Full or half-duplex operation The number of bits per character -- currently almost always 8-bit characters, but historically some transmitters have used a five-bit character code, six-bit character code, or a 7-bit ASCII. Endianness: the order in which the bits are sent The speed or bits per second of the line (equal to the Baud rate when each symbol represents one bit). Some systems use automatic speed detection, also called automatic baud rate detection.