What is the difference between SONET/SDH and DWDM?

SONET (Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy) are the American and international standards (respectively) for transmitting multiple digital streams over optical fiber at synchronous speeds. They define a rigid hierarchy of speeds: OC-1 (51.84 Mbps), OC-3 (155 Mbps), OC-12 (622 Mbps), OC-48 (2.5 Gbps), OC-192 (10 Gbps). SONET/SDH provide excellent operations, administration, and maintenance (OAM) capabilities and protection switching (recovery in 50ms). DWDM (Dense Wavelength Division Multiplexing) transmits multiple data streams simultaneously on a single fiber by using different wavelengths (colors) of light. Each wavelength carries an independent channel (100 Gbps or more per wavelength). 80-160 wavelengths per fiber = 8-16 Tbps per fiber pair. DWDM is the technology underlying modern long-haul and submarine fiber networks. Modern networks combine DWDM at the optical layer with OTN (Optical Transport Network) replacing legacy SONET/SDH framing, and Ethernet/IP at the packet layer.