Laser communication is another method of wireless communication. Laser communication with aliens

E. N. Chepusov, S. G. Sharonin

Today it is impossible to imagine our life without computers and networks based on them. Humanity stands on the threshold of a new world in which a single information space. In this world, communications will no longer be hampered by physical boundaries, time or distance.

Nowadays there are a huge number of networks all over the world that perform various functions and solving many different problems. Sooner or later, but there always comes a moment when throughput the network is exhausted and new communication lines need to be laid. This is relatively easy to do inside a building, but difficulties begin when connecting two adjacent buildings. Special permits, approvals, licenses to carry out work are required, as well as to carry out a number of complex technical requirements and satisfying the considerable financial demands of organizations managing land or sewerage. As a rule, it immediately becomes clear that the shortest path between two buildings is not a straight line. And it is not at all necessary that the length of this path will be comparable to the distance between these buildings.

Of course, everyone knows a wireless solution based on various radio equipment (radio modems, small-channel radio relay lines, microwave digital transmitters). But the number of difficulties does not decrease. The airwaves are oversaturated and obtaining permission to use radio equipment is very difficult, and sometimes even impossible. And the throughput of this equipment significantly depends on its cost.

We offer you to take advantage of the new economical form wireless communication, which arose quite recently, is laser communication. This technology received the greatest development in the USA, where it was developed. Laser communications provides a cost-effective solution to the problem of reliable, high-speed short-range communications (1.2 km) that can arise when connecting telecommunications systems from different buildings. Its use will allow integration of local networks with global ones, integration remote friend from other local networks, as well as provide the needs of digital telephony. Laser communication supports all interfaces necessary for these purposes - from RS-232 to ATM.

How is laser communication accomplished?

Laser communication, unlike GSM communication, allows for point-to-point connections with information transfer rates of up to 155 Mbit/s. In computer and telephone networks, laser communication ensures the exchange of information in the mode full duplex. For applications that do not require high speed transmission (for example, for transmitting video and control signals in technological and security television systems), there is a special cost-effective solution with half-duplex exchange. When you need to combine not only computer, but also telephone networks, models of laser devices with a built-in multiplexer can be used for simultaneous transmission of LAN traffic and digital group telephony streams (E1/ICM30).

Laser devices can transmit any network stream that is delivered to them using optical fiber or copper cable in the forward and reverse directions. The transmitter converts electrical signals into modulated laser radiation in the infrared range with a wavelength of 820 nm and a power of up to 40 mW. Laser communication uses the atmosphere as a propagation medium. The laser beam then hits a receiver that has maximum sensitivity within the wavelength range of the radiation. The receiver converts laser radiation into signals from the electrical or optical interface used. This is how communication is carried out using laser systems.

Families, models and their features

In this section, we would like to introduce you to the three families of the most popular laser systems in the USA - LOO, OmniBeam 2000 and OmniBeam 4000 (Table 1). The LOO family is basic and allows data transfer and voice messages up to 1000 m. The OmniBeam 2000 family has similar capabilities, but operates at longer distance(up to 1200 m) and can transmit video images and a combination of data and voice. The OmniBeam 4000 family can provide high-speed data transfer: from 34 to 52 Mbit/s over distances up to 1200 m and from 100 to 155 Mbit/s up to 1000 m. There are other families of laser systems on the market, but they either cover shorter distances, or support fewer protocols.

Table 1.

Each family includes a set of models that support different communication protocols (Table 2). The LOO family includes economical models that provide transmission distances of up to 200 m (the letter "S" at the end of the name).

Table 2.

The undoubted advantage of laser communication devices is their compatibility with most telecommunications equipment for various purposes(hubs, routers, repeaters, bridges, multiplexers and PBXs).

Installation of laser systems

An important stage in creating a system is its installation. The actual switching on takes a negligible amount of time compared to the installation and configuration of laser equipment, which takes several hours if performed by well-trained and equipped specialists. At the same time, the quality of operation of the system itself will depend on the quality of these operations. Therefore, before presenting typical inclusion options, we would like to pay some attention to these issues.

When placed outdoors, transceivers can be installed on roof or wall surfaces. The laser is mounted on a special rigid support, usually metal, which is attached to the wall of the building. The support also provides the ability to adjust the angle of inclination and azimuth of the beam.

In this case, for ease of installation and maintenance of the system, its connection is made through distribution boxes (RK). The connecting cables are usually fiber optic for data transmission circuits and copper cable for power and control circuits. If the equipment does not have an optical data interface, then it is possible to use a model with an electrical interface or an external optical modem.

The power supply unit (PSU) of the transceiver is always installed indoors and can be mounted on a wall or in a rack that is used for LAN equipment or cross-structured cable systems. A condition monitor can also be installed nearby, which serves to remotely monitor the functioning of transceivers of the OB2000 and OB4000 families. Its use allows for diagnostics of the laser channel, indication of the signal magnitude, as well as looping the signal to check it.

When installing laser transceivers internally, it is necessary to remember that the power of laser radiation decreases when passing through glass (at least 4% on each glass). Another problem is water droplets running down the outside of the glass when it rains. They act as lenses and can cause beam scattering. To reduce this effect, it is recommended to install the equipment near the top of the glass.

To ensure high-quality communication, it is necessary to take into account some basic requirements.

