Load Distribution In IEEE 802 . 11 Cells

A key issue in Wireless LANs (WLANs) is the management of user congestion at popular zones called “hot-spots”. At these sites, there are several access points (APs) with overlapped coverage and throughput is usually unevenly distributed among them. The reason is that the current IEEE 802.11 standard does not support a mechanism to distribute stations, thus they select APs based exclusively on the received signal quality. In addition, when the number of users per AP increases, the throughput per user decreases. As a result, the total network throughput is reduced producing under utilisation of the network resources. Several approaches have been suggested to solve this problem. Some of them are based on the modification or enhancement of the MAC layer, hence changes to the physical layer are required. This would imply that all deployed stations should be changed. Other approaches are based on adding Quality of Service (QoS) support to the standard. These solutions require that stations and APs cooperate, which makes its deployment difficult in existing WLANs. Recently, some vendors of WLAN devices have incorporated load-balancing capabilities within their products. Nevertheless, they also require cooperation between stations and APs. Another limitation of these load-balancing schemes is that they simply balance the number of associated users across APs. From the analysis of related work, we have identified in this thesis two groups of common issues that any load distribution scheme should deal with: architectural and algorithmic issues. Architectural issues deal with key points such as the cooperation between APs and stations, centralized versus distributed control or the most efficient load metrics to be used. Algorithmic issues refer to the four policies that a load distribution algorithm should include: transfer, which defines when an AP is suitable to participate in the load distribution; selection, which selects the user to transfer; location, which finds a suitable AP for the user and information, which specifies when, from where and what information is to be collected. To address these issues, we propose and evaluate a new group of mechanisms, called Load Distribution System (LDS), the goal of which is to provide higher utilization of the overall network resources. This is achieved by means of dynamically transferring users among APs. We consider as a load metric the throughput per AP and not only the number of associated users per AP. Each AP determines whether the network is balanced or not, calculating the balance index (β). This index, bounded between 0 and 1, indicates any slight change in the load of the APs and quantifies the fairness of the network. The LDS runs at each AP in a distributed manner; it does not require the modification of the standard and it is transparent to stations. Furthermore, our proposed LDS can be applied to any type of IEEE 802.11 networks (a, b or g) since they share the same architecture and MAC protocol. We evaluate the effectiveness of our LDS, building an experimental prototype. We perform three initial tests to set the necessary parameters of the LDS: the handover delay, the sampling time to monitor the traffic and the reactivity of the algorithms. After initial parameters are determined, we experimentally test the performance of the LDS. The results show that average packet delay per user can be decreased and total throughput in the network can be increased in comparison with a WLAN without our LDS. We also show that the LDS is stable and that it only transfers a station if this increases overall performance. Based on these results, we conclude that current WLANs will benefit from applying our LDS.