From the concept of "light" to optical transmission of information - lionzage. Signal transmission. Digital fiber optic systems

WORLD OF FIGURES AND GLASS

INTRODUCTION

Fiber optics have many well-known advantages over twisted pair and coaxial cables, such as immunity to electrical interference and unrivaled bandwidth.

Over the past quarter century, fiber optic communication has become a widespread method of transmitting video, audio, other analog signals, and digital data. Fiber optic communication has many well-known advantages over twisted pair and coaxial cables, such as immunity to electrical interference and unrivaled bandwidth. For these and many other reasons, fiber-optic information transmission systems are penetrating deeper and deeper into various fields of information technology.

Digital systems offer very high performance, flexibility and reliability, and cost no more than the analog solutions they replace.

However, despite these advantages, fiber optic systems have until recently used the same analog signal transmission technologies as their copper predecessors. Now, with a new generation of equipment based exclusively on digital signal processing techniques, fiber optic communications are taking telecommunications to a whole new level. Digital systems provide very high performance, flexibility and reliability, and cost no more than the analog solutions they replace.

This tutorial examines the digital signal transmission technique over fiber optic cables and its economic and technological benefits.

ANALOGUE FIBER TRANSMISSION

To fully appreciate the benefits of digital technology, let's first look at the traditional methods of transmitting analog signals over fiber. Amplitude (AM) and frequency (FM) modulation is used to transmit analog signals. In both cases, low-frequency analog audio and video signals or data are input to the optical transmitter and converted to an optical signal. This is done in different ways.

In AM systems, the optical signal is a luminous flux with an intensity that changes in accordance with changes in the input electrical signal. Either LEDs or lasers are used as a light source. Unfortunately, both are nonlinear, that is, in the full range of brightness from no radiation to the maximum value, the proportionality between the input signal and the light intensity is not observed. However, this is the control method used in AM systems. As a result, various distortions of the transmitted signal arise:

decrease in signal-to-noise ratio with increasing cable length;
nonlinear differential gain and phase errors in video transmission;
limitation dynamic range audio signal.

To improve the quality of operation of fiber-optic signal transmission systems, it was proposed to use frequency modulation, in which the light source is always either turned off completely or turned on at full power, and the pulse repetition rate changes in accordance with the amplitude of the input signal. For those familiar with frequency modulation of signals in radio engineering, the use of this term here may seem unreasonable, since in the context of fiber-optic systems it is perceived as a method of controlling the frequency of the light radiation itself. This is not so, and in fact it would be more correct to use the term "pulse-phase modulation" (PPM), but in the field of fiber-optic technology it is precisely this terminology that has settled. It should always be remembered that the word "frequency" in the name of the modulation method means the pulse repetition rate, and not the frequency of the light waves carrying them.

With amplitude modulation, the input signal level is represented by the intensity of the light beam

With frequency modulation, the input signal level is represented by the repetition rate of the light pulses
Figure: 1. Comparison of amplitude and frequency modulation

While frequency modulation eliminates many of the dimming problems in AM systems, it also has its own challenges. One of these is crosstalk known in FM systems. They are observed, in particular, when several signals with frequency modulation are transmitted over a single fiber, for example, when using a multiplexer. Crosstalk occurs in a transmitter or receiver as a result of tuning instability in important signal filtering circuits designed to separate carriers. If the filters are poorly tuned, then the frequency modulated carriers interact with each other and become distorted. Fiber-optic engineers can create FM systems in which the chance of crosstalk is minimized, but any design improvement will increase the cost of the instruments.

Another type of distortion is called intermodulation. Like crosstalk, intermodulation occurs in systems designed to carry multiple signals over a single fiber. Intermodulation distortion occurs in a transmitter most often as a result of non-linearity in circuits common to different FM carriers. As a consequence, before combining several carriers into one optical signal, they act on each other, reducing the transmission accuracy of the original signal.

DIGITAL SYSTEMS

As with analog systems, the transmitters digital systems low-frequency analog audio, video or digital data is input and converted to an optical signal. The receiver receives an optical signal and outputs an electrical signal of the original format. The difference lies in how signals are processed and transmitted from transmitter to receiver.

Figure: 2. Digital transmission system of analog signal

In purely digital systems, the input low-frequency signal goes directly to the analog-to-digital converter, which is part of the transmitter. There, the signal is converted into a sequence of logic levels - zeros and ones, called a digital stream. If the transmitter is multichannel, that is, it is designed to work with several signals, then several digital streams are combined into one, and it controls the on and off of one emitter, which occurs at a very high frequency.

At the receiving end, the signal is reversed. Individual streams are extracted from the combined digital stream, corresponding to the individual transmitted signals. They are fed to digital-to-analog converters, after which they are output to the outputs in the original format (Fig. 2).

Pure digital signal transmission has many advantages over traditional AM and FM systems, from versatility and better signal quality to lower installation costs. Let's take a closer look at some of the benefits and discuss the economics that are beneficial for both the installer and the user along the way.

ACCURACY OF SIGNAL TRANSMISSION

In analog AM systems, the signal degrades in proportion to the path traveled over the fiber. This fact, combined with the fact that AM systems operate only with multimode fibers, limits the use of such systems to relatively short transmission distances. FM systems work somewhat better: although the signal quality in them decreases, but in not very long lines it remains approximately constant, sharply decreasing only when a certain limiting length is reached. Only in fully digital systems is it guaranteed that signal quality is maintained during transmission over fiber optic line communication regardless of the distance between the transmitter and the receiver and the number of transmitted channels (of course, within the capabilities of the system).

Accuracy of reproduction of the transmitted signal is a significant problem in the development of systems for organizing multiple transmission channels over a single fiber (multiplexers). For example, in an analog system designed to carry four channels of video or audio, in order to meet the system bandwidth, you have to limit the bandwidth allocated to individual channels. In digital systems, there is no need to make such a compromise: one, four, or even ten signals can be transmitted over one fiber without compromising quality.

HIGHER SIGNAL QUALITY

Figure: 3

Transmission of analog signals in digital form provides higher quality than pure analog. Distortion of the signal with this method of transmission can occur only with analog-to-digital and reverse digital-to-analog conversion. Although no conversion is perfect, current technology is so advanced that even inexpensive ADCs and DACs provide much higher video and audio quality than can be achieved with analog AM and FM systems. This is easily seen from a comparison of signal-to-noise ratios and harmonic distortion (differential phase and differential gain) of digital and analog systems designed to transmit signals of the same format over optical fibers of the same type at the same wavelength.

Digital technologies provide engineers with unprecedented flexibility in fiber optic design. It is now easy to find the right performance level for different markets, tasks and budgets. For example, changing the bit width of the analog-to-digital converter can affect the system bandwidth required for signal transmission, and, as a result, the overall performance and cost. At the same time, other properties of the digital system - the absence of distortion and the independence of the quality of work from the line length - are preserved up to the maximum transmission distance. When designing analog systems, engineers are always in the grip between system cost and performance, trying to balance them without compromising the critical parameters of the transmitted signals. In digital systems, scaling systems and managing their performance and cost is much less difficult.

UNLIMITED TRANSMISSION DISTANCE

Another advantage of digital systems over analog predecessors is their ability to recover the signal without introducing additional distortion into it. This recovery is performed in a special device called a repeater or line amplifier.

The advantage provided by digital systems is clear. In them, the signal can be transmitted over distances that significantly exceed the capabilities of AM and FM systems, while the developer can be sure that the received signal exactly matches the transmitted one and meets the requirements of the technical specifications.

As light travels through the fiber, its intensity gradually decreases and eventually becomes insufficient for detection. If a linear amplifier is installed a little before reaching the place where the light becomes too weak, then it will amplify the signal to its original power, and it can be transmitted further over the same distance. It is important to note that the digital stream is restored in the linear amplifier, which does not have any effect on the quality of the analog video or audio signal encoded in it, no matter how many times the reconstruction is performed in the linear amplifiers along the signal path along the long fiber-optic line.

LOWER COST

Considering the many advantages that digital fiber optic systems have, it can be assumed that they should cost much more than traditional analog systems. However, this is not the case, and users of digital systems, on the contrary, save their money.

