The STS-1 frame.
STS-1 synchronous payload envelope (SPE).
As illustrated in Figure 19.1, the first three columns of the STS-1 frame contain the
Transport Overhead. These three columns have 27 octets (i.e., 9X3), of which nine are
used for the section overhead and 18 octets contain the line overhead. The remaining 87
columns make up the STS-1 Envelope Capacity as illustrated in Figure 19.2.
The STS-1 synchronous payload envelope (SPE) occupies the STS-1 envelope capacity.
The STS-1 SPE consists of 783 octets and is depicted as an 87-column by 9-row structure.
In that structure, column 1 contains 9 octets and is designated as the STS path overhead
(POH). In the SPE columns 30 and 59 are not used for payload but are designated as
fixed-stuff columns. The 756 octets in the remaining 84 columns are used for the actual
STS-1 payload capacity.
Figure 19.3 shows the fixed-stuff columns 30 and 59 inside the SPE. The reference
document (Ref. 1) states that the octets in these fixed-stuff columns are undefined and
are set to binary 0s. However, the values used to stuff these columns of each STS-1
SPE will produce even parity in the calculation of the STS-1 Path BIP-8 (BIP
The STS-1 SPE may begin anywhere in the STS-1 envelope capacity. Typically the
SPE begins in one STS-1 frame and ends in the next. This is illustrated in Figure 19.4.
However, on occasion the SPE may be wholly contained in one frame. The STS payload
pointer resides in the transport overhead. It designates the location of the next octet where
the SPE begins. Payload pointers are described in the following paragraphs.
19.2 SONET 491 Figure 19.1 The STS-1 frame. The STS-1 synchronous payload envelope (SPE) occupies the STS-1 envelope capacity. However, the values used to stuff these columns of each STS-1 SPE will produce even parity in the calculation of the STS-1 Path BIP-8 (BIP = path interleaved parity).