Reconfigurable Optical Add/Drop Multiplexer (ROADM)

An all-optical switching element for adding wavelengths to (and dropping them from) a DWDM stream.

The ROADM provides a way to do this in the optical domain, without having to terminate the traffic. More significantly, ROADMs gave carriers a way to nail up these wavelengths by only making changes at the endpoints. Previously, adding a wavelength meant manually configuring every node the wavelength was going to pass through.

ROADMs have room for improvement. For example, the configuration at those endpoints still has to be done manually. But they've become an integral part of the optical network.

ROADMs are deployed as modules -- cards slotted into optical transport equipment. They've become elemental to packet-optical transport systems (P-OTS). (See Optical's Packet Magic.) Sam Lisle of Fujitsu Network Communications Inc. explains as much in this 2008 video.

The heart of a ROADM is a Wavelength Selective Switch (WSS). WSSs are commonly available in sizes up to 1x9 -- capable of adding or dropping eight wavelengths. (The ninth is the "express lane" fiber that goes straight out of the ROADM, for traffic that doesn't need to be terminated at that node.)

The next large step could be a 1x23 WSS, which some companies began showing in 2010. (See ROADMs Get Sized Up and OFC/NFOEC: ROADM Roundup.)

This JDSU (Nasdaq: JDSU; Toronto: JDU) custom video from early 2010 notes the size trends in ROADMs -- and the possibility of going colorless and directionless, which we'll outline next.

Colorless, directionless, contentionless, gridless
Vendors would like ROADMs to be more automated. As Glenn Wellbrock of Verizon Communications Inc. (NYSE: VZ) put it in May 2010, an ideal feature would be a big "defrag" button that resets the network to its optimal state. (See Verizon: Give Us More Flexible ROADMs for 100G.)

Here's what they're asking for:

  • Colorless: Able to send any wavelength to any port, and to change these assignments automatically.

  • Directionless: Allowing output ports to send traffic in any direction.

  • Contentionless: Allowing one ROADM port to use a wavelength that got dropped by another port. This could be accomplished with an NxN switching element. Large all-optical switches, of sizes up to 768x768, could also play a role here. (See OFC/NFOEC 2009 Preview and Calient Gets Ambitious With Optical Switching.)

  • Gridless: Some carriers think they'll want ROADM to assign wavelengths off of the International Telecommunication Union (ITU) grid. When Terabit Ethernet gets invented, for example, it might be easiest to let it occupy two ITU grid spaces, or 1.5, or some other arbitrary proportion. (See OFC/NFOEC: For ROADMs, Less Is More.)

Here's a 2010 video where Wellbrock goes over these issues.

The business
As of 2009, JDSU was the market leader in ROADM components, with Finisar Corp. (Nasdaq: FNSR) coming on strong as a competitor. (See Finisar Climbs ROADM Ranks.)

Recent acquisitions and partnerships related to ROADMs:

Finisar CEO Jerry Rawls explains the Nistica deal at the end of this video, from OFC/NFOEC 2008.

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