Mesh algorithms enable the free-space laser revolution

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Conquering weather obstacles to improve uptime makes free-space laser solutions more viable for metropolitan areas.

DR. PIERRE HUMBLET AND WHITNEY WELLER, Astral Point Communications

For some time now, free-space lasers have had the potential to revolutionize the face of metropolitan networks. These powerful point-to-point beams can slip the bounds of physical fiber to send traffic without licensing fees or right-of-way requirements, with complete privacy, and with equal or better capacity. The biggest obstacle to full acceptance, however, has been their inherent 2% to 4% downtime due mostly to weather effects.1 Now, with the emergence of technologies using mesh algorithms to guarantee uptime, free-space lasers can jump the final hurdle to viability as a solution for delivering broadband services to metro areas.

In today's metro areas, only 5% of all buildings currently have actual physical fiber connections.1 Although the remaining 95% will eventually have either fiber or multimegabit radio-data services, the installation of these media will be a long, slow process.

Until recently, service providers have had three alternatives. The first was to wait for the physical connections, and in the interim, lose service opportunities. The second was to deployed licensed wireless services, and the third was to implement products employing free-space lasers to provide "virtual fiber" for delivering broadband voice and data services, risking the potential downtime. This alternative still allowed service providers to keep delivering services while awaiting fiber installation.

Downtime notwithstanding, some service providers have accepted the risks of the second alternative because of the many advantages free-space lasers provide. For example, free-space laser equipment takes up very little space and can even be placed on top of buildings and structures or inside the buildings themselves, with the laser beams aimed through the windows (see Figure 1). The beams are completely private, can be directed point-to-point, and offer more bandwidth [equal to multiple OC-3s (155 Mbits/sec), OC-12s (622 Mbits/sec), or OC-48s (2.5 Gbits/ sec)] than physical connections. In this way, free-space laser networks facilitate the provisioning of broadband services by providing bandwidth orders of magnitude greater than radio-frequency solutions such as microwaves.Th 0012lwfea01f1

Figure 1. Point-to-point broadband optical link supports WDM in free space. Mission-critical data can be protected using an optical service node mesh design.

In addition, the necessary equipment can be installed in hours instead of months, enabling high-bandwidth service for temporary or remote services. For instance, if there was an unexpected celebrity visit (i.e., by a presidential candidate) to a particular city, a service provider might have an immediate need for thousands of broadband connections for press, law enforcement, and secret-service people. By implementing free-space laser equipment, the service provider could have the necessary point-to-point connectivity, at speeds from 2.4 to 10 Gbits/sec, in just hours.

Free-space lasers can also bypass geographical obstacles such as rivers, highways, and places requiring environmental considerations that can cause problems for physical fiber installations. Free-space lasers use broadband spectrum-laser technology that meets all optical safety requirements and also ensures continuity of the beam in the presence of any obstacles (i.e., aircraft, birds, kites) that pass within one meter of the laser.

The technology is especially advantageous for competitive local-exchange carriers (CLECs), because it eliminates many constraints that have kept these carriers from realizing service opportunities. Free-space lasers operate completely independent of the physical connections owned by incumbent providers. This capability allows CLECs to bypass leased fiber and frees them from collocation, rights-of-way requirements, and radio-frequency-spectrum licensing fees. In this way, free-space laser solutions solve speed-of-deployment issues for CLECs and provide an economical alternative to more expensive fiber installations. The CLECs, in turn, can then pass along the savings to customers.

Despite all these advantages, free-space lasers have a serious drawback-the effects of weather. Although similar to radio signals in this respect, they do not have the capability of radio signals to switch to another frequency when the weather affects them. Weather sensitivity also limits the distances over which free-space lasers can work. Although free-space lasers will work over distances of about five miles, they are more apt to carry traffic more reliably over shorter distances.

These shortcomings can cause downtime and prevent service providers from taking full advantage of the many benefits free-space lasers can offer. A hybrid fiber and free-space solution allows service providers to control investment by allowing incremental network growth without sacrificing quality.Th 0012lwfea01f2

Figure 2. Spatial separation and diverse routing of point-to-point optics provides flexibility as well as an ability to provide asymmetric protection using the capability of the optical service node to take advantage of both land-based and free-space channels.

