Long-distance network trends involve latest fiber-optic technologies

Dec. 1, 1996

Long-distance network trends involve latest fiber-optic technologies

Although the interexchange carriers and new long-haul entrants have different network requirements, both carrier types are turning to dispersion-shifted fiber, fiber amplifiers and multiple wavelengths to strengthen their communications architectures

curt weinstein

corning inc.

Since the early 1990s, interexchange carriers and other telecommunications carriers have aggressively deployed new types of dispersion-shifted fibers, erbium-doped fiber amplifiers (edfas) and wavelength- division multiplexing (WDM) technology throughout their networks to take advantage of higher data rates, longer "reach" (transmission distance), and multiple optical wavelengths offered by these technologies. In addition, long-haul architectures and the emerging 1550-nm transmission technology alternatives for both new network builds and upgrades represent other important trends in the telecommunications market.

The passage of the Telecommunications Act of 1996 is greatly altering the competitive landscape for both local-access and long-haul carrier applications. A number of regional Bell operating companies have disclosed plans to enter the long-haul network market for toll service within local access and transport areas and for out-of-region services.

Some Bell companies are considering partnering arrangements to provide long-distance services to specific regions. Industry analysts speculate that the early manifestations of these activities will involve these companies` entry as resellers--essentially buying or leasing capacity or dark fiber from an existing interexchange carrier, called a carrier`s carrier.

In addition, tremendous growth in the long-haul segment of the telephony market is being fueled by the emergence of new carriers, capacity expansions in existing networks and new network builds. A key indicator of this growth is the North American deployment of cabled optical fiber (see Fig.1). This deployment is projected to surpass the growth of the original large interexchange carrier network builds of the mid-1980s. Growth in this sector of the industry had leveled off for several years, but has surged again as new entrants appraise the profitable telecommunications transport business. In addition to changing the players in the long-haul market, current and emerging user demand for bandwidth is changing telecommunications network architectures.

Optical tools

Boosted by an edfa, an optical signal generally can travel 120 km or more before amplification or regeneration is needed again. These amplifiers also enable the use of WDM technology to increase system capacity by simultaneously amplifying multiple wavelengths. Because optical wavelengths can be added incrementally, system designers have the flexibility to make the telecommunications network responsive to future demands by increasing capacity as needed.

To take advantage of multiple-channel WDM by using edfas, system designers must take into consideration nonlinear phenomena that might adversely affect optical network performance. The most intrusive nonlinear effect in WDM networks is four-wave mixing. When more than one wavelength is transmitted, the wavelengths can mix to produce new, unwanted wavelengths that can interfere with, or sap signal power from, the original wavelengths. This interference can degrade system performance. Other deleterious effects include self-phase modulation, cross-phase modulation, modulation instability and polarization mode dispersion (PMD).

Non-zero dispersion-shifted fiber (nz-dsf) designs, such as Corning`s smf-ls fiber, help suppress these nonlinearities. Nz-dsf designs are optimized to allow the transmission of multiple, high-bit-rate channels in the 1550-nm window--the passband region of edfas. Whereas conventional dispersion-shifted fiber is designed with zero dispersion across the 1550-nm operation range, smf-ls fiber has its zero dispersion characteristic moved above the 1530- to 1560-nm signal band. Consequently, its dispersion is a negative effect throughout the operating range. (Note that some manufacturers choose to move zero dispersion below the signal band, so that the dispersion is a positive effect.)

In the smf-ls fiber design, dispersion is kept high enough to suppress four-wave mixing effects, but low enough to allow for high-capacity, long-length systems. The slightly higher dispersion actually maintains the fiber`s WDM capabilities by suppressing the unwanted wavelengths generated by four-wave mixing.

In addition to nonlinearity effects, system designers have voiced concerns about the ramifications of PMD as the installed base of optical fiber is upgraded to OC-192 (9.6-Gbit/sec) rates. When suppliers began to specify this parameter on singlemode fiber and nz-dsf products a few years ago, much of the fiber already installed was manufactured before the industry was aware of the impact of PMD. To date, PMD field testing conducted on Corning matched-clad singlemode fibers has shown that fiber PMD should not significantly impact OC-192 performance.

Upgrading established networks

Carriers with established long-haul networks, such as interexchange carriers, are positioned to take full advantage of both high-rate time-division multiplexing and line-amplified WDM systems. The average cabled bandwidth for existing North American interexchange carrier networks is the highest in the world, by approximately 50%. Fiber counts for this application are low, and system lengths are generally long. Moreover, the installed base of fiber is predominantly SMF-type fiber. These factors are driving the deployment of 8-channel or higher OC-48 (2.4-Gbit/sec) WDM systems--a trend that began this year.

Interexchange carriers are likely to continue their deployment of multiwavelength OC-48 and single- and multichannel OC-192 systems for several years, with an emphasis on completing national Sonet networks. They are expected to use edfas and dispersion compensation to upgrade their installed base to OC-192. In this scenario, Sonet OC-192 technology coupled with dispersion-compensating modules and WDM ultimately will allow the highest aggregate bandwidth possible in established North American interexchange carrier networks.

