BAR-CODING FIBER-OPTIC SIGNALS / O-CDMA
A previous article appeared in America's Network Magazine back in July 1998 on the topic of optical CDMA, or O-CDMA, at:
americasnetwork.com
This is an earlier report of CTC's efforts in this cutting-edge space.
Enjoy, Frank Coluccio
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"Bar-coding fiber" - CDMA works for wireless— and may hold promise for optical networking. By Annie Lindstrom
Grocery stores have been using bar codes to speed up their checkout lines for years, but would you believe that they also can be used to add capacity to fiber optic cable?
Commercial Technologies Corp. (CTC; Culver City, Calif.)is hoping that carriers involved in optical networking will believe it. The company introduced bar code-enabled optical code division multiple access (O-CDMA) technology last month at Supercomm '98 in Atlanta. Like dense wavelength division multiplexing (DWDM) systems, CTC's CodeStream system lets carriers place multiple transmissions onto a single fiber. Unlike DWDM, O-CDMA uses a single optical source, rather than a laser for each wavelength.
Each O-CDMA transmitter bar codes and modulates an asynchronous signal onto a single fiber pair. At the receiving end, the receiver with the matching bar code connects to the transmitter and takes the signal destined for it, explains Bill Johnson, COO of CTC.
“Our first system can place 128 OC-12 [622 Mbps] signals on one fiber pair and perform add/drop functions,” Johnson explains. The second generation of equipment will employ a programmable bar code that makes it possible to perform optical cross-connect function.
CodeStream accommodates all types of traffic, asynchronous transfer mode (ATM), digital video, and asynchronous DS-1 (1.5 Mbps) and DS-3 (45 Mbps) signals.
A bright idea
CTC's parent company, Research and Development Laboratory (Culver City, Calif.), invented O-CDMA for the Air Force, which was looking for a way to reduce the weight of satellites by replacing copper with fiber optic cable. The telecom industry had long since dismissed CDMA as a way to add capacity to fiber, because there was not an economical means to derive the property of phase from a photonic signal, Johnson says. It could be done in the lab, but at great expense.
CTC has a patent pending on the method used to economically add phase to the optical signal.
Phase is a component of electrical and optical signals that allows a receiver to select one signal and reject all others. Phase is an easy component to extract from an electrical signal, because it consists of positive and negative properties, Johnson says. With an optical signal, photons travel at the speed of light and it becomes difficult to align them in phase. If one were to fire one photon each from two lasers sitting side by side on a fiber optic cable, it would be extremely difficult to have those two photons arrive at the far end of the fiber at exactly the same time. If they arrived at the same time, it would be easy to extract the phase component, he says.
“In essence, what we do is trick the photonic signal into thinking that it has both a positive and negative phase component,” Johnson explains.
“Phase is not an issue on DWDM systems, because each channel has its own laser,” Johnson continues. “In our system, one optical source powers the whole station, so it is a much cheaper solution.”
The bar-code technique has been demonstrated in CTC's lab, Johnson says. CodeStream does not produce different wavelengths.
“We don't care about colors; we just have broadband optical spectrum,” he says. We take the light source and run it through a grating mirror, which widens the beam out to a few tenths of an inch. Then, we aim that beam through a spatial filter and it looks just like a bar code.”
Fixed bar codes are a few tenths of inch in size with pixels etched into them. CTC also offers programmable bar codes built out of LCDs, which can be controlled by turning on or off each one of the pixels, Johnson explains.
Photons through and through
An all-photonic network must be able to do four things: accommodate a large number of users on a single fiber, add and drop traffic at short distances economically, cross connect traffic, and restore service, according to Johnson.
“We can do all four of those with O-CDMA,” he notes.
CTC's near-term goal is to introduce the technology to carriers and let them know that there is an alternative to DWDM, which provides a solution for the all-photonic network.
DWDM requires an additional technology, acousto-optic tunable filters (AOTF), to optically add and drop traffic, or do optical cross connects. O-CDMA does both, eliminating the requirement for carriers to buy external elements.
“With DWDM, it's not practical to drop traffic at each building down the street,” Johnson says.
Like wireless systems based on CDMA, O-CDMA reportedly is secure. That's because it uses the same spread spectrum technology, which allows only the recipient for whom the signal is intended to take it off the network.
The future
CTC aims to bring commercial systems to market by the first quarter of 1999. The company has purchased manufacturing facilities in Richardson, Texas, in which it is building a small number of complete systems, which will be available for pilot tests later this summer, Johnson says. Several incumbent local exchange carriers have shown an interest in testing the product, he adds.
“If O-CDMA is half as good as CTC makes it sound, it could make a big impact on WDM and optical networking in general,” says Mark Lutkowitz, principle of Trans-Formation Inc. (Birmingham, Ala.). But it depends on how fast product can get to market.” |
