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2 Transport aspects

15.6 OTU/ODU frame alignment OH description

15.6.1 OTU/ODU frame alignment overhead location
The OTU/ODU frame alignment overhead location is shown in Figure 15-9. The OTU/ODU frame alignment
overhead is applicable for both the OTU and ODU signals.

The OTUk/ODUk contains one instance of OTU/ODU frame alignment overhead. The OTUCn/ODUCn
contains n instances of OTU/ODU frame alignment overhead, numbered 1 to n.

Figure 15-9  OTU/ODU frame alignment overhead

15.6.2 OTU/ODU frame alignment overhead definition

15.6.2.1 Frame alignment signal (FAS)
A six byte OTU-FAS signal (see Figure 15-10) is defined in row 1, columns 1 to 6 of the OTU overhead. OA1 is
"1111 0110". OA2 is "0010 1000".
The OTUk contains one instance of OTU multi-frame alignment overhead. The OTUCn contains n instances
of OTU multi-frame alignment overhead, numbered 1 to n.

Figure 15-10  Frame alignment signal overhead structure
15.6.2.2 Multiframe alignment signal (MFAS)

Some of the OTU and ODU overhead signals will span multiple OTU/ODU frames. Examples are the TTI and
TCM-ACT overhead signals. These and other multiframe structured overhead signals require multiframe
alignment processing to be performed, in addition to the OTU/ODU frame alignment.
A single multiframe alignment signal (MFAS) byte is defined in row 1, column 7 of the OTU/ODU overhead
for this purpose (see Figure 15-11). The value of the MFAS byte will be incremented each OTU/ODU frame
and provides as such a 256-frame multiframe.
The OTUk contains one instance of OTU multi-frame alignment overhead. The OTUCn contains n instances
of OTU multi-frame alignment overhead, numbered 1 to n. All n MFAS bytes carry the same 256-frame
sequence and in each frame all n MFAS bytes carry the same value.

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