Vols (fiber-optic communication lines). Fiber optic communication lines

Optical fiber is currently considered the most advanced physical medium for transmitting information, as well as the most promising medium for transmitting large flows of information over long distances. The grounds for believing this stem from a number of features inherent in optical waveguides.

1.1 Physical features.

1. Broadband optical signals due to extremely high carrier frequency (Fo=10**14 Hz). This means that information can be transmitted over an optical communication line at a rate of about 10**12 bit/s or Terabit/s. In other words, one fiber can simultaneously transmit 10 million telephone conversations and a million video signals. The data transfer rate can be increased by transmitting information in two directions at once, since light waves can propagate in one fiber independently of each other. In addition, light signals of two different polarizations can propagate in the optical fiber, which makes it possible to double the throughput of the optical communication channel. To date, the limit on the density of information transmitted over optical fiber has not been reached.
Very low (compared to other media) attenuation of the light signal in the fiber. The best examples of Russian fiber have an attenuation of 0.22 dB/km at a wavelength of 1.55 µm, which makes it possible to build communication lines up to 100 km long without signal regeneration. For comparison, the best Sumitomo fiber at 1.55 µm has an attenuation of 0.154 dB/km. Optical laboratories in the USA are developing even more "transparent" so-called fluorozirconate fibers with a theoretical limit of about 0.02 dB/km at a wavelength of 2.5 μm. Laboratory studies have shown that such fibers can be used to create communication lines with regeneration sites over 4600 km at a transmission rate of about 1 Gbit / s.

1.2 Technical features.

1. The fiber is made of silica, which is based on silicon dioxide, a widely used and therefore inexpensive material, unlike copper.
2. Optical fibers have a diameter of about 100 microns, that is, they are very compact and light, which makes them promising for use in aviation, instrument making, and cable technology.
3. Glass fibers - not a metal, in the construction of communication systems is automatically achieved galvanic isolation segments. Using highly durable plastics, cable factories produce self-supporting overhead cables that are metal-free and therefore electrically safe. Such cables can be mounted on the masts of existing power lines, either separately or embedded in a phase conductor, saving significant funds for cable laying across rivers and other obstacles.
4. Communication systems based on optical fibers are resistant to electromagnetic interference, and information transmitted via optical fibers is protected from unauthorized access. Fiber optic communication lines cannot be eavesdropped in a non-destructive way. Any impact on the fiber can be registered by monitoring (continuous monitoring) of the line integrity. Theoretically, there are ways to circumvent protection by monitoring, but the costs of implementing these methods will be so great that they will exceed the cost of intercepted information.

   There is a method of covert transmission of information over optical communication lines. With covert transmission, the signal from the radiation source is modulated not in amplitude, as in conventional systems, but in phase. The signal is then mixed with itself, delayed by some time longer than the coherence time of the radiation source.

   With this method of transmission, information cannot be intercepted by an amplitude radiation receiver, since it will register only a signal of constant intensity.

   To detect the intercepted signal, you will need a tunable Michelson interferometer of a special design. Moreover, the visibility of the interference pattern can be weakened as 1:2N, where N is the number of signals simultaneously transmitted over optical system connections. Can be distributed transmitted information over a plurality of signals or transmit several noise signals, thereby worsening the conditions for intercepting information. Significant power take-off from the fiber is required to tamper with the optical signal, and this tampering is easily detected by monitoring systems.

An important property of optical fiber is durability. The lifetime of the fiber, that is, the preservation of its properties within certain limits, exceeds 25 years, which makes it possible to lay an optical fiber cable once and, as necessary, increase the channel capacity by replacing receivers and transmitters with faster ones.

Fiber technology also has its drawbacks:

1. When creating a communication line, highly reliable active elements are required that convert electrical signals into light and light into electrical signals. Optical connectors (connectors) are also required with low optical losses and a large connection-disconnection resource. The manufacturing accuracy of such communication line elements must correspond to the radiation wavelength, that is, the errors must be of the order of a fraction of a micron. Therefore, the production of such optical link components is very expensive.
2. Another disadvantage is that the installation of optical fibers requires precision, and therefore expensive, technological equipment.
3. As a result, in the event of an accident (break) of an optical cable, the cost of restoration is higher than when working with copper cables.

The advantages of using fiber-optic communication lines (FOCL) are so significant that despite the listed disadvantages of optical fiber, these communication lines are increasingly used to transmit information.

2. Optical fiber

The industry of many countries has mastered the production of a wide range of FOCL products and components. It should be noted that the production of FOCL components, primarily optical fiber, is characterized by a high degree of concentration. Most of the enterprises are concentrated in the USA. With major patents, American firms (primarily CORNING) have an impact on the production and market of fiber optic components throughout the world through licensing agreements with other firms and the creation of joint ventures.

The most important of the components of FOCL is optical fiber. Two types of fiber are used for signal transmission: single-mode and multi-mode. Fibers got their name from the way radiation propagates in them. The fiber consists of a core and a cladding with different refractive indices n1 and n2.

In a single-mode fiber, the diameter of the light guide core is about 8-10 microns, that is, it is comparable to the wavelength of light. With this geometry, only one beam (one mode) can propagate in the fiber.

In a multimode fiber, the size of the light guide core is on the order of 50-60 µm, which makes it possible to propagate a large number of beams (many modes).

Both types of fiber are characterized by two important parameters: attenuation and dispersion.

Attenuation is usually measured in dB/km and is determined by the absorption and scattering losses of radiation in an optical fiber.

Absorption loss depends on the purity of the material, scattering loss depends on the inhomogeneities of the refractive index of the material.

The attenuation depends on the wavelength of the radiation injected into the fiber. At present, signal transmission over fiber is carried out in three ranges: 0.85 µm, 1.3 µm, 1.55 µm, since it is in these ranges that quartz has increased transparency.

