Serial ATA - It's Time to Get in Line

Monday Sep 8th 2003 by John Vacca
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Serial ATA - It's Time to Get in Line

Originally Published: 10/15/02
Most Recent Revision: 09/08/03

Serial Advanced Technology Attachment (SATA) is the hottest technology being developed for desktop storage and low end server drives today. Serial ATA is the next generation personal computer (PC) storage interface. It replaces the Ultra ATA/100 interface (otherwise known as the latest-generation Parallel ATA interface) — which is used to connect most PCs to their primary storage, and which has now become a bottleneck because it has reached its maximum burst data transfer rate speed of 100 MB/sec.

Serial ATA, on the other hand, has a burst data transfer rate speed of 150 MB/sec in its initial iteration and is expected to double in speed roughly every three years — to 300 MB/sec in 2004, 600 MB/sec in 2007, and so on.

This article describes the primary benefits of the Serial ATA interface and the increase in data rate. It also includes an explanation of why this technology is needed, what the advantages are, the possible impact on the storage environment, and the possible barriers to implementation. Other features and benefits are also outlined, together with a comparison to alternative storage interfaces.

What Is Serial ATA?

Serial ATA (Advanced Technology Architecture) is an interface used to connect hard drives and other peripherals to a PC. It is the evolutionary replacement for the Parallel ATA (PATA) physical storage interface.

In other words, Serial ATA is a storage interface specification for the next-generation computing platform. This interface will be used to connect storage devices such as hard disc drives, DVDs, and CD-R/Ws to the motherboard and is the replacement for today's Parallel ATA physical storage interface.

Serial ATA technology allows for platform cost reductions and performance improvements while supporting a seamless transition from Parallel ATA technology. Serial ATA supplies storage interface headroom, beginning with 1.5 Gbps and scaling to 2x, 4x, and beyond. At the same time, Serial ATA is a drop-in solution that is compatible with existing ATA software drivers and runs on standard operating systems without modification. It provides for systems that are easier to design, with narrower cables that are simple to route and install, smaller cable connectors, improved silicon design, and lower voltages, which alleviate current design constraints in Parallel ATA. Configuration of Serial ATA storage devices is much simpler, with many of today's requirements for jumpers and settings no longer needed.

Serial ATA is 100% software compatible with today's ATA, but has a much lower pin count, enabling thinner, more flexible cables. Serial ATA's cables can be up to a meter in length, and are small and neat because they only need seven conductors or a seven-pin data connector. Even the polarity-keyed plug is just eight millimeters wide. Contrast this with parallel ATA, which uses a 40-pin connector. APT Technologies, Quantum, Dell, IBM, Intel, Maxtor, and Seagate are jointly leading this initiative, with broad industry support from nearly 300 companies that make up the Serial ATA Working Group.

The Serial ATA Working Group

The Serial ATA Working Group is an industry organization whose mission is to define, develop, and deliver the industry specification for the Serial ATA interface. The Serial ATA Working Group is comprised of two groups:

  • First, the Serial ATA 1.0 Working Group was established in February, 2000 to specify Serial ATA for desktop applications. Since that time, the organization has grown several-fold and now totals over 200 members.

  • Second, the Serial ATA II Working Group was formed in February, 2002, to further address the needs of servers and networked storage market segments, and to specify next generation transfer speeds. The Serial ATA II Working Group is made up of over 86 members.

Page 2: The Need for Change

The Need for Change

To understand the need for Serial ATA, we must take a look into the past. The Advanced Technology Attachment (ATA) interface (previously called Integrated Drive Electronics (IDE)) has existed in substantially the same form since 1989, and has become the highest-volume disk drive interface in production. Maxtor, in its role as the patent owner, has led continuous improvements to parallel ATA that has extended its data transfer rate from 3.3 Megabytes per second (MB/s) to 100 MB/s, with only one cable change.

