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Extending the Serial ATA Interface - TvTechnology

Extending the Serial ATA Interface

A drawback, at least for today, is that while eSATA can provide faster transfer rates than USB or FireWire, it requires its own power connector and external supply, something that is being addressed now within the standards community.
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The volume of choices in storage systems continues to expand in direct reaction to the growth in digital media content. Whether that media is housed on mission critical high-performance storage, in a datatape library or on a user’s notebook microdrive—the importance of protecting that investment, making it accessible and securing it are all driving forces that motivate the advancement in storage system performance, universally accessible interfaces and secured content.

The previous statements could have been made one, two or 10 years ago with the same purpose and a similar impact. What is different about today’s perspective is that the availability of the digital media production systems, i.e., the cameras—the editing software—and the hardware platforms are now available to all degrees of users, regardless if they are a consumer, a broadcaster or a professional services organization. None the less, the one variable that needs to be continually dealt with is storage—whether its capacity, accessibility, flexibility or performance—it seems there is never enough of any of them.

STORAGE DEMANDS RISE

Adding storage is inevitable. Adjusting the storage compliment for large, mission critical systems employing high-performance disk arrays with sophisticated fibre channel interconnect systems or multiple sets of specialized media controllers and encoder/decoder ports—look to the either the server manufacturer or an experienced systems integrator familiar with complex videoserver installations background.

On the other end of the spectrum is the consumer or prosumer. Here the number of system types and the pure volume of end users differ significantly. Thus, the scale of storage requirements is of a different proportion altogether, with the mainstream videographer and general users having a different set of needs. Nonetheless, the value of their content is no different and the management issues remain the same. For these users their budget constraints, project requirements and production schedules have guided the storage industry down a path that now makes large volume storage solutions affordable.


(click thumbnail)Interface speeds shown with overhead comparisons when using USB and 1394 interfaces for external ATA drives. Serial ATA interfaces have no overhead.What is emerging to address the needs of the nonmission critical organizations are storage subsystems and interfaces that make it easier and faster to connect, share and implement upwards of terabytes of storage; often on discrete locally connected PCs and workstations. These interfaces are not complicated fiber channel or gigabit Ethernet networked systems, or gigantic SANs or NAS arrangements. In fact, these types of systems now possess the ability to add, migrate to, or replace storage at a faster, cheaper and more reliable rate. As drive sizes increase or storage demands increase, it’s just a simple bolt-on attachment, and you’re ready for the next project or the move to HD-image resolution.

From the technology perspective, one of the more recent advances in external storage systems utilizes an extension of the early Advanced Technology Attachment (ATA) architecture—now referred to as Serial ATA. The parallel implementation (i.e., PATA) is rapidly becoming a legacy technology internal bus interface for hard drives. The original ATA has both physical interface issues and speed (i.e., performance) encumbrances. ATA is not a particularly well suited add-on drive interface for the large file size demands that occur in digital media production, which is unless the external storage system has been thoroughly optimized for the platform that those disk drives are mounted to.

SATA AND ESATA

Serial ATA, simply known as SATA, has seen increased popularity among both storage users and suppliers of external and internal drive subsystems. The SATA interface is far more streamlined than ATA, and provides a serial architecture that increases speed and performance well beyond the older parallel ATA technology. It is for this reason you’re also seeing implementations of SATA technologies in both nearline storage and mainstream video servers.

One of the most obvious advantages for SATA is that the interface cable is no longer a wide, parallel ribbon cable that consumes space inside the PC’s chassis. In the case of external storage, the impracticality of using a parallel cable for attaching drives is now gone, as the SATA interface cable can be up to a meter in length, uses only four signal lines, and is generally sheathed to shield data signals from EMI. Per the current specification, SATA provides performance up to 3.0 Gbps for each device, with a real speed of 300 MBps. SATA offers new features including hot-swapping and an optional Native Command Queuing (NCQ) —a technology that increases SATA hard drive performance under certain circumstances by allowing the individual hard disk to internally optimize the order in which received read and write commands are executed.

To facilitate the use of outboard storage systems, the “external” Serial ATA (eSATA) specifications were engineered. eSATA is an extension to the Serial ATA standard and was standardized in mid-2004. The eSATA spec defined separate cables, connectors, and revised electrical requirements for external applications. For example, the data transmit/receive voltages were changed: the minimum transmit potential was increased from 400–600 mV to 500–600 mV, and the minimum receive potential was decreased (240–600 mV instead of 325–600 mV). These changes allowed for a cable length increase of up two meters when a SATA interface port is used, even though USB and FireWire interfaces permit longer distances. Note that prior to eSATA, external hard drives had to be connected using USB 2.0 or FireWire, but transfer speeds were limited, thus creating an opening for the higher performance SATA interface.

A drawback, at least for today, is that while eSATA can provide faster transfer rates than USB or FireWire, it requires its own power connector and external supply, something that is being addressed now within the standards community.

Finally, the roadmap for SATA includes a plan for a 6.0 Gbps standard. For current PCs, the SATA 3.0 Gbps interface already exceeds the sustainable (nonburst) transfer rate of the very fastest hard disks. Even so, today there are uses for sharing the 6.0 Gbps port-bandwidth so as to allow multiple drives to be connected to a single SATA port. Through the use of port multipliers—devices that allow multiple drives to be connected to a single port—the value of the SATA 6.0 Gb standard makes good sense and increases the ability to add, remove or share larges storage systems simply and economically.