SONET and SDH standards are slightly different with respect to the behavior of K1 and K2 Bytes.
Table: Differences between SONET and SDH standards shows the differences between the two standards.
| SONET | SDH | Comments | |
|---|---|---|---|
SONET/SDH standards use different codes in the transmitted K1 byte (bits 1-4) to notify the far-end of a signal fail/signal degrade detection. |
1100 for signal fail 1010 for signal degrade 1101 unused 1011 unused |
1101 for signal fail 1011 for signal degrade 1100 unused 1010 unused |
None |
SONET systems signal the switching mode in bits 5-8 of the K2 byte whereas SDH systems do not signal at all. |
101 for bi-dir 100 for uni-dir |
Not used. 000 is signaled in bits 5 to 8 of K2 byte for both bidirectional as well as unidirectional switching |
SONET systems raise a mode mismatch alarm as soon as a mismatch in the TX and RX K2 byte (bits 5 to 8) is detected. SDH systems do not raise the mode mismatch alarm |