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WHITE PAPER
Successful SAS/SATA Equipment Design and Development: What You Need to Know for 6 Gb/s and Beyond
Abstract:
The new SAS-2 and SATA Gen-3 system protocols enable 6 Gb/s link speeds between storage units, disk drives, optical and tape drives, and protocol host bus adapters (HBAs). Carrying such high signal signals several meters over copper cables, however, tests the limits of signaling technology. As a consequence, the majority of design concerns are expected to arise at the physical layer. This whitepaper will describe the technical challenges in maintaining signal integrity at 6 Gb/s and how improper test setups can inadvertently degrade signals during product development and testing. Specifically, developers and systems engineers will learn how to deploy test equipment to minimize its impact on signal integrity. In this way, developers can avoid the time-consuming process of attempting to resolve signal integrity issues that are ultimately the result of improper testing practices. The introduction of 6 Gb/s SAS-2 and SATA Gen-3 promises new levels of performance for networks. At these higher speeds, however, signal integrity becomes a significantly more important design concern for equipment designers and network engineers than it was at 3 Gb/s as tolerances drop to the point where test equipment can adversely affect signal integrity. For example, a test setup that was already at the performance edge for 3 Gb/s will cause undesirable and misleading failures at 6 Gb/s. Apart from strictly adhering to each standard?s specifications, the key behind successful SAS/SATA product development and network debugging will be an understanding of the tighter tolerances at 6 Gb/s and the simple steps that one can take to minimize the impact of test equipment on the device under test. By understanding how attenuation and jitter impact signal integrity, developers and systems engineers can adjust test setups to minimize their impact during testing. Figure 1: 6 Gb/s SAS/SATA signals have increased sensitivity to attenuation and jitter. Higher frequencies attenuate faster than low frequencies over distance (A) and are more susceptible to jitter (B). Proper test setups reduce the additional attenuation or jitter introduced into a system during testing. A B Attenuation High Frequency Low Frequency Jitter
Attenuation and Jitter
The higher frequency signals used by SAS-2 and SATA Gen-3 have increased sensitivity to attenuation and jitter. Higher frequencies attenuate faster than lower frequencies over distance (see Figure 1A). Additionally, higher frequencies are more susceptible to jitter as jitter remains constant even while the signal period decreases (see Figure 1B). When attenuation and jitter become too pronounced, it becomes impossible to accurately sample and decode signals on the receive-side. The SAS-2 and SATA Gen-3 standards take different approaches to resolving attenuation and jitter issues. SAS-2 utilizes de-emphasis and equalization techniques to minimize the impact of attenuation and jitter on signal integrity (see description below). Alternatively, SATA Gen-3 employs neither, thus offering a lower-cost link technology for applications that don?t require these capabilities. The lack of these Distance ?D? Channel Channel High frequencies attenuate faster than low frequencies over distance High frequencies are more susceptible to jitter Source Destination
