Author: Chris Cicchetti, Marketing Director, SAS/SATA Analyzers and Testers,
The new SAS-2 and SATA Gen-3 system protocols bring 6Gb/s link speeds to a wide range of applications, including 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, making signal integrity a significantly more important design concern for equipment designers and network engineers than it was at 3Gb/s. For example, a test setup that was already at the performance edge for 3Gb/s will cause undesirable and misleading failures at 6Gb/s as tolerances drop to the point where test equipment can adversely affect signal integrity. 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 6Gb/s and the steps that developers can take to minimize the impact of test equipment on a device-under-test (DUT).
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. Additionally, higher frequencies are more susceptible to jitter as jitter remains constant even while the signal period decreases. 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 Decision Feedback Equalization (DFE) techniques to minimize the impact of attenuation and jitter on signal integrity. Most design and test issues relating to SAS-2 will be related to the introduction of these new, sophisticated techniques, and so it is critical to follow the specification exactly and set de-emphasis and DFE variables correctly. Failure to do so will not only make testing more difficult but also potentially lead to interoperability difficulties with other SAS-2 equipment.
Alternatively, SATA Gen-3 employs neither de-emphasis nor equalization, thus offering a lower-cost link technology for applications that don’t require these capabilities. This, however, makes SATA Gen-3 more susceptible to attenuation and jitter. As a result, it is even more important to carefully implement the suggestions in this article when working with SATA Gen-3 and its higher data rate. Specifically, test SATA Gen-3’s 6Gb/s signals with even shorter cables than were used to test 3Gb/s systems.
While a necessary debugging aide, inserting test equipment between devices-under-test introduces electrical discontinuities into the signal path which induce both jitter and attenuation that can adversely affect signal integrity. There are several methods available for connecting test equipment that reduce or compensate for these effects to varying degrees.
Analog Passthrough achieves the lowest impact to signal integrity of the available options by passing the signal through a solid-state switch (see Figure 1a). This method also keeps induced jitter to a minimum. Its primary disadvantage, however, is that it creates a discontinuity in the link, similar in effect to using a connector splice to join two cables, and so attenuates the signal.
Digital Retiming is a method where test equipment operates as a network device at the link layer, receiving signals, decoding them, re-encoding them, and then resending signals on to their destination (see Figure 1b). It is also important to note that digital retiming can add latency as well as alter clock-alignment commands at the link layer. For example, SAS/SATA ALIGN characters may be utilized to overcome clock skew between the tester and both the host and target. The tester drops or adds ALIGN characters to maintain clock alignment with the devices-under-test. Whether this subtle change to network traffic affects testing depends upon the specific test’s goals.
Buffered or Re-amplified connections reduce the attenuation induced by test equipment by electrically amplifying signals (see Figure 1c), thus enabling the use of longer cables. The primary disadvantage of buffering a signal, however, is that it introduces non-deterministic jitter. If the amount of jitter is too high, the advantages of using a buffered approach are lost. Additionally, by placing an electrical circuit in a signal pathway, buffering can mask channel issues, such as reflections.
Because analog passthrough tends to have the least impact on the electrical characteristics of a signal, it gives users the most accurate real-world representation of network signals. For SAS systems where de-emphasis and equalization manage attenuation, analog passthrough is the most commonly used connection method.
In some cases, however, analog passthrough fails to maintain sufficient signal integrity. If attenuation issues arise regardless of the shortness of cable length (i.e., if induced attenuation is discovered to be an issue), a buffered approach may provide better results. Likewise, if long cables are necessary, a buffered connection may eliminate attenuation concerns.
For systems where designers have a high degree of confidence in the physical layer of the network and whose concerns reside primarily in the protocol domain, a digitally retimed connection may provide the best results. Digital retiming is also appropriate when a physical setup requires long cables. For 6Gb/s SATA applications, where the channel model specifies a maximum 1 meter cable length, a passive method such as analog passthrough is not an option since the introduced attenuation would go over the link budget. SATA requires either a buffered or retimed signal.
An important element of testing is working from a stable foundation; if the network infrastructure has unresolved integrity issues, these may incorrectly appear to be caused by the device-under-test, complicating or delaying problem resolution. Pretesting physical infrastructure (i.e., cables and connectors) confirms the suitability and reliability of a test setup, and avoids the time-consuming process of attempting to resolve signal integrity issues that are ultimately the result of improper testing practices. This is especially important when first moving to 6Gb/s because test setups created for 3Gb/s applications may operate at just within the limits of what 3Gb/s systems can tolerate and so will fail if they are not updated to meet 6Gb/s requirements. By first characterizing the physical infrastructure between end-points as speed-capable, developers can more confidently assume problems found are with the device-under-test rather than the test setup.
