In synchronous transfer mode (STM), the Regenerator Section Overhead (RSOH) is a data field that precedes the pointer in the STM data frame. The first six bytes of this RSOH data field (A1, A2) are used for frame synchronization of the STM-1 data frame, followed by the sender or repeater identifier.
In addition, the RSOH has a parity byte for error detection, service channels for telephony, a payload channel for general information, and signaling for network component monitoring. The broader multiplexer sections are handled in the layer above by the multiplex section overhead (MSOH). In this layer, the plesiochronous and synchronous signals are combined into higher frequency SDH bit streams and signals are coupled in and out. This is where the virtual containers (VC) are formed.user data are transported and here the mapping of the tributary units (TU) of PDH or ATM signals via the 140-Mbit/s signal of VC-4 or of voice signals via 2-Mbit/s signal of VC-12 takes place.