SDH networks, or synchronous optical networks (SONET), currently allow radio link systems with capacities in synchronous transfer mode (STM) from 51 Mbit/s (STM-0) to 622 Mbit/s (STM-4). International Recommendations and Standards define the error performance and availability objectives for SDH networks. These objectives are media independent, and still have to be met whenever radio forms a part of the network.