Finisar recommends using a system test suite to pretest physical infrastructure. This test suite allows users to generate test patterns specifically designed to test the signal integrity limits of a network. Running a broad spectrum of stress-inducing traffic types across links tests both attenuation and jitter tolerances, revealing whether the physical layer will work at full line rates.
A critical element of any test setup is the quality of cables used and how they are connected to the analyzer and device-under-test. Problems arise when cables fail to meet the standard specifications or when multiple cables are connected such that they introduce discontinuities (i.e., impedance mismatches in the cable).
When standards are being developed, a common assumption used in the mathematical models is the use of single cable, one with no connectors or other impedance discontinuities between its endpoints. However, in practice, users will often make use of whatever cables they have on hand in the lab, cables which may be longer than necessary, are unshielded, or worse yet, contain multiple cables strung together. Alternatively, developers may assume the test setups used to test the previous generation of devices will work for the latest line rates. In these cases, the test setup may yield attenuation too great for 6Gb/s devices (i.e., cable length acceptable for 3Gb/s may be too long, and thus induce too much attenuation, for 6Gb/s).
Consider the test setup shown in Figure 2. In this example, the analyzer generates a signal out to the device-under-test, which is then captured by the analyzer. At the same time, the original signal is fed back into the analyzer so that the sent and received signals can be compared.
Figure 2: Test Setup to Avoid
There are several problems with this test setup. The 4-link Hydra cable coming out of the back of the analyzer sends the generated signal right back to the analyzer across a male-to-male cable segment or “splice”. As a result, the electrical characteristics of the link tend to attenuate the signal and make it look like an extended length of cable that is much longer than it actually is. Additionally, both the Hydra cable and splice are shielded much less than higher-quality cables, such as Mini SAS cables, and jitter could be further aggravated by this setup. Compare this to the test setup in Figure 3.
Figure 3: Appropriate Test Setup
A high-quality, shielded Mini SAS 4-lane cable connects the analyzer to the device-under-test. This cable individually shields each lane as well as the 4-lanes together. It is also a reasonable length and has no discontinuities. Regarding feeding the original generated signal back to the Analyzer, this is accomplished using a Loopback Plug. Rather than using two cables and a splice to create the signal feedback loop, the Loopback Plug keeps the loop as short as possible and minimizes discontinuities in the link. Internally, the Loopback Plug passes the signal over just a 1 mm trace of copper.
Other ways developers can improve signal integrity include:
- Co-locate the analyzer, host, and device-under-test: This is one of the simplest ways to reduce cable length. Rather than run long cables between workbenches, use shorter lengths of cable.
- Use high-quality shielded cables: Unshielded cables can induce jitter through Electro Magnetic Interference (EMI), reducing signal integrity and potentially leading developers to attempt to resolve phantom problems that would otherwise not be present with a shielded cable. It is important to note that the shielding of even standard cables may be less than ideal. For example, standard SATA cables have shielding around individual conductors but not around the cable itself. Higher quality cables can actually accelerate development – and quickly pay for themselves – by avoiding the problems caused by low-quality cabling.
- Use only 6Gb/s rated cables: 3Gb/s cables, such as those with SAS 4x connectors, were not designed to carry 6Gb/s. Using inappropriate cables, even if they should work theoretically, could significantly delay development with ‘red-herring’ signal integrity issues.
- Eliminate all discontinuities: Every discontinuity (connectors, splices, adapters, etc.) reduces signal integrity by causing reflections that attenuate signals by sending signal energy back in the opposite direction. Remove all but necessary discontinuities. In many cases, this may mean custom test cabling or introducing test fixtures such as JBODs.
- Use the shortest reasonable cable: The longer the cable, the greater the attenuation. The best way to reduce attenuation is by shortening cables. This is especially true for SATA, where the absence of the SAS specification’s de-emphasis and equalization mean that the link’s characteristics completely control signal attenuation and jitter.
- Don’t use too short a cable for SAS-2: While this may seem to contradict the previous point, SAS-2 employs de-emphasis and equalization to attempt to overcome the effects of attenuation and jitter. If a cable is too short, de-emphasis and equalization may actually end up overcompensating for a signal, actually reducing signal integrity. While the specification should work for any range of cable length, if issues arise with short cables, try experimenting with longer lengths to determine if this is the problem. Note that short cable issues do not apply to SATA as SATA does not support de-emphasis or equalization.
- Remember the 3 S’es: use Short, Shielded, and Single cables wherever possible.
Moving to 6Gb/s increases the difficulty of maintaining signal integrity between network devices. Many developers will struggle with signal integrity issues that unfortunately will arise from their own carelessness in how they manage their test setups. Those developers and systems engineers who respect the tighter tolerances of operating at 6Gb/s – taking care to use the appropriate connection method, pretesting their systems, and observing proper cable use – will find themselves free to accurately identify and resolve protocol system issues more quickly and painlessly.