“SDH differs from PDH in that the exact rates that are used to transport the data are tightly synchronized across the entire network, made possible by atomic clocks. This synchronization system allows entire inter-country networks to operate synchronously, greatly reducing the amount of buffering required between each element in the network.”
SDH = SONET
http://en.wikipedia.org/wiki/Synchronous_optical_networking#External_links
PDH
http://en.wikipedia.org/wiki/PDH
Overview (a bit old…)
http://www.gare.co.uk/technology_watch/sdh.htm