Page 105 - Optical Switching And Networking Handbook

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90 Chapter 4

As the data are prepared for placement in the frame, several dif-
ferent modes of transport are available. At the input, the data flow
into a container with a size designation. For example, C11 is the
equivalent of a T-1 transport mechanism at 1.544 Mbps. C12 will
carry the E-1 transport mechanism at 2.048 Mbps. Another trans-
port is C2, which is the equivalent of a DS-2 operating at 6.312
Mbps.
Each of these container levels is then input into a virtual con-
tainer (VC level n). In Figure 4-3, the inputs flow into the basic con-
tainer and then are mapped into the VC. This may sound very
similar to the SONET virtual tributary (VT). SDH defines a number
of containers, each corresponding to an existing plesiochronous rate.
The information from a PDH signal is mapped into the relevant con-
tainer. This is done in a manner similarly to the bit-stuffing proce-
dure used in conventional PDH multiplexers. Each container has
some overhead and control information contained in it, called path
overhead (POH). The bytes associated with the POH enable the car-

Figure 4-3
The VCs are TU 2
created.
C2
VC2

T2

TU 12
C12
VC12
E1

TU 11
C11
VC11

T1