Despite economics, WDM-PON remains in the mix
by Meghan Fuller Hanna
In its recent "PON and FTTH Subscribers" report, Infonetics Research (www.infonetics.com) noted that while GPON represents the most immediate growth opportunity, annual port shipments for the emerging WDM-PON segment are expected to grow at a 41% CAGR between 2007 and 2011. Much of this growth will come from the Asia-Pacific region, the analyst firm says, thanks to early WDM-PON adopters like Korea Telecom.
However, sentiment in some quarters regarding the prospects for WDM-PON appears mixed. Lightwave Europe recently spoke with three companies in the FTTx space—Alcatel-Lucent, Tellabs, and ADC—all of which agree that, given the cost of WDM optics, WDM-PON is not yet a legitimate residential triple-play technology. They also have vastly different opinions about when—and if—this will change.
WDM-based PONs allocate a separate wavelength for each subscriber, thus creating a point-to-point channel between the subscriber and the central office. The most aggressive vendor of WDM-PON systems to date has been Novera Optics (www.noveraop tics.com), which last year entered into a strategic partnership with ADC (www.adc.com). Per the agreement, Novera is supplying equipment to ADC for customers in North America, Europe, and South America.
When asked why ADC entered into the partnership, programme manager O.J. Johnston cites the recently announced French initiative to run fibre to every apartment. "There's no way you're going to be able to do that on separate, individual fibres to everybody," he says. "You're going to have to use some sort of WDM or DWDM. From that perspective, I looked at it and said, 'Ultimately, the market is going to go in this direction.' It's just a matter of when."
It makes sense for ADC to be in the WDM-PON business, he says, because one of the biggest issues facing operators deploying the technology is how to house the splitter in the field. As a provider of both connectivity products and PLC-type splitters, this falls squarely within ADC's sweet spot, Johnston confirms. "Plus, we didn't want to be a GPON vendor," he says. "We wanted to maintain our neutrality there but offer customers a way to upgrade beyond that later on."
Johnston says ADC is currently in major trials with vendors around the world for commercial service deployments. He says he believes the economic argument changes depending on the end user, and it's one of the key reasons why ADC is targeting carriers' commercial services offerings first.
"The cost of a WDM-PON is drastically less than, say, if I were going to try to deliver services over a SONET network," he says. "When you start comparing the kind of bandwidth that you're going to want to deliver, you can't even do it on the SONET network because you have OC-192, and then you're done. What we're doing today with our [WDM-PON system], we're up to 20 gig on a single fibre. Frankly, it changes the [service provider's] business model and lets them go after customers that maybe before they might have ignored."
Ultimately, though, even ADC recognises that changes have to be made before WDM-PON is cost effective for widespread residential deployment. "Density has to be increased," says Johnston. "And aggregation functions have to be built into the equipment."
He believes that ONT and OLT functions may eventually be incorporated into SFPs that could be plugged into existing switches or routers. (Fujikura has demonstrated just such a device for EPON.) Other than for aggregation, you don't need another switch in the network, he says, "so now it's just a matter of, 'How do I aggregate this traffic and get it into that switch?'
"I think what you'll see coming out in the near future is a different type of OLT—an OLT that essentially has the light source that is used for the wavelength locking, and it has the wavelength splitter built into it. Then you'll have the remote wavelength splitter that sits out in the field and SFPs on either end of the network," says Johnston, who notes that when you get down to that level of development, you'll start seeing sub-$1,000 per subscriber costs, placing WDM-PON squarely in the GPON realm.
"When you can get close to the cost but offer exponentially more bandwidth and the ability to upgrade it at any given time and put whatever you want on the end of it, WDM-PON is definitely a compelling way to go," he says.
Count Tellabs (www.tellabs.com) among the pro-WDM-PON camp. Like ADC, Tellabs believes there is not a mass market business case for WDM-PON today, "but there will be in probably a five-year horizon," says Mark Conata, director of product marketing for Tellabs' access portfolio. "And in a business environment where you are trying to deliver tens of gigabits in a very dense urban environment, I think it's reasonable to assume you'll see DWDM- or WDM-PON solutions deployed much sooner."
