The present invention relates to a control and monitoring telecommunication system for connecting and communicating between a site such as power plant and shopping mall where apparatuses to be controlled and monitored are located and a control room and monitor room where operators and observers are stationed, and also to a method of setting a modulation method for the communication.
A need for remote monitoring or remote maintenance of existing plant has been increasing in many plants including power plant. In a shopping mall or large-scale shop, a need for installing or adding monitor cameras is also increasing in view of higher security.
It therefore has become necessary to lay a dedicated cable newly between the site and monitor room or control room so as to transmit the data obtained by the sensors and actuators that have not been connected in a network or monitor cameras, sensors and actuators that have been newly installed from these apparatuses to the monitor room or control room. On the other hand, since there are large apparatuses or inverters installed at the site, eliminating the effect of the electromagnetic noise to be caused by driving these apparatuses needs to be considered. The construction cost needed to lay a dedicated cable in an existing installation so as to avoid the effect or eliminate the noise in the telecommunication turns to be higher than the construction cost of a telecommunication line for measuring and control points spent at the time of initial plant construction.
Generally, in a power plant, various sites including turbine room and boiler room are located far from each other by hundreds to thousand meters within an area, and even in a shopping mall, monitor cameras need to be installed as distributed within a relatively wide outdoor space. If a one-to-one communication modem is employed for these applications, a star type network, where individual telecommunication line is laid between each site (monitor camera in the shopping mole application) and the monitor room, is needed but the installation cost and maintenance cost of the total network increases.
In order to resolve this problem, there has been proposed a method for utilizing an unused line or frequency band out of the existing telecommunication lines installed between the site and monitor room so as to reduce cost needed to lay a new dedicated line or take an anti-noise measures (see Japanese Laid-open Patent Publication No. 2001-25000).
Although Japanese Laid-open Patent Publication No. 2001-25000 (paragraphs 0026-0035, FIG. 1 and FIG. 2) also discloses a telecommunication mode where multiple modems are connected with a single telecommunication line, it lists nothing else but generally known coordination methods (methods where one modem serving as master allocates the transmission right; comprising four methods: token passing, collision sensing, time division multiplex, and frequency division multiplex). The patent does not contain any concrete description about detailed physical layer of communication but simply refers to a reference literature on xDSL (x digital subscriber line). Detailed description about the physical layer of the x DSL is given in “Draft American National Standard for Telecommunications-Network and Customer Installation Interfaces-Asymmetric Digital Subscriber Line (ADSL) Metallic Interface” (U.S.A.), American National Standards Institute, Inc. ANSI (R) T1. 413-1998, pp. 95-135, for example. The document, which is written on the physical layer of ADSL (asymmetric digital subscriber line), a typical xDSL, describes that, for the modems communicating with each other, training to cope with noise or attenuation and adjusting the communication setting based on the training (determining a modulation method for every carrier) are needed, explaining how to realize it. In this explanation, carrier means the frequency of carrier wave signal.