Net processor vendors dragged into DRAM war
By Anthony Cataldo, EE Times
May 4, 2001 (1:32 PM)
URL: eetimes.com
SAN MATEO, Calif. — Network processor vendors are split over the choice of Rambus vs. double-date-rate (DDR) synchronous DRAM memories, as they keep a close eye on what happens in the PC market.
Because PCs still account for the majority of DRAMs sold, network processor vendors are dependent on the decisions of companies like Intel, Advanced Micro Devices (AMD) and third-party chip set makers. Their decisions will ultimately determine the fate of high-speed DRAMs.
"This is causing just as much angst as it has in the desktop business," said Bob Gohn, vice president of marketing for Motorola's C-Port network processor group. "Communications products tend to live a lot longer than PCs do. If you build your system on the latest whim of exotic technology and discover two years later that there aren't enough parts, you could be in deep trouble."
Yet from a product development standpoint, network-processing unit (NPU) vendors say they can't afford to stick with standard SDRAM when faster architectures abound. So far, they've spent most of their time evaluating DDR and Rambus DRAM (RDRAM), the two main contenders for high-speed main memory in PCs. Fujitsu's Fast-Cycle RAM low-latency device is under evaluation, and Infineon's Reduced-Latency RAM is expected to come on stream, but vendors say those architectures have yet to hit high-volume production.
The two camps
Intel Corp., for the most part, has thrown its weight behind the protocol-based Rambus for PC platforms based on its Pentium 4 processor. Rival AMD and a number of Taiwanese chip set suppliers are backing DDR, an evolutionary step after standard SDRAM.
The debate goes on over peak bandwidth, bandwidth efficiency, latency, pin count and cost issues. Generally, NPU vendors say Rambus has a distinct advantage when it comes to effective bandwidth and pin count, but loses ground to DDR in cost and latency. Some vendors are leaning toward DDR, an open standard, to avoid the extra licensing costs they would otherwise have to pay to Rambus Inc., while still others say licensing is something they have learned to accept as a normal industry practice.
PMC-Sierra Inc., for one, said it is joining the DDR camp and has promised to provide a MIPS-based processor for the control plane with an integrated DDR controller this year. The company chose DDR "purely because it's more mainstream and has as high or higher performance" than Rambus, said Tom Riordan, vice president and general manager of the MIPS processor division at PMC. The company's processors should benefit from DDR's low power consumption and higher maximum bandwidth, especially since the memory bits will be channeled directly into the processor.
PMC also had concerns about the first-access latency for Rambus DRAM, a sentiment expressed by several NPU providers. "Latency is much more important than bandwidth," Riordan said. "Networking is not pipeline-able; when the header comes in, you need to immediately look at it."
Motorola's C-Port operation, which put hooks to both SRAM and DRAM in its current network processor, faces similar concerns with Rambus. "The real performance parameter for DRAM memory technology is access time," said Dave Husak, chief technology officer at C-Port. "We're using the memory as a managed buffer system that requires haphazard random-access patterns. Paying the time penalty on access to various packets in buffers will drive performance to a fraction of the burst performance. Rambus doesn't address that problem."
Worst-case: 8 ns
Because RDRAM is a serial-access device, its worst-case latency is 8 ns. That's 2 to 3 nanoseconds higher than SDRAM, which uses a much wider bus. But Rambus claims that can be mitigated by integrating the controller directly onto the processor, or by ripping out some of the serializer/deserializer overhead on an external controller, as Intel did with its 850 chip set, said Michael Ching, infrastructure development manager for Rambus.
Vitesse Semiconductor Corp., for its part, chose RDRAM on the merits of its low pin count and high bandwidth, which the company believes provides for a more elegant and flexible solution. Customers using its RDRAM-compatible network processor could, for example, choose to solder down a fewer number of RDRAMs instead of using a module and still get equivalent bandwidth. "Peak bandwidth for DDR and RDRAM is equivalent, but RDRAM is much more pin efficient with its 16 pins vs. DDR's 64 pins," said Mark Fauber, senior product manager for Vitesse.
RDRAM also makes better use of the available bandwidth than DDR, said Fauber. "The effective bandwidth is less than 5.3 Gbytes per second for DDR-200, so it's in the 50 percent range. RDRAM utilization is in the 75 percent range, and you can get the effective utilization to approach 90 percent," he said.
Other companies remain noncommittal. While Intel's desktop processor group is backing Rambus in most cases, its network processor division is looking at both approaches, depending on the device and the market it targets.
Lexra, which provides licensable MIPS-compatible processor cores, said DDR is becoming a predominant memory architecture in the access market, while Rambus is making headway closer to the network core. Lexra plans to support both memory devices and is hoping that its foundries will eventually be able to sublicense the Rambus interface.
"The Rambus stuff is good," said Charlie Cheng, Lexra's president and chief executive officer, adding that most of the concerns people have about Rambus "are about the business model."
As DDR gains momentum, Rambus appears to be taking some steps to widen the appeal of its interface. Recently it allied with Denali Software Inc., which provides memory device models from most of the top DRAM vendors. Under the agreement, Denali is including the Rambus controller interface as part of its online tool for generating memory subsystems.
The move comes six months after Denali started offering a similar tool for DDR, demand for which "has exploded," said Sanjay Srivastava, president and chief executive officer of Denali. Any Denali customer that decides to use the RDRAM controller, however, must still work directly with Rambus to negotiate licensing terms.
At Vitesse, officials admitted that a year ago they had concerns about the availability of RDRAM when problems with its adoption in PCs began to surface. But those concerns have been allayed by recent announcements from memory vendors Elpida, Samsung and Toshiba that they plan to significantly boost RDRAM production this year, said Wade Appleman, vice president of the Advanced Networking Products Division of Vitesse.
Even so, Vitesse is hedging its bets. "We're using Rambus today and it's a great technology, but as new technology comes out we have to make sure that our architectures support memory controllers that may become more commonplace than Rambus," Appleman said. |
