Network operators are poised to pump up the metro portion of their 
networks with dense wavelength division multiplexing

Since its introduction, dense wavelength division multiplexing (DWDM) has been a technology applied primarily in the long-haul portion of the communications market–bringing much needed relief to overburdened backbone networks carrying long-distance traffic. Until recently, the use of DWDM in the metro space never took off. Common wisdom was that DWDM was too cost prohibitive for short distances.

But now, the tables have turned. Cahners In-Stat Group (owned by the same company as CED) reports that vigorous growth in DWDM metro applications will now make up for a 12 percent annual decline in the DWDM long-haul market for 2001. In-Stat projections show further that by 2005, the DWDM industry will be worth nearly $21 billion per year.

Suppliers couldn't be happier. While capital expenditures have certainly slowed since 2000, the DWDM metro market is a beacon of light, bringing hope to equipment manufacturers and suppliers that network operators are still committed to building out the network.

"The metro optical market is really the only optical market still expanding at a rigorous growth rate," says Demitri Elias, VP of marketing at Sorrento Networks, a supplier of end-to-end intelligent optical networking solutions for metro and regional applications. "Carriers now find themselves with excess capacity in the long-haul and are spending in the metro portion of the network."

This capacity imbalance must be corrected. Removing bottlenecks in the metro portion of the network and increasing capacity closer to the customer is of prime importance.

Rather than lay more fiber or increase the bit-rate using time division multiplexing (TDM), providers can use DWDM to exp nentially expand the capacity of existing fiber. Functioning as a capacity multiplier, DWDM also has the advantage of being scalable, bit-rate and format independent, and now, the most cost-effective alternative.

"The cost of DWDM is dropping rapidly, about 50 percent in the past 12 to 18 months, and the lead time for delivery of DWDM systems has gone down to four to six weeks," says Bob Scott, director of product marketing for transmission network systems at Scientific-Atlanta.

Telco market still main adopters

It isn't difficult to find a DWDM supplier for the metro. Anyone attending Supercomm or the National Fiber Optic Engineers Conference couldn't swing a stick without hitting a vendor catering to the burgeoning metro market. But, vendors tend to focus on either traditional copper-based carriers or prefer to target MSOs and utilities. This fact of life proves that supplying a DWDM system is only part of the equation. Vendors need to understand the operator's particular business needs and network architecture.

According to RHK estimates, ILECs, IXCs, RBOCs and greenfields will account for the lion's share of metro transport equipment. That's because when it comes to incumbent carriers, migrating from Sonet-based, connection-oriented networks to DWDM or pure wavelength switching networks is necessary to support IP traffic and next-generation services.

Naturally, cost is a factor, but scalability seems to be the common denominator. Carriers want solutions that allow them to expand capacity on a "grow as you go" basis, adding more channels as demand increases and supporting a range of high revenue services, such as storage area networks (SANs), virtual private networks (VPNs) and gigabyte Ethernet, to name a few.

Lucent Technologies, for instance, offers a metro solution, called metroEON, that allows operators to invest slowly in metro build outs. "We offer a low start-up cost. They can light up three nodes with three of four wavelengths and grow in-service to 16 and 32 wavelengths when needed. MetroEON is a flexible system that allows them to buy per node and per wavelength," says Rich Zoccollilo, general manager of the metro WDM business.

For those carriers that have a bigger appetite and want the SUV model, Sorrento Networks boasts that its GigaMux DWDM platform delivers the highest bandwidth on the market–supporting 64 wavelengths and a bit-rate up to 10 Gbps, or in a two-fiber ring, up to 128 wavelengths/1.28 Tbps. But, a network operator can still start with as few as four wavelengths and expand in-service from there. Again, scalability is key.

Perhaps simplifying the implementation process, Lightscape Networks, a wholly-owned subsidiary of ECI Telecom Ltd., offers what it calls a 5-in-1 platform. Called XDM, this one-size-fits-all product combines in a single shelf: multi-ring Sonet/SDH, data networking, a full suite of metro and long- haul DWDM, a distributed digital cross-connect system (DCS) and instant end-to-end provisioning. Lightscape also promises a flexible topology that allows networks to scale on a "when and where basis."

Interestingly, Lightscape Networks is also targeting this product to utilities and cable. "For utilities and other rights-of-way companies, we can convert their dark fiber to a revenue-generating network, integrating services on a single platform," says Emanuel Nachum, Lightscape's VP of marketing.

While most vendors that serve the telco market are certainly eyeing the cable TV market, the fact remains that there isn't much cross-over in a tight economy. "A lot of major vendors are pulling back and not pursuing new markets, like cable TV. They are focused on profitability in their current telco markets," says Dan Hanson, director of optical access at RHK.

Cable is a different animal

The slowing economy has forced telco-based operators to spend more cautiously and make sure end customers are lined up before building infrastructure. This more conservative approach is exactly what most MSOs have been preaching all along.

This wait-and-see stance has some advantages, according to Paul Gemme, VP of plant engineering at Time Warner Cable: "We tend to benefit from telcos pushing DWDM technology ahead. As more equipment is sold, prices come down and the technology improves."

