| The engineering of city water supply consists of 3 parts: Picking up natural water, making the natural water clear, Transporting the clear water to users. Because its geographical distributing is wide, communication plays important role in the monitor system of city water supply. The traditional methods are radio, telephone line and leased line. But they have such disadvantages as more cost, long time of construct and low speed. The development of GPRS technology overcomes the shortcomings mentioned above. GPRS is a value-added service based on GSM, covering widely. GPRS uses packet switching instead of circuit switching, providing high efficiency of frequency. It has a characteristic so said "always on line". It charges by flow rate, resulting in lower cost. So GPRS is the good choice for monitor system of city water supply. In this paper, a monitor system based on GPRS is designed to meet the most requests of engineering of city water supply.Because GPRS network access by IP address, it, theoretically speaking, will work if the host computer links to Internet and has an IP address assigned by Internet. Whereas, if the host computer links to internet by dial-up, ADSL or GPRS, its IP address is not fixed at every log in, also the Terminal's IP address is dynamic, they can't communicate with each other. So, the host computer must have a fixed IP address or they get the other's IP address by other means. Most manufacturer of GPRS MODEM supply a dynamic IP management server, we can use it to get IP address, this make it very easy to construct a monitor system.The master-call-slave-answer agreement is often used in most current monitor systems, that's because radio has fewer channels, we have to do so to avoid conflicts. There are disadvantages in this agreement, it cost a period of time to call all terminals circularly, and, the more terminals exist, the longer the time is. There is possibly no new data at calling time but there is at the other time. Thisweakens the reality and waste communications. It's bad for terminals powered by battery. If GPRS MODEM replaces radio, we get more channels and wide band width, conflicts are managed by GPRS network, so an improved communication protocol is put forward in this paper. Terminals can send data by themselves. A precision is set for every variable. If the variable changes bigger than the precision, the terminal will trigger the send procedure. Terminals send data to the host computer periodically for the system to see the failure of terminals in time. Concerning terminals of no up-sending function and terminals using a common GPRS MODEM, master-call-slave -answer agreement is still used.Because most of the terminals only monitor a pump or a pressure of pipe, this paper give a small size design of the terminal. When a terminal is not enough, several terminals are used together connecting to one field bus, they share a GPRS MODEM.Terminals may be from more than one manufacture, their communication protocols may be different, some of them don't support bus function, can not recognize address information. Though some of them do so, their address position, baud rate or parity may be different. It is difficult to put them together. So this paper designs protocol converter to solve this problem. |
