SONET Restoral

for 2-fiber UPSR (Uni-directional Path-switched Rings)

 and 4-fiber BLSR (Bi-directional Line-Switched Rings)

SONET fiber is built using rings, so that service can be restored if a line is cut.  The K1 and K2 bytes carry information that tells protection switches when a line is down, so that they can immediately restore service.