The most important of them, without which communication will be impossible, is that buildings must be within line of sight, and there should be no opaque obstacles in the path of beam propagation. In addition, since the laser beam in the receiver area has a diameter of 2 m, it is necessary that the transceivers are located above pedestrians and traffic at a height of at least 5 m. This is due to ensuring safety regulations. Transport is also a source of gases and dust, which affect the reliability and quality of transmission. The beam must not be projected in close proximity to or cross power lines. It is necessary to take into account the possible growth of trees, the movement of their crowns during gusts of wind, as well as the influence of precipitation and possible disruptions due to flying birds.

The correct choice of transceiver guarantees stable operation of the channel in the entire range of climatic conditions in Russia. For example, a larger beam diameter reduces the likelihood of precipitation-related failures.

Laser equipment is not a source electromagnetic radiation(AMY). However, if placed near devices with EMR, the laser's electronics will pick up this radiation, which can cause a change in the signal in both the receiver and transmitter. This will affect the quality of communication, so it is not recommended to place laser equipment near EMR sources such as powerful radio stations, antennas, etc.

When installing a laser, it is advisable to avoid oriented laser transceivers in the east-west direction, since several days a year sun rays can block the laser radiation for several minutes, and transmission will become impossible, even with special optical filters in the receiver. Knowing how the sun moves across the sky in a specific area, you can easily solve this problem.

Vibration can cause the laser transceiver to shift. To avoid this, it is not recommended to install laser systems near motors, compressors, etc.

Figure 1. Placement and connection of laser transceivers.

Several typical inclusion methods

Laser communication will help solve the problem of short-range communication in point-to-point connections. As examples, let's look at several typical options or methods of inclusion. So, you have a central office (CO) and a branch (F), each of which has a computer network.

Figure 2 shows a variant of organizing a communication channel for the case in which it is necessary to combine F and CO, using as network protocol Ethernet, but as physical environment- coaxial cable (thick or thin). In the CO there is a LAN server, and in F there are computers that need to be connected to this server. With laser systems such as the LOO-28/LOO-28S or OB2000E models, you can easily solve this problem. The bridge is installed in the central center, and the repeater in F. If the bridge or repeater has an optical interface, then an optical minimodem is not required. Laser transceivers are connected via dual fiber optics. The LOO-28S model will allow you to communicate at a distance of up to 213 m, and the LOO-28 - up to 1000 m with a “confident” reception angle of 3 mrad. The OB2000E model covers a distance of up to 1200 m with a “confident” reception angle of 5 mrad. All these models operate in full duplex mode and provide a transfer speed of 10 Mbit/s.

Figure 2. Connecting a remote Ethernet LAN segment based on coaxial cable.

A similar option for combining two Ethernet networks using twisted pair cable (10BaseT) as a physical medium is shown in Figure 3. Its difference is that instead of a bridge and a repeater, concentrators (hubs) are used that have the required number of 10BaseT connectors and one AUI interface or FOIRL for connecting laser transceivers. In this case, it is necessary to install a LOO-38 or LOO-38S laser transceiver, which provides the required transmission speed in full duplex mode. The LOO-38 model can support communication distances up to 1000 m, and the LOO-38S model can communicate up to 213 m.

Figure 3. Connecting a remote Ethernet LAN segment based on twisted pair.

Figure 4 shows a variant of combined data transmission between two LANs (Ethernet) and a group digital stream E1 (PCM30) between two PBXs (in the CO and F). To solve this problem, the OB2846 model is suitable, which provides data and voice transmission at a speed of 12 (10+2) Mbit/s over a distance of up to 1200 m. The LAN is connected to the transceiver using dual optical fiber through a standard SMA connector, and telephone traffic is transmitted via 75 Ohm coaxial cable via BNC connector. It should be noted that multiplexing data and speech streams does not require additional equipment and is performed by transceivers without reducing the throughput of each of them individually.

Figure 4. Integration of computer and telephone networks.

Embodiment high speed transmission data between two LANs (LAN "A" in the CO and LAN "B" in the F) using ATM switches and laser transceivers is shown in Figure 5. The OB4000 model will solve the problem of high-speed short-range communication in an optimal way. You will have the opportunity to transmit E3, OC1, SONET1 and ATM52 streams at the required speeds over a distance of up to 1200 m, and 100 Base-VG or VG ANYLAN (802.12), 100 Base-FX or Fast Ethernet (802.3), FDDI, TAXI 100/ 140, OC3, SONET3 and ATM155 with the required speeds - over a distance of up to 1000 m. The transmitted data is delivered to the laser transceiver using a standard dual optical fiber connected via an SMA connector.

Figure 5. Consolidation of high-speed telecommunications networks.

The examples given are not exhaustive possible options use of laser equipment.

Which is more profitable?

Let's try to determine the place laser communication among other wired and wireless solutions, briefly assessing their advantages and disadvantages (Table 3).

Table 3.

