Exar Beats the Clocks
Dubbed the XRT94L43, the new silicon is a so-called mapper chip, which aggregates up to 12 leased-line connections onto a single OC12 Sonet link. By mapping multiple low-speed links onto higher ones in a single chip, it shrinks the amount of components required to link older copper networks to optical ones by orders of magnitude, according to industry experts.
Specifically, Exar's chip groups 12 DS3/T3 (45 Mbit/s) or E3 (34 Mbit/s) into one Sonet/SDH OC12/STM4 (622 Mbit/s) link.
Exar isn't first out of the gate with this type of chip. The XRT94L43 competes with the Orinoco chipset released by Exar rival Applied Micro Circuits Corp. (AMCC) (Nasdaq: AMCC) early in 2001 (see AMCC Shrinks Sonet Equipment and AMCC Unveils New Framer).
Like the Orinoco chip, the XRT94L43 can add all of the characteristics to frame and unframe the different types of interfaces it supports, including DS3/E3 and Sonet/SDH. It is also a grooming chip, since it can selectively add up to four STM-1 channels onto one Sonet OC12/STM4 link. As well, it packs jitter attenuators, performance monitoring, and system diagnostics.
According to Exar, there are two key differentiators from Orinoco: First is the chip's programmable on-chip synthesizer, which the vendor says generates the necessary timing devices, or clocks, from one source.
This is a vital point, since framing chips must be able to set the rates at which the different protocols they support, such as Sonet, operate over the physical connection.
Exar says chips like AMCC's typically require several clocks -- usually in extra chips -- to ensure that all the interfaces are properly timed. The vendor says this can make framing chips more costly and complicated than the new Exar offering.
"Our serial oscillator is included on the chip. We don't require multiple external clocks, including expensive 622-MHz clocks," says High Wright, Exar director of marketing.
The second differentiator Exar claims is that its chip can support multiple and different low-speed interfaces at the same time in full duplex mode. That is, equipment manufacturers can use the chip to take in E3 and T3 streams at the same time. Exar says customers claim they can't do this with AMCC's chip, which supports only one type of incoming interface at a time.
AMCC did not return queries about Orinoco at press time.
Despite its apparent breakthrough, Exar faces a couple of hurdles. On the downside, the chip does not yet handle T1/E1 input. Further, some sources claim there are design downsides to high-density framing chips that require OEM customers to change their engineering design methods.
Others say Exar lacks the broad product portfolio of larger competitors. And in the world of networking chips, many OEMs prefer to buy all their system components from one provider. This allows them to save on the costs associated with having to integrate different elements into a coherent system.
Still, Exar has made progress in this regard. It recently forged a partnership with Internet Machines Corp.. Together, they are working to develop upper-layer components that compliment its physical-layer chips (see Exar Teams Up on 10-Gig Chips). Eventually, this should enable Exar to offer a more complete menu to its customers.
— Mary Jander, Senior Editor, Light Reading