Another the most important parameter optical fiber - dispersion. Dispersion is the time scattering of the spectral and mode components of an optical signal. There are three types of dispersion: mode, material and waveguide.

modal dispersion inherent in a multimode fiber and due to the presence of a large number of modes, the propagation time of which is different

material dispersion due to the dependence of the refractive index on the wavelength

waveguide dispersion is due to processes inside the mode and is characterized by the dependence of the mode propagation velocity on the wavelength.

Because the LED or laser emits a spectrum of wavelengths, dispersion causes the pulses to broaden as they propagate through the fiber, and thus generate signal distortion. When evaluating, the term "bandwidth" is used - this is the reciprocal of the pulse broadening when it passes a distance of 1 km along the optical fiber. The bandwidth is measured in MHz*km. From the definition of the bandwidth, it can be seen that the dispersion imposes a limit on the transmission distance and on the upper frequency of the transmitted signals.

If the propagation of light along a multimode fiber, as a rule, modal dispersion prevails, then only the last two types of dispersion are inherent in a single-mode fiber. At a wavelength of 1.3 µm, the material and waveguide dispersions in single-mode fiber cancel each other out, resulting in the highest throughput.

Attenuation and dispersion are different for different types of optical fibers. Single-mode fibers have the best attenuation and bandwidth characteristics, since only one beam propagates through them. However, single-mode radiation sources are several times more expensive than multi-mode ones. It is more difficult to introduce radiation into a single-mode fiber due to the small size of the fiber core; for the same reason, it is difficult to splice single-mode fibers with low losses. Terminating single-mode cables with optical connectors is also more expensive.

Multimode fibers are more convenient for installation, since the size of the fiber core in them is several times larger than in single-mode fibers. It is easier to terminate a multimode cable with optical connectors with low losses (up to 0.3 dB) at the junction. Emitters for a wavelength of 0.85 μm are designed for multimode fiber - the most affordable and cheap emitters produced in a very wide range. But the attenuation at this wavelength for multimode fibers is in the range of 3-4 dB/km and cannot be significantly improved. The bandwidth of multimode fibers reaches 800 MHz * km, which is acceptable for local communication networks, but not enough for trunk lines.

3. Fiber optic cable

The second most important component that determines the reliability and durability of FOCL is fiber optic cable (FOC). Today, there are several dozen companies in the world that produce optical cables. for various purposes. The most famous of them are: AT&T, General Cable Company (USA); Siecor (Germany); BICC Cable (UK); Les cables de Lion (France); Nokia (Finland); NTT, Sumitomo (Japan), Pirelli (Italy).

The determining parameters in the production of fiber optic cables are the operating conditions and the bandwidth of the communication line.

According to the operating conditions, cables are divided into:

• mounting
• station
• zone
• trunk

The first two types of cables are intended for laying inside buildings and structures. They are compact, light and, as a rule, have a small building length.

Cables of the last two types are intended for laying in cable communication wells, in the ground, on supports along power lines, under water. These cables are protected from external influences and have a construction length of more than two kilometers.

To ensure high throughput of communication lines, FOCs are produced containing a small number (up to 8) of single-mode fibers with low attenuation, and cables for distribution networks can contain up to 144 fibers, both single-mode and multimode, depending on the distances between network segments.

In the manufacture of FOC, two approaches are mainly used:

• designs with free movement of elements
• structures with rigid connection between elements

According to the types of structures, cables are stranded, bundled, cables with a profile core, as well as ribbon cables. There are numerous combinations of FOC designs, which, in combination with a wide range of materials used, allow you to choose the cable version that best meets all project conditions, including cost.

A special class is formed by cables embedded in ground wires.

Separately, we consider the methods of splicing the construction lengths of cables.

Splicing of the construction lengths of optical cables is carried out using special-designed cable glands. These sleeves have two or more cable glands, devices for fastening the strength elements of the cables and one or more splice plates. A splice plate is a structure for laying and securing spliced ​​fibers of different cables.

4. Optical connectors

After the optical cable is laid, it is necessary to connect it to the transceiver equipment. This can be done using optical connectors (connectors). Many types of connectors are used in communication systems. Today we will consider only the main types that are most widespread in the world. The appearance of the connectors is shown in the figure.

The characteristics of the connectors are presented in Table 1. When we say that these types of connectors are the most common, this means that most FOCL devices have sockets (adapters) for one of the listed types of connectors. I would like to say a few words about the last section of Table 1. It mentions a new commit type: "Push-Pull".

Table 1:

Fixation "Push-Pull" provides connection of the connector to the socket in the simplest way - on a latch. The locking latch provides a secure connection without the need to rotate the union nut. An important advantage of Push-Pull connectors is the high density of mounting optical connectors on distribution and cross panels and ease of connection.

5. Electronic components of optical communication systems

Now let's touch on the problem of transmission and reception of optical signals. The first generation of optical fiber signal transmitters was introduced in 1975. The transmitter was based on a light emitting diode operating at a wavelength of 0.85 μm in a multimode mode.

Over the next three years, the second generation appeared - single-mode transmitters operating at a wavelength of 1.3 μm.

In 1982, the third generation of transmitters was born - diode lasers operating at a wavelength of 1.55 μm.

Research continued and the fourth generation of optical transmitters appeared, giving rise to coherent communication systems - that is, systems in which information is transmitted by modulation of the frequency or phase of the radiation. Such communication systems provide a much greater range of signal propagation over optical fiber. NTT specialists built a regeneratorless coherent fiber optic STM-16 for a transmission rate of 2.48832 Gb / s with a length of 300 km, and in the NTT laboratories in early 1990, scientists for the first time created a communication system using optical amplifiers at a rate of 2.5 Gb / s over a distance of 2223 km.

The advent of optical amplifiers based on erbium-doped light guides, capable of amplifying signals passing through the light guide by 30 dB, gave rise to the fifth generation of optical communication systems. At present, long-distance optical communication systems over distances of thousands of kilometers are developing rapidly. Transatlantic communication lines USA-Europe TAT-8 and TAT-9, Pacific line USA-Hawaiian Islands-Japan TRS-3 are successfully operated. Work is underway to complete the construction of a global optical communication ring Japan-Singapore-India-Saudi Arabia-Egypt-Italy.

