Editing Synchronous optical networking (section)

==Basic transmission unit==
The basic unit of framing in SDH is a [[STM-1]] (Synchronous Transport Module, level 1), which operates at 155.520 [[Mbit/s|megabits per second]] (Mbit/s). SONET refers to this basic unit as an STS-3c (Synchronous Transport Signal 3, concatenated). When the STS-3c is carried over OC-3, it is often colloquially referred to as [[OC-3]]c, but this is not an official designation within the SONET standard as there is no physical layer (i.e. optical) difference between an STS-3c and 3 STS-1s carried within an OC-3.

SONET offers an additional basic unit of transmission, the STS-1 (Synchronous Transport Signal 1) or [[Optical Carrier transmission rates#OC-1|OC-1]], operating at 51.84 Mbit/s—exactly one third of an STM-1/STS-3c/OC-3c carrier. This speed is dictated by the bandwidth requirements for PCM-encoded telephonic voice signals: at this rate, an STS-1/OC-1 circuit can carry the bandwidth equivalent of a standard [[DS-3]] channel, which can carry 672 64-kbit/s voice channels.<ref name="td-hdbk"/> In SONET, the STS-3c signal is composed of three multiplexed STS-1 signals; the STS-3c may be carried on an OC-3 signal. Some manufacturers also support the SDH equivalent of the STS-1/OC-1, known as STM-0.

===Framing===
In packet-oriented data transmission, such as [[Ethernet]], a packet frame usually consists of a [[header (computing)|header]] and a [[payload (computing)|payload]]. The header is transmitted first, followed by the payload (and possibly a [[trailer (computing)|trailer]], such as a [[Cyclic redundancy check|CRC]]). In synchronous optical networking, this is modified slightly. The header is termed the ''overhead'', and instead of being transmitted before the payload, is interleaved with it during transmission. Part of the overhead is transmitted, then part of the payload, then the next part of the overhead, then the next part of the payload, until the entire frame has been transmitted.

In the case of an STS-1, the frame is 810 [[Octet (computing)|octets]] in size, while the STM-1/STS-3c frame is 2,430 octets in size. For STS-1, the frame is transmitted as three octets of overhead, followed by 87 octets of payload. This is repeated nine times, until 810 octets have been transmitted, taking 125&nbsp;[[microsecond|μs]]. In the case of an STS-3c/STM-1, which operates three times faster than an STS-1, nine octets of overhead are transmitted, followed by 261 octets of payload. This is also repeated nine times until 2,430 octets have been transmitted, also taking 125&nbsp;[[microsecond|μs]]. For both SONET and SDH, this is often represented by displaying the frame graphically: as a block of 90 columns and nine rows for STS-1, and 270 columns and nine rows for STM1/STS-3c. This representation aligns all the overhead columns, so the overhead appears as a contiguous block, as does the payload.

The internal structure of the overhead and payload within the frame differs slightly between SONET and SDH, and different terms are used in the standards to describe these structures. Their standards are extremely similar in implementation, making it easy to interoperate between SDH and SONET at any given bandwidth.

In practice, the terms STS-1 and OC-1 are sometimes used interchangeably, though the OC designation refers to the signal in its optical form. It is therefore incorrect to say that an OC-3 contains 3 OC-1s: an OC-3 can be said to contain 3 STS-1s.

===SDH frame===
[[File:SDH STM1 Frame.svg|thumb|upright=1.7 |An STM-1 frame. The first nine columns contain the overhead and the pointers. For the sake of simplicity, the frame is shown as a rectangular structure of 270 columns and nine rows but the protocol does not transmit the bytes in this order.]]

[[File:Stm 1.jpg|thumb|upright=1.7 |For the sake of simplicity, the frame is shown as a rectangular structure of 270 columns and nine rows. The first three rows and nine columns contain regenerator section overhead (RSOH) and the last five rows and nine columns contain multiplex section overhead (MSOH). The fourth row from the top contains pointers.]]
The Synchronous Transport Module, level 1 (STM-1) frame is the basic transmission format for SDH—the first level of the synchronous digital hierarchy. The STM-1 frame is transmitted in exactly 125&nbsp;[[microsecond|μs]], therefore, there are 8,000 frames per second on a 155.52&nbsp;Mbit/s OC-3 fiber-optic circuit.<ref group=nb>2,430 octets per frame × 8 bits per octet × 8,000 frames per second = 155.52&nbsp;Mbit/s</ref> The STM-1 frame consists of overhead and pointers plus information payload. The first nine columns of each frame make up the section overhead and administrative unit pointers, and the last 261 columns make up the information payload. The pointers (H1, H2, H3 bytes) identify administrative units (AU) within the information payload. Thus, an OC-3 circuit can carry 150.336&nbsp;Mbit/s of payload, after accounting for the overhead.<ref group=nb>2,349 octets of payload per frame × 8 bits per octet × 8,000 frames per second = 150.336&nbsp;Mbit/s</ref>

Carried within the information payload, which has its own frame structure of nine rows and 261 columns, are administrative units identified by pointers. Also within the administrative unit are one or more virtual containers (VCs). VCs contain path overhead and VC payload. The first column is for path overhead; it is followed by the payload container, which can itself carry other containers. Administrative units can have any phase alignment within the STM frame, and this alignment is indicated by the pointer in row four.

The section overhead (SOH) of a STM-1 signal is divided into two parts: the ''regenerator section overhead'' (RSOH) and the ''multiplex section overhead'' (MSOH). The overheads contain information from the transmission system itself, which is used for a wide range of management functions, such as monitoring transmission quality, detecting failures, managing alarms, [[data communication]] channels, service channels, etc.

The STM frame is continuous and is transmitted in a serial fashion: byte-by-byte, row-by-row.

====Transport overhead====
The transport overhead is used for signaling and measuring transmission [[bit error rate|error rates]], and is composed as follows:

:;Section overhead: Called regenerator section overhead (RSOH) in SDH terminology: 27 octets containing information about the frame structure required by the terminal equipment.
:;Line overhead: Called multiplex section overhead (MSOH) in SDH: 45 octets containing information about error correction and Automatic Protection Switching messages (e.g., alarms and maintenance messages) as may be required within the network. The error correction is included for STM-16 and above.<ref name="FECOptical"/>
:;Administrative unit (AU) pointer: Points to the location of the J1 byte in the payload (the first byte in the virtual container).<ref name="cisco-sdh"/>

====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.