1 / 26

Predicting FSO Link Availability: Key Factors and Techniques

Learn about FSO link availability, factors affecting it, and how to predict it accurately with clear air link margins, weather attenuation, and atmospheric models.

margarital
Download Presentation

Predicting FSO Link Availability: Key Factors and Techniques

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Predicting AvailabilityWhat is Link Availability? Link availability is the percentage of time over a year that the an FSO link will be operational. • 5 Nines = 99.999% = Down 5 min / year • 4 Nines = 99.99% = Down 53 min / year • 3 Nines = 99.9% = Down 8.75 hrs / year • 99.8% = Down 17.5 hours per year The Primary Driver of FSO Availability is Weather

  2. Predicting AvailabilityPart 1: Clear Air Link Margin • Difference between the optical power received in clear air versus the minimum power level required for a given error rate. • Expressed in terms of “dB” • 30dB of margin = 1 part in 1,000 required • 50dB of margin = 1 part in 100,000 required • Typical error rate 1e-9 (1 in a billion) • 1e-12 reduces margin by approximately 1 dB • 1e-6 increases margin by approximately 1 dB

  3. Predicting AvailabilityPart 2: Weather Attenuation • Curves based upon surface visibility • Seattle 99.99 attenuation = >212 dB/km • Seattle 99.9 attenuation = 98 dB/km • Denver 99.9 attenuation = 40 dB/km • Phoenix 99.9 attenuation = <5 dB/km

  4. Predicting AvailabilityThe Hard Part – Atmospheric Attenuation Curve The Inputs • Visibility information from nearby airport weather stations • 100m visibility corresponds to 128 dB/km at 1550nm • 400m visibility corresponds to 27 dB/km at 1550nm • Cloud ceiling information • Geographic location of airport vs. installation location • Urban heat island effects The Output • A four dimensional atmospheric model • Latitude • Longitude • Altitude • Time Visibility Sensor

  5. Predicting AvailabilityExamples of Visibility Visibility of 1 football field ~ 130 dB/km

  6. Predicting AvailabilityAirport Surface Visibility Data • September • Best – 3600m • Average – 400m • Worst – 200m • Visibility measured at 550nm

  7. Predicting AvailabilityCloud Ceiling Impact Stockholm, Sweden, 1982-97

  8. Predicting AvailabilityAttenuation Vs. Percentage Time Occurrence Three Altitudes in Seattle • At 99.8 Availability • Surface = 60 dB/km • 30th fl = 85 dB/km • 52nd fl = 105 dB/km • A link capable of operating at 50 dB/km • Surface = 99.78% • 30th fl = 99.72% • 52nd fl = 99.58%

  9. Predicting AvailabilityPutting It All Together – A LinkBudget • Installation Characteristics: • 1550nm Gigabit Ethernet link (1.25 Gbps) • Automatic pointing and tracking • 5 dB windows at each end • 400m range • 10m Altitude • Budget: • Transmit Power 1000 mW 30 dB • Receive Sensitivity (-36 dBm) 36 dB • Geometric Loss w/o fog at 0.4 km -2.0 dB • Window attenuation (2 x 5.0 dB) -10.0 dB • Mispoint Loss -1.0 dB • Total Optical System Losses -9.0 dB • Total Remainder for Weather 44.0 dB • Therefore, the maximum allowed weather attenuation is: • 44 dB/ 0.40 km = 110 dB/km

  10. Predicting AvailabilityFinal Result: Link Weather Availability • Best Year • = 99.995 • Average Year • = 99.94 • Worst Year • = 99.66

  11. Predicting Availability99.9% Attenuation Margins Attenuation margin (dB/km) required for 99.9% availability - City by City Notes: Data derived from surface visibility data (altitude effects not included) and Kruse formula Source: Terabeam Weather Group, Jan 2001

  12. 99.9 99.9 99.8 99.7 99.8 99.7 Predicting AvailabilityVariability w/ Location: Washington, DC Washington National Relative link footprints Dulles

  13. 6000m 4000m 2000m 0m New York Dallas Miami Seattle Denver Phoenix Predicting AvailabilityReal World 99.9% FSO Ranges

  14. Cornea Retina Eye SafetyThe Eye’s Response to Laser Light • Cornea • Wavelengths above 1400 nm almost completely absorbed by the cornea and not transmitted to the retina • Retina • Wavelengths below 1400 nm (close to visible light) focused onto the retina, so power levels must be lower to ensure safety • Maximum Permissible Exposure (MPE) limits established by ANSI

  15. Eye SafetyLaser Standards Organizations

  16. Viewing Condition Class One Eye-Safe (all conditions) aided I 1 1 Eye-Safe w/o Optical Aids unaided -- 1 1 M Class Two (Visible only 400 to 700 nm) < 0.25 sec (eye aversion) aided II 2 2 < 0.25 sec unaided -- 2 2 M Class Three “a” or “R” Minor Hazard aided IIIa (visible only) 3a 3 R (5 times Class 1) unaided -- 3a -- any IIIb 3 b 3 B Class Three “b” - Eye Hazard Class Four Eye Hazard any IV 4 4 Eye SafetyLaser Safety Standards • Most FSO systems are Class 1 or 1M

  17. Eye SafetyStandards and FSO Use • Class 1 systems can be installed in Unrestricted locations • Class 1M systems can be installed in Restricted areas • Class 3B and above only in Controlled locations

  18. Applications & Network Integration Emerging FSO Uses • “Sprinkler head” extensions of fiber • Closure of SONET/SDH metro rings • Spatial diversity • Emergency communications • Data outsourcing/mirroring/SANS • Wireless backhaul

  19. 2 3 1 5 6 4 5 Applications & Network IntegrationDeployments Single customer access 1 Point-to-point connection 2 Campus connection 3 4 Diverse lateral 5 Backhaul or ring closure 6 Multi-tenant building • and more… • Mobile network extensions • Mobile network backhaul • Spatial diversity

  20. Hudson River Jersey City Manhattan Merrill Lynch Office 2.6 km 1.8 km Merrill Lynch Office Merrill Lynch Office 1.6 km Applications & Network IntegrationDisaster Recovery – After 9/11 • Merrill Lynch urgently needed additional connectivity to three locations. • Terabeam installed Gig-E FSO units to build a ring topology to back up the SONET network • Installation was completed within 7 days of first call

  21. Applications & Network IntegrationHybrid Solutions RF FSO Copper or Fiber Automatic protection switching between FSO, Millimeter Wave, and/or terrestrial fiber can approach 99.999% availability

  22. FSO (30-400 THz) Applications & Network IntegrationHybrid Solutions: Non-Correlated Failures MMW (60 GHz)

  23. Applications & Network IntegrationFree-Space & 60 GHz MMW Hybrids

  24. The Future of FSOFSO Today • The high bandwidth of fiber with the speed and ease of installation of wireless • Unlicensed all over the world due to its inherent resistance to interference • Highly secure and safe • Allows through the window connectivity and single customer service • Fundamentally compliments fiber by accelerating the first and last mile

  25. The Future of FSOWhat’s on the Horizon? • All optical – “Fiber through the air” • Lighter, smaller units • “Innocuous” form factors (e.g., a pane of glass acting as a receive element) • Fundamentally the cheapest way to cross the street at 100 Mbps • Can be used to provision big bandwidth almost everywhere • Where fiber can’t offer an adequate ROI • Where 99.9 is acceptable • To the home?

  26. Thank You

More Related