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Table Of Contents  CertiGuide to A+ (A+ 4 Real)
 9  Chapter 10: The Small Computer System Interface, SCSI

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Serial Advanced Technology Attachment (SATA)

ATA (Advanced Technology Attachment) has been the main storage bus since the early days of personal computers. The low cost of ATA drives and the continued improvements in speed have helped maintain its dominance in the PC market. IDE/ATA disk drives interface with current systems using a parallel, shared-bus master/slave architecture. This parallel ATA interface has been in use for 10 years and has reached the end of its useful lifespan in terms of transfer rate improvement.

Serial ATA (SATA)134 is a standards based interface that improves on parallel IDE/ATA in several ways. Serial ATA's interface achieves a 50% transfer rate increase of 150M bps compared to parallel ATA's rate of 100M bps. This is particularly useful in applications with large data volume requirements, such as video editing. The improvements do not stop here…

Serial ATA works at lower voltages and only uses four data lines compared to parallel ATA's 32. SATA provides support for hot-swappable drives and ad-vanced features such as native command queuing. Command queuing lets the disk drive make multiple requests for data from the processor and rear-range the order of the data to maximize throughput. This feature was traditionally available only on SCSI and Fibre Channel drives. Serial ATA can support up to 128 devices per channel (compared to two for IDE/ATA and 15 for SCSI). Serial ATA extends the maximum supported cable length from 18 inches to 1 meter.

The first incarnation of SATA, ratified in the fall of 2002 and available in drives and controllers today, will not do much for end users. ATA disk speeds, at a maximum sustained throughput rate of about 75MB/sec, cannot make use of the bandwidth increase that SATA provides. (Though actually, my Asus A7V600 MB with VIA on-board RAID 0 in HW is hitting right about 140MB/sec.)

Vendors are beginning to ship disk drives with higher speeds. Serial ATA II, due by mid-2004, is expected to double transfer rates and a third generation standard could double them again by 2007.

Serial ATA's seven-pin connector cable will replace ATA's 40-pin, 2-in. wide ribbon cable. Ribbon cables can restrict airflow and increase heat in enclo-sures. Chips for SATA are smaller and only require 0.5 volts to operate. ATA requires 5-volts.

The mean time between failures (MTBF) of ATA drives (500,000 to 600,000 hours) still does not stack up to SCSI's 1.2 million hours.

Key Features of Serial ATA:

  • High-speed: 150Mbytes/sec now, 300 Mbytes/sec. (Soon)

  • Error correction

  • Point-to-point connections (no cable sharing)

  • Narrow cable for better airflow

  • Queuing capability, but not as much as SCSI

  • Backward compatibility to all software

  • Standard built for cost -effectiveness

Table 26 summarizes Serial ATA specifications and contrasts them to SCSI.


Table 26: Serial ATA Specifications

 

IDE/ATA

SATA 1.0

SATA II

SATA III

SCSI

Typical Drive Speed (RPM)

5,400 - 7,200

5,400 - 10,000

5,400 - 10,000

5,400 - 10,000

10,000 -15,000

Transfer Rate (MB/sec)

100

150

300

600

320

Status of Standard

Final

Final

Due in mid-2004

Due in mid-2007

Final


 __________________

134. http://www.serialata.org/

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