SONET Frame Format

SONET  uses  a basic  transmission  rate of STS -1 that  is equivalent to 51.84 mbps. Higher level  signals  are integer multiples of the base rate. For  example STS 3 is  three times the rate  of STS -1 ( 3 x 51.84 = 155.52) an STS 12 rate would  be 152 x 51.84= 622.08 mbps. SONET  is based on  the STS - 1 frame. STS- 1  frame format  is show.

The  synchronous payload envelope can also  be divided into two  parts the STS  path  overhead and the payload. Transport overhead  is composed of section overhead and line overhead. The STS -1  POH is part of the synchronous data  envelope. The  first  three Colum  of each STS-1 frame make up the  transport to overhead, and the last 87  columns make up SPE. SPEs  can have any  alignment within  the frame and this alignment  is indicated by  the H1 and H2  pointer bytes in the line overhead.