THE IMPACT OF NODE VELOCITY ON MOBILE ADHOC DATA NETWORK (MADNET) PERFORMANCE

Abstract

A Mobile Ad-hoc Data Network (MADNET) is a self-configuring, infrastructure-less network of mobile devices (nodes) where nodes communicate with  each other using multi-hop wireless links.  Node failures and arbitrary movement of nodes at higher node velocity break the routes and lead to the frequent operation of rebuilding routes that consume lots of the network resources and the energy of nodes. This study, analyses the impact of node velocity on performance of two MADNET Transport protocols i.e.  TCP  (Transmission Control  Protocol)  and  UDP  (User  Datagram  Protocol)  using Uniform Mobility Model. The network performance metrics for analysis in this study are throughput and packet delivery delay. Iperf was used as the network management and performance tool. The result got from the field study shows that the throughput decreases while the delay increases at higher nodal velocities.

CHAPTER ONE Introduction

Wireless  networks are  becoming  more  and  more  popular  among  recent  network technologies  as compared to the traditional wired network. Wireless networks are connected through the wireless channel. Generally there are two kinds of wireless networks. The cellular networks and WLANs that have a wired backbone in which the base stations are the boundary nodes and the extended connections between mobile users and the base stations are wireless channels. The other is wireless ad hoc network, which is an infrastructureless self-configuring network [3] with more than one hop wireless channels in the connection. This kind of topology is not widely implemented yet, but it is useful in emergency search such as rescue operations, and military applications [2].

1.1Description of MANET Transport Protocol

This section describes the transport protocols used in the evaluation of MANET; it includes the

Transmission Control Protocol and the User Datagram Protocol.

1.1.1 The Transmission Control Protocol (TCP)

The Transmission Control Protocol (TCP) [31] is a connection-oriented and reliable protocol, which provides reliable host-to-host data transmission in packet-switched computer communication networks. TCP  uses a  sliding  windowmechanism in  combination with timers  in order  to  adapt to  network conditions and retransmit lost packets to provide reliability. In TCP, the window size determines the number of bytes of data that can be sent before an acknowledgment from the receiver must arrive. T CP establishes a  full-duplex  virtual  connection  between  two  endpoints which is defined by the IP address and the port number of each endpoint. The byte stream is transferred in segments. Typically, TCP is the best transport layer protocol for applications that require guaranteed delivery of data [50]. The TCP algorithm as a whole is quite complex and there are many different versions and extension proposals available [30].

TCP congestion control: Slow start and congestion avoidance phase.

To avoid network overload and resolve network congestion, TCP provides window-based congestion control. It assumes network congestion upon the detection of packet loss. The figure below illustrates the different phases of TCP’s congestion control.

Figure 1.0 TCP congestion window illustration of [30].

When a new TCP connection is established between two end points, the Slow-Start (SS) mechanism takes place in order to probe the capacity of the network: Starting from a congestion window (cwnd) equal to one, the sender increases the cwnd by the current segment size when it receives a new ACK (i.e., non-duplicated) acknowledgment (ACK). Keeping to this principle, the window size increases to an estimated capacity, termed SS-threshold. Figure 2.2 illustrates the slow start algorithm for the first four packets (i.e., between packet number zero and three). When the SS-threshold is reached, TCP enters the Congestion Avoidance (CA) phase, as shown between packet numbers four and seven. In CA, the cwnd increases linearly up to the receiver’s maximum advertised window or until packet loss is detected. Regular TCP [30] assumes a packet loss when the retransmission timer expires before the respective segment is acknowledged. In this case, SS is unavoidable. In Figure 1.0, the sender detects a packet loss upon the transmission of packet number seven. Consequently, it enters SS: When the sender transmits packet number eight, the cwnd is reset to one and the SS-threshold drops by half of the current cwnd.

