DR. ADERO FREDERICK OMONDI

Wireless sensor networks (WSNs) form a large part of the ecosystem of the Internet of Things (IoT), hence they have numerous application domains with varying performance and availability requirements. Limited resources that include processing capability, queue capacity, and available energy in addition to frequent node and link failures degrade the performance and availability of these networks. In an attempt to efficiently utilise the limited resources and to maintain the reliable network with efficient data transmission; it is common to select a clustering approach, where a cluster head is selected among the diverse IoT devices. This study presents the stochastic performance as well as the energy evaluation model for WSNs that have both node and link failures. The model developed considers an integrated performance and availability approach. Various duty cycling schemes within the medium-access control of the WSNs are also considered to incorporate the impact of sleeping/idle states that are presented using analytical modeling. The results presented using the proposed analytical models show the effects of factors such as failures, various queue capacities and system scalability. The analytical results presented are in very good agreement with simulation results and also present an important fact that the proposed models are very useful for identification of thresholds between WSN system characteristics.

Wireless Sensor Networks have seen a tremendous growth in various application areas despite prominent performance and availability challenges. One of the
common configurations to prolong the lifetime and deal with the path loss phenomena
is having a multi-hop set-up with clusters and cluster heads to relay the information.
Although researchers continue to address these challenges, the type of distributions
for arrivals at the cluster head and intermediary routing nodes is still an interesting
area of investigation. The general practice in published works is to compare an empirical exponential arrival distribution of wireless sensor networks with a theoretical
exponential distribution in a Q-Q plot diagram. In this paper, we show that such comparisons based on simple eye checks are not sufficient since, in many cases, incorrect
conclusions may be drawn from such plots. After estimating the Maximum Likelihood parameters of empirical distributions, we generate theoretical distributions
based on the estimated parameters. By conducting Kolmogorov-Smirnov test statistics for each generated inter-arrival time distributions, we find out, if it is possible to
represent the traffic into the cluster head by using theoretical distribution. Empirical
exponential arrival distribution assumption of wireless sensor networks holds only
for a few cases. There are both theoretically known such as Gamma, Log-normal
and Mixed Log-Normal of arrival distributions and theoretically unknown such as
non-Exponential and Mixed cases of arrival in wireless sensor networks. The work is
further extended to understand the effect of delay on inter-arrival time distributions
based on the type of medium access control used in wireless sensor networks

Wireless Sensor Networks have seen a tremendous growth in various application areas despite prominent performance and availability challenges. One of the
common configurations to prolong the lifetime and deal with the path loss phenomena
is having a multi-hop set-up with clusters and cluster heads to relay the information.
Although researchers continue to address these challenges, the type of distributions
for arrivals at the cluster head and intermediary routing nodes is still an interesting
area of investigation. The general practice in published works is to compare an empirical exponential arrival distribution of wireless sensor networks with a theoretical
exponential distribution in a Q-Q plot diagram. In this paper, we show that such comparisons based on simple eye checks are not sufficient since, in many cases, incorrect
conclusions may be drawn from such plots. After estimating the Maximum Likelihood parameters of empirical distributions, we generate theoretical distributions
based on the estimated parameters. By conducting Kolmogorov-Smirnov test statistics for each generated inter-arrival time distributions, we find out, if it is possible to
represent the traffic into the cluster head by using theoretical distribution. Empirical
exponential arrival distribution assumption of wireless sensor networks holds only
for a few cases. There are both theoretically known such as Gamma, Log-normal
and Mixed Log-Normal of arrival distributions and theoretically unknown such as
non-Exponential and Mixed cases of arrival in wireless sensor networks. The work is
further extended to understand the effect of delay on inter-arrival time distributions
based on the type of medium access control used in wireless sensor networks.

In Wireless Sensor Networks (WSNs), performance and availability are important in providing Quality of Service (QoS). WSNs are prone to failures that may result from software and hardware malfunctions, battery drain, tampering and link failures. In addition, sensors are resource constrained in terms of inadequate processing capacity, limited storage memory and restricted power supply. Alternating sensor operations between sleep and active modes whilst saving energy, has also introduced more challenges to the performance and dependability of WSN systems. This paper therefore seeks to investigate performance and availability concerns resulting from link and node failures in addition to the buffer restrictions and challenges caused by alternating sensor operations between active and sleep states. First, a finite queue capacity analytical model integrating performance and availability in the presence of node and channel failures and repairs is developed. Using the model, we analyse performance and availability of WSNs in terms of network throughput, delays and dependability. Finally, the model is used to evaluate tradeoffs between performance metrics. The analytical solutions using Spectral Expansion and Kolmogorov Forward equations as well as the simulation results are in good agreement.

This thesis presents generic analytical models of homogeneous clustered Wireless Sensor Networks (WSNs) with a centrally located Cluster Head (CH) coordinating cluster communication with the sink directly or through other intermediate nodes. The focus is to integrate
performance and availability studies of WSNs in the presence of sensor
nodes and channel failures and repair/replacement. The main purpose is to enhance improvement of WSN Quality of Service (QoS).
Other research works also considered in this thesis include modelling
of packet arrival distribution at the CH and intermediate nodes, and
modelling of energy consumption at the sensor nodes