Tom Johannes Maria Coenen

Analysis of a polling system modeling QoS differentiation in WLANs

This paper investigates a polling system with a random polling scheme, a 1-limited service discipline and deterministic service requirement modeling WLANs with QoS differentation capability. The system contains high and low priority queues that are distinguished via the probability of being served next. We propose a new iteration algorithm to approximate the waiting time of customers in the high and low priority queues. As shown by simulation results, our approximation is accurate for light to moderately loaded networks.

Architectural and QoS Aspects of Personal Networks

Personal networks (PNs) are future communication systems that combine wireless and infrastructure based networks to provide users a variety of services anywhere and anytime. PNs introduce new design challenges due to the heterogeneity of the involved technologies, the need for self-organization, the dynamics of the PN composition, the application-driven nature, the co-operation with infrastructure-based networks, and the security hazards. This paper discusses the challenges of security, service discovery and QoS provisioning in designing self-organized PNs and combines them all into an integrated architectural framework

Modeling the impact of interference on wireless ad hoc network performance

This thesis presents a variety of mathematical models to model the impact of interference on wireless ad hoc network performance. Wireless ad hoc networks are characterized by their decentralized nature, they are self-configuring and dynamic. The wireless nature of these networks poses a number of challenges, where this thesis focusses on the impact of interference: As radio signals use a certain frequency, signals sent over the same channel can collide, disrupting the reception of the signal, causing packets to be re-sent or even lost. This has an impact on multiple performance measures of the network, including the capacity of the network, the delay packets experience and the throughput of the network. Chapter one introduces the setting of ad hoc networks. Chapter two analyses the lifetime of the network, depending on the strategy in which packets are transmitted within the network. Chapter three uses a polling model to take quality of service differentiation into account. Chapter four models a two node network, showing that when the impact of interference is strong, the capacity of the network should not be divided over both nodes, but should be allocated to a single node. Chapter five provides a model that analyses whether a network with given capacity of each link can accommodate a given traffic demand. Chapter six focuses on the parameter settings of the protocol in use and its impact on the throughput of the network. Chapter seven provides an algorithm that calculates where in the network bottlenecks can occur and the impact this has on the throughput of the network.

Combinatorial reasoning to solve problems

This study reports combinatorial reasoning to solve problems. We observed the mathematical thinking of students aged 14-16. We study the variation of the students’ solution strategies in the context of emergent modelling. The results show that the students are tempted to begin the problem solving process on the highest level and otherwise have difficulties transitioning from a lower to a higher level of activities. Qualitative analysis revealed some students’ preference of the use of formulas, while at the same time other students showed more insight by their systematic approach of the problems, leading to better results. We advocate matching emergent modelling with teaching combinatorial reasoning, stimulating students to create a relational network of knowledge.

An upper bound on multi-hop multi-channel wireless network performance

Given a placement of wireless multi-radio nodes in space and a traffic demand between pairs of nodes, can these traffic demands be supported by the resulting network? The key issue in this setting is the wireless interference between neighbouring nodes using the same channel, including self-interference along multi-hop paths. We extend previous work addressing this problem taking into consideration the availability of multiple channels. As such, this paper presents a generic model for sustainable network load in a multi-hop multichannel setting and recasts this model in a multicommodity flow problem with interference constraints. A theorem is stated which gives a necessary and sufficient condition for the feasibility of this multicommodity flow problem. From this theorem an upper bound is derived for the throughput that can be achieved by the network, which is illustrated by examples. We indicate how the results can be used as a basis for a channel allocating algorithm.