In recent years, along with coherent communication systems, an alternative direction has been developing: soliton communication systems. A soliton is a light pulse with unusual properties: it retains its shape and theoretically can propagate infinitely far along an "ideal" light guide. Solitons are ideal light pulses for communication. The duration of a soliton is approximately 10 trillionths of a second (10 ps). Soliton systems, in which a single bit of information is encoded by the presence or absence of a soliton, can have a throughput of at least 5 Gbit/s at a distance of 10,000 km.

Such a communication system is supposed to be used on the already built transatlantic line TAT-8. To do this, you will have to raise the underwater FOC, dismantle all the regenerators and splice all the fibers directly. As a result, there will be no intermediate regenerator on the underwater pipeline.

6. Application of FOCL in computer networks

Along with construction global networks communication optical fiber is widely used in the creation of local area networks (LAN).

The company "VIMCOM OPTIC", engaged in automation and electronic technologies, develops and installs local and trunk Ethernet networks, Fast Ethernet, FDDI, ATM/SDH using optical communication lines. Firm "VIMCOM OPTIC" does it for three reasons. First, it's profitable. When installing extended network segments, repeaters are not required. Second, it's reliable. Optical communication lines have a very low noise level, which makes it possible to transmit information with an error rate of no more than 10**(-10). Thirdly, it is promising. Fiber-optic communication lines allow you to increase the computing capabilities of the network without replacing cable communications. To do this, you just need to install faster transmitters and receivers. This is important for those users who are focused on the development of their LAN.

The cable for connecting network segments is inexpensive, but the work of laying it can be the largest cost item for installing a network. It will require the labor of not only cable technicians, but also a whole team of builders (plasterers, painters, electricians), which will be expensive, given the increasing cost of manual labor. Basic LAN topologies: "bus", "star", "ring". At present, optical fiber is difficult to use in the construction of a common bus, but it is convenient to use it for point-to-point communication used in star and ring topologies.

The FOCL scheme used, in particular, in the LAN, is arranged as follows:

The electrical signal comes from a network controller installed in a workstation or server (for example, an Ethernet network controller), then goes to the electrical input of the transceiver (for example, an ISOLAN 3Com optical transceiver), which converts electrical signal into optical. An optical cable (for example, OKG-50-2) is connected to the optical connectors of the transceiver using optical connectors (for example, ST).

Consider several options for the construction of FOCL.

1. FOCL inside one building. In this case, a two-fiber OK (Noodles type) is used for communication, which, if necessary, can be laid in a PND-32 tube under a raised floor or along walls in decorative boxes. All work can be carried out by the customer himself, if the supplied cable is terminated with the appropriate connectors.
2. FOCL between buildings is built with a fiber optic cable laid either along cable communication wells or by hanging a fiber optic cable between supports. In this case, it is necessary to ensure the pairing of a thick multi-fiber cable with optical transceivers. For this, cable boxes are used, in which the ends of the FOC are cut, the fibers are identified and the fibers are terminated with connectors corresponding to the selected transceivers. This work can be done in several ways.
   1. It is possible to order a wok in a special version of Break-Out. This is a more expensive option, but the cable can be immediately terminated with optical connectors, the terminated modules (cords similar to installation wires) can be removed from the coupling and connected to the transceiver equipment.
   2. It is possible to weld optical cords with connectors at one end (pig tail) to the fibers cut in the cable box. The length of the pig tail is chosen for user convenience (for example, 3 m).
   3. You can terminate the fibers with connectors and plug the connectors from the inside into optical sockets (coupling) mounted in the wall of the cable box. Outside, a connector of an optical cord is plugged into the coupling, leading to the transceiver equipment.

There are other ways of docking FOC with optical transceivers. Each method has its own advantages and disadvantages. In the practice of specialists of the company "VIMCOM OPTIC" the third method has become widespread, since it is economical, reliable, provides low optical insertion loss due to the use of sockets and connectors with ceramic elements, and is also convenient for users.

Of particular note is the need for an optical cross-connect.

It should be noted that several methods for splicing optical fibers have been developed in recent years. The method of splicing fibers by welding on a special apparatus is considered universal. Such devices are manufactured by BICC (Great Britain), Ericsson (Sweden), Fujikura, Sumitomo (Japan). The high cost of splicers has led to the creation of alternative technologies for splicing optical fibers.

For example, 3M mechanical splices are now used to quickly splice fibers. These are plastic devices measuring 40x7x4 mm, consisting of two parts: a body and a cover. Inside the case there is a special chute into which the fibers to be connected are inserted from different sides. Then a cover is put on, which is also a lock. The special "splice" design securely centers the fibers. It turns out a hermetic and high-quality connection of fibers with losses at the junction of ~ 0.1 dB. Such "splices" are especially convenient for the rapid restoration of damage to FOCL. The time for connecting two fibers does not exceed 30 seconds after the fibers are prepared (the protective coating is removed, a strictly perpendicular chip is made). Installation is carried out without the use of glue and special equipment, which is very convenient when working in a hard-to-reach place (for example, in a cable well).

SIECOR offers another fiber splicing technology in which the fibers are inserted into a precision sleeve. At the junction of the fibers inside the sleeve, a gel based on high-transparency silicone with a refractive index close to that of the optical fiber is placed. This gel provides optical contact between the ends of the spliced ​​fibers and at the same time seals the junction.

Other splicing methods are less common, we will not dwell on them.

