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Abstract-An ad hoc network is the network formed by the devices that are capable to communicate with each other by wireless medium without any centralized administration or control. Since the network consists of mobile nodes, path determination and topology composition are not easy issue to solve. Especially estimating the optimum number of nodes for the network area under consideration for certain application is very important for to cover the whole communication area.
In this paper we propose a decision method for selecting optimum number of nodes in mobile ad hoc networks covering whole communication area and collecting data from a network. From the optimum nodes we tried to find out the maximum number of simultaneous sources to have effective communication with better packet delivery ratio. The performance of proposed method is evaluated using ns-2 simulator.
Keywords-Wireless ad hoc network ,NS2, Simulation.
Wireless ad hoc networks are formed by devices that are able to communicate with each other using a wireless physical medium without having a pre-existing network infrastructure. These networks, also known as mobile ad hoc networks (MANETs), can form stand-alone groups of wireless terminals, or (some of) these terminals could also be connected to a cellular system or to a fixed network. A fundamental characteristic of an ad hoc networks is that they are able to configure themselves on-the-fly without the intervention of a centralized administration. In areas in which there is a little or no communication infrastructure or the existing infrastructure is expensive or inconvenient to use, wireless mobile users may still be able to communicate through the formation of an ad hoc network . In such a network, each mobile node operates not only as a host but also as a router, forwarding packets for other mobile nodes in the network that may not be within direct transmission range of each other. Each node participates in an ad hoc routing protocol that allows it to discover "multi-hop" paths through the network to any other node. The idea of ad hoc networking is sometimes also called infrastructureless networking.
The nodes in a MANET can dynamically join and leave the network, frequently, often without warning, and possibly without disruption to other node's communication.
A MANET can be configured by deploying nodes in various applications where a central processing unit is not there. Some of the applicaions of manet are : Tactical Networks, Sensor Networks, Emergency Services, Commercial Environments, Vehicular Services, Home and Enterprise Networking, Educational applications.
Nodes in MANET establish and maintain a route from a source to a destination by themselves. Because of frequent topology changes in mobile ad hoc networks, finding routing path become an essential issue . An effective decision method for the number of necessary nodes to cover whole communication area is important to establish effective topologies for communications.
Typical parameters for simulation environment consist of network area, the node transmission range, the number of nodes and node speed. In fact these simulation parameters can give direct effect to simulation results; especially the number of nodes used in the ad hoc network has significant influence to the output. Using too less number of nodes compared to the network area can decrease the path connection possibility. Using too many number of nodes can increase the control packet overhead and packet delays. Therfore accurate no.of nodes for a simulation is important.
The decision for number of simultaneous sources depends on reliable data delivery with less control overhead. In wireless ad hoc network there is no central control so as increasing number of simultaneous users increases control overhead thus reducing packet delivery and reducing the bandwidth per node. In order to have more delivery of data packets there should be alternate path like in the traditional circuit switched telephone network, where it reduces the call blocking by providing multiple network routes for the initial call-setup messaging. For the same purpose we have used the AOMDV protocol (Ad hoc on demand multipath distance vector).
The method for selection of the optimum value of the number of nodes for given network area and maximum number of simultaneous sources is proposed.
The Reminder of this paper is organized as follows. Section II describes the basic mechanism of AODV and AOMDV protocols. Section III describes the proposed method for deciding the optimum number of nodes to cover the arranged communication area and collecting data from an arranged network. Section IV describes the Simulation Model. Section V describes the Simulation Results. Section VI concludes the paper.
Description Of aodv and aomdv
The Ad hoc On-Demand Distance Vector (AODV) routing protocol is a reactive protocol designed for use in ad hoc mobile networks. AODV initiates route discovery whenever a source node needs a route, and it maintains this route as long as it is needed by the source. Each node maintains a monotonically increasing sequence number that is incremented whenever there is a change in the local connectivity information for the node. These sequence numbers ensure that the routes are loop-free.
Route discovery follows a route request/route reply query cycle. The Route Discovery process is initiated whenever a source node needs to communicate with another node for which it has no routing information in its table . Every node maintains two separate counters: a node sequence number and a broadcast_id. The source node initiates route discovery by broadcasting a route request (RREQ) packet to its neighbors.
This RREQ contains the IP address of the destination and last known sequence number for that destination. Nodes receiving the packet can respond to the RREQ either if they are the destination, or if they have an unexpired route to the destination whose corresponding sequence number is as least as great as that contained in the RREQ. If either of these two conditions satisfied, the node responds by unicasting a Route Reply (RREP) back to the source node. Otherwise node rebroadcast the RREQ. Additionally, each node receiving the RREQ creates a reverse route entry for the source node in its route table.
