INTRODUCTION:
Mobile ad hoc networks, which do not rely on any infrastructure such as access points
or base station, can be deployed rapidly and inexpensively even in situations with
geographical or time constraints. So ad hoc networks have attractive applications in both
military and disaster situations and also in commercial uses like sensor networks or
conferencing.
Dynamic Source Routing Protocol is a kind of on demand routing protocol based on
source routing. In DSR, when the sender sends a message to the destination node,the routing
information is present in the packet header which is composed of information of the number
of nodes in network address.The source node relays the data packet through these nodes to
the destination node.
Basic operations:
To send a packet to another host, the sender constructs a source route in the packets
header, giving the address of each host in the network through which the packet should be
forwarded in order to reach the destination host. The sender then transmits the packet over its
wireless network interface to the first hop identified in the source route. When a host receives
a packet, if this host is not the final destination of the packet, it simply transmits the packet to
the next hop identified in the source route in the packet’s header. Once the packet reaches its
final destination, the packet is delivered to the network layer software on that host.
Each mobile host participating in the ad hoc network maintains a route cache in which
it caches source routes that it has learned. When one host sends a packet to another host, the
sender first checks its route cache for a source route to the destination. If a route is found, the
sender uses this route to transmit the packet. If no route is found, the sender may attempt to
discover one using the route discovery protocol. While waiting for the route discovery to
complete, the host may continue normal processing and may send and receive packets with
other hosts. The host may buffer the original packet in order to transmit it once the route is
learned from route discovery, or it may discard the packet, relying on higher-layer protocol
software to retransmit the packet if needed. Each entry in the route cache has associated with
it an expiration period, after which the entry is deleted from the cache.
The two mechanisms that work together for the recovery and maintainance of source
routes in ad hoc network are
(a)Route discovery:
Route discovery allows any host in the ad hoc network to dynamically discover a
route to any other host in the ad hoc network, whether directly reachable within wireless
transmission range or reachable through one or more intermediate network hops through
other hosts. A host initiating a route discovery broadcasts a route request packet which may
be received by those hosts within wireless transmission range of it. The route request packet
identifies the host, referred to as the target of the route discovery, for which the route is
requested. If the route discovery is successful the initiating host receives a route reply packet
listing a sequence of network hops through which it may reach the target.
Fig 1: Route Request (RREQ) flooding
Consider a simple ad hoc network and node S wants to send packets to destination
D.As it does not know the path,it initiates a route discovery(RREQ) message and broadcast to
adjacent nodes.Each RREQ has senders address,destination address and unique ID
determined by the sender.Each node appends its own identifier while forwarding RREQ.If
intermediate node is not having a route to destination,it rebroadcasts message after adding its
address to source route.A node receiving a route request rebroadcast it only once.After
reaching the destination D,it does not broadcast further.
Fig 2: Route Reply (RREP) propagation
When destination received RREQ message, it returns the Route Reply that means
destination node must have a route to the source node. If the route is in the Destination
Node's route cache, the route would be used. Otherwise, the node will reverse the route based
on the route record in the Route Reply message header.RREP can be sent by reversing the
route in Route Request(RREQ) only if links are guaranteed to be bidirectional.
(b)Route Maintainance:
When sending or forwarding a packet to a destination,route maintainance is used to
detect if the network topology has changed such that the link used by this packet is broken.
Each node along the route,when transmitting the packet to the next hop, is responsible for
detecting if its link to the next hop has broken.
• Route Error (RERR) packets: Whenever a node encounters fatal transmission errors so that
the route becomes invalid, the source receives a RERR message.
• ACKs: ACK packets are used to verify the correct operation of the route links. This also
serves as a passive acknowledgement for the mobile node.
Evaluation Indicators of routing performance:
1)Throughput:
It is the measure of the number of packets or data successfully transmitted to their
final destination via a communication link per unit time. It is measured in bits per second
(bit/s or bps).
2)Packet Delivery Ratio :
Usually it refers to the proportion of correctly received packets to all sent packets in a
period of time. It is an evaluation indicator of the reliability of transmission in ad hoc
network. The smaller the packet delivery is, the worse the performance of routing
performance is.
N N
Packet delivery ratio=∑ R ni /∑ Sni
i=0 i=0
N is the total number of the nodes, Rn i is the number of packets received by node i,
Sni is the number of packets sent by node i.
3)Average End-to-End Delay:
It refers to the average time a packet used to be sent from the source node to the
destination node. The small delay shows that the response of the network is fast and the
quality of the network is satisfactory.
N
1
Average End-to-End Delay=
N ∑ Rt - St
I i
i=0
N is the total number of the packets received successfully, Rt i is the time when the packet i is
received, Sti is the time when the packet i is sent.
4)Packet Loss Rate:
Packet Loss ratio is calculated by subtracting the number of data packets sent to
source and number of data packets received at destination through the number of packets
originated by the application layer of the source (i.e. CBR source)
Simulation and analysis:
NS3 tool is used to simulate and determine performance of DSR.There are 50 mobile
nodes generated randomly in a 300x1500 scenario,The simulation time is 150 simulated
seconds and the first 80 are used for start up time.Nodes move according to random waypoint
mobility model with a speed of 20m/s and no pause time.The WiFi is in ad hoc mode with a 2
Mb/s rate(802.11b) and friis loss model is used.The transmit power is set to 7.5 dBm.By
default there are 10 source/sink data pairs sending UDP data at an application rate of 2.048
Kb/s each.Application data is started at a random time between 80 and 81 seconds and
continue till the end of simulation.
