Using data transmission via GSM standard cellular communication channels in security systems.

Using data transmission via GSM cellular channels in security systems..

Petrov Nikolay Nikolaevich, Candidate of Technical Sciences

USE OF DATA TRANSMISSION VIA CELLULAR COMMUNICATION CHANNELS
OF THE GSM STANDARD IN SECURITY SYSTEMS

The GSM standard provides the ability to transmit data between terminal equipment at a speed of up to 9600 bits per second. Data means: binary data in any format, fax messages and short messages in text format or in binary data format. The capabilities of the short message transmission mode correspond to the capabilities of a two-way pager, while providing greater flexibility. The small dimensions of GSM subscriber equipment and the high degree of its integration with modern digital control systems make it possible to recommend this type of communication for use in special-purpose technical equipment.

The use of cellular communication systems is justified in cases where it is necessary to reduce the dimensions of the equipment, the level of its own electromagnetic radiation (and, accordingly, the power consumed by the equipment from an autonomous power source or on-board network), and also when it is necessary to ensure a larger area of ​​the system. The parameters of the data transmission channel allow for the transmission of speech or low-frame video information, which allows for the implementation of additional security functions (transmission of coded speech, covert wiretapping or covert surveillance).

The ability to transmit data is theoretically built into any GSM subscriber device, however, access to these capabilities using standard voice communication devices may be difficult, and in some cases impossible without making changes to the design of the device. In addition, many functional accessories of a conventional device are redundant (for example, a keyboard, display, microphone-telephone) and can reduce some of the operational characteristics of the product (for example, an LCD display limits the minimum operating temperature to a few degrees below zero, while other electronic components can be used at temperatures below 20? C). In this regard, a number of companies produce special subscriber devices for data transmission. Most often, such devices are produced in the form of standard PCMCIA cards that can be connected to a portable computer. In addition, complete devices such as a personal digital assistant (PDA) are produced — a pocket computer with a built-in telephone. With the help of such a device, you can conduct voice conversations and exchange data. The most famous example is the Nokia 9000 series devices.

A number of companies produce equipment intended not for the end user, but for the development of original equipment on its basis (for example, modules M1, M20, A1 from Siemens, module GM12 from Ericsson, GSM Modem from Falcon, etc.). It is precisely such devices that are of the greatest interest from the point of view of special applications.

Let us consider the main possibilities of data transmission using the example of the GSM modem from Siemens, GSM Module M1, one of the first devices of this type.

The main technical characteristics of this module:

• dimensions 116? 67? 30 mm in a plastic case, on which there is an RS232 connector and a hybrid connector for connecting power and antenna;
• weight 157 g;
• supply voltage range +8 – +24 V;
• maximum consumption current 500 mA (in standby mode 70–150 mA, in transmission mode 220–500 mA, in station search mode – 130 mA);
• operating temperature range -20° C +55° C;
• storage temperature -40° C – +70° C.

There is a possibility of remote control of the module power supply: when applying a voltage of +5 — +24 V to the IGNITION contact, the module turns on, when applying a voltage of less than +1 V for 550 milliseconds, the module turns off. More modern modules from Siemens, such as GSM Module M20, in addition to data transmission functions, contain means for connecting a speech terminal.

The interaction of the terminal equipment with the module is carried out via the RS232 interface with a data rate of up to 19200 bits/sec (the module determines the rate automatically). The software interface is based on the standard set of AT commands. A number of commands defining the specifics of the data transmission environment have been added to the standard set of AT commands in accordance with the GSM 07.07 standard. Using these additional commands, you can block/unblock the module by setting a PIN code, select a communication network, obtain information about the parameters of the network radio field, and also perform a number of auxiliary actions typical of any GSM terminal. Commands for working with short messages comply with the GSM 07.05 standard. Data is transmitted in three main modes: in the mode of transmitting arbitrary data over a traffic channel (Transparent Data Transmission mode, this mode supports the V.22bis, V.32, V.110 ISDN protocols with a data transfer rate from 2400 to 9600), in the mode of transmitting facsimile data (Fax Transmission mode, Fax Group 3 protocol with a data transfer rate of 2400, 4800, 7200 and 9600) and in the mode of transmitting short messages (Short Message Service mode).

