Application of wireless transmission in real-time water level monitoring system

Application of wireless transmission in real-time water level monitoring system

With the advancement of China's informatization process, the water conservancy industry is also facing the problem of informatization construction [1]. At present, the informatization of the water conservancy industry is highly valued, and the information transformation of the water conservancy system has also achieved certain results, but at the same time, there are generally weaknesses such as weak network functions and data sharing capabilities, long data update cycles, and single data visualization methods. This makes the further improvement of the digital hydrological system an urgent task. This article originates from the renovation project of the Hangzhou River Flood Control Water Level Control and Monitoring System. The original system used the public telephone network (PSTN) as a data transmission network, and used dial-up methods for point-to-point communication to achieve water level data collection and transmission. The following problems existed in the actual use process :

(1) Terminal system function limitation: The system only has the function of regularly and actively polling each water level monitoring point, and cannot perform real-time online monitoring.

(2) Limitation of data transmission in PSTN dialing mode: the system cannot realize continuous water level monitoring of a certain point, nor can it achieve continuous water level detection of multiple key points or multiple monitoring units simultaneously detect a key point . Dial-up conflicts will occur when multiple departments, such as the Flood Control Headquarters, the Municipal Drainage Corporation, and the Municipal River Conservation Center, etc., simultaneously query the same water level monitoring point.

(3) Difficulties in installation: Since the monitoring points of the river water level are all outdoors, if additional monitoring points are needed, telephone lines need to be laid and installed, and individual monitoring points are difficult to achieve.

(4) There is no data management platform: so that the data of various departments are not synchronized and cannot be managed in a centralized manner.

In summary, considering the distribution of river detection points and the technical characteristics of existing wireless signal transmission, China Mobile GPRS or China Unicom CDMA 1x is used as the data transmission network to replace the original telephone dialing method to flexibly implement water level monitoring equipment Access, simple engineering installation, can well solve the problem of data transmission in remote areas without network and telephone lines. Compared with the traditional digital radio station, it is more convenient, flexible, and easy to operate, while also reducing costs. The wireless transmission scheme is the best choice for modern industrial field data transmission. Establish a unified data management platform, multiple units obtain data from the central server on demand, so that all units can obtain real-time data of water level monitoring points. Due to space limitations, this article focuses on the overall design of the system, and will not repeat how to implement Internet access for the embedded terminal system.

1 Overall system plan

The overall scheme of the system is shown in Figure 1. The terminal system uses GPRS / CDMA wireless access. The Internet communicates with the central server. The terminal stores the collected water level data locally and sends the data to the central server according to the instructions of the central server. The central server is responsible for the status management of each monitoring terminal and the water level data storage of each monitoring point, and provides a GIS-based data management platform. All relevant units can obtain real-time and historical data and related reports of each monitoring point provided by the central server through VPN authorized access.

1.1 Design of river water level monitoring system

The water level monitoring terminal is connected to the central server through wireless transmission. Currently available wireless data transmission networks are: China Mobile GPRS and China Unicom CDMA 1x. GPRS is a data transmission network of China Mobile based on the GSM network. At present, China Mobile generally uses CS-2 with a coding rate of 13.4kbps. The highest level of GPRS modules used by industry is Class10, that is, it supports 4 downlink time slots and 2 uplink times. At the same time, up to 5 time slots are used at the same time, so the current rate of GPRS transmission is 53.6kbps (4TImeslots) in the downlink and 26.8kbps (2TImeslots) in the uplink. CD-MA 1x is China Unicom's CDMA-based data transmission network. At present, the maximum data rate of the forward (downstream) channel defined by RC3 (wireless configuration) is 153.6kbps when supporting 1 SCH (Supplementary Service Channel) Release0. The maximum data rate for the (upstream) channel is 76.8kbps.

The reason to choose GPRS as the main and CDMA as the secondary networking solution is because in actual use, it is found that the coverage rate of GPRS (Rx≥-95dBm) in remote areas has a slight advantage over CDMA. However, the soft handover of CDMA and its related technologies are superior to the GPRS technology in the control of the drop rate [2].

1.2 Central server access method

There are two ways for the central station to connect to Internet: private network and public network.

Private network mode: The central station and the water level monitoring terminal are both inside the wireless network. By applying to the wireless operator for the APN private network service, the central station and the terminal use the intranet address (private address) to communicate with each other.

Public network mode: the central station accesses the Internet through other operators (such as China Telecom, China Netcom, etc.), and the water level monitoring terminal accesses the Internet through a wireless network, thereby achieving mutual communication between the terminal and the server.

Because this system will use GPRS as the main network structure and CDMA as the auxiliary network structure, it is not suitable for mobile private network access, so it uses a fixed IP to connect to the Internet in the public network. At the same time, the central station sets up a virtual private network VPN (Virtual Private Network) server to enable other relevant departments to authorize access to the central server through the Internet-based VPN. VPN uses unreliable public internet as the information transmission medium, through additional security tunnels, user authentication, access control and other technologies to achieve similar security performance as the private network, thereby achieving the secure transmission of important information. This method has low cost and overcomes the insecure characteristics of the Internet.

2 Water level monitoring terminal design

The water level monitoring terminal needs to implement the following functions [3]:

(1) In order to realize continuous and rapid real-time water level monitoring, the water level acquisition terminal first needs to have the function of rapid real-time water level acquisition and transmission.

(2) In the non-real-time monitoring working mode, it can perform timing water level collection according to the settings and can save the data locally for a long period of time.

(3) The historical water level data saved locally can be reported as required by the central server.

