Utilities and Cable: An Evolving Model for the Future
By John Dahlquist, Harmonic Lightwaves Inc.
The convergence of computer, television and telephone offers many new opportunities to companies that traditionally provided only one service, such as utility companies. New interactive services, such as video-on-demand and Internet access are in high demand, and utility companies have a unique advantage in this growing market. With their miles of “dark” or unused fiber-optic cable and substantial customer support infrastructures already in place, utility companies can successfully participate in an increasingly expanding communications market.
As utility companies investigate and pursue new opportunities, they may need to prepare their networks to provide these services. Upgrading an infrastructure in order to deliver the advanced services of tomorrow require strategic decision making to ensure that each part of the network is contributing to the whole. Once the services to be provided are determined, engineering for quality, reliability and cost are the three most important criteria for a successful network. Transmission devices and methods, forward and return path strategies and network management all contribute to this success.
Fiber-optic transmission systems are emerging as the key transport technology in broadband communications networks for future advanced services. Compared to traditional coaxial cable networks, fiber-optic technology provides higher bandwidth for information transmission, superior quality and reliability, and lower maintenance costs.
Hybrid Fiber-Coax Network
However, because coaxial cable is a viable resource for some network needs, especially for the last mile and in-home wiring needs, operators are using a combination of the two, creating a hybrid fiber-coax (HFC) network. The advantages of a hybrid architecture of fiber and coax are many. HFC offers robustness: 1 GHz bandwidth and the low cost of coax, combined with the low transmission loss, high performance and reliability of fiber. In addition, while standard fiber optic telecommunication systems cannot transmit analog signals, linear optical systems can transmit digital signals. Thus, HFC networks have the versatility of transporting either analog or digital communications, or both simultaneously, such as 80 channels of analog TV programming, with more than 500 compressed digital video channels and ultra high-speed data at 10 Mbps and higher to each subscriber. And HFC is ideal for providing high-performance, two-way communication to meet the needs of an emerging class of broadband interactive applications such as video-on-demand, Internet access and telecommuting.
The fiber-optics technology used in cable television architectures is substantially different from standard telephony and data communications fiber-optic transmission technology. In standard telecommunication fiber optic networks, lasers convert an electrical “pulse,” or on/off signal, into light pulses to convey digital signals. Radio frequency (RF) analog signals are waves with varying degrees of intensity, rather than a series of on/off pulses. To use a familiar metaphor, a typical light switch offers two discreet choices — on or off, like a digital signal. Some fancier “dimmer” switches offer both on and off, as well as a variety of light intensities in between, similar to RF analog signals.
In an HFC network, RF signals are converted into optical signals at the headend, where the network operator receives satellite and broadcast video signals and processes them for transmission through the network. The optical signals, modulated to transport information, are then transmitted through fiber-optic cables to optical nodes throughout the network. At each node or service area, typically designed to serve as many as 500 homes down to as few as 100 homes, the optical signal is converted back into RF signals. These signals are then transmitted over existing coaxial cable for local distribution to individual subscriber homes.
Because fiber can transport a signal much farther than can coax (typically 15 miles versus less than one-half mile for copper coax before re-amplification), networks can cover much larger geographical areas than traditional coax networks. Fewer amplifiers are needed in the overall architecture of the network, thus increasing system reliability and signal quality.
Optical Transmission Equipment
Network planners have the choice of two different types of optical transmission equipment, externally modulated 1,550 nm wavelength transmitters and optical amplifiers and directly modulated 1,310 nm optical transmitters. Today, there are no commercially available 1,310 nm optical amplifiers.
Generally, 1,550 nm transmission is used to transmit signals over long distances (over 40 km), while 1,310 nm transmitters are ideal for shorter distances of between 10 and 40 km. There are two basic reasons for this. First, the fiber`s attenuation to signal propagation in the 1,550 nm window is 0.25 dB per 1 km compared to 0.35 dB per 1 km for the 1,310 nm window (a 40 percent advantage for 1,550 nm transmission).
