i/oZONE Products for the week of December 22, 2003
Vitesse Says…
Vitesse Supercharges The Backplane With 6.5 Gbit/s
Data Transmissions
Highest Switching Density on the Market at a Competitive Cost
Vitesse Semiconductor Corp. has announced a pair of fast and versatile
crosspoint switches, setting a new record for the highest density switches
on the market. The eight-port VSC3108 delivers 0.8 Gbits of switching capacity
per square millimeter of board space.
The VSC3104 is a four-port version of the same product. Both come in an 8mm square package and operate at data rates up to 6.5 Gbps.
These fast, compact, and cost effective crosspoints are an everywhere solution for digital video, Internet commerce, and the explosive needs of data storage. Vitesse's crosspoint development provides a 'solder and go' solution for system implementation improvements: preventing data loss, correcting distortion, decreasing component size, and increasing speed. The result is the easiest to use and broadest range of switch products that are best-in-class for signal integrity and function.
"Vitesse crosspoint switches add value while reducing space consumption and costs for our customers," says Gary McCormack, Vitesse's circuit switch product line director. "Vitesse excels at taking advanced circuit technology and making it simple to use so networks deliver high-quality data expected by today's user."
The VSC3108 and VSC3104 maintain unprecedented reliability and signal integrity by utilizing Vitesse's industry-preferred advanced equalization capability. Unlike other products, all of the chips' functions are independent of data rate, require no re-tuning as operating conditions change, and have no need for a clock. These switches take a fixed rate design and transform it into an any-rate solution that is both forward and backward compatible, allowing developers to upgrade current generation equipment or cut new trails in the next-generation development in Storage, Enterprise, and Metro applications. At $3 per port, the VSC3108 is priced competitively against less capable alternatives, and at a significantly lower cost than PLL-based products.
Both products have multiple purposes such as fan-out, loop-back, and
protection switching; and both come in small 8x8mm, 69-pin CBGA packages
with 500ps latency, for minimal impact when board space and timing budgets
are limited.
analogZONE Says . . .
With the average data transmission speed steadily increasing, it's no wonder that designers are facing traffic jams of unprecedented proportions as they try to move those incoming multi-Gigabit streams from the line cards to the switch fabrics, traffic managers, and other parts of their box. For this reason, Vitesse may well have a winner in its new low-cost 310x family that integrates a set of SerDes transceivers with a low-latency crosspoint switch.
They have been a strong player in the SerDes market for some time now -- both in standalone transceivers, and in more highly integrated switch and traffic management products that use these same cores as inter-component links. These new products reviewed here have lifted a very cost-effective transceiver technology from their high-end products and put it in an inexpensive chip that can be used either as a "straight-through" backplane transceiver or as a cross-connect to switch signals between channels. The nice thing here is that the switch fabric is there for applications such as protection switching, but adds so little to its cost that it can be used almost anywhere that you have a "problem signal" that needs to be cleaned up and boosted.
The VSC3104/3108 SerDes switches seem to hit a market sweet spot with a careful balance of performance, price, and power consumption that should address a significant fraction of today's SerDes applications. The transceiver has a combination of transmit pre-emphasis and receive equalization technologies to thread its signals across the less-than-ideal conditions found on most FR4-based PC boards attempting to carry multi-Gigabit signals. Internal control registers allow you to select one of 16 pre-emphasis settings that vary overshoot and the time constant of the signal decay curve. This allows you to tailor the signal, sharpening and equalizing its leading edge.
But since excessive pre-emphasis can cause excessive power consumption and unwanted crosstalk, Vitesse also includes a significant amount of equalization in the receiver. They employ a straight analog equalization technique in this part, with simple digital controls for level, but no fancy DFE or other sophisticated adaptive technologies. The EQ can be selected in four steps (off, low, med, high). In hindsight, four steps of equalization may have been overkill according to Gary McCormack, Vitesse's circuit switch product line director. He says that, for the most part, the full-on setting seems to produce results that are close to optimum for nearly every application.
The part is nominally rated at 6.5 Gbit/s, but McCormack says it has demonstrated performance to around 8 Gbit/s or more in channels with less demanding characteristics. He says it exhibits a "soft crash" as the EQ circuitry loads more heavily with increased attenuation, while maintaining around 50 ns or less of added jitter.
While simple in comparison to some of the great SerDes parts I've reviewed here, I'm reasonably comfortable with Vitesse's claims that their all-analog approach allows them to closely fit the target EQ curves with minimum power and silicon real estate. This was a strategic decision because they felt they could deliver more than sufficient performance with this, and have a small enough circuit that it can be replicated 100 or more times on their larger application-specific products. The result is a solution that should solve a good chunk of the everyday signal integrity problems faced by designers and leaves the "corner case" or "PhD problems" to lower-volume higher-cost parts.
This combination of a robust SerDes and a simple cross bar switch is a good candidate for applications such as protection switching in edges of a backplane. In this case, the crossbars serve as edge switches that re-route signals to either side of a double fabric, enabling switching and protection at a minimal cost.
And even if you don't need the crossbar functions, they serve as cost-effective SerDes transceivers that can compensate for most channel impairments. Vitesse likes to think of it as a generic tool for serial transmission, or "signal integrity insurance". Applications in this role include switching multi-Gigabit signals across lossy twin-ax (or less lossy InfiniBand cable) that serves as a high-capacity interconnect between chassis. Vitesse also says that some designers use these parts to equalize and boost signals across thin, long trace runs between chips sharing the same circuit board.
From what I can see here, the VSC3104/3108 family offers a carefully-engineered mix of programmable transmit pre-emphasis and analog equalization that should address a significant fraction of the signal integrity problems you'll encounter in a "modern" backplane or other serial link.
Of course, the definition of what the characteristics of a "modern" backplane are, and what, exactly constitutes a "challenging" transmission environment is subject to considerable debate. I think that much of the wrangling over how much equalization and other signal processing techniques are necessary for 2.5-, 5- and 10-Gbit/s systems boil down to honest differences between what manufacturers believe to be the "real world" environments.
Hopefully, the preliminary test results we got on the VSC3108 when it ran in our "Equalize This!" Gigabit Backplane Challenge will shed some light on this murky subject. We are running chips from several manufacturers (including Vitesse) in a series of challenging backplane environments. Since we only ran the VSC3108 with its pre-emphasis circuitry enabled for the first part of the test series, I'm very curious about Gary's clams of a "one-setting-fits-all" solution for equalization -- especially since this is exactly the opposite of what I've heard from some other companies who are offering more complex adaptive solutions.
We'll know more after we get the chip back into the lab sometime in January when we hope to run further tests to explore its performance with various combinations of equalizer and pre-emphasis. Stay tuned, and keep checking the i/oZONE every week as we hope to have a full set of test results on the VSC3108 some time in January.
The VSC3108 (8x8) and VSC3104 (4x4) are available today for production shipments, and are priced at $24 and $16 per unit, respectively.
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