Optical communication is carried out by transmitting information using electromagnetic waves in the optical range. As an example optical communications One can cite the transmission of messages used in the past using fires or semaphore alphabet. In the 60s of the 20th century, lasers were created and it became possible to build broadband systems optical communications, transmitting not only telephone, but also television and computer signals.
Optical communication systems are divided into open, where the signal is transmitted in the atmosphere or space, and closed, that is, using light guides . Below, only open atmospheric communication lines are considered.
An optical atmospheric communication system between two points consists of two paired transceiver devices located within line of sight at both ends of the line and directed towards each other. The transmitter contains a laser generator and a modulator of its optical radiation by the transmitted signal. Modulated laser beam is collimated optical system and is directed towards the receiver. In the receiver, the radiation is focused onto a photodetector, where it is detected and the transmitted information is isolated. Since the laser beam is transmitted between communication points in the atmosphere, its distribution is highly dependent on weather conditions, the presence of smoke, dust and other air pollutants. In addition, turbulent phenomena are observed in the atmosphere, which lead to fluctuations in the refractive index of the medium, beam oscillations and distortions of the received signal. However, despite these problems, atmospheric laser communication turned out to be quite reliable at distances of several kilometers and is especially promising for solving the problem of "the latter". The propagation of laser radiation in the atmosphere is accompanied by a number of phenomena of linear and nonlinear interaction of light with the medium. However, none of these phenomena manifests itself separately. Based on purely qualitative characteristics, these phenomena can be divided into three main groups: absorption and scattering by air gas molecules, attenuation by aerosols (dust, rain, snow, fog) and radiation fluctuations by atmospheric turbulence. The main limiters of the ALS range are thick snow. and dense fog, for which the aerosol attenuation is maximum.The propagation of the laser beam is also strongly influenced by atmospheric turbulence, that is, random spatiotemporal changes in the refractive index caused by the movement of air, fluctuations in its temperature and density. Therefore, light waves propagating in the atmosphere experience not only absorption, but also fluctuations in transmitted power.
Atmospheric turbulence leads to distortions of the wave front and, consequently, to oscillations and broadening of the laser beam and redistribution of energy in its cross section. In the plane of the receiving antenna, this manifests itself in a chaotic alternation of dark and bright spots with a frequency from fractions of a hertz to several kilohertz. In this case, signal fading sometimes occurs (the term is borrowed from radio communications) and the connection becomes unstable. Fading is most pronounced in clear sunny weather, especially in the hot summer months, during the hours of sunrise and sunset, and in strong winds. ALS systems can be used not only in the “last mile” of communication channels, but also as inserts in fiber networks. optical lines in some difficult areas; for communication in mountainous conditions, at airports, between individual buildings of one organization (government bodies, shopping centers, industrial enterprises, university campuses, hospital complexes, construction sites, etc.); when creating spatially dispersed local computer networks; when organizing communications between switching centers and base stations cellular networks; for quickly laying a line with limited installation time. Therefore in lately the interest of domestic producers in this new and promising sector is growing

The functional diagram of a laser communication system is very simple:

· the processing unit receives signals from various standard devices(telephone, fax, digital PBX, local computer network) and converts them into a form acceptable for transmission by a laser modem;

· the converted signal is transmitted by the electro-optical unit in the form of infrared radiation;

· on the receiving side, the light collected by the optical system falls on a photodetector, where it is converted back into electrical signals;

· reinforced and processed electrical signal goes to the signal processing unit, where it is restored to its original form.

Transmission and reception are carried out by each of the paired modems simultaneously and independently of each other. Laser modems are installed in such a way that the optical axes of the transceivers coincide. The main difficulty is adjusting the direction of the optical axes of the transceivers. The divergence angle of the transmitter beam is y different models from a few arc minutes to 0.5°, and the adjustment accuracy must correspond to these values.

After installing the transceiver units, you need to connect them to the cable networks in both buildings. There are many models of devices with a wide variety of interfaces, however, unlike suppliers of radio communication equipment, manufacturers of wireless optics systems adhere to the following general connection ideology: a laser communication line is an emulation of a piece of cable (two twisted pairs or two fibers of an optical cable). Connected via wireless optics local networks function as if they were connected by a dedicated cable. Some models of laser modems have combined interfaces to Ethernet networks and E1 streams. As a result, one atmospheric link can connect the LAN and telephone networks of buildings without the use of a multiplexer.

This is what it looks like installed system atmospheric laser communication. System throughput is 100Mbit/sec at a distance of up to 3! kilometers. photo:

Some wireless remote bridges use infrared laser light to transmit data. Typically, such a device contains a traditional wired Ethernet bridge and a laser modem that provides physical communication. In other words, laser device only sends data bits, and the rest of the work is done by a regular bridge. Laser modems generate radiation with a wavelength of 820 nm, which cannot be detected without special instruments. Obviously, for laser bridges, the emitter and receiver must be located on a straight line visibility. The typical distance between bridges is slightly more than 1 km and is limited by laser power.
One of the main advantages of such systems is their high throughput. Second the advantage is sufficient noise immunity, since infrared radiation does not interact with radio waves. Like fiber optic systems, laser bridges provide high level security. To intercept information, it is necessary to place the appropriate device on the beam line, which, firstly, can be easily detected, and secondly, this is very difficult to implement, since such systems are installed on the roofs of high-rise buildings. The disadvantages of laser-based systems are the influence of weather conditions on the stability of communications. Heavy rain, snow or fog will cause significant beam scattering and signal weakening. The connection can also be affected by sunrise or sunset if the channel is oriented east to west.
Wireless bridges used to permanently connect networks, as a backup link, or as a temporary solution. Many companies are involved in their production. Prices, depending on bandwidth and communication distance, range from 5 to 75 thousand dollars per channel. Expensive, but over time this decision can pay off.

2.5 Gbit/s over laser beam

fSONA Communications presented new system wireless optical communication SONAbeam 2500-M, allowing to achieve data transfer rates of about 2.5 Gbit/s. The system is based on four redundant transmitters operating at a wavelength of 1550 nm with a laser signal output power of 560 mW. At a five-kilometer test site in clear weather, the system worked at maximum speed and virtually no errors.