While telcos might be purchasing the majority of DWDM systems, MSOs are in a favorable position to adapt the technology to the HFC architecture and cost-effectively enter profitable new markets. "Cable operators are fortunate to have extra glass available close to the home and newer fiber in the ground. While we might not push the R&D envelope, we work hard to apply new technology and make it work," says Wayne Hall, VP of engineering at Comcast Cable.

Applying DWDM in the cable world means using the technology for the transport portion of the cable network: moving traffic from the headend to the hub. How an operator chooses to implement DWDM in the transport network depends on a number of factors: planned services, target markets, space, take rate, server location and cost per stream.

Some MSOs, like Time Warner Cable, see VOD and SVOD as the main drivers for adopting DWDM technology. "At this point, we still have sufficient fiber without having to use WDM. But VOD will drive our need for this technology. We first have to finalize deals with the movie houses and then make a system-by-system decision," explains Gemme.

In general, VOD systems would use an analog/DWDM approach. As Tim Brophy, director of advanced photonics technology at Motorola, points out in a recent DWDM white paper, "The signals sent over DWDM transmitters are actually QAM signals, where a digital bit stream is encoded onto an analog subcarrier."

Time Warner Cable's implementation of DWDM would go something like this: DWDM transmitters would transport the VOD signals in the 1550 nm optical window from the headend to the hub using 16 or 32 colors of light per fiber. From the hub, signals would be routed on 1310 nm wavelengths to nodes passing 500 homes.

For Scientific-Atlanta, it's not just a question of what applications are being planned, but where an MSO places its file servers.

Its Prisma II solution is an analog DWDM product for operators centralizing both file servers and QAM modulators in the headend. Prisma DT or Prisma IP, on the other hand, is digital transmission DWDM used when file servers are centrally located, but QAM modulators are at the hub. And, in a third scenario where hub-based file servers house content, but master file servers located in the central headend perform back-up, Prisma DT is the choice.

"None of these options is better than another," says Scott of Scientific-Atlanta. "It's simply a matter of cost and availability of space."

Leveraging HFC for businesses

Many MSOs are aggressively courting businesses that will pay a premium for dedicated wavelengths. According to Quantum Bridge Communications, a privately-held company based in Andover, Mass., the HFC network of today that serves residential customers will evolve to a DWDM over passive optical network (PON) of the future.

Figure 2: Quantum Bridge Communications addresses bandwidth bottlenecks with three DWDM topologies: Point-to-multipoint DWDM over PON, point-to-point and DWDM add/drop ring.

Described as a point-to-multipoint DWDM over PON approach, MSOs would split fiber at the node and divide wavelengths to serve multiple end points, from residences to small, medium and large businesses. "Right now, we are the only vendor to ship product that enables fractional wavelength services over PON," claims Vincent Lucarini, business development manager at Quantum.

Comcast Business Communications, a wholly-owned subsidiary of Comcast Corp., just activated a large PON network in the Baltimore market using Quantum equipment. In this instance, DWDM transmissions are purely digital, versus the analog/DWDM approach described earlier, and the underlying structure is Sonet-based.

"This is an efficient fiber consumption plan where they can put a lot of traffic, as high as OC-192, on each wavelength," says Hall of Comcast Cable.

With stints at AT&T, Lucent and Bell Labs, to name a few, Quantum's management team is no stranger to HFC and optical access. Again, while MSOs may be open to considering a wide field of vendors, those that can demonstrate intimate knowledge of the HFC architecture have a leg up on the competition.

How about telephony-over-cable? Delivering voice services reliably over HFC might be the toughest lion to tame. But, DWDM might just come to the rescue here, as well. As demand for packet-based voice services, such as Voice-over-IP (VoIP), increases in the future, MSOs could use DWDM to transport voice with enough capacity and reliability to make it profitable.

Bridging the telco/cable gap

Focusing on one market at a time makes perfect sense for Motorola. To keep its attention fixed squarely on MSOs, Motorola Broadband Communications Sector struck an arrangement with Geyser Networks in 2000 to provide cable operators with an optimized Sonet platform for the transport of video, voice and data traffic.

"This is a complementary arrangement where Geyser provides solutions for the telco side, and we provide really flexible bandwidth services that look like a telco solution, but they're for broadband operators. We might piggy-back on the telco side, but products deployed for MSOs have their own twist," says Brophy of Motorola.

It looks like DWDM has a bright future in cable, but it will not be a "one-size-fits-all" application. At the end of the day, MSOs prefer vendors steeped in HFC architecture to help them integrate optical transport technology. Who knows? Adapting DWDM to HFC networks may not be such a big deal.

"MSOs have had to deal with the difficult task of transporting a high density signal such as the QAM modulation signal. As a result, MSOs will have an easier time moving to voice and data services than it will (be) for telcos to make video work," remarks Scott of Scientific-Atlanta.

Even so, it always comes down to economics. Putting DWDM systems in the metro area is still a hefty investment. So, despite recent price decreases, it will only be deployed when there is a strong business case.

"Whoever can crack the economic nut, whether telco or cable TV operator, will be the first to deploy DWDM," insists Martin Nyman, director for ADC's advanced photonics integration center (APIC).