SDH (Synchronous Digital Hierarchy) and SONET (Synchronous Optical NETwork) are standards (European and U.S., respectively) for using optical media as the physical transport for high speed long haul networks. They support data rates from 51.84 Mbps to 13.22 Gbps, with 155 Mbps (STM-1 or OC-3) and 622 Mbps (STM-4 or OC-12) the most common rates. Bi-directional ring topology provides redundancy in case a link fails.
Quality of Service (QoS) capabilities and aggregation of various rates make it the preferred choice for metropolitan access.
