The basic unit of framing in SDH is a STM-1 (Synchronous Transport Module, level 1), which operates at 155.520 megabits per second (Mbit/s). SONET refers to this basic unit as an STS-3c (Synchronous Transport Signal 3, concatenated). When the STS-3c is carried over OC-3, it is often colloquially referred to as OC-3c, but this is not an official designation within the SONET standard as there is no physical layer (i.e. optical) difference between an STS-3c and 3 STS-1s carried within an OC-3.