Container position in the SDH structure
An SDH container, denoted C-n, is a fixed-capacity field intended to carry a tributary signal. The value of n identifies the hierarchy level. It is not a sequential container number and does not state how many channels the container holds.
A tributary can be a PDH stream such as E1, E3, or E4. The container defines the space available for that stream. A separate mapping procedure specifies the positions assigned to client bits and clock-adaptation information.
Container and virtual container
A container provides the field for client information. Adding Path Overhead creates a Virtual Container VC-n. POH carries information used to supervise the path between the points where the VC-n is created and terminated.
High-order containers receive high-order path overhead. Low-order containers receive their own path overhead before being grouped into a parent structure. This arrangement allows a particular path to be monitored independently of the other signals carried in the same STM-N frame.
Low-order containers
Low-order containers serve tributaries whose rates are below the capacity of C-3 or C-4. After the corresponding VC-n is formed, these structures are combined through tributary-unit groups.
- C-11 is associated with the DS1 signal and is used mainly in systems derived from the North American digital hierarchy.
- C-12 carries the E1 signal. It is the principal low-order container in SDH systems derived from European PDH.
- C-2 is defined for a tributary. It is less common in practical configurations than C-11 and C-12.
High-order containers
- C-3 occupies 9 rows by 84 columns, giving 756 bytes in the recurring frame structure. It can carry an E3 signal at or a DS3 signal at . Those clients use different mapping variants.
- C-4 occupies 9 rows by 260 columns, giving 2340 bytes. Its capacity supports an E4 signal at. Adding one POH column creates a VC-4 that is 261 columns wide.
Selecting a container
Container selection follows the nominal rate and format of the client signal. The C-n capacity exceeds the client bit rate because positions are also needed for timing adaptation. Container utilization therefore cannot be calculated only as the ratio between the client rate and the STM-N line rate.
are separate stages. A container describes payload capacity, while the TU, TUG, AU, and AUG hierarchy describes how virtual containers are carried in the transport frame.