A brief introduction to cable television and the evolution to satellite distribution
Cable television originated in the United States from the simple desire to extend broadcast television to areas where off-air reception was proving a problem. Due to weak signals transmitted by distant transmitting stations, the high cost of building additional relay transmitters to reach these communities, and unfavorable geographical conditions, large pockets of potential audiences could only receive poor or no television pictures.
The solution was seen in Community Area Television (CATV) where large towers topped with high-gain antenna systems pulled in otherwise weak signals from transmitting stations, amplified them and distributed them to these communities via a co-axial cable distribution network. These cables could be strung overhead on existing telephone and power lines to reach individual households. System operators set up receiving stations far away from his coverage area and fed the programs received via microwave relay to his local station, to be distributed via cable. (See Fig. 1)
The distribution of program material to thousands of CATV systems throughout the United States itself relied on videotape and microwave relay, but this method soon proved inefficient and impractical to cope with the number of stations and the distances involved in the US CATV industry.

And so borne out of practical (and economical) need, the satellite television market was developed where firstly communications satellites and later custom launched satellites were used to distribute program material to CATV operators for distribution to authorized subscribers in their locality.
Communication satellites : 36 000 meter-high antennas
Satellites are modern communication tools which enables the exchange of information on a global basis. These spacecraft carry receiving and transmitting equipment "relay" signals between ground-based stations which can be as far away as opposite ends of the globe.
Most modern satellites are geostationary satellites. This means that these satellites appear to stay in a fixed spot above the earth 24 hours a day. In reality, these satellites are actually 35 900 kilometers above the Earth, travelling at a speed of 11 000 km/h in what is called geostationary orbital positions or slots. Its motion is thus synchronized with the Earth's rotation, and remains stationary in the sky in relation to an observer on Earth. This enables the satellite to communicate with its ground stations in its coverage area 24 hours a day. Based on this principle, 3 satellites placed equidistant around the earth will be able to provide communications coverage to every point on the earth.
Modern communication satellites are spacecraft equipped with numerous two-way transmission systems called transponders, devices that conduct two way relays of high-frequency microwave signals. Satellites provide communications links via microwave radio, most commonly in the frequency band of 3 to 30 gigahertz (3 billion to 30 billion hertz, or cycles per second). These frequencies correspond to wavelengths ranging from 10 cm to 1 cm. Radio waves this short diverge along straight lines in narrow beams, rather than propagating in an expanding spherical wavefront in the manner of longer wavelengths. In order to communicate via microwave radio, therefore, transmitters and receivers must be situated within line of sight of one another. On land, this can be achieved by using towers or hilltop locations, but microwave communication across oceans is impossible without the use of satellites. Up until the early 80s, commercial satellite transmissions where conducted in the 4/6 GHz range known as the C-band. However the C-band is prone to interference as it is also shared with ground-based microwave systems. Recently, satellites have started using the K-band (12/14 GHz), a more powerful downlinking frequency, which enables ground receiving stations to utilize smaller receiving dishes, and is ideal for highly urbanized areas like New York City where terrestrial microwave networks abound.
A primary function of many of these satellites is to provide television service. Programs are transmitted up to a satellite (the uplink), where they are received, modified, and retransmitted by transponders back to the Earth (the downlink). A satellite's transmission is focused and falls on a specific region of the earth, and this reception area, the "footprint", can be shaped to conform to a country's particular geographical outline.

Because the majority of such systems operate at low power, the dish-shaped receiving antenna (see Fig. 2) must be large, 3 or more meters in diameter. By 1992, however, improved technology allowed smaller antennas to be used. These systems were called VSATs (very small aperture terminals), and the dishes were only 1.2 m in diameter.
Although most of the downlinks are terminated at a commercial cable TV company's facility, it is possible for individuals to subscribe directly to TV service by capturing a signal from one of the downlinks. This, however, requires the purchase and installation of an antenna. Most people consequently choose the commercial cable TV companies. These organizations receive signals from the downlink and distribute them to subscribers via a coaxial cable network. Satellite-distributed television programming is the backbone of the cable television industry in the US. A company, such as Home Box Office or Cable News Network, uplinks it's programming to a satellite where it is subsequently downlinked and received by cable companies throughout the country. The programming is then locally distributed to individual subscribers via cable.

