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Transmission Control Protocol (TCP) is a reliable, end-toend transport protocol, which is most widely used for data services and is very efficient for wired networks. However, experiments and research showed that TCP’s congestion control algorithm performs very poorly over Wireless Ad Hoc Networks with degraded throughputs and severe unfairness among flows.In this paper, a deep study has been conducted in order to study the factors that cause congestion in an ad hoc wireless network. Main focus in this report has been to simulate and study the effect of change in topology and number of users on network congestion. Apart from that, the effects of network congestion and the importance of its study have also been highlighted. Congestion is a critical factor, in determining the quality of network. It also determines the dependability and sustainability of a network. Deploying new network infrastructure to tackle congestion problem is not economically viable solution, hence it is important to understand the reasons behind such network operation conditions and then design suitable methods to overcome them. In this paper, various network behaviors’ have been simulated using OPNET Modeler 14.5 to study how node’s buffer space gives impact to the in-flight packets in ad hoc environment by also taking mobility and power consumption into consideration. With a controlled size of users ( ) the network condition has been simulated. Performance has been measure on several parameters such as throughput, number of packets dropped, and retransmission count and end number of users changed. General Terms Network Congestion,Network Parmeters. Keyword Ad hoc network,Power Consumption,Buffer size,Opnet 1.INTRODUCTION Wireless Ad Hoc Networks are multi-hop wireless networks. A Wireless Ad Hoc Network consists of mobile platforms (e.g., a router with multiple hosts and wireless communication devices) here in simply referred to as “nodes” – which are free to move about arbitrarily [1]. Wireless Ad Hoc Networks can be easily deployed with or without the support of existing infrastructure. Figure 1 is an example Ad Hoc network which uses gateways as the connection between the wired and wireless parts, which is referred to as a Wireless Ad Hoc Access Networks. Figure 1:An Example Wireless Ad Hoc Access Network. 1.1 Ad Hoc WLANs The word “ad hoc” refers to making or happening only for a particular purpose or need, not planned in advance [3]. In networking context, ad hoc network is an IEEE 802.11 networking framework, in which nodes communicate directly with each other without the use of an access point, by which it can connect or communicate with the network. An ad hoc mode is also referred to as a peer-to-peer mode that is useful for establishing a network where infrastructure does not exist or where services are not required [5]. In ad hoc wireless LANs, all nodes working ad hoc mode and form a network a dynamically without any existing infrastructure or topology. The nodes adjust accordingly with the topology change and hence are very robust. Also, since it does not utilize expensive network switches or other access and control points, it is a low cost solution. Due to the flexibility, robustness, and dynamic structure of such networks, ad hoc wireless LANs have made a way significantly into the business, military and personal communication sectors in a very short time [2]. The early groundbreaking research for ad hoc wireless LANs was supported by the Defense Advanced Research Projects Agency (DARPA) and the Navy in US [4, 6]. Despite many advances over the last several decades in wireless communications, in general, and ad hoc wireless International Journal of Computer Applications (0975 – 8887) Volume 67– No.22, April 2013 2 networks, in particular, the optimal design, scalability, performance, and fundamental capabilities of these networks remain poorly understood, at least in comparison with their wireless network paradigms and a lot of “daylight” remains in this field of research. However, with enormous potentials for such networks, ad hoc networks primarily support data networks, but it has been envisioned recently to enter to home networks, wireless device networks, distributed control systems, and sensor networks etc. In the following subsections, the network architecture, routing and scalability of an ad hoc network are described. 1.2 Congestion Control Congestion is an unwanted situation in networked systems, where the part of the network is being offered more traffic than its rated (desired) capacity. Congestion can be disastrous for a data transmission system as it manifests itself as depletion of resources that are critical to the operation of the system. These resources can be CPU, buffer space, bandwidth etc. Resource crunch will lead to lengthening of various queues for these resources. Due to finite length constraint, many packets may eventually get dropped, which, in turn, will deteriorate the response time of the system beyond permissible limits due to retransmission requests. “Congestion Control” refers to the mechanism of combating congestion, which makes sure the resources are used optimally and the system has maximum data throughput with the given conditions.The main objective of congestion control is to make sure the system is running at its rated capacity, even with the worst case overload situations. In certain systems, this is ensured by restricting certain nodes to transmit at the maximum capacity or to make use of certain resources monotonously. Doing this enables optimal usage of resources for all the nodes in the system with a measurable quality-of-service (QOS). In some systems, there are built-in mechanisms that does not allow congestion situation to take place and every node keeps track of system statistics and resources. This is often knows as “congestion prevention” or “Congestion Avoidance”.Congestion control is necessary for systems, whose nodes do not keep track of such statistics or do not keep resource information. In such systems, the nodes participate in the network, in which the topology changes very often and the network statistics also vary randomly. As such the control of congestion becomes an issue of the nodes that act as bridges. Ad hoc networks are examples of such scheme. In this thesis, the terms congestion control and congestion avoidance will be used synonymously with the ultimate aim to keep the total networked system free of congestion. Congestion control can either be rate-based control or buffer-based control depending on how the actual control is done. Most of the rate-based congestion control algorithms are applied during routing of data from node to node. In multi hop routing, thus, congestion takes place on every hop and is termed hop-by-hop congestion control. However, for single hop routing congestion is only an end-to-end issue and more of rate adjustment of the source rather than destination. A major open challenge for research still remains for congestion control of large ad hoc wireless networks, where single hop routing is virtually impossible [1]. 2.OBJECTIVE A network link is said to be congested when the offered load on the link reaches a value close to the capacity of the link. In a similar manner, wireless network congestion can be defined as the state in which the transmission channel is close to being completely utilized. Due to popularity of wireless ad hoc network technology in the market it has become necessary to study and manage the effects of congestion in the wireless networks. The objective of our research is to study how node’s buffer space gives to the in-flight packets in ad-hoc environment by also taking the mobility and power consumption into consideration. This will also enable us to conduct a deeper study to understand network congestion in ad-hoc networks.In wireless networks, the congestion control protocols and methods are very different from the conventional wired networks. Specially, in the case of ad-hoc networks it is hard to control network congestion, as every node behaves like a router. The congestion will vary with different topologies, methods of network deployment, mobility, channel utilization and power consumption at node level. We will try to simulate various network environments with different number of users and then study the network on parameters like throughput, delay retransmission attempts, packets dropped etc. 2.1 Objectives include: 1. To simulate various network topologies of adhoc network and vary the number of users in various cases. 2. Study the effect of node’s buffer space on in flight packets of the network. 3. Vary the power consumption and topology of networks to study the effect of network congestion, and how they may or may not be an important contributor towards network congestion. 4. Represent the results on various standard parameters such as throughput,retransmission attempts , packets dropped etc. 5. Conduct a deep study to understand network congestion in ad-hoc networks. 3.LITERATURE REVIEW Congestion is an unwanted situation which takes place in the access points in the networks that have limited resources, such as buffer length. Also, the fact that large networks often have nodes having different input and output rates, congestion can take place in such access points, as well. Congestion control has been a serious issue in communication networks and is a key to network performance. Early works in congestion control are based on the modern Internet technology, where the access points are well defined and are administered by dedicated nodes. But due to lack of these infrastructures, congestion in ad hoc wireless LANs cannot be dealt in exactly the same way as that in the Internet, even though the basic purpose is the same. In the following sections, we look into different literatures to describe how congestion control algorithms have matured from that in Internet to ad hoc wireless LANs and how these

A Systematic Approach for Congestion Control in Wireless Ad hoc Network using Opnet