It is part of SPE and contain followings information: Performance monitor of synchronous transport , signal , path , track ,parity ,checks, and path status.
Figure Path Overhead
a.Path trace byte : The JI bytes in consecutive frames form a 64 kbps channel used for tracking the path . the JI byte send a continuous 64 bits string to verify the connection. The choice of the string is left to the application programs.
b.Path parity byte : Byte B3 is for bit interleaved parity like byte B1 and B2. But calculated for the path header.
c.Path signal label byte > Byte C2 is the path identification byte. It is used to identify different protocols used at higher levels .
d.Path status byte > Byte G1 is sent by the receiver to communicate its status to the sender.
e.Path user channel byte : The byte 2 bytes in consecutive frames like the F1 bytes form a 64 kbps channel is reserved for user needs,, but at the path level.
f.Virtual tributary indicate byte H4 is the multi frame indicator. It indicates payloads that cannot fit in single frame. Virtual tributaries are discussed in the next section.