A communication system includes a control terminal device and one or more communication devices. The control terminal device transmits a search message to entire network when the communication device is registered into the communication system. When the communication device is connected to the network, the communication device determines whether a connection position of the connected communication device on the network is the predetermined connection position. When the communication device is not positioned at the predetermined connection position, the communication device cancels the connection with the network and then connects to the predetermined connection position of the network and responds to the search message through the network after the communication device directly connects to the control terminal device and obtains information for connecting to the predetermined connection position of the network.