A new generation of optical testing emerges.

Gen-O aims at eliminating O-E conversion as much as possible.

The first generation of optical networks culminated in the development of dense wave division multiplexing (DWDM), which uses new equipment to leverage existing fiber plant infrastructure, making possible bandwidth traffic increases by factors of up to 80 times. This allows the optical network to accommodate the recent dramatic increase in demand for bandwidth, largely driven by the Internet and wireless technology.

Testing the first wave of optical networks revolves around verifying the ability of point-to-point DWDM systems to deliver multiple wavelengths error free and without wavelength crosstalk across a single fiber. Current test solutions involve primarily analog techniques, such as optical spectrum analysis--an indirect method of predicting whether wavelength channels can be expected to function error-free. While necessary for testing certain aspects of DWDM systems, analog techniques do not measure whether transmission errors actually occur, nor will they detect every potential transmission problem.

Drawbacks can also be found in existing digital methods, such as bit error rate test (BERT), for testing optical networks. Channelized testing typically limits tests to a single channel within a multiplexed signal. For example, while a user may be able to configure an OC-192 BERT to test OC-48 or OC-12 channels, only one of those channels may be tested at any given time.

Most devices that allow multichannel channelized operation only allow a single pattern to be propagated through the various channels, which fails to vigorously test the equipment used to build and propagate multiplexed signals. Multichannel channelized devices that allow simultaneous and independent testing of all channels provide emerging capabilities that will become more critical as the channel density of optical networks continues to increase.

To get an idea of the radical increase in capacity on a single fiber, assume a single DS-0 voice channel is a can of soda. If you drink one soda a day, you could consume a DS-3 in less than two years, but it would take more than 353 years to drink an OC-192. Commercially available DWDM systems offer a capacity that would take more than seven millennia to drink at the rate of one DS-0 a day.

Given the increased potential for signal degradation in systems of this density, the ability of all wavelengths in a DWDM system to function error-free without interactions should be measured directly by running simultaneous independent BERT patterns on all the channels. This is impractical to do with traditional BERT testers and requires implementing optical networking test equipment only just now becoming available.

In Gen-D (DWDM) optical networks, everything of significance happens electrically. With the exception of optical repeaters, every time the signal is touched, it undergoes optical-to-electrical (O-E) conversion. The second wave of the optical networking revolution--Gen-O--aims at eliminating O-E conversion as much as possible. Electrical processing cannot be totally eliminated until science finds some way of doing calculations with photons instead of electrons. In the meantime, much can be done, such as the development of optical switches.

Optical switching is accomplished by communication between Layer 1 optical transmissions and upper layer optical signaling protocols. Signaling information is passed between switches, either through out-of-band connections or in-band encoding in the transport overhead.

Unlike legacy switches, optical switches do not set up virtual paths to establish an end-to-end circuit. Instead, wavelengths are switched to create an actual wavelength path through the optical transport network. Legacy methods of testing optical circuits that are limited only to Layer 1 are not able to test this type of equipment. In addition to performing BERT, the test device must be able to generate and respond to signaling messages in order to test the ability of the optical switch-to-switch wavelengths, while simultaneously carrying user payload traffic.

An optical switch test solution utilizes multiple low-cost BERT testers generating streams that are multiplexed with DWDM equipment and transmitted to the optical switch. The switch and the test device use an overlaying routing protocol stack that exchanges messages to set up, modify and tear down wavelength paths. Both the control and data planes of the switch are tested concurrently with the associated DWDM equipment. Ultimately, tunable lasers will allow the data and control planes of optical switches to be tested directly as a standalone network element.

Currently, wavelengths are preserved as they pass through an optical switch. Future technology will allow wavelength translation, permitting much more efficient switching and wavelength utilization of the switched optical network. Test requirements for this phase build on the BERT capabilities needed for DWDM point-to-point systems. Needed capabilities include the verification that many optical wavelengths are switched rapidly and correctly to the proper destination, and can carry streams without bit errors.

Optical network designers will need the advantage of integrated test platforms that can incorporate testing between all network layers and multiple protocols. These test platforms will be able to test across multiple protocol layers in existing networks, and leverage the investment by building a foundation for testing of Gen-O networks as they are deployed.

[ILLUSTRATION OMITTED]

www.spirentcom.com

Circle 268 for more information from Spirent

Whittington is a product manager at Spirent Communications, Adtech Division, Honolulu, HI.