Application of gprs modem in remote measurement and control system

1、 Analysis of communication requirements in remote measurement and control system

The data transmission of remote measurement and control system includes various detection, monitoring, production and alarm. The industry involves various departments such as oilfield, environmental protection, earthquake, water conservancy, electric power, meteorology, gas, pipeline and tap water. The data of remote measurement and control system has the characteristics of small amount of data, timing or non timing and real-time transmission. On the one hand, it is necessary to upload the data, on the other hand, it is also necessary to issue various transmission, control transmission and control instructions, that is, two-way data and instruction transmission. At the same time, it must be combined with its own measurement and control system. Traditional data transmission is mainly solved by several transmission radio stations, telephone dialing, satellite communication and other methods: these transmission methods have problems such as coverage, real-time, investment and operation cost, and have great difficulties in the unattended operation of industrial control equipment.

At present, GPRS communication mode mainly promoted by China Mobile is a new data transmission technology, which will fundamentally solve the disadvantages of traditional transmission mode and be closer to the needs of professional data transmission.

2、 Overview of GPRS Technology:

GPRS is the English abbreviation of general packet radio service. It is a new bearer service developed from the existing GSM system. Its purpose is to provide GSM users with packet data service. GPRS adopts the same wireless modulation standard, the same frequency band and the same TDMA frame structure as GSM. This new packet data channel is very similar to the current circuit switched voice service channel, so the existing base station subsystem can provide GPRS service as long as some modules are added.

GPRS allows users to send and receive data in packet switching mode, which provides 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 in the relay mode of storage and forwarding through different routes, and then 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 transmission "share" transmission bandwidth: bandwidth is occupied when there is data and not occupied when there is no data, so as to share resources. In GSM wireless system, wireless channel resources are very valuable. If circuit switching is adopted, the communication needs to establish an end-to-end connection, and the channel can only be monopolized by one user, which is obviously lack of feasibility in wood forming efficiency. GPRS with packet switching can flexibly use wireless channels. Each user can have multiple wireless channels, and the same channel can be shared by several users, which greatly improves the utilization of wireless resources. In theory, GPRS can combine up to 8 time slots to provide users with up to 171 2KB / s bandwidth. Since the data communication fee of GPRS users is based on data traffic without considering the communication time, the access of GPRS for IP service will be more acceptable to users.

GPRS has many advantages for wireless data access. First, it has a wide access range, which can make full use of the national telecommunications network to provide the deployment of remote access network for user data terminals conveniently, quickly and at low cost. Second, it has high transmission rate, and the data transmission speed can reach 57 6kbps, up to 171kbps, twice the ideal speed of commonly used wired 56K modems. The speed of the next generation GPRS service can even reach 384kbps, which can fully meet the application needs of users: 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, provide real-time online function. Now use gprs modem to surf the Internet, and users will always be connected and online, which will make the access service very simple and fast; Fifth, charging by traffic. Users only occupy resources during the period of sending or receiving data. Users can always be online and charge according to the number of data packets received and sent by users. When there is no transmission of data traffic, users will not charge even if they are connected to the Internet. The above advantages make GPRS very suitable for industrial and enterprise wireless access applications.

3、 GPRS data transmission mode and network structure

The development of wireless mobile communication network, especially the emergence of GPRS, provides a new means for the communication of industrial automation equipment. Through GPRS service, the equipment can exchange data with the server on the Internet in the standard way of Internet. GPRS is based on the transmission of data in the form of IP packets. The method of GPRS wireless terminal accessing GPRS network is similar to that of ordinary wired modem, which adopts the establishment of PPP (point to point protocol) connection. PPP protocol is a widely used method of transmitting datagrams on serial point-to-point link, including LCP, PAP, IPCP, NCP and so on. Gprs modem obtains the dynamically allocated IP address through PPP protocol. After the connection is established, the data is transmitted on the basis of PPP protocol

Protocols, such as TCP, UDP, etc., realize data communication with other computers on the Internet.

The following is the typical application diagram of measurement and control system network based on GPRS Technology:

Typical application diagram of measurement and control system network based on GPRS Technology

At present, the GPRS data service opened by China Mobile is mainly to provide wireless internet access for mobile phone and computer users. For the transmission of professional data, the following problems must be solved in order to realize the transmission of measurement and control dataTimely, accurate and safe transmission.

1. Implementation of PPP and TCP / IP protocol on terminal equipment.

For computer users, it is easy to realize dial-up access to the Internet, because there are very mature software on PC, which can easily realize PPP and TCP / IP protocols. However, for embedded devices For the prepared terminal, especially for the single chip microcomputer, due to the restriction of hardware resources and the complexity of the protocol, it is still difficult to implement.

2. Access mode of server

Since the working mode of GPRS network is based on IP address addressing, theoretically, the server only needs to access the Internet and have the IP address assigned by the public network (Internet). However, in the actual system application, if the server is accessed by dial-up or ADSL, the IP address obtained by each access is not fixed, and gprs modem also obtains the IP address dynamically. The two dynamic IP address devices cannot communicate directly on the Internet. Therefore, the server is required to have a static IP address or through other channels

Transfer mode. In this way, the access mode of the server is limited.

In the GPRS service opened by China Mobile, only point-to-multipoint transmission service is provided at this stage. Point to multipoint transmission means that multiple GPRS wireless terminals transmit data to a fixed data processing center. The point-to-point transmission between GPRS terminals is feasible in theory, but it needs better support in technology.

3. Interconnection of mobile network and public network (Internet)

The IP address dynamically obtained by gprs modem is the address in the mobile backbone network and cannot be directly parsed by the public network server. When the connection state is not entered, the server cannot communicate directly with gprs modem. Gprs modem must actively send data to the server for connection. After the connection route is established, the server and terminal equipment can carry out data transmission on both sites. Therefore, the communication must be first initiated by gprs modem, which should be taken into account when formulating the communication mode of measurement and control system.

4. Identification of terminal equipment

The IP address dynamically obtained by gprs modem can not judge the identity of the terminal device by the IP address or MAC address like the ordinary computer network, and can not accurately locate the terminal device. In addition, from the perspective of network security, this problem also exists. Therefore, an ID recognition 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. The location is remote, scattered and unattended. The equipment must be able to work for a long time and stably in a special environment. The system must have the functions of automatic dialing, disconnection and redial. When the GPRS network is unstable, it has the ability to automatically restore communication to ensure the stable operation of the system without human intervention. In addition, the anti-interference and heat dissipation of power supply must also be specially designed to make the terminal equipment adapt to the harsh working environment.