Page 82 - Optical Switching And Networking Handbook

04_200023_CH03/Batesx 1/17/01 9:42 AM Page 67

SONET 67

Inside the STS-1 Frame

The SPE can begin anywhere inside the STS-1 envelope. Normally,
it begins in one STS-1 frame and ends there. However, it may begin
in one STS-1 frame and end in another. The STS payload pointer
contained inside the transport overhead points to the beginning byte
of the SPE. The possibilities, then, are that the frame can carry a
locked payload or a floating payload. Floating in this regard refers to
the payload floating between two frames. The overhead associated
with SONET is designed to let the receiver know where to look for its
payload and extract the information at a starting point. A floating
frame is shown in Figure 3-10.

Figure 3-10
A floating payload
inside two frames
P
O
H

P
O
H

Transport
O/H