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Short for Synchronous Digital Hierarchy, an international standard for synchronous data transmission over fiber optic cables. The North American equivalent of SDH is SONET.

SDH defines a standard rate of transmission at 155.52 Mbps, which is referred to as STS-3 at the electrical level and STM-1 for SDH. STM-1 is equivalent to SONET's Optical Carrier (OC) levels -3.

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