<title>Wireless optics protection of fiber via SONET ring closure</title>

12 A free-space laser link closes an otherwise all-fiber SONET ring, demonstrating for the first time the feasibility of using wireless optics as a back-up to fiber in an application demanding the highest levels of statistical availability and sub-50-ms protection-restoral times. This experiment demonstrates that protocol-transparent wireless optical links can be readily internetworked with industry- standard fiber-based protection protocols to achieve SONET restoral times in the event of a fiber cut. By using the wireless optics as a back-up to fiber rather than as the primary link, end-users are normally protected from the unavoidable burst errors and outages that can arise on a wireless optical link in the event of anomalously poor atmospheric visibility or unanticipated line-of-sight obstructions. While an all-fiber SONET ring operating over physically diverse paths is generally preferred, hybrid fiber/air rings operating over physically-diverse paths (fiber as one path and air as the other) will easily meet or exceed existing Bellcore availability standards for SONET rings. The hybrid part-fiber, part-air ring advantageously protects customers from fiber cuts (a.k.a. `backhoe fade') and may be preferable to over service via either an unprotected fiber spur or over a `collapsed' fiber ring made up of fiber segments sharing a common conduit. The experiment is performed at an OC-12 (622 Mbps) data rate in a point-to-consecutive point configuration which demonstrates the use of a relay site to work-around a line- of-sight obstruction.