WDM for the short haul

As network operators roll out fiber to the premises, they are already making plans for future generations of equipment. Increasingly, operators are considering the possibility of using wavelength division multiplexing to play the same bandwidth-boosting role in the access network that it previously played in long-haul and metro networks.

Korea Telecom already has begun deploying a WDM-passive optical network (PON) system from Novera Optics, a Santa Clara, Calif.-based company with a South Korean subsidiary. That system uses a 32-wavelength WDM transmission from an optical line terminal (OLT) in the central office (CO) to a neighborhood node, where signals are split onto individual fibers for delivery to optical network units (ONUs) at 32 customer locations.

Novera's WDM-PON system provides 1 Gb/s or 100 Mb/s of symmetrical bandwidth, both significantly more than other PON technologies such as Ethernet PON (EPON) or broadband PON (BPON) that are based on Ethernet or ATM. Bernd Hesse, vice president of marketing and business development for Novera, said the company envisions even greater capacity in the future.

WDM-PON also offers another type of flexibility, Hesse said. With other PON systems, “It's hard to do mixed services. With EPON, you can't do ATM,” he said. “You should keep the optical layer transparent. The operator should be able to offer any service or service mix and any bit rate.”

WDM-PON also could offer a higher level of security than other PON systems, Hesse said. With traditional PON systems, signals destined for all 32 customers in a neighborhood are sent to each location. Although customers control which signals they actually view through a network interface, some manufacturers have had to encrypt data to address security concerns. But WDM-PON avoids this by using what Hesse calls a “virtual point-to-point” architecture that dedicates a private path from each customer to the CO.

Hesse hastened to add, though, that the system uses a “physical point-to-multipoint solution.” That means it uses the same physical architecture as GPON, BPON and EPON, which also use a single fiber to feed a neighborhood node that then makes the individual connections to customers. As a result, Hesse said, network operators that have deployed those other PON systems will be able to easily upgrade to WDM-PON. To do so, they will need to install an arrayed wavelength grating (AWG) at the neighborhood node in place of, or in addition to, the optical power splitter already there. Customers who will be served through the AWG also need a different ONU.

Novera's secret sauce is an optical module that is installed in the ONU and the OLT, which eliminates the need to use ONUs tuned to specific wavelengths. Instead, each ONU is locked onto a specific wavelength through a process controlled at the CO. In addition to using the optical module in its own products, Novera supplies it to other WDM-PON system manufacturers, Hesse said.

Although Korea Telecom is the only network operator that has actually deployed WDM-PON, Hesse said operators in various world regions are evaluating the technology.

One of the operators evaluating WDM-PON is Verizon. “We're looking at it in the lab, as well as within the standards bodies,” said Vincent O'Byrne, director of technology for Verizon's wireline access group. While declining to name specific manufacturers, he said the company is looking at a range of products with a variety of approaches, including some that provide asymmetrical bandwidth.

The first place Verizon would likely use WDM-PON would be as an overlay to handle larger businesses, O'Byrne said. Accordingly, the company would want an easy upgrade path from the GPON equipment it is currently deploying. “We're interested in ensuring that, as standards get developed, they take into account how we've already deployed our GPON, and that, as we deploy GPON, we take into account the benefits that we could get from WDM-PON,” O'Byrne said.

An important concern, O'Byrne said, is that “some WDM systems aren't taking into account the fact that we have a wavelength at 1550 nanometers for broadcast television. Some systems may already use that frequency or use a frequency that may cause some interference.” Novera's current WDM-PON system operates in WDM's C Band, which includes the 1550 nanometer band, as well as in the E Band. But Hesse said that other optical bands could be used.

O'Byrne is hopeful that the Full Service Access Network (FSAN) standards body will take broadcast video requirements into account as it develops specifications for WDM-PON. “We're at the point in time where the standards have to define the wavelength plan,” he added.

One industry analyst who specializes in access technologies sees compelling advantages in WDM-PON. “Long term, every operator or vendor will tell you they believe giving each individual customer their own wavelength is the best way to deliver services and offer quality of service and differentiation,” said Teresa Mastrangelo, principal analyst for broadbandtrends.com.

As a result, she believes all manufacturers of traditional PON systems are looking closely at the technology. She cautioned, however, that high costs are a prohibitive factor at present.

Korea Telecom is in a unique position, Mastrangelo added. The company's WDM-PON deployment, she said, “is being driven by what Korea as a country is trying to do.” The South Korean government has set a goal that every household will get between 50 megabits and 100 megabits of bandwidth, and WDM-PON, she said, “is one of the technologies that has been identified to make that happen.”

But without that kind of government mandate, it could take several years before other network operators see a clear business case for deploying WDM-PON, Mastrangelo said. She added that network operators have other alternatives for boosting PON bandwidth. One option is a Fujitsu system that uses coarse WDM to increase the bandwidth between the CO and the neighborhood node, while retaining existing GPON technology from the neighborhood node to the customer premises.

That approach is similar to one several companies in Korea are taking to try to drive down the cost of WDM-PON, said Even Zimmer, chief technology officer for Ignis Photonyx, an AWG manufacturer that supplies LG-Nortel (which has developed a WDM-PON system but has not yet announced any customers). Unlike Fujitsu's system, the Korean method would use DWDM rather than CWDM for the connection between the CO and the neighborhood node for higher overall bandwidth.

Hesse declined to provide pricing details on Novera's WDM-PON system, but said that the system is “cost competitive.” He also touted the fact that the system is based on low-cost Fabry-Perot lasers.

Verizon has not yet received pricing from potential vendors, O'Byrne said, but in theory, the technology should be cost-competitive with Ethernet, Sonet or other technologies that deliver equivalent bandwidth. WDM-PON, he said, “has the promise of savings given that it makes more economical use of fiber that's already deployed.”