Figure 8 shows a simple SCADA system consisting of a master unit (MTU) and a remote unit (RTU). The MTU and RTU must be equipped with communication equipment to establish communication with each other.
Fig. 8
In terms of communications, the equipment needed to establish communication between the MTU and the RTU is known as DCE (Data Communication Equipment). This equipment has the ability to generate the signals necessary to establish communication; that is, to encode and decode the signals according to the requirements.
An example of this communication equipment is the modem. Figure 9 shows a diagram of the equipment used.
Fig. 9
Information travels over a medium from one DTE to another. Modems use a physical medium such as a telephone line or a dedicated line.
The International Organization for Standardization (ISO) has developed an open interconnection system model, the OSI model. This model consists of seven layers. The function of each layer is defined, and the appropriate number of layers is used depending on the requirements. The lower layers are sufficient for most SCADA systems. Note that there is not necessarily a direct correspondence between an OSI layer, which defines a function, and a physical piece of hardware.
Fig. 10 The seven levels of the ISO-OSI model
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