In conditions where there is no developed infrastructure of communication networks, the use of radio equipment for data transmission is often the only reasonable option for organizing communication. A data transmission network using radio modems can be quickly deployed in almost any geographic region. Depending on the used transceivers and antennas, such a network can serve its subscribers in an area with a radius from units to tens or even hundreds of kilometers. Radio modems are of great practical value where it is necessary to transmit small amounts of information (documents, certificates, questionnaires, telemetry, answers to queries to databases, etc.). Especially if it is necessary to guarantee the reaction time (response) of the remote device.
Radio modems are often called packet controllers (TNC - Terminal Node Controller) because they include a specialized controller that implements the functions of exchanging data with a computer, managing frame formatting procedures and accessing a common radio channel in accordance with the implemented multiple access protocol. These radio modems are a lot like smart modems for PSTN telephone circuits. Their main difference is that radio modems are designed to work in a single radio channel with many users (in a multiple access channel), and not in a point-to-point channel.
The algorithms for the operation of packet radio networks are regulated by Recommendation AX.25.
AX.25 standard
The AX.25 Recommendation establishes a unified packet exchange protocol, i.e. the order of data exchange, obligatory for all users of packet radio networks. The AX.25 standard is a version of the X.25 standard specially revised for packet radio networks.
A feature of packet radio networks is that the same radio channel is used for data transmission by all network users in multiple access mode. The AX.25 exchange protocol provides for multiple access to the communication channel with busy control. All users (subscribers) of the network are considered equal. Before starting the transmission, the radio modem checks if the channel is free or not. If the channel is busy, the transmission of its data by the radio modem is postponed until it becomes free. If the radio modem finds the channel free, then it immediately starts transmitting its information. It is obvious that at the same moment any other user of the given radio network can start transmitting. In this case, the signals of the two radio modems overlap (conflict), as a result of which their data is highly likely to be seriously distorted under the influence of interference interference. The transmitter radio modem learns about this after receiving a negative acknowledgment for the transmitted data packet from the receiving radio modem or as a result of exceeding the timeout time. In such a situation, he will be obliged to repeat the transmission of this packet according to the already described algorithm. Since the pause before the next communication attempt is set randomly for each device, the probability that the next time the modems will start transmitting at the same time is extremely low.
In packet communication, information in the channel is transmitted in the form of separate blocks - frames. Basically, their format corresponds to the frame format of the well-known HDLC protocol, but there are differences, which are discussed below.
Frame format
| FLAG | ADRES | CONT | CRC-16 | FLAG |
| 011111110 | 14-70 bytes | 1 byte | 2 bytes | 011111110 |
| FLAG | ADRES | CONT | INFORM | CRC-16 | FLAG |
| 011111110 | 14-70 bytes | 1 byte | up to 256 bytes | 2 bytes | 011111110 |
The beginning and end of the frame are marked with FLAG flags, i.e. combinations of the form "011111110", which makes it easier to receive a frame against the background of interference. The ADRES address field contains the addresses of the sender, recipient, and relay stations, if any. The size of the address field can be from 14 to 70 bytes.
The CONT control field defines the type of frame: informational or service. Service frames, in turn, can be subdivided into supervisory and unnumbered ones. Supervisor frames serve to acknowledge receipt of interfering frames or to request retransmission of corrupted frames. Unnumbered frames are intended for establishing a logical connection and in cases of exchange control in the network.
The length of the INFORM information field, which is a network layer packet, in packet radio networks usually does not exceed several hundred bytes. An increase in the length of the information field leads to an increase in the probability of being hit by interference and an increase in the waiting time for packet transmission by other users.
When implementing the network (third) layer of the AX.25 protocol, the protocol definition field is used, which acts as part of the information field and is optional.
The frame check field (CRC-16) is designed to detect errors in the frame during its transmission.
Address field can contain from two to ten logical addresses. The simplest case is an address field of two addresses (two users). If users are out of range, they can use radio modems of other network users as repeaters. There can be up to eight such repeaters for one logical channel. The repeater addresses are also present in the address field of the frame. Thus, the address field is divided into three subfields: recipient, sender, and relay. The format of the address field is as follows:
The addresses entered in it can consist of no more than six characters. If the address is less than six characters long, it is padded with the appropriate number of spaces.
After the address in each subfield there is a secondary user (subscriber) identifier SSID (Secondary Station IDentifier). This is some number from 0 to 15. It determines the level of service of this user, for example, that he has several stations of packet radio communication operating in different bands, supports the functions of electronic mailbox BBS, or is a network node - a NET / ROM relay. A regular user works without a secondary identifier or with an identifier equal to 1. The BBS and gateway identifier can be equal to values from 2 to 9. When the frame passes through the NET / ROM node, the secondary identifier receives values from 10 to 15, depending on whether through how many nodal stations he passed.
The value of the identifier in binary form occupies four bits - from the second to the fifth in the byte following after each address. The first bit of this byte is used as the end of the address field. If it is equal to one, then this is the sign of the last bow of the address field. There is no specific purpose for the sixth and seventh bits of the byte in question, and they can be used on separate networks at the discretion of its users or the network administrator, if any.
The eighth bit in the last byte of the sender and receiver subfield is always set to zero. In the repeater subfield, it is set to one if the frame passed through the repeater, and to zero if not. The setting of the repeater bit is necessary so that the repeaters located in the radio visibility zone of each other follow the sequence of transmitting frames through themselves and perform this procedure strictly in the order indicated by the sender of the frame.
Control field contains information about the frame type, which is used to determine the purpose of the message. The AX.25 protocol uses three main types of frames: I - informational, containing information of a user or an application process; S - supervisory (service), confirming the correct reception of the frame or containing a request to issue the next information frame; U - Unnumbered frames that control connect-disconnect requests.
