This document discusses different modes of data transmission between digital devices. It describes parallel transmission, which transfers all data bits simultaneously over separate lines and is faster but more costly than serial transmission. Serial transmission sends each data bit in sequence over a single line, making it less costly but slower. Transmission can be synchronous, sending multiple bytes at once without start/stop bits, or asynchronous, sending one byte at a time with start/stop bits. Modes of transmission include simplex (one-way), half duplex (one-way at a time), and full duplex (two-way simultaneous).