Data communication method

A method of connecting a plurality of communication devices to an IP network through communication lines to each of which a unique identification number is assigned, registering in each of the plurality of communication devices the identification numbers of other communication devices allowed to communicate with the each of the plurality of communication devices, uniquely connecting the communication lines of the communication devices by a three-way handshake based on the identification numbers, and performing data communication between the connected communication devices, comprising:

• (1) activating from the communication line of one of the plurality of communication devices to the communication line of another of the plurality of communication devices, and, when a connection by three-way handshake is established between an activating and activated connection lines, disconnecting the connection without data communication;
• (2) sending a connection request signal from the activated communication line to the activating communication line within a predetermined time after the disconnection;
• (3) when the connection request signal arrives at the activating communication line, confirming that the communication device of the activated communication line is true if when an identification number extracted from the connection request signal corresponds to the identification number of the communication line which has activated in the step (1) and the reception of the connection request signal is done within the predetermined time after the disconnection, and sending an acknowledge and connection request signal from the activating communication line to the communication line from which the connection request signal has been sent;
• and
• (4) when the acknowledge and connection request signal arrives at the activated communication line, determining that the communication device of the activating communication line is true, and sending an acknowledge signal from the activated communication line to the communication line from which the acknowledge and connection request signal has been sent, so that the connection by three-way handshake is established again between the activating and activated communication lines so as to perform the data communication.