Photonic network routes gigabits

Oct 1st, 1995

Photonic network routes gigabits

dave wilson

Although it is relatively easy to increase the traffic load in fiber-optic cables, a more challenging goal is to increase the capacity and speed of the electronic switches that perform the routing operations. To accomplish this, researchers are looking at ways to process and transmit information in optical form.

Researchers at British Telecom Laboratories in Martlesham Heath, Suffolk, UK, recently demonstrated a fiber-optic network that uses optical time-division multiplexing that enables many high-speed data channels to be sent simultaneously along the same fiber by allocating each channel its own time slot.

They also demonstrated that the network could avoid the electronic bottleneck at switching centers by switching the optical signals directly, without conversion to and from electrical signals. In the British Telecom demonstration, optical technology was used for large-capacity routing of long-distance transit traffic between the network switching centers, leaving electronic processors the task of "fine-grain" routing of traffic for the local connections to users.

In the network, long-distance traffic passed directly through the switching centers in the form of optical pulses. This promises to be less expensive and more flexible than the traditional approach based on using powerful computers at switching centers.

Andrew Ellis, a senior professional in networks research at British Telecom, says that "the complementary strengths of optics and electronics were combined to create an efficient network. The key advance shown in this experiment is not the long-distance capability or even the high capacity that can be achieved, but the ability to route traffic flexibly on-demand without costly electronic switching."

Higher data rates

By performing the channel multiplexing and extraction processes at high speed using optical devices rather than electronic ones, much higher aggregated data rates can be attained--as high as 100 gigabits per second or beyond. Although high-speed point-to-point optical transmission using optical time-domain multiplexing over global distances has already been proven in several research laboratories, British Telecom claims that this experiment is the first to demonstrate the important networking potential of switching and routing optical signals.

The company claims that such a network can provide simple, flexible routing of multigigabit-per-second capacity between specified switching centers, at a lower cost than an equivalent network that uses all-electronic switching technology.

The experimental network simulates 690 switching centers interconnected by optical fibers using a transmission speed of 20 Gbits/sec in each fiber. The individual bits of data are represented by solitons, which are short optical pulses that can propagate over long distances without suffering excessive spreading or dispersion.

Each switching center performs a "drop and insert" function in which data addressed to that center is extracted from the optical signal stream; new data to be sent elsewhere is inserted in its place. In this way, users can obtain long-distance connections at any speed to 10 Gbits/sec. Optical time-domain multiplexing time slots are re-used to allow maximum network efficiency.

Global network

In the laboratory, a global network was simulated by connecting the output of an experimental network switch back to its input via 100 kilometers of optical fiber and allowing the data signals to circulate around the loop many times. On each round trip, the equipment dropped and inserted randomly selected data channels. After 690 circuits, or 69,000 km, the high-speed data remained error-free. The demonstration indicated that such a network could span Europe or encircle the globe, providing interconnection of all the world`s major cities.

An optical time-domain multiplexing network is usually located at major towns and cities where switching centers can receive and transmit high-speed data. The incoming stream of soliton pulses at 20 Gbits/sec is split into two transmission paths--one allows data to be accessed by the electronic processor, and the other provides a bypass for data intended for onward transmission to other parts of the network.

"We were using the best of both worlds--the photonic side and the electronic side. Each has its individual strengths," says British Telecom Laboratories` researcher Terry Widdowson. Selecting which data to access and which to bypass is performed by the photonic processor. "The photonic processor comprises two electro-absorption modulators that perform drop and insert functions and demultiplex the data stream," he adds.

Although they are driven by electrical signals, the two modulators process the electro-optic signals in the optical domain. One electro-absorption modulator was built at British Telecom, while the other was purchased. The photonic processor performs a key additional function for the data that bypasses the electronic processor: It suppresses timing jitter and noise on the pulses that pass through, thereby allowing error-free transmission over tens of thousands of kilometers.

For its part, the electronic processor performs timing extraction and clock recovery. That could be done optically, but as Widdowson admits, it is "difficult to stabilize." In the British Telecom design, those functions are performed electronically with a phase-lock loop that derives all the timing signals. Those signals are then fed to the photonic processor that performs the drop and insert functions.

"Electro-absorption modulators such as the ones used in the demonstration will be used in future transatlantic communications systems due online at the end of this year," says Widdowson. "Simple modulators to encode data are already seeing their way into the more advanced networks," he adds. More advanced functions, however, could take five years to implement.

At the output of the electronic processor, the bypassed signal is combined with locally generated traffic for transmission through the network. For operation at even higher transmission speeds--perhaps as high as 100Gbits/sec--the electro-optic devices used in the current version of the photonic processor could be replaced by all-optical devices, which are currently under development at British Telecom Laboratories. q

Dave Wilson writes from London.

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