Hybrid fibre-coaxial
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Hybrid fibre-coaxial (HFC) is a telecommunications industry term for a broadband network which combines optical fiber and coaxial cable. It has been commonly employed by US and Canadian cable TV operators since the 1990s. See diagram below for a typical architecture for an HFC Network.
The fiber optic network extends from the cable operators' master headend, sometimes to regional headends, and out to a neighbourhood's hubsite, and finally to a fiber optic node which serves anywhere from 25 to 2000 homes. A master headend will usually have satellite dishes for reception of distant video signals as well as IP aggregation routers. Some master headends also house telephony equipment for providing telecommunications services to the community. A regional or area headend will receive the video signal from the master headend and add to it the Public, Educational and/or Governmental (PEG) channels as required by local franchising authorities or insert targeted advertising that would appeal to a local area. The various services are encoded, modulated and upconverted onto RF carriers, combined onto a single electrical signal and inserted into a broadband optical transmitter. This optical transmitter converts the electrical signal to a downstream optically modulated signal that is sent to the nodes. Fiber optic cables connect the headend or hub to optical nodes in a point-to-point or star topology, or in some cases, in a protected ring topology.
A fiber optic node has a broadband optical receiver which converts the downstream optically modulated signal coming from the headend/hub to an electrical signal going to the homes. Today, the downstream signal is a radio frequency modulated signal that typically begins at 50 MHz and ranges from 550 MHz to 1000 MHz on the upper end. The fiber optic node also contains a reverse/return path transmitter that sends communication from the home back to the headend. In North America, this reverse signal is a modulated radio frequency ranging from 5 to 42 MHz while in other parts of the world, the range is 5 to 65 MHz.
The optical portion of the network provides a large amount of flexibility. If there are not many fiber optic cables to the node, Wavelength division multiplexing can be utilized to combine multiple optical signals onto the same fiber. Optical filters are used to combine and split optical wavelengths onto the single fiber. For example, the downstream signal could be on a wavelength at 1310nm and the return signal could be on a wavelength at 1550nm. There are also techniques to put multiple downstream signals on a single fiber by putting them at different wavelengths.
The coaxial portion of the network connects 25 to 2000 homes (500 is typical) in a tree-and-branch configuration. Radio frequency amplifiers are used at intervals to overcome cable attenuation and passive losses caused by splitting or "tapping" the cable. Trunk coaxial cables are connected to the optical node and form a coaxial backbone to which smaller distribution cables connect. Trunk cables also carry AC power which is added to the cable line at usually either 60V or 90V by a power supply and a power inserter. The power is added to the cable line so that trunk and distribution amplifiers do not need an individual, external power source. From the trunk cables, smaller distribution cables are connected to a port of the trunk amplifier to carry the RF signal and the AC power down individual streets. If needed, line extenders, which are smaller distribution amplifiers, boost the signals to keep the power of the television signal at a level that the TV can accept. The distribution line is then "tapped" into and used to connect the individual drops to customer homes. These taps pass the RF signal and block the AC power unless there are telephony devices that need the back-up power reliability provided by the coax power system. The tap terminates into a small coaxial drop using a standard screw type connector known as an “F” connector. The drop is then connected to the house where a ground block protects the system from stray voltages. Depending on the design of the network, the signal can then be passed through a splitter to multiple TVs. If too many TVs are connected, then the picture quality of all the TVs in the house will go down requiring the use of a "drop" or "house" amplifier.
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[edit] Transport over HFC network
By using frequency division multiplexing, an HFC network may carry a variety of services, including analog TV, digital TV (standard definition and HDTV), Video on demand, telephony, and high-speed data. Services on these systems are carried on Radio Frequency (RF) signals in the 5Mhz to 1000MHz frequency band.
The HFC network is bi-directional, meaning that signals are carried in both directions on the same network from the headend/hub office to the home, and from the home to the headend/hub office. The forward-path or downstream signals carry information from the headend/hub office to the home, such as video content, voice and internet data. The return-path or upstream signals carry information from the home to the headend/hub office, such as control signals to order a movie or internet data to send an email. The forward-path and the return-path are actually carried over the same coaxial cable in both directions on the same network from the headend/hub office to the home, and from the home to the headend/hub office. In order to prevent interference of signals, the frequency band is divided into two sections. In countries that have traditionally used NTSC, the sections are 52 MHz to 1000 MHz for forward-path signals, and 5 MHz to 42 MHz for return-path signals. Other countries use different band sizes, but are similar in that there is much more bandwidth for downstream communication instead of upstream communication.
As detailed above, much more of the frequency band is dedicated to the forward-path than the return-path. Traditionally much more information is sent in the forward-path due to video content only needing to be sent to the home, so the HFC network is structured to be non-symmetrical, meaning that one direction has much more data-carrying capacity than the other direction. Years ago, the return-path was only used for some control signals to order movies, etc., which required very little bandwidth. As additional services have been added to the HFC network, such as internet data and telephony, the return-path is being utilized more.
Multiple System Operators (MSOs) developed methods of sending the various services over RF signals on the fiber optic and coaxial cables. The original method to transport video over the HFC network and, still the most widely used method, is by modulation of standard analog TV channels which is similar to the method used for transmission of over-the-air broadcast Analog television channels.(See Broadcast television system for more information.) One analog TV channel occupies a 6 MHz-wide frequency band in NTSC-based systems, or an 8 MHz-wide frequency band in PAL or SECAM-based systems. Each channel is centered on a specific frequency carrier so that there is no interference with adjacent or harmonic channels. Digital TV channels offer a more-efficient way to transport video by using MPEG-2 or MPEG-4 coding over Quadrature amplitude modulation (QAM) channels. To be able to view a digitally modulated channel, home, or customer-premises equipment (CPE), e.g. digital televisions, computers, or set-top boxes, are required to convert the RF signals to signals that are compatible with display devices such as analog televisions or computer monitors. The Federal Communication Commission (FCC) has ruled that consumers can obtain a cable card from their local MSO to authorize viewing digital channels. By using digital compression techniques, multiple standard and high-definition TV channels can be carried on one 6 or 8 MHz frequency carrier thus increasing the channel carrying-capacity of the HFC network by 10 times or more versus an all analog network. Note that a digital tuner (i.e. TV set-top box) is not required for standard analog TV channels since most televisions have integrated analog tuners that can decode the signal, unless some type of scrambling is used.
[edit] Competitive network technologies
Digital subscriber line (DSL) is a technology used by traditional telephone companies to deliver advanced services (high-speed data and sometimes video) over twisted pair copper telephone wires. It typically has lower data carrying capacity than HFC networks and data speeds can be range limited by line lengths and quality.
Satellite television competes very well with HFC networks in delivering broadcast video services. It usually does not compete well in delivering Internet data, telephony and interactive services (i.e. VOD) because it does not have a good method to transport return-path information.
Analogous to HFC, Fiber In The Loop technology is used by telephone local exchange carriers to provide advanced services to telephone customers over the POTS local loop.
[edit] Weblinks
[edit] See also
- Broadcast television system
- Cable television
- National Television System(s) Committee
- National Cable & Telecommunications Association (NCTA)
- Society of Cable Television Engineers (SCTE)
- Radio Frequency (RF)
- Quadrature amplitude modulation
- DOCSIS
- MPEG-2
- Video on demand
- cable modems
- Passive optical network
