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I Want to Believe - Alien Wavelengths

by Juniper Employee ‎02-29-2016 04:10 PM - edited ‎02-29-2016 04:31 PM

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.

Comments
by Juniper Employee
on ‎03-01-2016 11:34 AM

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

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  • I started work at a router company, moved to an optical company adding MPLS, moved to mobility company, moved to a packet optical company who got bought by a router company. Full Circle!!
  • David Song is a Sr Staff Engineer within Juniper's Optical Engineering team where he is responsible for the design of packet-optical solutions for routing and switching platforms. He joined Juniper in 2004 and has been designing networking software in control plane and data plane on various platforms. Prior to Juniper, he held various software development positions at Ciena and Nortel Networks.
  • David is a Distinguished Engineer in Juniper's Optical Engineering team, having joined Juniper as part of the acquisition of WANDL Inc in January 2014. David is working on routing and optimisation software for multilayer networks to help plan and design networks using the new generation of packet-optical technology from Juniper. In "previous lives", David worked on soliton propagation; diffractive optic device design and network design software and algorithms in the Optics Research Group in BT's Adastral Park Laboratories in Ipswich. He holds a BA and PhD in Mathematics from the University of Cambridge.
  • Steven Keck is a Distinguished Engineer within Juniper's Optical Engineering team where he is responsible for architecture and implementation of packet-optical solutions for routing and switching platforms. Steve has been designing telecom hardware and optical systems for nearly 20 years. Prior to Juniper, he has held positions at Stanford Telecom, StrataLight Communications and Cisco Systems. He holds a B.S. in Computer Engineering from University of the Pacific.