In a competitive market, there will always be a manufacturer offering digital quality at the price of an analog system

The cost of digital components has dropped significantly in recent years, and OEMs have been able to develop and market products that cost the same, or even less, than previous generation analog instruments. Of course, some firms want to convince the public that superior quality digital systems can only come from additional fee, but in reality they just decided not to share the savings with their clients. But in a competitive market, there is always a manufacturer that offers digital quality at the price of an analog system.

Digital systems allow more information to be transmitted over one cable, thereby reducing the need for it

Other factors also affect the cost of installing and operating a fiber optic system. The most obvious of these is the cost of the cable. Digital systems allow more information to be transmitted over one cable, thereby reducing the need for it. The advantage is especially noticeable where it is necessary to simultaneously transmit signals of different types, for example, video and sound or sound and data. Engineers can easily design a cost-effective digital system that carries multiple types of signals over a single fiber, such as two video channels and four audio channels. When using analog technology, most likely, you would have to do two separate systems, or at least use two separate cables for the audio and video signals.

With fewer components that can fail over time, digital systems are much more stable and reliable.

Even in cases where several signals of the same type must be transmitted over one fiber optic, digital systems are preferable, since they work more reliably and provide a higher signal quality. For example, in a digital video multiplexer, ten channels can be transmitted with the same high quality, but in an analog system this is generally impossible.

It should also take into account the inevitable maintenance and repair costs over the years of operation of fiber-optic systems. And here the advantage is with digital systems. First, they do not require initial setup after installation - the transmitter and receiver are simply connected by fiber optic cable and the system is ready to go. Analog systems usually need to be adjusted to the parameters of a specific transmission line, taking into account its length and signal intensity. Additional adjustment time entails additional costs.

Transmitters and receivers for digital systems are cheaper, cable consumption is less, and operating costs are lower

With fewer components that can fail over time, digital systems are much more stable and reliable. They do not require re-tuning and are much less time-consuming to troubleshoot because they are free of crosstalk, parameter drift, and other disadvantages associated with traditional analog systems.

Summarize. Transmitters and receivers for digital systems are cheaper, cable consumption is lower, and operating costs are lower. Digital fiber optic systems provide clear economic benefits at all levels.

CONCLUSIONS

Just as fiber-optic technology has many advantages over traditional copper wires and coaxial cables, digital communication takes fiber-optic technology a few steps upward, giving users a whole new set of benefits. Digital systems have unique characteristics: accuracy of signal transmission along the entire length of the communication line, minimal introduced distortion (including the absence of cross-distortion and intermodulation), the ability to repeatedly recover a digital stream during its transmission over a long line without compromising the quality of the analog signal encoded in it. This guarantees a level of fidelity in analog signal reproduction unattainable for analog systems.

Component prices for digital and analog fiber optic systems are comparable, and when the costs of installation, operation and maintenance are taken into account, digital systems provide clear economic benefits.

When designing a new fiber optic system, do not waste time analyzing the advantages and disadvantages of digital and analog systems, since the choice is clear: digital systems are better from every point of view. It will be much more useful to limit yourself to only them and select those products that best suit your needs. Even among digital systems, there is a huge variety of solutions. Here are some questions to help you assess them:

how easy is the installation of the system?
- if the transmitter and receiver are user configurable, how easy is it and what are the problems?
is the design of the devices compact, robust and reliable?
are the instruments available in desktop enclosures or rack-mountable? Are there options in both body types?
- are the fixtures suitable for use with both singlemode and multimode fibers?
- does the manufacturer have sufficient experience and reputation in the market for the products it offers?
- how does the price of a product compare to the price of traditional analog systems? (Digital devices in production are not more expensive than analog ones and their cost should not be higher).

Market analysis and comparison of the characteristics of similar products will allow you in the end to select elements of digital fiber optic systems that will serve you faithfully for many years.

Optics opens up wide opportunities where high-speed communications with high bandwidth are required. This is a well-proven, clear and convenient technology. In the Audiovisual field, it opens up new perspectives and provides solutions not available with other methods. Optics penetrated all key areas - surveillance systems, dispatch and situational centers, military and medical facilities, and areas with extreme operating conditions. Fiber-optic communication lines provide a high degree of protection of confidential information, allow transferring uncompressed data such as high-resolution graphics and video with pixel precision. New FOCL standards and technologies. Fiber - the future of SCS (structured cabling systems)? We are building an enterprise network.

Fiber optic (aka fiber optic) cable - this is a fundamentally different type of cable compared to the two types of electrical or copper cable considered. Information on it is transmitted not by an electrical signal, but by a light signal. Its main element is transparent fiberglass, through which light travels over huge distances (up to tens of kilometers) with insignificant attenuation.

The structure of the fiber optic cable is very simpleand is similar to the structure of a coaxial electrical cable (Fig. 1.). Only instead of a central copper wire, thin (about 1-10 microns in diameter) fiberglass is used, and instead of internal insulation, a glass or plastic sheath is used, which does not allow light to go outside the glass fiber. In this case, we are talking about the so-called total internal reflection of light from the interface of two substances with different refractive indices (the refractive index of the glass shell is much lower than that of the central fiber). There is usually no metal braid on the cable, as shielding from external electromagnetic interference is not required here. However, sometimes it is still used for mechanical protection from the environment (such a cable is sometimes called armored; it can combine several fiber-optic cables under one sheath).

Fiber optic cable has exceptional performance on noise immunity and secrecy of transmitted information. In principle, no external electromagnetic interference is capable of distorting the light signal, and the signal itself does not generate external electromagnetic radiation. It is almost impossible to connect to this type of cable for unauthorized eavesdropping on the network, as this violates the integrity of the cable. The theoretically possible bandwidth of such a cable reaches 1012 Hz, that is, 1000 GHz, which is incomparably higher than that of electrical cables. The cost of fiber optic cable has been steadily decreasing and is now roughly equal to the cost of thin coaxial cable.

Typical Fiber Optic Attenuation at frequencies used in local networks, it is from 5 to 20 dB / km, which roughly corresponds to the performance of electric cables at low frequencies. But in the case of a fiber-optic cable, with an increase in the frequency of the transmitted signal, the attenuation increases very slightly, and at high frequencies (especially above 200 MHz) its advantages over an electric cable are undeniable, it simply has no competitors.

Fiber-optic communication lines (FOCL) allow transmitting analog and digital signals over long distances, in some cases, over tens of kilometers. They are also used at shorter, more "manageable" distances, such as inside buildings. Examples of solutions for building SCS (structured cabling systems) for building an enterprise network are here: Building an enterprise network: Scheme of building SCS - Optics horizontally. , We build an enterprise network: SCS construction scheme - Centralized optical cable system. , We build an enterprise network: SCS construction scheme - Zone optical cable system.

The advantages of optics are well known: they are immunity to noise and interference, small cable diameters with huge bandwidth, resistance to hacking and interception of information, no need for repeaters and amplifiers, etc.
There were once some problems with terminating optical lines, but today they are mostly solved, so that working with this technology has become much easier. There are, however, a number of issues that need to be considered exclusively in the context of application areas. As with copper or radio transmission, the quality of fiber optic communication depends on how well the output signal of the transmitter and the front end of the receiver are matched. Incorrect signal power specification leads to an increase in the transmission bit error rate; the power is too high - and the receiver amplifier “oversaturated”, too low - and there is a problem with noise as it starts to interfere with the desired signal. The two most critical parameters of a fiber-optic link are the output power of the transmitter and transmission loss - the attenuation in the optical cable that connects the transmitter and receiver.

There are two different types of fiber optic cable:

* multimode or multimode cable, cheaper, but of lower quality;
* single mode cable, more expensive but has best performance compared to the first.

The type of cable will determine the number of propagation modes or "paths" that light travels within the cable.

Multimode cableMost commonly used in small industrial, residential and commercial projects, it has the highest attenuation and only works over short distances. The older type of cable, 62.5 / 125 (these numbers represent the inner / outer diameters of the fiber in microns), often referred to as "OM1", has a limited bandwidth and is used for data transfer rates up to 200 Mbps.
Recently, 50/125 "OM2" and "OM3" cables have been adopted, offering speeds of 1Gbps at distances up to 500m and 10Gbps at distances up to 300m.

