All-optical test beds prove national networking

All-optical test beds prove national networking

george kotelly

At the Optical Fiber Communication (ofc `97) conference held recently in Dallas, researchers of the Multiwavelength Optical Networking (monet) Consortium detailed a national-scale fiber-optic network made up of three all-optical test beds. These test beds are proving the future technical feasibility of high-capacity commercial and defense communications systems using wavelength-division multiplexing (wdm), crossconnect, and dispersion-compensated fiber-optic cable technologies.

The monet Program, established in 1994, is funded in part by the Defense Advanced Research Projects Agency (darpa) for $5 million over five years. Additional funds are being provided by the monet Consortium, which includes Bell Atlantic, BellSouth, Pacific Telesis, and Southwestern Bell Technology Resources Inc. in cooperation with the National Security Agency and the Naval Research Laboratory.

The consortium`s objective is to define and demonstrate the best way to achieve national-scale, high-capacity, high-performance, cost-effective, reliable, transparent, multiwavelength optical networking by integrating network architectures, advanced technologies, network management schemes, and business drivers.

Therefore, the monet test beds represent the vision of transparent, reconfigurable optical networking layers capable of supporting all currently employed or proposed telecommunications standards, including Synchronous Optical Network (Sonet) services, ranging from OC-1 (52 Mbits/sec) to OC-192 (10 Gbits/sec), and Asynchronous Transfer Mode (atm) broadband, multimedia, and high-speed networking services.

The all-optical network would support virtually all future telecommunications standards. Its support of a variety of format-, bit-rate, and protocol-independent services could offer increased flexibility and economic advantages in commercial networks and is of particular interest to U.S. defense agencies.

"The monet vision is not simply very-high-speed, multiwavelength optical transmission links," states Adel Saleh, head of the broadband access research department at AT&T Laboratories. "It`s about reliable, flexible, high-capacity networking on a national or global scale."

Three test beds

Presently, the three network test beds are all located in New Jersey (see figure on page 1). One test bed functions as a local exchange network located at Bell Communications Research (Bellcore) in Red Bank. It is a mock-up of an operating local-exchange-carrier network using wdm technology and has recently been brought online.

Part of this test bed has been structured by Bellcore scientists as a multiwavelength self-healing ring network. This experimental network can deliver traffic across a regional multiwavelength network even when a portion of the network is disabled. It will later include several wavelength switches and wavelength-routed star networks, all designed to determine the best mix of equipment for local public networks.

"The greatest benefits of multiwavelength technology will be realized in local- exchange networks when it`s possible to dynamically set up an optical path to meet a customer`s service request, supporting whatever bit rate or signal format the customer wishes to transmit," says Joseph Berthold, Bellcore executive director, network systems research.

"It will be even more attractive if the path can be made survivable in the event of an optical-fiber cut. In the complex operating environment of local networks, the management systems must have open interfaces, be interoperable with existing systems, and be constructed in such a way that they are easily modified and enhanced. They must also be reliable.

"[This] local-exchange ring-network prototype marks the first demonstration of the type of networking equipment that multiwavelength optical networking technology will make possible and the management support systems that will be necessary to meet the needs of the local-exchange network," declares Berthold.

Brought online last fall, the second test bed is situated in Crawford Hill and is serviced by a team of scientists from both Lucent Technologies` Bell Laboratories and AT&T Laboratories. This long-distance networking test bed--more than 2000 km long--is running at 2.5 Gbits/sec on eight wavelengths to achieve an aggregate capacity of 20 Gbits/sec. The optical layer of the test bed is format- and bit-rate-independent, and the researchers plan to test multiple formats next.

The third test bed is also online and employs a crossconnect system for configuring networks by switching wavelengths from place to place. It is being structured at Lucent Technologies` Bell Laboratories in Holmdel by researchers from Bell Laboratories and AT&T Laboratories.

Transmission barriers

The key transmission issues to be confronted in a national-scale optical network are chromatic dispersion, optical-amplifier gain flatness, and optical nonlinearity. The precise design of erbium-doped optical-fiber amplifiers and selection of optical-fiber types can mitigate the effects of all three factors on the performance of signals transmitted over the 2000-km length of the monet long-distance network.

