optical payload unit (OTN) (OPU)
The Optical Payload Unit (OPU) is a container that is transported in the Optical Data Unit( ODU) via optical transport networks. In the Optical Payload Unit (OPU), the user data is combined with the overhead. The payload data can be from ATM, Multi-Protocol Label Switching( MPLS), Synchronous Digital Hierarchy( SDH), Ethernet or other protocols. The Optical Payload Unit, in conjunction with Tandem Connection Monitoring( TCM) and another header, forms the Optical Data Unit (ODU).
Optical Data Units can be converted to Optical Payload Units relatively easily by mapping. The OPU units are assigned to different Optical Payload Units depending on the Optical Data Units used, Tributary Slots of 1.25 Gbit/s and 2.5 Gbit/s, the Payload Type, as well as Generic Mapping and Asynchronous Mapping. There are OPU units OPU-1, OPU-2, OPU-3 and OPU-4, with OPU-2 and OPU-3 operating with tributary slots of 1.25 Gbit/s as well as 2.5 Gbit/s.
The original mapping concept corresponded to the STM-n synchronous transport modules with multiplications of 16, 64, and 256, and adaptation was possible using a simple asynchronous mapping. This is no longer possible with 100 Gigabit Ethernet. Therefore, with the high data rate data services in G.709, a different mapping was introduced, the Generic Mapping Procedure (GMP).