scenarios are generated. The simulation results are there-
after statistically presented by the mean of the perform-
ance metrics. This reduces the chances that the observa-
tions are dominated by a certain scenario which favors
one protocol over another. As we have interest in the
case of high mobility (i.e. high link status and topology
changes) we have reduced the HELLO interval and TC
interval at 0.5s and 3s, respectively, for quick updates of
the neighbors and topology data bases.
In particular, for the PRD-OLSR protocol related to
the product criteria, we have choose THRESH-
OLD_Link= 0.05 as a threshold for evaluating the aver-
age mobility of links.
6.2. Results and Discussion
To show how the modified version of the OLSR protocol
is more adapted to the link status and topology changes
comparing to the original OLSR protocol, we have made
there performance comparison based on the three per-
formance metrics cited in Section 5. Moreover, with the
supposed configuration cited above, we have run simula-
tions in different mobility levels by varying maximum
speed of nodes between 0m/s (no mobility) to 50m/s
(very high mobility) in steps of 10m/s. For given the
same importance of mobile nodes leaving and joining the

ENHANCING DELAY IN MANET USING OLSR PROTOCOL
Copyright © 2009 SciRes. Int. J. Communications, Network and System Sciences
397
communication range at each node in the network we
have choose the mobility coefficient equal to

=1/2.
According to the Figure 6, the original OLSR,
PRD-OLSR and DIR-OLSR protocols ensure in the
whole the same packet delivery fraction for all maximum
speeds. Indeed, it can be seen that the number of packets
dropped along the path is quite similar for all maximum
speed being approximately 42% at worst. Moreover, the
ratio is worse for a continuously changing network (i.e.
high maximum speed) than for the static path conditions,
because the number of link failures grows along with the
mobility. However, it is interesting to notice that even
with static topology conditions, sending nodes do not
achieve 100% packet delivery but only 81%-83%. This
clearly shows the impact of the network congestion and
packet interference as the load on the network increases.
Figure 7 shows that PRD-OLSR protocol ensures a
good enhancement in terms of delay comparing to the
DIR-OLSR and original OLSR protocols, where have
globally the same delay for all maximum speeds. Pre-
cisely, the original OLSR protocol delay is around 2.7
seconds with higher mobility rate (maximum speed equal
to 50m/s) and decreases to almost 1.2 seconds with static
topology conditions. For DIR-OLSR protocol the delay
gets more than twice as large being almost 2.65 sec for
high mobility and surprisingly increasing to over 1.2
seconds when the mobility is decreased. For the interme-
diate speed (from 10m/s to 40m/s) al lightweight differ-
ence between them is found. This allows us to conclude
that original OLSR and DIR-OLSR protocols ensures
approximatively the same delay.
Unlike to the protocols above (i.e. original OLSR and
DIR-OLSR protocols), the PRD-OLSR protocol delay is
about 2.6s (enhancement by 0.05s and 0.1s comparing to
DIR-OLSR and original OLSR, respectively) with high
mobility, increasing to almost 0.9s-1.1s (unlike the
DIR-OLSR and original OLSR protocols that their
minimum delay is found at 1.2s) with lower maximum
Figure 6. Comparison of the three versions of the OLSR
protocol in terms of packet delivery fraction.
Figure 7. Comparison of the three versions of the OLSR
protocol in terms of Average end-to-end delay.
Figure 8. Comparison of the three versions of the OLSR
protocol in terms of normalized routing load.
speed. Moreover, this enhancement is more shown for all
intermediate maximum speeds and particularly for the
two maximum speeds (10m/s and 30m/s). In short, we
can say that the PRD-OLSR protocol is more adapted to
all levels of mobility from 0m/s (no mobility) to 50m/s
(very high mobility).
Figure 8 illustrates the normalized routing load (NRL)
introduced into the network for the three versions of
OLSR protocol, where the number of routing packets is
normalized against sent data packets. A fairly stable
normalized control message overhead would be a desir-
able property when considering the performance as it
would indicate that the actual control overhead increases
linearly with maximum speed of nodes due to the num-
ber of messages needed to establish and maintain con-
nection. The OLSR protocol produces the lowest amount
of NRL when compared to PRD-OLSR and DIR-OLSR
protocols during all maximum speed values. Moreover,
the PRD-OLSR protocol produces a lightweight routing

N. ENNEYA ET AL.
Copyright © 2009 SciRes. Int. J. Communications, Network and System Sciences
398
load comparing to the DIR-OLSR protocol that produces
more routing load. In the worst case (at the maximum
speed value equal to 50m/s), the NRL increases to 5.5%
for DIR-OLSR protocol, 4.8% for PRD-OLSR and
4.25% for the original OLSR. Precisely, comparing to
the original OLSR protocol, the PRD-OLSR and
DIR-OLSR protocols produce 12.94% and 29.41% rout-
ing packets, respectively. This explains that our two
proposed criterions request more routing packets to es-
tablish and maintain routes in the network.
8. Conclusions and Perspectives
This paper presents two versions of the original OLSR
protocol, in the goal to adapt and enhance its perform-
ance to the dynamic nature of MANETs characterized by
the link status and topology changes. These versions are
based on a mobility degree that is quantified and evalu-
ated in time by each mobile node in the network.
In the future works, we plan to continue this study by
considering different configurations of MANETs for
well understanding the behavior of each OLSR protocol
version. Moreover, it is important to study the impact of
the mobility coefficient

(

=1/2 in this work) by
varying them into (0.00, 0.25, 0.75, 1.00). Finally, to
implement an extension of the OLSR protocol supporting
QoS, assuming that QoS requirements are expressed in
terms of less mobility.
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