Optical/IP Networks

DWDM Goes Pluggable

Vendors of dense wavelength-division multiplexing (DWDM) systems are continuing their push for pluggable optics, a movement that's swept the 2.5-Gbit/s generation and is encroaching on 10-Gbit/s as well.

Those plans extend all the way to the XFP multisource agreement (MSA), the most compact of the 10-Gbit/s pluggable options. ADVA Optical Networking (Frankfurt: ADV) claims to support XFP interfaces in its FSP 2000 systems. Ciena Corp. (Nasdaq: CIEN) says it's going to have XFP-based DWDM interfaces by September, and Foundry Networks Inc. (Nasdaq: FDRY), working with Finisar Corp. (Nasdaq: FNSR), plans to produce an XFP interface "later in the year," according to Foundry product line manager Bill Ryan.

Not to be outdone, Cisco Systems Inc. (Nasdaq: CSCO) says it expects to offer XFP DWDM interfaces next year.

Vendors are already shipping DWDM interfaces based on the larger Xenpak form factor. Foundry began shipping one such Xenpak, for 80km DWDM, in December, and Cisco says it's been shipping a Xenpak DWDM interface since last summer.

At the 2.5-Gbit/s level and below, the move to pluggables is even further along, with small-form-factor pluggable (SFP) interfaces for 2.5 Gbit/s and gigabit interface converter (GBIC) modules at 1 Gbit/s. Cisco offers both, as does Ciena; in fact, Ciena is "moving 100 percent to SFPs for all service ports below 10 Gbit/s across our entire portfolio," says David Payne, senior director of product management.

Why the interest? Compared with older modules, pluggable optics are smaller and interchangeable, even (theoretically) between different vendors' equipment (see Use Our Optics, or Else!). And they're widely in use for routers and switches; strength of the SFP market even helped displace a DWDM-specific MSA previously developed (see DWDM Gets Smaller -- and Cheaper).

The latter point is particularly important to Cisco, which is moving its DWDM equipment to use the same pluggable optics as its routers. "We're trying to push DWDM pluggable interfaces," says Rajiv Ramaswami, VP and general manager of Cisco's optical networking group.

But packing a DWDM module into pluggable form can be tricky. Because DWDM transmission can involve multiple wavelengths that are relatively close to one another, the laser's wavelength can't be allowed to waver -- something that happens naturally with temperature changes and time. And the traditional cooled lasers used in DWDM were too bulky to fit in pluggable packages.

"Usually, people need to use wavelength lockers in order to get wavelength stability. The challenge is to either fit that in or go without," says John Trail, director of product management at transponder vendor Big Bear Networks.

When it comes to the 10-Gbit/s level, only a few vendors offer pluggable DWDM modules. Optillion AB was one, and it even scored Foundry as a customer. But Optillion fell into receivership, prompting Foundry to turn to Opnext Inc., which announced its Xenpak DWDM module last year (see Optillion Runs Aground and Opnext Intros 10-GigE Module).

XFP, being smaller than Xenpak, provides an even higher degree of difficulty. That's particularly true at longer distances, where controlling heat in the XFP package becomes difficult. In crafting an 80km DWDM XFP module, "it's the 80km part that's the killer, not the DWDM part," says Foundry's Ryan.

Still, a couple of vendors are taking on the challenge. Finisar made the conscious decision to skip Xenpak initially and jump straight to XFP modules, and the company demonstrated a 40-km XFP DWDM unit at Supercomm in 2004. An 80km version followed, debuting at last month's OFC/NFOEC. Bookham Inc. (Nasdaq: BKHM; London: BHM) likewise is promising an XFP DWDM module (see Finisar Gets XFP Happy and Bookham to Launch XFP Transceivers ).

Not all DWDM is going pluggable just yet, however. Optical transport vendor Xtera Communications Inc. sees a definite use for pluggable DWDM interfaces in client-facing interfaces, but the company doesn't feel pluggables are advanced enough for all its transport needs. "On the long-haul side, we wouldn't use pluggables at first. For the tunability and performance we need, the pluggable interfaces are not there yet," says Herve Fevrier, Xtera's vice president of photonic systems.

— Craig Matsumoto, Senior Editor, Light Reading

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