Communication between two device called data flow. Data flow are three type. i) simplex : in simplex mode the communication is unidirectional, as on a one way sreet. Only one of the two device on a link can transmit the other can only receive. Example – radio, keyboard, monitor. ii)half duplex : in half duplex, each station can both transmit and receive but not at the same time, when one device is sending the other can only receive and vice versa. Example – walkies-talkies. iii) full duplex : in full duplex mode , both station can transmit and revive at a same time simultaneously. Mobile phone.