Fiber standards help ATM designers - includes related article on UNI fiber specifications

The letters on everyone's lips are A-T-M. But the buzz isn't over cash machines, although many are banking on asynchronous transfer mode to become the protocol of choice for the next generation of data communications networks. What makes ATM hot is its ability to integrate data, video and voice service in one physical infrastructure.

Moreover, ATM is scalable, allowing for speed upgrades without changing the network cabling plant.

But "ramping up" places formidable bandwidth demands on the network transmission medium. For this and other reasons, optical fiber is the medium of choice to support ATM network operation at 155 Mb/s and beyond.

To succeed in today's network technology market, however, ATM faces several challenges, including standards development. Fortunately, work has been undertaken by the ATM Forum to overcome these obstacles. Among other things, the ATM Forum has worked out specifications for optical fiber which offer a simple and cost-effective migration path to ATM. Still, ATM network planners should specify fiber parameters to ensure reliable network operation.

Fiber's greatest benefit to the ATM network may be its enormous information-carrying capacity, or bandwidth. With fiber, speed upgrades can be achieved simply and cost-effectively by changing electronics only. Yet fiber offers further benefits, including low attenuation, virtually error-free transmission and high reliability, which is crucial with our increasing dependence on computer networks.

Because it is dielectric, fiber is immune to electromagnetic noise and ground loops, which cause a significant percentage of the downtime on copper-based networks. Fiber also provides secure transmission because it is difficult to tap without detection.

Finally, its small size and light weight make fiber easy to install, particularly in crowded ducts.

With so many potential benefits from ATM, and with optical fiber providing a nearly perfect physical medium, why isn't ATM being implemented now?

ATM has to overcome several challenges. Costs must come down, but this typically happens quickly as volume increases. Also, ATM products will need to be compatible. providing interoperability among private and public networks. Therefore, standards must be developed.

The ATM Forum is a consortium of 445 domestic and international suppliers working to speed the development and deployment of ATM products and services. It has published the User-to-Network Interface (UNI) Specification, defining the interface between an ATM user and an ATM switch.

The UNI lists different physical medium specifications. Those for optical fiber interfaces match those of established protocols and standards, making the migration path to ATM simple and cost-effective--and allowing for possible use of today's installed base of fiber-optic cable.

System designers will ensure network success and reliability by specifying critical fiber parameters: bandwidth, attenuation and geometry. The longer the network link, the greater the attenuation, or signal loss over distance. As link distances continue to increase in public and private networks, low fiber attenuation is ever more crucial in keeping links within their loss budgets.

All standards documents are revised periodically, so system designers should check current versions when writing an RFQ.

Fiber geometry, the physical characteristics of a fiber, also is important. When two fibers are joined, consistency in cladding diameter (the outside diameter of the cladding glass) and core/clad concentricity (how well the core is centered in the cladding glass) is crucial, because most connectors use the outside surface of the fiber as a guide to align the cores.

Differences in these parameters can cause core offset, resulting in increased signal loss. Better dimensional matches help keep systems within their loss budgets.

ATM system designers can get the best return on investment in their physical infrastructure by selecting their optical fiber carefully. Moreover, specifying key fiber parameters helps ensure reliable performance throughout the life of the network.

[RELATED ARTICLE:] UNI FIBER SPECIFICATIONS

1 A 100 Mb/s multimode optical fiber data link, up to 2 km, taken from the FDDI physical medium dependent specifications: Nominal core diameter: 62.5 Cladding diameter: 125.0 [+ or -] 3.0 Modal bandwidth (1,300 nm): 500 MHz/km Attentuation for link, including component loss (1,300 nm): 11.0 dB Nominal numerical aperture: 0.275

2 A 155 Mb/s multimode optical fiber link (topology and distance requirements stated in EIA/TIA 568 Commercial Building Telecommunications Wiring Standard): Nominal core diameter: 62.5 Cladding diameter: 125.0 Modal bandwidth (1,300 nm): 500 MHz/km Fiber attenuation (1,300 nm): [is less than or equal to] 1.5 dB/km Zero dispersion wavelength range: 1,295 nm to 1,365 nm Zero dispersion slope: [is less than or equal to] 0.110 pl/[nm.sup.2]/km (Allowances are made for other fiber sizes, but these impact the maximum 2-km link length and may not be compatible with ATM equipment from different suppliers.)

3 A 55.52 Mb/s link based on Sonet specifications, and referencing documents written by ANSI and Bell Communications Research (Bellcore). For single-mode fiber, both documents refer to Bellcore's Generic Requirements for Optical Fiber and Optical Fiber Cable, commonly known as TR-20. The Bellcore document also references Generic Requirements for Intrabuilding Optical Fiber Cable, or TR-409. For multimode, the ANSI document references the FDDI specification (1, above) with fiber attentuation specified at less 1.5 dB/km at 1,300 nm.

COPYRIGHT 1995 Nelson Publishing
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