Packet-Optical Technologies
Packet-Optical Technologies
I Want to Believe - Alien Wavelengths

Do aliens really exist? I'm not sure if Mulder & Scully definitively answered this one, but in the world of packet-optical transport the answer is decidedly 'yes'. Alien wavelengths, that is.
I want to believe.jpg
Confirmed Sightings
In the optical transport world, the term 'alien wavelength' is used to describe wavelengths in a DWDM line system that traverse the network but aren't sourced/terminated by the line system vendor's equipment. A quick refresher on key parts of a line system:
Alien Wavelength.png
Source: ADVA
  • Optical Shelf - A collection of all-optical gear that typically includes mux/demuxes, amplifiers, optical channel monitors (OCM) and/or reconfigurable optical add-drop muliplexers (ROADMs). These pieces are often integrated into a custom, dedicated chassis.
  • Line System - A combination of the Optical Shelf components plus the optical fiber and amplifiers that make up the transmission infrastructure.
  • Transponder - A device that translates a 'grey' client interface signal (e.g. 100GBASE-LR4) to a colored DWDM line interface signal (e.g. coherent DP-QPSK). Such a device is typically a line card that plugs into an optical shelf.
Every DWDM network operating over ~80km is going to require many of the line system components described above. Optical multiplexing, amplification and sometimes switching are fundamental building blocks of transport infrastructure. Of course, there are numerous optical-centric companies that would love to sell you all this gear - especially the transponders.
Transponders are important - but they're also the most complicated, expensive and profitable piece in an optical vendor's portfolio. When implementing a packet-optical based network you can eliminate transponders. In doing so you're maximizing the primary value of router-integrated DWDM interfaces. Nonetheless, a packet-optical interface alone is not magic - it still needs to traverse a line system.
Legal Aliens
When a colored router interface is connected to a line system, it is considered an alien wavelength. The term 'alien' is used since the wavelength is not sourced from the transponders within the optical shelf. Now you might ask yourself "will alien wavelengths blow up my line system like those pesky guys that blew up stuff in Independence Day?"
The answer is no - but you do need to do your homework:
  1. Channel Plans - The line system and router interfaces need to have compatible min/max tuning ranges and ITU grid spacing. Provision both sets of gear accordingly.
  2. Power Levels - Compare the power levels (in dBm) specs for the optical shelf ADD/DROP ports and compare them to the input/output power level specs on your router interface. Ensuring that power levels are compatible is important to maintain full performance. It may be necessary to adjust the equipment output power levels and/or add fixed attenuation on each port to get everything dialed in.
  3. Friendly Neighbors - Coherent interfaces (e.g. 100G DP-QPSK) generally play nicely when packed in next to other coherent channels, but be careful when mixing with older 10G wavelengths using on-off keying (OOK). If you have existing legacy 10G channels present then extra care is needed in channel usage to prevent performance degradation.
Don't Believe the FUD
When it comes to upgrading your DWDM infrastructure, the alien wavelength concept is a powerful one. Leverage the inherit capex and opex advantages of packet-optical solutions without ripping out existing DWDM infrastructure. A transponder vendor's loss is your gain. You may hear plenty of FUD, but remember: The Truth is Out There.

Juniper Employee

Can you point me to further basic studies on Gray vs Colored signals?

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