Single mode cable used in high-speed connections (above 10 Gbps) or over long distances (up to 30 km). For audio and video transmission, the most expedient is the use of "OM2" cables.
Reiner Steil, vice president of marketing for Extron Europe, says fiber optic lines have become more affordable and more commonly used for networking inside buildings, driving an increase in the use of optical-based AV systems. Steil says: “In terms of integration, fiber-optic communication lines already have several key benefits.
Compared to similar copper cabling infrastructure, optics allow both analog and digital video signals to be used simultaneously, providing a single system solution to work with existing as well as promising video formats.
In addition, since the optics offer very high bandwidth, the same cable will work at higher resolutions in the future. FOCL easily adapts to new standards and formats that appear in the process of development of AV technologies. "

Another recognized expert in this field is Jim Hayes, President of the American Fiber Optics Association, formed in 1995, which fosters the growth of professionalism in the field of fiber optics and, among other things, has more than 27,000 qualified optical installation and implementation specialists in its ranks. He says the following about the growing popularity of fiber-optic communication lines: “The benefit is in the speed of installation and the cheapness of components. The use of optics in telecommunications is growing, especially in Fiber-To-The-Home * (FTTH) systems with wireless access, and in the field of security (surveillance cameras).
The FTTH segment appears to be growing faster than other markets in all developed countries. Here, in the USA, networks of traffic control, municipal services (administration, firefighters, police), educational institutions (schools, libraries) are built on optics.
The number of Internet users is growing - and we are rapidly building new data processing centers (DPCs), for the interconnection of which fiber is used. Indeed, when transmitting signals at a speed of 10 Gbit / s, the costs are similar to those of "copper" lines, but optics consume significantly less energy. For years, fiber and copper adherents have fought each other for priority in corporate networks. Wasted time!
Today, WiFi connectivity has become so good that users of netbooks, laptops and iPhones have opted for mobility. And now, in corporate LANs, optics are used for switching with wireless access points. "
Indeed, the fields of application of optics are becoming more and more, mainly due to the above advantages over copper.
Optics penetrated all key areas - surveillance systems, dispatch and situational centers, military and medical facilities, and areas with extreme operating conditions. Reducing the cost of equipment made it possible to use optical technologies in the traditionally "copper" areas - in conference rooms and stadiums, in retail and at transport hubs.
Extron's Rainer Steil comments: “Fiber optic equipment is widely used in healthcare facilities, for example, for switching local video signals in operating rooms. Optical signals have nothing to do with electricity, which is ideal for patient safety. Fiber-optic communication lines are perfect for medical educational institutions, where it is necessary to distribute video signals from several operating rooms to several classrooms, so that students can observe the progress of the operation "live".
Fiber-optic technologies are also preferred by the military, since the transmitted data is difficult or even impossible to "read" from the outside.
Fiber-optic communication lines provide a high degree of protection of confidential information, allow transferring uncompressed data such as high-resolution graphics and video with pixel precision.
Long distance transmission capability makes the optics ideal for digital signage systems in large shopping malls, where cable lines can reach several kilometers. If for twisted pair the distance is limited to 450 meters, then for optics and 30 km is not the limit. "
When it comes to the use of fiber in the Audio-Visual Industry, there are two main factors driving progress. First, this is the intensive development of IP-based audio and video transmission systems that rely on high-bandwidth networks - fiber-optic communication lines are ideal for them.
Second, there is a widespread requirement to transmit HD video and HR computer images over distances greater than 15 meters — the limit for HDMI over copper.
There are cases when the video signal simply cannot be “distributed” over a copper cable and it is necessary to use fiber - such situations stimulate the development of new products. Byeong Ho Park, VP of Marketing at Opticis, explains, “UXGA, 60Hz data bandwidth and 24-bit color require a total speed of 5 Gbps, or 1.65 Gbps per color channel. HDTV has a slightly lower bandwidth. Manufacturers are pushing the market, but the market is simultaneously pushing players to use images more high Quality... There are specific areas of application where displays are required that can display 3-5 million pixels or 30-36-bit color depth. In turn, this will require a transfer rate of about 10 Gbps. "
Today, many manufacturers of switching equipment offer versions of video extenders (extenders) for working with optical lines. ATEN International, TRENDnet, Rextron, Gefenand others release different models for a range of video and computer formats.
At the same time, service data - HDCP ** and EDID *** - can be transmitted using an additional optical line, and in some cases - via a separate copper cable connecting the transmitter and receiver.
As a result of HD becoming the standard for the broadcasting market,other markets — installation, for example — have also begun to use anti-tampering protection for DVI and HDMI content, ”said Jim Jacetta, senior vice president of engineering at Multidyne. - With our company's HDMI-ONE device, users can send the video signal from a DVD or Blu-ray player to a monitor or display up to 1000 meters away. Previously, no device with multimode lines supported HDCP copy protection. ”

Those who work with fiber-optic communication should not forget about specific installation problems - cable termination. In this regard, many manufacturers produce both the actual connectors and assembly kits, which include specialized tools, as well as chemicals.
Meanwhile, any element of the FOCL, be it an extension cord, a connector or a place where cables are connected, must be checked for signal attenuation using an optical meter - this is necessary to assess the total power budget (power budget, the main calculated indicator of the FOCL). Naturally, it is possible to assemble fiber cable connectors manually, "on the knee", but really high quality and reliability is guaranteed only when using ready-made, "cut" cables produced at the factory, subjected to rigorous multi-stage testing.
Despite the huge bandwidth of fiber-optic communication lines, many still have a desire to "cram" more information into one cable.
Here, development is going in two directions - wavelength division multiplexing (optical WDM), when several light beams with different wavelengths are directed into one fiber, and the other is data serialization / deserialization (English SerDes), when parallel code is converted into serial and back.
At the same time, WDM equipment is expensive due to the complex design and use of miniature optical components, but does not increase the transmission speed. The high-speed logic devices used in SerDes equipment also increase the expense of the project.
In addition, today equipment is being produced that allows control data to be multiplexed and demultiplexed from the total light flux - USB or RS232 / 485. In this case, the light flux can be sent along the same cable in opposite directions, although the cost of performing these "tricks" of the devices usually exceeds the cost of an additional light guide for data return.

Optics opens up wide possibilities where high-speed communications with high bandwidth are required. This is a well-proven, clear and convenient technology. In the Audiovisual field, it opens up new perspectives and provides solutions not available with other methods. At least without significant labor and money costs.

Depending on the main area of \u200b\u200bapplication, fiber optic cables are divided into two main types:

Indoor cable:
When installing fiber-optic communication lines in closed rooms, a fiber-optic cable with a dense buffer is usually used (to protect against rodents). It is used to build SCS as a trunk or horizontal cable. Supports data transmission over short and medium distances. Ideal for horizontal cabling.

External cable:
Tight buffer fiber optic cable, armored with steel tape, moisture resistant. It is used for external laying when creating a subsystem of external highways and connect individual buildings to each other. Can be laid in cable ducts. Suitable for direct burial.

External self-supporting fiber optic cable:
Self-supporting fiber optic cable with steel cable. It is used for external laying over long distances within telephone networks. Supports cable TV signal transmission as well as data transmission. Suitable for cable ducts and overhead installations.

