How to future-proof a hybrid fiber/coaxial-cable network
Network planners at telephone and cable-TV companies have different issues to solve as they build or upgrade their network infrastructures to deliver broadcast video, interactive digital video and voice/data services
adc telecommunications inc.
The key trend in the communications industry is the use of broadband or hybrid fiber/coaxial-cable networks rather than switched digital video/digital-loo¥carrier links, which are familiar to cable-TV operators and telephone companies, respectively. Although each method has distinctive benefits from a technology perspective, the most compelling factor concerns near-term economics.
Hybrid fiber/coaxial-cable networks are powerful and flexible tools that enable network planners and service providers to profitably deliver today`s telephony and video entertainment services while offering flexible migration to future service possibilities.
This perspective poses differentiating design issues for network planners so they will not have to install additional fiber in the future as increased interactive service usage drives fiber serving areas smaller and smaller.
The design issues of fiber counts, active vs. passive devices, network powering and optical node sizing are some factors that confront network planners as they study the deployment of hybrid fiber/coaxial-cable networks. By working closely with service providers and equipment vendors, planners can intelligently and comprehensively structure their needs.
The starting point
The logical place to begin the network planning process is by looking at fiber counts and the transmission equipment needed to deliver video, voice and data services from the central office or headend to the customer. If plans call for sharing the same lasers for video and voice/data, one fiber is required for downstream transmission (to the customer) and one fiber for upstream transmission (to the central office or headend).
In some networks, planners may choose not to combine video, voice and data services over one laser because of its prohibitive cost. Or, in some network configurations, video transmission systems may not be located in the same facilities as voice and data systems. Therefore, an additional downstream fiber is needed to separate digital and analog services. Less expensive Fabry-Perot lasers can be used for data transmission if services are separated.
Availability of lifeline services, such as 911 emergency telephone service, requires a high level of reliability. Therefore, the electronics portion of the network should include a diverse routing scheme, and a redundant module should provide protection. For this type of network architecture, two fibers must be added in each direction--for a total of five fibers. Planners may want to consider adding a spare, or dark, fiber for future use.
Two choices are available for transmitting analog or combined services--distributed feedback or yttrium-aluminum garnet lasers at either 1310 or 1550 nanometers. For simplicity and consistency, network planners prefer distributed-feedback lasers.
Because hybrid networks convert optical signals to electrical signals, network planners must specify the coaxial cable based on the number of homes passed in the serving area and the number of active components deployed in the network.
Running fiber as close to a home or business as possible improves overall network performance. Decreasing the number of radio-frequency amplifiers, which are required approximately every 500 to 1000 feet to boost signal strength in the coaxial-cable plant, greatly enhances the quality of analog video services. Voice and data services, which also demand high reliability, benefit as well.
However, some obstacles arise when fiber is run dee¥into the neighborhood. No matter how close to the home fiber comes, for instance, the network still needs to be powered to maintain service, even when commercial power goes out. The high signal quality and reliability that result from the addition of optical receiving equipment often encourage fiber-dee¥networks. Still, as the last link to the customer, coaxial cable remains more cost-effective.
A hybrid fiber/coaxial-cable architecture provides a practical solution at low cost, allows the network to carry all the services offered and delivers the necessary power to operate all the active devices on the same conductor. With a hybrid system, both the radio-frequency signals and the alternating-current voltage can be delivered via the coaxial cable from the optical node to the home or business.
The economic impact of upgrading the hybrid network is minimized, permitting planners to run fiber as close to the customer as possible within today`s budget parameters. More important, service providers can profitably deliver today`s broadband services and be ensured of easy migration to future services by matching network upgrade costs with generated revenues.
Key design issues
Eventually, network planners have to face the prospect of running fiber even closer to the customer as their networks evolve. At that time, several network design issues need evaluation. Underlying each issue is how many homes will be passed per optical node or how close to the customer can fiber be run economically? The configurations are typically 2000, 500 and 125 homes passed per optical node.