However, as PC processor performance has increased, so have the read/write data rates of hard disk drive (HDD) heads and media. This disk rate is projected to exceed today's 100 MB/s interface bandwidth by 2004. Parallel ATA has kept pace in the past, but is nearing its limit and therefore becoming a performance bottleneck. Serial ATA will eliminate this bottleneck by initially offering 150 MB/s and will also provide significant headroom for future improvements.

In other words, Serial ATA is scalable and allows future enhancements to the computing platform. Parallel ATA (PATA) has been a solid interface — allowing performance scalability and reliable data transmission, but drive rates continue to climb and system designs continue to demand more flexibility and robustness from components. Serial ATA allows the performance and growth to continue without adding cost or utilizing extraordinary means to achieve the requirements.

So, with the preceding in mind, what are the real user benefits of Serial ATA? Let's take a look at some of the evolutionary improvements.

Benefits of Serial ATA

As previously explained, Serial ATA is a high-speed serial link replacement for the parallel ATA attachment of primary internal storage devices. This breaks down to overall system reliability improvements to airflow and thermal dynamics, as well as easier installations and upgrades.

Evolutionary Improvements

Constant evolutionary improvements in the ATA interface have enabled it to remain competitive with other storage interface technologies, despite a number of limitations. Improvements include:

  • ATAPI for support of other peripheral devices, such as CD-ROM drives and tape drives
  • Backward compatibility with older ATA storage devices
  • Cyclic redundancy checking (CRC) for improved data protection and greater overall data integrity
  • Enhanced Integrated Drive Electronics (EIDE) extensions for faster HDD access and logical block addressing (LBA)
  • Multiple data-transfer modes, including Programmed Input/Output (PIO), direct memory access (DMA), and Ultra DMA (UDMA)

Page 3: Ultra ATA-100

Ultra ATA-100

Ultra ATA-100 was the latest-generation Parallel ATA interface. With its maximum burst data transfer rate of 100 MB/sec, it superseded the Ultra ATA-66 interface. Before the industry completes its final transition to Serial ATA, Ultra ATA-100 is the last Parallel ATA interface.

Parallel ATA Interface Limitations

The Parallel ATA interface has a long history of design issues in spite of its success. Most of these issues have been successfully worked around, overcome, or simply ignored. They include:

  • The 5-volt signaling requirement and high pin count (40-pin cable connectors)
  • The 18-inch cable length limitation; cable width and cable routing problems
  • Data robustness issues

5-Volt Signaling Requirement

Since the industry continues to reduce chip core voltages, Parallel ATA's 5-volt signaling requirement is increasingly difficult to meet. Parallel ATA has 26 5-volt signals per ATA channel, requiring the use of large physical chip pads to accommodate the high pin count. The large pads will ultimately dominate the chip as chip sizes are reduced.

18-Inch Cable Length Limitation

With the current Parallel ATA interface, the 18-inch cable length limitation can be a serious issue. The limited cable length complicates peripheral expansion choices, making some internal drive configurations impossible to implement. Of course, this depends on PC chassis size and the design and location of internal media bays.

The wide, flat ribbon cables of the Parallel ATA bus are difficult to route, and their shape and bulk can restrict air flow and create hot spots inside the chassis.

Data Robustness

With Parallel ATA, data robustness has been a long-standing issue. During its early development, no form of data checking was designed into the Parallel ATA interface. However, a degree of data protection was added in the form of CRC, which enabled the verification of interface data, for the first time when the first UDMA mode was introduced. Unfortunately, ATA command data is still not checked and remains a potential error source.

Page 4: The Serial ATA Solution

The Serial ATA Solution

As previously explained, Serial ATA has eliminated the limitations of the Parallel ATA interface. Serial ATA maintains register compatibility and software compatibility with Parallel ATA because its architecture changes the physical interface layer only. No device driver changes are necessary, and the Serial ATA architecture is transparent to the BIOS and the operating system.

