AMTC '98 Yields Technical Insight After Rough Start
By Erik Kreifeldt
ATLANTA—At the first Annual Multiplexed Technology Conference underway here in Atlanta this week (August 23-27), the technical program got off to a rough start with a few no-shows, but went on to offer a comprehensive look at fiber optic networking. Attendees gleaned new twists on metro applications from papers on optical code division multiple access (CDMA), wavelength division multiplexing (WDM), and OC-192, plus a run-down of passive WDM components.
Faced with the absence of scheduled keynote speaker George Gilder (an infamous prognosticator on fiber-related "telecosm technologies"), the AMTC plenary went on with a talk about bandwidth demand, optical networking, and optical CDMA. Commercial Technologies Corp. (Culver City, CA) CEO Raj Dutt delivered the talk, revealing that he had just wrapped up a 16-day world tour promoting the technology.
"An all-optical network is not more than 12 to 18 months away," Dutt says, dismissing five and 10-year predictions. He fails to elaborate on who might deploy such a network, which he defines as a single technology—presumably optical CDMA—that will enable photons in and photons out of the network.
A novel approach
Although not abundantly clear from the keynote talk, optical CDMA does offer a novel perspective on the topic of optical networking—a topic dominated by discussions of wavelength division multiplexing (WDM). "Does just WDM solve all our problems? I don't believe so," posits Dutt.
"Don't think of optical CDMA as just another remedy for fiber exhaust in the local loop—it's a new network architecture that can potentially reduce the cost of every aspect of an optical network," advises Commercial Technologies' Pete Kayashima. The concept, he adds, is quite simple. "Place hardware where the traffic originates and terminates on the network, and perform all provisioning and restoration functions within the network using photons instead of electrons."
How it works
Analogous to WDM, which divides the fiber spectrum into narrow optical wavelengths with individual lasers, optical CDMA divides the spectrum into individual codes, derived from a single broadband source, Kayashima explains. The broadband source sends light through a splitter to multiple channels. Each channel passes through a spatial filter and then an optical modulator. The spectrally encoded and modulated signals are then combined onto a network fiber (A 28 nm source can generate 256 codes and 120 Gbps aggregate data rate, Kayashima reports). With all signals going to all parts of the network, a receiver on any point in the network receives only the signal for which it is coded.
"The concept is very nice," observes Xin Cheng, president of Osicom Technologies (San Diego, CA), which makes WDM, datacom, and video equipment. Nice enough, apparently, to inspire his colleagues. "I'm interested in carrying [an optical CDMA signal] in our WDM," says Don Buell, product manager for Osicom's WDM.
Going commercial
The trick to implementing a commercial optical CDMA system is coming up with a cost-effective way to carry a phase component of the optical signal, without actually operating lasers in phase, Kayashima says. Although his company performs a simple solution in the lab, he says it can not yet be mass-produced.
Because optical CDMA is inherently secure, the technology may find a niche in defense-oriented applications. As for the public network, using a single broadband source appeals to carriers that are not too keen on keeping spare lasers on hand for a 128-channel WDM system, but the source requires lots of power. Loss and dispersion limit span lengths to 25 km.
Upgrading the metro network
While optical CDMA may be well suited for local, secure network applications, vendors continue vying for the best solution to upgrade metro networks. OC-48 WDM options have been picking up steam, due in part to the high initial cost of OC-192. But Hitachi's (Norcross, GA) Steve Carter makes a case for OC-192 that's 15% cheaper than OC-48 WDM.
Carter compared OC-192 4-fiber bidirectional lineswitched rings (BLSR) with OC-48 2-fiber BLSR and metro WDM for a metro ring with five nodes, less than 50 miles in circumference, and ultimately carrying 276 DS-3s, 5 OC-12s, and 3 OC-48s (modeled over 12 growth stages). Carter finds that OC-192 BLSR generates 11% savings in equipment, 16% less revenue lost to time-to-market delays, 27% savings in space, power, and maintenance, and 47% in engineering and installation costs.
"The big thing that surprised me was that one OC-192 4-fiber BLSR can serve the equivalent of 11 wavelengths of OC-48," Carter says, attributing the phenomena to efficient use of bandwidth.
More metro WDM
In the exhibit, attendees found another WDM system vying for the metro market. Synchronous is showing a 32-channel WDM system with multimedia interfaces for wide-area networks. Leveraging its experience in the cable TV industry, Synchronous integrates codecs into the system's transponders to carry analog video—a unique feature in the metro WDM space. With an eye towards expensive long-haul systems that currently dominate the WDM market, "we're shooting for $17,000 per wavelength," reports Synchronous President Albert Johnson.
WDM component options
The proliferation of WDM system offerings includes several approaches to multiplexing (mux) and demultiplexing (demux) the signals. Yuan Shi of E-TEK Dynamics Inc. (San Jose, CA) offers a run-down of the passive optical components that perform these key tasks.
Widely used mux and demux devices are arrayed waveguide gratings, thin film dielectric coated filters, and fiber Bragg gratings. With an eye to the advantages and disadvantages of each (see chart), Shi sees reports improvements in crosstalk and polarization independent performance with a hybrid Bragg grating/dielectric filter solution. "If we combine these guys together, we'll have some very nice devices," he says.
The hybrid components enable 8-channel devices with 0.4 nm spacing, 25 dB channel isolation, and 7.5 dB insertion loss, Shi reports, adding that the devices can be expanded to 32 or 64 channels.
AMTC '99 heads west to San Diego, CA, July 18-22. The next fiber show update will come from the National Fiber Optics Engineers Conference in Orlando, FL, September 13-17.