DWDM heralds era of bandwidth abundance.

Speeding to the all-optical network.

Bandwidth abundance is the underlying promise of the optical revolution that is transforming the communications-services industry in the U.S. Optical fiber has always augured a bandwidth bonanza, but with the advent of dense wavelength division multiplexing (DWDM), the prize is even greater than expected. Today's DWDM systems already allow carriers to boost each fiber's capacity 160-fold, and this number is likely to increase considerably in the years ahead.

Given these benefits, it's hardly surprising that the optical revolution is spreading throughout the world. In Korea, GNG Networks is expanding its network infrastructure with an optical DWDM backbone capable of providing a throughput of 320 Gbps. In Hong Kong, New T&T has just completed its first trial of an optical networking system for metropolitan users, while in Australia, Nextgen Networks is developing a national fiber network extending 8,400 km from Brisbane to Perth via Sydney, Canberra, Melbourne and Adelaide.

The action is thick and fast in Europe, too. Sonera is bringing the first optical Internet solution based on DWDM for long-haul networks to Finland and Russia. France Telecom is testing DWDM technology to extend optical networking to metropolitan customers, while British Telecom is upgrading its core national network with an optical DWDM transport layer to support a new data and Internet protocol (IP) architecture. Global Crossing is also leveraging DWDM in a ring-based pan-European metropolitan network.

Enthusiasm over DWDM technology stems from a number of technical, economic and operational trends. Faced with exponential growth in data traffic, service providers are increasingly confronting fiber exhaust in existing long-haul net works. At the same time, the highly competitive market for communications services is forcing service providers to reduce network infrastructure costs, and DWDM offers an attractive alternative to laying new fiber.

DWDM offers more than a way to avoid laying new cable, however. On the one hand, it could provide the basis for a more efficient network architecture that eliminates the SONET (synchronous optical network) and ATM (asynchronous transfer mode) layers; on the other, it could be a replacement for SONET, a means of long-term capacity expansion and a cornerstone of next-generation optical networks.

DWDM is an attractive solution for both incumbent service providers and those building data-centric, next-generation networks. Incumbents appreciate its ability to optimize the capacity of their existing fiber systems, including the long-haul, interoffice and metro portions of their: network. Also, because DWDM is both protocol and bit-rate independent, it protects carriers' existing investments in other technologies, such as ATM, which are easily transportable by DWDM on a per-wavelength basis.

For so-called "greenfield players," who are engaged in building next-generation, data-centric networks, DWDM has a somewhat different value proposition. It allows them to build flatter, more streamlined networks that not only lay the foundation for easy capacity expansion but also require lower capital costs than incumbent networks. Improved serviceability and maintainability, coupled with lower administrative costs, means that service providers can attract customers with more competitive offerings.

VIRTUAL FIBERS

Wave division multiplexing (WDM) is an optical technology that expands fiber-optic capacity in carrier networks by simultaneously carrying multiple optical signals over a single fiber at different wavelengths. Each wavelength represents a separate channel, or "lambda" of optical information, which can be either voice, data or video. In essence, WDM creates a series of virtual fiber's from a single fiber, each with its own logical domain as a partitionable data stream. When WDM technology creates eight or more virtual fibers, it is referred to au "dense" WDM, or DWDM.

Prior to WDM, if a carrier needed more fiber capacity, it had two choices, both expensive: one was to lay new fiber' in the ground; the other was to deploy a higher SONET transmission speed, which involved expensive forklift upgrades. With WDM, carriers can take an existing OC-48 point-to-point connection, for example, and expand its capacity by creating additional channels.

WDM systems employ optical amplifiers to regenerate the optical signal, allowing it to traverse longer spans in the network backbone. For point-to-point applications, WDM systems typically include a pair of terminals, one or more optical amplifiers, and, in some cases, an optical add/drop multiplexer.

WDM is a relatively new technology. The first commercial, but nonstandard, products became available in 1994. Ciena, the bellwether startup that became synonymous with higher-capacity DWDM, only introduced its first product, the MultiWave Sentry, in April 1996. But Ciena's tinting was excellent--a rare case of appropriate technology meeting strong market demand at just the right time.

Previously, SONET systems were used to propagate optical signals through a backbone network. With SONET optical signals are multiplexed into a framed and synchronized transmission based on the time division multiplexing (TDM) system used extensively throughout the nonoptical or electrical portions of the public switched telephone network (PSTN). While SONET is usually viewed as an optical technology, its value proposition lies in its ability to span the optical and electrical worlds.

In the U. S., SONET is now deployed extensively throughout the PSTN at different fixed bandwidth levels in the North American digital hierarchy. In Europe, SONET's counterpart is known as SDH.

