Networking technologies compete for broadband services delivery in England
Dave Wilson
Emerging telecommunications technologies, multimedia services and legislative rulings are influencing the future deployment of fiber-optic networks to businesses and homes in the United Kingdom.
Perhaps the most publicized catalyst, though, centers on the high bandwidth needed to deliver a wide variety of video, entertainment, shopping and educational services.
Considering the lucrative market potential, alternative copper technologies have begun to ally with or compete against the installation of fiber-optic networks for delivering future multimedia services to homes and businesses.
British telephone and cable-TV companies are therefore crystallizing their networking strategies to compete and gain market share, but the telcos are being limited by government decisions.
Government restrictions
Recent government policies have mandated that the telephone companies, such as British Telecommunications, do not have sole rights to provide plain old telephone service; the cable-TV companies can also compete. One cable-TV vendor, Yorkshire Cable, already serves more than 4000 telephony customers in Sheffield.
Legislation also prohibits telephone companies from providing entertainment services, whereas the cable-TV companies can furnish these services. Consequently, the implementation potential of any fiber network scenario has to factor that status into the cost equation. The technical advantages and disadvantages of any fiber network approach might not be sufficient to determine deployment.
Add to this scenario the impact of new digital compression technologies that lessen the burden on the bandwidth needed for video delivery to the home. Mix in the several competing transmission technologies for broadband service delivery and the result is a complex and intriguing telecommunications market landscape that is unfolding in the United Kingdom.
Hybrid fiber/coaxial cable
The most popular option calls for a hybrid fiber/coaxial-cable network alongside the existing twisted-pair copper network and installed within a common duct. Both network technologies involve worldwide implemented methodologies that suit the analog television broadcast market.
The conventional hybrid fiber/coaxial-cable network that cable-TV companies are presently deploying is based on an analog modulation scheme. Worldwide acceptance of equipment that supports this analog methodology has been driven by the fact that most television sets sold must be cable ready. In network operation, signals are fed from analog modulators to lasers at the headend and then through a point-to-point fiber feed to an optical amplifier at an active node. From the optical receiver, a series of line amplifiers boost the signal for short-distance delivery, allowing a cable-TV operator to pass around 2000 homes.
Because cable-TV companies in the United Kingdom are allowed to supply telephony services, they are building narrowband delivery systems on hybrid fiber/coaxial-cable networks that almost mirror the networks installed by British Telecom.
In the future, hybrid fiber/coaxial-cable networks will be upgraded to support digital television. This upgrade will be achieved by adding a digital modulation scheme that will work alongside the conventional analog network. At the receiver end of the network, the user will require a set-top box to decode the digital signals. Each digital modulator will support 20- to 30-megabit-per-second transmissions--translating to 10 digital channels for every analog channel supported.
Such an upgraded system would provide a 1-gigahertz capability that could be divided between the upstream and downstream directions. It could be used for integrated narrowband and broadband deliveries. Furthermore, the system would take advantage of the large market for equipment generated by cable-TV network upgrades.
Detractors to hybrid fiber/coaxial-cable networks, however, state that although these networks are well-suited to television-set delivery, overall costs could escalate with the addition of more services. According to British Telecom, upgrading beyond 1 GHz is difficult, and such systems have a short optical reach. Furthermore, these networks are difficult to deploy in less populated areas.
Speaking at a recent Institute of Electrical Engineers seminar on Practical Developments in Optical Fiber Technology, David Payne at BT Labs adds that hybrid fiber/coaxial-cable networks require the correct installation of splitters and equalizers throughout the network. He contends that these networks also prove difficult to achieve optimal usage of plant; for example, positioning the equipment cabinets becomes an arduous task in network design.
Fiber oriented
Two other fiber-oriented technology options have gained increased favor among network providers. In the first option, telecommunications providers can supply a fiber-to-the-curb architecture and use existing copper drops for telephony and either enhanced asymmetrical digital subscriber line technology over copper or fiber to the home for broadcast services.
Although FTTC avoids disturbing plain-old-telephone-service-only customers and reuses the copper drop, it must be competitive on service price, capability and quality. Fiber to the curb is ranked as a future-proof network; it is perceived as meeting the service demands predicted for availability during the next decade.
In the second method, fiber can be routed directly to the home. Here, though, a new fiber platform would need to be built to carry all the expected multimedia services.
Passive optical network
One broadband fiber-to-the-home alternative would use a passive optical network architecture. This approach would use switching centers connected via fiber-optic cable, which, in turn, would connect to an optical amplifier located in a local exchange. Fiber could then be used to route signals to a cabinet for splitting. The signals would then be transmitted to a local distribution center for delivery over hybrid fiber/coaxial cable, radio links or fiber.
Such a passive optical network would accommodate service upgrades easily, provide service scalability and use the existing duct network. Moreover, it would suit bidirectional services. Because it has long-distance and multiple splitting capabilities, the passive optical network could prove economical for rural area installations, an acute deployment problem for cable-TV vendors.
Asymmetrical digital technology
A technology option for telephone companies is to enhance their existing copper networks by using asymmetrical digital subscriber line technology. This approach provides 2- to 6-Mbit/sec downstream trans mission capabilities.
Embracing copper-wire technology, ADSL has been proposed by many telecommunications providers to transmit such services as interactive digital television. It provides a 2- to 6-Mbit/sec unidirectional link over as long as 5 kilometers of twisted-pair copper with an upstream capability of 600 kilobits per second. An enhanced ADSL can support 10 to 50 Mbits/sec over 200 meters to 1 km of twisted-pair copper and an upstream capacity of 2 Mbits/sec.
Major ADSL market issues need to be resolved, however, such as adequate volumes to drive down costs and acceptable channel capacities. Another obstacle concerns electromagnetic interference, which could limit copper deployment capability. q
Dave Wilson writes from London.