Electronic Design

Switch-Fabric Architecture Buffers Asynchronous And Isochronous Information Independently

While the bus was the interface of the 1990s, the switch-fabric architecture may become the interface of the new decade. StarGen Inc. of Marlborough, Mass., certainly hopes so. Over the past year and a half, it has been engaged in "heads down" engineering, designing its version of a switch-fabric architecture aimed at telecommunications applications.

Currently developing several ASIC switch-fabric designs in parallel, StarGen hopes that its technology will become an open standard. The company plans on publishing specifications that will enable other silicon vendors to produce products that are complementary, and even potentially competitive, with its own. "Actually, we expect to see a lot of participants," says Tim Miller, vice president of marketing at StarGen.

Switch-fabric architectures may very well supplant bus structures in telecommunications equipment. "The bus-based approaches are simply running out of gas," Miller says. "In such arrangements, agents have to wait their turns, requiring arbitration of some kind, since only one agent can talk at a time. There are also physical scaling constraints with bus approaches."

Miller acknowledges that standards-based interconnects are attractive when it comes to cost as well as component and software availability. Yet these devices don't meet the next-generation requirements for converged data, voice, and video networks. There is no distinction between traffic types. They have limited scalability and reliability, too.

"As an alternative to cranking up the clock, or making the bus wider to meet the demand for more bandwidth, we are proposing a switched point-to-point approach," Miller says. "Our circuit switches have been designed to interface seamlessly with existing telecommunications modules and thereby bring a large number of benefits, freeing telecommunication systems from a number of constraints due to limited bandwidth and the independent handling of voice and data on different buses like H.110 and PCI." StarGen hopes to provide off-the-shelf telecom equipment solutions.

Over the last ten years, PCI-based bus systems have progressed from 32-bit, 33-MHz systems with 133-Mbyte/s capability to 64-bit, 133-MHz systems with 1.066-Gbyte/s capability—as seen in last year's PCI-X system. StarGen's approach will boost performance from tens to hundreds of gigabits per second and above by simply aggregating switch ports (Fig. 1).

"What we propose is that each device be connected in a point-to-point fashion through a series of switches that have the ability to scale up. Our intended products, which will include a switch and a bridge to PCI, solve the scalability problem," Miller adds.

There may be other benefits to a switch-fabric approach. According to Miller, a switch-fabric architecture can scale to any bandwidth within its upper bound while keeping the latency bounded. It also enables flexible routing. By their very nature, switches are point-to-point and therefore friendly to device removal and insertion. Longer spans become possible, along with lower power, lower pin counts, and lower cost than comparable bus structures.

To scale, simply add more switches into the mesh. Then there could be hundreds of switches and thousands of end points in a single system. They also can be configured to establish redundancy. This contributes to reliability, letting the system simply reroute around a failed node.

A bus structure requires a different strategy. Designers have to build in hot-standby, totally redundant systems. This technique is expensive and tricky. If a switch fabric has a faulty component, though, designers could simply route around it.

In larger star topologies, the latency through the switches requires holding the hop count low (Fig. 2). As for cable spans, StarGen's switch technology uses standard Category 5 cabling to handle spans of 5 to 10 m. And, this switch architecture is scalable up to room-size equipment.

"Voice traffic, which is 8 bits of data every 125 µs, is deterministic, but not particularly demanding from a bandwidth point of view," says Todd Comins, the company's chief technical officer. "But when someone tries to move a 100-Mbyte file, you don't want to get in the way of voice delivery. The challenge is to be able to deliver real-time data traffic in the presence of asynchronous, best-effort, delivery traffic that is common in the computing/data-networking world," he continues. "You want deterministic delivery of this isochronous traffic, even though there are devices in the system that are trying to use every bit of bandwidth."

Comins believes this switch-fabric architecture delivers a common interconnect that can deliver asynchronous, isochronous, and multicast traffic across a common interconnect. "That's one of the great benefits of our technology," he says. "Buffering for the different traffic classes is independent so that asynchronous transfer is not going to interfere with the movement of isochronous data."

He also believes a move to switch-fabric architecture is really essential to drive interconnect costs down. Systems today have separate data and control interconnects—a whole set of separate costs—along with two sets of connectors, buffers, and silicon interconnects. "There is a tremendous advantage in being able to collapse all that into a common interconnect," Comins concludes.

This switch-fabric architecture will employ well-established, widely available, cost-effective technology. For example, 622-Mbit/s low-voltage differential signal (LVDS) devices with a 400-mV differential will serve as the basis of the physical serial connection. Such a move would lead to a design where the physical layer meets IEEE and EIA standards. The basic port, consisting of four differential pairs in each direction, delivers 2.5 Gbits/s in each direction.

"Our choice of 622 Mbits/s is well below the bit-rate ceiling for getting signals off a die, through a bond wire, out the package pin and through the connector," Comins explains. "We have confirmed that exotic mediums simply aren't necessary. So, signals may be transferred via FR-4 circuit boards, CompactPCI connectors and RJ-45 connectors, Category 5 cables, and the like."

For more information, call StarGen at (508) 786-9950, or point your browser to www.stargen.com.

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