FOCL advantages:

Information transmission over FOCL has a number of advantages over copper cable transmission. The rapid introduction of Wols into information networks is a consequence of the advantages arising from the peculiarities of signal propagation in optical fiber.
Wide bandwidth - due to the extremely high carrier frequency of 1014Hz. This gives the potential for the transmission of information streams of several terabits per second over one optical fiber. High bandwidth is one of the most important advantages of optical fiber over copper or any other media.
Low attenuation of the light signal in the fiber. Industrial optical fiber currently produced by domestic and foreign manufacturers has an attenuation of 0.2-0.3 dB at a wavelength of 1.55 microns per kilometer. Low attenuation and low dispersion make it possible to build sections of lines without retransmission up to 100 km or more.
Low noise level in fiber optic cable allows to increase the bandwidth by transmitting various signal modulations with low code redundancy.
High noise immunity. Since the fiber is made of a dielectric material, it is immune to electromagnetic interference from surrounding copper cabling systems and electrical equipment that can induce electromagnetic radiation (power lines, electric motors, etc.). Multi-fiber cables also do not have the EM crosstalk problem inherent in multi-pair copper cables.
Light weight and volume. Fiber optic cables (FOCs) are lighter and lighter than copper cables for the same bandwidth. For example, a 900-pair telephone cable with a diameter of 7.5 cm can be replaced with a single fiber with a diameter of 0.1 cm. If the fiber is “dressed” in multiple protective sheaths and covered with steel tape armor, the diameter of such a FOC will be 1.5 cm, which several times less than the telephone cable under consideration.
High security against unauthorized access. Since the FOC practically does not radiate in the radio range, it is difficult to eavesdrop on the information transmitted over it without disrupting reception and transmission. Monitoring systems (continuous control) of the integrity of the optical communication line, using the properties of high sensitivity of the fiber, can instantly disable the "compromised" communication channel and give an alarm. Sensor systems using the interference effects of propagated light signals (both through different fibers and different polarizations) have a very high sensitivity to vibrations and small pressure drops. Such systems are especially needed when creating communication lines in government, banking and some other special services that impose increased requirements for data protection.
Galvanic isolation of network elements. This advantage of optical fiber lies in its insulating property. Fiber helps avoid electrical ground loops that can occur when two network devices non-insulated computer networks, connected by copper cables, are grounded at different points in the building, for example, on different floors. In this case, a large potential difference can occur, which can damage the network equipment. For fiber, this problem simply does not exist.
Explosion and fire safety. Due to the absence of sparking, optical fiber increases the safety of the network at chemical, oil refineries, when servicing high-risk technological processes.
Profitability of FOCL. The fiber is made of silica based on silica, a widespread and therefore inexpensive material, unlike copper. Currently, the cost of fiber relative to copper pair is 2: 5. At the same time, FOC makes it possible to transmit signals over much longer distances without retransmission. The number of repeaters on long lines is reduced with the use of FOC. With the use of soliton transmission systems, ranges of 4000 km have been achieved without regeneration (that is, only with the use of optical amplifiers at intermediate nodes) at a transmission rate above 10 Gbit / s.
Long service life. Fiber degrades over time. This means that the attenuation in the laid cable gradually increases. However, due to the perfection of modern technologies for the production of optical fibers, this process is significantly slowed down, and the service life of the FOC is approximately 25 years. During this time, several generations / standards of transceiving systems may change.
Remote power supply. In some cases, a remote power supply to the node is required information network... Optical fiber cannot function as a power cable. However, in these cases it is possible to use a mixed cable, when, along with optical fibers, the cable is equipped with a copper conductive element. This cable is widely used both in Russia and abroad.

However, fiber optic cable also has some disadvantages:

The most important of them is the high complexity of installation (when installing connectors, micron accuracy is required, the attenuation in the connector greatly depends on the accuracy of the cleavage of the fiberglass and the degree of its polishing). To install the connectors, welding or gluing is used using a special gel that has the same refractive index of light as fiberglass. In any case, this requires highly qualified personnel and special tools. Therefore, most often, fiber optic cable is sold in the form of pre-cut pieces of different lengths, on both ends of which the connectors of the required type are already installed. It should be remembered that a poorly installed connector dramatically reduces the allowable cable length, which is determined by attenuation.
It should also be remembered that the use of fiber optic cable requires special optical receivers and transmitters that convert light signals to electrical and vice versa, which sometimes significantly increases the cost of the network as a whole.
Fiber optic cables allow signal splitting (for this purpose, special passive couplers are produced for 2-8 channels), but, as a rule, they are used to transfer data in only one direction between one transmitter and one receiver. After all, any branching inevitably weakens the light signal, and if there are many branches, then the light may simply not reach the end of the network. In addition, there is an internal loss in the splitter, so the total signal power at the output is less than the input power.
Fiber optic cable is less durable and flexible than electrical cable. Typical bending radii are around 10 - 20 cm; at smaller bending radii, the center fiber may break. Poorly tolerates cable and mechanical stretching, as well as crushing effects.
The fiber-optic cable is also sensitive to ionizing radiation, due to which the transparency of the glass fiber decreases, that is, the signal attenuation increases. Sudden changes in temperature also adversely affect it, fiberglass can crack.
Fiber optic cable is used only in networks with a star and ring topology. In this case, there are no problems of matching and grounding. The cable provides perfect galvanic isolation of network computers. In the future, this type of cable is likely to supersede electrical cables, or at least strongly suppress them.

FOCL development prospects:

With the growing demands of new network applications, the use of fiber optic technologies in structured cabling systems is becoming increasingly important. What are the advantages and features of using optical technologies in a horizontal cable subsystem, as well as at user workplaces?
Having analyzed the changes in network technologies over the past 5 years, it is easy to see that the SCS copper standards lagged behind the "network arms" race. Not having time to install the SCS of the third category, the enterprises had to switch to the fifth, and now to the sixth, and not far off the use of the seventh category.
Obviously, the development of network technologies will not stop there: gigabit per workplace will soon become a de facto standard, and later de jure, and for LANs (local computer networks) of a large or even medium-sized enterprise, 10 Gbps Etnernet will not be uncommon.
Therefore, it is very important to use such a cable system that would make it easy to cope with the increasing speeds of network applications for at least 10 years - this is the minimum service life of the SCS defined by international standards.
Moreover, when changing the standards for LAN protocols, it is necessary to avoid re-laying new cables, which previously caused significant expenses for the operation of SCS and is simply not acceptable in the future.
Only one transmission medium in SCS meets these requirements - optics. Optical cables have been used in telecommunication networks for over 25 years, and recently they have also found widespread use in cable TV and LAN.
In a LAN, they are mainly used to build trunk cable channels between buildings and in the buildings themselves. , while providing high speed data transfer between segments of these networks. However, the development of modern network technologies actualizes the use of fiber as the main medium for connecting users directly.

New FOCL standards and technologies:

In recent years, several technologies and products have appeared on the market that make it possible to significantly facilitate and reduce the cost of using fiber in a horizontal cable system and connecting it to user workstations.

Among these new solutions, first of all, I would like to highlight optical connectors with a small form factor - SFFC (small-form-factor connectors), plane laser diodes with a vertical cavity - VCSEL (vertical cavity surface-emitting lasers) and optical multimode fibers of a new generation.

It should be noted that the recently approved type of multimode optical fiber OM-3 has a bandwidth of more than 2000 MHz / km at a laser wavelength of 850 nm. This type of fiber provides serial transmission of 10 Gigabit Ethernet data streams over a distance of 300 m. The use of new types of multimode fiber and 850 nm VCSEL lasers provides the lowest cost of implementing 10 Gigabit Ethernet solutions.

The development of new standards for fiber optic connectors has made fiber optic systems a serious competitor to copper solutions. Traditionally, fiber optic systems have required twice as many connectors and patch cords as copper - telecom sites required a much larger area to house optical equipment, both passive and active.

Small form factor optical connectors, recently introduced by a number of manufacturers, provide twice the port density of previous solutions, since each such connector contains two optical fibers at once, rather than one as before.

At the same time, the size of both optical passive elements - crosses, etc., and active network equipment, which can reduce installation costs by four times (compared with traditional optical solutions).

It should be noted that the American standards bodies EIA and TIA in 1998 decided not to regulate the use of any specific type of optical connectors with a small form factor, which led to the appearance on the market of six types of competing solutions in this area at once: MT-RJ, LC, VF-45, Opti-Jack, LX.5 and SCDC. There are also new developments today.

The most popular miniature connector is the MT-RJ type connector, which has one polymer ferrule with two optical fibers inside. Its design was designed by a consortium of companies led by AMP Netconnect based on the MT multi-fiber connector developed in Japan. AMP Netconnect has already presented more than 30 licenses for the production of this type of MT-RJ connector.