As the RREP is forwarded to the source node, an intermediate node that receives the RREP creates a forward route entry for the destination in their route tables before transmitting the RREP to the next hop. Once the source node receives a RREP, it can begin using the route to send the data packets. If the source node does not receive a RREP before its discovery timer expires, it rebroadcast the RREQ. It attempts discovery up to some maximum number of attempts. If it does not discover a route after this maximum number of tries, the session is aborted.
Movement of nodes not lying on active path does not affect the routing to that path's destination. If the source node moves during an active session, it can reinitiate the route discovery procedure to establish a new route to the destination. When either the destination or some intermediate node moves, a special RREP (RRER) is sent to the affected source nodes. Periodic hello message can be used to ensure the symmetric links, as well as to detect link failures. A link failure is also indicated if attempts to forward a packet to the next hop fail.
Once the next hop becomes unreachable, the node upstream of the break propagates an unsolicited RREP with fresh sequence number and hop count of âˆž to all active upstream neighbors and so on. This process continues until all active source nodes are notified.
Upon receiving notification of broken link, source nodes can restart the discovery process if they still require a route to the destination.
The main idea in AOMDV is to compute multiple paths during route discovery. It is designed primarily for highly dynamic ad hoc networks where link failures and route breaks occurs frequently. When single path on demand routing protocol such as AODV is used in such network, a new route discovery is needed in response to every route break. Each route discovery is associated with high overhead and latency. This inefficiency can be avoided by having multiple redundant paths available. Now, a new route discovery is needed only when all paths to the destination break.
A noteworthy feature of the AOMDV protocol is the use of routing information already available in the underlying AODV protocol as much as possible. Thus little additional overhead is required for the computation of multiple paths. The AOMDV protocol has two main components:
1. A route update rule to establish and maintain multiple
loop-free paths at each node.
2. A distributed protocol to find link-disjoint paths.
When an intermediate node receives copies of a RREQ packet, it compares a hop count field in a packet with minimum hop count, called advertised_hopcount, store in routing table for previous RREQ packets. Only packet with minimum hop count is accepted to avoid routing loops. Furthermore, a firsthop field in RREQ packet is then compared with firsthop_list in routing table. When node with node disjoint property (new firsthop) is found, new reverse route is recorded. A destination returns RREP packet accordingly, and multiple routes with link-disjoint property are establish at source node.
optimum number of nodes
Minimum Number of Necessary Nodes:
The Number of Necessary nodes to cover all of the given network area for complete communication is given by equation
Nmin = (a/ r ) * (b /r)
To decide the number of necessary nodes to cover the network area, information about size of network area and the radius of transmission range of the nodes is enough.
Average Number of Neighbours:
The Average number of neighbours in the network is calculated by the following equation
NN = ( N * Ï€* R2 ) / A
Where as A- Area of network
R - Radius of transmission range of the node.
For the connectivity in network optimum value of Average number of neighbours in the network should be 7 to 8 .
Optimum Number of nodes :
From the above equation the proposed equation for calculating the range of the optimum value of the number of the node in network is given by following equation.
(7 * A) / (Ï€ * R2 ) < Noptimum â‰¤ ( 8 * A ) / ( Ï€ * R2 ) ----(1)
For the network with area 1000m x1000m theoretically optimum numbers of nodes in a network is 40.
We use a detailed simulation model based on ns-2 in our evaluation. NS-2 support for simulating multi hop wireless networks complete with physical, data link and medium access control (MAC) layer models. The Distributed Coordination Function (DCF) of IEEE 802.11 for wireless LANs is used as a MAC layer protocol. The 802..11 DCF uses Request-To-Sent (RTS) and Clear-To-Sent (CTS) control packets for unicast data transmission to a neighboring node.
Traffic sources are continuous bit rate (CBR). The source-destination pairs are spread randomly over the network. Only 512-byte data packets are used. The mobility model uses the random waypoint model in a square field.
Table I. Simulation Parameters.
1000 m x1000 m
0 and 100 sec
0 - 25 m/sec
No. of connection
Â¾ th of the no. of nodes
Data Packet size
Packet sent rate
Performance Metrics and Results
Packet Delivery Ratio (PDR):- It is the ratio of number of data packets received by destination to number of packet sent by source. It is important as it describes the loss rate that will be seen by the transport protocols, which in turn effects the maximum throughput that the network can support.
Average end to end delay (AVD):- This includes all possible delays caused by buffering during route discovery, queuing delay at the interface, propagation and transfer time. It is important for best effort traffic.