Security questions

1. What technologies are used to create wireless networks?

2. List the main technologies of radio networks.

3. What is t access points(access point)?

4. Describe 802.11 technology. What is a directional and omnidirectional antenna?

5. What is roaming(roaming).?

6. List technologies alternative to the IEEE 802.11 standard;

7. Characterize the technology Bluetooth.

8. Characterize the technology HiperLAN.

9. What are optical networks?

10. What are microwave systems?

11. Describe the IEEE 802.16 (WiMAX) standard?

12. What is wireless networks based on low-orbit Earth satellites?

13. What devices are included in the infrared system?

14. What is IR radiation?

15. What is atmospheric laser communication?

16. How does reception and transmission occur during atmospheric laser communication?

Laser data transmission systems are designed to organize one-way and duplex communication between objects located within line of sight.
Free Space Optics - FSO technology, which includes atmospheric optical communication (AOLC) and wireless optical communication channel (BOX) is a way wireless transmission information in the short-wave part of the electromagnetic spectrum. It is based on the principle of transfer digital signal through the atmosphere (or outer space) by modulating radiation (infrared or visible) and its subsequent detection by an optical photodetector.
The current state of wireless optical communications makes it possible to create reliable communication channels at distances from 100 to 1500-2000 m in the atmosphere and up to 100,000 km in outer space, for example, for communication between satellites. As an alternative solution to optical fiber, atmospheric optical data transmission lines (AODL) allow you to quickly create a wireless optical communication channel.

1. Atmospheric optical communication link

The rapid development of the telecommunications market requires high-speed data transmission lines. However, laying optical fiber requires a significant investment, and in principle is not always possible.
A natural alternative in this case is wireless communication lines in the microwave range, but the problem of quickly obtaining frequency permissions sharply limits the prospects for their use, especially in large cities.
Another method of wireless communication is optical communication lines (laser or optical communication), using a point-to-point topology or point-to-multipoint access mode. Optical communication is carried out by transmitting information using electromagnetic waves in the optical range. An example of optical communication is the transmission of messages used in the past using bonfires or semaphore alphabet. In the 60s of the 20th century, lasers were created and it became possible to build broadband optical communication systems. The first atmospheric communication line (ALC) in Moscow appeared in the late 60s: it was launched telephone line between the Moscow State University building on the Lenin Hills and Zubovskaya Square with a length of more than 5 km. The quality of the transmitted signal fully complied with the standards. In those same years, experiments with ALS were carried out in Leningrad, Gorky, Tbilisi and Yerevan. In general, the tests were successful, but at that time experts considered that bad weather conditions made laser communication unreliable, and it was considered unpromising.
The use of signals with continuous (analog) modulation, which was used in those years, led to abnormal attenuation optical signal due to the influence of the atmosphere.
The modern widespread use of ALS in many countries around the world began in 1998, when inexpensive semiconductor lasers with a power of 100 mW or more were created, and the use digital processing signal made it possible to avoid abnormal signal attenuation and retransmit the information packet when an error is detected.
At the same time, the need for laser communications arose, as they began to develop rapidly information Technology. The number of subscribers requiring the provision of telecommunications services such as Internet, IP telephony, cable television With a large number channels, computer networks etc. As a result, the “last mile” problem arose (connecting a broadband communication channel to end user). Laying new cable networks requires large capital investments, and in some cases, especially in dense urban areas, is very difficult or even impossible.
The optimal solution to the problem of the last section is the use of wireless transmission lines.
The advantages of wireless communication lines are obvious: they are cost-effective (no need to dig trenches to lay cables and rent land); low operating costs; high throughput and quality digital communications; rapid deployment and change of network configuration; easy overcoming of obstacles - railways, rivers, mountains, etc.
Wireless communications in the radio range are limited by congestion and scarcity frequency range, insufficient secrecy, susceptibility to interference, including intentional and from adjacent channels, increased power consumption. In addition, radio communications require lengthy approval and registration with the assignment of frequencies by the State Communications Supervision Authority of the Russian Federation, rent for the channel, and mandatory certification of radio equipment by the State Commission for Radio Frequencies. The use of laser means eliminates this difficult issue. This is due to the fact that, firstly, the radiation frequency of laser communication systems goes beyond the range in which coordination is necessary (in Russia), and secondly, the lack practical possibilities their detection and identification as means of information exchange.
Basic properties of laser systems:
almost absolute security of the channel from unauthorized access and, as a consequence, a high level of noise immunity and noise immunity due to the possibility of concentrating the entire signal energy in angles from fractions of arc minutes (in laser space communication systems) to tens of degrees (fully accessible indoor communication systems);
high information containers channels (up to tens of Gbit/s)
there are no delays in the transmission of information (ping<1ms) как у радиолиний
the absence of pronounced unmasking signs (mainly collateral electromagnetic radiation) and the possibility of additional camouflage, which makes it possible to hide not only the transmitted information, but also the very fact of information exchange.
In addition, many experts note the biological safety of these systems, since the average radiation power density in laser systems for various purposes is approximately 3-6 times less than the irradiation created by the Sun, as well as the simplicity of the principles of their construction and operation, and the relatively low cost compared to traditional means of transmitting information for a similar purpose.
Design:
The laser communication line consists of two identical stations installed opposite each other within line of sight (Fig. 1).

