FANETs have evolved using the concept of
MANETs and VANETs. To apply routing protocols
to FANETs is one of the common challenges.
Unlike FANETs, generic ADHOC networks
are not capable of sending information
quickly and correctly is some situations like at
the time of natural disaster like flooding, earthquakes
and even in military battlefield.
Optimized Link-State Routing (OLSR), and
Predictive-OLSR (P-OLSR) are the techniques
for dynamic topology adaptation in accordance
with the mobility pattern. OLSR is a routing
protocol where every node maintains one or
more tables representing the entire topology
of the network. P-OLSR which is designed for
FANETs is an extension of Optimized Link-State
Routing. OLSR have a benefit of having routes
quickly available whenever required. Simulation
results verify that the proposed scheme
could significantly increase the performance of
flying ad hoc networks.
1 INTRODUCTION
A Wireless ad hoc network is a set of different
network nodes forming the temporary networks
without the aid of any infrastructure or
any centralized administrator. They do not
have gateway, each node can act as the gateway.
The wireless arena has been experiencing exponential
growth in 21st Century. The IEEE
Standard 802.11 is used for ad hoc networks
which specifies Medium Access control and
Physical layer. In ad hoc network every node
has its own transmission range, which in turn
combine to form a bigger transmission area.
Nodes use hopping technique to transmit data.
To make this process more effective , an appropriate
routing algorithm should be implemented.
Routing protocols are designed based on the
requirement such as when the data is to be exchanged,
what data should be exchanged, how
routes are calculated etc. In proactive algorithms
routes are calculated in the before hand
, so nodes use these routes for information ex1
change. OLSR routing algorithm is an example
of proactive. Whereas in reactive algorithms
route is calculated only when there are nodes
that need to be communicated. AODV is an
example of reactive algorithm. Multiple paths
between any two given nodes can have better
throughput and recovering connection failure
is also easy , but overhead of route discovery in
multi path routing more when compared with
a single path routing algorithm.
In table driven routing protocol information
about each node to other node is updated.
Where as in on demand routing ,nodes calculate
if they have to exchange data from each
other. They don’t have to update information.
For a routing algorithm to be best, it should
overcome the routing problems in ad hoc network.
Some of the routing problems are given
below:
• Asymmetric Link: If node 1 can hear node
2, that does not imply vice versa, hence
routing algorithms should be asymmetric.
• Redundant connection: It consumes lot of
time to update routing table , if there are
many redundant connections.
• Interference: Interference is very common
as the data transfer is done trough waves.
Natural effects like weather, scattering can
cause interference.
• Dynamic topology : Since nodes can enter
and leave network freely, it causes node
to make frequent changes in their routing
table.
FANETs are the special form of MANETs and
VANETs. They have much mobility degree and
because of this topology changes more frequently
than that of MANETs and VANETs. Distance
between the nodes is also longer than
that of the VANETs and MANETs. Hence communication
range in FANETs also should be
longer than that of others. Since the topology
changes in an unpredictable manner, robust
algorithm with dynamic routing are better to
maintain communication among the nodes in
the FANETs. Dynamic routing in ad hoc network
is an reactive algorithm where the nodes
recover the routes each time a node transfers
data to other node. Since FANETs have high mobility
nodes, route breakage is very common.
Hence route is maintained in order to minimize
the breakage.
Network Simulator widely known as NS2, is
an event driven simulation tool that has proved
useful in studying dynamic nature of FANETs.
NS2 enables users by providing with ways to
specify network protocols and simulate their
behavior. The paper has three sections. In the
first section , OLSR protocol is analyzed , in the
second part P-OLSR is analyzed. Third section
is about the simulation results.
2 DESCRIPTION
2.1 OLSR
OLSR is a proactive routing protocol that obtain
the strength of link state algorithm. By proactive,
it refers that the entire topology of the network
is maintained by every node. Some of the
OLSR performance include: 1. It declares only
links with its multipoint relay selectors. 2. Every
node select from its neighbor, who can read
2
or who can broadcast messages, those nodes
are termed as multipoint relays (MPRs). 3. The
link state information is partially distributed in
the network, where the link is obtained only between
an MPR and its MPR selectors. Only MPR
can retransmit in this protocol. For instance,
consider a node A having nodes B, C, and D
as its neighbors. So, if A considers C as MPR,
then whatever packet A sends is received by
all the other three nodes. However, only C can
rebroadcast that packet. Hence, this protocol
significantly reduces the number of retransmissions
in a broadcast procedure and can play a
significant role for dense ad hoc networks.
OLSR is not a central entity based but works
on a completely distributed manner. Since the
nodes send periodic control messages, it is not
compulsory to have a reliable transmission of
messages. Types of packets:
1. Hello packets: Hello packets are mainly
used to know the state of the link. Every
node transmits their neighbor’s information
and the information about their link
to its neighbors.