In addition, the control field contains the number of the frame that the radio modem of the receiving correspondent expects to receive. For retransmission of corrupted frames, GBN and SR ARQ mechanisms are used.
Information field frame contains an information packet up to 256 bytes. When transmitting text information in the terminal mode, the information field is a sequence of user characters, which, when received, are displayed on the correspondent's computer screen.
Sometimes the first byte of the information field acts as an independent subfield-identifier of the protocol. This happens when using the network (third) layer of the AX.25 protocol when the packet passes through the NET / ROM stations.
Frame control field, as in other protocols, it serves to check the correctness of data transfer. The formation of the control field of the frame occurs when using the generating polynomial CRC-1 b ^ x ^ = - c + x + x +1 in accordance with the algorithm given in the ISO 3309 Recommendation, similar to the rules for the formation of the control field of the frame of the HDLC and V.42 protocols. On reception, the control field is also calculated and compared with the received value. If the control sequences do not match, a request for retransmission of the frame is made.
Physical implementation of radio modems
A typical packet communication station includes a computer (usually a portable notebook type), a radio modem (TNC), a VHF or HF transceiver (radio station).
Modern intergal radio modems are made in a single housing containing a port controller, a transmitter control controller, a specialized transceiver with a short receive / transmit switching time.
The computer interacts with the radio modem through one of the well-known DTE-DCE interfaces. The serial RS-232 interface is almost always used.
The data transmitted from the computer to the radio modem can be either a command or information intended for transmission over a radio channel. In the first case, the command is decoded and executed, in the second, a frame is formed in accordance with the AX.25 protocol. Before the direct transmission of the frame, its bit sequence is coded with a line code without returning to zero NRZ-I (Non Return to Zeroln-verted). According to the NRZ-I coding rule, the drop physical layer signal occurs when zero occurs in the original data sequence.
A timing diagram explaining the NRZ-I encoding process is shown in the following figure:
A packet radio modem is a combination of two devices: the actual modem and the actual TNC controller. The controller and the modem are interconnected by four 
lines: TxD - to transmit frames in the NRZ-I code, RxD - to receive frames from the modem also in the NRZ-I code, PTT - to send a signal to turn on the modulator and DCD - to send a channel busy signal from the modem to the controller. Typically, the modem and packet controller are designed in the same housing. This is the reason why packet radios are called TNCs.
Before transmitting the frame, the controller turns on the modem using a signal on the PTT line, and sends a frame in the NRZ-I code over the TxD line. The modem modulates the received sequence in accordance with the adopted modulation method. The modulated signal from the modulator output is fed to the MIC input of the transmitter.
When frames are received, the carrier modulated by a sequence of pulses is fed from the EAR output of the radio station receiver to the demodulator input. From the demodulator, the received frame in the form of a sequence of pulses in the NRZ-I code enters the controller of the packet radio modem.
Simultaneously with the appearance of a signal in the channel, a special detector is triggered in the modem, which generates a channel busy signal at its output. The PTT signal, in addition to turning on the modulator, also performs the function of switching the transmission power. It is usually implemented by means of a transistor switch that switches the transceiver from receive mode to transmit mode.
In packet radio communication based on typical radio stations, two modulation methods are used for short and ultrashort waves. The KB uses single sideband modulation to form the voice frequency channel in the radio channel. For data transmission, frequency modulation of the subcarrier is used in the telephone channel frequency band of 0.3 to 3.4 kHz. The value of the subcarrier frequency can be different, and the frequency spacing is always 200 Hz.
This mode provides a transmission rate of 300 bps. In Europe, the commonly used frequency is 1850 Hz for "0" and 1650 Hz for "1".
In the KB range, they often operate at a speed of 1200 bit / s when using frequency modulation with a spacing of subcarriers of 1000 Hz. It is assumed that "0" corresponds to a frequency of 1200 Hz, and "1" - 2200 Hz. Less commonly, in the VHF range, relative phase modulation (OFM) is used. In this case, baud rates of 2400, 4800, and sometimes 9600 and 19200 bps are achieved.
As an example, the following table shows comparative characteristics some commercially available packet radio modems.
| Characteristic | RC-88 | RK-900 | DSP-2232 | STACK | ATMA |
| Transfer rate, Kbps | 0,3,0,6,1.2, 2,4, 4,8. 9,6 | 0,3-19,2 | 0,3-19,2 | 1,2 | 2,4 |
| ROM size, Kbit | 32 | 256 | 384 | ||
| RAM, Kbit | 64 | 64 | |||
| Output level, mV | 5300 | 5-100 | 5-100 | ||
| Weight, kg | 1,1 | 2,84 | 1,7 | 4,5 | 1,5 |
| Dimensions, mm | 191x152x38 | 300x305x89 | 305x249x74 | 330x270x90 | 220x270x45 |
10.4. Application of radio modems
To successfully use the radio modem, you need the correct
Application of radio modems
To successfully use the radio modem, it must be correctly connected to the computer on the one hand, and to the radio station on the other.
To connect a radio modem to a computer when using the RS-232 serial interface, you must pay attention to the correctness (the same) setting of the exchange parameters between the computer and the radio modem: speed, information symbol size (7 or 8 bits), parity (Even - even bit, Odd - odd , Mark is always 1, Space is always 0) and the number of stop bits (1, 1.5 or 2). These parameters in radio modems are set by DIP switches, less often by jumpers or by software.
Many modern models of radio modems implement automatic setting to the required exchange rate with the computer. Pay particular attention to the flow control protocol used, whether it be hardware or software. In this case, each of the protocols must correspond to its own connecting cable with the corresponding pinout.
