363-206-285
A SONET Overview
Issue 3 June 2001
A-7
Line Overhead 1
â– Line Pointer (H1, H2)
— Two bytes indicate the offset in bytes between the pointer action
byte (H3) and the first byte (J1) of the STS-1 synchronous payload
envelope (SPE).
â– Pointer Action (H3)
— One byte is allocated for frequency justification.
â– Line Bit-Interleaved Parity (BIP-8) (B2)
— This byte is for line performance monitoring. This byte is provided in
all STS-1 signals within an STS-
N
signal.
â– Line Automatic Protection Switching (APS) (K1, K2)
— Two bytes used for APS signaling between line level entities. In
addition, bits 6, 7, and 8 of K2 are used for line alarm indication
signal (AIS) and line far-end receive failure (FERF). Defined only for
STS-1#1ofanSTS-
N
signal.
â– Line Data Communications Channel (D4 - D12)
— Is a 576 kb/s message-based channel.
â– Synch. Status (S1)
— In STS-1 #1, the S1 byte is for synchronization status messages,
and only bits 5 through 8 are used.
â– Line REI (M0)
— The M0 byte is for STS-1 line far-end block error (FEBE), and only
bits 5 through 8 are used.
â– Line Orderwire (E2). Defined only for STS-1 #1 of an STS-
N
signal.
— One byte is allocated to be used as an express orderwire between
line terminating equipment.
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