1.1.2 The User Datagram Protocol (UDP)

Unlike TCP, the user datagram protocol (UDP) is a connectionless transport protocol that is usually employed on top of packet switched IP networks. UDP assumes that Internet Protocol (IP) is used as

the underlying protocol. It offers a minimal transport service, which allows applications to directly access the datagram service of the IP layer. UDP has some limitations which results in its inability to provide  reliability  or  error  recovery,  but  it  provides  services  like  checksum  calculation  and multiplexing by port number. The speed of data transmission in UDP is relatively high because it introduces only minimal overhead but offers no guaranty for data delivery and duplicate protection. Real-time applications like IP telephony or video conferencing with some specific requirements make use of UDP. These applications do not require reliability or congestion control, but rather aggressively use the network according to their bandwidth and delay requirements [30].  For real-time applications, packet  loss up  to  a  certain  limit  is  tolerable  because  human perception is  not  sensible to  small interruptions (i.e., depending on the codec, assuming that the codec is able to cope with packet loss). In real-time communications, the retransmission of a lost packet would just waste bandwidth and increase cost because the receiver drops the packet when it  is too late (i.e after the data stream has been presented to the receiver). In addition, real-time applications require a constant transmission rate, which conflicts with  transport  layer  services,  such  as  congestion control.  A drop  in throughput  due  to congestion control could result in a quality decrease or even link failure.

1.2 Description of MANET Routing Protocol

In an ad hoc network, mobile nodes communicate with each other using multi-hop wireless links. There is no stationary infrastructure such as base stations. Each node in the network also acts as a router, forwarding data packets to other nodes. A central challenge in the design of ad hoc networks is the development of dynamic transport and routing protocols that can efficiently find routes between two communicating nodes [6]. The routing protocol must be able to keep up with the high degree of node mobility that often changes the network topology drastically and unpredictably. The routing protocols for  mobile  adhoc  network that  are  available  in  the  literature  are  classified  into  two  categories: Proactive (table driven) and Reactive (on demand) protocols [1].

1.2.1 Proactive (table driven) Routing Protocols

Each node in the network has routing table for the broadcast of the data packets and want to establish connection to other nodes in the network. These nodes record all the presented destinations, number of hops required to arrive at each destination in the routing table. The routing entry is given a sequence number which is created by the destination node. To retain the stability, each station broadcasts and modifies its routing table from time to time. How many hops are required to arrive that particular node and which stations are accessible is result of broadcasting of

packets between nodes [1]. Each node that broadcasts data will contain its new sequence number and for each new route, node contains the following information:

i. The number of hops required to arrive at that particular destination node. ii. A new sequence number marked by the destination node

iii. The destination address

The proactive protocols are appropriate for networks with less number of nodes because its periodic update over  floods the  network with  much  more  Routing overhead resulting  in  more  bandwidth consumption [1]. An example of Proactive Routing Protocol is Destination Sequenced Distance Vector (DSDV).

Destination-Sequenced Distance-Vector (DSDV)

Destination-Sequenced Distance-Vector Routing protocol is a proactive table driven algorithm based on classic Bellman-Ford routing. In proactive protocols, all nodes learn the network topology before a forward request comes in. In DSDV protocol each node maintains routing information for all known destinations. The routing information is updated periodically. Each node maintains a table, which contains information for all available destinations, the next node to reach the destination, number of hops to reach the destination and sequence number. The nodes periodically send this table to all neighbors to maintain the topology, which adds to the network overhead. Each entry in the routing table is marked with a sequence number assigned by the destination node. The sequence numbers enable the mobile nodes to distinguish stale routes from new ones, thereby avoiding the formation of routing loops [2].

1.2.2 Reactive (On Demand) Routing Protocol

Reactive Protocol has lower overhead since routes are determined on demand. It employs flooding (global search) concept [1]. Constant updating of route tables with the latest route topology is not required in on demand concept. Reactive protocol searches for the route in an on-demand manner and set the link in order to send out the packet from a source node to destination node [6]. Route discovery process is used in on demand routing by flooding the route request (RREQ) packets throughout the network. Examples of reactive routing protocols are Dynamic Source Routing (DSR) and Ad hoc On- demand Distance Vector routing (AODV).

Dynamic Source Routing (DSR)

Dynamic Source Routing protocol is a reactive protocol i.e. it determines the proper route only when a packet needs to be forwarded [6]. The node floods the network with a route-request and builds the required  route  from  the  responses  it  receives.  DSR  allows  the  network  to  be  completely  self-

configuring without the need  for any existing  network infrastructure or administration. The DSR protocol is composed of two main mechanisms that work together to allow the discovery and maintenance of source routes in the ad hoc network. All aspects of protocol operate entirely on-demand allowing routing packet overhead of DSR to scale up automatically.

