This section provides detailed information on the locations and functions of various overhead bytes for each of the following SDH path and line sections:
The following table identifies the location and function of each regenerator section overhead byte.
|
Bytes |
Function |
|---|---|
|
A1, A2 |
Frame alignment A1 = 1111 0110 ; A2 = 0010 1000 ; These fixed-value bytes are used for synchronization. |
|
B1 |
BIP-8 parity test Regenerator section error monitoring; BIP-8 : Computed over all bits of the previous frame after scrambling; B1 is placed into the SOH before scrambling; BIP-X: (Bit Interleaved Parity X bits) Even parity, X-bit code; first bit of code = even parity over first bit of all X-bit sequences; |
|
B2 |
Multiplex section error monitoring; BIP-24 : B2 is computed over all bits of the previous STM-1 frame except for row 1 to 3 of the SOH (RSOH); B2 is computed after and placed before scrambling; |
|
Z0 |
Spare bytes |
|
D1 - D3 (= DCCR) D4 - D12 (= DCCM) |
Data Communication Channel (network management information exchange) |
|
E1 |
Orderwire channel |
|
E2 |
Orderwire channel |
|
F1 |
User channel |
|
K1, K2 |
Automatic protection switch |
|
K2 |
MS-AIS/RDI indicator |
|
S1 |
Synchronization Status Message |
|
M1 |
REI (Remote Error Indication) byte |
|
NU |
National Usage |
The Path Overhead (POH) is generated for all plesiochronous tributary signals in accordance with ITU-T Rec. G.709. The POH provides for integrity of communication between the point of assembly of a Virtual Container VC and its point of disassembly. The following table shows the POH bytes and their functions.
|
Byte |
Location and Function |
|---|---|
|
J1 |
Path Trace Identifier byte |
|
B3 |
Path Bit Interleaved Parity (BIP-8) Provides each path performance monitoring. This byte is calculated over all bits of the previous payload before scrambling. |
|
C2 |
Signal Label All "0" means unequipped; other and "00000001" means equipped |
|
G1 |
Path Status Conveys the STM-1 path terminating status, performance, and remote defect indication (RDI) signal conditions back to an originating path terminating equipment. |
|
F2, F3 |
User Data Channel Reserved for user communication. |
|
H4 |
Multiframe Indicator Provides a general multiframe indicator for VC-structured payloads. |
|
K3 |
VC Trail protection. |
|
N1 |
Tandem connection OH |
The AU pointer together with the last 261 columns of the STM-1 frame forms an AUG (Administrative Unit Group). An AUG may contain one AU-4 or three byte-multiplexed AU-3s (an AU-3 is exactly one third of the size of an AU-4). AU-3s are also compatible with the SONET standard (Synchronous Optical NETwork) which is the predecessor of SDH (and still the prevailing technology within the USA). Three byte-multiplexed STS frames (SONET frame), each containing one AU-3 can be mapped into one STM-1.
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