The installation of optical communication lines is carried out by the company "VIMKOM OPTIC" using a welding machine of the company "Sumitomo" type 35 SE. This device allows you to weld any type of fiber in manual and automatic modes, tests the fiber before welding, sets the optimal operating parameters, evaluates the quality of the surfaces of the fibers before welding, measures the loss at the fiber junction and, if necessary, gives the command to repeat the welding. In addition, the device protects the welding site with a special sleeve and checks the strength of the welded joint. The machine can splice single-mode and multi-mode fibers with a loss of 0.01dB, which is an excellent result. I would especially like to say about a specially developed method for assessing the quality of welding. In devices of other designs, for example, BICC, the fiber is bent, and laser radiation is emitted at the bend of the fiber to be welded, which is recorded at the bend of the second fiber to be welded by a photodetector. With this method of measurement, the fiber is subjected to excessive bending deformation, which can lead to the formation of cracks in this section of the fiber. Sumitomo conducts measurements in a non-destructive way based on the processing of video information using specially developed algorithms.

For some special applications, optical fibers are available with a special sheath coating or with a complex refractive index profile at the core-sheath interface. It is very difficult to introduce probing radiation into such fibers in the bend region. For Sumitomo devices, working with special fibers is not difficult. Such devices are quite expensive, but we work on such devices. This achieves two goals: 1) high quality welding, 2) high speed of work, which is important when fulfilling important orders (urgent elimination of an accident on the main communication line).

During the installation of FOCL, the line is tested using an optical reflectometer. In the opinion of VIMCOM OPTIC experts, one of the most suitable devices for these purposes is the Ando AQ7220 mini-reflectometer. Light and compact (340x235x100 mm, 4.6 kg with built-in battery for 3-4 hours of operation), it is especially suitable for field work. The device has internal memory, 3.5" floppy drive, hard disk (optional).

The increase in sales leads to a significant reduction in the cost of all FOCL components, and new technologies for building optical networks make it possible to create highly reliable telecommunications.

Computing networks of enterprises in our country are developing at an ever faster pace. Therefore, usually a company faces 2 main problems over time: modernization existing network in the direction of increasing the power of all its components (workstations, active and network equipment) and reorganization of information processing. A situation where a company needs to combine several departments into a single network, such as a warehouse, head office, remote office, accounting department, design department etc. and so on, often does not come immediately, but begins to be solved when disparate processing of information becomes economically unprofitable and leads to loss of time. Only then information department, a service or a similar subdivision begin to rack their brains as to the most economical, with the least time costs, without loss of quality, to combine into a single corporate network Enterprises have multiple local area networks and remote information processing centers.

In addition, the need to transfer data at high speed and without loss of quality comes to the fore. The solution to this problem requires, in addition to the purchase of active network equipment, the organization of communication lines. For this, cable wiring based on copper or fiber optic cable is usually used. However, well-established solutions for organizing short-range communications using copper or fiber optic lines are not always convenient.

Cable laying often entails significant difficulties:

• inability to obtain permission to lay a cable, especially in urban areas;
• not available for rent telephone lines from the operator, or poor communication quality over leased lines;
• high costs of funds and time for laying new communications, as well as due to the high rent for the use of existing communications;
• the use of old communications, which, due to their high workload, can no longer cope with new additional traffic.

From the foregoing, it follows that in some cases the use of wireless connections may be cost effective.

Advantages of wireless data networks:

• a possible alternative to using leased lines;
• economy. For example, for the organization of temporary networks with frequent structural changes in the organization associated with a change in the configuration of the cable network;
• networking of computers where cabling is often technically impossible.

If a few years ago the first wireless network devices were just beginning to appear on the market, now solutions based on wireless access offered by all major system integrators. It is worth mentioning that we are talking about radio access.

Majority wireless devices support Ethernet configuration. From a physical point of view, when organizing wireless network either the point-to-point scheme is used or the networks operate in the point-to-multipoint access mode. In the first case, communication is provided between two devices remote from each other, in the second, several devices are combined into a network.

Technologies and devices used in building wireless networks:

• cellular communications with circuit switching;
• packet radio;
• use of space satellites ( satellite connection);
• usage wireless bridges for LAN connection;
• using the radio interface;
• paging radio communication;
• using laser equipment;
• using optical equipment, etc.

Wireless optical communication channels

To organize the connection of individual LANs, optical lines operating in the infrared part of the spectrum can be used. In the domestic market, several companies offer special optical equipment.

There are solutions for organizing optical communication channels using domestic equipment. Experience in the use of BOKS (wireless optical communication channels) has shown their high reliability, the ability to operate in almost any weather conditions. The use of BOKS for organizing a corporate network of a brewery in Tula made it possible to reduce the total cost of the project by 70% (see Networks and communication systems No. 9 p. 8).

In general, the use of wireless optical communication channels is advisable in the following cases:

• creation of the main and/or backup communication channel;
• association of several local computer networks;
• to solve the problem " last mile";
• emergency communications when rapid deployment is needed;
• for "point-to-point" communication with a maximum distance between "points" up to 1 km;
• creation of main canals;
• for organizing access to general and departmental data transmission networks or for access to the Internet.

The most typical use of optical communication channels is to create wireless connections between individual buildings separated by barriers: roads, squares, railway lanes, water barriers, industrial areas, etc.

Who might be interested in such solutions? These can be companies:

• located in several separate buildings at a distance of up to 1 km from each other;
• having heavy data traffic;
• having several local area networks and remote terminals;
• placing high demands on the reliability of the entire network;
• solving the problem of distributed information processing in a single corporate network.

Optical data transmission

Let us briefly consider the process of data transmission using an optical channel. Through the interface device, network traffic from the cable ( twisted pair or fiber optics) is delivered to the LED operating in the infrared range of the spectrum. The signal is transmitted by a narrow beam of light to a receiving photodiode at the other end of the network. The received light signal is demodulated and converted into a communication protocol.

It should be borne in mind that to organize duplex configurations, a set of equipment is required, consisting of 2 receivers and 2 transmitters BOKS-10MPD.

