A Fault-Tolerant Optical Network
So-called backbone networks, such as Metropolitan Area Networks, or MANs, and wide Area Networks, or WANs, which span relatively large geographical areas, are often implemented using high-speed optical fiber cables. Backbone networks carry large amounts of network traffic, so they must be reliable and capable of recovering quickly in the event of a hardware failure.
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Optical Versus Electronic Networks
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In an optical fiber network, as in an electronic network, failure can occur if a communications link is disconnected or if a device connected to the network becomes inoperable. However, the data transfer speed, or bandwidth, of an optical fiber network is many times higher than the bandwidth of an electronic network, which makes physically monitoring the network difficult. Monitoring methods typically rely on the presence of light in one signal and the absence of light in another, otherwise known as “on-off keying.”
Redundancy
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Schemes for protecting an optical fiber network from hardware faults, or restoring the network in the event of a fault, rely on redundant physical links in the network. During normal operation, light travels along just one path from point to point on the network, but enough network capacity is reserved to redirect it along another path if one of the original physical links fails.
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Strategies
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The two main strategies for fault-tolerant optical networks involve techniques known as link restoration and path restoration. Link restoration redirects network traffic between the devices at either end of a failed optical link by any means possible, whereas path restoration redirects network traffic along one or more specific paths across the network.
Pros and Cons
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Physical protection and restoration schemes don't require complicated management procedures, so they guarantee that normal operation of an optical fiber network can be restored quickly. Even a short disruption to a MAN or WAN can result in a substantial loss of data. Physical protection schemes, in which redundant links, or routes, across the network are predetermined, typically allow faster recovery than restoration schemes that search dynamically for redundant routes when a failure occurs. The Synchronous Optical Network specification defines a recovery time of less than 60 milliseconds for protection schemes, whereas restoration schemes may take several seconds or even several minutes to recover from network failure. The main disadvantage of both types of scheme, however, is that they require a significant proportion of network resources to be set aside for recovery, and are consequently expensive to implement.
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References
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