The cable distribution network
Cable-television systems originated in the United States in the early 1950s and were designed to improve reception of commercial network broadcasts in remote and hilly areas. During the 1960s they were introduced in many large metropolitan areas where local television reception is degraded by the reflection of signals from tall buildings.
In a cable system, signals are sent along wires rather than through the air. Signals originate from a headend from which they are distributed throughout the service area by co-axial cables used in trunk lines, feeder lines, and drops connecting individual subscribers with the system. The wires are either as previously mentioned strung overhead or underground. The headend is where distant signals are received from distant television stations, or satellites. Here, signals are amplified, equalized, and assigned to a channel. These signals are then sent to co-axial cables and then to feeder lines strung next to buildings or through neighborhoods and then to drops which connect individual subscribers. Signals lose their strength very quickly as they pass through the cables, and so amplifiers and hubs are required at regular intervals.
Headends also possess television receive-only (TVRO) antennas that receive satellite transmissions from distant broadcast stations. These transmissions are then

distributed along the cable networks. Ted Turner's HBO started using satellites for distribution of their signals in the 1970s and they set a pattern that is used today by nearly 100 satellite-delivered cable networks (see Fig. 3). While different satellites are used, the basic approach is the same. Turner's Atlanta "superstation" WTBS is picked up by Southern Satellite Systems, a satellite carrier, and the signal is uplinked to a Hughes Galaxy I satellite. The signal is beamed down by satellite transponders for pickup at TVROs located at or near cable headends across the country to be distributed to subscribers.
Since the mid-1970s there has been a proliferation of cable-television systems offering special services. Besides bringing high-quality signals to subscribers, the systems provide additional television channels. Some of these systems can deliver 50 or more channels because they distribute signals occurring within the normal television broadcast band as well as non-broadcast frequencies. A frequency-conversion device is connected to the television set of the subscriber to accommodate these signals of non-broadcast frequencies. The increased number of channels allows expanded programming, including broadcasts from distant cities, continuous weather and stock-market reports, programs produced by community groups and educational institutions, and access to pay-TV program materials such as recent motion pictures and sports events not telecast by other broadcasters.
With dropping costs of satellite equipment, cable news networks have picked up on this technology to have remote satellite uplinks situated on vans, trucks or portable satellite dishes. This form of satellite newsgathering (SNG) provides transmission and reception of national and international news events at a moment's notice. This provides news networks with mobility and speeds not previously available without satellite technology. They are able to uplink news feeds from anywhere within a satellite's coverage area to be downlinked at the station's headends across the country.
However, as technology progresses, consumers are now bred on interactivity and superfast broadband connections to media sources, as well as hundreds of television channels on existing cable networks. The development of high definition television (HDTV) and interactive television amongst other digital technologies have pushed the limits of the co-axial cables in terms of bandwidth. With so much data squeezing into today's networks, an alternative has to be sought.
Fiber optics
Fiber optics is the science of transmitting data, voice, and images by the passage of light through thin, transparent fibers. In telecommunications, fiber optic technology has virtually replaced copper wire in long-distance telephone lines, and it is used to link computers within local area networks. Fiber optics is also the basis of the fiberscopes used in examining internal parts of the body (endoscopy) or inspecting the interiors of manufactured structural products.
The basic medium of fiber optics is a hair-thin fiber (see fig. 4) that is sometimes made of plastic but most often of glass. A typical glass optical fiber has a diameter of 0.125-mm (0.005-inch). This is actually the diameter of the cladding, or outer reflecting layer; the core, or inner-transmitting cylinder may have an even smaller diameter. Through a process known as total internal reflection, light rays beamed into the fiber can propagate within the core for great distances with remarkably little attenuation, or reduction in intensity, unlike traditional copper cables.

In analog transmissions, the strength of an original signal is converted into light, which strength and intensity is relational to the original signal. In digital transmissions, the optical signal is generated as pulses which signify "off" and "on" (or "1" and "0", in computer speak).
Light, due to its higher frequency range and bandwith, can accommodate and enormous amount of information. A single 0.75-inch fiber cable can replace 20 conventional 3.5-inch coaxial cables. Fiber cables are also immune to electromagnetic and radio frequency interference because it neither uses nor radiates electricity. Also, it is impossible to tap or splice a fiber cable due to its thinness, and this provides it with a high degree of data security. Unlike co-axial cables, which need amplifiers and hubs to amplify signals over distance, fiber cables can relay data for over 100 kilometers without repeaters. In practical terms, fiber cables, because of size, weight, bandwidth, and cost efficiency (no need for repeaters) look poised to take over the communications industry.
Fiber optics in the CATV and satellite industry
In the satellite-dominated age where most long-distance signals are beamed via satellite, fiber optics may provide a more practical and efficient mode of communications.
Compared to satellite transmissions, fiber optics has a larger channel capacity, and longer lifetime, and is more cost effective. Satellite communication entails a time delay in the uplinking and downlinking process, due to the great distance the signal has to travel in that time. Also, the signal transmitted it susceptible to atmospheric disturbances. A fiber cable, however, is not. Unauthorized persons can intercept satellite

