DWDM RISING

In an industry where "divide and conquer" has become the mantra of more than a few telcos, it's perhaps no surprise that dense wave division multiplexing has secured its place in the public network. The act of splitting light into wavelength components not only enables carriers to increase network capacity, but it allows them to meet customer demand for bandwidth and provide the basis for quality of service guarantees.

The primary reason carriers use DWDM is to pack more wavelengths into each fiber, says Luc Ceuppens, senior director of product marketing for Omaha-based Level 3 Communications. However, according to Ceuppens and many others, the real promise of DWDM is its ability to route traffic on the optical wavelengths.

So what about Sonet? Some technical and economic arguments indicate that Sonet's days are numbered.

"Look at what Sonet was designed to do," says Denny Bilter, director of marketing for Ciena Corp. "It was designed to carry voice circuits, and it can't be beat for voice networks. But look at what has happened with the growth in data traffic."

Today, most networks consist of layers, including the physical layer, the Sonet layer, the asynchronous transfer mode layer and Internet protocol traffic on top of that. It is this move to IP and ATM that could conceivably spell the end for Sonet-or at least Sonet as we know it. As carriers add DWDM capabilities to their fiber networks to equip them to carry higher volumes of data traffic, the functions that Sonet technology previously performed for voice transmission are becoming obsolete.

Now it's a question of whether DWDM will snuff out Sonet entirely-or force it to evolve with the public network.

DWDM casts its shadow You don't have to look very hard to find evidence of DWDM's increasing penetration-or its effect on the Sonet layer. Carriers are moving huge volumes of data over their networks, and network architects figure they can chop their long-haul networking costs by roughly one-third by adding DWDM to other optical equipment (Figure 1).

One example is Tulsa, Okla.-based Williams, which is rolling out ATM directly over fiber early this spring.

"Conservatively, we'll save 20% to 40% on the electronics," says Mark Allen, Williams' manager of technology. As an added bonus, he figures Williams will get access to almost double the bandwidth on its existing pipes. Here's why: A typical Sonet-based network includes a working amplifier chain and a protection amplifier chain. Any carrier using Sonet under a traditional setup does not have full-time access to the backup or protection part of the bandwidth. With Williams' scheme, the protection bandwidth will be available for general transport chores 99.9999% of the time-anytime the network is up and running normally.

Williams is working to lay 30,000 miles of fiber, building from its original base of 11,000 miles of single-strand fiber with OC-12 (622 Mb/s) and OC-48 (2.5 Gb/s) capacity obtained from MCI WorldCom. Williams' latest buildout uses non-zero dispersion-shifted fiber that enables more power to be pumped onto the network, giving the carrier the highest possible count of OC-192 signals. Put into perspective, an OC-192 network, running at 10 Gb/s, could download the entire movie "Titanic" in less than a second.

The ability to move that kind of volume across oceans also is attractive to multinational carriers that are moving to WDM.

Level 3 is an emerging global carrier, with POPs in London, Amsterdam, Frankfurt and Paris. Domestically, it is building out a 16,000 mile DWDM-based network that will link 50 U.S. cities by December 2000. As it builds out its network, Level 3 is putting in multiple fiber conduits to allow it to follow any unexpected change or advance in technology without disruption to the customers.

Moreover, competitive local exchange carriers such as Coudersport, Pa.-based Hyperion Communications are well on the way to realizing their vision of an all-optical network by using DWDM. Earlier this year, Hyperion signed a $200 million agreement with Lucent Technologies to install high-capacity DWDM optical networking equipment. The DWDM gear will serve Hyperion's fiber optic network in the eastern half of the United States.

Hyperion hopes to begin service in several markets later this year, says Jeff Miller, director of business development for Hyperion. The CLEC is engineering and designing a 9000-mile communications network, and it will use Lucent's WaveStar OLS 400G DWDM system in its long-haul backbone to boost capacity and service capabilities. Installation of the DWDM equipment is slated to begin this spring and be completed over the next two years.

Frontier GlobalCenter (a division of Frontier Corp.), Sunnyvale, Calif., has begun deploying DWDM technology on a OC-48 Internet backbone network. The first section, between Los Angeles and San Francisco, is now up, and a cross-country route that will link Cleveland, Washington and New York is in the works. Frontier GlobalCenter is using about 15,000 miles of fiber from Qwest Communications.

