Reliability in single, double and N2R ring network structures Thorbjørn Jørgensen [thjo02@kom.aau.dk], Lars Pedersen [lbpe02@kom.aau.dk] and Jens Myrup Pedersen [jens@control.aau.dk] Center for Network Planning (CNP) and Center for TeleInfrastruktur (CTIF), Aalborg University DK-9220 Aalborg East, Denmark, Fax: +45 98151739 Abstract— This paper studies the properties of single, double and N2R ring network structures during link errors. The structure of the network infrastructure must be redesigned in order to fulfil the requirements of services using the Internet in the future; hence, N2R structures have been suggested. N2R structures are found to be superior regarding network properties compared to the other more traditionally ring structures but the deployment of this type of structure is more complicated. Therefore, this paper suggests a deployment scheme, denoted N2R tube deployment, which is a compromise between the easy deployable dobule ring structure, and the better properties of the N2R structure. The properties of single ring, double ring and N2R structures have never been compared during errors in previous research. Nevertheless, this is an important part of selecting the topology of the network infrastructure for the future. This paper compares single, double, N2R and N2R tube structures when one and two errors are introduced. The results show that even when errors are introduced in the network structures, the N2R structure remain superior. Furthermore, this is also valid with a smaller margin for the N2R tube structures. Keywords— Ring network structures, network planning, net- work reliability, N2R structures and network infrastructure. I. I NTRODUCTION Recently, several new services using the Internet have been introduced which set higher requirements to the network infrastructure: Online television/video streaming requires a significant amount of bandwidth; telerobotics [1] [2] require low delays; the increasing use of the Internet for reliable ser- vices implies that the reliability of the network infrastructure must be increased [3]. There exists several other examples but those mentioned state the main challenges for the new network infrastructure: More bandwidth, lower delays and more reliable networks. The bandwidth challenge can be addressed by replacing existing copper lines with optical fibers, which are upgradeable simply by replacing terminal equipment. The deployment of fiber optical lines is relatively expensive, mainly because of the expenses for digging ducts. Furthermore, the expected operation time of the network infrastructure is very high, which stresses the importance of choosing a suitable topology. Reducing delays and increasing the reliability of the net- work infrastructure can be achieved by considering other network topologies than those typically used. This implies that it is necessary to deploy a suitable network structure to support the end-users’ demand for increasing reliability. Therefore, it is crucial to choose a suitable network structure, which guarantees the demands of the future. Single rings (SR) are often used in network infrastructure since they make routing and restoration uncomplicated, and still have redundancy in case of failure. Recently, more ad- vanced network structures like the double ring (DR) and the N2R structures [8] have been proposed as replacements for the SR structure. The DR and N2R structures are more complex regarding deployment and routing, but have better network properties [6], which imply higher capacity and lower delays. In the United States, an average of 1 cable cut per year on 370km [4] of deployed 1 cable can be expected. Cable cuts are the most frequent failure in a widespread network and the most time consuming to repair [4]. The fact that it is impossible to avoid errors in networks stresses the importance of studying how the different network structures behave during errors, in order to ensure that the requirements to the networks are still guaranteed. Two basic error types can occur in the network structures: A node can fail, or a link can fail. Node errors can be caused by equipment or power failure, but usually the recovery time or time to start emergency systems is very low [4], compared to the time it takes to locate and repair a link error. Link errors are usually caused by links being dug over. These types of errors will happen randomly in the network with some probability, depending on the size of the network and the way it is deployed. This paper compares certain properties of the SR, DR and N2R network structures in scenarios where link errors occur. In case of a node error the network is divided into two separate parts since the defect node is not connected to the network anymore; hence it is not possible to evaluate the structure as a whole. Furthermore, a traffic generating element is removed from the network in case of a node error; hence, from the network’s perspective the overall traffic load in the network is reduced, which reduces the impact of node errors. Therefore node errors are not evaluated in this paper. The comparison of the network structures in scenarios with or without errors can support the selection of the network infrastructure topology of the future. 1 Deployed in both ducts and masts.