A radio modem with an integrated controller is an intelligent device. It performs many functions and has its own command system. For this reason, it is not necessary to connect to it Personal Computer, in the simplest case, a terminal is sufficient. The computer is more convenient in that it allows you to write received information into memory, prepare data for transmission and perform a number of other service functions.
For working together radio modem and computer, the latter must be switched to terminal mode using any of the available terminal programs. Such programs exist for all types of computers. The most famous terminal programs for IBM PC-compatible computers are TELIX, PROCOMM, MTE, QMODEM, etc. You can use any of them. There are also specialized terminal programs for batch communication, for example, PC-Pacratt for Windows, Mac-RATT for Macintosh computers, COM-Pacratt for Commodore computers. Also, programs for the transmission of faxes in packet radio networks have been developed and are available for sale. These are AEA-FAX, AEA WeFAX and a number of others. Sold radio modems, as a rule, are completed with a floppy disk with a terminal program.
A limiting factor in the use of the entire spectrum of software developed for conventional modems for radio modems is the radio modem control command system, which is different from the AT command set.
A single recipe for connecting radio modems and radio stations different types no, it cannot be. However, a few general comments can be made.
The easiest way is to connect a radio station that has a connector for an external headset - a device that combines the functions of a microphone, a telephone (loudspeaker) and a radio station transmission / reception control switch. In this case, the connection is reduced to the manufacture of a connecting cable from the radio modem to the transceiver. At the same time, as in any other case, it is necessary to carefully study the technical documentation for both the radio modem and the radio station, especially concerning the switching circuits.
If the radio station does not have a connector for an external headset, then you will have to either refuse to use it, or open the case and connect directly to station diagram, again guided by the documentation. Such modernization of a radio station is a rather difficult and risky business and should be carried out by qualified specialists.
Magazine "Radio" No. 12 2002
Rakovich N.N.
Let's start a review of ICs for transmitting / receiving data in the radio band of super-regenerative receivers of the RRn-xxx series. These are functionally complete devices (block diagram in Fig. 1), made using hybrid thick-film technology. The receiver includes: a high-frequency preamplifier, a high-frequency generator, an oscillation stall circuit, a low-frequency filter that does not pass the high-frequency oscillations to the output in the absence of an external signal, a low-frequency amplifier and a comparator for generating a signal with TTL levels. That is, one of the variants of the super-regenerative receiver circuit (the comparator does not count), but only without the "strapping". A typical connection diagram is simple and is shown in Fig. 2. Let's note some features of the IS in this series, which, I hope, will help developers.
Rice. 1. Block diagram of super-regenerative receivers of the RRn-xxx series
Rice. 2. Wiring diagram for super-regenerative receivers of the RRn-xxx series (for example, RR3-xxx)
The use of laser edge trimming in RR3, RR4, RR6, RR10, RR11 products made it possible to improve the tuning accuracy to ± 0.2 MHz, which is 2.5 times better than in RR1 or RR8 products. The RR4-xxx device has a cascode input and the lowest level of the emission spectrum (-70 dBm) is obtained. In cases where low consumption is required, Telecontrolli recommends using RR6 or RR11 (current consumption 0.5 mA and 0.3 mA, respectively), but you will lose a little in sensitivity. And some deterioration in the parameters of the RR8 in comparison with other ICs in this series is a payment for 3V power supply.
The last microcircuit in the RRn-xxx series is the RR15 product, the parameters of which are the most attractive: tuning accuracy - ± 75 kHz; -3 dB bandwidth is - ± 250 kHz, the level of the emitted frequency spectrum is -75 dBm, metal shield. Only one "but" - the only operating frequency of 433 MHz.
Concluding our conversation about this group of devices, we present some of their technical parameters.
Table 1.
| RR3 | RR4 | RR6 | RR8 | RR10 | RR11 | RR15 | |
| Supply voltage, V | 5 | 5 | 5 | 3 | 5 | 5 | 5 |
| Consumption current, mA | 2,5 | 2,5 | 0,5 | 0,5 | 1,2 | 0,3 | 4 |
| Working frequency, MHz | 200-450 | 200-450 | 200-450 | 280-450 | 200-450 | 280-450 | 433,9 |
| Tuning accuracy, MHz | ± 0.5 | ± 0.2 | ± 0.2 | ± 0.2 | ± 0.2 | ± 0.2 | ± 75 kHz |
| 2 | 2 | 2 | 2 | 2 | 2 | 4.8 ÷ 9.6 kbps | |
| Sensitivity, dBm | -105 | -105 | -95 | -90 | -102 | -95 | -102 |
| Radiation level, dBm | -65 | -70 | -65 | -65 | -65 | -65 | -75 |
| -25…+80 | -25…+80 | -25…+80 | -25…+80 | -25…+80 | -25…+80 | -25…+80 |
The disadvantage of direct conversion receivers is their low selectivity, especially at a high intensity of the electromagnetic field. To obtain a higher quality radio reception, superheterodyne receivers of the RRSx-xxx series with amplitude modulation and the RRFx-xxx series with frequency modulation are designed.
The block diagram of the superheterodyne RRS1-xxx ÷ RRS3-xxx is shown in Fig. 3. The signal from the antenna enters the input of the SAW filter and, passing through the mixer, which also receives the signal from the local oscillator, passes through the IF filter. Next, an AM signal demodulator and a comparator that forms a digital signal await it. Among these devices, the RRS2 microcircuit has a higher sensitivity and a higher radiation level (the absence of a SAW high-frequency filter affects), but also a lower cost. The input filter with a preamplifier in the RRS3 device made it possible to obtain a narrow bandwidth at the same level of -3 dB and the lowest noise level (the main parameters of these ICs are given in Table 2).
