One of the things that SDH can cope with is incoming bit streams that are not synchronised with the SDH clock.
It can do this through the use of bit stuffing - when an incoming bit stream arrives and is fed into the containers, and during the multiplexing, empty bits are added to the bit stream, so that the relative timing of the incoming bit stream and the SDH containers are brought into line. To accomodate these extra bits, the containers are designed with a larger capacity than that required to accomodate the incoming bit stream. So the incoming bit stream can "float" around in time within the containers.