Synchronous Digital Hierarchy (SDH) is a standardized technology used in optical communications to transmit digital signals over long distances with high reliability and efficiency. A SONET SDH SFP module is a compact optical transceiver designed specifically for equipment that operates on these synchronous transport standards. At low transmission rates, data can also be. This tutorial provides an overview of SDH/SONET, covering basics, HDLC framing, terminologies, rates, and the SONET STS-1 SDH Frame.