Path virtual envelope

Payload overhead (POH): 9 octets used for end to end signaling and error measurement. Payload: user data (774 bytes for STM-0/STS-1, or 2340 octets for STM-1/STS-3c).For STS-1, the payload is referred to as the synchronous payload envelope (SPE), which in turn has 18 stuffing bytes, leading to the STS-1 payload capacity of 756 bytes.

The STS-1 payload is designed to carry a full PDH DS3 frame. When the DS3 enters a SONET network, path overhead is added, and that SONET network element (NE) is said to be a path generator and terminator. The SONET NE is said to be line terminating if it processes the line overhead. Note that wherever the line or path is terminated, the section is terminated also. SONET regenerators terminate the section but not the paths or line.

An STS-1 payload can also be subdivided into 7 VTGs (virtual tributary groups). Each VTG can then be subdivided into 4 VT1.5 signals, each of which can carry a PDH DS1 signal. A VTG may instead be subdivided into 3 VT2 signals, each of which can carry a PDH E1 signal. The SDH equivalent of a VTG is a TUG2; VT1.5 is equivalent to VC11, and VT2 is equivalent to VC12.

Three STS-1 signals may be multiplexed by time-division multiplexing to form the next level of the SONET hierarchy, the OC-3 (STS-3), running at 155.52 Mbps. The multiplexing is performed by interleaving the bytes of the three STS-1 frames to form the STS-3 frame, containing 2,430 bytes and transmitted in 125 microseconds.Higher speed circuits are formed by successively aggregating multiples of slower circuits, their speed always being immediately apparent from their designation. For example, four STS-3 or AU4 signals can be aggregated to form a 622.08 Mbps signal designated as OC-12 or STM-4.

The highest rate that is commonly deployed is the OC-192 or STM-64 circuit, which operates at rate of just under 10 Gbps. Speeds beyond 10 Gbps are technically viable and are under evaluation. [Few vendors are offering STM-256 rates now, with speeds of nearly 40Gbps]. Where fiber exhaustion is a concern, multiple SONET signals can be transported over multiple wavelengths over a single fiber pair by means of wavelength-division multiplexing, including dense wavelength division multiplexing (DWDM) and coarse wavelength-division multiplexing (CWDM). DWDM circuits are the basis for all modern transatlantic cable systems and other long-haul circuits.