Differences between SONET and SDH

The hierarchy of the standard SDH corresponds to that of SONET for ATM interfaces.
The hierarchy of flows is different on three continents (USA, EC, Japan) is a rate of 51.84 Mbps, which was chosen for form the first level of SONET: STS-1 (Synchronous Transport Signal, level 1). The other levels are called SONET OC-n (Optical Carrier level n). So there is just a gap between the levels of SDH and SONET: Level 1 SDH (155.52 Mb / s) is level 3 and level SONET 2 of SDH (622.08 Mb / s) is the level 12 of SONET.