Depending on their geographic location, interexchange carrier networks contain a mixture of short- and long-route network lengths. As a result, system designers can employ two different telecommunications network architectures (see Figs. 2 and 3), depending on the number of add/drop nodes needed.

Established interexchange carriers typically are the first to deploy new technologies such as nz-dsf and bidirectional optical line amplifiers. The bidirectional approach couples lightwaves in both directions on the same fiber. This approach includes optical line amplifiers, which integrate a four-port coupler with edfa technology. These devices are simplifying the planning and installation of multiwavelength Sonet networks.

Clearly, interexchange carriers are expecting to realize large and immediate cost savings from mass deployment of booster amplifiers and, especially, line amplifiers. These amplifiers eliminate intermediate regenerator sites and free up dark fiber for further network transmission opportunities. Both unidirectional and bidirectional amplified systems can form redundant, low-cost Sonet ring networks.

Emerging networks

Presently, the Bell companies and new-entrant interexchange carriers have the opportunity to build state-of-the-art 1550-nm networks largely from scratch. The strategy for the Bell companies to establish a regional (or even national) interexchange presence is foreseen to include

upgrading and extending the capability of existing fiber infrastructure,

building new network routes,

leasing transport capacity or dark fibers from interexchange carriers,

acquiring or partnering with other service providers.

This strategy mix will likely depend on expected traffic volumes, available capital, infrastructure costs, and the desired speed to market for offering services within local access and transport areas. For example, the planned mergers of Bell companies SBC-Pacific Telesis and Bell Atlantic-nynex already account for a significant percentage of all U.S. subscriber lines. Both new combined companies are likely to target the business of completing the high percentage of calls within local access and transport areas, which constitutes 40% to 60% of all telecommunications traffic. The Bell companies clearly want to make up for an anticipated reduction in access charges by acquiring new long-haul business customers and by providing end-to-end switched services.

Many Bell companies today have a small percentage of links that include regenerators. These regenerators are needed because of the close spacing of hubs wherever add/drop multiplexers are located; typical interoffice span lengths are 25 to 30 km or less. Although most interoffice traffic uses 1310-nm single-channel transport systems, metropolitan interoffice ring networks are expected to evolve as 1550-nm OC-48 and OC-192 systems. Both unidirectional and bidirectional multichannel amplified approaches will also be used to cost-effectively provide bandwidth over the existing installed base of singlemode fiber.

As the Bell companies develop their own large regional rings, they likely will mimic and parallel established interexchange carrier networks. Whether these companies lease out dark fiber or build new networks, the amount of traffic and the density of add/drop points throughout the network are important for determining the transport strategy. For example, the distribution of node lengths on new regional ring networks likely will call for combinations of edfa booster amplifiers and line amplifier technology. The eastern region of the United States, for example, will probably have a higher density of booster amplifiers than line amplifiers. Networks in this region will likely use standard singlemode fiber due to the density of add/drop multiplexers (see Fig. 2).

On the other hand, new builds in the western United States may contain a higher density of line amplifier sections and be ideal for use of nz-dsf to maximize the distance between lightwave terminating equipment (see Fig. 3). For those Bell companies planning to use existing singlemode fiber for OC-192 transport over longer distances, a combination of line amplifiers with dispersion compensation is figured to be the most likely alternative. Four major nationwide interexchange networks already exist in the United States.

New-build interexchange carrier networks, being installed by new market entrants, typically have lower initial and current cross-sectional bandwidth than the networks provided by established interexchange carriers. The system route lengths generally are longer, as these companies are spanning the western and southern parts of the United States and the provinces of Canada. In these networks, system designers currently use Sonet OC-48 systems and soon are expected to install OC-192 systems for transporting data between distant node sites.

Although these systems are expected to use multiple wavelengths, the wavelength count probably will not ap proach the number used in current singlemode fiber networks for some time. nz-dsf is estimated to dominate these new long-haul installations to accommodate the expected multiplexing of several simultaneous OC-48 and OC-192 systems. The re sults of recent experiments show that these fibers can transmit at least 80 Gbits/sec (eight 10-Gbit/sec channels) over 360 km.

The common thread

Even though the new entrants to the long-haul market and the existing interexchange carriers have slightly different network requirements, they are universally turning to 1550-nm technology as the solution for their networks. Across North America, network operators are installing new 1550-nm-based systems on existing standard singlemode fiber routes and building new routes with 1550-nm optimized nz-dsf. Known as the "power data transporters" in North America, established interexchange carriers are transforming their networks on a foundation of multiwavelength edfa technology. Whether for upgrades or new builds, 1550-nm technology clearly is the technological solution that will take the U.S. telecommunications industry into the twenty-first century. u

Curt Weinstein is market manager, telephone company and new markets, Telecommunications Products Div., at Corning Inc., Corning, NY.

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