As the name implies, a cable television system is simply a medium that transports television signals to the home by means of a cable, instead of receiving the signals that arrive by air through a receiving antenna.
There are different types of architectures for this system, each with advantages and disadvantages. Its choice by the operator will depend on the characteristics of the signals to be transported, in addition to other considerations such as number of subscribers, available physical space, capital to invest and above all future expansions both in coverage and in other services.
The traditional tree-branch architecture, where a large number of cascading trunk amplifiers are used – in large networks up to 50 are used, depending on the bandwidth to be used and the technology used in the amplifier stations – which feed signal to the distribution amplifiers, which will have the mission of delivering signal to the line extenders to, subsequently, be taken to the users.
Fiber optic architectures
Fiber optics have become popular as a means of transporting cable TV signals..
The fiber architecture was born as an improvement to the traditional type, developing in three levels: optical fiber, up to the nodes, from there trunks and finally distribution.
Origin of the optical fiber: from the observation that light could be transported in a jet of water, it was proposed to achieve the same in a solid medium, such as glass. To achieve this there had to be a way for light not to escape from the glass. This requirement to achieve "a mirror" to reflect the light would allow bending in the glass tube and conserve light inside.
To obtain a total internal reflection it is enough to surround the crystal with another of different density and direct the light within a defined angle.
What is fiber optics? in short, it is a mechanical means to transport light. For this reason it suffers from some effects that alter the optical signal.
One of the great advantages of fiber transmission, compared to other means, such as coaxial cable, is the great difference in the bandwidth to carry information. A single fiber is capable of carrying several thousand GHz of information, that is, 6 million TV channels, each with a bandwidth of 6 MHz. In addition to the above, the fiber has low attenuation, immunity to radio frequency (RF) and electromagnetic interference (EMI), low weight and safety.
To generate and receive optical signals, transmitters are located in the headend or central and am optical receivers at regular intervals, thus decreasing the number of cascading trunk amplifiers. The difference with the tree-branch architecture is that with the fiber you can see waterfalls with a maximum of 8 to 12 amplifiers, in addition to the distances from the header to the subscriber are very similar to each other, so the signals will be the same for different users located at distances from the dissimilar headend.
A receiver or node is generally used for eight fibers, a minimum amount for each service area of approximately 2,000 subscribers.
The growth plan will be different for each system depending on the type of services you want to offer in the future. If an ideal route crosses a commercial or service sector, it is convenient to think that within a period not exceeding 10 years these businesses may need a data link or telephony, so more fiber will be needed.
The life of a fiber optic cable is not less than 25 years, so it is convenient to leave fibers to spare.
Currently the FTF (Fiber to the Feeder) architecture is widely used, based on two levels: primary from the header to the node or receiver with optical fiber, and secondary from the node to the subscriber with coaxial cable. The difference with the previous ones is that there are no trunk amplifiers here, because all the transport is carried out by optical signals.