In the mode of transmitting data over a traffic channelthe transmission is carried out as follows:

• a connection is established with the subscriber's modem («dial-up»);
• after the connection is established, a virtual «dedicated full-duplex communication channel» is created for the subscribers (as in the case of connecting a telephone line);
• when the connection is established, all data received at the modem input is transmitted to the communication channel;
• control over the correctness of the transmitted data is carried out at the software level in the terminal equipment of each subscriber; the GSM system only controls the quality of the communication channel and can break the connection if the level of errors and interference in the communication channel is exceeded;
• if it is necessary to terminate the data transfer, a special sequence of characters is issued (Escape sequence, in the standard version +++), after which the modem goes into command mode and can execute the command to terminate the communication session.

During experiments with M1 modules on data transmission via a traffic channel, the following results were obtained:

1. The modems provide stable data transmission at a speed of 9600, including when one of the subscribers is moving.
2. The equipment at the GSM–900 network switching centers in Moscow (the operator is Mobile TeleSystems) is not fully equipped with data transmission modems, as a result of which in some cases it is difficult to establish a connection between two modems (when dialing a subscriber’s number, the station responds “No Carrier — no signal”).
3. The connection establishment time is on average about 20–30 seconds (excluding the time it takes to reach the subscriber’s base station kit, which has a modem).
4. The M1 module probably contains some error, as a result of which the module periodically switches to the subscriber registration loss mode in the network (the modem response is NO DIALTONE). The error usually disappears after the AT command is issued again.

The standard data transfer mode is most suitable for transferring large amounts of information, when the information transfer time is not less than the connection establishment time.

Since March 1998, Mobile TeleSystems has been operating a short message service center. This has made it possible to test this data transfer mode. According to GSM standards, a short message is an array of text data presented in 7- or 8-bit alphabets. A data block in a 7-bit alphabet can contain up to 160 characters, in an 8-bit alphabet — up to 140 characters. Data in a 7-bit alphabet can only contain standard ASCII characters. When using an 8-bit alphabet, it is possible to transfer arbitrary binary data.

The short message transfer mode differs significantly from the standard data transfer mode from the point of view of the end equipment.

In the short message transfer modethe user's terminal equipment must form a special data block in the SMS format — Submit PDU (Protocol Data Unit), defined by the GSM 03.40 standard. This data block contains information about the subscriber's address, the duration of storage of the message in the Short Message Service Center until it is read by the addressee, the format of the transmitted data (7 or 8 bits), and the message itself in the selected alphabet. For a 7-bit alphabet, the memory area allocated for the message is considered as a bit field divided into 7-bit sections (i.e. the bits defining the symbol code can be located in either one or two bytes, with the initial bits of the current byte containing information from the next data byte).

The generated data block in PDU format can be directly transmitted to the short message service center, or it can be written to the modem memory located in the SIM card, and then transmitted to the switching center by a separate command. This mode can be useful when generating a predetermined set of messages that can be issued by a separate operator command.

The data block received from the modem by the short message service center is then converted into the SMS-Deliver PDU format and transmitted, if possible, to the recipient's equipment. The converted SMS-Deliver block, which is received by the recipient's equipment, additionally contains information about the message sender (his/her telephone number) and the time the message was received by the service center. When a message is received from the service center in the recipient's subscriber terminal, the service center automatically sends information about the time of data receipt to the sender's terminal.

Since special means of the GSM system (control channel or signaling channels) are used to transmit short messages, error-free transmission of the message from one subscriber to another is guaranteed.

The performance of the short message service center, which the Moscow GSM operator is equipped with, is currently 1 message per second. In practice, this performance can be increased to 300 messages per second.