Fortunately, the current situation is changing, thanks to the availability of new multiservice transport terminals that include support for traditional ring architectures. In addition, these terminals support self-healing mesh algorithms that can provide free-space lasers with the capability to reroute traffic in real time. That ensures customer traffic is not interrupted even in the case of a link failure.

These algorithms are now available in equipment that enables service providers to build a logical mesh of optical-service nodes (see Figure 2). This mesh can provide a backup for each transport path and protection in the event of a laser failure or, in this case, a weather event. Using a mesh system, service providers can mix and match land-based fiber with free-space lasers to form a "self-healing" mesh with 99.999% availability for high quality-of-service (QOS) applications (see Figure 3). Th 0012lwfea01f3

Figure 3. Mesh and ring topologies are fully supported with seamless transition between them.

By interconnecting free-space lasers with the newly available mesh-enabled equipment, service providers can automatically reroute traffic to other alternate paths (either fiber or free-space lasers) in the event of a failure. Multiservice equipment allows service providers to draw on the best protection solution, either ring or mesh, as appropriate, to meet their service-level agreements (SLAs). An added protection benefit of this network is that it enables service providers full use of available bandwidth, since none of the capacity must be reserved for protection. That provides a preemptive model in which failures will not affect high QoS traffic. The mesh model gives service providers the ability to manage and scale their free-space laser networks as needed.Th 0012lwfea01f4

Figure 4. The mesh protection algorithm meets or exceeds current SONET recovery times.

Mesh algorithms provide laser networks with the same sub-50-msec protection and restoration levels as physical fiber (see Figure 4). This capability will enable service providers to write SLAs that enable them to increase profits by providing uninterrupted service options to customers. If SLAs can be written to provide protected services for specified portions of available bandwidth, then service providers can protect OC-48 point-to-point interconnects using free-space lasers by leasing lower-speed protection paths. By aggregating all traffic onto optical-service nodes connected to free-space lasers, service providers can purchase DS-3s (44.736 Mbits/sec) for protection, while getting OC-48 capacity and above.

Although these developments are very new, they are just the latest chapter in the very long history of free-space lasers. Despite its futuristic applications, this technology goes back to the birth of telecommunications. The technology was first patented in 1880 by Alexander Graham Bell, who demonstrated the ability to transmit articulate speech over a beam of light (Patent No. 235199). The technology then languished until 1961, when NASA began research on free-space optical modulation aimed at connecting satellites to one another.

Here on earth, free-space lasers take the form of bidirectional telescopes and transmitters/receivers that work at four times OC-48 rates point-to-point and take advantage of WDM. To implement free-space lasers, service providers simply place a telescope/transmitter/receiver at one interior or exterior location and another telescope/transmitter/receiver at another interior or exterior location and turn the systems on. The systems have the built-in capability to perform self-monitoring for quality and performance and to automatically align themselves.

Once in place, free-space laser networks will support secure, point-to-point multichannel (up to 2.5-Gbit/sec) transmissions through the air. These transmissions typically range over distances of one to five miles, depending on the weather statistics for the area. Because they require only minimal equipment, free-space laser networks are highly portable and can be easily moved to meet changing needs.

Free-space laser networks must be deployed with multifunction optical-service transport equipment that gives service providers the option of combining any mix of 1,330-nm and 1,550-nm traffic and provides industry-standard interfaces. The multifunctional capability will enable service providers to combine traffic at collocation sites and transport it optically using either landlines or free-space lasers, depending on which is available at the time. The standard interfaces will enable service providers to choose the service format (i.e. DS-1, DS-3, OC-3, OC-12, OC-48) that will best meet customer requirements.

Dr. Pierre Humblet is chief technology officer and Whitney Weller is director of business development at Astral Point Communications (Chelmsford, MA).


  1. Liu, E., "Extending the Metro Through the Air," NFOEC 2000.
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