Consequently, special erbium-doped fiber amplifiers have been designed in a mid- amplifier-pumped configuration with a highly inverted counter-pumped first stage to obtain low noise figures; a co-pumped second state produces high output power levels. These amplifiers also offer the opportunity to add passive optics, such as dispersion-compensation or gain-equalization filters in the mid-amplifier region, where the impact of excess losses on system performance is minimal.

The monet network consists of three different types of fiber segments:

Twenty-five percent of its length consists of conventional singlemode fiber that produces 17 psec/nm-km chromatic dispersion.

Forty percent of its length contains Lucent Technologies` TrueWave positive-dispersion fiber (see Lightwave, December 1994, page 1), which yields 1.5- to 4-psec/nm-km chromatic dispersion.

Thirty-five percent of its length includes TrueWave fiber that furnishes alternating positive and negative dispersions.

As a result of the amplifier and cable designs, the monet network operates within the following guidelines:

Total chromatic dispersion is close to the dispersion limit for 2.5-Gbit/sec transmissions.

All eight channels are transmitted with less than a 1-dB transmission penalty.

All eight channels have signal-to-noise ratios in excess of 20 dB referred to a 0.1-nm bandwidth.

Gain variations over the channels are held to less than 5 dB without intermediate trimming.

In coming months, researchers are planning to incorporate dispersion-compensating and additional components using both routers and multilayer film devices. They are expecting to deploy dispersion-compensating fiber (dcf) after each 80-km span of conventional fiber; dcf with less compensation in the positive-dispersion TrueWave fiber segments; and self-compensation in the alternating-dispersion TrueWave fiber segment.

The monet local-exchange networking test bed demonstrates, for the first time, a 4-fiber, 6-node, survivable wdm ring network. It is now performing transport experiments with 8-wavelength, Sonet OC-48 signals and a mixed format of digital and frequency-modulated analog services.

The reconfigurable and survivable wdm ring network provides high-transport capacity at low cost for local-exchange-carrier networks and, at the same time, can meet the rapid capacity growth required by new high-bandwidth video and data services for future telecommunications and Internet networks.

To facilitate communication among network elements and the network management system, the researchers have constructed and tested a data communication network for the local exchange network test bed using an atm overlay network and an OC-3c (155-Mbit/sec) signaling channel at 1310 nm. The connection setup uses a 4-channel, frequency-modulated cable-TV video signal.

Network integration

Concerning overall network integration, Rod Alferness, head of the photonic networks research department, Bell Laboratories and Lucent Technologies program manager for the monet Program, says, "The all-optical New Jersey wdm network links Bell Atlantic fiber-optic cables to the Red Bank local-exchange test bed, through the crossconnect test bed in Holmdel, and to the long-distance test bed in Crawford Hill.

"It carries eight channels or wavelengths, each running at 2.5 Gbits/sec. Signals entering the network are sent over the local- exchange test bed, routed around that ring and through the optical crossconnect test bed, and on to the long-distance test bed. The bidirectional path then runs back through the long-distance, crossconnect, and local-exchange test beds. It essentially demonstrates the vision of a national-scale wdm optical network that is more than 2000 km long.

"Everything is done completely in the optical domain without going to electronics at any time," adds Alferness. "The signals never go into electrical format until they are dropped out of the network."

monet all-optical network equipment, such as crossconnects, add/drop multiplexers, receiver arrays, and multiple-wavelength sources, have been custom-built for reconfigurability and testing. However, some devices in the long-distance network are off-the-shelf units.

According to darpa directives, monet results will be made public to accelerate all-optical wdm networking. This information is anticipated to encourage telephone companies to build transparent and configurable-wavelength networks for themselves and for Department of Defense use. One of darpa`s goals is that the monet equipment lead to commercially viable all-optical wdm networking.

"The technologies demonstrated by the monet long-distance and local-exchange test beds will be the foundation of the next-generation Internet, where the demands for quality of service, bandwidth, configurability, and scalability will far exceed the capability of today`s network infrastructure," says Bert Hui, program manager of darpa`s information technology office. q

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