A communication system comprising: communication devices (4-1 - 4-8) configured to be connected to one another through a tree-like network, wherein in the tree-like network a root device (4-1) is a communication device (4-1 - 4-8) connected to a wired Local Area Network , LAN, and node devices (4-2 - 4-3) are communication devices (4-1) connected to the root device (4-1) by a wireless LAN, and leaf devices (4-4 - 4-7) are communication devices (4-2 - 4-8) that are connected to a node device (4-2 - 4-3) via a wireless LAN, wherein the root device (4-1) is one of the communication devices (4-1 - 4-8) that stores system information including a system management table with address information and information on availability of communication of each of the communication devices (4-1 - 4-8) and a connection form of the communication devices (4-1 - 4-8) in the tree like network, wherein the root device (4-1) is configured to transmit the system information to each of the communication devices (4-1 - 4-8) periodically; wherein each of the communication devices (4-1 - 4-8) is configured to send an acknowledgement message to the root device (4-1) when receiving the system information; and wherein the root device (4-1) is configured to determine whether the acknowledgement message has been sent from each of the communication devices (4-1 - 4-8) or not, and, when an acknowledgement message from one of the communication devices (4-1 - 4-8) has not been received by the root device (4-1), the root device (4-1) is configured to rewrite the information on availability of communication of the communication device (4-1 - 4-8) from which the acknowledgement message has not been received by the root device (4-1), as incommunicable, and to update the system information accordingly. communication devices (4-1 - 4-8) configured to be connected to one another through a tree-like network, wherein in the tree-like network a root device (4-1) is a communication device (4-1 - 4-8) connected to a wired Local Area Network , LAN, and node devices (4-2 - 4-3) are communication devices (4-1) connected to the root device (4-1) by a wireless LAN, and leaf devices (4-4 - 4-7) are communication devices (4-2 - 4-8) that are connected to a node device (4-2 - 4-3) via a wireless LAN, wherein the root device (4-1) is one of the communication devices (4-1 - 4-8) that stores system information including a system management table with address information and information on availability of communication of each of the communication devices (4-1 - 4-8) and a connection form of the communication devices (4-1 - 4-8) in the tree like network, wherein the root device (4-1) is configured to transmit the system information to each of the communication devices (4-1 - 4-8) periodically; wherein each of the communication devices (4-1 - 4-8) is configured to send an acknowledgement message to the root device (4-1) when receiving the system information; and wherein the root device (4-1) is configured to determine whether the acknowledgement message has been sent from each of the communication devices (4-1 - 4-8) or not, and, when an acknowledgement message from one of the communication devices (4-1 - 4-8) has not been received by the root device (4-1), the root device (4-1) is configured to rewrite the information on availability of communication of the communication device (4-1 - 4-8) from which the acknowledgement message has not been received by the root device (4-1), as incommunicable, and to update the system information accordingly. <EOS>