(4) The current water level can be compared with the set water level upper and lower thresholds. If the limit is exceeded, the central server should be automatically alerted.

(5) It can automatically detect the status of the network, and if there is an exception, it can redial to ensure real-time online. At the same time, it has an automatic time adjustment function to ensure the clock synchronization with the central server, and then achieve the clock synchronization of the entire water level monitoring system.

(6) The terminal's working status (including power status) can be transmitted to the central server in time for management and maintenance.

According to the above functions, the terminal system is divided into power module, central processing module, storage module, user interface module, input module and data transmission module, as shown in Figure 2.

3 Central station system design

3.1 Water level monitoring terminal management subsystem

The design of the water level monitoring terminal management subsystem mainly lies in the application layer protocol to achieve the stable and efficient operation of the entire system. The application layer protocol first needs to realize the data communication between the central station and the monitoring terminal, monitor the status of the communication link, and realize the management of the data collection terminal at the central station.

3.1.1 Data communication function

The data collection terminal can transfer data to the central station through two transport layer protocols: IP-based TCP or UDP. TCP provides reliable end-to-end services to the upper layer, but this reliability is at the cost of system resources and network overhead. Using UDP transmission will save a lot of system resources. GPRS / CDMA are all charged by flow rate, so UDP, which takes up less system resources, is more suitable for real-time transmission. However, the use of UDP transmission will also cause new problems, such as the problem of the fixed communication port of the monitoring terminal caused by the dynamic allocation of the GPRS / CDMA gateway, and because UDP does not provide reliable communication transmission, it is also necessary to ensure the reliable transmission of data in terminal management .

3.1.2 Dynamic terminal management function

Because UDP transmission does not establish a connection with the destination host before transmission, but the dynamic address mapping (DNAT) is performed by the GPRS / CDMA gateway. The IP address and port number mapped to the public network may change after a period of time, so the central station The public network address and port number of the current terminal need to be obtained in order to communicate with the terminal normally. This requires the terminal to send data packets regularly, that is, the so-called heartbeat packets to maintain the communication link with the central station. In order to avoid unnecessary waste of heartbeat packets, heartbeat packets should be designed as useful data packets (such as terminal configuration information, etc.) in the upper layer protocol. The station sends a heartbeat packet response to inform the terminal of the existence of the central station. At the same time, the central station continuously updates the IP address and port number of the corresponding terminal in the station status table (such as Table 1) according to the source address and port number of the heartbeat packet sent by the terminal, and communicates with the terminal through the address and port. If the central station does not receive the heartbeat packet of the terminal after timeout, it is considered to be disconnected; if the terminal does not receive the heartbeat response from the central station, it is considered that the communication link is abnormal, and the PING telecom server task (such as the DNS server) is started for link test. When receiving a response, it is considered that its own network connection is normal and waiting for the central station to recover. If there is no response, it is considered that its own network connection is abnormal and a redial is adopted to connect. The central station manages each terminal by maintaining the site status table.

3.1.3 Reliable transmission of data

The application layer protocol must also ensure the reliable transmission of data. Due to the limited resources of the terminal system, the complexity of the terminal system should be reduced as much as possible in the design. In order to ensure reliable transmission of data, overtime retransmission should be implemented at the central station, that is, any command sent by the central station requires a terminal response.If the central station does not receive a terminal response after timeout, the command is deemed to have failed and needs to be retransmitted. The frame should be consistent with the original command frame, but the consequences of the retransmission mechanism should be fully considered in the protocol design of each functional module; secondly, in order to prevent data errors, the frame check function must be implemented on both the central station and the terminal. Therefore, it is necessary to include the check digit in the frame structure.

3.1.4 Frame structure

The command frame sent by the central station (as shown in Figure 3) includes: command type, destination terminal number, command number, command length, command content, and CRC check. The terminal determines which command is based on the CmdType of the received command frame, such as 0x00. Heartbeat packet response, 0x11 real-time data query command, 0x12 historical data query command, 0x18 clock calibration command, etc.

The data frame sent by the terminal (as shown in Figure 4) includes: data type, terminal status, terminal number, data content and CRC check. Based on the DataType and StaTIonID of the received data frame, the central station determines which data is from which terminal, for example: 0x00 configuration information, 0x0l real-time data, 0x02 historical data, 0x04 command response, 0x0F system alarm, etc.

As mentioned above, a unified frame format is used in the system to represent different central commands and terminal data, and the minimum amount of bytes is used to express as much information as possible, so that the central server can timely understand the working status of each terminal and realize Dynamic management of the terminal.

3.2 Water level data management subsystem

The main function of the data management subsystem is to establish a water level database of monitoring points, regularly send historical data query commands to each monitoring point, and maintain the integrity of the water level database; display the current status of each monitoring point in real time through the GIS module (as shown in Figure 5) ; When the client authorized by VPN requests to query data of a certain monitoring point, the central server automatically generates related reports (as shown in Figure 6), and according to whether the client requires real-time monitoring of a monitoring point, to the monitoring point Send real-time commands to achieve centralized and unified management of data.

Through the use of GPRS / CDMA transmission system and the upper layer software system based on C / S architecture, multiple monitoring units can simultaneously monitor one or more water level monitoring stations in real time, improving the reliability of the entire river flood control water level control monitoring system , Real-time and stability, achieved the desired transformation goal. However, because some sites are powered by accumulators and solar cells, low energy consumption is not a design goal in the design, and the cost of accumulators and solar cells at individual sites has been increased.In the next step, energy consumption reduction will be an important goal. achieve.

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