Chip maker Mindspeed Technologies has unveiled a crosspoint switch fabric that will help systems vendors build bigger optical switches using significantly less board space (see Mindspeed Intros Switches).

A crosspoint switch is neither Sonet nor packet specific. It's function is simply to interconnect different ports in a nonblocking manner, as might be required in a wavelength crossconnect. It's also a building block from which systems integrators can piece together proprietary solutions.

Mindspeed's contribution to this space is the M21155, which it claims is the industry's most highly integrated crosspoint switch. It has 144x144 ports, each delivering a data rate up to 3.125 Gbit/s, giving it an aggregate capacity in excess of a terabit.

But not only does it cram 144x144 ports onto the switch; it also integrates 144 CDRs (clock-and-data recovery circuits), Mindspeed says. The job of the CDRs is to remove errors in the timing of the data signal.

Including the CDRs is a big deal, according to Babak Nabili, director of marketing for the crosspoint switch products. Compared to the alternative -- 36 external quad CDR chips -- it gives a power reduction of 60 percent and a 90 percent saving in board space, he claims. And, in his view, the board space comparison is conservative, since it only considers the package size and doesn't take into account the space needed for routing traces.

Mindspeed also introduced a version of the fabric without the CDRs, M21150, for applications that don't require it -- for example, when building a multistage switch fabric.

At 144x144 ports, Mindspeed's new chips are among the largest crosspoint switch chips on the market. Vitesse Semiconductor Corp. (Nasdaq: VTSS) also has a 144x144 device, but without the integrated CDRs. The main competitor for the M21155 is Velio Communications Inc.'s 140x140 chip with onboard CDRs (see Velio Thinks Big on Optical Switches).

But while the products from Mindspeed and Velio tackle the same problems, the inner workings of the chips are worlds apart.

Mindspeed's chip includes a separate reference clock for each CDR, according to Nabili. And the data rate of each CDR is continuously variable from 1 to 3.2 Gbit/s.

Velio, on the other hand, has adapted its grooming switch chip architecture. Like the grooming chip, its crosspoint fabric employs SerDes (serializer/deserializer) functions on both inputs and outputs, which break down the signals into tiny STS1 (51.4 Mbit/s) channels before they are sent through the core. And, since grooming switches are synchronous, groups of CDRs operate from the same reference clock.

Crosspoint switches, however, are meant to be asynchronous, which means that each channel can operate at its own data rate. That leads Mindspeed's Nabili to speculate that there might be problems with Velio's architecture. "As a result of breaking up the signal and putting it back together, it introduces interference, for want of a better word," he contends.

But Velio's VP of marketing, Bill Woodruff, says that's not the case. "To allow every lane to operate in its own time domain requires some innovation," he says. That innovation is clock forwarding, where the clock signal is sent along with the data signal.

To a customer, however, details of the architecture don't matter greatly, providing the chip performs as advertised. What matters are the features.

On power, the two chips are close -- 18 watts for Mindspeed and "under 20 W" for Velio -- although Mindspeed appears to have extra power saving features. In addition to being able to power down lanes on an individual basis, as Velio can, it also bypasses the CDRs if required and powers down sections of the switch core.

Mindspeed's Nabili also claims that the company is stealing business from Vitesse on account of the low power operation of its chips -- something that's very difficult to verify.

All chips include additional signal conditioning: Vitesse and Mindspeed have input equalization, while Velio employs pre-emphasis on the output.

And in terms of price, they there's not a lot to chose from, with Mindspeed's M21155 carrying a price tag of $2,995 per unit in orders of 1,000. Velio's chip was priced slightly higher, at $3,200, when it was introduced six months ago.

"If you stand back far enough, all three vendors look the same," says Velio's Woodruff. "But they all have some things that they do differently." A customer's choice ultimately depends on the application they have in mind, he says.

— Pauline Rigby, Senior Editor, Light Reading
http://www.lightreading.com