Two way communication systems

An information transmitting system capable of two way communication, suitable for use in transmitting information signals between a central control unit (100, 200) and a plurality of terminal apparatus units (35). Each terminal apparatus unit (35) may be located at a different passenger seat in an aircraft, a train, a bus, or the like. The signals transmitted from each terminal apparatus unit (35) to the central control unit (100, 200) are transmitted by way of two channels via a respective transmitter (38) and a cable (21). In one operating mode, one channel is used as a random access channel, and the other is used as a polling channel. When polling transmission is not being effected, the polling channel is used as a second random access channel, so that the system may efficiently transmit a maximum number of information signals on two random access channels.