Cable TV networks for the smallest. Part 8: Optical backbone network

For many years, the basis of data transfer has been the optical environment. It is difficult to imagine a habuchitel 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 my fingers and simplistically, I’ll tell you about a couple of commonplace things (do not throw slippers, this is for those who do not know at all): an optical fiber is a glass that is pulled into a thread thinner than a hair. A beam formed by a laser propagates along it, which (like any electromagnetic wave) has its own specific frequency. For convenience and simplicity, speaking of optics, instead of the frequency in hertz, use the inverse value of the wavelength, which in the optical range is measured in nanometers. For the transmission of cable television signals, λ = 1550nm is usually used.

Parts of the line are interconnected by welding or connectors. You can read more about this in the excellent article by @stalinets . I can only say that KTV networks use almost always oblique polishing of APC.


Image from fiber-optic-solutions.com

It introduces a slightly larger attenuation than the direct one, but has a very important property: the signal reflected at the junction does not propagate along the same axis as the main signal, and therefore has a smaller effect on it. For digital transmission systems with embedded redundancy and restoration algorithms, this seems unimportant, but the television signal started its way as an analogue (including fiber optics), and for him it is very critical: everyone remembers ghosting or crawling of the image on old TVs with uncertain reception. Similar wave phenomena both on the air and in cables take place. The digital television signal, although it has increased noise immunity, nevertheless, does not have many buns of packet data transfer and can also suffer at the level of physics, but can no longer be restored through a re-request.

In order for a signal to be transmitted over a considerable distance, a high level is required, therefore, amplifiers cannot be dispensed with in a chain. The optical signal in KTV 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. In a nutshell: when a ray passes through a fiber mixed with erbium, conditions are created under which each photon of the initial radiation creates two of its clones. Such devices are used in all data transmission systems over long distances. They are, of course, not cheap. Therefore, in cases where 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 structural diagram, performs double signal conversion 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 kilometer attenuation of the cable. The greatest losses occur when the signal is divided between the network branches. The division is performed using passive devices, which, depending on the need, can have a different number of taps, as well as divide the signal both symmetrically and not.



Inside the divider is either fibers connected by side surfaces, or is etched like tracks on a printed circuit board. For deepening I recommend NAGru articles about weldedand planar dividers, respectively. The more the divider has taps, the more it attenuates the signal.

If we add filters for separating beams with different wavelengths to the divider, then we can transmit two signals in one fiber at once.



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

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