Actual Benefits of Serial ATA

Serial ATA offers a number of benefits over Parallel ATA, including:

  • Improved performance — Serial ATA is faster than parallel ATA
  • Reductions in voltage and pin count
  • Smaller, easier-to-route cables; elimination of the cable-length limitation
  • Improved data robustness
  • Backward compatibility
  • Increased disc drive data rates
  • Serial ATA integration
  • Bundled costs

Serial ATA Is Faster

A few years ago, if someone would have said that "Serial ATA is fast," or that it was faster than a "parallel port," he or she would have gotten some strange looks. The COM port was never known for its speed. Let's not forget, however, that today's most important standards (USB 2.0, Firewire, Ethernet, V-Link, MuTIOL, HyperTransport, RapidIO) are all serial-based, yet they are fast and provide high performance.

Thanks to serial transfer, Serial ATA needs only two data channels — one for sending and one for receiving. These are supplied with a more modern 250 mV, in contrast to the 5 V typically used with IDE. With differential signaling, interference on one signal affects the other signal by the same amount. Because the signals run phase reversed, interference is self-canceling. Twisting the wires is no longer necessary.

Reduction in Voltage

Serial ATA's low-voltage requirement (500 millivolts [mV] peak-to-peak) effectively alleviates the increasingly difficult-to-accommodate 5-volt signaling requirement. This requirement hampers the current Parallel ATA interface.

Cabling

The Serial ATA architecture replaces the wide Parallel ATA ribbon cable with a thin, flexible cable that can be up to 1 meter in length. The serial cable is smaller and easier to route inside the chassis. The small-diameter cable can help improve air flow inside the PC system chassis and facilitates future designs of smaller PC systems. The lower pin count of the smaller Serial ATA connector eliminates the need for the large and cumbersome 40-pin connectors required by Parallel ATA.

Improved Data Robustness

Serial ATA offers more thorough error checking and error correcting capabilities than was available with Parallel ATA. The end-to-end integrity of transferred commands and data can be guaranteed across the serial bus.

Backward Compatibility

Serial ATA provides backward compatibility for legacy Parallel ATA and ATAPI devices. This can be accomplished by two methods. First, you can use chip sets that support Parallel ATA devices in conjunction with discrete components that support Serial ATA storage devices. These discrete components are now available. An integrated chip set, which supports a mix of serial and parallel channels, is also available. Second, you can use serial and parallel dongles, which adapt parallel devices to a serial controller or adapt serial devices to a parallel controller.

Increased Disc Drive Data Rates

Since disc drive data rates have not yet exceeded ATA100 limits, why should you switch to Serial ATA? The maximum internal data rate on an IDE disc drive today is ~72MB/sec. The ATA/100 data transfer rate has not been reached. But one of the reasons IDE performance is where it is today is due to the expandable data path PATA has allowed.

That data path in PATA has reached its limit. Serial ATA allows disc drives to continue to offer performance and reliability at cost parity to Parallel ATA. In addition, the Serial ATA interface requires less voltage, meaning better power consumption and management in both desktop and mobile applications. The thinner cable allows for flexible designs and improved airflow in smaller form-factors.

Page 5: Serial ATA Integration

Serial ATA Integration

So, what do you need to integrate Serial ATA? The Serial ATA adoption will first take place with the introduction of Serial ATA drives and Host Bus Adapters in early 2003. By the second quarter of 2003, you will start to see Serial ATA motherboards integrated into desktop systems. One of the main objectives of the Serial ATA working group was that Serial ATA would not require any software changes. Serial ATA basically is 100% software compatible — meaning no changes are needed to current operating systems or applications. All you need is a Serial ATA drive, Serial ATA HBA, and Serial ATA interface cable and power adapter.

ATA Device Connectivity

By using a master/slave communication technique, Parallel ATA allows up to two devices to be connected to a single port. Both devices are daisy-chained together via one ribbon cable that is an unterminated multidrop bus. The standard parallel ATA software and device driver access the Serial ATA subsystem in exactly the same manner as parallel ATA and functions correctly. However, for Serial ATA the software views the two devices as if they were masters on two separate ports. The drive interface section of the host adapter uses a new design that converts the normal operations of the software into a serial data/control stream. The Serial ATA structure connects each of the two drives with individual cables in a point-to-point fashion.