transmissions, by virtue of its broad "footprint" which is aimed over a large area. Thus, fiber cables eliminate this risk by offering secure and fast point-to-point transmissions.
Optical fiber has successfully replaced copper cables in the backbone networks of most local and long-distance telephone carriers, cable television operators and utility companies, to facilitate transmissions between their facilities. However, there is still the problem of the last mile - the cost of installing fiber cables to homes is restrictively high. Connecting fiber cables to each customer, along with the electronics needed to receive the optical signals, is very expensive. For a typical home, the installation cost is about US$1,500. But it is still higher, in most cases, than the cost of connecting a comparable home with existing co-axial cable.
The cable industry is upgrading its coaxial cable architecture into a hybrid fiber optic/coaxial cable network (HFC). Cable companies have installed fiber optic technology in trunk lines to upgrade these major arteries of the cable architecture with wider bandwidth (higher capacity) links
Because backbone networks are multiplexed (a single fiber in the network can carry many independent channels of signals) the cost per channel is relatively low. Thus, telephone and cable TV companies make use of fiber's huge capacity by installing it partway to homes. In these systems, often called fiber-to-the-node, a few fibers connect a service provider to an enclosure near a group of homes. This enclosure holds the equipment for converting the optical signal in the fiber to an electrical signal that can travel on existing cables. This has changed the architecture of cable systems from the traditional tree and branch to the node design used for fiber optic networks (see Fig.5a & 5b). Nodes provide service to small groups of customers and were originally placed to meet engineering requirements. Still, the transmission rates are lower than those of fiber-to-home, because the final copper cabling act as bottlenecks.

The earliest successful fiber-to-home connections took place in Higashi-Ikoma, Japan, in 1977. Now it is becoming commercially viable, thanks to recent advances in fiber architectures and technology. The first important change was the development of the passive optical network (PON), in which a single fiber extends from an optical transceiver at the provider's location to an optical splitter near a small group of homes. The splitter divides the light signals equally among 16 or 32 output fibers, which then carry the signals to the customers' homes (see Fig.6)..

With further upgrades, HFC will also be able to support two-way telecommunications. Therefore, a broadband cable network that is capable of delivering more channels as well as high-quality voice, video, and data can be created without replacing the feeder and drop lines with fiber optic technology.
Cable companies began widespread installation of fiber technology into the trunk lines of the cable architecture during the late 1980s, which improved signal quality and lowered maintenance costs. HFC are capable of delivering a variety of high-bandwidth, interactive services for a lower cost than fiber-to-home. In 1993, the CATV industry deployed 221% more fiber (428,700 miles) than it did in 1992 (193,800 miles). Fiber-optic cable that is not presently used, called dark fiber, is also being installed to meet future needs. Changes in the channel capacity for each fiber have increased capacity from 18 to 80 video channels per fiber.
In recent years, the cable industry has begun constructing regional fiber optic networks to link headends and "regional hubs," so that cable operators in the same region can share services with one another in order to eliminate headends. The regional hub would also allow cable operators to interconnect with other telecommunications services so that cable could provide video, audio, and textual data to homes and businesses from a variety of sources. With this Integrated Services Digital Network (ISDN), subscribers could request the delivery of specified services (such as electronic newspapers, home shopping, or video teleconferencing). Regional hub systems are being built in San Francisco, Denver, Florida, Boston, Long Island, and Pennsylvania, to name a few. For CATV services, this greater capacity means more channels for viewers and increased growth potential. Picture quality will also improve because there will no longer be numerous amplifiers along the trunk line to maintain signal strength.
As the industry moves into new areas of services and products, fiber has other advantages. It enables cable operators to at least prepare for the surge in bandwidth demand amongst customers who need data and phone services. Fiber optic cable is also one of the key elements in developing interactivity between a cable customer and the cable system. Someday, the broadband capability of fiber will be available in both directions for duplex communications, so customers may have full interactivity with services. By using their own fiber link, customers may be able to order movies, conduct shopping and banking transactions, and access a wide variety of information and education services.
Speed is certainly a factor in all telecommunications. The ability to smoothly transmit data, whether digital video or computer data streams, depends in part on how fast the data can be sent. Theoretically, digital transmission of full-motion, color video requires a data transmission rate of 10 to 100 megabits per second (Mbps). The data transmission rate of twisted-pair phone lines is much less than that, closer to 1.5 Mbps. But even once the network is HFC, video switches that can transmit compressed digital video signals must be in place.
Singapore Cable Vision's broadband network is based on HFC. Optical fiber links the system headend to network nodes, which serve between 500 to 2000 subscribers, and coaxial cables connecting individual homes to each node. Inside the home, cable signals can be split between various TV or PC-based devices like a cable TV set-top box and a cable modem. The HFC architecture automatically results in a network that offers greater channel capacity, improved signal reliability and far-superior two-way transmission capabilities.
Conclusion
In the past, CATV's primary market was residential neighborhoods, because it sold entertainment that was used in the home. Now, to compete, CATV must penetrate the areas it previously passed by: the places where people work. To do that and provide the kinds of services businesses need, the use of fiber optic cable must be expanded.
These advanced fiber systems are still rare. Most telephone and cable television companies rely on fiber only as a "backbone" technology for transmitting signals between their own facilities. In fact, fibers are the standard links to and from the switching offices serving each community, and often stretch from there to large business customers or neighborhood distribution nodes. A single pair of fibers now can carry up to hundreds of gigabits per second, with each fiber transmitting separate signals at dozens of wavelengths in one direction. Yet the rest of the distribution network is virtually all copper-an investment worth well over $100 billion and phone and cable companies are not eager to abandon it.

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