This protocol uses a central station to monitor other stations in the network. It is used predominantly in star networks. The master station sends a message to each slave station in turn. If the slave station has information to send, it passes it to the master station. The master station then sends the information to the desired slave station, or keeps it if the information is for itself.
Slave stations are very simple and have limited functionality.
It is appropriate for industrial and process control situations. This section describe some common network facilities and connections. This allows a number of devices to share an expensive line. Only one device can use the line at any one time, and the line is often managed using polling. Dial-up telephone circuits are an example of switched circuits.