Wireless optical communication channels have a number of advantages:

• relatively low cost of equipment;
• high reliability of information transfer. Testing the work of BOXES when organizing a wireless LAN connection in Tula showed that the reliability and quality of data transmission is the same as with conventional cable transmission;
• Compactness and low weight, which greatly facilitates both installation and dismantling of the system. Devices can be easily attached to building walls, poles, etc.;
• Ease of operation (all that is required is to periodically (not often) wipe the lenses);
• Minimum terms installations - fast entry in operation (2-3 hours);
• data transfer rate up to 10 Mbps
• installation of BOKS does not require approval by the Gossvyaznadzor bodies;
• increased resistance to interference;
• work in all weather conditions (snow, rain, etc.);
• infrared radiation is harmless to humans.
• Transferability a large number data;
• High connection speed
• No need to occupy frequencies;

It should be noted that despite the short installation time of the system, a certain skill is needed. Therefore, it is better to contact specialists who will be able to fulfill all the requirements for installation and will be responsible for the operation of the system.

The range of BOXES is wide enough and suitable for a wide range of tasks. The table contains a list of devices offered on the Russian market and their brief characteristics.

BOX equipment

As can be seen from the table, the working distance depends on specific model. All devices provide continuous operation of the communication channel in rain, snow, fog. It is worth noting that the theoretical (calculated) distance exceeds the working distance by 3 times. Consider the characteristics of several devices.

The basic product of the family - BOKS-10M, is designed to create a data transmission channel of the Ethernet standard. The device converts electrical signals of the IEEE 802.3 standard (Ethernet) into optical infrared range (850 - 890 nm), transmits them in the atmosphere with a highly directional beam, with subsequent reception on the other side and converts the optical signal into an electrical one.

BOKS-10M consists of two identical transceivers (optical tubes) installed on both sides of the communication channel.

Each unit consists of a transceiver module, a visor, an interface cable (5 meters long), a guidance system, a bracket, a power supply and an access unit.

The transceiver module is a transmitter of highly directional infrared radiation, consisting of an infrared semiconductor LED and a receiver - a highly sensitive LED. LEDs operate at a wavelength of 0.87 microns. The characteristics of the device are presented in the table:

Specifications

Wireless Optical Communication Channel BOKS-10MPD

The principle of operation of this model is similar to BOKS-10M.

• long working distance
• diversity receiver and transmitter (independent housings);
• full duplex

Specifications

Wireless optical communication channel BOX - E1

The principle of operation and composition of this model is similar to BOKS-10MPD. Significant differences are a large working and maximum distance, it complies with the CCITT G.703 specification.

BOKS-E1 is designed to connect equipment with standard digital interfaces to E1 (or T1) channels implemented according to G.703 recommendation. These channels are used in digital systems transmission (for example, in IKM-30, the most common in Russian telephone networks).

Specifications

Mounting

The transceivers can be installed on the surface of roofs or walls. BOX is mounted on a metal support, which provides the ability to adjust the angle of inclination horizontally and vertically. In both planes, the angle of inclination does not exceed 45 degrees, which is quite enough for precise guidance of 2 pipes relative to each other.

The transceiver is connected via a special access block. Commonly used as connecting cables twisted pair category 5 (UTP). On the one hand, the access block is connected to a computer or to a network device if there is a connection to a LAN. The network device is either a router or a switch. From the side of the optical channel, the access unit is connected to the transceiver with an interface cable. An ordinary twisted pair cable equipped with special connectors is used as an interface cable.

Both the access unit and the transceiver power supply are always installed indoors and next to each other. Both are mounted on a wall or in a rack that is used for LAN equipment.

For successful installation, a number of requirements must be met:

• buildings must be within line of sight. The beam must not encounter any opaque obstacles along its entire path.
• devices should be located at some elevation above the ground. This requirement is especially relevant for urban conditions. It is better if no one can touch such a device. This can end badly for both the pedestrian and the BOX. Given the passion of our citizens for free-standing (hanging) equipment, it will be better if the device is located as high as possible above the ground and in a place that is difficult to access;
• when installing the system, the orientation of the transceivers in the east-west direction should be avoided. Such, at first glance, specific requirement it is explained quite simply: the sun's rays can block the radiation for several minutes and the transmission may stop;
• Vibration may affect the operation of the BOX. The presence of a working generator near the device can cause the pipe to shift and break the connection. Therefore, when choosing a mounting location, make sure that there are no motors, compressors, etc. nearby.

Typical Applications

point to point

The length of the point-to-point connection varies depending on the specific equipment model. When creating such a connection, you should always choose the path in such a way that there are no insurmountable obstacles in the future, such as tree growth. Installation of transceivers can be carried out both on the roof of the building and on the wall. An ideal alternative to any cable solution in terms of price, installation speed, investment liquidity.

Access point

Highway

The Ethernet standard (IEEE 802.3) has determined that between two LAN nodes there can be no more than 4 active devices: HUBs, repeaters. However, this limitation is easily eliminated with the help of more intelligent devices: switches, bridges, routers.

Our equipment (for local networks) does not belong to the class of active or passive Ethernet devices, but is a converter of electrical signals into optical ones. Therefore, when creating trunks, the restriction to 4 active devices will not apply if a cross-over cable is used to connect two transceivers at the connection point of two trunk sections. Subject to this rule, the length of the highway is theoretically unlimited.

Combination

In practice, this method is probably the most common. It allows you to model the communication infrastructure in accordance with the task being solved, expediency, cost and efficiency. The skillful application of all methods and technologies in practice is the art of system integration.

Conclusion

So what to choose? Perhaps the table below can help answer this question.

Practical experience of the St. Petersburg company " Computer systems Akropolis", which, within the framework of a long-term project with OJSC "Tulskoe Pivo Brewing Company", used BOXes to integrate into a single corporate network computing facilities factory showed that:

• the equipment works stably in conditions of direct visibility of the connected objects at distances up to 500 m (model BOKS-10M) and up to 1000 m (model BOKS-10MPD);
• at the same time, reliable communication is ensured in almost any weather conditions;
• the achieved communication quality is similar to using a conventional copper or fiber optic cable;
• the channel allows you to exchange data at speeds of 10 Mb / s (set 10M), or 20 Mb / s (for 10MPD);
• The decision to install IR equipment made it possible to reduce the total cost of the project (including the cost of equipment and work done) by 60-70%.

