PSI (Payload Structure Identifier)

The PSI Message

The PSI (Payload Structure Identifier) message is a 256-byte message that an OTN terminal will transport via the PSI byte, over the course of 256 consecutive OPUk/ODUk/OTUk frames.

PSI[0] – PT (Payload Type)

The first byte of the PSI Message (e.g., PSI[0]) carries the Payload Type (or PT).  The PT identifies the type of client data that the OPUk structure is transporting via its payload.  Table 1 presents a list of PT values along with the corresponding client data types (being transported within the OPUk Structure).

Table 1a, PT (PSI[0]) Values, and the Corresponding Client Data within the OPUk Structure – Part I

Table 1b, PT (PSI[0]) Values, and the Corresponding Client Data within the OPUk Structure – Part II

Table 1c, PT (PSI[0]) Values, and the Corresponding Client Data within the OPUk Structure – Part III

Table 1d, PT (PSI[0]) Values, and the Corresponding Client Data within the OPUk Structure – Part IV

Other posts contain detailed information on how ITU-T G.709 recommends that the System Design map each of these client signals into their corresponding OPUk structure.

The Remaining Bytes within the PSI Message

PSI bytes 1 and 3 through 255 are for “Mapping and Concatenation Specific” roles and are discussed in another post.  We also use these bytes to transport MSI (Multiplex Structure Identifier) information throughout the OTN.  We will use the MSI information for applications in which we are mapping/multiplexing lower-speed ODUj signals into higher-speed OPUk signals.

PSI byte 2, Bit 1 is the CSF (or Client Signal Fail) indicator.  PSI Byte 2, Bits 2 through 8 are currently reserved for “future standardization”.

 

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