Analogous to the layering models seen in packet networks [Stallings03], the TDM optical networks can also be layered according to the functions performed. Figure 2-17 shows layer diagrams of SONET and SDH, with the overhead usage and functionality illustrated. The VT path (lower-order VC) and the STS path (higher-order VC) level signals were previously described as being the envelopes containing the end-to-end user information. The physical layer is used to map the SONET frames into standardized optical signals. Each layer transports information from the layer immediately above it transparently, that is, without changing the content while adding some type of transport networking functionality.