Rice. 3. Block diagram of the superheterodyne RRS1-xxx ÷ RRS3-xxx
Table 2.
| RRS1 | RRS2 | RRS3 | RRQ2 | RRFQ1 | |
| Supply voltage, V | 5 | 5 | 5 | 5 | 5 |
| Consumption current, mA | 3.7 ÷ 5 | 3.7 ÷ 5 | 5 | 5 | 5,5 |
| Working frequency, MHz | 315/418/433 | 315/418/433 | 433,92 | 433,9/868,35 | 315/418/433 |
| Intermediate frequency, kHz | 500 | 500 | 500 | 10.7 MHz | 1000 |
| Data transfer rate, kHz | 3 | 3 | 3 | 4.8 kbps | A: 2.4 kbps B: 4.8 kbps C: 9.6 kbps |
| Sensitivity, dBm | -100 | -102 | -106 | -107/-102 | -90 |
| Radiation level, dBm | -65 | -50 | -70 | -70 | -70 |
| Operating temperature range, ° С | -25…+80 | -25…+80 | -25…+80 | -25…+80 | -25…+80 |
The connection diagram for RRS1-xxx ÷ RRS3-xxx receivers is practically the same as for super-regenerative receivers.
The block diagram of a receiver with frequency modulation RRF1-xxx differs from RRSх-xxx by an input filter with a preamplifier and an FM demodulator instead of AM (Fig. 4). Parameters - in table 2.
Rice. 4. Block diagram of a receiver with frequency modulation RRF1-xxx (the difference from RRSх-xxx is an input filter with a preamplifier and FM demodulator instead of AM)
Concluding short review receivers, I will mention two more: RRQ2-xxx and RRFQ1-xxx (parameters - in the same table 2). In both receivers (with AM and FM, respectively), a phase-locked frequency synthesizer and a quartz resonator are used instead of a local oscillator (RRQ2-xxx block diagram is shown in Fig. 5).
Rice. 5. Block diagram of RRQ2-xxx and RRFQ1-xxx receivers (phase-locked frequency synthesizer and quartz resonator instead of heroin)
Telecontrolli manufactures transmitters (pair to the aforementioned receivers) both with amplitude modulation (RTx-xxx series) and frequency modulation (RTFx-xxx series) (the main parameters are in Table 3).
Table 3.
In view of the relative simplicity of the circuit of the RTx-xxx series transmitters and their functional completeness, I will give only their structural diagrams (Fig. 6 - 8). A typical connection diagram can be seen in Fig. 9 (using RT4-xxx as an example).
Rice. 6. Block diagram of the transmitter RT4-xxx
Rice. 7. Block diagram of the transmitter RT5-xxx
Rice. 8. Block diagram of the transmitter RT6-xxx
Rice. 9. Wiring diagram for RTx-xxx series transmitters
We do not consider the two junior ICs of this series (RT1 and RT2), due to their simplicity and the lack of normalized parameters for noise, output power and input voltage level.
Concluding our brief overview of Telecontrolli components operating in the microwave range, let us focus on two transmitters with a built-in crystal oscillator: RTQ1-xxx and RTFQ1-xxx. Block diagrams of the transmitters are shown in Fig. 10 and 11 respectively. To expand the possibilities to reduce consumption in the "standby" mode, the output of the synthesizer and output amplifier operation permission is provided. The connection diagram in Fig. 12.
Rice. 10. Block diagram of a transmitter with a built-in crystal oscillator RTQ1-xxx
Rice. 11. Block diagram of a transmitter with a built-in crystal oscillator RTFQ1-xxx
Rice. 12. Connection diagram RTQ1-xxx
RTFQ1 is remarkable in that it has a frequency deviation of ± 30 kHz (total !!! at an operating frequency of 433 MHz), and the frequency adjustment accuracy is ± 25 kHz (typical value is 0).
Readers must have noticed that all examples are considered for the 433 MHz range. This is due to the fact that according to Decision No. 64 dated 01.03.2000 “On the allocation of the frequency band 433.050 - 434.790 MHz for low-power radio stations”, citizens and business entities of the Republic of Belarus are allowed “1. ... secondary use of the 433.050 - 434.790 MHz frequency band by legal and individuals for the development, production, import from abroad and operation of portable low-power (up to 10 mW) radio stations with an integrated antenna intended for voice communication: 3. ... Registration and obtaining permits for the operation of such radio stations is not required. " This solution actually opened up a new range for use in all areas of industry and everyday life. However, the company supplies instruments for the 315 bands; 418; 443.92; 868.35 MHz.
Having familiarized ourselves with the dry theory, and inspired by the decision No. 64, let's move on to practice: where and how these microcircuits can be applied.
Enough has been said about traditional security and safety applications, including automotive and remote control systems. National manufacturers of such complexes can now use inexpensive Telecontrolli devices to create competitive products. Let's pay special attention to the developers of various security sensors: it becomes possible to manufacture them in a wireless version. So far, such devices, which are in demand due to their ease of installation, are completely imported.
It is also obvious that an inexpensive and stable radio channel is interesting in monitoring systems for climatic parameters as an element of transmission in the system for collecting and transmitting readings of any number of geographically distributed sensors that can be located in greenhouses, greenhouses, incubators, poultry houses, elevators and other objects of the agro-industrial complex. The main task of systems of this class is to measure climatic parameters, register their exceeding the set thresholds and control the corresponding equipment.
