SATA 3.0 specification no relation 8b/10b encoding
SATA revision 3.0 (6 Gbit/s, 600 MB/s, Serial ATA-600)
Serial ATA International Organization (SATA-IO) presented the draft specification of SATA 6 Gbit/s physical layer in July 2008, and ratified its physical layer specification on August 18, 2008. The full 3.0 standard was released on May 27, 2009.Third-generation SATA interfaces run with a native transfer rate of 6.0 Gbit/s; taking 8b/10b encoding into account, the maximum uncoded transfer rate is 4.8 Gbit/s (600 MB/s). The theoretical burst throughput of SATA 6.0 Gbit/s is double that of SATA revision 2.0. It is backward compatible with SATA 3 Gbit/s and SATA 1.5 Gbit/s.The SATA 3.0 specification contains the following changes:Physical layer
The physical layer defines SATA's electrical and physical characteristics (such as cable dimensions and parasitics, driver voltage level and receiver operating range), as well as the physical coding subsystem (bit-level encoding, device detection on the wire, and link initialization).
Physical transmission uses differential signaling. The SATA PHY contains a transmit pair and receive pair. When the SATA-link is not in use (example: no device attached), the transmitter allows the transmit pins to float to their common-mode voltage level. When the SATA-link is either active or in the link-initialization phase, the transmitter drives the transmit pins at the specified differential voltage (1.5 V in SATA/I).
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.
Also, SATA uses some of the special characters defined in 8b/10b. In particular, the PHY layer uses the comma (K28.5) character to maintain symbol-alignment. A specific four-symbol sequence, the ALIGN primitive, is used for clock rate-matching between the two devices on the link. Other special symbols communicate flow control information produced and consumed in the higher layers (link and transport).
Separate point-to-point AC-coupled low-voltage differential signaling (LVDS) links are used for physical transmission between host and drive.
The PHY layer is responsible for detecting the other SATA/device on a cable, and link initialization. During the link-initialization process, the PHY is responsible for locally generating special out-of-band signals by switching the transmitter between electrical-idle and specific 10b-characters in a defined pattern, negotiating a mutually supported signalling rate (1.5, 3.0, or 6.0 Gbit/s), and finally synchronizing to the far-end device's PHY-layer data stream. During this time, no data is sent from the link-layer.
Once link-initialization has completed, the link-layer takes over data-transmission, with the PHY providing only the 8b/10b conversion before bit transmission.
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