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Transport aspects 2

11.1 OTUk frame structure

The OTUk (k = 1,2,3,4) frame structure is based on the ODUk frame structure and extends it with a forward
error correction (FEC) as shown in Figure 11-1. 256 columns are added to the ODUk frame for the FEC and
the reserved overhead bytes in row 1, columns 8 to 14 of the ODUk overhead are used for an OTUk specific
overhead, resulting in an octet-based block frame structure with four rows and 4080 columns. The MSB in
each octet is bit 1, the LSB is bit 8.

NOTE – This Recommendation does not specify an OTUk frame structure for k=0, k=2e or k=flex. See Annex G for the
specification of OTU0LL.

.......
1 3824
1
2
3
4
ODUk
1 ....... 14 15 ....... 3824 3825 ....... 4080
1 FA OH OTUk OH
2 OTUk FEC
3
4 (4 256 bytes)
OTUk G.709-Y.1331(12)_F11-1

Figure 11-1  OTUk frame structure

The bit rates of the OTUk signals are defined in Table 7-1.
The OTUk (k=1,2,3,4) forward error correction (FEC) contains the Reed-Solomon RS(255,239) FEC codes.
Transmission of the OTUk FEC is mandatory for k=4 and optional for k=1,2,3. If no FEC is transmitted, fixed
stuff bytes (all-0s pattern) are to be used.

The RS(255,239) FEC code shall be computed as specified in Annex A.
For interworking of equipment supporting FEC, with equipment not supporting FEC (inserting fixed stuff all-
0s pattern in the OTUk (k=1,2,3) FEC area), the FEC supporting equipment shall support the capability to
disable the FEC decoding process (ignore the content of the OTUk (k=1,2,3) FEC).
The transmission order of the bits in the OTUk frame is left to right, top to bottom, and MSB to LSB (see
Figure 11-2).

Column
Row 1 4080
1

2
3
4
MSB LSB
1 2 3 4 5 6 7 8
G.709-Y.1331(12)_F11-2

Figure 11-2  Transmission order of the OTUk frame bits

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