Switching grows up: Where we're going

How much faster and cheaper can data switching get? In eight years, switch throughput has gone from 150,000 packet/sec at Layer 2 to more than 50 million packet/sec at Layer 3, and there is no sign that the electronics driving these advances are running out of steam. In fact, recent announcements of terabit-speed switches indicate the rate of improvement may be accelerating.

Every time it starts to look as if the industry might have to go to optical technology in order to increase capacity, electronics makes another leap. The new terabit switches don't even incorporate the latest silicon technology. They use .25-micron silicon -1/600th the width of a human hair - and .18-micron technology is in the works. Such linear decreases in size translate into a geometric progression in the number of circuits that can be squeezed onto a chip. And the closer together the circuits, the faster they can operate.

"I tend to think there isn't a limit," says Diane Myers, a senior analyst who follows the semiconductor industry for In-Stat, a consultancy in Scottsdale, Ariz.

Six or seven years ago, Application Specific Integrated Circuits ran at 20 MHz and had 20,000 gates, or groups of transistors that implement some logic. Today, .25-micron technology has pushed those numbers to 100 MHz and 500,000 gates, and .15-micron silicon should bump them to 400 MHz and two million gates.

This is assuming chip designers use standard libraries that have been developed at the gate level. While using such prepackaged logic is a lot cheaper than starting from scratch, it wastes a lot of space.

"If they took the design down to the transistor level, they could do 400 MHz at .25 micron," says Donal Byrne, vice president of marketing for Berkeley Networks, Inc. in Milpitas, Calif.

The capacity of the basic materials is just one aspect of switch performance. Throughput can also be boosted dramatically through architectural innovations.

For example, typical shared-memory switches have trouble scaling beyond 30G bit/sec because the ports have to access the memory through a bus. Packet Engines, Inc., of Spokane, Wash., has eliminated the bus and come up with what it calls parallel access shared memory.

"In a multicast, we put each packet in shared memory, and any port that is supposed to take it does so," says Jeff White, vice president of marketing for Packet Engines. "There is no risk of oversubscribing the backplane - a typical problem for crossbar architectures, where you have to replicate all the packets in a multicast."

Power X, Ltd., a semiconductor start-up in Manchester, England, is addressing this problem and other crossbar limitations with a serial crossbar chipset that has separate matrix, control and application-interface modules. The first set, scheduled for release this September, can be used to build 80G bit/sec switches.

"We have developed a scheduling and arbitration mechanism to eliminate the head-of-line blocking associated with crossbar architectures," says Russell Johnson, vice president of sales and marketing for Power X. He expects the technology to support switch speeds of 320G bit/sec next year using existing software and to scale to the terabit range with more advanced software that's in the works. The Power X chipsets can be used in ATM and Ethernet switches.

If and when the electronics wizards run out of tricks, optical technology presents some possibilities. A 1988 patent describes a 30G bit/sec photonic-array backplane. But that technology emanated from the defense industry, in which checkbooks tend to be a bit larger than the ones available to Gigabit Ethernet start-ups.

"Affordable photonics technology for intelligent switching doesn't exist yet and probably won't for at least 10 years," says Mukesh Chatter, president and CEO of Nexabit Networks, a Westborough, Mass., start-up that recently announced a 6.4 terabit/sec switch aimed at the service-provider market.
One problem is that while electronics have memory, there is no method of storage for photonics yet. "You can do simple switching with devices that use mirrors to move the beams from one path to another, but that's a far cry from true optical routing," says Joe Ferguson, director of marketing for start-up Juniper Networks, Inc., a Nexabit competitor in Mountain View, Calif.