Route Discovery frequency (RDF):- The total number of route discoveries initiated per seconds. Routing overhead is very important metric, as it measures scalability of a routing protocol, the degree to which it will function in congested or low bandwidth environment.
Normalized routing load (NRL): - The total number of routing packets transmitted for each delivered data packet. Each hop-wise transmission of these packets is counted as one transmission. The routing load metric evaluates the efficiency of the routing protocol.
We have considered the two scenarios for simulation.
Scenario 1.Varying Mobility and Number of Nodes
The first set of experiment uses differing number of nodes with a moderate packet sent rate and varying the mobility (speed) with pause time of 100sec. The nodes are varied between 10 to 100 uniformly. This experimentation is done to decide the optimum number of nodes in the network.
Figure 1. Packet Delivery Ratio by varying number of nodes
Fig.1 shows the packet delivery ratio for different speed by varying number of nodes. The packet delivery ratio for network with nodes 40 to 100 is almost same i.e. within the range of 90-100%. Packet delivery ratio for network with 10 and 16 nodes is less than 60 %.
Figure 2. Route discovery frequency with various numbers of nodes
Fig. 2 shows the route discovery frequency for different speed by varying number of nodes. Route discovery frequency increases with increase in number of nodes. RDF for network with 100 nodes is much larger.
Figure 3. Normalized Routing Load by varying number of nodes
Fig. 3 shows the Normalized Routing Load for different speed by varying number of nodes. Normalized Routing Load increases with number of nodes and also increases gradually with increasing mobility.
Figure 4. Average end to end delay by varying number of nodes
Fig. 4 shows the Average End to End Delay for different speed by varying number of nodes. Average Delay for network with 10 and 16 nodes is large compared to network with 40-100 nodes. Average end to end delay for network with 40-100 nodes is almost less than 0.1 sec.
Scenario 2. Varying Mobility and Number of simultaneous Sources.
The second set of experiment uses differing number of simultaneous sources with a moderate packet sent rate and varying the mobility (speed) continuously. With a network of 1000x1000 we have varied number of simultaneous sources from 1-10 for 40 number of nodes which is the optimum nodes for the 1000x1000 network area. This experimentation is done to decide the maximum number of simultaneous sources in the network.
Figure 5. Packet Delivery Ratio with various simultaneous sources.
Fig. 5 shows the Packet Delivery Ratio for different speed by varying number of simultaneous sources. Packet Delivery Ratio decreases with increasing number of simultaneous sources. PDR decreases drastically after 5 number of simultaneous sources for network with 0 m/sec.
Figure 6. Route discovery frequency with various simultaneous sources
Fig. 6 shows the route discovery frequency for different speed by varying number of simultaneous sources. Route Discovery Frequency increases with increasing number of simultaneous sources. RDF is increasing drastically after 6 simultaneous sources. RDF for AOMDV protocol is less than RDF for AODV protocol.
Figure 7. Normalized Routing Load with various simultaneous sources
Fig. 7 shows the Normalized Routing Load for different speed by varying number of simultaneous sources. Normalized routing load increases with increasing simultaneous sources except in the begining. NRL for AOMDV protcol is less than NRL for AODV protcol except for network with 0 m/sec mobility.
In this paper we have proposed the method for selection of
optimum number of nodes and number of simultaneous sources. From Fig. 1-4, Performance metrics like 1. Packet delivery ratio 2. Route Discovery frequency 3. Normalized routing load 4. Average end to end delay it can be concluded that for area 1000mx1000m optimum number of nodes should be 40.
From Fig. 5-7, it can be concluded that number of simultaneous sources should be 1/8th of the optimum value of the node. For steady state communication or for maximum connectivity simultaneous sources should be 5 to 6. By using multi path routing (AOMDV) the performance of the simultaneous sources is improved with respect to PDR, RDF, and NRL as compared to AODV.
From table II, it can be seen that network with 40 nodes has highest packet delivery ratio (90.29%) compared to other nodes. Normalized routing load for network with 10 nodes has lowest value (8.42) and it increases gradually. Route discovery frequency for network with 10 nodes has lowest value (4.33) and it increases with increasing number of nodes. Average end to end delay for network with 40-70 nodes has low value compared to others.
It can be seen from table III that packet delivery ratio for network with simultaneous sources with AOMDV is more (91.75) compared AODV (90.30). Route discovery frequency for network with simultaneous source with AOMDV is less (12.98) compared o AODV (16.87). Normalized routing load for network with simultaneous source with AOMDV is less (21.71) compared o AODV (22.67).
Table II. Performance comparison of network with varying no. of nodes (25 m/sec)
Table III. Comparison of AODV & AOMDV for varying Simultaneous sources