Survivable routing serves as one of the most important issues in optical backbone design. Due to the high data rates enabled by the wavelength division multiplexing technology, any interruption in the service results in the loss of a large amount of application data. Thus, making efforts to calculate and signal the protection resources promptly after the failure occurred would lead to an unacceptable high delay. As the main purpose of this chapter, the principles of pre-planned protection approaches in mesh optical backbone networks are discussed. The Shared Risk Link Group (SRLG) concept is introduced modeling physical and geographical dependency among seemingly unrelated link failures. Finally, methods are presented for calculating the exact end-to-end availability of a connection.
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Communication network design serves as an important issue for service providers among the rapidly changing and emerging technologies. It is particularly critical when an all-optical backbone is in place due to its high data rate along each fiber and transparency in the data plane. The transparency – lack of O/E/O conversion at the intermediate nodes – enables very high data rates exceeding 10 or even 40 Gbps. There has been an increasing interest in providing high data-rate services such as video-conferencing or multimedia internet access recently. The rapidly increasing thirst for bandwidth and the spread of multicast technology provide new challenges for engineers. The persistent change of the underlying technology (e.g. Wavelength Division Multiplexing (WDM) networks, wavelength conversion capability, dynamically switched multi-layer networks) always requires new design goals and methodologies. However, the main design goals and Quality of Service (QoS) requirements of the network are permanent: low capital expenditure (CAPEX) and operational expenditure (OPEX), throughput efficiency, and survivability. Survivability – the capability of a network to recover ongoing connections disrupted by a failure of a network component (Mouftah and Ho 2003) – has emerged to be the most important aspect in designing the control and management planes for next-generation networks. The techniques proposed for survivability in optical network infrastructures can be classified into two general categories: pre-designed protection and dynamic restoration.
In circuit switched and virtual circuit switched mesh networks, like the extensively deployed wavelength-division multiplexing networks, one of the key quantifiable properties of survivability is the end-to-end availability provided by the network to the connection during its lifetime. Thus, we give a special attention to this metric in this chapter. In WDM networks each optical fiber carries a large number of wavelength channels modulated at even 40 Gbps, thus a short transport level interruption may lead to an enormous loss of application data. Availability refers to the probability of a reparable system to be found in the operational state at some time t in the future. End-to-end connection availability refers to the case when the source and destination nodes are connected by at least one path of operating edges and nodes, given that the connection was established at time t=0 (Sterbenz et al. 2010). Faults possibly cause the disruption of a connection if the users’ data is carried only along one path in the network, which might not be sufficient to fulfill the required connection availability defined in the Service Level Agreement (SLA) contracted between the service provider and customers. Providing optical backbone network services high connection availability is essential for service providers, as they gain more profit from higher rates on reliable transfer. In the SLA, the operator declares the minimal service conditions able to carry the customer’s data in the network for a given charge. The customer states his/her required bandwidth – if it is known – and chooses one of the connection availability classes offered by the provider.
The chapter gives insight into various pre-designed protection techniques, as one possible way of designing a survivable network. First, we discuss the pre-designed single failure resilient protection methods proposed for survivable mesh networks. In order to handle the protection methods independently from the underlying technology, the edges used for carrying users’ data in the absence of failures are referred to as working (or active) path. To deal with unexpected interruptions caused by accidental events (such as rodent bite on communication fibers), pre-planning protection (or backup) resources during connection setup with sufficient bandwidth for each working path has been widely accepted as the most effective solution. The strategy is also known as survivable routing in the path selection stage. For each type of protection, we briefly explain its distinctive characteristics, such as cost versus efficiency tradeoffs, resource utilization (fibers, wavelength, switches, etc.), speed of recovery, and other features that set it apart from other types.