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This article describes DWDM ring networks from three viewpoints. The first viewpoint is the network architecture level. Here, the architecture of two–fiber–rings and of four–fiber rings, protection schemes, and the layer network structure of all–optical transport networks are discussed. In the second viewpoint, the network element level that comprises of the optical add/drop multiplexers (OADM) and their characteristic features (i.e., add/drop capacity, static and dynamic configuration, wavelength routing capability, transparent and opaque applications) is discussed. The third viewpoint is the component level. Tunable filters, switching fabrics, WDM multiplexers and demultiplexers, and interleaver filters are the areas discussed under this viewpoint.