| Summary: | Network failures can originate from or be located in any one of several network layers as
described by the OSI model. This investigation focuses on the role of physical topological design
parameters in determining network reliability and performance as can be expected from the
point of view of a typical client-server based connection in an Ethernet local area network. This
type of host-to-host IP connection is found in many commercial, military and industrial network
based systems. Using Markov modelling techniques reliability and performability models are
developed for common network topologies based on the redundancy mechanism provided by
IEEE spanning tree protocols. The models are tested and validated using the OPNET network
simulation environment. The reliability and performability metrics calculated from the derived
models for different topologies are compared leading to the following conclusions. The reliability
of the entry-nodes into a redundant network is a determining factor in connection availability.
Redundancy mechanisms must be extended from the entry-node to the connecting hosts to
gain a significant benefit from redundant network topologies as network availability remains
limited to three-nines. The hierarchical mesh network offers the highest availability (sevennines)
and performability. Both these metrics can be accurately predicted irrespective of the
position of the entry-node in the mesh. Ring networks offer high availability (five to sevennines)
and performability if the ring remains small to medium sized, however for larger rings
(N≥32) the availability is highly dependant on the relative position of the entry-node in the ring.
Performability also degrades significantly as the ring size increases. Although star networks offer
predictable and high performability the availability is low (four-nines) because of the lack of
redundancy. The star should therefore not be used in IP networked systems requiring more than
four-nines availability. In all the topologies investigated the reliability and performability can be
increased significantly by introducing redundant links instead of single links interconnecting the
various nodes, with the star topology availability increasing from four-nines to seven-nines and
performance doubling. === MIng (Computer and Electronic Engineering), North-West University, Potchefstroom Campus, 2014
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