Bundled costs

Is Serial ATA more costly than Parallel ATA? Initially, Serial ATA is an added cost to the overall system since integrated motherboards are not always available. However, there are Serial ATA host bus adapter card solutions bundled or available from multiple vendors who are working with HDD vendors to derive compatible solutions.

Differences in Serial ATA Solutions by Different HDD Vendors

There are two main methods for establishing the Serial ATA interface on the disc drives and hosts, "native" and "bridge." One method called "native," allows maximum throughput, bypassing the legacy Task File reads and writes as well as the limitation of 133MB/sec for Ultra DMA Mode 6 transfers to enable the maximum 150MB/sec transfer rate for first-generation Serial ATA storage devices.

A bridge solution enables the adoption of a parallel device to the Serial ATA interface. Because the Serial ATA information flow occurs at 1.5Gbps, it is not always possible for the Link state machines to keep up when using a bridge device. The link layers on a bridged system must incorporate buffering to allow for throttling the interface if one side gets behind.

Performance Differences in Serial ATA Drives

You may see a 1% to 5% performance increase from a PATA drive to a Serial ATA drive, but the main performance benefit is in the long run. This is because with Serial ATA the hard drive throughput will not bottleneck the system performance. In the meantime, system integrators and original equipment manufacturers (OEMs) will enjoy a big reduction in assembly time and reductions in handling damage due to connector and pin issues.

Serial ATA and Serial Attached SCSI

Serial Attached SCSI (SAS) compliments Serial ATA by adding device addressing and offers higher reliability and data availability services, along with logical small computer system interface (SCSI) compatibility. It will continue to enhance these metrics as the specification evolves, including increased device support and better cabling distances.

Serial ATA is targeted at cost-sensitive non-mission critical server and storage environments. Most importantly, these are complementary technologies based on a universal interconnect, where Serial Attached SCSI customers can choose to deploy cost-effective Serial ATA in a Serial Attached SCSI environment.

Serial ATA Drives and Controllers

HDD manufacturers all plan to have Serial ATA drives out by calendar quarter 1 in 2003. The integrated Serial ATA host motherboards will start to appear around late 2003.

Page 6: Summary and SATA Road Map

Summary and Conclusions

Parallel ATA was the primary storage interface for the past 11 years. An association of seven leading PC technology companies has developed a Serial Advanced Technology Attachment (ATA) storage interface for hard-disk drives (HDDs) and ATA Packet Interface (ATAPI) devices that has replaced the current Parallel ATA interface.

Compared with Parallel ATA, Serial ATA has lower signaling voltages and reduced pin count, is faster and more robust, and has a much smaller cable. Serial ATA is completely software compatible with Parallel ATA. To provide a framework for comparing the two interfaces, this article has reviewed the current Parallel ATA interface technology, described its strengths and weaknesses, and then introduced Parallel ATA's successor: Serial ATA.

Finally, let's briefly look at the projected next steps for the Serial ATA program. It is expected that drives and PC motherboards incorporating Serial ATA will be available in 2003.

Projected Serial ATA Road Map

Serial ATA is planned as the foundation of a new storage interface replacement architecture that is as cost-effective as Parallel ATA and has greater performance improvement potential. Serial ATA releases will generally follow this road map:

  • First-generation Serial ATA: Shipped in 2001. The first release of the interface supports data transfer rates of up to 150 MB/sec.
  • Second-generation Serial ATA: When second generation Serial ATA becomes available, it will support data transfer rates of up to 300 MB/sec.
  • Third-generation Serial ATA: When third generation Serial ATA becomes available, it will support data transfer rates of up to 600 MB/sec.

Serial ATA will allow the performance of internal storage devices to continue to increase unabated for generations to come.


About the Author :John Vacca is an information technology consultant and author. Since 1982, John has authored 36 technical books including The Essential Guide To Storage Area Networks, published by Prentice Hall. John was the computer security official for NASA's space station program (Freedom) and the International Space Station Program, from 1988 until his early retirement from NASA in 1995. John can be reached at jvacca@hti.net.

» See All Articles by Columnist John Vacca

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