EXFO FTB-8510B Guia Del Usuario página 1275

Glossary
SONET - Line Overhead (LOH)
K1 and K2: Automatic Protection Switching (APS)
The K1 and K2 bytes communicate APS between two LTE. This byte is only
defined for the first STS-1 frame of a composite signal.
D4 - D12: Data Communications Channel (DCC)
The D4 through D12 bytes provides a 576 Kbps data communications
channel between two LTEs for administration, monitoring and other
communications. These bytes are only defined for the first STS-1 frame of a
composite signal.
S1: Synchronization Status
The S1 byte is used to carry the synchronization status of the SONET
device. This byte is only defined for the first STS-1 frame of a composite
signal (STS-1 of an electrical or OC-N signal).
Z1: Growth
The Z1 byte is allocated for future growth. This byte is located in the second
STS-1 through STS-n frame of a composite signal (STS-1 #2, STS-1 #3, up to
STS-1 #N of a OC-N (N>3) signal).
M0: STS-1 REI-L
Bits 5 through 8 of M0 byte are used for line Remote Error Indication
(REI-L). Bits 1 through 4 are currently undefined. The M0 byte is defined
only for the STS-1 in an STS-1 electrical signal.
M1: STS-n REI-L
The M1 byte is used for line Remote Error Indication (REI-L). This byte is
located in the third STS-1 of an STS-n signal (n > 3).
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SONET/SDH Application