The optical network layer is structured into three layers: the access layer, the aggregation layer, and the core layer. This overall framework works together to realize the network's efficient and robust data transmission function. ODU Layer – Multiple Service Transport At the top of. An optical transport network (OTN) is a digital wrapper that encapsulates frames of data, to allow multiple data sources to be sent on the same channel. Moving upward, the. The text provides a comprehensive overview of the functional architecture of Optical Transport Networks (OTNs) as defined by ITU-T Recommendations.