Jazzing up fiber - includes related article on Microsoft's use of DWDM technology in its Redmond campus network and technology of Lucent's DWDM - dense wavelength division multiplexing - Technology Information

DWDM promises all the bandwidth you can handle.

Dense wavelength division multiplexing (DWDM) arrived with a bang earlier this year with the announcement that Cable and Wireless had applied the new optical technology to its trans-Atlantic fiber trunks. Cable and Wireless did this by multiplying the number of infrared bands that the trunks can support. Many experts believe that DWDM is a major step toward a National Information Infrastructure (NII) that one day could accommodate a limitless amount of information.

In essence, DWDM increases dramatically the information-handling ability of single-mode fiber. It weaves this magic by dividing the infrared light emitted by the laser into multiple wavelengths and by assigning an end-user payload to each wavelength. The long-term implications for both the public carriers and the customers they serve are both exciting and far reaching.

To shed some light on the new technology, Communications News asked Marty Kaplin, chief technology officer for Sprint, to share his thoughts on DWDM with our readers.

INITIAL IMPACT

Sprint is banking on the belief DWDM will soon move out of the backbone and all the way to the customer. "Five to seven years from now you could see passive optical networks provide a beam of light that hits a neighborhood and is divvied up among end users. This will be done using either a bus technique or as part of a local area ring that provides access to a broadband infrastructure shared by a multitude of carriers," Kaplin says.

Can DWDM transport data directly from the content source without the need for switching? "This is a question network architects are asking themselves today," answers Kaplin. "We are in discussions with vendors on a new generation of products that can handle data applications over an optical layer. We plan to utilize IP (Internet protocol) over optical paths. The old terminology was a combination of transport and switching; the new one will be integrated optical networking.

"The technological ingredient that's missing today is being able to identify the services carried by the individual DWDM channels. That is why running IP packets directly over an optical channel is still a couple of years off. You need visibility into those payloads, and that's what needs to be developed. But it's going to happen soon," Kaplin adds.

When will DWDM impact end users? "Products are already being deployed in the core of the backbone," explains Kaplin. "In fact, all new long-haul optical circuits will use wavelength division. This year you'll see it move into the MAN (metropolitan area network) environment and the next generation of networks will be DWDM based. The final step is to move closer and closer to the customer." Where will it eventually end up? "That's a question for the financial number crunchers rather than the technologists," Kaplin says.

The most visible application for DWDM will be connecting ISPs (Internet service providers) to the World Wide Web. This will permit entire communities to have direct T1 access to the WWW. "My take on wavelength division is that it could move the information bottleneck from the network transport system to the Internet content hosts and the application servers that connect to them," says Kaplin.

FUTURE IMPACT

At some point WDM techniques eventually may evolve to a point where they could replace future generations of SONET (synchronous optical network), predicts Kaplin. "At that stage SONET could become simply an aggregator of different bit streams at the network access point. A synchronous multiplexer could take both electrical and optical payloads from end users and aggregate them as optical channels for transfer across the network using DWDM."

For example, Chicago-based 21st Century, the city's first competitive cable franchise holder, is currently connecting multi-tenant office buildings along its lakeshore franchise to its fiber backbone. For a provider like this to make the decision to use DWDM it would have to decide whether it's better to dedicate a fiber to each customer or whether to leverage that fiber by multiplying its channel capacity.

"Some people are already talking about 256 windows on single fiber at a passband of 1,550 nanometers to 1,570 nm, and the impact on the CLECs (competitive local exchange carriers) and their users could be substantial," explains Kaplin. "It's getting to the point where, at the end of the day, we may have to start asking ourselves just how much bandwidth is enough."

"We must be careful not to waste bandwidth simply because we've got it," he muses. "Dense wavelength division makes bandwidth into a commodity, and we must learn to adapt to that transformation. For example, I believe business customers that have enterprise networks today that are predominantly private line could tomorrow turn to their public carriers and say `provide me with a dynamic bandwidth allocation on the public network.'"

OPPORTUNITIES

Sprint is well positioned to take advantage of the technology. The robustness of its backbone is predicated on all the non-dense WDM that this IEC (inter-exchange carrier) has placed in service over the past few years. "This is going to allow us to make a different play on offering total services to the customer," Kaplin says. "We're ready with our capabilities much earlier than others, and the industry soon will see how this is going to pay dividends for us."

One thing the carriers must ensure is that when they eliminate the bottleneck problem adjacent to the end user that they do not move it back into the network. "We want to solve once and for all the network congestion issue and then stay ahead of that curve," affirms Kaplin. "If you look at the international cables and what is happening to the cost of those, all the new ones coming up are all DWDM based. The impact on the prices of end-to-end T1 circuits has been dramatic."