In an active plant configuration, every additional cascaded device adds noise and distortion to the services being delivered. As a result, planners need to be familiar with such performance specifications as carrier-to-noise ratio, composite second-order and composite triple-beat characteristics imposed by the last active device in the distribution network.
The impact of an electronic failure on network reliability must also be considered. If the network must power such active devices as optical nodes and amplifiers, battery backu¥might be an adequate solution in the event of a commercial power failure. If so, the battery-provided power has to maintain and monitor that portion of the network. Otherwise, other powering options must be explored.
The number of cascaded amplifiers is determined by the desired performance characteristics and the service reliability needs. For example, if the network will initially carry only analog video, an optical node size of 2000 or more homes passed with five amplifiers in cascade is the likely choice.
There is still a question as to which distributed powering scheme is better--centralized or a distributed powering. The standard cable-TV approach is to use pole- or pedestal-mounted power supplies. These supplies convert the utility 110V AC to 60V AC quasi-square-wave output and add that output to the RF signals onto the coaxial-cable plant.
If the commercial power fails, deep-cycle batteries are used to power the network by inverting the voltage to 60V AC. An alternative powering backu¥is to use gas-powered generators collocated at the power supply. It is imperative to determine how many hours of backu¥power are needed for the services being delivered. For example, to ensure adequate lifeline-type services, such as 911 telephony, a minimum eight hours of standby power is needed.
Another important issue involves the amount of upstream bandwidth needed for interactive services. Most of the network equipment expected to be available in the near future should modulate from one to three bits of information per hertz. The calculation of take-rates (the percentage of homes purchasing services to the total homes passed), along with the number of bits of information required for service delivery, should determine how much bandwidth is required in the upstream portion of the network. For example, a 500-home node with three modulated bits per hert¥will have enough bandwidth in the upstream path for two telephony lines per home. Downstream bandwidth should not be a factor because of the large amount available in the hybrid network.
Another decision concerns service offerings--video first, telephony first, or both. If network deployment plans call for passing 2000 or 500 homes per optical node, the active plant question has been answered. Five amplifiers should be deployed in cascade for the 2000-home node, and two or three amplifiers in cascade for the 500-home node.
125-home optical nodes
For network planners dealing with fiber-rich networks with a node size of 125 homes, a passive, or non-amplified, distribution system may be deployed. The need to power the remote telephony/data electronics, however, would remain an issue. A passive network provides high reliability, requires less maintenance and possesses greater bandwidth availability, all contributing to less need for upgrading.
Optical node sizes markedly affect the services provided. For example, a 2000-passed-home node best serves cable TV, video only or out-of-region low penetration video (such as telephone companies that provide cable-TV-type services). In this scenario, 96 is the ideal number of fibers to install at initial deployment. This number provides a fully protected network with room to upgrade the network to 125-home optical nodes.
A 500-passed-home node suits telephone and cable-TV providers involved in high interactive video take-rates. In this case, 24 fibers provide a fully protected and upgradable network.
A 125-passed-home node is ideal for service providers that have high telephony take-rates or providers that prefer a passive coaxial-cable network. Six fibers are required in this setu¥to accommodate the serving area with a fully protected network.
Service providers that choose 2000- or 500-home optical node sizes need to consider the future potential for upgrading to smaller nodes. Some key issues surrounding future upgradability are architecture, power distribution and amplifiers.
If voice and data services are added to a video-first application with a 2000-home node with centralized power, there may be enough upstream bandwidth for a 10% to 15% take-rate. However, once the upstream bandwidth capability is exceeded, network planners must consider either upgrading to smaller node sizes or adding additional bandwidth. These approaches are virtually impossible to take in the coaxial-cable plant without sacrificing downstream bandwidth, as the same conductor is used for both upstream and downstream transport. However, once electrical-to-optical conversion is performed at the optical node, two fibers with unlimited bandwidth become available. Frequency-shifting technology takes advantage of this situation. u
Rich Henkemeyer is global market manager in the access platforms systems division at ADC Telecommunications Inc. in Minneapolis.