As defined in G.709 an ODUk container consist of an OPUk (Optical Payload Unit) plus a specific ODUk Overhead (OH). OPUk OH information is added to the OPUk information payload to create anOPUk. It includes information to support the adaptation of client signals.Within the OPUk overhead there is the payload structure identifier (PSI) that includes the payload type (PT). The payload type (PT) is used to indicate the composition of the OPUk signal.
When an ODUj signal is multiplexed into an ODUk, the ODUj signal is first extended with frame alignment overhead and then mapped into an Optical channel Data Tributary Unit (ODTU). Two different types of ODTU are defined in G.709:
– ODTUjk ((j,k) = {(0,1), (1,2), (1,3), (2,3)}; ODTU01,ODTU12,ODTU13 and ODTU23) in which an ODUj signal is mapped via the asynchronous mapping procedure (AMP), defined in clause 19.5 of G.709.
– ODTUk.ts ((k,ts) = (2,1..8), (3,1..32), (4,1..80)) in which a lower order ODU (ODU0, ODU1, ODU2, ODU2e, ODU3, ODUflex) signal is mapped via the generic mapping procedure (GMP), defined in clause 19.6 of G.709.
When PT is assuming value 20 or 21,together with OPUk type (K=1,2,3,4), it is used to discriminate two different ODU multiplex structure ODTUGx :
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