For many years, the basis of data transmission has been the optical medium. It is difficult to imagine a 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.
Contents of the article series
- Part 9: Headend
- Part 10: Troubleshooting on the cable TV network
To complete the picture on the fingers and in a simplified way, I will talk about a couple of banal things (do not throw slippers, this is for those who are not at all in the know): an optical fiber is a glass that has been pulled into a thread thinner than a hair. A laser-shaped beam propagates through it, which (like any electromagnetic wave) has its own specific frequency. For convenience and simplicity, when talking about optics, instead of the frequency in hertz, it is customary to use the reciprocal of the wavelength, which is measured in nanometers in the optical range. For transmission of cable television signals, Ξ»=1550nm is usually used.
Parts of the highway are interconnected by welding or connectors. You can read more about this in . Let me just say that APC oblique polishing is almost always used in cable TV networks.
Image from fiber-optic-solutions.com
It introduces slightly more attenuation than a straight line, but it has a very important property: the signal reflected at the junction does not propagate along the same axis as the main signal, due to which it has less effect on it. For digital transmission systems with built-in redundancy and recovery algorithms, this seems unimportant, but the TV signal began its journey as an analog one (including fiber optics), and for it this is very critical: everyone remembers ghosting or image creep on old TVs with uncertain reception. Similar wave phenomena both in the air and in cables take place. The digital TV signal, although it has increased noise immunity, nevertheless, does not have many goodies of packet data transmission and can also suffer at the physics level, but can no longer be restored through a re-request.
In order for the signal to be transmitted over a considerable distance, its high level is required, so amplifiers are indispensable in the chain. The optical signal in CATV systems is amplified by erbium amplifiers (EDFA). The operation of this device is an excellent example of the fact that any sufficiently advanced technology is indistinguishable from magic. In a nutshell: when a beam passes through a fiber doped 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 block diagram, performs a 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 signal amplification and regeneration are necessary not only to compensate for kilometer cable attenuation. The greatest losses occur when the signal is divided between the branches of the network. The division is carried out 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 etched like tracks on a printed circuit board. For deepening, I recommend articles about ΠΈ dividers respectively. The more taps the divider has, the more attenuation it introduces into the signal.
If we add filters to the divider to separate beams with different wavelengths, then we can transmit two signals at once in one fiber.
This is the simplest version of multiplexing on optics - FWDM. By connecting cable TV equipment and the Internet to the TV and Express inputs, respectively, we will get a mixed signal in the common COM output, which can be transmitted over one fiber, and on the other side, it can also be divided 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 the optical signal, in addition to optical receivers with two inputs, which I wrote about an electromechanical relay can be used, which can switch from one source to another according to the specified signal parameters.
When one fiber is degraded, the device will automatically switch to another. The switching time is less than a second, so for the subscriber it looks at worst like a handful of artifacts on the digital TV image, which immediately disappear with the next frame.
Source: habr.com