Much of the success of the MT-RJ connector is due to its external design, which is similar to that of the 8-pin RJ-45 modular copper connector. Recently, the performance of the MT-RJ connector has improved markedly - AMP Netconnect offers MT-RJ connectors with keys to prevent erroneous or unauthorized connection to the cable system. In addition, a number of companies are developing single-mode versions of the MT-RJ connector.

The company's LC connectors are in high demand in the optical cable solutions market Avaya (http://www.avaya.com). The design of this connector is based on the use of a ceramic ferrule with a diameter reduced to 1.25 mm and a plastic housing with an external lever-type latch to lock into the socket of the connecting socket.

The connector is available in both simplex and duplex versions. The main advantage of the LC connector is its low average loss and its RMS deviation of only 0.1 dB. This value ensures the stable operation of the cable system as a whole. Standard epoxy bonding and polishing procedures are used to install the LC fork. Today, connectors have found their way into 10 Gbps transceiver manufacturers.

Corning Cable Systems (http://www.corning.com/cablesystems) manufactures both LC and MT-RJ connectors at the same time. In her opinion, the SCS industry has made its choice in favor of MT-RJ and LC connectors. The company recently released the first single-mode MT-RJ and UniCam versions of MT-RJ and LC connectors, which feature fast installation times. At the same time, for the installation of UniCam connectors, there is no need to use epoxy glue and poly

Russian State Pedagogical

University named after

abstract

on computer architecture

on the topic:

"Fiber Optic Networks"

Performed: Yunchenko T.

student II course

f-that IOT, group 2.2

Checked:

Saint Petersburg 2004

1. Optical cable device

2. Optical fiber classification

3. Fiber optic transmission of information

4. DWDM and traffic

5. DWDM tomorrow

6. Literature

Fiber optic networks and technologyDWDM

Optical cable device

The main element of an optical cable (OC) is an optical waveguide - a round rod made of an optically transparent dielectric. Because of their small cross-sectional dimensions, optical waveguides are commonly referred to as optical fibers (WF) or optical fibers (OF).

The dual nature of light is known: wave and corpuscular. On the basis of studying these properties, quantum (corpuscular) and wave (electromagnetic) theories of light have been developed. These theories cannot be opposed. Only in their totality do they allow one to explain the well-known optical phenomena.

An optical fiber consists of a core, through which light waves propagate, and a cladding. The core is used to transmit light waves. The purpose of the cladding is to create better reflection conditions at the “core - cladding” boundary and to protect against energy radiation into the surrounding space.

In general, three types of waves can propagate in an optical fiber: guided, outgoing, and radiated. The action and predominance of any type of waves are primarily associated with the angle of incidence of the wave at the “core - cladding” OF boundary. At certain angles of incidence of the rays on the OF end face, the phenomenon of total internal reflection at the “core - shell” boundary of the OF takes place. Optical radiation is, as it were, locked in the core and propagates only in it.

Optical fiber classification

Distinguish between single-mode and multimode modes of transmission of radiation over the OF. In the multimode mode of radiation propagation along the OF, the condition of total internal reflection is satisfied for an infinite set of beams. This is possible only for OFs, whose cores are much larger than the propagated wavelengths. Such OFs are called multimode.

In single-mode optical fibers, unlike multimode, only one beam propagates, and, therefore, signal distortion caused by different propagation times of different beams is absent.

All optical fibers are divided into groups according to the type of propagating radiation, into subgroups according to the type - according to the type of refractive index profile, and into types - according to the material of the core and shell.

The following OM groups are distinguished:

Multimode (M)

Single-mode polarization-free radiation (E)

Single-mode with polarization conservation (P)

The group of multimode optical fiber is subdivided into two subgroups:

With stepped refractive index (C)

Gradient refractive index (G)

In addition, OM are subdivided into the following types:

Quartz core and cladding

The core is quartz, and the shell is polymeric

Multicomponent glass core and cladding

Polymer core and sheath

By purpose, optical communication cables are divided into:

Urban

Zone

Trunk

Depending on the installation conditions, a distinction is made between fixed and line optical cables. The latter, in turn, are divided into cables intended for laying in sewers and collectors, in the ground, for hanging on supports and racks, for underwater laying.

Fiber optic transmission of information

When compared with other methods of transferring information, the order of magnitude of TB / s is simply unattainable. Another plus of such technologies is transmission reliability. Fiber optic transmission does not have the disadvantages of electrical or radio signal transmission. There is no interference that could damage the signal and there is no need to license the use of radio frequency. However, not so many people imagine how information is transmitted over optical fiber in general, and even more so they are not familiar with specific implementations technologies. We will consider one of them - DWDM technology (dense wavelength-division multiplexing).

First, let's look at how information is transmitted over optical fiber in general. Optical fiber is a waveguide through which electromagnetic waves propagate with a wavelength of the order of a thousand nanometers (10-9 m). This is a region of infrared radiation that is not visible to the human eye. And the main idea is that with a certain choice of fiber material and its diameter, a situation arises when for some wavelengths this medium becomes almost transparent and even when it hits the interface between the fiber and the external environment, most of the energy is reflected back into the fiber. This ensures that radiation passes through the fiber without any significant losses, and the main task is to receive this radiation at the other end of the fiber. Of course, for such brief description hides a huge and difficult work of many people. Don't think that such material is easy to create or that the effect is obvious. On the contrary, this should be treated as a great discovery, since now it provides the best way transmission of information. It should be understood that the material of the waveguide is a unique development and the quality of data transmission and the level of interference depend on its properties; Waveguide insulation is designed to minimize energy output.

Specifically, a technology called multiplexing means you are transmitting multiple wavelengths at the same time. They do not interact with each other, and when receiving or transmitting information, interference effects (imposition of one wave on another) are insignificant, since they are most pronounced at multiple wavelengths. Here we are talking about the use of close frequencies (frequency is inversely proportional to the wavelength, so it doesn't matter what to talk about). A device called a multiplexer is a device for encoding or decoding information into waveforms and back. After this short introduction, let's move on to a specific description of the DWDM technology.

The main characteristics of DWDM multiplexers that distinguish them from just WDM multiplexers:
using only one transparency window of 1550 nm, within the amplification region of EDFA nm (EDFA - optical amplification system; EDFA - optical repeater, it allows you to recover the optical power of the signal lost during its passage along a long line, without converting into an electrical signal and vice versa. Optical fiber doped with the rare earth element erbium, has the ability to absorb light at one wavelength and emit at another wavelength.An external semiconductor laser sends infrared light with a wavelength of 980 or 1480 nanometers into the fiber, exciting erbium atoms.When an optical signal with a length of waves from 1530 to 1620 nanometers, excited erbium atoms emit light with the same wavelength as the input signal. Elimination of the conversion of light signals into electrical signals and vice versa simplifies and reduces the cost of amplifying equipment and allows not to introduce additional distortions during conversions. EDFA amplifiers are used on " long-range "off" lines where it is difficult to install complex intermediate amplifier equipment (for example, a submarine cable). For reference, let's say that the wavelength of visible light is 400-800 nm.

In addition, since the name itself speaks of a dense transmission of channels, the number of channels is greater than in conventional WDM schemes, and reaches several dozen. Because of this, there is a need to create devices that are capable of adding or removing a channel, in contrast to conventional schemes, when all channels are encoded or decoded at once. Associated with such devices operating on one channel out of many is the concept of passive wavelength routing. It is also clear that dealing with a large number of channels requires greater precision in signal encoders and decoding devices and places higher demands on line quality. Hence the obvious increase in the cost of devices - while reducing the price for the transmission of a unit of information due to the fact that now it can be transmitted in greater volume.
This is how the demultiplexer works with a mirror (diagram in Fig. 1a). The incoming multiplex signal goes to the input port. This signal then passes through the plate waveguide and is distributed over a plurality of waveguides, which are an arrayed waveguide grating (AWG) diffraction structure. As before, the signal in each of the waveguides remains multiplexed, and each channel is represented in all waveguides, that is, so far only parallelization has occurred. Further, the signals are reflected from the mirror surface, and as a result, the light fluxes are again collected in the waveguide-plate, where they are focused and interfered. This leads to the formation of an interference pattern with spatially separated maxima, and usually the geometry of the plate and mirror is calculated so that these maxima coincide with the output poles. Multiplexing occurs in the opposite way.

