SATA physical coding uses a line encoding system known as 8b/10b encoding. This scheme serves multiple functions required to sustain a differential serial link. First, the stream contains necessary synchronization information that allows the SATA host/drive to extract clocking. The 8b/10b encoded sequence embeds periodic edge transitions to allow the receiver to achieve bit-alignment without the use of a separately transmitted reference clock waveform. The sequence also maintains a neutral (DC-balanced) bitstream, which lets transmit drivers and receiver inputs be AC-coupled. Generally, the actual SATA signalling is half-duplex, meaning that it can only read or write data at any one time.