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2 Transport aspects

Figure 15-13  OTU section monitoring overhead

15.7.2 OTU overhead definition
15.7.2.1 OTU section monitoring (SM) overhead

One field of OTU section monitoring (SM) overhead is defined in row 1, columns 8 to 10 to support section
monitoring.
The OTUk contains one instance of OTU SM overhead. The OTUCn contains n instances of the OTU SM
overhead, numbered 1 to n (SM #1 to SM #n).
The SM and SM #1 field contains the following subfields (see Figure 15-13):
 trail trace identifier (TTI);

 bit interleaved parity (BIP-8);
 backward defect indication (BDI);
 backward error indication and backward incoming alignment error (BEI/BIAE);
 incoming alignment error (IAE);

 status bits indicating the presence of an incoming alignment error or a maintenance signal (STAT);
 bits reserved for future international standardization (RES).
The SM #2 to #n fields contain the following subfields (see Figure 15-13):
 bit interleaved parity (BIP-8);

 backward error indication and backward incoming alignment error (BEI/BIAE);
 bits reserved for future international standardization (RES).

15.7.2.1.1 OTU SM trail trace identifier (TTI)
For section monitoring, a one-byte trail trace identifier (TTI) overhead is defined to transport the 64-byte
TTI signal specified in clause 15.2 or a discovery message as specified in [ITU-T G.7714.1].
The OTUk and OTUCn contain one instance of OTU TTI overhead.

The 64-byte TTI signal shall be aligned with the OTU multiframe (see clause 15.6.2.2) and transmitted four
times per multiframe. Byte 0 of the 64-byte TTI signal shall be present at OTU multiframe positions 0000
0000 (0x00), 0100 0000 (0x40), 1000 0000 (0x80) and 1100 0000 (0xC0).

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