For the time being DWDM needs things like SONET and ATM (asynchronous transfer mode). Kaplin says, "SONET provides a payload focused ability to lots of equipment, lots of network elements, lots of interfaces, and so forth. Dense WDM developers have yet to look at the challenges of what I call synchronization. This gives you the ability to look inside a payload, to transfer information from electrical and optical, and vice versa. The DWDM experts have a lot of work to do in that area."

LIGHT AT THE END OF THE TUNNEL?

A technology such as DWDM is both a threat and a promise to established providers. The threat lies in the fact that one day DWDM may make many of the component parts of their SONET and ATM long-haul networks, and the supporting legacy equipment they hold dear, obsolete. Its promise is that it can solve once and for all the bottleneck that faces powerful workstations each time they access the public information highway.

As Kaplin makes clear, deployment of DWDM will not happen overnight. Although optical wavelength division allows carriers to multiply the information-handling capability of existing fiber cable many times over, much R&D remains to be done on the optical components required. In the meantime the majority of local exchange carriers will-continue to purchase "non-dense" systems.

Dense WDM will be used in fiber MANs and enterprise networks that operate at 1,550 nm. This will be a major business market over the coming years. The key point is that although DWDM involves physical layer transport now, IP packets will soon be loaded directly on the optical beam. Most experts believe that lost packets can be duplicated and restored due to the enormous number of alternative channels made available by DWDM.

RELATED ARTICLE: Microsoft tests DWDM in its Redmond campus network

In an event of potential significance to end users, US West Communications and Lucent Technologies are installing and testing optical networking equipment aimed at vastly increasing the bandwidth in Microsoft's Redmond, Wash., campus network. The trial marks one of the first reported examples of bringing the speed and high capacity of DWDM technology to the corporate environment.

Microsoft typically transmits more than 25 trillion bits of information per day. The company employs the latest ATM (asynchronous transfer mode), IP, and optical networking technologies in its campus network. By adding Lucent's WaveStar OLS 40G metro system to a network jointly designed by Microsoft and US West, Microsoft can quickly meet its employees' demand for high-speed data communications and ad network capacity without having to overlay additional equipment. The trial is expected to run through mid-September.

"As a vendor that knows networks inside out, Lucent is well equipped to meet Microsoft's needs for a powerful system that supports the most sophisticated data services for today's corporate campus environment," says Gerry Butters, president of Lucent's Optical Networking Group. "As corporations need more capacity to enhance their employees' productivity and creativity, carriers are looking for ways to provide added services. US West is responding to this market, recognizing that our DWDM technology and network bandwidth management pave the way to faster, more streamlined campus networks."

Designed by Bell Labs, Lucent's WaveStar OLS 40G uses DWDM technology to allow up to 16 wavelengths of light or channels, to be transported simultaneously over a single optical fiber, thereby increasing the amount of traffic carried over a fiber-optic network. Network service providers using Lucent's system can economically carry a wide range of date, advanced voice, or video services while providing unsurpassed flexibility for rapid service deployment.

RELATED ARTICLE: How Lucent's DWDM works

Unlike traditional fiber-optic systems that transmit one optical channel at either 1,550 nm or 1,310 nm, DWDM enables multiple wavelengths to be transported over a single fiber. The reason this technology is so hot today is because current DWDM systems can support many more channels than this-up to 100 separate paths in some cases.

The basic parts of a DWDM system consist of precisely controlled fixed and variable wavelength transmitters, optical multiplexers (or filters), and optical receivers. Because filtering reduces the energy content of the wave, optical amplification must be used to remove this loss. One problem with dense systems is that amplifiers introduce problems, particularly when cascaded.

The first component in Lucent's DWDM communications link consists of a solid-state transmitter consisting of a Lithium Niobate laser constructed from a 1" x 0.5" wafer. The wavelength this transmitter emits depends on the dimensions and tolerances of the substrate. The number of individual wavelengths are only limited by the number of transmitters installed.

Information from the transmitters is modulated on the laser with a device that interrupts the radiation like the shutter of a high-speed camera. The individual channels are combined into a single beam of infrared in the fiber by means of a low-loss wavelength division mux.

Long-haul fiber may contain up to eight optical amplifiers spaced at 80 km intervals, providing a total reach of 640 km. At the receiving end a demultiplexer divides the beam back into its individual wavelengths. The final component is an optical detector that demodulates the imbedded information and puts it back into electrical form.

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