Free Space Optics (FSO) : Optical Wireless Link Availability

Free space optics wireless link availability can be a confusing topic. In actuality, it can be approached very objectively and factually.

First, one must accept that no "A to B data path" is likely to be available 100% of the time unless it is two fiber runs, each in a separate trench.

Then the question really becomes, "How much less than 100% am I willing to plan for?" Which interpreted means, "How much am I willing to spend to acquire the availability that I need?"

To "objectify" the question, let's take a look at what 3-9's, 4-9's, or 5-9's availability really means…

Many FSO links provide suitable availability standalone. The following displays the relationship between three percentage availability levels and the minutes down per month for various applicable hours of operation.

 

Link Unavailable per Month @ Availability %

 

Hrs. Open:

M-F 9-5

M-F 6-6

7 Days 6-6

24 x 7

Hrs/Month:

174

261

365

730

 

 

Outage/Mo:

Outage/Mo:

Outage/Mo:

Outage/Mo:

99.999%

 

6 sec

9 sec

13 sec

26 sec

99.990%

 

1 min

2 min

 2 min

4 min

99.900%

 

10 min

16 min

22 min

44 min

The decision to invest $X.XX to move from 99.99% to 99.999% for a 9-5 operation is a question of the value of the elimination of 1 minute of down time per month (see chart). 

For FSO links in excess of approximately 200-300 meters, adverse atmospheric events, fog in particular, can compromise link availability. SSS can provide  an approximate availability estimate for deployment of specific link equipment at a given location based on local weather records (50 year histories are available in some areas) and equipment ratings.  

Weather can interfere with the beam when the link is >200m. As with all connections (Telco included) availability is never 100%, but rather should be provided to match your requirements. Typical Telco lease line availability usually is somewhere between 99.9% and 99.995%.

Next...

The following chart is an excellent way to interpret various atmospheric events' impact on two representative FSO link models. From the data, it is evident that at distances of less than about 200-300 meters or so, even very extreme weather is not particularly significant. From about 400 meters and up, depending on your local climatic environment, weather, especially fog, increases in significance. 

 


*From "Availability of Free Space Optics (FSO) and Hybrid FSO/RF Systems", Isaac I. Kim and Eric Korevaar, Optical Access Incorporated , 10343 Roselle Street , San Diego, CA 92121. 

To comprehensively consider availability for a given FSO link model, in a specific 
local environment, you should consider the variables (your proposed link distance and your particular local weather) in an objective format:

Combine Historical Visibility Data with Link range vs. visibility curve to give Availability vs. Link Range curve.

And here are a few examples of availability calculations for a couple of shorter-distance links (400m TS1000 and 1000m TS1000X) for different parts of the globe...

 

 

 

 

      

 

For longer distance links, consider a low cost RF Fusion backup option

-TereScope Integrated "Fusion" Redundant Port                           

The new TereScope Fusion Redundant Port option, now available on all of the "Protocol Select" TereScope models, is truly a breakthrough from both a cost and simplicity perspective. The reason is that a backup is a really good idea on longer "mission critical" links (>300m or so) to accommodate fog, but in many cases, by the time a failover is added (switch or router based) the increased cost and complexity become excessive. With the new fully integrated, protocol transparent Fusion option, you just plug in ANY backup path (e.g. 802.11a RF), any protocol, and it's done. The link fails over when the monitored receive light intensity falls below threshold (50 mW), and returns when the incident is over (150 mW). 100% availability, yet simple, very fast, and very low cost ($600 USD/link).  more >>

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