FOCL is a system based on data transmission via optical fiber.

Fiber-optic communication line contributes to reliable data transmission, has high communication quality. The system is able to operate regardless of the presence of electromagnetic interference, and also operates over long distances without amplifiers.

This method of information transmission is based on the use of fiber optics technology, when light is the data carrier.

Components of FOCL

It is customary to divide FOCL equipment into active and passive elements.

A simplified diagram of the operation of all components is to find at one end of the cable an LED or a laser diode that transmits a signal.

During data transmission, the infrared diode generates a pulse according to the type of signal. The photocoder at the other end of the fiber receives and converts the light signal into an electrical signal.

The active components of the system include:

• multiplexer - a device that connects several signals into a single one;
• amplifier - allows you to increase the power of the transmitted signal;
• LEDs and laser diodes - light source in the cable;
• photodiode - signal receiver on the final part of the fiber, converts the received signal;
• modulator - a device for converting a signal from electrical to optical.

Passive elements of FOCL:

• fiber optic cable - the medium through which the signal is transmitted;
• optical coupler - connects several fibers;
• optical cross - a device at the end of the cable that connects it to active elements;
• adhesions - splice fibers;
• connectors - devices for disconnecting or connecting a cable;
• couplers - devices for distributing the power of optics from several fibers into a single one;
• switches - equipment for the redistribution of optical signals.

FOCL construction

Before starting work related to the construction of FOCL, it is necessary to carry out a number of preliminary works, that is, to create a FOCL project.

Its tasks are to determine the capacity of future communication lines; study of the environment through which the system will run; calculation of mass, volumes and total cost of the entire FOCL; creation of a protective system for the communication line; ensuring the security of transmitted data.

The design and construction of FOCL provides for the installation of equipment, the preparation of an environment for cable installation, and the purchase of equipment. Arranging the receipt specifications for the installation of communication lines.

After carrying out the above stages of design and preparation for work, the installation of equipment is carried out: cable laying in the ground, sewers, collectors; installation of modules, fastening of couplings, installation of all active components. After installing the necessary equipment, measures are taken to create a safe environment for the cable.

The finished section of the communication line is tested for basic properties.

Measurement types

Testing a fiber optic communication line is performed by carrying out two types of measurements. The first type evaluates the attenuation of the signal from one end of the cable to the other. On the one hand, a laser is connected, on the other, a photodiode. A change in data current between two components is indicative of fiber loss. The device by which signal attenuation is detected is called an optical tester.

The disadvantage of this equipment is the inability to determine the location of the damage due to which losses occur.

The second type of FOCL measurements is with the help of an optical reflectometer. The device determines the location of defects in the cable, makes measurements of signal loss in any part of the fiber. The data is displayed on the screen in the form of graphs, which show signal levels and distances between different points of the entire system.

Optical budget

The optical budget characterizes the maximum attenuation in the line, which is possible in the communication line. Functioning is possible if the budget is not exceeded. All elements of the system are divided into those that create a signal in the cable and those that reduce it, contributing to the attenuation of the data flow.

Signal generating elements are transceivers and amplifiers. All other elements and equipment create interference and affect signal loss.

Manufacturers of systems indicate the calculation of FOCL in the documentation.

The work of calculations is based on taking into account the sources of attenuation in the fiber, multiplexers, modules, connection sections, the presence of branches. To calculate the optical budget of an FOCL, it is necessary to have data on the length of the measured fiber section in km, the number of connections on optical panels, and the number of welding fasteners.

To ensure the reliability of the entire system, it is necessary to take into account the possibility of increasing signal losses due to external factors independent of the line itself, as well as due to equipment aging.

FOCL (fiber-optic communication lines) uses waves in the optical range (most often in the near infrared) to transmit a signal. The main component in this case is an optical cable, and in addition to it, the network includes active and passive components for amplifying, filtering, protecting and modifying the signal.

FOCL application

To date, FOCL (FOCL) are gradually replacing traditional cable wiring, as they have much better characteristics, in particular, greater throughput, immunity to environmental influences, lower signal attenuation, etc.

The main scope of FOCL are information signal transmission networks ( computer networks, video surveillance, telecommunication access control systems, etc.).

At the same time, at the level of backbone (up to intercontinental) signal transmission lines, fiber optics already occupies a dominant position, while in the subsystems of internal backbones, FOCL is used along with twisted pair.

Characteristics of optical fiber types

Comparison of types of optical cables (to enlarge the image - ):

The main advantages of FOCL

1. Low signal attenuation (about 0.15 dB/km in the 3rd transparency window). This makes it possible to transmit information over significantly greater distances relative to traditional wiring without the use of amplifiers. For optical lines, amplifiers are usually installed after 40-120 km, which is determined by the class of terminal equipment;
2. low weight and dimensions;
3. high level of shielding of lines from interfiber influences (more than 100 dB).
   

   Thus, the radiation of neighboring lines practically does not interact with each other and does not exert mutual influence;

4. high explosion and fire safety in situations of changing chemical or physical parameters;
5. Information Security. Through fiber, information is transmitted from point to point, and it is possible to intercept or eavesdrop on the signal only with physical interference in;
6. optical fibers are highly reliable and durable. Optical fibers are not subject to oxidation, weak electromagnetic effects and destruction under the influence of moisture;
7. high throughput. Other ways of transmitting information lag behind the optical medium in this indicator.

Disadvantages of FOCL

1. low resistance of a standard fiber against radiation (there are doped fibers with high radiation resistance);
2. high cost of optical terminal equipment compared to systems used for traditional lines. Although when compared with the final cost in terms of distance and bandwidth costs, fiber today shows the most top scores relative to competing systems;
3. the difficulty of restoring communication in cases of line breaks;
4. complexity of signal conversion (for interface equipment);
5. complex fiber manufacturing technology, as well as other components of the FOCL network;
6. brittleness of the fiber. With significant deformations, for example, bends, the fibers can be destroyed, cracked and clouded.
   