Rice. 1. ALS design

The structure of all ALS stations is almost the same: interface module, modulator, laser, transmitter optical system, receiver optical system, demodulator and receiver interface module. The transmitter is an emitter based on a pulsed semiconductor laser diode (sometimes a regular LED). The receiver in most cases is based on a high-speed pin photodiode or an avalanche photodiode.
The transmitted data stream from the user equipment goes to the interface module and then to the emitter modulator. The signal is then converted by a highly efficient injection laser into infrared optical radiation, collimated by optics into a narrow beam and transmitted through the atmosphere to the receiver. At the opposite point, the received optical radiation is focused by a receiving lens onto the site of a highly sensitive high-speed photodetector (avalanche or pin photodiodes), where it is detected. After further amplification and processing, the signal is sent to the receiver interface, and from there to the user equipment. Similarly, in duplex mode, counter data flow occurs simultaneously and independently.
Since the laser beam is transmitted between communication points in the atmosphere, its distribution is highly dependent on weather conditions, the presence of smoke, dust and other air pollutants. However, despite these problems, atmospheric laser communication has proven to be quite reliable over distances of several kilometers and is especially promising for solving the “last mile” problem.
Let's consider the influence of the atmosphere on the quality of wireless infrared communications. The propagation of laser radiation in the atmosphere is accompanied by a number of phenomena of linear and nonlinear interaction of light with the medium. Based on purely qualitative characteristics, these phenomena can be divided into three main groups:
1. absorption (direct interaction of a photon beam with atmospheric molecules);
2. scattering by aerosols (dust, rain, snow, fog);
3. fluctuations of radiation due to atmospheric turbulence.

Laser beam communication through the atmosphere has now become a reality. It ensures the transmission of a large amount of information with high reliability over distances of up to 5 km and solves many difficult problems. Therefore, interest in this type of communication has recently increased.

¹Fluctuations (from Latin fluctuatio - fluctuation), random deviations of physical quantities from their average values.
²Internet source: http://laseritc.ru/?id=93

2. Wireless optical communication channel

Wireless optical communication channel (BOX) is a device that transmits data through the atmosphere. It is designed to create a data transmission channel of the Ethernet standard. BOXING consists of two identical transceivers (optical pipes) installed on both sides of the communication channel. Each unit consists of a transceiver module, a visor, an interface cable (5 m long), a guidance system, a bracket, a power supply and an access unit.
The transceiver module includes a transmitter of highly directional optical radiation in the IR range (consisting of an infrared semiconductor LED) and a receiver - a highly sensitive LED. LEDs operate at a wavelength of 0.87 microns. Several examples of domestic manufacturers of BOX systems and their characteristics are described in Table 1.
Table 1. Devices for creating optical communication channels

Figure 2 clearly shows BOX-10M.

Rice. 2. BOX-10M

Working principle:
Let's consider the process of data transmission using an optical channel (Fig. 3). The electrical signal from the Ethernet port travels through the interface cable to the transmitter, where the LED converts it into IR radiation, which passes through the beam splitter and is focused by the lens into a narrow beam. Having passed through the atmosphere, part of the radiation hits the lens of another transceiver, is focused and sent to the receiver by a beam splitter. The receiver converts IR radiation into an electrical signal, which is sent via an interface cable to the Ethernet port. The power supply powers the transmitter, receiver, display unit, and lens anti-fog/icing system.

Rice. 3. General operating principle of the BOX family device.

Transmission reliability is achieved primarily through correct guidance and energy reserves. With proper aiming, the energy reserve of the system should be fourfold for the BOX-10ML and BOX-10M models (in other words, by covering 4/5 of the objective lens, we have a reliable 100% channel in good weather). The BOX-10MPD model has a 16-fold energy reserve. In this case, the availability of the channel throughout the year will be 99.7-99.9%. The higher the energy reserve of the system, the higher the reliability of the channel, which ideally reaches 99.99%.
In addition, reliable system operation is due to the CSMA/CD media access method used in Ethernet networks. Any collision - worsening weather conditions or the appearance of a short-term obstacle leads to retransmission of the packet at the physical level, but even if it happens that the collision will not be heard (this is possible, for example, in the BOX-10ML and BOX-10M models due to the fact that that the switching time from reception to transmission is, of course, equal to 4 μs) and the packet is lost, then higher-level protocols that work with a delivery guarantee will track this incident and the request will be repeated.
A connection through the atmosphere never gives a 100% guarantee of connection, so it is possible that, for example, in bad weather conditions (heavy snowfall, very dense fog, heavy rain, etc.) the channel will not work. But in this case, the cessation of communication will be temporary, and after conditions improve, the connection will be restored on its own. To reduce the likelihood of loss of communication due to weather conditions, it is necessary to install models with a larger operating distance, which increases the energy of the light flux and, as a result, the reliability of the system as a whole.
Another condition for reliable and stable operation of the system is the coincidence of the center of the geometric spot of illumination of the transmitter with the center of the receiver lens. Wind loads, as well as mechanical and seasonal vibrations of the support can remove the system from the light spot area, as a result of which the connection will disappear. The entire design of the systems and the size of the illumination spot from the transmitter are coordinated in such a way that the likelihood of loss of communication due to the above reasons is minimized. When pointing, the following geometric problem is solved: from the point obtained during rough pointing, it is required to move the system to the geometric center of the illumination spot from the light flux of the emitter, finally fixing the pointing system in this position. Using a standard guidance system, this problem is solved in 35 iterations.
Installation:
Transceivers can be installed on roof or wall surfaces. The BOX is mounted on a metal support, which allows you to adjust the angle of inclination horizontally and vertically (Fig. 4). The transceiver is connected through a special access unit; twisted pair category 5 (UTP) is usually used as connecting cables. On the optical channel side, the access unit is connected to the transceiver by an interface cable, which uses a regular twisted pair cable equipped with special connectors. On the other hand, the access unit connects to a computer or network device (router or switch).
The access unit and the transceiver power supply are always installed indoors next to each other. They can be mounted on the wall or placed in the same racks that are used for LAN equipment.
For reliable operation, the following recommendations must be taken into account:
buildings must be within line of sight (the beam must not encounter opaque obstacles along the entire path);
it is better if the device is located as high above the ground as possible and in a hard-to-reach place;
when installing the system, you should avoid orienting the transceivers in the east-west direction (this specific requirement is explained quite simply: the sun's rays at sunrise or sunset can block the radiation for several minutes, and the transmission will stop);
There should be no motors, compressors, etc. near the mounting point, since vibration can lead to the pipe shifting and breaking the connection.