Like most other networks, the Frontier network is designed in rings to provide a route diversity and survivability. Sonet multiplexed traffic is protected using four-fiber bidirectional line switched ring (BLSR) technology.

"DWDM technology is used to maximize the bandwidth capability of our fiber," says Jim Watts, director of network engineering at Frontier. "The technology has enabled Frontier to build a cost-effective national transport system."

Frontier GlobalCenter is using the SpectralWave DWDM system from NEC America and has deployed NEC's Intelligent Optical Network ITS-2400 on a series of Sonet four-fiber BLSRs to access more redundant fiber spans. The carrier also is using routers from Cisco Systems and ATM switches from Ascend Communications.

Frontier GlobalCenter uses the NEC equipment for transport on its OC-48 four-fiber BLSR, coupled with equipment from NEC and Alcatel to connect more than 100 add/drop sites, Watts says. The equipment is also used to transport IP and ATM traffic connected directly to the DWDM at OC-48 rates. "Our Sonet rings use DWDM for the optical layer transport between Sonet terminal site locations," he says.

"Data is the driver," Watts explains. "Using DWDM, we have been able to provide OC-48 channels connected directly to our ATM and IP network equipment without the added cost of providing Sonet multiplexers to provision low-speed interfaces. WDM provides a high bandwidth capacity solution at a lower cost than a Sonet multiplexed solution for data network applications."

But the real cost of Sonet to carriers may not be in the equipment overhead. Any carrier using add/drop multiplexers (ADMs) connected to IP routers or similar technology ends up paying twice for its switching. This may be the fatal objection to Sonet.

A bidirectional footnote Sonet serves two basic functions: multiplexing and network restoration. If Sonet's survivability features are bypassed by WDM, what future is there for Sonet as a multiplexing technology?

Sonet looks at the network's circuit level, but because most data networks are interested only in packets and cells, they do not worry about the circuit level. "Network architects will have to decide whether to go with IP and have survivability built into the network," says Ciena's Bilter.

Some would argue that Sonet's bidirectional capability will become a 1990s-era historic footnote as the massive transmission capacity and optical switching provided by WDM does away with the need to worry about bandwidth availability. The network's ability to use optical switching will become the hallmark of the all-optical network.

In February, Monterey Networks introduced its Wavelength Router architecture for the point-to-point long-haul optical pipes created by DWDM. These wavelength routers interconnect IP routers and switches across a ringless optical core, providing fast provisioning and restoration of end-to-end paths. Service providers can traffic-engineer and rapidly scale up survivable mesh optical cores without the 100% bandwidth penalties of rings-and without introducing intermediate ATM switches or proliferating legacy Sonet multiplexers and cross-connects.

Level 3's Ceuppens says the day is not far off when there will be only two optical layers: the WDM and the IP layers. "The Sonet layer will be here for another two or three years," he says. Networks will go through a "thin Sonet" period, where the Sonet capabilities are reduced, before it disappears altogether and IP runs over glass directly, he says. "It will be a gradual migration."

In networks where restoration is not an issue-IP routed networks, for example-DWDM is being deployed without the Sonet layer (Figure 2). However, where the reliability of Sonet is required, both in voice and in some data networks, Sonet remains the technology of choice. Once DWDM can provide the equivalent of ring switching on optics and provide QOS guarantees, IP over fiber will be a natural migration path.

Carriers will continue to use Sonet to interface with lower line rate services for restoration, says James Frodsham, vice president of marketing for Nortel Networks' Optera Solutions. "When the service rate equals the line rate-and you will see that in the next 12 to 24 months-you will look to direct connections into the WDM system," he says.

Sonet offers 50-msec. restoration, Frodsham says. With DWDM and IP, restoration is figured in full seconds, or even minutes. Until carriers can provide separate restoration capabilities with each service, they will need a Layer 1 protection scheme. Unless they can stream data flows, carriers are going to have one high-speed pipe for voice and another for data. That won't happen.

"It is inconsistent with current strategy to integrate on a single platform," Frodsham says, noting that carriers like to design for the highest grade of service. "The optical infrastructure must be flexible, with different grades of restoration, allowing the evolution of different grades of service. IP must be consistent with the architecture in the transport layer," he says.

