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If you only knew the [optical] power of the Dark Side
Jul 26, 2015

Darth Vader may have been an optics engineer earlier in his career. His prowess with optical devices is well- documented. I'm pretty sure his light saber was rated at nearly +80dBm [most certainly not Class 1, as my colleague points out]. While I don't know the exact numbers, I'm sure he did. You should know your optical numbers as well when planning your next optical transport link. 

 

vader.jpeg 

Transport links can be divided into two basic categories: amplified and and unamplified spans. Vader (and his management) clearly focused on the latter category, so let's take a closer look at estimating optical reach for this application.

 

Come to the Dark Side

 

Unamplified spans are sometimes known as 'dark fiber'. Why? At a physical level this means that there are no inline optical amplifiers (e.g. EDFAs or Raman amps) to boost the signal from point A to point B. Amplified spans are a must for metro and long haul DWDM links and the amps are always active regardless of what's connected at the endpoints. Unamplified spans, by contrast, are 'dark' until you hook up a transmitter at one end. Such a link is not necessarily simple: it may include multiple spans from one or more 3rd-party carriers that lease the actual infrastructure. However simple or complex it may be, let’s look at the numbers.

 

Estimating reach - What you need to know

 

Now that we have an idea what dark fiber is, how do you estimate reach? Let's assume for a moment you’re using a robust 100G coherent interface like the new ZR CFP module for MX & PTX or the high-performance PTX DWDM PIC. These interfaces are purpose-built for long distance utilizing DSP-based chromatic dispersion (CD) compensation and forward error correction (FEC).

 

Having said all that, before we start running the numbers we need to know the following interface parameters:

  • Transmit Output Power - The optical power level from the router's TX port (dBm)
  • Receive Sensitivity - The minimum input optical power level that the router RX port can accept (dBm)
  • Loss Budget - This is simply the transmit output power minus the receive sensitivity (dB)

For Juniper 100G coherent interfaces, here are the relevant specs:

 

Model Number

TX Output Power

RX Sensitivity

Loss Budget

CFP-100GBASE-ZR

+5 dBm

-20 dBm

25 dB

P1-PTX-2-100G-WDM

-2 dBm

-28 dBm

25 dB

 

...and lastly, the link details: 

  • Fixed Loss - Attenuation (in dB) from passive components such as patch panels, splitters/couplers or AWG-based mux/demuxes
  • Path Loss Ratio - This is the optical attenuation of the fiber (in dB/km)

Run the Numbers

 

With the key specs in-hand, now we can finally calculate the reach and explore few real-world example:

 

Reach = (Loss Budget - Fixed Losses) / Path Loss Ratio

 

Example 1: ZR CFP over point-to-point link

  • Loss Budget: 25dB
  • Fixed Loss: 2x patch panels @ 0.5dB/ea = 1dB
  • Path Loss: Typical SMF-28 fiber, 0.25dB/km

Estimated Reach: (25dB - 1dB) / 0.25dB/km = 96km

  

Your Mileage May Vary

 

You can see now why simple industry labels such as '80km optics module' can be misleading. Also consider that while loss budgets may be similar between products, a full-featured PTX DWDM PIC is also designed to handle much greater reach amplified span DWDM applications that ZR CFP cannot.

 

Using the methods above should provide a reasonable estimation of your expected reach. To improve your estimate, an optical power meter is a valuable tool to measure actual TX output optical power levels and transmission line losses. Now you can run the numbers and actually calculate the power of the Dark Side. 

 

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