Measurement and Modeling of the congestion-controlled traffic over CSMA based multihop Wireless Mesh Networks

wireless mesh network is a well suited future technology. Its properties like low cost, high bandwidth and significant progress has been made in understandingthe behavior of TCP and congestion-controlled traffic over CSMA based multihop wireless networks. Despite these advances, however, no prior work identified severe throughput imbalances in the basic scenario of mesh networks, in which a one-hop flow contends with a two-hop flow for gateway access. In this paper, we demonstrate via real network measurements, testbed experiments, and an analytical model that starvation exists in such a scenario; i.e., the one-hop flow receives most of the bandwidth, while the two-hop flow starves. Our analytical model yields a solution consisting of a simple contention window policy that can be implemented via standard mechanisms defined in IEEE 802.11e. Despite its simplicity, we demonstrate through analysis, experiments, and simulations that the policy has a powerful effect on network-wide behavior, shifting the network’s queuing points, mitigating problematic MAC and transport behavior, and ensuring that TCP flows obtain a fair share of the gateway bandwidth, irrespective of their spatial location. Introduction Wireless Mesh Networks(WMN) are next generation wireless networks with theirpromising and low-cost technology. It provides high-speed Internet access forfuture broadband applications. WMNs are flexible, mobile, reliable and scalablewireless networks. WMNs reduce the initial investment and deployment time compared to traditional broadband Internet access technologies [1]. In WMNs802.11 based multi-hop communication is used for delivering fast services to end-users,WMNs are not used for any fixed Access point networks likeWireless LocalArea Networks(WLAN). WMNs are extremely reliable with its mesh connection.If any node fails or drop of packet in the network occurs, it uses another routeas activce route.Wireless mesh networks (WMN) is an emerging technology. Existing definitions are simply based on Mobile Ad hoc Networks (MANETs), which arevariants of WLAN. WMNs are considered as special MANETS [3,5]. Wireless Mesh Networks(WMNs) are self-organized, selfconfigured, ease and can rapidity change with the network deployment [2, 4, 5]. Wireless Mesh Network consists of three types of nodes. Mesh Points(MP), Mesh Routers(MR), andMesh Clients(MC). In WMN, some Access Points(APs) have wired connections known as mesh points(MPs). APs that don’t have wired connections arecalled Mesh Routers(MRs), they can form multihop communication with theMPs. Both MPs and MR form the backhaul network, which is used to forwardtraffic between the nodes. MP and MR are usually equipped with multiple wireless interfaces built on either same or different wireless technologies and theyestablish a permanent infrastructure. New MP and MR are easily added, sincethe connection is wireless connection [5]. Compared to conventional wireless routers, a mesh router can achieve the same radio range with much lower transmission power through multi-hop communications.Mesh router and conventionalrouters are usually built based on a similar hardware platform [5]. MPs and MRshave the similar design but MPs are connected to wired networks. Mesh Clients(MC) usually have only one wireless interface and hardware as well as softwareare much simpler than MPs or MRs. MCs do not have routing capabilities andworks as Mesh Router(MR) and as a router [4, 5]. Due to high mobility of meshclients they can leave and join the Mesh Router(MR) at any time [5]. There is an increasing need towards portable and mobile computers or workstations with the development of wireless technology and Internet. Wireless networks need to provide communications between mobile terminals, in addition, access to high speed wired networks needs to be provided too. Wireless Figure 1: Wireless Mesh Networks Venkatesh.Donepudi, vahiduddinshariff / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera. com Vol. 3, Issue 2, March -April 2013, pp.242-252 243 | P a g e Local Area Networks(WLANs), which provides better flexibility and convenience than their wired counterpart, are being developed to provide high bandwidth access for users in a limited geographical area. IEEE Project 802 recommends an international standard 802.11 for WLANs. The standards include detailed specifications both for MediumAccess Control(MAC) Layer and Physical(PHY) Layer. In WLANs, the physical media, which is shared by allstations and has limited connection range, has significant differences when compared to wired media. The design ofWLAN MAC protocol is further complicated by the presence of hidden terminal and capture effects. Currently, the IEEE 802.11 WLAN standards include a basic medium access protocol Distributed Coordination Function(DCF) and an optional Point Coordination Function(PCF). In 802.11, the DCF is the fundamental