Cisco exec says standards can bust I/O bottleneck

Cisco exec says standards can bust I/O bottleneck
SAN JOSE, Calif. — Standards groups need to come to grips with a rising "I/O gap" and the proliferation of diverse technologies trying to address it, a Cisco Systems Inc. executive said. Specifically, the Jedec standards organization needs to define a standard interface that delivers 1 Gbit/second or faster signaling at 1 volt or less, said Andy Bechtolsheim, general manager of Cisco's gigabit systems business.

Although some Cisco systems will adopt the HyperTransport I/O backed by Advanced Micro Devices Inc., that specification is competing for attention with other interconnects such as RapidIO, PCI Express and PCI-X. "We were glad to see AMD license HyperTransport, but we would have wished to see less than three choices," Bechtolsheim said in a keynote Monday (Oct. 28) at the inaugural Chip2Chip Conference here.

"The real challenge is the next generation. The real question here is how low can we go in voltage and how fast can we go in data transfer rates," Bechtolsheim said.

"We need to get Jedec working on a 1-volt standard for gigabit signaling. Right now each vendor is doing his own thing, and Jedec is taking its own sweet time addressing it," Bechtolsheim said in an interview with EE Times following his keynote.

"By moving from 1.5 to 1 V we were able to save 20 watts on a recent chip design. I could hardly believe it. But the problem is we have to get chip vendors' support for the 1-V approach on both sides of the link, making sure there are TSMC Taiwan Semiconductor Manufacturing Co. Ltd. macros for it, and so on," he said.

"People need to talk about not just electrical and interface levels, but perhaps look at IP intellectual property blocks so we can have standards for what we integrate into a chip," he added.

Worsening crisis

In his keynote, Bechtolsheim said the number of gates on a chip has increased exponentially in the last five years while the amount of I/O has increased only about 50 percent.

"There is an I/O gap. The I/O-to-gate ratio is increasing to the point where we have a crisis on our hands, and it's rapidly getting worse," said Bechtolsheim, who was a founder of Sun Microsystems Inc. "I've seen one design where I/O takes up a third of the die size."

In addition to eating up more die size, I/O is also consuming more power and generating increasing signal integrity problems, Bechtolsheim said. "It is key to have bit error rates not just at 10^-3 but down to 10^-30 — something that basically never happens."

Bechtolsheim talked about the I/O problem in three areas. Parallel chip I/O is scaling well in bandwidth as terabit/second chip-to-chip connections loom on the horizon. "Don't write off conventional parallel I/O yet, but the issue here is if it can scale in voltage and make improvements in skew control," he said.

In high-speed serial I/O, Cisco is shipping systems with 2.5-Gbit/s to 3.125-Gbit/s chip-to-chip links and has working models as fast as 10 Gbits/s in the lab.

Too few efforts are grappling with backplane I/O issues that are important for communications OEMs like Cisco, Bechtolsheim said. "Most of the I/O we deal with is sending Ethernet or Sonet over a backplane, but outside of Xaui there is little standardization there. Xaui is significant, but it was not specified for how to optimize it for the backplane.

Bechtolsheim also cited improvements in what he called advanced serial I/O using multilevel or PAM coding, citing the work of Accelerant Networks.

Nevertheless, the whole area of serializer/deserializer (serdes) technology underlying serial interfaces remains a technically challenging one. "We've worked with dozen of serdes suppliers and at different times they have all been disqualified," he said.

Optical I/O will not be a contender for chip-level or board-level interconnects for the foreseeable future, he said. "While optics have been a clear winner in long distance I/O it is a clear loser at the backplane level. It's not just costs but the problems of keeping connectors clean and dust free. So I don't see optics in the box," he said.