Rice. 4. Guidance system diagram

Connection types:
Figure 5 shows the possible types of BOX connections.

Rice. 5. Types of BOX connections

In various sources there are a large number of names of equipment for wireless data transmission in the infrared wavelength range. Abroad, this class of systems is usually called FSO - Free Space Optics; in the post-Soviet space, there are a number of designations for wireless optical communication systems. As a basis, you should take the abbreviation BOX - wireless optical communication channel, as reflected in the certificate of the Communication system (CCS).

The advantages of a laser channel over a radio channel are that, firstly, it does not create radio interference; secondly, it is more confidential; thirdly, it can be used under conditions of exposure to high levels of electromagnetic radiation.

The schematic diagram of the transmitter is shown in Fig. 1. The transmitter consists of a command encoder made on an ATtiny2313 microcontroller (DD1), an output block on BC847V transistors (VT1, VT2) and an RS-232 interface, which, in turn, consists of a DB9-F connector (for cable) (XP1) and level converter - on MAX3232 (DD3).

The microcontroller reset circuit consists of elements DD2 (CD4011B), R2, C7. The output unit is an electronic switch made on transistor VT1, in the collector circuit of which a laser pointer is connected through a current limiter on transistor VT2. The transmitter is powered by a constant stabilized voltage of 9 - 12 V. Microcircuits DD1, DD2, DD3 are powered by a voltage of 5V, which is determined by the 78L05 stabilizer (DA1).

The DD1 controller is programmed in the BASCOM environment, which allows it to send commands from a personal computer (PC) via the RS-232 interface, from the Bascom terminal using the “echo” function.

The microcontroller has a clock frequency of 4 MHz from an internal oscillator. Packs of pulses with a frequency of about 1.3 KHz from the OS0A (PB2) output are supplied to the output block. The number of pulses in a packet is determined by the number of the command received from the PC.
To enter a command, you need to press any key on the PC keyboard, then when the words “Write command” and “Enter No. 1...8” appear, enter a number from 1 to 8 and press the “Enter” key.

The program for the transmitter microcontroller “TXlaser” consists of a main loop (DO...LOOP) and two interrupt processing subroutines: for reception (Urxc) and for timer 0 overflow (Timer0).

To obtain an output frequency of 1.3 KHz, the timer is configured with a frequency division factor (Prescale) = 1024. In addition, counting starts from the lower value Z = 253 (at a high level on PB2) and reaches 255. A timer overflow interrupt occurs when the processing of which switches the output of PB2, and the timer is again set to the value Z = 253. Thus, a signal with a frequency of 1.3 KHz appears at the output of PB2 (see Fig. 2). In the same subroutine, the number of pulses on PB2 is compared with the specified one, and if they are equal, the timer stops.

In the reception interrupt processing subroutine, the number of pulses that need to be transmitted is set (1 – 8). If this number is greater than 8, the message “ERROR” is displayed in the terminal.

While the subroutine is running, there is a low level at pin PD6 (LED HL1 is turned off), and the timer is stopped.
In the main loop, pin PD6 is high, and the HL1 LED is turned on.
Text of the "TXlaser" program:

$regfile = "attiny2313a.dat"
$crystal = 1000000
$hwstack = 40
$swstack = 16
$framesize = 32

Config Pind.0 = Input "UART - RxD
Config Portd.1 = Output "UART - TxD
Config Portd.6 = Output "LED HL1
Config Portb.2 = Output "output OC0A

"timer configuration 0-division factor=1024:
Config Timer0 = Timer, Prescale = 1024
Stop Timer0 "stop the timer

Dim N As Byte "variable definition"
Dim N0 As Byte

Const Z = 253 "lower limit of the timer count for output frequency = 1.3 KHz
Timer0 = Z

On Urxc Rxd "reception interrupt processing subroutine
On Timer0 Pulse "overflow interrupt routine"

Enable Urxc
Enable Timer0

Do "main loop
Set Portd.6 "turn on LED HL1
Loop

Rxd: "receive interrupt processing subroutine
Stop Timer0
M1:
Print "Write commad"
Input "Enter No. 1...8:" , N0 "command input
If N0 > 8 Then "limit the number of commands
Print "Error"
Goto M1
End If
N0 = N0 * 2
N0 = N0 - 1 "set value of the number of pulses in a packet
Toggle Portb.2
Start Timer0 "start the timer
Return