Figure: 1. Schemes of DWDM-multiplexers: a) with a reflective element; b) with two waveguides-plates

Another way of constructing a multiplexer is based not on one, but on a pair of waveguide plates (Fig. 1b). The principle of operation of such a device is similar to the previous case, except that an additional plate is used here for focusing and interference.
DWDM multiplexers, being purely passive devices, introduce a large signal attenuation. For example, the loss for a device (see Fig. 1a) operating in demultiplexing mode is 10-12 dB, with long-range crosstalk less than –20 dB and a signal half-width of 1 nm (based on Oki Electric Industry). Due to high losses, it is often necessary to install an optical amplifier in front of the DWDM multiplexer and / or after it.
The most important parameter in dense wavelength division multiplexing technology is undoubtedly the distance between adjacent channels. Standardization of the spatial arrangement of channels is necessary if only because on its basis it will be possible to begin conducting tests for the mutual compatibility of equipment from different manufacturers. The Telecommunications Standardization Sector of the International Telecommunication Union ITU-T has approved a DWDM frequency plan with 100 GHz channel spacing, which corresponds to a wavelength difference of 0.8 nm. The issue of information transmission with a difference in wavelengths of 0.4 nm is also being discussed. It would seem that the difference can be made even smaller, thereby achieving a higher throughput, but this creates purely technological difficulties associated with the manufacture of lasers generating a strictly monochromatic signal (constant frequency without interference), and diffraction gratings, which separate maxima in space corresponding to different wavelengths. When using 100 GHz separation, all channels evenly fill the used range, which is convenient when setting up equipment and reconfiguring it. The choice of the separation interval is determined by the required bandwidth, the type of laser and the degree of noise on the line. However, it should be borne in mind that when working even in such a narrow range (nm), the effect of nonlinear interference at the boundaries of this area is very significant. This explains the fact that with an increase in the number of channels it is necessary to increase the laser power, but this, in turn, leads to a decrease in the signal-to-noise ratio. As a result, the use of a more rigid seal is not yet standardized and is under development. Another obvious disadvantage of increasing density is a decrease in the distance over which the signal can be transmitted without amplification or regeneration (more on this below).
Note that the nonlinearity problem mentioned above is inherent in silicon-based amplification systems. More reliable fluorine-zirconate systems are now being developed, providing greater linearity (over the entire nm region) of the gain. With an increase in the EDFA working area, it becomes possible to multiplex 40 STM-64 channels with an interval of 100 GHz with a total capacity of 400 GHz per fiber (Fig. 2).

Figure: 2. Spectral placement of channels in fiber

The table shows specifications one of the powerful multiplex systems using a 100/50 GHz frequency plan manufactured by Ciena Corp.

System level	Capacity, Gbps	channels of 2.5 Gbps)
	OC-48 / (STM-16) / OC-48c / STM-16c
Frequency plan
Possible configurations	5 spans of 25 dB - (500 km) 2 spans of 33 dB - (240 km)
System Error Rate (BER)
Channel interfaces		Short / intermediate distances, STM-16 / G.957 I-16 & S.16.1, intra-office applications
Input signal level, dBm	-18 to -3
Output signal level, dBm
Wavelength of injected radiation, nm
Network control	Control system	WaveWatch by CIENA via SNMP or TMN
Standard interface	VT100 (TM) asynchronous RS-232, remote access via Telnet, ITU TMN, TL-1, SNMP
Channel health monitoring	Channel bit errors via SDH B1 overhead, optical power control on each channel
Remote interfaces	RS-422 / X.25 (TL-1 interface), IP / 802.3 over 10Base-T
Optical service channel	2.048 Mbps at 1625 nm
Nutritional characteristics	Supply voltage, V, direct current	-48 to -58
Power consumption at 40 channels, W	800 typical, 925 (maximum) - rack 1, 1000 typical, 1250 (maximum) - rack 2

Let's take a closer look at the optical amplification system. What is the problem? The signal is initially generated by a laser and sent to the fiber. It spreads along the fiber undergoing changes. The main change to deal with is signal dispersion (dispersion). It is associated with nonlinear effects arising from the passage of a wave packet in a medium and is obviously explained by the resistance of the medium. This raises the problem of long distance transmission. Large - in the sense of hundreds or even thousands of kilometers. This is 12 orders of magnitude longer than the wavelength, so it is not surprising that even if the nonlinear effects are small, then in total at such a distance they must be reckoned with. Plus, nonlinearity can be in the laser itself. There are two ways to achieve reliable signal transmission. The first is the installation of regenerators, which will receive the signal, decode it, generate a new signal, completely identical to the incoming one, and send it further. This method is effective, but such devices are very expensive and it is difficult to reconfigure the system to increase their capacity or add new channels that they must handle. The second method is simply optical signal amplification, completely analogous to sound amplification in a music center. This amplification is based on EDFA technology. The signal is not decoded, but only its amplitude is increased. This allows you to get rid of speed losses in the amplification nodes, and also removes the problem of adding new channels, since the amplifier amplifies everything in a given range

On the basis of EDFA, the power loss in the line is overcome by optical amplification (Fig. 3). Unlike regenerators, this "transparent" gain is not tied to the bit rate of the signal, which allows information to be transmitted at higher rates and increases throughput until other limiting factors such as chromatic dispersion and polarization mode dispersion come into play. EDFA amplifiers are also capable of amplifying a multi-channel WDM signal, adding another dimension to the bandwidth.

Figure: 3. Optical communication systems based on: a) a cascade of regenerative repeaters; b) a cascade of optical amplifiers EDFA

Although the optical signal generated by the original laser transmitter has a well-defined polarization, all other nodes along the path of the optical signal, including the optical receiver, should exhibit a weak dependence of their parameters on the direction of polarization. In this sense, EDFA optical amplifiers, characterized by a weak polarization dependence of the gain, have a tangible advantage over semiconductor amplifiers. In fig. 3 shows the schemes of both methods.
Unlike regenerators, optical amplifiers introduce additional noise that must be taken into account. Therefore, along with the gain, one of the important parameters of the EDFA is the noise figure. EDFA technology is cheaper, for this reason it is more often used in real practice.

Since EDFA, at least in terms of price, looks more attractive, let's break down the main characteristics of this system. This is the saturation power, which characterizes the output power of the amplifier (it can reach or even exceed 4 W); gain, defined as the ratio of the power of the input and output signals; the amplified spontaneous emission power determines the level of noise generated by the amplifier itself. Here it is appropriate to give an example of a musical center, where you can trace analogies in all these parameters. The third (noise level) is especially important, and it is desirable that it be as low as possible. Using an analogy, you can try turning on the music center without starting any disc, but still turn the volume knob to the maximum. In most cases, you will hear some noise. This noise is generated by amplification systems simply because they are powered. Similarly, in our case, spontaneous emission occurs, but since the amplifier is designed to emit waves in a certain range, photons of this particular range are more likely to be emitted into the line. This will create (in our case) light noise. This imposes a limitation on the maximum line length and the number of optical amplifiers in it. The gain is usually adjusted to restore the original signal level. In fig. 4 shows the comparative spectra of the output signal in the presence and absence of a signal at the input.

Figure: 4. The output spectrum of the EDFA, taken by the spectral analyzer (ASE - noise spectral density)

Another parameter that is convenient to use when characterizing an amplifier is the noise factor - this is the ratio of the signal-to-noise parameters at the input and output of the amplifier. In an ideal amplifier, this parameter should be equal to unity.
There are three applications for EDFA amplifiers: preamplifiers, line amplifiers, and power amplifiers. The former are installed directly in front of the receiver. This is done to increase the signal-to-noise ratio, which allows simpler receivers to be used and can reduce equipment costs. Linear amplifiers are designed to simply amplify the signal in long lines or in the case of branching of such lines. Power amplifiers are used to amplify the output immediately after the laser. This is due to the fact that the laser power is also limited and sometimes it is easier to just install an optical amplifier than to install a more powerful laser. In fig. 5 shows schematically all three applications of EDFA.

