Synchronous Digital Hierarchy (SDH)
SDH is the international standard for synchronous digital transmission, which replaced the earlier, less flexible PDH.
What is Synchronous Digital Hierarchy (SDH)?
SDH's base rate STM-1 is 155 Mbit/s. PDH multiplexed E1 (2 Mbit/s) circuits upwards but with separate clocks. Both are being replaced by packet networks.