A section can be defined as the part of the transmission media limited by two successive network elements. A section between two regenerators is known as a Regeneration Section (RS). This layer is managed using the RSOH (Regeneration Section Overhead) bytes of the STM-N frame with which they detect and give warnings of errors and alarms.

It must be made clear that every SDH network element has a regenerator as part of its hardware, although the actual name of Regenerator (REG) is only given to those devices that solely perform this function. For this reason, a regeneration section can also be the section between a multiplexer of any kind (or a DXC) and a regenerator. Or between two multiplexers, when no intermediate regenerators are required.