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.