There are 3 layers that control the transport functions
in the SONET frame. The first layer is the Section layer that moves
data across the media (OSI - 1). The second layer is the Line layer
which maps the payload into the electrical STS signal to send
to the peer terminating network elements (OSI - 2). The third layer
is the Path layer which maps sevices into the preliminary payload prior
to STS framing. (OSI - 3).