What is SATA?
Maxtor, the patent holder for the ATA standard, has regularly improved the standard to the point where ATA and Ultra ATA can now achieve rated burst transfer speeds of 100 mbps and 133 mbps respectively. The term ATA is interchangeable with the term IDE, and the ATA standard is also sometimes referred to as Parallel ATA (PATA) because PATA transmits data along its data path in a parallel manner.
However, because PATA burst transfer rates are limited, it is fast becoming a bottleneck in data transfer as drive manufacturers market products with ever-higher data transfer rates and increasing drive speeds. SATA, which in its earliest incarnation is already rated at 150 mbps, is set to eclipse PATA’s burst transfer rates. Enhancements in SATA are not just to be found in performance; they extend to items such as scalability, easier installations, and better airflow. SATA is aimed primarily at the high-end PC market and the low-end server market (or, in other words, non-mission critical production servers, although this may well change).
The Serial ATA Working Group is an industry body that designs, develops, and delivers specifications for the SATA interface. There are two parts to the working group. The first convened in 2000 to set standards for SATA with respect to desktop applications. The second was established in February 2002 to focus on the needs of the server and network storage market segments.
To fully appreciate the improvements SATA brings to storage technology, we need to briefly examine the evolution and limitations of Parallel ATA. When it first went to market, PATA was achieving transfer rates of 3.3 mbps, and over the years this has gradually increased to transfer rates in the 100 mbps range. There were other improvements over the years as well, such as:
- The development of ATAPI to support other storage devices, such as tape drives and CD-ROMs
- Backwards compatibility to earlier ATA versions
- Cyclic redundancy checking for data integrity
- EIDE extensions for faster drive access
- Multiple transfer modes, including DMA and UDMA
As welcome as these improvements have been, the PATA interface nevertheless still suffers from some inherent design weaknesses, which are now coming to a head thanks to faster drive speeds and higher data transfer rates. There are three main limitations to PATA:
- Cable length
- Voltage requirements
- Data integrity issues
Cable length on PATA devices is limited to 18 inches (45 cm) because of signal attenuation. This is problematic in larger computer chassis with longer distances between connection points and can make certain physical drive configurations impossible to implement. Furthermore, the wide, 40-pin ribbon cables used to connect PATA devices are cumbersome and tend to restrict airflow inside a computer. This can lead to unwelcome hotspots inside a given machine. Because the cables are unwieldy, they are difficult to route; in any case, the length issue usually makes creative wiring impossible.
PATA devices require a 5-volt signal. The trend in chip design is towards smaller chips with lower voltages. The large chip pads needed to accommodate the 26 x 5-volt signals per ATA channel mean that, eventually, the chip pads would dominate the size of the chip. Higher voltages also mean that the machine’s overall power consumption is higher.
Data integrity in PATA does offer cyclic redundancy checks (CRC), but this does not extend to command data. SATA offers more in the way of end-to-end integrity of command data.
SATA benefits
Let’s move on to SATA and examine the benefits of this newly developed bus interface. Chief amongst these is the speed of data transfer. Earlier, I mentioned that SATA currently offers burst speeds up to 150 mbps. Well, by 2004/5 and 2007/8, SATA data transfer rates are set to go to 300 mbps and 600 mbps, respectively. Eventually, we may see burst transfer rates nearly 10 times faster than today’s PATA devices.
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