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OTDR Dynamic Ranges


Introduction

The OTDR is one of the most powerful tools you will purchase to support your fiber network. This article discusses one key but sometimes confusing OTDR specification, Dynamic Range.

Definitions

Dynamic Range

The informal definition of Dynamic Range is “how far can my OTDR see?” There are at least two formal definitions of Dynamic Range, the Telcordia definition (the most widely accepted and specified), and IEC definition. Each of these specify the amount of (one way) fiber loss from the initial backscatter (fiber) signature to a given Signal to Noise Ratio (SNR) level on the OTDR trace.

OTDR Dynamic Range Specification (SNR Limit)

  • IEC              98%
  • Telcordia     50%
The IEC range will usually be about 6.6 dB LESS than the Telcordia figure, and it’s also a good estimate of the OTDR’s “useful” range. The term “useful” is used somewhat subjectively here but means exactly what it sounds like. You won’t be able to “see” or measure a 0.02 dB splice at the IEC or useful range, but if there is a high loss splice or break at that range, you should know it.
 
For most modern OTDRs, if you subtract ~6 dB from the Telcordia specification for Dynamic Range, you will have a good estimate of its “useful” Dynamic Range.  [Warning: on some older OTDRs and perhaps some from non-mainstream manufacturers, there is so much filtering loaded into the electronics that the “useful” range (where you can see events) can be as much as 14 dB below the Telcordia range, i.e.  these filter out noise to report good specs, but they can filter out “events” of interest.]

“Gaming” the Dynamic Range Specification

Note that the Telcordia specification is based on an SNR of 1 using a 3 minute average, a 1km (10µs) pulse and G.652 fiber.  Sometimes results reported on brochures and spec sheets are not “apples to apples”. Look for a note at the bottom of the page that may say, “Using 10 min average and 2km pulse on Dispersion Shifted fiber.” This means the manufacturer is giving the best case specs and not complying with the Telcordia test conditions.  This might increase their reported spec by 1 to 3 dB.

OTDR Application Examples

To offer the best cost/performance to the user, manufacturers may optimize an OTDR for a particular task. This is to ensure satisfactory performance for it’s intended purpose while omitting features that are likely unnecessary for that task.

Fiber to the Antenna

These will have the lowest Dynamic ranges as they are intended for testing only a few kilometers or a few hundred meters. They may also have some software features designed for easy interpretation of “up antenna/down antenna” loopback testing.

Fiber to the Home

These will have significant dynamic ranges usually in the mid 30 dB range and higher. This is to allow them to test through splitters (15-18dB loss) when required. They are optimized to have short Event and Attenuation Dead zones through splitters.  A “PON” OTDR or “FTTx” OTDR may have much better performance through splitters than an IOF or Long Haul OTDR of similar Dynamic Range.

IOF (Interoffice) Medium or Long Haul 

These are optimized for testing moderate to long spans and may have Dynamic Ranges of 33 to 40 dB for Medium haul and up to 45 dB or more for long haul. Very long haul OTDR’s are optimized for both Dynamic Range and Linearity to ensure accurate readings at long distances.

The Dead Zone Paradox

Be careful worrying about just one specification like Dead Zone (DZ). A Fiber to the Antenna OTDR may spec an Event Dead Zone of 0.5 meter, while another OTDR might spec a 1meter DZ. But remember that both Dead Zone and Dynamic Range (DR) are dependent on Pulse Width. So, while a low DR OTDR may have a pulse width of 0.3m and a DZ of 0.5m, this may only be useful for 1 or 2 km.  At 10 km, this OTDR might require a very large pulse which creates a dead zone of 5 or 10 meters. A high Dynamic Range OTDR with a minimum Dead Zone of 1 meter, may be able to “see” 10 or 20 km with its lowest pulse width and dead zone. So, from 2 to 10km the OTDR with the longer DZ spec has a shorter DZ than an OTDR with a shorter DZ spec. 

Summary

Before selecting an OTDR, define the task or tasks you need it to perform. Examine specification sheets side by side to determine its suitability to each task. If possible, “sample” or rent a unit to use it on your network. Consider ease of use, support, and price. Be sure you understand which software features are included or optional at additional cost. Depend on trusted sources, possibly distributors which carry several brands and your own expertise. If you can, take an advanced OTDR training course to understand how powerful and versatile these instruments can be.