LEARN: Lightwave Exchangeable Add/Drop Ring Network


As the Internet expands and handles more applications, growing numbers of requirements are being imposed on Optical Metropolitan Area Networks (MAN). MAN provides links to a diverse customer base and thus needs to support a variety of services including IP, ATM, Frame Relay, Gigabit Ethernet and SONET. Moreover, since each customer will have a different capacity and QoS requirement, MAN also needs to support bandwidth provisioning. To meet the potential needs, new trends become necessary in the design of MAN such as optical layer reconfigurability, which allows a node to add/drop any WDM channel, enabling provisioning, services over wavelengths, and restoration. 

The LEARN project, launched in July 1996, consisted of three nodes interconnected by a single 84 km single-mode fiber ring carrying payload wavelengths in the 1550nm window and a control wavelength in the 1300nm window. Novel reconfigurable Optical Add/Drop Multiplexers (OADM) and an Optical Wavelength Translating Crossconnect (OWTC) were designed and built, using a single AWG, several optical switches and transponders. A node housing the OWTC at Sprint ATL in Burlingame forms independent WDM circuits with two Stanford OADM nodes using two 1550nm wavelengths. 

We have demonstrated the format and bit-rate independence of the network by transmitting error-free multiple data formats including: 2.5 Gb/sec pseudo random traffic generated by a BER tester; 622 Mb/sec SONET OC-12; 90 Mb/sec Manchester coded DS3 with encoded video and audio; 125 Mb/sec FDDI; and 20 Mb/sec Manchester coded MPEG2 video. We have also determined the network’s signal degradation for 2.5 Gb/sec traffic. In the worst case (two loops around the 84 km ring with two wavelength translations), the sensitivity penalty was less than 4 dB. 


Visiting scholars