Application of GPRS in Remote Measurement and Control System

Analysis of communication requirements in remote measurement and control systems

The data transmission of the remote measurement and control system includes various detection, monitoring, production, alarm, etc. The industry involves various parts such as oil fields, environmental protection, earthquakes, water conservancy, electricity, meteorology, gas, pipelines, tap water, etc. The remote measurement and control system has the characteristics of small data volume, scheduled or non-scheduled and real-time transmission. On the one hand, it is necessary to realize data upload, and on the other hand, it is also necessary to issue various transmission, control and control instructions, that is, two-way data and instruction transmission, and at the same time, it must be combined with its own measurement and control system. Traditional data transmission is mainly solved by digital radio, telephone dialing, satellite communication and other methods; these transmission methods have problems such as coverage, real-time, investment and operating costs, and at the same time, there are great difficulties in the unattended operation of industrial control equipment. At present, the GPRS communication method promoted by China Mobile is an emerging data transmission technology, which will fundamentally solve the drawbacks of traditional transmission methods and be closer to the needs of professional data transmission.

GPRS Technology Overview

GPRS is the abbreviation of General Packet Radio Service. It is a new bearer service developed on the existing GSM system. Its purpose is to provide GSM users with packet data services. GPRS uses the same wireless modulation standard, frequency band and TDMA frame structure as GSM. This new packet data channel is very similar to the current circuit-switched voice service channel. Therefore, the existing base station subsystem can provide GPRS services by adding some modules. GPRS allows users to send and receive data in packet switching mode, thus providing an efficient and low-cost wireless packet data service. The basic process of packet switching is to divide the data into several small data packets, which can be transmitted to the destination through different routes in a store-and-forward relay mode and assembled into complete data. Packet switching is basically not a real-time system, and the delay is not fixed, but it can make different data transmissions "share" the transmission bandwidth: occupy the bandwidth when there is data, and do not occupy it when there is no data, thereby sharing resources. In the GSM wireless system, wireless channel resources are very valuable. If circuit switching is used, communication needs to establish an end-to-end connection, and the channel can only be monopolized by one user, which is obviously not feasible in terms of cost efficiency. GPRS, which uses packet switching, can flexibly use wireless channels. Each user can have multiple wireless channels, and the same channel can be shared by several users, thereby greatly improving the utilization rate of wireless resources. In theory, GPRS can combine up to 8 time slots to provide users with a bandwidth of up to 171.2kb/s. Since the data communication fee of GPRS users is based on data traffic, regardless of the communication duration, GPRS for access to IP services will be more acceptable to users.

GPRS has many advantages in wireless data access. First, it has a wide access range and can make full use of the national telecommunications network to provide user data terminals with remote access network deployment conveniently, quickly and at low cost. Second, it has a high transmission rate. The data transmission speed can reach 57.6kbps and the highest can reach 171kbps, which is twice the ideal rate of the commonly used wired 56k Modem. The speed of the next generation GPRS service can even reach 384kbps, which can fully meet the user's application needs. Third, the access time is short. The GPRS access waiting time is short and the connection can be established quickly, with an average time of two seconds. Fourth, it provides real-time online function. Now when using GPRS MODEM to access the Internet, users will always be connected and online, which will make access services very simple and fast. Fifth, it is charged according to traffic. Users only occupy resources during the period of sending or receiving data. Users can be online all the time and the fees are charged according to the number of data packets received and sent by users. When there is no data traffic transmission, users are not charged even if they are online. The above advantages make GPRS very suitable for the development of industry and enterprise wireless access applications.

GPRS data transmission method and network structure

The development of wireless mobile communication networks, especially the emergence of GPRS, has provided a new means of communication for industrial automation equipment. Through GPRS services, equipment can exchange data with servers on the Internet in a standard way using the Internet. The basis of GPRS is to transmit data in the form of IP packets. The method of connecting GPRS wireless terminals to the GPRS network is similar to that of ordinary wired MODEMs, both of which use the PPP (Point-to-Point Protocol) connection method. The PPP protocol is a widely used method for transmitting datagrams on serial point-to-point links, including LCP, PAP, IPCP, NCP, etc. GPRS MODEM obtains a dynamically assigned IP address through the PPP protocol. After the connection is established, data communication with other computers on the Internet is achieved through data transmission protocols such as TCP and UDP based on the PPP protocol.

