The growth of mobile networks and the imminent deployment of third generation networks and services will require signalling networks to maintain their integrity during increased unanticipated traffic volumes. As signalling networks become larger and more complex, an analysis ofprotocol operation is necessary to determine the effectiveness of the current protocol implementation and to evaluate the applicability of the proposed enhancements. The objective of this study ,is to develop analytical models to analyse the impact of Signalling Transfer Point congestion on network performance when simple message discard schemes are used as the primary flow control mechanism, and to investigate suitable congestion and flow control mechanisms to help alleviate the congestion. Unlike previous studies, that are localised and only concentrate on the nodes around the congested entity, the models presented here examine the impact of network wide and focused overloads on the entire network. The study considers both the fixed-line and mobile network environments, and analyses the performance of the ISDN User Part and Mobile Application Part protocols. The call completion rate and location update success rate are used to measure performance, instead of message throughput, since these parameters provide a more appropriate measure of the grade-of-service and more accurately reflect the level of service provided to a customer. The steady state equilibrium models, derived here, can be used to quickly estimate the safe operating regions of a signalling network, while the transient models provide a more intuitive perspective of the traffic processes that lead to congestion . Furthermore, these models can be used to examine the network performance for different message priority schemes, routing algorithms, overload scenarios and network configurations. The performance of various congestion control mechanisms that incorporate non-linear throttling schemes is also evaluated, together with an examination of the impact of congestion on multiple user parts in a mobile network environment. Message priority schemes are found to offer little or no advantage in a fixed network environment, but in a mobile network they can be used to maintain the network's performance at an optimum level during periods of overload. Network performance is also improved if congestion controls block load-generating traffic at the initial onset of congestion and then gradually restore traffic as the performance improves . / Thesis (Ph.D.)-University of Natal, Durban, 2002.