Ong Beng Hui wrote:

The problem of been LoS is a big problem in metro as far as I know. You can't just put a pair of FSO gear without going to the building owner to talk about rights and cost. Not forgetting lighting protection and other stuff.

Murphy, Brian S CTR USAF ACC 83 NOS/Det 4 wrote:

...

I haven't seen any mention of the possible use of FSO (Free Space Optics) by the provider to restore some reasonable amount of connectivity during an outage due to a fiber cut. I would expect that having 2 or 3 pairs of FSO boxes to provide a "reduced failover capacity" in metro areas would be a reasonable measure to ensure service for extended physical (fiber break, cut, backhoe) outages - although not necessarily for power. Yes, it would take some time to roll them out and set them up, but less time than the crew working the splices, and the folks handling the FSO boxes should be different from the fiber splice truck roll crew.

Note that a power outage would not allow microwave to be an effective remediation method either.

Plus, FSO's use of lasers (vice microwaves) means no issues with spectrum (AFAIK). Granted, they have limited distance and require LoS, but using two or more pairs can probably handle the 80% situation in the metro (unless there is data to indicate otherwise).

murph

Based on my experience with operating FSOs as infrastructure some years ago, the major limiting factor for FSOs is weather. In good weather, they should work just fine even at quite long ranges, providing that there is no obstruction or source of heat shimmer in the path, and you have carefully aimed your link to avoid sun outages. Bad weather (rain, snow, sandstorms, fog) causes very high levels of attenuation, with particularly bad weather reducing effective range to a few hundred meters at most. When this happens, the effect is area-wide, with a typical rain cell being a few km in size, so adding extra FSO links for redundancy is useless. If you've got a local airport nearby, you should be able to get good historical data for the frequency and duration of such weather conditions from METAR visibility data. For long-term standby installations, you've got to watch out for building work and cranes, which can pop up unexpectedly. However, if the link is being used solely as a protection path for rare failures in otherwise reliable fiber, and the alternative is either no protection path or a prohibitively expensive protection path, this may be perfectly acceptable: quite long ranges can be achieved with around 95-99% availability in typical European climates. You should expect installing and aiming a couple of FSO links at one another to take about a day in practice, unless you have a crack team of mobile laser ninjas trained and in readiness at all times (although the USAF may have greater access to ninjas, compared to to the rest of us). There is still the matter of getting permission for physical access, safety approval, access to power and network connectivity to the vantage points you will need to install the FSOs on, which can take much longer unless you already have it pre-planned. For truly rapid temporary links, I've seen one major UK operator actually just manually grout fiber in place along a kerbside to cover a few hundred meters of (presumably) temporary fiber run. This is probably faster to install than FSOs, even if the lifespan of such a link might be measured in days before someone crunches the fiber. -- Neil