Overview

Overview

HYBRID FIBER COAXIAL NETWORK (HFC)

Overview

The cost and performance of fiber optic components for CATV systems have reached a point where they can be economically applied in today’s systems. Fiber optic systems have many advantages over coaxial cable. First, very great bandwidths are possible – several times that of coaxial cable. Second, ingress and egress problems are nonexistent. Third, illegally tapping a fiber optic cable is much more difficult than is tapping into a coaxial cable or telephone wire. Fourth, signal loss through optical fiber is substantially less than through coaxial cable, requiring fewer repeaters. Finally, some time in the future, fiber optic systems are likely to be cheaper than coaxial cable systems. Fibers are made of glass, a relatively inexpensive material. Fiber cost has been falling, where as coaxial cable costs have been rising over years. Fiber is also lightweight and small.

Most fiber optic development in the past has been in the telephone industry. The Telephone Company presently utilizes copper wire or coaxial cable to tie together switching centers within cities. In the major cities, these cables are in ducts located beneath the streets. As ducts fill up, and new ducts must be installed, Tremendous capital investment is necessary. Since a single optical fiber can carry as many telephone conversations as a massive cable with several thousand individual wire circuits with fibers to increase the communication capacity in existing ducts. In addition, fiber optic lines are now cost-effective with microwave and coaxial cable for long-distance telephone lines. As a result, a high percentage of new inter city phone interconnection is now done with fiber optic links.

There has been some use of optical fibers in cable systems. Most have been "super-trunk" situations where two headends are to be tied together or headends are to be interconnected with hubs.

An innovative way of utilizing fiber optics is called Fiber to the Feeder (FTF).
Hybrid fiber Coaxial Network is the only truly integrated access network that can provide multimedia broadband services (Video, Audio and data). The term HFC signifies a network, which delivers broadband signals to subscribers through a combination of amplitude modulated (AM) optical transmission equipment, Coaxial cable and RF amplifiers. The quantities and arrangements of these components, however, can very significantly from one system to another, because different architectures will be found to be better suited to some areas than others factors such as population density, size of the fiber service area, and the interconnectedness of the street plan will each have a profound impact on the architecture selection process.

This project report is intended to provide the CATV system operator with an understanding of the FTF architecture, its applications and implementation, cost implications and the future utilization of a system constructed with FTF.

The Fiber to the feeder (FTF) architecture (Some times called Fiber trunk and Feeder) is a radically different approach to combining AM fiber optic transmitters fiber optic cables, and strand mounted AM fiber optic receivers with RF coaxial components. No trunk amplifiers are used in this architecture, and a different approach to the feeder is taken.

In an FTF system, fiber optic cables, an AM laser transmitter, and receivers replace the trunk amplifiers and trunk coaxial cable of a conventional system. The fiber optic cables leave the headend in bundles (fiber construction costs are reduced if common routes are used). The number of fiber cables in the bundle decreases as AM fiber optics receivers are reached. Each fiber optic receivers is called a node. Each fiber node feeds RF amplifiers (typically mini trunks and line extenders). Coaxial cable and multi port taps. Feed forward and/or power-doubling technology is used in the mini trunk amplifiers. Typically the maximum cascade of amplifiers fed from a node is four.

The coax network also collects return path signals from the neighborhood, which are transmitted back to the headend or central office via a return path optical link.
System reliability is significantly improved as only five or six active devices are used to feed the most distant subscribers. The FTF system architecture allows for a star- type delivery system, with each star point servicing a relatively small number of subscribers (2,000 or so). This facilitates a number of benefits including pay-per-view, neighborhood programming, and non-video services. 2,000 or more subscribers may be fed from a one node to make a more economically valuable.

Advantages

þ To improve system reliability by reducing the number of actives between the headend and the subscribers.

þ To segment system into subscriber's areas suitable for the delivery of enhanced services.

þ Better Data transmission through HFC Network.

þ To provide Internet Services.

þ To provide Telephone services in feature.

þ To be more competitive with telephone companies in utilization of advanced technology.

þ Reducing signal interruptions due to long cascade trunk line.

þ Reducing Powering up Systems in the Network.

þ Low transmission loss as compared to coaxial cable.

þ Nonradiative, noninductive, nonconductive, low crasstalk.

þ Complete isolation from nearby electrical systems etc.

Optical Splitting

Optical splitting is highly desirable for an FTF system. To maximize the utilization of the expensive transmitters, Optical splitting is used to share one transmitter’s output among a number of receivers. The combination of optical splitter loss and fiber optic cable loss must be less than or equal to the total path loss allowed for the fiber optic link. The path loss is the maximum optical loss allowed between the transmitter and receiver. If consistent fiber link performance is desired, splitting can be used most effectively in the areas nearest to the Headend, where the fiber optic cable loss is lower.

Facts of FTF Life

· End of line performance requirements 46dB CNR and 53dB CTB min.
· Cost effective FTF designs use express feeder between the fiber receiver and the first amplifier. FTF designs should use an untapped cable between the fiber    receiver and the first RF amplifier. This untapped cable is called express feeder.
· Express feeder is even more effective if cable sizes larger than equal to 500 series used.
· With express feeder and wide temperature swings, ALC in the first amplifier after the fiber node is recommended.
· An underground system rebuild might not be Economical with FTF.
· The power passing capability of taps is a limitation to the powering design. Because of more complex amplifiers (Feed forward, With ALC) used in FTF         designs, higher currents need to be carried on the feeder lines. The Power passing capability of taps (Typically 6Amps) can necessitate additional power supplies.
· The number of fiber transmitters is critical to the cost of an FTF system.

·        The following techniques can minimize the number of transmitters.

þ Use of optical splitting to feed multiple receives, from one transmitter.

þ Maximizing the area and number of subscribers covered from fiber receivers thus reducing the fiber receivers.

þ Selecting appropriate system performance criteria.

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