Editing Synchronous optical networking (section)

====Path virtual envelope====
Data transmitted from end to end is referred to as ''path data''. It is composed of two components:
:;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 2,430 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.<ref>{{cite web
 |url= http://www.iec.org/online/tutorials/sonet/topic03.html
 |archive-url= https://web.archive.org/web/20080407212239/http://www.iec.org/online/tutorials/sonet/topic03.html
 |archive-date= 7 April 2008
 |title= Synchronous Optical Network (SONET)
 |work= Web ProForums
 |publisher= International Engineering Consortium
 |access-date= 21 April 2007
 |year= 2007
}}</ref>

The STS-1 payload is designed to carry a full PDH [[T-carrier|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 ''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 seven ''virtual tributary groups'' (VTGs). Each VTG can then be subdivided into four [[VT1.5]] signals, each of which can carry a PDH [[Digital Signal 1|DS1]] signal. A VTG may instead be subdivided into three [[VT2]] signals, each of which can carry a PDH [[E-carrier level 1|E1]] signal. The SDH equivalent of a VTG is a TUG-2; VT1.5 is equivalent to VC-11, and VT2 is equivalent to VC-12.

Three STS-1 signals may be [[Multiplexing|multiplexed]] by [[time-division multiplexing]] to form the next level of the SONET hierarchy, the [[OC-3]] (STS-3), running at 155.52&nbsp;Mbit/s. The signal is multiplexed by interleaving the bytes of the three STS-1 frames to form the STS-3 frame, containing 2,430 bytes and transmitted in 125&nbsp;[[microsecond|μs]].

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&nbsp;Mbit/s signal designated [[OC-12]] or [[STM-4]].

The highest rate commonly deployed is the [[OC-768]] or [[STM-256]] circuit, which operates at rate of just under 38.5&nbsp;Gbit/s.<ref name="oc768"/> Where fiber exhaustion is a concern, multiple SONET signals can be transported over multiple wavelengths on 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 [[submarine communications cable]] systems and other long-haul circuits.