2. Topology Control Packets: Topology control
packets are used to give information
about the MPRs of that particular node.
3. MID packet: If a node has multiple interface,
then these MID packets are used to
send the information of those interfaces
A HELLO message consists information of its
neighbors and their link status. The addresses
of all the neighbors to which there exists a valid
bi-directional links. And addresses of the neighbor
that received the HELLO messages. All onehop
nodes receive the message. Every node
in the network periodically sends the HELLO
messages to its neighbors. As a node receives
a HELLO message, it updates its MPR selector
table.
The question is how one can select the multi
point relay. It is selected independently by each
node on the network. The set of MPRs is selected
in such a way that all the two hop neighbors
are selected, and this information can be
retrieved from the HELLO messages received
by this node. Only the information of bi directional
link nodes is taken. Whenever the node
of the network is updated or a node fails, the
multipoint relay is recalculated.
The information of MPR should also be mentioned
to other nodes. This can be sent with
topology control packets. The TC messages
3
sent by the MPR includes its own sequence
number to the list. The topology control messages
that are sent throughout the network
helps in building the topology of the network.
The interval of TC messages may depend on the
MPR selector set, that if a change occurs then
sent early. A node records information about
all the multipoint relays, using the information
obtained from TC messages. The information
of all the multipoint relays in a topology table.
The table consists of 1. MPR selector set sequence
number, 2. Destination address, 3. Last
hop node address. This topology table is executed
in the following way:
The entry in the topology table is discarded
if the MPR selected sequence number is greater
than the sequence number than sequence
number in the received message. Second, if
there is an entry of last-hop address is smaller
than the sequence number in the received, the
entry is removed. By using the
Third, for each MPR selector received in the
TC message, a new topology entry is recorded
in the topology table.
Each node consists of a routing table, to send
the packets for other destinations in the network.
Routing has information of all the nodes,
hence if any of the nodes are updated or declined
then this routing table needs to be recalculated.
The following procedure
1. All the entries from the routing table are
deleted.
2. Starting with one hop neighbors as destination
nodes, new entries are added into
the table.
3. The topology table records that are not
used while calculating the routing table
are removed.
2.2 P-OLSR
Since FANETs have no central infrastructure,
they are prone to isolated attacks and node failures.
UAV move rapidly with high speeds hence
the topology varies very rapidly. Nodes must
react to this rapid change and be able to update
their routing tables automatically. It can
be achieved by using a fast and reactive algorithm.
OLSR fails to track fast topology changes
of a FANET. In P-OLSR routing algorithm relative
speeds are used to calculate the weighted
ETX factor that is expected transmission count.
Consider node A and node B , ETX is used as
measurement for quality of the wireless network
link. It is defined as
ET X A,B =
1
?f ?r
where ?
f
is the probability that the data
packet is successfully sent. ?
r
is probability
that the acknowledgement packet is successfully
sent. ETX of a particular route R is sum of
ETX metrics of links composing the routes A,B.
ET X R =
P
(A,B)?R
ET X A,B
?
f
,?
r
are the receiving ratios which are measured
using a link probe packet that is hello
packet. The frequency in which hello messages
are broad-casted is called Hello interval. A
node takes a while before noticing that wireless
link quality have reduced, and before realizing
about the broken or a poor link it sends the data
packets on a broken link causing interruption
4
to the service. Relative speeds are used to learn
how the link quality is going to change. By assuming
that each node has knowledge about
its neighboring nodes,
ET X A,B =
e
v
A,B
l
?
r
f
r
r
Where v˜
A,B
l
is relative speed between nodes
A,B and BETA is a non negative value. If nodes
moves towards each other relative speeds will
be negative making ETX value less than 1 and
vice versa if the nodes are moving away from
each other.Hence the link between the nodes
that are moving closing towards each other is
preferred to that of the nodes moving away
from each other.
GPS information is collected by hello messages,
each time ETX is calculated it will have
its upgrade GPS information. Following is the
formula to calculate instantaneous relative velocity
between A,B

A,B
l
=
d
A,B
l
?d
A,B
l?1
tl ?tl?1
where tl and ti?1 are time of arrival for last
and second last hello messages respectively.
Implementation of P-OLSR is done through
sharing hello messages through which each
node will have to share GPS coordinates. Then
each node uses its neighboring co-ordinates
to calculate its relative speeds and share them
through hello and TC messages. TC messages
are topology control messages that are used to
construct routing table. Figure1 is the structure
of hello message in OLSR. It has the 1st block
which is 8 bytes carries information about node.
Where as in the figure2 which is format of modified
P-OLSR hello message, have 16 bytes. It has
Figure 2.1: structure of hello message in OLSR
Figure 2.2: structure of hello message in OLSR
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2 empty bytes that are used to communicate
the relative speeds between the nodes. Similarly
in TC message format the OLSR and POLSR
differ by one byte which is used to communicate
relative speeds between nodes.