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Exar Aims to Kill Jitter Bugs

Its new chip integrates a line-interface unit with a desynchronizer for applications in Sonet-to-DS3 conversion

April 2, 2002

3 Min Read
Exar Aims to Kill Jitter Bugs

Exar Corp. (Nasdaq: EXAR) is developing more integrated components that help bridge the gap between optical Sonet networks and smaller-bandwidth electrical circuits.

Exar today announced a new T3/E3 chip (see Exar Intros Desynching Gizmo). Mindspeed Technologies, a direct competitor of Exar, is likely to announce something in the same space soon, but it declines to comment on specifics at this time.

Exar's product addresses the task of converting high-speed Sonet signals, which are carried over optical fiber, into lower-speed DS3 signals, which are carried over copper wiring such as T3/E3 leased lines. This is a common requirement in data centers, points of presence (POPs), and traffic-aggregation sites.This process is more complicated than it might appear at first, because Sonet is a synchronous protocol, while T3/E3 are asynchronous protocols, operating at slightly different line rates from STS-1, the basic building block of Sonet. The line rates of E3, T3 and STS-1 are 34 Mbit/s, 45 Mbit/s, and 51.84 Mbit/s, respectively.

Therefore, in addition to mapping from one protocol to another, another component or set of components is needed to cope with all the timing changes. The signal is then sent to a line-interface unit (LIU), which creates the voltage and current levels for transmitting it over copper cable. Exar's new chip integrates a LIU with a jitter attenuator and desynchronizer. Jitter attenuators and desynchronizers correct for timing errors in the data signals that occur when converting between different data rates.

Last year, Exar stole a lead on the competition when it announced an integrated circuit that could correct for timing errors. Until then, systems vendors had been required to build this functionality onto their boards using discrete components, says Hugh Wright, Exar's VP of marketing.

"You can see that this becomes a real issue when connecting multiple channels, because each channel has to have its own set of discrete components," he notes.

As an aside, there are several sources of jitter in the network, and some products called jitter attenuators deal with these other sources. Timing jitter caused by changing line rates is specified in Telcordia Technologies Inc.'s standard GR-253-CORE. Wright claims that Exar is compliant with this standard, as well as a slew of other standards far too numerous to name here.

So far, Exar appears to be the only vendor with a single-chip desynchronizer, although that could change if Mindspeed is adding the same functionality. Mindspeed would not release details of unannounced products for comparison.

Last year, Exar also released a product integrating the desynchronization capability with an OC12 (622 Mbit/s) mapper, which aggregates 12 T3/E3 or STS-1s (see Exar Beats the Clocks). But not everyone buys OC12 mappers from Exar, so the company designed its latest chip, which integrates a desynchronizer with an LIU, to appeal to the other half of the market.

Using the new chip, XRT75L00D, a customer can connect a single chip directly to their choice of framer to provide a complete Sonet-to-T3/E3 solution, Wright boasts. One Exar chip is needed per full-duplex T3/E3 port. Wright expects this product to start sampling before the month is out, but to be on the safe side, the company claims 60-day availability in its press release.

A three-port version of the integrated LIU-desynchronizer is also in the works, although no specific date was given for its availability.

According to the press release today, Ocular -- which was recently acquired by Tellabs Inc. (Nasdaq: TLAB; Frankfurt: BTLA) (see Tellabs Nabs Ocular) -- is a customer for Exar's desynchronizer chips, but it is unclear whether it would also purchase the newest chip.

Other companies making LIUs include TDK Corp. and TranSwitch Corp. (Nasdaq: TXCC).

— Pauline Rigby, Senior Editor, Light Reading

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