GPRS transmission mode: At the remote terminal, the collected data is sent to the GSM/GPRS wireless communication control terminal through the A/D interface. The GSM/GPRS wireless communication control terminal first packages the data into TCP/IP data packets, then converts them into GPRS data packets, and transmits them to the wireless data exchange center (MDEC) through the wireless link. MDEC then strips the GPRS data packets and transmits the TCP/IP data packets to the Internet through the GGSN gateway, and then transmits them from the Internet to the communication control computer. The communication control computer receives the TCP/IP data packets through the Socket socket and restores it to the original data. When the data is transmitted from the wireless data transmission monitoring center to the remote terminal, the situation is similar.

SMS transmission method: SMS transmission method is a point-to-point transmission method. The collected data is sent to the control center via SMS. The control center receives the SMS through the SMS sending and receiving equipment, and sends it to the computer through the computer serial port for processing.

Currently, the GPRS data service launched by China Mobile mainly provides wireless Internet access for mobile phone and computer users. For the transmission of professional data, the following issues must be solved to achieve timely, correct and safe transmission of measurement and control data.

<1>Implementation of PPP and TCP/IP protocols on terminal devices

For computer users, dialing into the Internet is easy, because there are very mature software on the PC that can easily implement PPP and TCP/IP protocols. However, for terminals using embedded devices, especially single-chip microcomputers, it is still difficult to implement due to the constraints of hardware resources and the complexity of the protocol.

<2>Server access method

Since the working mode of GPRS network is based on IP address addressing, the server side theoretically only needs to access the Internet and have an IP address assigned by the public network (Internet). However, in actual system applications, if the server is accessed by dial-up or ADSL, the IP address obtained each time is not fixed, and the GPRS MODEM also obtains the IP address dynamically. Two dynamic IP address devices cannot communicate directly on the Internet. Therefore, the service user is required to have a static IP address or transfer through other methods. In this way, the access method of the server is subject to certain restrictions. At present, the GPRS service launched by China Mobile only provides point-to-multipoint transmission services. Point-to-multipoint transmission refers to multiple GPRS wireless terminals transmitting data to a fixed data processing center. Point-to-point transmission between GPRS terminals is theoretically feasible, but it still needs better support in terms of technology.

<3> Mobile network and public network (Internet) interconnection

The IP address dynamically obtained by GPRS MODEM is an address within the mobile backbone network, which cannot be directly resolved by the public network server. When it is not in the connected state, the server cannot communicate directly with the GPRS MODEM. The GPRS MODEM must first actively send data to the server to connect. After the connection route is established, the server and the terminal device can perform data transmission in duplex mode. Therefore, the communication must be initiated by the GPRS MODEM first, which should be considered when formulating the communication method of the measurement and control system.

<4>Identity recognition of terminal equipment

The IP address obtained dynamically by GPRS MODEM cannot be used by the server to determine the identity of the terminal device like ordinary computer networks do by IP address or MAC address, and cannot correctly locate the terminal device. In addition, from the perspective of network security, this problem also exists. Therefore, an ID identification mechanism must be established.

<5> Long-term stability of GPRS MODEM

The terminal equipment in the measurement and control system is generally installed in the field environment, in remote and scattered locations, without anyone on duty. The equipment must be able to work long-term and stably in special environments. The system must have automatic dialing and disconnection redialing functions, and the ability to automatically restore communication when the GPRS network is unstable, to ensure the system works stably without human intervention. In addition, special designs must be made in terms of power supply anti-interference and heat dissipation to enable the terminal equipment to adapt to harsh working environments.