A count of APS Protection Switching Byte failures. A
Protection Switching Byte defect occurs when either an
inconsistent APS byte or an invalid code is detected. An
inconsistent APS byte occurs when no three consecutive K1 bytes
of the last 12 successive frames are identical, starting with
the last frame containing a previously consistent byte. An
invalid code occurs when the incoming K1 byte contains an unused
code, or a code irrelevant for the specific switching operation
(e.g., Reverse Request while no switching request is
outstanding) in three consecutive frames. An invalid code also
occurs when the incoming K1 byte contains an invalid channel
number in three consecutive frames.