Pulse: "overflow interrupt processing routine"
Stop Timer0
Toggle Portb.2
Reset Portd.6 "turn off the LED
Timer0 = Z
N = N + 1 "increment in the number of pulses
If N = N0 Then "if the number of pulses = specified
N=0
N0 = 0
Waitms 500 "delay 0.5s
Else
Start Timer0 "otherwise, continue counting
End If
Return
End "end program

The transmitter is made on a printed circuit board measuring 46x62 mm (see Fig. 3). All elements, except the microcontroller, are SMD type. The ATtiny2313 microcontroller is used in a DIP package. It is recommended to place it in the panel for DIP chips TRS (SCS) - 20 in order to be able to “painlessly” reprogram it.

The transmitter circuit board TXD.PCB is located in the "FILE PCAD" folder.
The schematic diagram of the laser channel receiver is shown in Fig. 4. At the input of the first amplifier DA3.1 (LM358N), a low-pass filter formed by elements CE3, R8, R9 and having a cutoff frequency of 1 KHz attenuates background noise of 50 -100 KHz from lighting fixtures. Amplifiers DA3.2 and DA4.2 amplify and increase the duration of received pulses of the useful signal. The comparator on DA4.1 generates an output signal (one), which is supplied through the inverters of the CD4011D (DD2) chip - DD2.1, DD2. The signal synchronously arrives at the contacts of the microcontroller ATtiny2313 (DD1) – T0 (PB4) and PB3. Thus, Timer0, operating in the external pulse counting mode, and Timer1, measuring the time of this counting, are launched synchronously. Controller DD1, performing the function of a decoder, displays received commands 1...8 by setting log.1 on the PORTB pins, respectively PB0...PB7, while the arrival of a subsequent command resets the previous one. When command “8” arrives at PB7, log.1 appears, which, using an electronic switch on transistor VT1, turns on relay K1.

The receiver is powered with a constant voltage of 9 -12V. The analog and digital parts are powered by 5V voltages, which are determined by stabilizers of type 78L05 DA5 and DA2.

In the RXlaser program, Timer0 is configured as a counter of external pulses, and Timer1 as a timer that counts the period of passage of the maximum possible number of pulses (command 8).

In the main cycle (DO...LOOP), Timer1 is turned on when the first command pulse is received (K=0), the condition for enabling the inclusion of timer Z=1 is reset.
In the interrupt processing subroutine, when the Timer1 count coincides with the value of the maximum possible count, the command number is read and set in PORTB. The condition for enabling inclusion of Timer1 is also set - Z=0.
Text of the RXlaser program:

$regfile = "attiny2313a.dat"
$crystal = 4000000
$hwstack = 40
$swstack = 16
$framesize = 32

Ddrb = 255 "PORTB - all outputs
Portb = 0
Ddrd = 0 "PORTD-input
Portd = 255" pull-up PORTD
Config Timer0 = Counter , Prescale = 1 , Edge = Falling "as pulse counter
Config Timer1 = Timer, Prescale = 1024, Clear Timer = 1" as timer
Stop Timer1
Timer1 = 0
Counter0 = 0

"variable definition:
Dim X As Byte
Dim Comm As Byte
Dim Z As Bit
Dim K As Bit

X =80
Compare1a = X "number of pulses in the match register
Z=0

On Compare1a Pulse "interrupt routine by coincidence

Enable Interrupts
Enable Compare1a

Do "main loop
If Z = 0 Then "first condition for turning on the timer
K = Portd.3
If K = 0 Then "second condition for turning on the timer
Start Timer1
Z=1
End If
End If
Loop

Pulse: "subroutine interrupt processing by coincidence
Stop Timer1
Comm = Counter0 "reading from the external pulse counter
Comm = Comm - 1 "definition of the bit number in the port
Portb = 0 "port zeroing
Set Portb.comm "set the bit corresponding to the command number
Z=0
Counter0 = 0
Timer1 = 0
Return
End "end program

The programs "TXlaser" and "RXlaser" are located in the Lazer_prog folder.

The receiver is located on a board measuring 46x62 mm (see Fig. 5). All components are SMD type, with the exception of the microcontroller, which must be placed in a panel for DIP chips of type TRS(SCS) - 20.

Setting up the receiver comes down to setting the end-to-end transmission coefficient and the response threshold of the comparator. To solve the first problem, you need to connect an oscilloscope to pin 7 of DA4.2 and by selecting the value of R18, set the end-to-end transmission coefficient at which the maximum amplitude of noise emissions observed on the screen will not exceed 100 mV. Then the oscilloscope switches to pin 1 of DA4.1 and selecting a resistor (R21) sets the zero level of the comparator. By turning on the transmitter and directing the laser beam to the photodiode, you need to make sure that rectangular pulses appear at the output of the comparator.
The receiver circuit board RXD.PCB is also located in the FILE PCAD folder.

It is possible to increase the noise immunity of the laser channel by modulating the signal with a subcarrier frequency of 30 – 36 KHz. Modulation of pulse trains occurs in the transmitter, while the receiver contains a bandpass filter and an amplitude detector.

