server 306 need not be exclusively dedicated to transmitting but may be a multifunction server the transmitter server 306 is in turn connected to a transmitter interface 310 and controls the operation of the transmitter interface 310 in response to commands from the transport controllers 304 a 304 c the server 306 also transfers data from the transmitter interface 310 to the transport controllers 304 a 304 c each of the transports 302 a 302 c may suitably establish a session with the operator control units 312 a 312 c each of the transport controllers 304 a 304 c sends messages to the transmitter server 306 if a message is an availability message it includes an identifier for the transport and if the message is part of a communication session with a control unit it includes an appropriate session identifier the transmitter server 306 formats each message for transmission and broadcasts it using the transmitter interface 310 each of the control units 312 a 312 c displays availability messages if disengaged or if engaged in a session responds to messages including the appropriate session identifier when an exception occurs for example affecting the transport 302 a while the transport is engaged in a session with the control unit 312 a the transport controller 304 a prepares an operator message including the session identifier being used by the transport controller 304 a and the control unit 312 a the transport controller 304 a then passes the message to the transmitter server 306 which sends the message using the transmitter interface 308 and antenna 309 because the message is identified as belonging to the session going on between transport controller 304 a and the operator control unit 312 a the operator control units 312 b and 312 c.