Container : VCn
The virtual container (VC) is the unit of transport between the input and output points for the tributaries, whether they are plesiochronous signals, ATM signals or signals from metropolitan Ethernet networks. A basic SDH frame has a capacity of 155 Mbit/s, which means that it can either transport one VC-4 virtual container (designed for plesiochronous signals at 140 Mbit/s) or three VC-3 virtual containers (designed to transport plesiochronous signals at 34 Mbit/s or 45 Mbit/s).
Points to be noted:
VC-n: Virtual Container, it is made up of a C-n and a POH
POH: Path OverHead
The POH is an additional transport-capacity designed for the container:
it carries details on e.g. the payload contents.
VC-n = C-n + POH n = 11, 12, 2, 3, 4