Page 112 - Optical Switching And Networking Handbook

05_200023_CH04/Batesx 1/17/01 8:18 AM Page 97

Synchronous Digital Hierarchy 97

ITU G.709 defines different combinations of VCs that can fill the
payload of an STM-1 frame. The process of data filling in the con-
tainers and the addition of overhead is repeated many times at dif-
ferent levels in the SDH. This results in the nesting of smaller VCs
within larger ones. The nesting process is repeated until the largest
bundle (VC4) is filled.After this, the filled VC4 is loaded into the pay-
load of the STM-1 frame, as shown in Figure 4-11. The pointer pro-
cessing is different here in the TSM-1 frame than in an STS-3 frame
in SONET.
After the payload envelope of the STM-1 frame, additional control
information is added to the frame to form the section overhead. This
overhead stays with the payload as it propagates along the fiber
route between two multiplexers. The reasons this overhead is used
include the following:

It aids in the provisioning for operations, administration, and
maintenance (OA&M).
It creates user channels for diagnostics.

Figure 4-11 RSOH
The data fill based 27 Bytes
on G.709
AU-4 Pointer
9 Bytes

MSOH
45 Bytes

VC-4
POH