Cable TV networks for the smallest. Part 8: Optical Trunk Network

For many years, the basis of data transmission is the optical environment. It is difficult to imagine a hacker reader who is not familiar with these technologies, but it is impossible to do without at least a brief description in my series of articles.


To complete the picture on the fingers and in a simplified way I will talk about a couple of trivial things (do not throw slippers, this is for those who are completely unaware): optical fiber is glass that is pulled into a thread thinner than a hair. The laser-generated beam propagates through it, which (like any electromagnetic wave) has its specific frequency. For convenience and simplicity, speaking of optics, instead of the frequency in hertz, use the reciprocal of the wavelength, which is measured in nanometers in the optical range. For transmission of cable television signals, λ = 1550nm is commonly used.

Parts of the line are interconnected by welding or connectors. Details about this can be found in the excellent article @stalinets . Let me just say that in the networks of cable TV, almost always, skewed polishing of the APC is used.
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Image from fiber-optic-solutions.com

It introduces a little more attenuation than a direct one, but it has a very important property: the signal reflected at the junction does not spread along the same axis as the main signal, which has less impact on it. For digital transmission systems with redundant redundancy and restoration algorithms, this seems unimportant, but the television signal began its journey as analog (including fiber), but for him it is very critical: everyone remembers the doubling or slipping of the image on old TVs with an uncertain reception. Similar wave phenomena both on the air and in cables have a place to be. Digital TV signal, although it has increased noise immunity, nevertheless, it does not have many packet data buns and can also suffer at the level of physics, but it can no longer be recovered through a re-request.

In order for a signal to be transmitted over a considerable distance, its high level is required, so it is impossible to do without amplifiers in the chain. The optical signal in CATV systems is amplified by erbium amplifiers (EDFA). The operation of this device can serve as an excellent example of the fact that any sufficiently developed technology is indistinguishable from magic. If in a nutshell: when a beam passes through a fiber with an erbium impurity, conditions are created under which each photon of the original radiation creates two of its clones. Such devices are used in all data transmission systems over long distances. They, of course, are not cheap. Therefore, in cases when signal amplification by a significant amount is not required and there are no strict requirements for the amount of noise, signal regenerators are used:



This device, as can be seen from the block diagram, performs a double conversion of the signal between the optical and electrical media. This design allows you to change the wavelength of the signal, if necessary.

Such manipulations as amplification and signal regeneration are necessary not only to compensate for the kilometer attenuation of the cable. The greatest losses occur when the signal is divided between the branches of the network. The division is made with the help of passive devices, which, depending on the need, may have a different number of taps, as well as divide the signal both symmetrically and not.



Inside, the divider is either fiber, connected by side surfaces, or etched like tracks on a printed circuit board. For deepening, I recommend NAGru articles on welded and planar dividers, respectively. The more divider has taps, the more it introduces attenuation in the signal.

If you add filters to the divider to separate rays with different wavelengths, then we can transmit two signals at once in one fiber.



This is the simplest variant of multiplexing on optics - FWDM . By connecting the KTV and Internet equipment to the TV and Express inputs, respectively, we will receive a mixed signal in the common COM output, which can be transmitted over one fiber, and on the other side we can also divide it between an optical receiver and a switch, for example. This happens in much the same way as a rainbow appears in a glass prism of white light.

In order to reserve an optical signal, in addition to optical receivers with two inputs, about which I wrote in the last part , an electromechanical relay can be used, which can be switched from one source to another according to specified signal parameters.
At degradation of one fiber the device will automatically switch to another. The switching time is less than a second, so for the subscriber it looks at worst as a handful of artifacts on a digital TV image, which immediately disappear with the next frame.