A fax machine can receive a message and then send a message back. However it cannot receive and send messages at the same time. Simultaneously Full Duplex Transmission allows two-way simultaneous signal transmission. An example of this transmission type would be a traditional telephone. Transmission Mode Asynchronous transmissions send only one character at a time using parity bits to designate where a message starts and stops.
Synchronous transmissions send more than one character.
In this transmission mode, data packages are sent with header and flags to denote where a message starts and stops. Modems will also convert analog signals into digital so that the data will be compatible to computer technology. Multiplexers allow more than one telecommunication signal to be sent over line by concentrating data over a single communications channel. Front-end Processors are communications devices that poll terminals for data and information that is waiting to be transmitted. They can also a call other sites and answer calls from other sites automatically.