Serial ATA - It's Time to Get in Line Page 5
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.
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.