access method used to support asynchronous data transfer on a best effort basis. As specified in the standards, the DCF must be supported by all the stations in a basic service set(BSS). The DCF protocol is based on Carrier Sense Multiple Access with Collision Avoidance(CSMA/CA). CSMA/CD is not used because a station is unable to listen to the channel for collision while transmitting. In 802.11 CS is performed both at physical layer, which is also referred to as physical carrier sensing, and at the MAC layer, which is known as virtual carrier sensing. The PCF in the 802.11 is a polling-based protocol, which is designed to support collision free and real time services. This paper focuses on the performance analysis and modeling of DCF in 802.11 WLAN. There are two techniques used for packet transmitting in DCF. The default one is a two-way handshaking mechanism, also known as basic access method. A positive MAC acknowledgement(ACK) is transmitted by the destination station to confirm the successful packettransmission. The other optional one is a fourway handshaking mechanism, which uses request-tosend/clear-to-send(RTS/CTS) technique to reserve the channel before data transmission. This technique has been introduced to reduce the performance degradation due to hidden terminals. However, the drawback of using the RTS/CTS mechanism is increased overhead for short data frames. The modeling of 802.11 has been a research focus since thestandards has been proposed. In this,the effect of capture and hidden terminal and paper gives the theoretical throughput limit of 802.11 based on a ppersistent variant. However, none of these captures the effect of the Contention Window(CW) and binary slotted exponential back-off procedure used by DCF in 802.11. Unlike those ones, we use Markov process to analyze the saturated throughput of 802.11 and show that the Markov analysis works well. We believe that the Markov chain analysis method is fit for examining the performance of IEEE 802.11, which is based on binary slotted exponential backoff. In this also uses Markov chain and considers the frame retry limits to analyze the saturated throughput; therefore, a more exact model isproposed in this thesis. On the other hand, with the prosperity of Internet, Transport Control Protocol(TCP), which is the widely used reliable protocol in the Internet, is supposed to work well in heterogeneous environment. Since the WLAN MAC has its own characteristics, such as MAC Layer ACK frame, MAC retransmissions, which are different from traditional wireless medium, the performance evaluation and enhancement will be somewhat different from the research before. The performance of TCP over WLAN is being studied recently; however, none of these give a TCP performance enhancement based on the WLAN MAC layer solutions. In fact, when TCP runs over WLAN, where a shared channel is used for multiple access, the forward TCP data and the backward TCP ACK will compete the channel, which may cause collisions and degrade the overall performance. Meanwhile, 802.11 has been standardized and any proposed enhancement scheme must keep backward compatibility with 802.11, i.e., it can work with 802.11 without introducing performance degradation. Large-scale mesh network deployments are planned and underway in cities across the world. According to In-Stat, the market will grow from 248 cities in 2005 to 1,500 cities in 2010, becoming a $1B industry in mesh access point sales alone. The prevailing architecture for large-scale deployments is a two-tier architecture in which an access tier connects end users’ PCs and mobile devices to mesh nodes and a backhaul tier forwards traffic to and from high-speed gateway nodes. Directional antennas terminating at the gateway are also used to expand coverage and capacity. We have deployed and are operating UrbanMesh,1 a two-tier mesh network serving a user population of nearly 2,000 users in a 3 km2 urban community. As most deployments are in the planning or early deployment stages, to the best of our knowledge, this is the highest density urban mesh network operating to date. Unfortunately, we have observed that under heavy load, flow starvation will occur in which onehop flows obtain high throughput whereas competing multi-hop flows obtain near zero throughput. The phenomena occurs under two hardware/software platforms as well as in simulation. Clearly, for mesh networks to be successful, it is critical that network resources are distributed fairly among users, irrespective of their spatial location. In this paper, we (i) perform extensive measurements in UrbanMesh to characterize starvation, (ii) studystarvation’s protocol origins, (iii) develop an analytical model to capture the interaction of medium access and Venkatesh.Donepudi, vahiduddinshariff / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera. com Vol. 3, Issue 2, March -April 2013, pp.242-252 244 | P a g e congestion control that leads to starvation and (iv) design, justify, a