Abstract: A source position detecting method and a communications system, communications equipment and relay suitable for implementing this method. When relaying a coded data signal from one individual network to another individual network, a first relay, for instance, increases the length of preambles by 1 and when relaying to the next individual network, a second relay, for instance, decreases the length of preambles by 2. Increment/decrement processing of the length of preambles can be carried out without decoding the main body of the coded data signal. Alternatively, preamble duration shortening accompanying the relay can be utilized and signal distortions accompanying signal transmission such as group delay and signal strength attenuation can also be utilized. Without processing the data at the relay, the destination equipment can obtain data concerning the position of the source equipment which transmitted the data signal, such as data concerning the transmission route of the data signal.

Abstract: Preventing signals from undesirably entering and enabling interactive communication. Signals from a main line or branch line are received by receiving circuits. Signal detecting circuits detect signals, and then transmit the detected signals (detection B and detection A) to a first-arrival judging circuit. The first-arrival judging circuit determines which signal is the first arrival between the signals from the detecting circuits, and then allows one of transmitting circuits to output a signal. Accordingly, only one of the transmitting circuits generates signals, preventing signals of the other transmitting circuit from entering and enabling interactive communication.