What is one of the primary overhead fields associated with the Optical Payload Unit (OPU)?

A.
path monitoring

B.
tandem connection monitoring activation deactivation (TCM ACT)

C.
Payload Structure Identifier (PSI)

D.
multiframe alignment signal (MFAS)

E.
section monitoring

Explanation:
Optical Payload Unit (OPU)
In order to begin describing the OTN as defined by the ITU G.709 standard, we must first
enumerate its critical elements, their termination points, and the way they relate to one another in
terms of hierarchy and function.
The primary overhead field associated with the OPU is the payload structure identifier (PSI). This
is a 256-byte multiframe whose first byte is defined as the payload type (PT). The remaining 255
bytes are currently reserved. The other fields in the OPU overhead are dependent on the mapping
capabilities associated to the OPU. For an asynchronous mapping (the client signal and OPU
clock are different) justification control (JC) bytes are available to Application Note 153Telecom
Test and Measurement compensate for clock rate differences. For a purely synchronous mapping
(client source and OPU clock are the same), the JC bytes become reserved. Further details on
mapping are available in ITU G.709.