In SDH, each protocol data unit (PDU) is pictured as a two-dimensional array of bytes with a specified number of rows and columns. The smallest PDU that concerned us during the design of our SDH framer was a Container-4 (C-4). A container of size 4 is a 2D array structure with 260 columns and 9 rows that carries the payload of the SDH frame. Thus, the C-4 can accommodate up to 2340 bytes.
For information on why we chose this size container for our SDH Framer, please see Justifications for C-4 Size.