363-206-285
A SONET Overview
Issue 3 June 2001
A-13
Two key points should be noted at this time. First, the SONET frame is a fixed
time (125 µs) and no bit-stuffing is used. Second, as shown in Figure A-10, the
synchronous payload envelope can
float
within the frame using byte-stuffing. This
is to permit compensation for small variations in frequency between the clocks of
the two systems that may occur if the systems are independently timed
(plesiochronous timing). The SPE can also drift across the 125-µsframe
boundary. SONET STS pointers are used to locate the SPE relative to the
transport overhead.
Figure A-10. STS-1 Synchronous Payload Envelope in Interior of STS-1 Frame
125 s
STS-1 Synchronous Payload Envelope
125 s
Pointer
info
Transport Overhead
STS-1 POH
Pointer
info
Start of STS-1 SPE
STS-1 Frame Format
(SPE can start
at any byte boundary)
STS-1 SPE
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