High-density fiber network eliminates active devices
Although all-passive fiber-network upgrades have yet to be financially proven in the U.S. cable-TV industry, Vision Cable Communications, a multiple system operator in Charlotte, NC, has taken a giant ste¥in that direction.
Last March, the company completed a 360-mile network rebuild that reduced the number of radio-frequency active devices in cascade from as many as 22 to two, including those in the optical node. This Bergen County, NJ, rebuild included enough fibers to allow Vision Cable to extend two fibers to every amplifier location in the system, eliminating all active devices between the optical node and the subscribers` homes.
According to Richard White, Vision Cable`s vice president for engineering, the company and Englewood, CO-based Antec -- which supplied equipment and engineering and technical training support for the project--examined six deployment options before they decided on Tri-star, a modification of Antec`s Star-star-bus-500 architecture.
The Tri-star network routes AT&T Accuribbon fiber from the headend to regional hubs that serve approximately 6500 homes. From there, fiber sub-trunks feed local hubs sized for 1625 homes each. In turn, the local hubs are linked by fiber feeders to neighborhood fiber nodes, which handle 430 homes each.
Eighteen fibers terminate at each fiber node, which feed six coaxial cables, each containing an amplifier. This setu¥provides enough fibers for Vision Cable to deliver two dedicated fibers to each amplifier location should they be needed to support future services.
The primary trunk routes contain as many as 216 fibers, packaged 12 fibers per ribbon--a cable design that was selected "primarily for restoration purposes," says White. He adds that each of Antec`s 58 750-megahert¥Laser Link II Plus transmitters serves an average of 3.1 nodes. Thanks in part to the system`s high density, the average optical link budget is 3 decibels, with the highest being 8 dB. As a result, explains White, the rebuilt system can deliver "near studio-quality video pictures to customers` homes," without the need for optical or radio-frequency amplification at the primary or local hub sites. The minimum end-of-line specifications for the system, he notes, are 48.8-dB carrier-to-noise ratio, 56-dB composite triple beat, 56-dB cross modulation and 58-dB composite second order.
Today, says White, the 80-cubic-foot cabinets that house the primary hubs contain a passive fiber splice and three empty racks. He adds that the latter will eventually contain equipment to support telephony and other two-way services.
The project, which took 14 months to complete, including the design phase, involved only a minuscule cost penalty relative to less fiber-rich options, explains White. This is mainly because Bergen County, a bedroom community in the New York City metropolitan area, has an average density of 220 homes per mile.
According to Antec vice president and general manager of broadband transmission systems, Mike Sparkman, who was involved in the design of the Bergen County rebuild, the cable industry has in recent years experienced a 30% annual decline in the per-mile cost of fiber and the per-link cost of opto-electronics. Nevertheless, he says, "we are not at the cost targets yet" for totally passive networks.
In a paper he presented last year at a conference sponsored by the Society of Cable Television Engineers, Sparkman compared the cost of a star-star-bus architecture serving 500 homes per node with that of a passive optical network. Excluding construction expenses, the bus architecture costs approximately $11,200 per mile; two passive optical network approaches--the less expensive of which employs optical transition nodes--in volved per-mile costs ranging from $19,800 to more than $25,300.
The Bergen County rebuild included 152 sheath miles of fiber and dense fiber bundles. According to John D`Allesandro, Antec`s senior sales vice president, his company claims the Bergen County system is "the most fiber-dense system in the cable-TV industry." With a total of 11,573 fiber miles, the system has an average fiber count of more than 76, with greater than 2600 fibers coming out of the headend.
To handle these high fiber counts, Vision Cable`s contract with Antec included the installation and customization of AT&T`s fiber management system--a computerized system that organizes, tracks and manages all the system`s fibers
Much of the Bergen County system, which passes 80,000 homes, serves upscale subscribers. It enjoys nearly 66% penetration (subscribers per number of homes passed), which is slightly above the average for the New York metropolitan area, and pay-service penetration of approximately 100%. The system`s 360 miles of cable plant includes less than five miles underground. More than one-third of the system connects to multiple dwelling units.
The rebuild increased system bandwidth to 750 megahertz, which, according to system manager Ed Rose, allowed Vision Cable to increase channel usage immediately from 54 to 65 channels and still have room for growth.
According to White, Vision Cable has been planning to use 50 to 550 MH¥for analog services and the 550- to 750-MH¥range for digital. When asked about when digital services would be deployed, White suggested the company is anticipating deliveries of digital set-to¥boxes from General Instrument, Hatboro, PA, during the first quarter of 1996. q
Mitch Shapiro is a freelance writer based in Encinitas, CA.