Figure: 5. Application different types optical amplifiers

In addition to the direct optical amplification described above, an amplifying device using the Raman amplification effect and developed at Bell Labs is currently being prepared for the market. The essence of the effect lies in the fact that a laser beam of a certain wavelength is sent from the receiving point towards the signal, which shakes the crystal lattice of the waveguide in such a way that it begins to emit photons in a wide spectrum of frequencies. Thus, the overall level of the useful signal rises, which makes it possible to slightly increase the maximum distance. Today this distance is 160-180 km, compared to 70-80 km without Raman amplification. These devices, manufactured by Lucent Technologies, will hit the market in early 2001.

What was discussed above is technology. Now a few words about the implementations that already exist and are actively used in practice. First, let us note that the use of fiber-optic networks is not only the Internet, and perhaps not so much the Internet. Voice and TV channels can be transmitted over fiber optic networks. Second, let's say that there are several different types of networks. We are interested in long-distance trunk networks, as well as localized networks, for example, within one city (the so-called metro solutions). At the same time, for backbone communication channels, where the rule “the thicker the pipe, the better” works perfectly, DWDM technology is the optimal and reasonable solution. A different situation arises in urban networks, in which the requests for traffic transmission are not as large as for backbone channels. Here carriers use good old SDH / SONET transport operating in the 1310 nm wavelength range. In this case, to solve the problem of insufficient bandwidth, which, by the way, is not very acute for metropolitan networks, you can use the new SWDM technology, which is a kind of compromise between SDH / SONET and DWDM (for more information about SWDM technology, see our CD-ROM ). In accordance with this technology, the same fiber optic ring nodes support both single-channel data transmission at 1310 nm and WDM at 1550 nm. Savings are achieved by “switching on” an additional wavelength, which requires adding a module to the appropriate device.

DWDM and traffic

One of the important points when using DWDM technology is the transmitted traffic. The fact is that most of the equipment that exists today supports the transmission of only one type of traffic at one wavelength. As a result, a situation often arises when the traffic does not completely fill the fiber. Thus, less "dense" traffic is transmitted over a channel with a formal bandwidth equivalent to, for example, STM-16.
Currently, equipment appears that implements the full load of wavelengths. In this case, one wavelength can be "filled" with heterogeneous traffic, for example, TDM, ATM, IP. An example is the equipment of the Chromatis family manufactured by Lucent Technologies, which can transmit all types of traffic supported by I / O interfaces on one wavelength. This is achieved through the built-in TDM cross-connect and ATM switch. Moreover, the additional ATM switch is not pricing. In other words, the additional functionality of the equipment is achieved at practically the same cost. This allows us to predict that the future belongs to universal devices capable of transmitting any traffic from

optimal use of bandwidth.

DWDM tomorrow

Smoothly moving on to the development trends of this technology, we will certainly not discover America if we say that DWDM is the most promising optical data transmission technology. This can be largely attributed to the rapid growth of Internet traffic, the growth rates of which are approaching thousands of percent. The main starting points in development will be the increase maximum length transmission without optical amplification of the signal and the implementation of a larger number of channels (wavelengths) in one fiber. Today's systems transmit 40 wavelengths, which corresponds to a 100 gigahertz frequency grid. Next in line to enter the market are devices with a 50-gigahertz grid supporting up to 80 channels, which corresponds to the transmission of terabit streams over a single fiber. And today one can already hear statements from development laboratories such as Lucent Technologies or Nortel Networks about the imminent creation of 25 GHz systems.
However, despite such a rapid development of engineering and research thought, market indicators are making their own adjustments. The past year was marked by a serious drop in the optical market, as evidenced by a significant drop in the share price of Nortel Networks (29% in one day of trading) after it announced difficulties in selling its products. Other manufacturers found themselves in a similar situation.
At the same time, while some saturation is observed in the western markets, the eastern ones are just beginning to unfold. The most striking example is the Chinese market, where a dozen nationwide operators are racing to build backbone networks. And if "they" have questions of construction backbone networks have been practically resolved, then in our country, sadly enough, there is simply no need for thick channels to transmit our own traffic. Nevertheless, the exhibition "Departmental and Corporate Communication Networks" held in early December revealed the great interest of domestic communications workers in new technologies, including DWDM. And if such monsters as "Transtelecom" or "Rostelecom" already have transport networks on a national scale, then the current power engineers are just beginning to build them. So, despite all the troubles, optics is the future. And DWDM will play a significant role here.

Literature

1. http: // www. ***** / production. php4? & rubric97

2. ComputerPress magazine # 1 2001

Signal transmission methods different types, data and control commands over fiber-optic communication lines began to be actively implemented in the last decade of the last century. However, for a long time they could not make serious competition (at least in the TSB segment) coaxial cable and twisted pair. Despite such disadvantages as high resistance and capacitance, which significantly limits the range of signal transmission, coaxial cable and twisted pair were prevalent in security systems. Today the situation is beginning to change, and I would venture to say that these changes are cardinal. No, in small systems where video and control signals need to be transmitted to short distances, coaxial cable and twisted pair are still indispensable. In large and especially distributed systems, fiber has practically no alternative.
The fact is that fiber-optic equipment has become much more affordable today and the trend towards its further reduction in price is quite stable.
So fiber optics now makes it possible to offer a security system customer not only a reliable, but also a cost-effective solution. Using a light beam for signal transmission, wide bandwidth allows high quality signal to be transmitted over long distances without the use of amplifiers and repeaters.
The main advantages of using fiber optics are known to be:
- wider bandwidth (up to several gigahertz) than copper cable (up to 20 MHz);
- immunity to electrical interference, no "earth loops";
- low losses during signal transmission, signal attenuation is about 0.2-2.5 dB / km (for coaxial cable RG59 - 30 dB / km for a 10 MHz signal);
- does not cause interference in adjacent "copper" or other fiber-optic cables;
- long transmission distance;
- increased security of data transmission;
– good quality the transmitted signal;
- The fiber optic cable is miniaturized and lightweight.

How the fiber optic line works
Fiber optics is a technology in which light is used as an information carrier, no matter what type of information we are talking about: analog or digital. Infrared light is commonly used and fiberglass is the transmission medium.
Fiber optic equipment can be used to transmit analog or digital signal of various types.
In its simplest form, a fiber optic link consists of three components:
- a fiber-optic transmitter for converting an input electrical signal from a source (for example, a video camera) into a modulated light signal;
- a fiber-optic line through which the light signal is transmitted to the receiver;
- a fiber-optic receiver that converts a signal into an electrical one that is almost identical to the source signal.
Light emitting diode (LED) (or semiconductor laser - LD) is the source of light propagated through optical cables. At the other end of the cable, a receiving detector converts the light signals into electrical signals. Fiber optics relies on a special effect - refraction at the maximum angle of incidence, when total reflection occurs. This phenomenon occurs when a ray of light leaves a dense medium and enters a less dense medium at a certain angle. The inner core (strand) of a fiber optic cable has a higher refractive index than the sheath. Therefore, the light beam passing through the inner vein cannot go beyond its limits due to the effect of total reflection (Fig. 1). Thus, the transported signal goes inside the closed environment, making its way from the signal source to its receiver.
The rest of the cable elements only protect the fragile fiber from damage by the external environment of various aggressiveness.

