The nodes of the SDH network can be formed by multiplexers, add/drop multiplexers( ADM) or digital crossconnect( DXC). Between each node are the different transport sections, which are named according to the nodes that bound them. Thus, the section between two regenerators( REG) is called the regenerator section, and the section between two multiplexers is called the multiplexer section. The transmission section between the network nodes is the path.
In SDH networks, optical transmission technology is predominantly used. Due to the distances that can be bridged, only single- mode fibers are used, operating at 1,310 nm and 1,550 nm. In addition, SDH transmission links can also be implemented via directional radio or satellite links, but with limited transmission speeds.
In Recommendations G.957 and G.691, the International Telecommunication Union( ITU) has specified five distance classes for optical interfaces, the distances of which depend on the optical windows and the optical fib ers. The distance ranges are between 2 km and 60 km at 1,300 nm and between 25 km and 160 km at 1,550 nm. The different distance classes are known as Short Haul (S), Long Haul (L), Very Long Haul (V) and Ultra Long Haul (H).