The diagram of such a transmitter (transmitter 2) is shown in Fig. 6. Unlike transmitter 1 discussed above, transmitter 2 has a subcarrier generator tuned to a frequency of 30 KHz and assembled on slots DD2.1, DD2.4.. The generator provides modulation of bursts of positive pulses.

The laser channel receiver with a subcarrier frequency (receiver 2) is assembled on the domestic K1056UP1 (DA1) microcircuit. The receiver circuit is shown in Fig. 7. To isolate command pulses, an amplitude detector with a low-pass filter and a pulse normalizer, assembled on logic elements DD3.1, DD3.2, a diode assembly DA3 and C9, R24, are connected to the output of the DA1 10 microcircuit. Otherwise, the circuit of receiver 2 coincides with the circuit of receiver 1.

This chapter discusses laser communication network technology, as well as its advantages, such as cost-effectiveness; low operating costs; high throughput and quality of digital communications, as well as rapid deployment and change of network configuration.

Laser devices can transmit any network stream that is delivered to them using optical fiber or copper cable in the forward and reverse directions. The transmitter converts electrical signals into modulated laser radiation in the infrared range with a wavelength of 820 nm and a power of up to 40 mW. Laser communication uses the atmosphere as a propagation medium. The laser beam then hits a receiver that has maximum sensitivity within the wavelength range of the radiation. The receiver converts laser radiation into signals from the electrical or optical interface used. This is how communication is carried out using laser systems.

The optical range has many characteristic features and, due to its short wavelength, makes it possible to achieve high radiation directivity, significantly reduce the size of antenna systems, form extremely narrow laser beams and obtain a high concentration of electromagnetic radiation in space.

When transmitting information by modulated electromagnetic waves, it is necessary that the modulation frequency be 10...100 times less than the carrier frequency. In addition, modulation frequencies occupy a certain frequency band, and its width is determined by the amount of information transmitted per unit time. For example, the transmission of telegraph text requires a frequency band of 10 Hz, and for television images a frequency band of 107 Hz and a carrier frequency of at least 108 Hz are required. The radio range occupies the frequency band 104…108 Hz and is fully mastered. The information capacity of the communication channel in the microwave range (109..1012 Hz) is higher, but due to the characteristics of the propagation of microwave radiation in the atmosphere, communication between microwave stations is possible only at a line-of-sight distance. In the optical range, only the visible region occupies the frequency band from 41014 to 1015 Hz. Using a laser beam, it is theoretically possible to transmit 1015/107 = 108 television channels, which is several orders of magnitude higher than modern needs, or 1013 telephone conversations. Thus, one of the advantages of optical communication lines is the ability to transmit large amounts of information due to the ultra-wide frequency band. Mastering the optical range: creating laser light sources, sensitive semiconductor optical radiation receivers and developing low-loss fiber LEDs opens up new opportunities for creating communication systems.

The optical range opens up the possibility of creating information and control systems with characteristics that are fundamentally unattainable in the radio range. To date, a variety of ground, aviation and space systems for optical communications, laser ranging, laser systems for aerospace monitoring of the natural environment, aerial reconnaissance systems, collision avoidance systems for moving objects, laser systems for docking spacecraft, laser guidance and laser weapon control systems have been developed.

The potential capabilities of laser information systems, as well as optical methods of information transmission and processing in general, are very great. In many problems, the maximum achievable characteristics are limited only by quantum effects. However, in reality, the potential capabilities of the optical range cannot always be effectively realized in practice. There are many reasons for this.

The performance characteristics of real laser systems are greatly influenced by inevitable fluctuations in laser radiation sources, random changes in the parameters of information processes, the effects of various interferences, and the probabilistic nature of the photo detection operation. Many optical range information systems are built using an open (most often atmospheric) channel. For laser radiation, the atmospheric channel is a channel with a randomly inhomogeneous propagation medium. The effects of absorption of optical radiation by atmospheric gases, molecular and aerosol scattering, distortions of the spatio-temporal structure and disruption of the coherence of laser radiation - all this has a noticeable impact on the energy potential, principles of processing information signals and the range of the created systems. All of the listed features show that the analysis of laser information systems and the assessment of their potential and actually achievable characteristics cannot be carried out without a probabilistic study of the structure of information signals and interference.

To date, numerous results have been accumulated on the probabilistic analysis of various laser systems. However, most of these results seem to be very disparate, they are not based on a unified approach, and they are quite difficult to use in practical problems. The need for additional detailed studies of the probabilistic structure of signals, interference and, in general, information processes in radio optics is associated with the need to improve mathematical models, solve problems of optimizing the structure of signals and systems, and develop new promising algorithms for transmitting, receiving, converting and processing information in optical information systems.

Laser communication is an alternative to radio, cable and fiber optic communications. Laser systems make it possible to create a communication channel between two buildings located at a distance of up to 1.2 km from each other, and transmit telephone traffic (speed from 2 to 34 Mbit/s), data (speed up to 155 Mbit/s) or their combination. Unlike wireless radio systems, laser communication systems provide high noise immunity and transmission secrecy, since unauthorized access to information can only be obtained directly from the transceiver.

A company that uses laser communications to create a main (backup) short-range communication channel will not only avoid the need to lay new wire communications, but also the need to obtain permission to use the radio frequency. In addition, the low level of costs for organizing a high-performance communication channel, as well as the short time it takes to put it into operation, will ensure a quick return on investment. Thus, a wide range of capabilities and undoubted advantages of laser equipment make its use the best solution to the problem of organizing a reliable communication channel between two buildings.