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%.

The below is a representative chart of MRV FSO link models at representative distances for various weather conditions (light, moderate, heavy, and very heavy) rain, snow and fog...

rain snow fog

A multi-beam system (TS5000) can reach to over 5000 meters in clear air. A smaller single beam system (TS700) could have outage at as short as 150 meters in a "London fog".

But, in real world deployments (we have done over 400) we have observed that with correct model selection (one that is appropriate for the link distance it is installed at) FSO links do provide average availabilities that meet or exceed expectations.

In fact, over the nearly ten years we have been deploying Free-Space Optics links, we can count on < five fingers the number of times we've had an availability complaint.

Conversely, there aren't enough fingers in a crowd to match the number of times the first words on the phone have been,

"Our 802.11 RF link was cheap but never had adequate throughput, and it just sort of quit working, any idea what could be wrong?"

 

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 most 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.  more >>

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