A striking example of the effective use of a radio channel is a complex for measuring temperature in a greenhouse (greenhouse, incubator, etc.). The measuring complex inside each greenhouse consists of a Recorder and the required number of autonomous sensors. Each self-contained sensor contains a direct temperature meter, controller, transmitter and battery power supply. As a temperature meter, it is logical to use a digital thermometer DS1920 or similar manufactured by Dallas Semiconductor (see Chip News No. 8, 2000, pp. 8-10), equipped with a built-in battery. Such a thermometer automatically records temperature values in the non-volatile memory at specified time intervals, while the sensor controller is in standby mode (minimum power consumption). It periodically activates, establishes communication with the Recorder (receiver with a range of up to 250 m) and transmits over the radio channel all temperature readings accumulated since the last communication session. All sensors installed inside one greenhouse are polled in the same way. Data transfer throughout the entire facility can be carried out by wired means, for example, over a microLAN network.
The main advantages of such a measuring complex are the simplicity of deployment and configuration changes (the sensor can be located anywhere), as well as the reduction in the cost of implementation and maintenance due to the absence of wired communication.
Of course, the entire measuring complex in the greenhouse can be built on a wired connection. However, there are situations when the wire cannot be stretched: registration of miners who are underground, registration of the movement of vehicles, control of the patrol service.
The registration of miners is an urgent problem due to the fact that the registration of personnel located underground in emergency situations must be carried out instantly and reliably. However, due to aggressive environmental conditions, registration means must be reliably protected, and registration must be performed passively, without deliberate actions of personnel. Such conditions can be met if the radio-identifiers of the personnel are located inside the battery of the miner's lamp.
Telecontrolli devices can be effectively used to keep track of compliance with the schedules of regular passenger or freight transport. Such tasks arise when enterprises lease transport for transporting employees to places of work, when accounting for the production and control of drivers' working time (transportation of building materials, raw materials). Equipping cars with electronic identifiers with a radio channel and placing the recorders along the routes, you can confidently control the schedules and routes without imposing restrictions on the speed and order of routes.
A similar solution is applicable to the control of the patrol and guard service, when you need to be sure that the officers on duty bypass the specified routes in set time... Radio-based identification means will allow solving this problem and guaranteeing high-quality protection of objects.
Let's summarize. The use of Telecontrolli microcircuits for data transmission in the 400-900 MHz range allows not only to reduce the total cost of the product as a whole, but to create original systems with new consumer properties.
Modern concepts and the level of development of technology make it possible to create a wide variety of complex-branched systems of security television surveillance. The main technical problem solved by a video surveillance system is the transmission of a video signal from a source (object of observation) to a receiver (viewing / recording / storage equipment). In our progressive time, there are many solutions to the issue of video signal transmission, each of which has its own pros and cons, subtleties and equipment composition.
Most popular solutions:
1. Transmission of a video signal through a cable line. (The basis of any system).
- Coaxial cable (RK, RG ..) (Analog signal, TVI, AHD).
- Twisted pair (UTP, FTP, CCI ...) (Analog signal with transceivers, IP digital signal).
2. Signal transmission by radio channel. (The method is not available to everyone by law).
3. Signal transmission over fiber-optic lines or LAN. (IP digital signal).
| Video signal transmission over coaxial cable (RK, RG). | |
|---|---|
| Pros: | Minuses: |
| Transmits the signal from the video camera to the receiver (video recorder) directly, without the use of additional equipment, because transmitting and receiving equipment initially provides for just such a method of signal transmission. | The transmission range of a reliable signal is limited to 200-250m, depending on the external conditions and the cable products used; Low noise immunity of the cable. In some cases it is necessary to use isolation transformers and special noise filters. |
| Transmits TVI, AHD signal from a video camera to a receiver (video recorder) directly, without the use of additional equipment. The method has been mastered by all manufacturers and is positioned as a way to transfer old systems to a new level in the FullHD format and higher, without replacing the cable line. Noise immunity is higher than that of analog systems. | The transmission range of a reliable signal is limited to 200-250m, depending on the external conditions and the cable products used. Usually video cameras of TVI, AHD format work only with recorders of their own manufacturer. |
Here are several ways of a simple system configuration using video signal transmission over RK and RG cables.
Analog method (The very beginning of the development of video surveillance)
Performs visual detection of violation of the guard line without video registration (recording).
Analog way and new TVI and AHD transmission formats.
Performs visual detection with video recording (digitization or signal conversion, archive formation). System capacity 4, 8 or 16 channels. The video recorder is installed at a security post or in another room with limited access.
The diagram shows two types of twisted pair transceivers: passive and active. The passive transmitter does not require power, easy to install, but the signal transmission range from the b / w camera is up to 600 meters, from color to 400 meters. An active transmitter requires power, most often it is combined with a video signal amplifier, a corrector and an isolator, the range of video signal transmission up to 2400 meters and the noise immunity of the system are noticeably increased.
To such a solution, you can add (+), UTP cable is cheaper than RK or RG per meter.
This method is not applicable to complex systems and is used on rare occasions when there is a need to identify repeated misconduct or theft. And even in such cases, the law is on the side of the offender. But nevertheless, equipment for transmitting a signal over a radio channel exists and is being successfully sold.
You can read more about the method of transmitting a video signal over a radio channel in the article Wireless video surveillance.
Below are the options for building a video surveillance system using IP cameras.
Transfer of a digitized signal from a video camera
it the simplest way formation of video surveillance on IP cameras over a structured cable network. Let's add (+) to the solution for the absence of any interference. The video signal is digitized in a video camera, which excludes interference to high-frequency cables. Software is installed on the server, the task of which is to communicate with cameras, display video information and save.
Transfer of a digitized signal from recorders
This method is most suitable for translation. old system video surveillance to the modern level in the case when the server equipment is not satisfied with the quality of the recording or is out of order. A "encoder" device and a packet shaper are added to analog video cameras.