The experiments conducted using short message transmission channels allow us to draw the following conclusions:

1. The M1 module operates stably in the short message transmission and reception mode.
2. The delivery time of a short message from a subscriber to the service center is up to 5 seconds.
3. The delivery time of a short message from one subscriber to another is generally 10–20 seconds.
4. The hardware and software of the M1 module do not allow the fact of receiving a short message to be detected in the signaling mode, therefore the software of the terminal equipment must operate in the cyclic polling mode of the M1 module for unread short messages.
5. The memory of the M1 module is designed to store up to 10 short messages. To free up memory, read short messages must be deleted using a special command.

It is advisable to use the short message transmission mode when issuing single blocks of small data (up to 1120 bits) that must be quickly delivered to the addressee (for example, tracking the location of a mobile object in near-real time, transmitting an alarm signal or control command).

Let's consider possible options for using data transmission equipment via GSM communication channels for security systems.

1. A system for tracking the movement of especially important cargo, objects, and people, together with ensuring their safety and security.

The monitored vehicles are equipped with on-board navigation equipment and a number of sensors (door opening/closing sensors, shock sensors, rollover sensors, thermal sensors, hidden microphones and video cameras). The on-board controller processes the information from the sensors and, in accordance with a specified algorithm (for example, when a deviation from the standard state of the sensors occurs, at specified intervals, when passing specified checkpoints, when pressing the panic button), generates information messages for the control and tracking center (CTC). To protect the vehicles, rapid response teams are organized, which are on the move throughout the city or at stationary bases. The vehicles of the teams are equipped with on-board computers, on the screens of which the location of the monitored objects, the nearest teams, and possible routes to the scene of the incident are displayed in real time against the background of an electronic map.

Such systems can be deployed in large cities and along major highways to ensure the safety of long-distance freight transportation. National and international roaming capabilities allow for territorial coverage similar to that of long-distance satellite communications systems.

Information about the location of vehicles and the status of on-board sensors is transmitted in the form of short messages. An approximate diagram of such a system is shown in Fig. 1.

Fig. 1. An approximate diagram of a tracking system.

2. A public or special transport dispatching system.

It is intended to control routes and/or schedules of motor transport, issue control commands to drivers via voice or information terminals. Possible users: bus depots, route taxi companies, ambulances, companies engaged in freight transportation in the city.

3. Automation system for operational mobile groups of public order protection units.

On-board computers installed on task force vehicles implement the functions of automatic determination of their current location and transmission of this information to the Duty Unit (DU). The computers have the means to automate the preparation of protocols, provide access to the DU information base for the purpose of checking persons, vehicles, addresses. All the capabilities of GSM channels are used to transmit data: large volumes of information from the database are transmitted via traffic channels or in the form of fax messages, information about the location and short requests to the database are received in the form of short messages.

4. Security system for stationary objects (cottages, apartments, offices, mansions, warehouses)

The equipment placed on a stationary object enables connection of various types of security sensors (for monitoring protected perimeters and volumes, monitoring network voltage, fire sensors, intercoms and video intercoms, etc.). Flexible algorithms for polling sensors, complete autonomy of the system's operability make it extremely resistant to deliberate action by intruders, and reduce the likelihood of false alarms. The control center monitors the system's operability by periodically polling the sensor status. The alarm signal is sent to the control panel with a delay of no more than 20 seconds.

5. VIP-class car security systems and stolen car search systems.

The small dimensions of the on-board equipment and low power consumption requirements make it easy to camouflage the security equipment on the car. The functions of arming and disarming the equipment with registration in the control center are implemented, which practically excludes the possibility of unauthorized shutdown of the system. Various algorithms for responding to an attempted theft can be implemented: immediate activation of the immobilizer or blocking of the engine, blocking of the interior doors on command from the control center, listening to the interior, activation of the immobilizer in a specified location (for example, near a traffic police post or police station) or when leaving the coverage area of ​​the communication network, etc. With the help of navigation equipment in the control center, continuous tracking of the location of the car is carried out.