Obvious advantages Outside the core carrier network, there are other advantages to WDM. WDM technology is the basis for a new paradigm for transport and switching, says Bryan Zwan, chairman and founder of Digital Lightwave. He predicts that the capacity and the quality realized by deploying DWDM will allow CLECs and other carriers to guarantee QOS levels to customers.

Bypassing Sonet will allow CLECs and other carriers to improve latency figures by using ATM. In addition, customers will send data on networks with fewer points of failure. Cheaper transmission costs also should flow through to the user in the form of lower rates. Further savings accrue from reduced requirements in the management center because there are fewer platforms to manage. There are sizable savings in the cost of switching fabrics because the carrier does not need to pay for both ADMs and switching. And the obvious savings in route costs add up fast.

Uptime is another advantage of DWDM that will make a powerful marketing tool. Some carriers may offer guaranteed 100% availability on their DWDM-based networks, predicts Level 3's Ceuppens, who says that it is irrelevant with today's network quality whether a carrier offers four nines or five nines reliability-there will still be the occasional outage. "At some point something will happen. But I see a model where some carriers will offer 100% uptime guarantees, including local access, and concede the penalty," Ceuppens says.

Level 3 plans to control quality on its network by controlling the long-distance, metro-area and last-mile fiber to the customer premises-taking it even to the riser. "We will put our own electronics on the network and put our own restoration on it," Ceuppens says. Coupled with the confidence the company has in DWDM, that will allow Level 3 to provide service guarantees on huge pipes.

Tastes great...less filling For those who talk of pushing Sonet out of the network, there is a silver lining. "Moving forward, it will be a framing technology. It allows a carrier to do bandwidth management using ADMs for OC-3 (155.5 Mb/s) or OC-12," says Allen of Williams. That carrier's game plan is to use ATM directly over fiber. "We'll be using it as a framing technology, not to manage bandwidth," he says. "Sonet does not buy you anything as a bandwidth manager."

This is in keeping with recent discussion of a "Sonet-lite" protocol. Sonet-lite has gone beyond the talking stage and is being hashed about in several development and standards groups. Steve Cortez, marketing manager for transport products at NEC, says he expects it to find a home in the data world where synchronization and jitter control are not needed.

Sonet-lite will incorporate the convergence of Sonet and DWDM, says Ron Mackey, executive vice president/technology for Osicom Technologies. "Sonet-lite will be used as a way to frame packets over WDM networks," he says. Sonet-lite will offer a cheap optical device with WDM capability, providing 95% of what carriers want from WDM without the 5% of expensive extras, he says.

Even if this particular version of Sonet is never standardized, many carriers and vendors predict Sonet will still be around for a while.

"Sonet is alive and well," maintains Bill Gartner, vice president of product development for Lucent Technologies. While acknowledging that DWDM is penetrating the metro market as a replacement for Sonet, he says that any wholesale changeover will be a long time coming. "There is a lot of 2.5 Gb/s and 10 Gb/s Sonet deployment," Gartner says. "We see that continuing." Like many others, Gartner says that no carrier is eager to dump a legacy network that is doing a good job.

NEC's Cortez points out that carriers are uniformly interested in protecting their core infrastructure. "All the basic benchmarks still are Sonet-based," he says. And even if Sonet is banished from the core of the network, it still promises to thrive closer to the customer .

Nortel's Frodsham agrees. "Sonet and DWDM are complementary technologies," he says. "Sonet [multiplexers] give you the means to get to that optical multiplexer. Until the network catches up to the 10 Gb/s speeds, we'll need Sonet multiplexers to provide that link."

Frodsham envisions a metro network with islands of transparency connected by a format-dependent, Sonet-framed, high-capacity backbone. Sonet multiplexers will aggregate narrowband traffic. "

Given the demand-push from the edge, today's fiber infrastructure will not scale economically to meet the demand. WDM is the technology that will provide the scalability to manage in a metro environment," Frodsham says. He puts the economic point for crossover from to a wavelength-based system from Sonet at 155 to 622 Mb/s. "The demand in the metro network is there now," he says.

Indeed, the future for DWDM networking equipment is not set in stone.

"Sonet is not going to go away. There will be applications for Sonet and carriers will not throw their Sonet [multiplexers] out overnight," says Ciena's Bilter. "DWDM won't blow Sonet out of the water. But when you look at the predominance of high-speed data in the IXCs' and CLECs' networks, you have to ask whether they will continue to use a circuit-based voice product."