Transmission of a digitized signal via FOCL
With such a solution, any distance is not the limit. It is best used in complex projects where video surveillance is formed from 150-200 cameras. Suitable for any type of objects of varying complexity in architecture and area. Using the solution allows you to build a video surveillance system at the lowest cost at distributed sites or at separate sites, where it is more convenient to conduct local video recording. For example, ATMs, gas stations, power and transformer substations, payment and information terminals.
In recent years, the thesis that information Technology have the most direct impact on the state and development of the economy, has become almost universally recognized. The computer world became networked a few years ago. The network infrastructure makes it possible to quickly exchange data and access information resources, both locally and globally. The Russian problem is the weakness of the telecommunications infrastructure (especially its public, civilian part) compared to similar infrastructure in the West. In many cases, the use of wired or fiber-optic communication lines is impossible or economically impractical. In this situation, one of the most effective solutions to the communication problem, and often the only possible one, is the use of radio data transmission networks.
The distinctive properties of wireless data transmission technologies include:
- Mobility. The inability to connect mobile subscribers is a fundamentally insurmountable limitation of cable networks. Nurses, doctors, assembly line workers, stock brokers and warehouse workers are constantly on the move. For them wireless technology represents a channel that does not block their movements into a wired network, opening up access to all information available in this network.
- Possibility of organizing a network where cable laying is technically impossible. For example, in buildings that are architectural monuments.
- The ability to network remote subscribers. If the subscribers are scattered over a vast sparsely populated (or hard-to-reach) territory, then in many cases it is not economically feasible to lay the cable. In Russia, almost 90% of radio equipment is used for outdoor communication, at distances of many kilometers. Radio networks connect settlements that are simply not accessible by telephone lines. If, nevertheless, they do, the telephone exchanges are in no hurry to lease communication lines, and the quality of communication is low. But the main thing is even different - the throughput of telephone channels leaves no hope for organizing effective data exchange.
- Urgency. Reliable communications are needed now, immediately, and laying a cable network requires colossal investments and a long time. Radio equipment allows you to deploy a network in just a few hours. Radio equipment can also be used to organize temporary networks. For example, exhibitions, election campaigns, etc.
Consider radio equipment that can be used to create radio networks for data transmission, and the tasks that a particular class of equipment can solve.
Radio equipment can be classified according to the frequency used. The range in which the equipment operates depends on such indicators as communication range, information transfer rate, dependence on weather conditions, the requirement to ensure "line of sight".
1.6-30 MHz(Shortwave range). Systems operating in this range allow data transmission and voice messages over distances of up to several thousand kilometers, which provides a unique opportunity to cover large areas, including those with mountainous terrain, which is absolutely impossible for traditional solutions in the VHF and microwave bands with a commensurate investment. The transmission speed in HF systems is relatively low, up to 6 Kbps. For the implementation of radio systems for data transmission in the HF band, the "Barret 923" complex, which is manufactured by Barret Communications Pty Ltd., can be used. The complex "Barrett 923" implements adaptive methods for analyzing the radio channel, which allows it to optimally select the frequency range, protocol and data transfer rate.
136-174 MHz- data transfer rate up to 19.2 Kbit / s, communication range up to 70 km, communication can be carried out "from behind" the angle and beyond the horizon due to the curvature of the path of the radio beam at the ground. Radio modems operating in this range are used to transfer files and e-mail, allow you to organize mobile access in the database. They are used in geographically distributed networks, in telemetry and telecontrol systems, and can be very useful for organizations such as the traffic police, ambulance service, etc. Integrated radio modems operating in this frequency range are produced by companies such as Pacific Crest, Maxon, Young Design, etc.
SPC "Dateline" has developed the "Jaguar" system for building packet radio networks for data transmission, which has been successfully operated by the territorial branches of Sberbank of the Russian Federation for a long time. The Jaguar system provides high reliability of data transmission, flexibility in management, the ability to easily expand the network at distances of up to 300 km. The hardware complex of the system can be built on the basis of a wide range of FM radio stations and packet controllers. Dayline recommends the use of Uniden IMH4100 transceivers and Paccom Spirit 2 controllers, which provide the best price / performance ratio.
400-512 MHz- data transfer rate up to 128 Kbps, communication range up to 50 km. Line-of-sight is desirable, but operation with reflected signals is also possible. Narrowband synchronous radio modems RAN manufactured by Wireless, Inc (formerly Multipoint Networks) (9.6, 19.2, 64, 128 Kbps) can operate in this range.
RAN 64 / 25,128 / 50 radio modems use 16 QAM modulation, which allows data transmission at 64 kbps in 25 kHz bandwidth or 128 kbps in 50 kHz bandwidth. Radio modems of this type are used to build high-speed point-to-point channels for multiplexed transmission of data, voice, video images and other information. On their basis, it is also possible to organize multi-node geographically distributed networks. RAN radio modems can also operate in the 820-960 MHz range.
Above 2GHz- it is possible to organize data transmission channels with a speed of more than 2 Mbit / s, while the condition of direct visibility between the antennas is mandatory. Radio-Ethernet equipment (IEEE 802.11 standard) operates in this part of the radio frequency spectrum. The Radio-Ethernet standard has two main uses. The first of them is a wireless LAN within the walls of one building or on the territory of the enterprise, thus solving the problem of "limited mobility" within the same enterprise (an employee with laptop moving from one room to another has access to the network from everywhere). The second application of the Radio-Ethernet standard solves the problem of connecting subscribers to big network data transmission or, as the signalmen say, the problem of the last mile.
