Traditionally, telecommunications networks have always been described using a model of layers in order to facilitate their design, implementation and management. On the one hand, the protocols and information exchange units are described within each layer, while on the other hand, the interfaces between the different layers are defined.

This same model, adapted for transmission, is used by SDH to describe its architecture. Each layer communicates with its counterpart at the other end of the transmission by making use of specific overheads (OH). As in any communications system, the overheads allow us to manage the information carried, detect errors, determine the type of payload and follow up the path established.

In the SDH hierarchy, four layers are defined:

• Two lower layers relating to the network transmission medium. These are the Regeneration Section (RS) and the Multiplex Section (MS). The associated overheads are called RSOH (Regeneration Section Overhead) and MSOH (Multiplex Section Overhead) respectively.
• Two upper layers relating to the paths of the tributary transported. These are the Higher Order Path (HOP) and the Lower Order Path (LOP), and the associated overheads are called HO-POH (Higher Order Path Overhead) and LO-POH (Lower Order Path Overhead) respectively.

This way, SDH segregates its functions into different transport layers, each providing a series of services to the next layer up. In other words, SDH adopts the classic client-server model of division by layers and functions associated to each layer defined.

The physical framework in which the layers are implemented is the collection of telecommunications equipment or network elements. Each element reaches a certain height in the tower of layers and they exchange perfectly standardized units of information with each other.