Siemens SRA 4 User Manual page 37

Fig. 1.8
SRA 4 - 6/7/8/10/11/13 GHz (WB ODU) - UMN
911-381/02C0000 - Issue 3, July 2004
The tributary units are synchronized by timing signals from the VC-4 in order to tie their
timing together.
The TU structures contain virtual containers (VC) as signals to be transported, which in
turn are composed of a container C into which the actual payload signal is loaded, and
of the POH required for channel related service transmissions (BER, alarms, load
identification etc.).
The composition of the TU is shown schematically in Fig. 1.8.
Composition of the TU structure
TU3
TU2
TU12
The virtual containers VC can be divided into two groups, depending on their capacity
and structure: VC of a higher order (VC-3 and VC-4), and VC of a lower order (VC-2,
VC-11 and VC-12).
Containers of a higher order have in fact a field defined in one single STM-1 frame (125
µs), those of a lower order have a field in a multiframe composed of four STM-1 frames
(500 µs).
Anyway, this kind of structure can be assembled and disassembled only at the input or
output of the synchronous SDH network.
+ VC3
PTR
+ VC2
PTR
+
VC12
PTR
+ VC11
PTR
VC-11 is alternative to VC-12
1 - PRELIMINARY INFORMATION
POH
+
POH
+
POH
+
POH
+
C3
C2
C12
C11
1-19