Fiber transport networks key to broadband service growth

Fiber transport networks key to broadband service growth

Fiber has opened the door to new services. But the bandwidth demands some of these services will present could test transport networks.

Donald T. Gall, Mitch Shapiro

Among the pivotal roles played by optical technology in the evolution of the cable industry has been its use as a transport medium. The deployment of optical transport networks has been an essential ingredient in the industry`s transformation from a checkerboard of small-town cable-TV operations to today`s large multiple system operators (msos), which are preparing to offer a full range of video, voice, and data services to highly clustered metropolitan markets.

Transport networks played an important role even in the early days of cable, when its main business was to deliver broadcast TV signals to towns that could not receive them over the air. These early analog transport systems fell into three categories:

Coaxial-cable-based systems carried at

relatively low frequencies--from 7.0 to 43.0 MHz. While these allowed cable systems within 10 to 15 mi of each other to interconnect, they were not redundant and operated at the whim of the weather, power outages, and the unreliable electronics of the day.

Frequency-modulated (FM) video mi-

crowave systems were leased through a common carrier. Though they typically transported single channels imported from a major television market, some private FM microwave systems multiplexed up to 24 video channels.

Amplitude modulated link (aml) mi-

crowave, which emerged in the late 1960s, was the first system to allow cable operators to extend their reach economically between multiple communities within a greater than 20-mi radius. aml was relatively reliable (as long as it did not rain more than 1 inch per hour) and was used extensively across the United States and abroad.

During the 1970s, satellites emerged as the key transport vehicle for nationwide distribution of new networks such as hbo and cnn. This changed the industry`s fundamental economics and helped trigger a long-term evolution from local mom-and- pop organizations to consolidated system "clusters" located in major metropolitan markets and owned by large msos.

As these clusters grew, cable operators found themselves attracted to new business opportunities that required the ability to reliably and cost-effectively target services to increasingly smaller market segments. Unfortunately, the transport network technologies the industry had been relying on were not well suited for extensive deployment of such "narrowcast" services. Were it not for developments in fiber-optic technology, the cable industry may never have been able to make the transition from its one-way broadcast mode of operation to today`s two-way networks, which can deliver a range of broadcast, narrowcast, one-way and two-way video, voice, and data services.

Under pressure

When it first embraced optical technology in the 1980s, the cable industry was under growing pressure to increase channel capacity and reliability. Existing coaxial-cable networks lost operating headroom with every additional channel. Efforts to improve customer service and the advent of improved electronics helped incrementally, but the industry was losing ground as it confronted the fundamental limitations of its all-coaxial networks. Though optical costs were initially higher than many operators would have liked, it was clear that costs were heading down a steep curve and that the technology`s low losses and large bandwidth capability were the industry`s key to future growth. Fiber also enabled the reliable use of the reverse frequency band, which opened the potential for a truly interactive multimedia network.

One of the earliest uses of fiber optics for video transport was to provide interconnection for cable`s advertising sales business, which had long suffered from fragmentation and anemic pricing relative to its broadcast counterpart. In such applications, video programming is usually received via satellite and routed to local commercial insertion equipment, which strips the national commercial out of the video channel and inserts a new commercial with local content. The repackaged channel is digitized and transported via fiber to distribution sites, each of which serves a local advertising zone. By marrying fiber`s huge capacity and a new generation of digital insertion gear, cable systems are today finally beginning to realize their potential as a vehicle for targeted video messages.

Most optical transport systems installed in metropolitan markets during the last five years or that are on the drawing boards today use a ring architecture. The ring allows for both path and electronic diversity, which assures reliability (see figure). Fiber counts are typically between 36 and 48--even higher in markets where fiber rings also support an alternate access business. These fiber counts are expected to support all near- to medium-term growth in existing or new services without the need for wavelength-division multiplexing (wdm). Should services such as video-on-demand (vod) and high-speed data really take off in the market, however, some wdm may be needed.

Video, audio, telephone, and Internet services are processed in the headends. Each headend serves 100,000 to 140,000 homes; multiple headends are interconnected by a primary ring. These rings often employ all-digital systems that provide a higher level of signal quality and functionality than analog alternatives. Though digital systems are more expensive, their cost premium does not amount to much on a per-subscriber basis when deployed for headend interconnects. As the figure suggests, multiple secondary fiber rings are then used to distribute a mix of analog and digital services, often using lower-cost 1550-nm analog transport systems and optical amplifiers. From these distribution hubs, each of which might serve 20,000 homes, the hybrid fiber/coaxial-cable (hfc) network is deployed.

As part of their core broadcast video business, cable operators use two to four fibers to transport as many as 160 analog channels of local, satellite, government, school district, pay-per-view, and add/insertion content. The high-speed Internet business usually originates from one of the headends and is distributed using either a Synchronous Optical Network OC-1 (52-Mbit/sec) or 100Base-T (100-Mbit/sec) Ethernet link. Although these "cable modem" services can deliver much higher data rates than dial-up services, and operators hope to see them achieve healthy double-digit penetration rates, their demands on transport network capacity are likely to be constrained by their "bursty" nature.

If the business and technical environment improves, telephony may also be deployed on this same network, though the transport needs for voice service will demand only a small portion of transport network capacity.

Managing the demand

The heaviest demands for bandwidth will be driven by compressed digital video and, in particular, vod. To get an idea of the magnitude of the required capacity, let`s use 4 Mbits/sec as an average broadcast video stream. True vod requires one stream for each video watched. If we assume a prime-time peak of 25% of customers watching a movie, then a system with 250,000 subscribers would need to deliver 250 Gbits/sec across the network.

While there are ways to bring this figure down to a more manageable number, it seems clear that, in the future, the role of vod could be a critical factor in pushing the capacity limits of the cable industry`s fiber transport networks. Though vod fell out of favor after being over-hyped several years ago, interest has reawakened as long-awaited digital deployments are finally becoming real and as companies such as diva Systems Corp. (Menlo Park, CA) unveil new generations of more cost-effective vod systems.

Today cable operators are aggressively deploying optical transport networks across the country and are beginning to use them to deliver high-speed data, alternate access, and compressed digital video services. The industry`s growing appetite for fiber transport rings has been a boon to companies specializing in key enabling technologies. By 1996, the combined digital and 1550-nm transport markets accounted for more than one of every four dollars spent by the U.S. cable industry on optoelectronic gear.

Multiple system operators typically rely on a ring structure that combines analog and digital transmission systems using a fiber-optic backbone.