G.709 OPTICAL TRANSPORT NETWORK - Optical Payload Unit (OPU)


The optical transport network (OTN) was created with the intention of combining the benefits of SONET/SDH technology with the bandwidth expansion capabilities offered by dense wavelength-division multiplexing (DWDM) technology.

In addition to further enhancing the support for operations, administration, maintenance and provisioning (OAM&P)
functions of SONET/SDH in DWDM networks, the purpose of the ITU G.709 standard (based on ITU G.872) is threefold.

First, it defines the optical transport hierarchy of the OTN; second, it defines the functionality of its overhead in support of multiwavelength optical networks; and third, it defines its frame structures, bit rates and formats for mapping client signals.

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.
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