Differences in SONET/SDH Standards for K Bytes

SONET and SDH standards are slightly different with respect to the behavior of K1 and K2 Bytes.

Table 1 shows the differences between the two standards.

Table 1. Differences Between SONET and SDH 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