Route Discovery: this is the mechanism used by a node to find out the route leading to the destination node that will receive the packet it wants to send. Route Discovery is used only when Source node attempts to send a packet to Destination node and has no information on a route to the Destination node.

Route Maintenance: When there is a change in the network topology, the existing routes can no longer be used. In such a scenario, the source node can use an alternative route to the destination node, if it knows one, or invoke Route Discovery. This is called Route Maintenance.

Adhoc On-demand Distance Vector Routing (AODV)

AODV uses a very special technique to maintain routing information. AODV protocol is both an on- demand and a table-driven protocol. It adopts flat routing tables, one entry per destination unlike DSR, which can maintain multiple route cache entries for every one destination. Unlike DSR The packet size in AODV is uniform [1]. In AODV there is no need for system-wide broadcasts due to local changes, unlike DSDV. AODV has multicasting and unicasting routing protocol property within a uniform framework. Source node, destination node and next hops are addressed using IP addressing. AODV builds routes using a route request /route reply cycle. AODV uses sequence numbers maintained at each destination to determine freshness of routing information and to prevent routing loops. Sequence number for both destination and source are used. These sequence numbers are carried by all routing packets. Maintenance of timer-based states in each node, regarding the use of individual routing table entries is an important feature of AODV. If routing table entry is not used recently then routing table entry is expired [6]. When the next-hop link breaks nodes are notified with repeat request (RERR) packets. Each predecessor node, forwards the RERR to its own set of predecessors, thus effectively erasing all routes using the broken link. Route error propagation in AODV can be visualized conceptually as a tree whose root is the node at the point of failure and all sources using the failed link as the leaves. It is loop free, self-starting, and scales to large numbers of mobile nodes.

1.3 Problem Statement

Among recent research works, wireless mobile adhoc network has received a major attention from different researchers because of its unique operations and importance. It is self-configuring and uses no centralized control among member nodes. The mobility of the member nodes is expected to have a

significant effect on the performance of the networkresulting in difficulty in achieving high data throughput and low packet delivery delay as widely suggested from simulation results in the literature. Hence, this research work is intended to experimentally validate the impact of varying node velocity on the performance of TCP and UDP of a wireless mobile adhoc data network using data throughput and packet delivery delay as the performance metric. In general, throughput and packet delivery delay are the most important performance metric for any wireless network systems [1]. The capacity represents the throughput (bits per second) of the whole system including all nodes, and the delay represents the average time duration of a packet transmitting in the network from a source to the destination.

1.4 Objective of the study

Generally, it is difficult to achieve both high throughput and low packet delay because as the node velocity increases link failures and network overhead increases [2]. Furthermore, our objective is to evaluate and validate the impact of mobility rates on the performance of MADNET transport protocols TCP and UDP using Uniform mobility model and make recommendations based on the findings so as to achieve high throughput and low packet delay.

1.5 Significance

This study will be employed to improve capacity and performance of such computer to computer networks employed in areas struck by disaster for the rescue of the victims. Also, this kind of network is useful for sharing data among members in a conference. Furthermore this study will help in decision making when designing new protocols or when improving an existing one.

1.6 Scope of the Study

There are many network performance metrics which can be employed to evaluate mobile adhoc data network performance. The evaluation in this study will use network throughput, and packet delays to evaluate the performance of MADNET using TCP and UDP at different nodal velocities.

1.7 Methodology

In this research we intend to carry out an experiment using two identical laptops, designating one as our sender and the other as the receiver. We put the sender in motion and the receiver stationary during which we measured the amount of TCP and UDP data sent and the delay experienced at different velocities. We also reversed the process by keeping the sender stationary and the receiver in motion at corresponding nodal speeds. We adopted Iperf as our performance tool which we installed in the

laptops and configured one as sender and the other as client using Iperf commands in each section of the experiment. We plotted the graph of throughput with speed and delay with speed from our records from where we made useful recommendations based on our observations.

1.8 The Structure of the Work

This study will address the problem in these chapters: chapter     one     will     introduce     the     study, chapter 2 will review some literatures available on the topic, Chapter 3 will describe the methodology, chapter 4 will give out the main results and explain their meanings to the capacity and the packet delay based on the MADNET

performance. Finally in chapter 4 we conclude and make necessary recommendations.

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