Synchronous Transport Signal. STS is a term that describes a SONET data transfer rate. For example, STS-1 is 51.84 Mbps, STS-3 is 155.52 Mbps, STS-12 is 622.08 Mbps, and STS-48 at 2.488 Gbps. The STS designation refers to the interface for electrical signals. The optical signal standards are correspondingly designated OC-1, OC-2, etc.