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A SONET Overview
A-8 Issue 1 December 1997
Path Overhead 1
There are two types of path overheads:
— STS-1 path overhead
— VT path overhead.
STS-1 Path Overhead 1
The STS-1 path overhead is assigned to and remains with the STS-1 SPE until
the payload is extracted and is used for functions that are necessary to transport
all synchronous payload envelopes.
■ STS-1 Path Trace (J1)
— Repetitively transmits a 64 byte, fixed length, string so that an STS-1
path receiving terminal can verify its continued connection to the
intended transmitter.
■ STS-1 Path Bit-Interleaved Parity (BIP-8) (B3)
— Provides each STS-1 path performance monitoring. This byte is
calculated over all bits of the previous STS-1 SPE before
scrambling.
■ STS-1 Path Signal Labels (C2)
— Indicates the construction of the STS-1 SPE. A value of 00000000
indicates an unequipped STS-1 SPE. Values for various payload
mappings are defined in TR-NWT-000253, Issue 2.
■ STS-1 Path Status (G1)
— Conveys the STS-1 path terminating status, far end block errors
(FEBE), and yellow alarm signal conditions back to an originating
STS-1 path terminating equipment.
■ STS-1 Path User Channel (F2, F3)
— User communication channel between Path elements.
■ VT Multiframe Indicator (H4)
— Provides a general multiframe indicator for VT-structured payloads.
■ STS-1 Path Automatic Protection Switching (K3)
— Path Automatic Protection Switching
■ TCM - Tandem Connection Maintenance (N1)
— Bits 1-4 used for incoming error monitoring. Bits 5-8 used as
communications channel.
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