AT&T advocates dual fiber modes

June 1, 1995

AT&T advocates dual fiber modes


As network planners debate the merits of fiber-to-the-curb versus hybrid fiber/coaxial-cable technologies for delivering diverse multimedia services to homes and businesses, AT&T Network Cable Systems in Phoenix, AZ, is aiming to satisfy virtually all system needs by promoting both infrastructures.

Representing the two leading service delivery systems, FTTC technology competes head-on with hybrid fiber/coaxial-cable technology. Both approaches provide network planners with the fiber-optic framework needed to deliver voice, video and data services. The two technologies differ mainly in the depth or penetration of the installed fiber-optic network to the customers.

Hybrid fiber/coaxial-cable networks, for example, connect fiber-optic cables to optical nodes that serve approximately 500 homes. In contrast, fiber-to-the-curb networks push fiber deeper into the neighborhood to optical nodes that link approximately 25 homes. The medium used in the last leg of the delivery infrastructure--optical node to an individual customer--is either coaxial cable or twisted-pair wiring.

One such FTTC infrastructure is being jointly developed with Broadband Technologies Inc., Research Triangle Park, NC. This system marries AT&T`s SLC-2000 digital subscriber loop carrier system with Broadband`s fiber loop access switching and transport technology. The combination permits telephone and cable companies to connect homes and offices with interactive multimedia services. Broadband`s technology adds switched video, or broadband, capability to AT&T`s access system, which is, traditionally, a narrowband transmission service for voice and data. The resulting product can be deployed in a central office or in a neighborhood optical node or cabinet.

No commercial rollout date of the fiber-to-the-curb SLC-2000 access system with FLX SDV technology has been set. However, AT&T and Broadband have agreed to jointly develop and market the system.

Although AT&T has a financial interest in seeing its fiber-to-the-curb technology deployed, it also provides the alternative hybrid fiber/coaxial-cable technology, known as the HFC-2000 broadband access system. In this manner, the company is able to adapt readily to market trends, demands and applications, and play a leadership role in both technologies.

The combined AT&T/Broadband system can deliver 51.84-megabit-per-second asynchronous transfer mode-type delivery services to the home. These services permit a host of initial deployment options, including telephone to switched digital services and high-speed data services; 16-carrierless amplitude-modulation, phase-modulation technology for the transmission of video services over coaxial cable or twisted-pair wiring used in existing telephone lines; private and secure information to suppliers and service providers through switched, point-to-point digital services with delivery of only one signal per subscriber; and two-way, bandwidth provisions for future customer interactive services.

To date, however, just a few field trials are developing this fiber-to-the-curb technology. In one trial, AT&T is partnering with SBC Communications Inc. in Richardson, TX, to link 10,000 homes. The trial is scheduled to begin in January 1996, according to Brian Schoenherr, AT&T technical manager for consumer broadband applications.

When activated, the trial network is expected to deliver a mix of video services. Some sites will receive analog video from a cable-TV headend, while other sites will be served digital video using 16-carrierless amplitude-modulation, phase-modulation technology.

Technology toss-up

Whereas several companies have expressed interest in the AT&T/Broadband approach, no provider has committed to deploy the fiber-to-the-curb architecture, says Schoenherr. On the other hand, several cable-TV providers and telephone companies have opted for the hybrid fiber/coaxial-cable approach. Both technologies have received support because the methodologies have been proven, numerous companies are interested and the available equipment accommodates popular network standards.

According to Andy Paff, executive vice president at Antec Corp., which designs, builds and integrates hybrid fiber/coaxial-cable networks, "Once fiber-to-the-curb technology is installed, the system bandwidth is defined for some time. What fiber to the curb comes down to is making bets in 1995 that may or may not play out during the next 20 years."

Donald Crowe, technical manager at AT&T, says that while hybrid fiber/coaxial-cable and fiber-to-the-curb technologies differ in structure, both are capable of delivering identical services. He adds, "Neither architecture inhibits what kind of services can be deployed over the network."

Frank Dzubeck, president at Communications Network Architects, a telecommunications consultancy based in Washington, contends there are several advantages for deploying an all-digital versus hybrid network, including manageability and flexibility.

However, the jury is still out on the cost of deploying a fiber-to-the-curb network. Initial industry estimates suggest fiber-to-the-curb networks should cost approximately $800 to $1500 per home versus $150 per home for hybrid fiber/coaxial-cable networks. Of course, as more systems are deployed, costs are expected to decline.

One determining network consideration involves the delivery of video-on-demand services. These services call for digital-signal compression techniques that allow the transmission of six to eight digital signals in the same bandwidth as needed for one analog signal. However, the user world is not ready for all-digital service delivery. For example, the installed base of almost 200 million analog television sets should resist replacement for at least a decade.

In practice, the embedded architecture must be evaluated to help determine which network technology to deploy. Cable-TV providers, for example, employ mixed fiber and coaxial-cable networks, whereas the telephone companies prefer fiber and twisted-pair wiring. Moreover, the telephone companies work in a switched environment.

Still other considerations will steer companies to either hybrid fiber/coaxial cable or fiber to the curb, suggests Schoenherr. One determining factor involves a company`s philosophy on how deep it wants to drive fiber into the network. The more homes that share the optical node--such as 500 homes for the hybrid fiber/coaxial-cable approach--the greater the operational savings.

Another factor deals with the delivery of interactive video and upstream bandwidth, such as using asynchronous transfer mode technology, to the home. "A major issue we run into is that no one has a crystal ball that will tell what services customers want and when," declares Schoenherr. Therefore, system scalability is critical. He claims AT&T is trying to tailor the services to meet a variety of applications based on customer needs. Schoenherr adds, "All solutions start with telephony and analog video and migrate to digital video. Flexibility is critical." n

Lynn Haber is a freelance writer based in Boston.

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