The fundamental signal in SONET is the STS-1, which operates at a rate of about 51 Mbps. The fundamental signal for SDH is STM-1, which operates at a rate of about 155 Mbps (three times the STS-1 rate). The signals are made of contiguous frames that consist of two parts: the transport overhead (TOH) contained in the header, and the payload. For synchronization purposes, the data can be allowed to shift inside the payload inside a Synchronous Payload Envelope (SPE) for SONET and inside the Virtual Container for SDH. The SPE inside the payload is referenced using a pointer. Figures A-1 and A-2 show the SONET and SDH frames.