Redundant fibers offer more protection against outages.
High-bandwidth networks use fiber links to carry large volumes of mission-critical traffic. Outages often result in large financial losses, so carriers and enterprises have used a variety of mechanisms, such as SONET rings in the core and MPLS in the access network, to boost availability. A new protection mechanism based on all-optical switching offers a more cost-effective solution for high-speed links, however.
Protection from fiber failures is achieved through the use of redundant fibers. When the primary link fails, traffic is rerouted to an alternate link. To use the redundant fibers, operators have two options: use two sets of equipment at each end (for example, two router line cards at each end of the link), or use a single set of equipment and route traffic to the appropriate link through the use of a protection switch. High data-rate line cards are fairly expensive, costing tens and even hundreds of thousands of dollars. Protection systems typically cost less than $10,000, making their use cost-effective.
The new protection technology is based on optical switching modules. Optical signals are routed to one of the fiber links without undergoing any electro-optical conversion. Unlike other technologies, optical switches don’t use expensive signal conversion circuitry, greatly reducing the cost, space and power consumption of this solution.
Optical protection systems may be deployed in several ways. In a single-ended 1+1 protection scheme, the transmitted signal is split at the bridge and routed in parallel over two fibers (working and protection) to the receiving end. An optical switch at the receiving end is responsible for routing one of the streams to the equipment side (main port), based on signal quality.
Under normal circumstances, when the working fiber is operational, traffic arriving over this fiber is selected. During failure of the working fiber, traffic from the protection fiber is selected. Because failure detection, isolation and protection are all handled on the receiving end of the link, switchover is simple and therefore fast, offering almost instant recovery.
Today’s optical switches have a loss of less than 1 decibel (not including connectors, taps and losses related to fiber routing). Optical splitters, such as those used in the bridge, have a nominal loss of 3 decibels; in practice this could be even higher.
The end-to-end loss associated with a 1+1 protection system could exceed 5 decibels, with the largest loss component being the bridge. A 1:1 protection scheme can reduce the end-to-end loss by more than 2 decibels, replacing the splitter at the bridge with an optical switch.
Unlike the 1+1 protection scheme, in which traffic is carried in parallel over the two fibers, in the 1:1 protection scheme the traffic – consisting of most of the optical power – is carried over a single (active) path. Under normal conditions the working fiber carries the traffic. Upon failure of the working fiber, traffic is switched to the protection fiber if it is operational.
1:1 protection systems require synchronization of both ends of the link, as switchover takes place on both ends. This scheme is therefore termed dual-ended protection.
A new breed of all-optical protection systems has a place in high-availability fiber networks.
Queller is vice president of customer applications and support for Lynx Photonic Networks. He can be reached at aqueller@lynxpn.com.
