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The overhead carries information that provides OAM&P (Operations, Administration, Maintenance, and Provisioning) capabilities such as framing, multiplexing, status, trace, and performance monitoring.

There are 4 Types of Overhead (2 types of TOH: SOH and LOH and 2 types of POH: STS POH and VT POH):

Section Overhead

Section overhead contains 9 bytes of the transport overhead accessed, generated, and processed by section-terminating equipment. This overhead supports functions such as the following:

This might be two regenerators, line-terminating equipment and a regenerator, or two sets of line-terminating equipment. The section overhead is found in the first three rows of columns 1 to 9

SOH - Section OverHead
Byte	Description
A1 and A2	framing bytes—These two bytes indicate the beginning of an STS–1 frame. Their value is constant (0xF628 - or Hex F628), so that the receiving station knows where the start of the frame is. NOTE: this is the start of the FRAME – not the SPS !!! The start of the SPE is revealed by the offset bytes, H1 and H2.
J0	section trace (J0)/section growth (Z0)—The byte in each of the N STS–1s in an STS–N that was formally defined as the STS–1 ID (C1) byte has been refined either as the section trace byte (in the first STS–1 of the STS–N), or as a section growth byte (in the second through Nth STS–1s).
B1	section bit-interleaved parity code (BIP–8) byte—This is a parity code (even parity), used to check for transmission errors over a regenerator section. Its value is calculated over all bits of the previous STS–N frame after scrambling then placed in the B1 byte of STS–1 before scrambling. Therefore, this byte is defined only for STS–1 number 1 of an STS–N signal.
E1	section orderwire byte—This byte is allocated to be used as a local orderwire channel for voice communication between regenerators, hubs, and remote terminal locations.
F1	section user channel byte—This byte is set aside for the users' purposes. It terminates at all section-terminating equipment within a line. It can be read and written to at each section-terminating equipment in that line.
D1, D2, and D3	section data communications channel (DCC) bytes—Together, these 3 bytes form a 192–kbps message channel providing a message-based channel for OAM&P between pieces of section-terminating equipment. The channel is used from a central location for alarms, control, monitoring, administration, and other communication needs. It is available for internally generated, externally generated, or manufacturer-specific messages.

LOH - Line OverHead
Byte	Description
H1 and H2	STS payload pointer (H1 and H2)—Two bytes are allocated to a pointer that indicates the offset in bytes between the pointer and the first byte of the STS SPE. The pointer bytes are used in all STS–1s within an STS–N to align the STS–1 transport overhead in the STS–N and to perform frequency justification. These bytes are also used to indicate concatenation and to detect STS path alarm indication signals (AIS–P).
H3	pointer action byte (H3)— The pointer action byte is allocated for SPE frequency justification purposes. The H3 byte is used in all STS–1s within an STS–N to carry the extra SPE byte in the event of a negative pointer adjustment. The value contained in this byte when it is not used to carry the SPE byte is undefined.
B2	line bit-interleaved parity code (BIP–8) byte—This parity code byte is used to determine if a transmission error has occurred over a line. It is even parity and is calculated over all bits of the line overhead and STS–1 SPE of the previous STS–1 frame before scrambling. The value is placed in the B2 byte of the line overhead before scrambling. This byte is provided in all STS–1 signals in an STS–N signal.
K1 and K2	automatic protection switching (APS channel) bytes—These 2 bytes are used for protection signaling between line-terminating entities for bidirectional automatic protection switching and for detecting alarm indication signal (AIS–L) and remote defect indication (RDI) signals.

D4 to D12	line data communications channel (DCC) bytes—These 9 bytes form a 576–kbps message channel from a central location for OAM&P information (alarms, control, maintenance, remote provisioning, monitoring, administration, and other communication needs) between line entities. They are available for internally generated, externally generated, and manufacturer-specific messages. A protocol analyzer is required to access the line–DCC information.
S1	synchronization status (S1)—The S1 byte is located in the first STS–1 of an STS–N, and bits 5 through 8 of that byte are allocated to convey the synchronization status of the network element.
Z1	growth (Z1)—The Z1 byte is located in the second through Nth STS–1s of an STS–N (3 <= N <= 48) and are allocated for future growth. Note that an OC–1 or STS–1 electrical signal does not contain a Z1 byte.
M0	STS–1 REI–L (M0)—The M0 byte is only defined for STS–1 in an OC–1 or STS–1 electrical signal. Bits 5 through 8 are allocated for a line remote error indication function (REI–L, formerly referred to as line FEBE), which conveys the error count detected by an LTE (using the line BIP–8 code) back to its peer LTE.
M1	STS–N REI–L (M1)—The M1 byte is located in the third STS–1 (in order of appearance in the byte-interleaved STS–N electrical or OC–N signal) in an STS–N (N >= 3) and is used for a REI–L function.
Z2	growth (Z2)—The Z2 byte is located in the first and second STS–1s of an STS–3 and the first, second, and fourth through Nth STS–1s of an STS–N (12 <= N <= 48). These bytes are allocated for future growth. Note that an OC–1 or STS–1 electrical signal does not contain a Z2 byte.
E2	orderwire byte—This orderwire byte provides a 64–kbps channel between line entities for an express orderwire. It is a voice channel for use by technicians and will be ignored as it passes through the regenerators.