Hybrid fiber/coaxial-cable networks pass telephony reliability standards

Hybrid fiber/coaxial-cable networks pass telephony reliability standards


Industry observers previously suggested that a local telecommunications network using hybrid fiber/coaxial-cable technology might not be able to meet Bell Communications Research network availability standards. These viewpoints appear to be changing because of increased use of route diversity and component redundancy, component-level advances and improved craft practices.

Meeting Bellcore standards, which limit network downtime to no more than 53 minutes per customer, per year, is difficult. However, there is growing optimism among industry watchers that those specifications can be met by properly designed and engineered hybrid fiber/coaxial-cable networks.

Technologists at Rogers Communications, in Don Mills, Ontario, Canada, which owns the largest Canadian cable-TV operation, were discussing hybrid fiber networks` shortcomings in 1994. Using a model developed by Louisville, CO-based Cable Television Laboratories Inc.--the research and development organization for North American cable-TV companies--Nick Hamilton-Piercy, engineering and technical services vice president, and Robb Balsdon, network planning director, compared a variety of network topologies and suggested theoretical network availabilities ranging from 99.94% to 99.98%.

That study suggested that an optical ring network supporting a non-redundant node with no more than three radio-frequency amplifiers in series had the best chance of reaching telephony-grade availability. However, Hamilton-Piercy and Balsdon did not, as part of that analysis, evaluate the availability of a network using a path-diverse, component-redundant node.

Reliability issue

Further experience intimates that concerns about hybrid fiber/coaxial-cable network reliability over telephony networks can be dispelled. This point of view--which is generally advanced by technologists looking at cable-TV legacy networks--suggests that each optical transceiver (also known as a node or optical network unit) must be fed by a route-diverse optical cable.

According to another school of thought, reliability can be improved to such an extent that optical transceiver redundancy is unnecessary. This point of view tends to be taken by local telephone company technologists who have a "green field" or "new build" construction opportunity. The difference is the ability to build the entire network using more-advanced monitoring capabilities, components and more-stringent construction techniques.

David Large, principal with Media Connections Group, a San Francisco-based consulting company, says that 99.99% availability (no more than 53 minutes of network outage per customer, per year) is theoretically possible for a video-only hybrid fiber/coaxial-cable network where an optical transceiver serves 500 homes.

The reference model assumes the use of one power supply for the area, four coaxial-cable distribution cables, three radio-frequency amplifiers maximum between any customer location and the optical transceiver, 100 homes-per-mile density and 160 four-port customer taps in the serving area.

Engineers at Scientific-Atlanta Inc., in Norcross, GA, also contend that telephony-capable hybrid fiber/coaxial-cable networks will probably require redundant optical fiber routes and greater attention to amplifier reliability.

According to Farr Farhan and Lee Thompson, Scientific-Atlanta engineers, a hybrid fiber/coaxial-cable network with three amplifiers in series and supporting voice transmission might be expected to achieve network availability of 99.96%. This figure includes estimated reliability of customer interface units and a headend switch interface. Adding a path-diverse and redundant optical cable to the network design may boost availability to 99.98%.

Use of radio-frequency amplifiers

To get performance to the 99.99% standard, the final requirement is the use of radio-frequency amplifiers with failure modes in the 0.3%-per-year range, instead of devices with a predicted annual failure rate of 2%, which is typical for current devices.

It is possible to achieve telephony network performance that meets Bellcore`s standards using a hybrid fiber/coaxial-cable network, especially when radio-frequency active cascades are limited to six devices and use status monitoring and optical rings to interconnect hubs, say executives at Philips Broadband Networks Inc., in Manlius, NY.

Chuck Merk, Philips` engineering vice president, and Walt Srode, the company`s director of quality assurance, predict that most hybrid fiber architectures--ranging from networks with 11 amplifiers in cascade to passive networks that use no amplifiers--will meet the Bellcore specification if the optical portions of the network are redundant.

Atlanta-based Cox Communications Inc., for example, is considering this path-diverse, component-redundant approach. On the other hand, Tele-Communications Inc., in Englewood, CO, is leaning toward route diversity and component redundancy for two layers of optical rings, but no redundancy for the node locations.

Equipment failure and repair times suggest that networks using optical fiber regional rings to connect hubs, as well as networks that do not feature such redundancy, can meet telephony availability requirements as long as status monitoring is available for each radio-frequency and optical active device.

But not all technologists agree that redundant optical transceivers are necessary. In fact, neither Pacific Bell nor Southern New England Telephone plans to take this path. Pac Bell, for example, is confident of the integrity of the optical portions of its hybrid fiber/coaxial-cable network, which uses a 500-home serving area in the forward path and a 100-home serving area for return-path communications (see Lightwave, September 1995, page 1).

Approximately 80% of Pac Bell`s customers will be served by two-amplifier cascades, and almost 20% by 1-ampere cascades, says Keith Cambron, the company`s chief network architect. Furthermore, Pac Bell`s network "can evolve to a fiber-to-the-curb or fiber-to-the-home network," says Harry Bosco, AT&T Network Systems chief technology officer. To ensure that capability, Pac Bell is pulling conduit to every active device location and to each customer`s premises, says Cambron. If fiber is pulled deeper into the network, no more digging is necessary.

Time Warner engineers agree. "A purely passive coaxial bus is expensive and does not yield significant gains in reliability over a hybrid fiber/coaxial-cable design with short cascades," says Don Gall, Time Warner senior project engineer. "In fact, the difference between a passive and a near-passive network may be as little as three minutes per customer, per year," he adds.

The most significant reliability issues will concern the coaxial portion of the plant--not the optical feeder. For that reason, Pac Bell is specifying RG-6 rather than RG-59 drop cable, ordering custom connectors and connectorization tools, and incorporating automated switching into its taps. All of those measures are intended to protect the integrity of the transmission path for digital traffic and improve network availability.

Although there are varying opinions concerning optical route diversity and component redundancy in the nodes, both telephone and cable-TV companies contend that hybrid fiber/coaxial cable will be a viable and reliable method for carrying telephone traffic. q

Gary Kim is senior vice president at Probe Research Inc. in Cedar Knolls, NJ.

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