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Figure: 1 Fiber optics are based on the effect of total reflection

Physical parameters of optical fibers
All common fiber types are characterized by two important parameters: attenuation and dispersion.
Distinguish between mode and material dispersion - signal distortions caused by the characteristics of the propagation of light waves in the medium.
Material dispersion is caused by the fact that waves of different lengths propagate at different speeds, which is associated with the peculiarities of the physical structure of the fiber. This effect is especially noticeable when using single mode fiber. Decreasing the source bandwidth and choosing the optimal wavelength leads to a decrease in material dispersion.
Mode dispersion occurs in multimode fiber due to the difference in path lengths traveled by beams of different modes. A decrease in the diameter of the fiber core, a decrease in the number of modes, and the use of a fiber with a gradient profile lead to its decrease.
Signal attenuation in fiber optic cable depends on material properties and external influences. Attenuation characterizes the loss of power of the transmitted signal at a given distance, and is measured in dB / km, where decibel is the logarithmic expression of the ratio of the power coming from the source P1 to the power entering the receiver P2, dB \u003d 10 * log (P1 / P2). A 3 dB loss means half the power is lost. A loss of 10 dB means that only 1/10 of the source power reaches the receiver, a loss of 90%. Fiber optic lines are generally able to function normally at 30 dB loss (receiving only 1/1000 power).
There are two fundamentally different physical mechanisms causing this effect. Loss on absorption. Associated with the transformation of one type of energy into another. An electromagnetic wave of a certain length causes a change in the orbits of electrons in some chemical elements, which, in turn, leads to heating of the fiber. Naturally, the process of wave absorption is the smaller, the shorter its length and the cleaner the fiber material.
Scattering loss. The reason for the decrease in the signal power in this case means that a part of the light flux leaves the waveguide. This is due to inhomogeneities in the refractive index of materials. And with decreasing wavelength, scattering losses increase.

Figure: 2 Optical fiber transparency windows

In theory, the best overall attenuation can be achieved at the intersection of the absorption and scattering curves. The reality is somewhat more complicated and is related to the chemical composition of the environment. In quartz fibers (SiO2), silicon and oxygen are active at a certain wavelength and significantly degrade the transparency of the material in two surroundings.
As a result, three transparency windows are formed (Fig. 2), within which the attenuation has the smallest value. The most common wavelengths are:
0.85 μm;
1.3 μm;
1.55 μm.
For analog transmission, wavelengths of 850 and 1310 microns are more commonly used.
It is for such ranges that special heterolasers have been developed, on which modern FOCLs (fiber-optic communication systems) are based.
Fiber optic with this characteristic is now considered obsolete. The production of optical fiber of the AllWave ZWP type (zero water peak, with a zero peak of water), in which hydroxyl ions are eliminated in the composition of quartz glass, has been mastered quite a long time ago. Such glass no longer has a window, but an aperture in the range from 1300 to 1600 nm.
All transparency windows are in the infrared range, i.e., the light transmitted through the fiber-optic link is not visible to the eye. It is worth noting that radiation visible to the eye can also be introduced into a standard optical fiber. To do this, use either the small blocks found in some OTDRs, or even a slightly modified Chinese laser pointer. With these devices, you can find fractures in the cords. Where the fiber is broken will be seen bright glow... This light decays quickly in the fiber, so it can only be used over short distances (no more than 1 km).

Analog transmission

The simplest video transmitters use amplitude modulation (AM): the intensity of the emitted light changes in proportion to the change in the amplitude of the video signal. To obtain a more stable result, increase the transmission distance of signals, achieve a better signal-to-noise ratio, frequency modulation (FM) is used.
Amplitude modulation (AM) is a type of modulation in which the variable parameter of the carrier signal is its amplitude. The intensity of the emitted light changes in proportion to the change in the amplitude of the video signal. Since it is rather difficult to control the radiation intensity at a high level, even small changes in it introduce significant distortions in the transmitted signal.
Frequency modulation (FM) is a type of analog modulation in which an information signal controls the frequency of light pulses. Compared to amplitude modulation, the amplitude remains constant.
The analog method is used to transmit video and audio signals, control signals, 10 / 100M Ethernet, monitoring the status of contacts.
It should be noted that analog devices are not the best choice for transmitting video or audio information. It can be quite difficult to transmit and receive it via FOCL using analog equipment. In addition, the price differences between analog and similar digital equipment are negligible.
Equipment of this type is present in the assortment of many market players; readers can familiarize themselves with some models in the review part of the article.

S732DV (GE Security, Fiber Option)
The set of analog transceivers is designed to transmit video and data over 1 single-mode or multi-mode fiber at a distance of up to 60 km. Distinctive features devices are a wide range of operating temperatures (from -40 C to +75 C), Plug-and-Play technology, CWDM, SMARTSä diagnostics, which allows testing the system in real time. The equipment is guaranteed for 5 years.

DE7400 (GE Security, EtherNAVä series of the IFS line)

The 2-port series of transceivers are designed to transmit and receive data at 10/100/1000 Mbps over multimode, single-mode fiber, or Cat 5 electrical cable. The DE7400 offers increased climatic protection for operation in extreme temperatures (from -40 C to +85 C). A standard function is contact actuation to initiate a remote alarm in the event of loss of optical communication. The RJ-45 connector has LED indicators for power status and baud rate. It also supports RSTP, QoS / CoS, IGMP, VLAN, SNMP. Supports IEEE 802.3 standards, which makes it possible to connect any devices of the organization local area networks... The equipment comes with a lifetime warranty.
The IFS line of equipment includes equipment with various port configurations.

Receiver / transmitter OVT / OVR-1 ("BIK-Inform")
The equipment of the OVT / OVR-1 series (receiver / transmitter) is designed to transmit analog video signals in real time in video surveillance systems at industrial and long-distance facilities. The device allows transmitting high-quality color and b / w video signals over multimode optical fiber at a distance of up to 5 km in the 25 Hz - 10 MHz frequency band with a signal-to-noise ratio of at least 5 dB. The equipment has a high noise immunity. There is a built-in generator of test signals, AGC systems (automatic level control by the sync signal level), low current consumption - no more than 85mA for the transmitter and 75mA for the receiver. Compact dimensions allow devices to be placed both in DIN-rail mounting cabinets and in small junction boxes. The equipment does not require additional settings and can be operated in the temperature range from -40 ° C to +50 ° C.

SFS10-100 / W-80 (SF&T)

The set, consisting of two analog transceivers, is intended for organizing the 1st 10 / 100M Ethernet data channel over the 1st single-mode fiber. This device, the latest in the SFS10-100 / W-xx series, can extend the signal transmission distance up to 80 km. Operating modes: duplex and half duplex.
Thanks to the support of IEEE 802.3 10 Base-T / 100Base-Tx / 100Base-Fx standards, it is possible to connect most IP-devices used for organizing local networks, as well as for building video surveillance systems.
A wide range of operating temperatures (from -10 to +70 ° C), Plug-and-play support, no need for additional settings and the use of attenuators, as well as compact dimensions (165 x 144 x 33 mm) make the installation of devices as fast and convenient as possible. The modular design allows the SFS10-100 / W-80 to be used as standalone modules and rack-mountable.
All SF&T equipment comes with a 3-year warranty.

SVP-11T / 12R
SVP-13T / 14R ("Spetsvideoproekt")
The devices are designed for signal transmission in television surveillance systems at distances up to 6–12 km. Transmitter and receiver kits provide transmission of one composite video signal over multimode optical cable at wavelengths of 850 and 1310 nm.
The video signal resolution is 570 TVL, the signal-to-noise ratio at the maximum range is no worse than 50 dB, the frequency band is 50 Hz - 8 MHz. System automatic adjustment gain constantly maintains a video signal swing of 1 V. The light signaling indicates the presence or absence of a video signal. The devices have small dimensions, low power consumption, and are equipped with wall mounting elements.
The devices are protected from power reversal - they do not fail if turned on incorrectly. They work in plug and play mode - setup and adjustment are not required during installation.
Signal receivers are also made in a housing designed for installation in standard 19 ”racks.

SVP-21T
SVP-22T ("Spetsvideoproekt")

The SVP-21T and SVP-22T fiber-optic video transmitters are designed to work with outdoor television surveillance cameras. The sealed casing is equipped with sealed glands and has a degree of protection against weathering IP66. Working temperature from -35 to +50 ° С. The signal is transmitted over long distances: up to 6-12 km.
Transmitters SVP-21T and SVP-22T complete with receivers SVP-12R, SVP-14R, SVP-12-2Rack, SVP-14-2Rack provide transmission of one composite video signal over multimode optical cable at wavelengths 850 and 1310 nm. The devices are available with AC power supply with a voltage of 220 V or 24 V. They work in plug and play mode - setup and adjustment during installation is not required. The receiver's automatic gain control system maintains 1 Vp-p at the output at all times.
The pressurized housing has free space for crossing cables of other equipment. Overall dimensions: 200 x 150 x 55 mm.