   To avoid damage to the fiber, the manufacturer's recommendations must be followed, which lists, among other things, the minimum bending radius.

Currently, as optical communication lines use:

• a) optical lines using a fiber optic cable - fiber optic communication lines (FOCL);
• b) optical communication lines without the use of a fiber-optic cable.

Fiber-optic communication lines have the best indicators in terms of data transfer rate, noise immunity, and protection from unauthorized access.

Fiber-optic communication lines (FOCL)

The block diagram of a fiber-optic communication line is shown in fig. 7.11.

Rice. 7.11.

The electrical signal is sent to a transmitter - a transceiver, which converts the electrical signal into a light pulse. The latter is fed into the optical cable through an optical connector. At the receiving point, the optical cable is connected to a receiver-transceiver using an optical connector, which converts the beam of light into an electrical signal.

Depending on the purpose of the FOCL, its length, the quality of the components used structural scheme may change. With significant distances between the points of transmission and reception, a repeater is introduced - a signal amplifier. With a short length of the optical cable (if the building length of the optical cable is enough), cable welding is not needed. Construction length is understood as the length of a single piece of cable supplied by the manufacturer.

Fiber-optic communication lines have the following advantages:

• 1. High noise immunity from external electromagnetic interference and from inter-channel mutual interference.
• 2. A wide range of operating frequencies allows information to be transmitted over such a communication line at a rate of 10 | 2 bit / s = Tbit / s.
• 3. Protection from unauthorized access: FOCL almost does not emit radiation into the surrounding space, and it is almost impossible to manufacture optical energy taps without destroying the cable. And any impact on the fiber can be recorded by monitoring (continuous control) of the integrity of the line.
• 4. The possibility of covert transmission of information.
• 5. Potentially low cost due to the replacement of expensive non-ferrous metals (copper) with materials with unlimited raw materials (silicon dioxide).
• 6. Galvanic isolation of line segments is automatically provided.

However, fiber optic technology also has its drawbacks:

• 1. High cost of equipment.
• 2. Expensive technological equipment is required, both during installation and during operation. When an optical cable breaks, the cost of its restoration is much higher than for the restoration of a copper cable.
• 3. Optical cables are not resistant to radiation.

FOCL is based on optical cables made of

individual light guides - optical fibers.

optical fiber is a thin two-layer thread consisting of a core and a sheath with different refractive indices. To protect the fiber from atmospheric and mechanical influences, a protective coating is applied over the reflective sheath. The design of an optical fiber with a protective coating is shown in Fig. 7.12.

Rice. 7.12.

3 types of optical fibers are used: polymer optical fibers (POF = Plastic Optical Fiber), quartz-polymer optical fibers (PCF = Polymer Cladded Fiber), quartz optical fibers (GOF = Glass Optical Fiber).

Polymer optical fibers are made from polymer materials with high optical properties. Fiber optic cables made of polymer optical fibers are characterized by good flexibility (with a fiber diameter of 1.5 mm, the allowable fiber bending radius is 8 mm) and provide a throughput of up to 2.5 Gbps, which is significantly higher than that of twisted pair (max 1 Gbps). Data transmission range - up to 80 m.

POF is currently widely used. It is used for decorative, architectural and landscape lighting systems, for lighting pools, for safe lighting of hazardous areas. Another area of ​​application can be considered the use of POF for the manufacture of visual indication systems for information panels in consumer, automotive, industrial and medical electronics. SOV is used to create high-speed, inexpensive, electromagnetic interference-free data transmission lines on short distances(automation systems technological processes, transmission of signals from video cameras, optical sensors; local computer networks). For example, POV cables are used in the industry standard PROFIBUS. Figure 7.13 shows the appearance of such a cable with a connector installed.

Quartz-polymer optical fibers are made with a quartz core and a polymer reflective sheath and are designed for intra- and inter-object communication systems. Data transmission range up to 400 m, radius of multiple cable bends - not less than

75 mm. PCF cables are shipped pre-cut with connectors installed. The appearance of one of these cables is shown in fig. 7.13.

Rice. 7.13.

Quartz optical fibers are made of high-purity quartz glass (core and reflective sheath) and are used where large amounts of data must be transmitted at high speeds and over long distances - up to several kilometers (long-distance, intra- and inter-object communication systems: local computer networks LAN (Local Area Networks), MAN networks (Metropolitan Area Networks), WAN networks (Wide Area Networks)).

The transmission of optical energy through an optical fiber is provided by the effect of total internal reflection. Quartz optical fiber is a two-layer cylindrical light guide (Fig. 7.14).

Rice. 7.

in fiber

The material of the inner core has a refractive index n and and the material of the outer layer is n 2, wherein n > n 2, i.e., the inner core material is optically denser than the sheath material. For radiation entering the cylinder at small angles with respect to the axis of the cylinder, the condition of total internal reflection is satisfied: when radiation is incident on the boundary with the cladding, all the radiation energy is reflected into the core of the fiber. The same thing happens with all subsequent reflections; as a result, the radiation propagates along the fiber axis without exiting through the cladding. The maximum off-axis angle at which there is still total internal reflection is given by

Value A 0 is called the numerical aperture of the optical fiber and is taken into account when matching the optical fiber with the emitter. Radiation incident on the end face at angles y>yo(out-of-aperture rays), when interacting with the shell, it is not only reflected, but also refracted; part of the optical energy leaves the fiber. Ultimately, after multiple encounters with the core-sheath boundary, such radiation is completely scattered from the fiber.

Optical fiber is characterized by two important parameters: dispersion and attenuation.