In fact, the vendor is so bullish on the technology, it announced at June's NXTcomm conference its participation in the European Commission-funded SARDANA project, short for Scalable Advanced Ring-based passive Dense Access Network Architecture. A three-year project, SARDANA aims to radically increase the capacity and reach of FTTP networks through a combination of WDM-PON and remote amplification technology.
"If you look very broadly, one of the things we're trying to do with this basic research is to look holistically at the metro and the access part of the network and kind of collapse that," says Conata. Remote amplification of the fibre can extend the reach of the PON network by 100 km or 60 miles, thereby effectively merging the metro and access network.
The SARDANA initiative is also looking to expand bandwidth more than 128 times per OLT interface. "It's a real quantum leap in bandwidth to the end user," notes Conata. The project has a Tier-1 champion in France Telecom, which will test the resultant WDM-PON architecture in a field trial in France. This may be a research initiative, says Tellabs' spokesperson Ariana Nikitas, but "we are committed to demonstrating the results."
"We will have access to [SARDANA's WDM-PON technology] as time goes on," adds Conata, "and we do believe that ultimately, this wavelength of light to every subscriber is the endgame and solves whatever bandwidth problem that may crop up."
Marcus Weldon, CTO of Alcatel-Lucent's (www.alcatel-lucent.com) Fixed Access Division, believes WDM-PON is once again a topic of interest thanks to the next-generation PON standardisation efforts in the IEEE and ITU FSAN. Ironically, these efforts may actually hinder the emergence of a WDM-PON market because they also include 10-Gbit/sec PON variations. The issue, says Weldon, is one of both economics and timing; throw 10G PON into the mix, and WDM-PON may not seem so advantageous.
Retaining the current split ratio, 10G PON will provide guaranteed bandwidth of 300 Mbits/sec per subscriber with the ability to burst up to the full line rate of 10 Gbits/sec. WDM-PON, by contrast, currently tops out at 100 Mbits/sec per wavelength because some of the technologies—such as injection seeding the laser to give it its proper colour—do not work past 100 Mbits/sec, says Weldon. He notes that some folks are working on a 1-Gbit/sec version, "but 1-gig WDM still isn't any better than the 300-Mbit/sec to 10-Gbit/sec TDM-PON where you have much more dynamic range of bandwidth," he says.
Alcatel-Lucent recently performed an economic analysis of the technologies and concluded that the cost of a WDM-PON per user is still twice the cost of a GPON today. "And it's still much more expensive than an NGPON, even," says Weldon. "We're thinking NGPON might be, say, a 50% premium over GPON, whereas WDM-PON is a 100% premium over GPON."
With WDM-PON, you need a pair of coloured optics per home, one at the ONT and the other in the CO, whereas GPON requires a capital cost of one optic at the ONT plus 1/32 of an optic at the OLT; the shared cost here helps dilute the overall cost of the optics in a GPON architecture.
Weldon believes TDM-PON will continue to prevail for the better part of the next decade, with WDM-PON relegated to a niche technology for backhaul applications or certain office or campus parks where it can find "an appropriate deployment model at tolerable price points.
"One of the dangers of always being the technology of the future is that you may never actually hit the right point in the market with the speed you can offer at the cost points you can offer," he says. "I believe that WDM-PON is still in danger of always being the technology of the future and never quite exceeding the capability of the TDM PON—or, at least, not at the right cost points. It's still very much up in the air as to whether WDM-PON will ever find a residential niche or market that it can deploy in as cost-effectively as a 10G TDM-PON."
When pressed for a timeframe in which WDM-PON might be economic enough for residential deployments, Weldon suggests 2020: "That's far enough out that it means not now.
"It has to exceed what you can do in the TDM space and if WDM-PON can't solve its problems and exceed what's available for 10G PON, then it's missed its window once again. You can think of it as a race between WDM-PON and NGPON for who gets there first with the right bandwidth, cost points, and reach," he says."Currently, I think 10G PON is winning."