Hybrid Fibre-Coax Networks

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.

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.

External Links

• Hybrid Fibre Coaxial (HFC)