SDH Overhead Bytes | Teleinf Edu

SDH frame overhead and payload

An STM-N frame contains a payload area and an Overhead (OH) area. The payload carries client information. The OH bytes carry framing, trace, parity, protection, synchronization, maintenance, and management information used by SDH network elements.

G.707 identifies three main regions in the STM-N frame: Section Overhead (SOH), the Administrative Unit pointer row, and the information payload. The SOH serves section functions, while Path Overhead (POH) belongs to a Virtual Container and follows its path through the transport network.

Path Overhead and Section Overhead

SDH assigns overhead to a transport scope. Path Overhead (POH) belongs to one Virtual Container and is examined at the path endpoints. Section Overhead (SOH) belongs to the STM-N transmission section and is processed at the relevant section boundaries.

Path Overhead (POH)
POH is associated with a specific . It travels from the path source to the path sink with that VC. Path termination functions read fields such as J1, B3, C2, and G1 to check trace, signal type, status, and end-to-end errors.
Section Overhead (SOH)
SOH is associated with the STM-N signal on one transmission section. In an STM-1, rows 1 to 3 and 5 to 9 of the first nine columns form SOH, while row 4 carries the AU pointer. RSOH is handled at regenerator section boundaries and MSOH at multiplex section boundaries, so these fields are replaced or processed at the appropriate termination points.

Section Overhead (SOH) in STM-1

In an STM-1 frame, SOH occupies rows 1 to 3 and 5 to 9 of the first nine columns. Rows 1 to 3 are the Regenerator Section Overhead (RSOH). Rows 5 to 9 are the Multiplex Section Overhead (MSOH). Row 4 is reserved for Administrative Unit pointers.

1. Regenerator Section Overhead (RSOH): rows 1 to 3

RSOH is processed at each regenerator section endpoint, including a regenerator. It covers frame alignment, section trace, section error monitoring, orderwire, user, and DCCR functions for one span between adjacent network elements.

A1, A2 (frame alignment): A1 contains $11110110$ and A2 contains $00101000$ , which gives $0xF628$ when the pair is written in hexadecimal. The receiver uses this fixed word to locate the STM-N frame boundary.
J0 (regenerator section trace): J0 repeatedly transmits a section access-point identifier. The standardized form uses a 16-byte trace frame whose first byte contains a CRC-7 result and whose remaining bytes carry T.50 characters. This lets the section receiver check that it remains connected to the intended transmitter.
B1 (BIP-8): The transmitter calculates even-parity BIP-8 over all bits of the previous STM-N frame after scrambling. It places the result in B1 of the current frame before scrambling. The receiving endpoint compares its own calculation to detect errors on this regenerator section.
E1 (orderwire): E1 is an optional 64 kbit/s voice channel in RSOH. A regenerator can access it for maintenance communication along the section.
F1 (user channel): F1 is reserved for user purposes, such as a temporary data or voice connection used during maintenance.
D1 to D3 (DCCR): These bytes provide a 192 kbit/s regenerator-section Data Communications Channel for management messages to equipment that can access RSOH.

2. Multiplex Section Overhead (MSOH): rows 5 to 9

MSOH is processed at multiplex section endpoints, such as terminal multiplexers and Add-Drop Multiplexers (ADMs). It monitors the STM-N signal across the multiplex section and is terminated and regenerated when a new multiplex section begins. The AU pointer in row 4 is described separately from MSOH.

AU pointer (H1, H2, H3 in row 4): The pointer occupies the row between RSOH and MSOH. It identifies the position of the VC-4 or another high-order payload relative to the STM-N frame.
B2 (BIP-N×24): B2 uses even parity over all bits of the previous STM-N frame except its first three SOH rows. The B2 bytes in the current frame report multiplex-section error blocks. STM-1 carries BIP-24 and STM-4 carries BIP-96.
K1, K2 (automatic protection switching): K1 and K2 bits 1 to 5 carry the APS protocol for multiplex section protection. K2 bits 6 to 8 carry the multiplex section RDI code.
D4 to D12 (DCCM): These bytes provide a 576 kbit/s multiplex-section Data Communications Channel for management messages. A regenerator cannot access this channel because it belongs to MSOH.
S1 (synchronization status message): Bits 5 to 8 of S1 carry a four-bit code for the synchronization quality of the transmitted signal. The code also supports the states quality unknown and do not use for synchronization.
M1 (multiplex section REI): M1 reports the number or presence of BIP violations detected at the far end of the multiplex section. The near end can use this remote information with its local B2 count.
E2 (orderwire): E2 is an optional 64 kbit/s voice channel in MSOH. It is accessible at multiplex section terminations.

Path Overhead (POH) in a VC-4

POH is part of the Virtual Container and follows it from the path source to the path sink. G.707 defines nine bytes for the VC-4, VC-3, and VC-4-Xc high-order path overhead. Their functions cover trace, error monitoring, signal labeling, path status, multiframe information, and tandem connection monitoring.

VC-4 POH bytes

J1 (path trace): J1 repeatedly carries a path access-point identifier. The receiving path termination uses it to verify that the VC-4 remains connected to the intended transmitter.
B3 (BIP-8): B3 carries an even-parity result calculated over all bits of the previous VC-4. The path endpoint compares the result to detect errored blocks along the high-order path.
C2 (signal label): C2 identifies the composition or maintenance status of the VC. Codes distinguish, for example, a TUG structure, asynchronous E3 or E4 mapping, ATM mapping, and GFP mapping.
G1 (path status): G1 carries path status fields, including the remote error and remote defect indications. The far end uses these fields to report the result of its B3 and defect processing toward the source.
H4 (multiframe indicator): H4 carries payload-specific position information when a VC contains a multiframe structure, such as lower-order tributary units carrying E1 signals.
F2, F3, and K3 (user and path-level functions): These fields carry functions assigned by the applicable path structure. Their exact use depends on the transported payload and the relevant G.707 definition.
N1 (tandem connection monitoring): N1 supports monitoring of a selected segment within a larger path. It can be overwritten within an operator domain without changing the end-to-end B3 function.

J1 and B3 support end-to-end path identification and error monitoring, C2 identifies the payload or maintenance status, and G1 reports path status toward the source. H4 supplies payload-dependent multiframe position information. Path termination interprets these fields at the VC boundary, while section functions can process SOH without terminating the POH.

SOH fields are terminated or regenerated at their defined section boundaries. POH fields remain with the Virtual Container, so a section failure can be localized without replacing the path identity carried by J1.