Radio-Ethernet can use noise-like signal technology or wideband signal technology (BSS). Narrowband devices emit a signal with a spectrum width of 12.5-200 kHz on the air, and the width of the emitted spectrum increases with an increase in the information transfer rate. Narrowband systems have a very significant drawback: if interference appears in the frequency range of such a system, then the quality of communication drops sharply. It was this vulnerability to interference of narrowband systems that led to the development, first for military applications, of broadband technology.
Systems based on noise-like signals have the following advantages:
- Interference immunity
- No interference with other devices (Low signal strength)
- Confidentiality of transmissions
- Low cost in mass production (Low signal strength - cheap high frequency hardware components)
- Noise-like signal allows for operation in the range already occupied by other radio transmission systems
- High transfer rate
The idea behind wideband signal technology is that a much wider bandwidth is used to transmit information than is required when transmitting in a narrowband channel. The 802.11 standard provides Direct Sequence Spread Spectrum-DSSS and Frequency Hopping Spread Spectrum-FHSS for noise-like signals.
Frequency hopping (FHSS) splits the entire 2400 MHz to 2483.5 MHz band into 79 subchannels. The receiver and transmitter are synchronously tuned to different carrier frequencies every few milliseconds in accordance with a pseudo-random sequence algorithm. Only a receiver using the same sequence can receive the message. It is assumed that other systems operating in the same frequency range use a different sequence and therefore practically do not interfere with each other. For cases where two transmitters try to use the same frequency at the same time, there is a collision resolution protocol in which the transmitter attempts to resend data on the next frequency in sequence.
The Direct Sequence Method (DSSS) splits the 2400 MHz to 2483.5 MHz band into three wide subchannels that can be used independently and simultaneously over the same territory. The principle of operation of DSSS systems is as follows: significant redundancy is introduced into the transmitted radio signal by transmitting each bit of information simultaneously in several frequency channels. If any of them (or several at once) appear interference, the system determines the correct data flow by choosing the greatest number identical streams.
Most major manufacturers Radio-Ethernet equipments are Proxim, BreezeCom, Aironet, Cylink, Lucent Technologies, Solectek, WaveAccess. It is pleasant to note that in recent years, domestic developments have also begun to appear. For example, the Impulse enterprise produces a cross-8 wireless Ethernet bridge for a point-to-point configuration, which operates in the relatively unloaded range of 37.0-39.5 GHz, providing a transmission speed of 10 Mbit / s and a range of 10 km.
For a long time, direct sequence transmission (DSSS) has been the dominant technology in the Russian market. However, recently the domestic market has begun to show more and more interest in FHSS. The main reason for this is the "overpopulation of the ether".
In the same space, no more than three DSSS networks can coexist without interfering with each other. When trying to increase the number of users, such wasteful use of ether can turn into problems. FHSS allows you to define for each network its own set and sequence discrete frequencies... Another significant feature of the frequency hopping technology is that the entire broadband is split into 79 separate subchannels. FHSS equipment (for example, from BreezeCom) does not allow using all 79 channels, but any number of frequencies from this set, up to one frequency. In DSSS systems, the use of wide bandwidth is essential. 
ShPS-technology, in addition to Radio-Ethernet equipment, is used in high-speed synchronous radio modems in the 2.4 and 5.7 GHz bands. These radio modems are used to organize duplex trunk synchronous radio data transmission channels with speeds up to 2048 Kbit / s. Equipment of this class is produced by such companies as Wireless, Inc (models RAN64ss, RAN128ss, RAN2048ss), BreezeCom (BreezeLINK series), Wave Wireless (SpeedCOM). 
ShPS-technology is used in another interesting and very useful product of Wireless, Inc - the WaveNet IP radio router. Unlike radio-Ethernet devices, this equipment includes an IP router and is specially designed for organizing radio networks of urban and regional scale at a distance of up to 30-40 km from the central station. In addition, the WaveNet IP design solves the so-called long cable problem. The problem is that quite often the point of connection to the local network and the point of installation of the antenna on the roof are at a sufficiently large distance from each other. Radio-Ethernet equipment is usually designed for indoor use and can only be used in normal climatic conditions. Since the RF signal experiences significant cable attenuation, this imposes severe restrictions on maximum length cable between the device and the antenna. WaveNet IP has an external weatherproof design and is installed in the immediate vicinity of the antenna, which allows placing the high-frequency unit at a distance of up to 100 m from the physical point of entry into the network without signal loss.
Introduction
1. Analytical overview
1.1 Overview of methods of encoding-decoding information
1.2 Comparative analysis of coding methods for decoding information
1.3 Analysis of hardware implementation
1.4 Comparative analysis of hardware implementation methods
1.5 Findings from the Policy Brief
2. Development of a structural diagram
3. Synthesis of an electrical circuit diagram
3.1 Choosing a digital signal processor
3.2 Choosing a codec
3.3 Selecting the RS-232 interface driver
3.4 UV erase memory selection
3.5 Selection of auxiliary circuit elements
4. Development of the program algorithm
4.1 Initialization block
4.2 Receive / transmit interface
5. Development software
6. Feasibility study
7. Labor protection
Application
Introduction
The need to receive and transmit information has always worried humanity. In a modern, busy computer technology the world, it has received the most widespread. The ability to connect several computers at a distance allowing you to connect their email. wire, and access to their data, has added a qualitatively new step to the use of opportunities modern computers... This connection is called a local area network. Also after this, the concept appeared global network, while computers may not be nearby, but let's say in different cities. This connection uses a special device called a "modem". In this case, communication is provided via a telephone line.
Modem is short for MODULATOR - DEModulator.