Dispersion, i.e., the dependence of the signal propagation velocity on the radiation wavelength, is the most important parameter of an optical fiber. Since an LED or laser emits a certain spectrum of wavelengths when transmitting information, dispersion leads to a broadening of the pulses when propagating along the fiber and thereby generates signal distortion. When assessing the dispersion, the term "bandwidth" is used - the reciprocal of the pulse broadening when it passes a distance of 1 km along the optical fiber. The bandwidth is measured in megahertz per kilometer (MHz km). Dispersion imposes restrictions on the transmission range and the upper value of the frequency of transmitted signals.

attenuation is determined by the losses due to absorption and scattering of radiation in the optical fiber. Absorption loss depends on the purity of the material, and scattering loss depends on the inhomogeneity of its refractive indices. The attenuation also depends on the wavelength of the radiation introduced into the optical fiber.

Attenuation is quantified by the formula

where P in is the power of the input optical signal; R ex- power of the output optical signal; / - the length of the fiber.

The unit of attenuation is the decibel per kilometer (dB/km).

The values ​​of attenuation and dispersion differ for different types of quartz optical fibers.

Depending on the diameter and profile of the refractive index in the direction from the center to the periphery in the cross section of the fiber, they are divided into multimode fibers with a stepped refractive index profile, single-mode fibers, and multimode fibers with a gradient change in the refractive index. On fig. 7.15 shows the propagation paths of light in various types optical fiber.

Rice. 7.15.

The fiber in (Fig. 7.15, a) is called a fiber with a stepped refractive index profile and multimode, since there are many possible ways, or mod. This multiplicity of modes results in pulse dispersion (broadening) because each mode travels a different path through the fiber, and therefore different modes have different transmission delays as they travel from one end of the fiber to the other. The result of this phenomenon is a limitation on the maximum frequency that can be efficiently transmitted for a given fiber length. Increasing either the frequency or the length of the fiber beyond the limit values ​​essentially leads to the merging of successive pulses, making them impossible to distinguish. For a typical multimode fiber, this limit is approximately 15 MHz km. This means that a video signal with a bandwidth of, for example, 5 MHz can be transmitted over a maximum distance of 3 km (5 MHz? 3 km = 15 MHz km). Attempting to transmit a signal over a greater distance will result in a progressive loss high frequencies. In multimode fiber, the diameter of the light strand is 50; 62.5; 85; 140 µm.

Single-mode fibers (Fig. 7.15, b) very effectively reduce dispersion, and the resulting bandwidth - many GHz km - makes them ideal for long links. Ideally, only one wave propagates through single-mode fibers. They have a much lower attenuation coefficient (depending on the wavelength by 2 ... 4 and even 7 ... 10 times) compared to multimode ones and the highest bandwidth, since the signal is almost not distorted in them. But for this, the diameter of the fiber core must be commensurate with the wavelength. Practically, the diameter is 8 ... 10 microns. Unfortunately, a fiber of such a small diameter requires the use of a powerful, precisely aligned, and therefore relatively expensive laser diode emitter, which reduces their attractiveness for many applications.

Ideally, a fiber with the same order of bandwidth as a single-mode fiber, but with a diameter similar to a multimode fiber, is required to allow the use of low-cost LED transmitters. To some extent, these requirements are met by a multimode fiber with a gradient change in the refractive index (Fig. 7.15, c). It resembles the step index multimode fiber discussed above, but the refractive index of its core is non-uniform - it smoothly changes from a maximum value at the center to lower values ​​at the periphery. This leads to two consequences. First, the light travels along a slightly curving path, and second, and more importantly, the differences in propagation delay between the different modes are minimal. This is because the high modes that enter the fiber at a higher angle and travel a longer path actually start to propagate at a faster rate as they move away from the center into the zone where the refractive index decreases, and generally travel faster. than lower-order modes remaining close to the fiber axis, in the region of high refractive index. The increase in speed just compensates for the greater distance traveled.

Gradient multimode optical fibers are preferable, because, firstly, fewer modes propagate in them and, secondly, their angles of incidence and reflection differ less, and, consequently, the transmission conditions are more favorable.

Although multimode graded-index fibers are not ideal, they still show very good bandwidth. Therefore, in most systems of short and medium length, the choice of this type of fibers is preferable.

The optical signal attenuates in all fibers at a rate that depends on the wavelength of the light source transmitter. There are three wavelengths at which the attenuation of an optical fiber is usually minimal - 850, 1310 and 1550 nm. These are known as transparency windows. For multimode systems, the 850 nm window is the first and most commonly used (lowest cost fiber link). At this wavelength, good quality graded multimode fiber exhibits an attenuation of the order of 3 dB/km, which makes it possible to communicate over distances in excess of 3 km.

At a wavelength of 1310 nm, the same fiber shows even lower attenuation - 0.7 dB / km, thereby allowing a proportional increase in the communication range to about 12 km; 1310 nm is also the first operating window for single-mode fiber optic systems, with attenuation of about 0.4 dB/km, which, in combination with laser diode transmitters, allows you to create links over 50 km long. The second transparency window - 1550 nm - is used to create even longer communication lines (fiber attenuation - less than 0.24 dB/km).

The attenuation values ​​in different transparency windows in multimode and single-mode fibers are given in Table 1. 7.3.

Table 7.3

Attenuation values ​​in multimode and single-mode fibers

To connect the receiver and transmitter, a fiber optic cable (FOC) is used, in which optical fibers are supplemented with elements that increase the elasticity and strength of the cable, and protect the cable from external factors. There are cables for indoor laying, cables for outdoor use (cables that can be buried in the ground; cables that are laid in special sewers; cables that are suspended in open space), cables for long underwater communication lines.

Almost all European manufacturers apply markings on fiber optic cables that comply with the DIN VDE 0888 system. According to this standard, each type of cable is assigned a sequence of letters and numbers that contain all the characteristics of fiber optic cables. Domestic manufacturers use their own classification and their own notation.

Temporary failure of an optical cable or the inability to lay a cable, the need for high protection against electromagnetic interference and interception led to the creation of cableless optical communication lines with different communication ranges.

Optical communication lines without the use of fiber optic cable are divided into long-range optical lines and local wireless optical lines.

The ideology of cableless optics is based on the fact that the optical channel replaces the cable.