Second, optical amplifiers are available for the 1,550 nm wavelength enabling re-amplification of a weak signal directly in the optical domain. To re-amplify a week signal in the 1,310 nm window, the optical signal would have to be converted back to RF, then amplified and injected back into a 1,310 nm transmitter.
The optical to RF to optical conversion process will degrade the quality of signals being transmitted, and therefore, will not provide the high level of picture quality that can be achieved with 1,550 nm wavelength transmission over long distances and/or when a large amount of signal splitting is called for in the network design.
However, a 1,550 nm transmission system also has disadvantages. Transmitters for a 1,550 nm based system are more expensive than for 1,310 nm systems (about $50,000 for a 1,550 nm transmitter and optical amplifier compared to about $5,000- $15,000 for a 1,310 nm transmitter). In addition, 1,550 nm transmission systems provide some limitations. Because information sent over 1,550 nm transmitters remains in the light domain, services cannot be added or dropped throughout the network, a process requiring conversion to RF signals. Using a 1,310 nm transmitter will necessitate the conversion to RF, thereby enabling services such as video or data narrowcasting to be added or dropped throughout the network.
In many networks a combination transmission system is the best solution. This type of system is one in which both 1,550 nm and 1,310 nm transmitters are used together to maximize the benefits of each. For example, operators can utilize 1,550 nm transmitters to transport video and data over long distances, from the headend to the hub for example, and use 1,310 nm transmitters to transport the services to the node or 500 home service areas. This method allows operators to provide a cost-effective solution and enables the flexibility to add or drop additional services at the local hubs.
In a fiber-optic system the biggest weakness is the fiber itself, as the RF/opto electronics are extremely reliable. Fibers can be accidentally damaged or cut by natural disasters, construction activities or many other causes. Because the forward and return paths are typically placed in the same sheating, it is likely that damage to fibers will affect signals to and from subscribers. Therefore, it is important to utilize alternate routing on long fiber routes and to have intelligent receivers that can detect signal failure and automatically switch to backup routing. This type of architecture maintains service to subscribers, and through the element management system, reports the nature and location of the system fault.
The return path as a separate component has some unique characteristics that should also be considered. The return path enables interactive services, and although the bandwidth requirement is not nearly as high as for the forward path, it is important that the signal not be interrupted. In redundant architectures, some return path receivers can be designated as “primary” and “backup” so that transmission is automatically and instantaneously routed through the optimal unit if there is a system failure, thus working in a similar fashion to the forward system. In addition, operators can also set the unit to a specific level at which they want switching from primary to backup route to occur. This feature deals with system faults that are not hard system failures.
The decision to deploy a system architecture based on 1,310 nm, 1,550 nm or a combination of these technologies requires a thorough analysis of the service goals for the network, geography considerations, performance specifications and operation strategy to determine the right choice.
The network management system is a high-level monitoring and managing system that, at its fullest implementation, can efficiently oversee the entire operation: the transmission network, addressable subscriber premise equipment, customer service, operations, etc. The network management system communicates to specialized element managers via a standard networking protocol called simple network management protocol (SNMP). The element management systems (EMS) are tailored to monitor and control specific types of equipment and/or computer-based services. The EMS systems communicate to their respective network elements continuously to maintain real-time knowledge network performance to immediately identify faults and to provide remote control of various aspects of the equipment.
The network operator is typically provided a complete view of both the physical and logical views of the network. This dual view approach allows the operator to simultaneously see all possible problems in the network from one location. The network management system enables the operator to efficiently and cost-effectively operate and maintain an advanced communications network.
These are just a sampling of the many issues to consider when preparing a network to deliver the advanced, interactive services of today and tomorrow. System operators have many choices that depend on the services offered, the extent of reliability, quality required, and the topology and demographics of the service area.
John Dahlquist is Harmonic Lightwaves Inc.`s marketing vice president in Sunnyvale, Calif. Dahlquist has more than 25 years of worldwide experience in the cable television industry. He holds a bachelor`s of science degree in electrical engineering and a master`s degree in business administration from Drexel University.