There is also a way to receive and transmit information between computers over a radio channel. In this case, a modulation / demodulation device (modem) is also used. At the same time, a separate device is also used with a computer and a modem - a unit for receiving and transmitting information over a radio channel. This is a rather cumbersome device and every computer user, of course, cannot afford to purchase it. But such a combination of technical means is very effective when communicating between two objects located at a very large distance and not having access to the telephone line. For example, it can be a ship on a voyage and a home port transmitting information from a satellite about an impending storm.
Of course, the modem in this case will differ in function from the modem working with a telephone line. Because there is no concept of dialing to a subscriber, duplex communication is also not allowed here. In principle, the functions of dialing and others are taken over by the unit for receiving and transmitting information over the radio channel. The modem is just waiting for the signal to be received, demodulates it, forming a digital code, and transmits it to the computer. During transmission, the modem receives a digital code, modulates it, converts it into an analog signal and transmits it to the information transmission unit via a radio channel.
Nowadays, the technology for the production of integrated circuits, microcontrollers, etc. is on very high level, is constantly being improved and invents all new types of microchips. One such microchip is the DSP - digital signal processor. It is ideal for signal processing. Having a built-in programming language, it allows you to customize it for any job the electronics engineer needs. Almost all modern modems, regardless of the purpose, have DSP installed.
In this diploma project, we will design a device that will receive and transmit data over a radio channel, while performing encoding and decoding of information using a digital signal processor (DSP).
1. Analytical overview
1.1 Overview of methods of encoding - decoding information
To select the necessary path for designing a device, it is required to analyze modern methods and means of encoding-decoding information.
From the beginning, we will consider ways to solve encoding-decoding of information. To do this, consider modern ways modulation - demodulation of the signal.
As mentioned above, modems modulate the signal for transmission over telephone or radio channels, but the signal can be modulated in different ways.
Modulation - changing one or more carrier parameters sinusoidal oscillation(amplitude, frequency, phase) in accordance with the values binary information transmitted by the source.
Modems use a kind of modulation, the so-called "keying", in which the specified modulated parameters can only have fixed values from a certain set.
Modulation allows you to match the spectrum of the transmitted information signal with the bandwidth of the telephone or radio channel. At low transmission rates (up to 1200 bit / s), frequency modulation is used in modems, the implementation of which at such rates is the simplest. At medium bit rates (1200 - 4800 bps), differential difference modulation is used with a number of possible changes in phase positions from two (1200 bps) to eight (4800 bps) (phase modulation). The transmitted digital information values are contained in the phase increments between the data and the previous modulated signal element. At high transmission rates (> 4800 bit / s) and when transmitting over switched channels with frequency division of the transmission directions, starting from 2400 bit / s, combined amplitude-phase modulation is used). When using this type of modulation, digital information is contained both in the amplitude value and in the phase increments of the carrier frequency. With amplitude-phase and multi-position phase modulation, the number of possible positions of the modulated signal (or the number of signal vectors) is more than two. In this case, one element of the modulated signal contains several bits of digital information (this number is equal to the binary logarithm of the number of possible vectors of the modulated signal).
Phase modulation:
When using the so-called relative phase shift keying (PSK), i.e. modulation, in which the carrier phase takes only fixed values from a series acceptable values(for example, 0, 90, 180 and 270 degrees), and the information is embedded in changes in the phase of the carrier wave. With the above set of possible phases, each phase change corresponds to a certain dibit value, i.e. two consecutive bits of information. Phase Shift Keying refers to two-way modulation techniques, i.e. the spectrum of the modulated signal is located symmetrically with respect to the carrier frequency, and the spectrum width in Hz at the level of 0.5 of its value at the carrier frequency is equal to the modulation linear velocity, expressed in Baud. The most commonly used types of phase shift keying in modems are relative phase shift keying (OFM) / 1200 bit / s rate, two phase positions /, four-position (or quadrature phase shift keying / 2400 bit / s, four phase positions /) and eight-position (4800 bit / s, eight phase positions). Sometimes in the literature, these types of manipulation are called, respectively, PRM (phase difference modulation), DOPM (two-fold phase modulation) and TDFM (three-fold phase modulation). A further increase in the number of positions in order to increase the speed leads to a sharp decrease in noise immunity, therefore, for more high speeds Combined amplitude-phase modulation methods began to be used.
Amplitude - phase modulation:
In this type of modulation, to increase the throughput, the simultaneous manipulation of two parameters of the carrier wave is used: amplitude and phase. Each possible element of the modulated signal (signal vector or signal space point) is characterized by the value of the amplitude and phase.
To further increase the transmission rate, the number of "points" of the modulated signal space is increased by a multiple of two. Currently, modems use methods of amplitude-phase modulation with the number of possible signal positions up to 256. This means that the information transfer rate exceeds the modulation linear rate up to 7 times.
To ensure maximum noise immunity, the points of the signal space are placed at an equal distance from the envelope of all points in the form of a square (16-position quadrature AM), octagon, etc. An increase in the number of signal positions leads to a rapid decrease in the noise immunity of reception.
The use of a combination of modulation with "trellis" coding has become a radical means of ensuring noise-immune transmission. When using this method, some redundancy is introduced into the signal space and due to this, correlations are created between the transmitted symbols. Due to this, on the basis of the analysis of the sequence of the received elements of the modulated signal, it is possible to detect and correct errors at the reception. In practice, this gives a significant increase in the noise immunity of reception.
A kind of amplitude-phase modulation - 16-position quadrature AM (signal space of 4x4 points in the form of a square, the points are equidistant from one another, and 4 points in each square) is used in duplex modems.
Frequency shift keying (FSK)
The modems use the so-called frequency shift keying, in which each value of the information bit ("1" and "0") corresponds to a certain frequency of a sinusoidal signal.
The spectral characteristics of FSK signals allow relatively simple implementation of modems up to 1200 bps.
Minimum Shift Modulation (MSK)
