Wss up? Are ROADMs ready to take the next step?
Reconfigurable optical add/drop multiplexers, or ROADMs, are very rapidly becoming a must-have for equipment vendors serving carriers' metro network needs. And while the buzz surrounding metro ROADMs seemed to reach critical mass only last year, the next generation of ROADMs already is emerging, and it includes wavelength-selective switching, or WSS, a technology that promises even greater flexibility and control than that promised by last year's ROADMs.
WSS was the “top topic” at a recent Supercomm planning meeting, said Michael Kennedy, a meeting participant and industry consultant.
“It's being positioned as one step beyond ROADM,” he said. “I think the carriers will definitely endorse this.”
ROADMs gained carrier interest for their ability to allow carriers to reconfigure and deploy wavelengths remotely, using software, without having to make hardware adjustments on site. After deploying them in long-haul networks, the industry recently moved to apply them to metro networks.
Last year's generation of metro ROADMs have a relatively simple decision to make regarding each wavelength that passes through them on a network ring: either pass it along or divert it (i.e., add it to, or drop it from, the ring).
Most metro ROADMs do this using “blockers,” simply blocking wavelengths that need to be diverted. Blockers are often made with relatively inexpensive liquid crystal devices (also known as LCDs, used for decades in watches and calculators), but it's not the most scalable approach because it involves splitting each wavelength among all possible routes and blocking only the undesired paths. Using several blockers in a cascading series would deplete the wavelength's power along the way, requiring more money to be spent on amplifiers to get power levels back up.
ROADMs with WSSs have more options: Either pass the wavelength along or divert it, and if diverting it, either drop it from the ring or switch it to one of perhaps three other rings. LCDs can be used for this, too, as can microelectrical mechanical systems (MEMS). The result is greater flexibility in managing wavelengths and, theoretically, reduced costs.
“It's more than a ring,” said Stuart Elby, vice president of network architecture for Verizon Communications. “It's some kind of mesh.”
Verizon currently has a request for proposals for a next-generation DWDM optical transport platform that requires ROADM functionality as well as WSS or wavelength cross-connection. The company expects to pick a platform in the second half of this year and deploy it next year.
“Typically, I'd have a bunch of wavelengths stacked together coming in and a bunch of digital cross-connects to do grooming,” Elby said . “It would take a lot of on and off rings to do that, a lot of jumpers, a lot of complexity and a lot of local transponders. A WSS would remove all that. I could specify a certain wavelength carrying traffic destined for a different ring just be switched as that wavelength to that other ring. No need to go through back-to-back transponders and jumper cables. It's a tremendous savings in big offices.”
In fact, the application may be needed only in Verizon's largest offices, he said — the “superhubs.” While evaluating wavelength switches for its long-haul network a few years back (a search that led Verizon to deploy Lucent Technologies' LambdaExtreme DWDM), Verizon found that only 10% of its offices truly could be optimized with wavelength switching.
“Ninety percent of them do just fine on rings,” he said. That being the case, Verizon is ideally looking for a product that can function as a metro ROADM in some offices and as a WSS in the superhubs.
Earlier this month, Fujitsu Network Communications unveiled exactly such a product (or so the company claims) with the latest release of its Flashwave 7500 ROADM, version 4.1. The announcement came just weeks after research analysis firm RHK declared Fujitsu the leader of the metro ROADM market in 2004, taking more than 80% of the North American market and 75% of the worldwide market, thanks to customers in Japan and the North American cable industry.
Movaz Networks introduced a large all-optical switch based on MEMS in 2002 that it named iWSS for “integrated wavelength selective switch,” but its ROADM is a separate product, and the company didn't respond to inquiries about future plans. However, in February 2004, Movaz's partner, Lucent, told Telephony the duo would likely introduce a new generation of ROADM in late 2004 or early 2005. Last month, Bruce Nelson, chief technology officer for Lucent's multimedia network solutions group, said he expected to see ROADMs allowing any wavelength to be dropped to any port (a fair description of WSS) “by next year.”
Nortel Networks promises to unveil a WSS ROADM this year. Tropic Networks, whose TRX-24000 DWDM platform was designed to be upgradable from blockers to WSS, said it will migrate to WSS when the technology becomes more “mature [and] robust.”
A spokesperson for Mahi Networks claims its customers already have deployed “probably 50” of its WSSs and are likely to deploy a total of close to 100 by mid-year. The company inherited a ROADM and WSS through its June 2004 acquisition of Photuris Networks, which already had boasted perhaps the first metro ROADM deployment, through Verizon's enterprise solutions group, the previous summer. Mahi expects to announce at least three more customers for the WSS in the weeks leading up to Supercomm, the spokesperson said, adding, “I've got these deployed in the field for almost two years; they're working like a charm. From our perspective, this technology is pretty mature. All the other vendors are playing catch-up.”
Some subsystem vendors may even be preparing to release WSS and ROADM subsystems that could be integrated with carriers' existing DWDM systems, thus avoiding the hassle and expense of replacing existing gear altogether, which could be especially enticing given that much of that DWDM gear may not be more than a few years old.
“[Carriers] would like to have — and component people would like to see because they'll sell more ROADMs — a retrofittable ROADM device,” said Clif Holliday, an industry consultant who authored a report on ROADMs for IGI Consulting in March. “Some component vendors are working toward that. What they want is to retrofit the market.”
However, the largest carriers are unlikely to buy gear directly from subsystem vendors without going through a larger system vendor.
“These WSSs are one piece of the overall system,” said Glenn Wellbrock, MCI's director of network technology development. “A few small vendors — plus JDS [Uniphase] — have WSSs that are relatively mature. They work; they're good devices. But now you've got to have transient control, wavelength management, boxes that can handle [all-optical switching], back-office systems that support it and everything else that goes into it to make it a system. [The systems aren't] necessarily as far along as perhaps the devices are. That's what usually gets us into trouble.”
Wellbrock believes metro ROADMs in general are not yet mature enough or cost-effective enough for widespread deployment, at least for MCI's purposes. The carrier plans to deploy them in the first half of 2006, he said.
“I think we'll deploy ROADMs when we can get them with the functionality we want and the cost points to where we need to have them,” he said. “ROADMs were available three years ago, and yet they've seen very limited deployment, not because they're not useful but because the cost structure wasn't right. They aren't [yet cost-effective], and there aren't enough suppliers. Introducing WSS [is a] step in the right direction, but it's not compelling enough to use everywhere in wide-scale deployment.”
A Mahi spokesperson refuted the characterization of metro ROADMs as uneconomical.
“While it is very true that the ROADM component subsystem is more expensive than a fixed filter, what comes with fixed filters is more expensive amplifiers and optical power monitoring,” he said. “The ROADM allows the rest of the system to be much cheaper.”
WSSs are just the latest step in the evolution of metro ROADM technology. Further on the horizon, ROADMs probably will be able to actually translate traffic from one wavelength to another under the same remote control through which WSSs allow wavelength switching. (In other words, a 1550 nanometer wavelength becomes a 1562 nanometer wavelength as it switches ports.) But that technology is a long way from commercial viability, and it remains to be seen how quickly and successfully carriers will take up WSSs in the meantime.
ROADM component sales projections, U.S. market (in units)
|Year||Blocker||Mux/Splitter||DeMux/Combin||Tun Laser||Tun Filter||Switch Pts||Monitor Points|
|Source: IGI Consulting, March 2005|
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