Browsing by Subject "Local area networks (Computer networks)--Standard."

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Open Access
An analytical model of IEEE 80211 DCF for multi-hop wireless networks and its application to goodput and energy analysis
(2010) Aydoğdu, Canan
In this thesis, we present an analytical model for the IEEE 802.11 DCF in multihop networks that considers hidden terminals and works for a large range of traffic loads. A goodput model which considers rate reduction due to collisions, retransmissions and hidden terminals, and an energy model, which considers energy consumption due to collisions, retransmissions, exponential backoff and freezing mechanisms, and overhearing of nodes, are proposed and used to analyze the goodput and energy performance of various routing strategies in IEEE 802.11 DCF based multi-hop wireless networks. Moreover, an adaptive routing algorithm which determines the optimum routing strategy adaptively according to the network and traffic conditions is suggested. Viewed from goodput aspect the results are as follows: Under light traf- fic, arrival rate of packets is dominant, making any routing strategy equivalently optimum. Under moderate traffic, concurrent transmissions dominate and multihop transmissions become more advantageous. At heavy traffic, multi-hoppingbecomes unstable due to increased packet collisions and excessive traffic congestion, and direct transmission increases goodput. From a throughput aspect, it is shown that throughput is topology dependent rather than traffic load dependent, and multi-hopping is optimum for large networks whereas direct transmissions may increase the throughput for small networks. Viewed from energy aspect similar results are obtained: Under light traf- fic, energy spent during idle mode dominates in the energy model, making any routing strategy nearly optimum. Under moderate traffic, energy spent during idle and receive modes dominates and multi-hop transmissions become more advantageous as the optimum hop number varies with processing power consumed at intermediate nodes. At the very heavy traffic conditions, multi-hopping becomes unstable due to increased collisions and direct transmission becomes more energy-efficient.The choice of hop-count in routing strategy is observed to affect energyefficiency and goodput more for large and homogeneous networks where it is possible to use shorter hops each covering similar distances. The results indicate that a cross-layer routing approach, which takes energy expenditure due to MAC contentions into account and dynamically changes the routing strategy according to the network traffic load, can increase goodput by at least 18% and save energy by at least 21% in a realistic wireless network where the network traffic load changes in time. The goodput gain increases up to 222% and energy saving up to 68% for denser networks where multi-hopping with much shorter hops becomes possible.
Open Access
A distributed air-time fair MAC for multi-rate IEEE 80211 networks using multiple distributed coordination functions
(2010) Karataş, Fırat
In a multi-rate IEEE 802.11 network, the conventional Distributed Coordination Function (DCF) Medium Access Control (MAC) aims to ensure max-min throughput fairness and equal channel access in scenarios with multiple nodes, while failing to satisfy air-time fairness. Consequently, nodes that have relatively poor channels or longer packets to transmit invade the channel substantially more than others, hence decreasing the throughput of nodes which have better channels or shorter packets. This phenomenon is known as the performance anomaly problem in the existing literature. In this thesis, we propose a novel distributed air-time fair algorithm to cope with the performance anomaly problem without having to change the conventional IEEE 802.11 DCF MAC. In the proposed algorithm, each node in the system runs multiple instances of the conventional IEEE 802.11 DCF back-off algorithm where the number of instances for the particular node is calculated independently from other nodes using only local information such as packet lengths and transmission rates. Both analytical and simulationbased results are provided to validate the effectiveness of the distributed air-time fair algorithm we propose.
Open Access
Optimal access point selection in multi-channel IEEE 80211 networks
(2008) Aydınlı, Mustafa
A wireless access point (WAP or AP) is a device that allows wireless communication devices to connect to a wireless local area network (WLAN). AP usually connects to a wired network, and can relay data between the wireless devices (such as computers or printers) and wired devices on the network. Optimal access point selection is a crucial problem in IEEE 802.11 WLAN networks. Access points (APs) cover a certain area and provides an adequate bandwidth to the users around them. When the area to be covered is large, several APs are necessary. Furthermore in order to mitigate the adverse effects of interference between APs, multi channels are used. In this thesis, a service area is divided into demand clusters (DCs) in which number of users per DC and average traffic rates are known. Next, we calculate the congestion of each AP by using the average traffic load. With our Optimal Access Point Selection Algorithm, we balance the traffic loads in APs using a mixed integer linear programming formulation. This algorithm guarantees that each DC is assigned an AP and there is sufficient received power. Furthermore, the interference between the adjacent APs is controlled so that the received signal to interference and noise ratio at each AP satisfies a minimum level. Interference control is accomplished by using a multi-channel WLAN. In this thesis, both orthogonal (non-overlapping) and non-orthogonal (overlapping) channel assignment schemes are considered. The total interference is computed taking into account both co-channel and inter-channel interferences. The developed AP selection methodology is applied to WLAN designs for several buildings. It is observed from the designated networks that a DC shouldnot need to connect to the closest AP but it may be connected to an AP which may be farther away but less congested. DCs are assigned to APs such that all DCs are covered. The effects of the parameter such as traffic load, receiver sensitivity, number of APs, etc are also studied.
Open Access
Uplink scheduling algorithms for the rtPS traffic class for IEEE 80216 networks
(2008) Ertürk, Mustafa Cenk
IEEE 802.16 MAC provides extensive bandwidth allocation and QoS mechanisms for various types of applications. However, the scheduling mechanisms for the uplink and downlink are unspecified by the IEEE 802.16 standard and are thus left open for vendors’ own implementations. Ensuring QoS requirements at the MAC level for different users with different QoS requirements and traffic profiles is also another challenging problem in the area. The standard defines five different scheduling services one of them being the real-time Polling Service (rtPS). In this thesis, we propose an uplink scheduler to be implemented on the WiMAX Base Station (BS) for rtPS type connections. We propose that the base station maintains a leaky bucket for each rtPS connection to police and schedule rtPS traffic for uplink traffic management. There are two scheduling algorithms defined in this study: one is based on a simpler round robin scheme using leaky buckets for QoS management, whereas the other one uses again leaky buckets for QoS management but also a proportional fair scheme for potential throughput improvement in case of varying channel conditions. The proposed two schedulers are studied via simulations using MATLAB to demonstrate their performance in terms of throughput, fairness and delay. We show that the leaky bucket based scheduler ensures the QoS commitments of each user in terms of a minimum bandwidth guarantee whereas the proportional fair algorithm is shown to opportunistically take advantage of varying channel conditions.