Hardware Reference
In-Depth Information
SATA Express devices are expected to be available in 2014.
Table 7.9. SATA Transfer Modes
SATA Express
The advent of high-performance SSD (solid-state drive) storage has pushed the need for greater and
greater interface bandwidth. SATA 3.0 offers up to 600MBps throughput, which by 2011 many SSDs
could deliver. Since then the development of faster and faster drives has been mostly limited by the
interface bandwith, and that SATA had become the bottleneck. To eliminate this bottleneck, the Serial
ATA International Organization (SATA-IO) first studied doubling the SATA 6Gbps rate to 12Gbps;
however, they found that doing so required extensive (and expensive) changes in cabling and
signaling, not to mention that development would take some time. Instead, they decided to take a much
easier way out by using the existing PCI Express interface. SATA-IO first announced in 2011 that it
was developing a faster version of SATA called SATA Express, which was finally completed and
published in 2013 as part of the SATA 3.2 specification.
SATA Express combines PCI Express signaling with the SATA software protocol (command set),
plus a new set of cables and connectors that are backward compatible with SATA. When using PCIe
3.0 signaling, SATA Express offers up to 16Gbps in raw data throughput, which translates to nearly 2
gigabytes per second of actual data bandwidth. That is nearly 3.3 times faster than conventional
SATA at 600MBps.
Not only is the SATA Express signaling speed much faster, but it is also more efficient, resulting in
even higher bandwiths than the raw signaling rate would imply. Conventional SATA uses 8b/10b
encoding, which is 80% efficient. That means that 8 out of every 10 bits (or 80%) in the raw data
stream are actual data; the other 2 bits (or 20%) are overhead. SATA Express uses the more
advanced 128b/130b encoding scheme found in PCI Express 3.0, which is an incredible 98.5%
efficient, with only 1.5% overhead. This is achieved by scrambling the raw data to be sent using a
known binary polynomial and then unscrambling it at the other end using the inverse polynomial.
Because SATA Express uses two PCIe lanes with up to 8Gbps per lane, combined with the more
efficient encoding, the end result is a whopping 1,969MBps maximum throughput as compared to
600MBps for conventional SATA.
SATA Express uses a wider cable with 18 conductors vs. the 7 conductors in a standard SATA
cable. SATA Express motherboard connectors are backward compatible with SATA, meaning you
can plug one or two standard SATA cables into a single SATA Express connector (see
Figure 7.9
).
Connecting conventional SATA drives to a SATA Express port causes the port to shift down to
conventional SATA mode.