SONET is not only deployed in long-haul. fiber-based backbone routes used by interexchange carriers (IXCs) and other carriers. It is also used in metropolitan fiber rings deployed by regional Bell operating companies (RBOCs) and competitive local exchange carriers (CLECs), in interoffice routes between LEC central offices, and in long-haul transoceanic fiber routes. DWDM opportunities in the marketplace map closely to the areas of the network in which SONET is now deployed.

LARGER ROLE

DWDM's initial success over SONET solutions was largely based on economics. SONET, which was designed and optimized for voice, is expensive and inefficient for data-centric applications, does not scale well and is costly to upgrade. Increasing the capacity of an OC-12 metro ring, for example, generally means a forklift upgrade for all of the add/drop multiplexers on the network. It is not simply a matter of changing circuit cards. Each of the devices can cost $50,000 to $70,000 or more, and six to eight may be required. These cumbersome upgrades also cost carriers time and revenue because making the new network operational can take months.

New DWDM solutions aimed at SONET replacement address these limitations and, at the same time, seek to retain the restoration capabilities available via SONET's multiple-ring redundancy. Other network-critical functions that SONET provides include network and traffic management, circuit provisioning, bridging between electrical and optical signals, and basic channel multiplexing and demultiplexing.

Although DWDM was introduced to allow IXCs to cope with fiber exhaust in their backbone networks, it has since grown to assume a much larger role and mindshare in the thinking of network planners and managers. DWDM technology has opened the door to a new approach for building network infrastructure using packet-over-fiber technology.

Starting with packet-over-SONET the technology has evolved to packet-over-DWDM and to IP-over-DWDM, or IP-over-glass. By eliminating the role of the ATM and SONET layers in the innermost core of the network, "new wave" IP carriers such as Williams and Level 3 are creating innovative, low-cost network design alternatives, allowing them to provide more competitive network services to businesses and other customers.

The advent of erbium-doped fiber amplifiers as part of a total DWDM solution was an important development, since they are far less expensive to deploy than the regenerators used in SONET systems. Other' related optical devices making important contributions toward the migration to all-optical networks include optical add/ drop multiplexers (OADM) and optical cross-connect systems.

The OADM is similar in function to a SONET add/drop multiplexer and allows the optical signal to be split such that any number of separate channels can either be dropped from or added to a multiplexed signal. Some systems are standalone devices, and others are functionally integrated into other products, such as optical amplifiers.

Optical cross-connects perform essentially the same functions as nonoptical cross-connects or DACS (digital access cross-connect) systems. These functions include grooming and connecting large pipes in the carrier's backbone network. A number of startup companies are working on the development of next-generation cross-connect systems that can replace the costly add/ drop multiplexers associated with SONET, as well as traditional DACS equipment. In addition to being far less expensive than both types of traditional devices, these new systems offer a vastly reduced footprint in the central office environment, where space is increasingly at a premium.

OPTICAL MIGRATION

As DWDM evolves, it will become more "intelligent," changing from a simple protocol-transparent transmission technique into one, which, like SONET adds important secondary channel information that can be used for traffic management and provisioning. With higher sophistication and intelligence, WDM systems will increasingly be used in applications beyond simple point-to-point configurations, finding their way into mesh and ring networks.

Mesh networks will eventually free optical technology from dependence on SONET for restoration. They will do this by enabling more complex intelligent routing to take place within the mesh topology, allowing a failed link to be easily bypassed. This routing will supplant SONET's current means of restoration, which is based on a resource-intensive dual-ring concept.

These trends, as separate and distinct as they seem now, will eventually congeal in a recognizable major shift in the communications industry: the migration toward an all-optical network. This migration has already begun and will take place over the next five to 10 years.

In this all-optical world, what will happen to SONET? That, of course, is a key question for service providers and equipment suppliers alike. In general, IDC expects that SONET as a technology will remain in place for many years to come. Eventually, however, as backbones migrate to higher capacity levels, the purer optical portion of the network will be supplanted by DWDM. SONET "big iron"--the add/ drop multiplexers that pose scalability problems for data-centric networks--will be replaced by next-generation optical systems.

SONET as an interface and transmission technology, however, will live on in next-generation routers and even state-of-the-art optical devices being delivered over the next few years, until such time as its full functionality can be duplicated in the optical realm. In addition, SONET equipment will continue to be necessary at the lower OC capacity levels to serve as the interface and conversion point between TDM and optical transmission.

Next month: Major players vie for optical crown.

Edwards manages communications and network consulting for IDC, a global IT market research and consulting firm headquartered in Framingham, MA.