The first three columns of the frame are the transport overhead, which is divided into the section overhead composing the first three rows, and the line overhead composing rows four through nine. The fourth column contains the path overhead, as shown in Figure 1.

STS-1 Synchronous Payload Envelope (SPE)

The payload is defined as the actual data to be transported across the SONET path. Payloads can take many forms, such as typical T-carrier channels (such as DS3), or virtual tributaries (VTs) of various sizes. The SPE is defined as 783 bytes and is shown as 87 columns $\times 9$ rows. The first column of the SPE is the path overhead and columns 30 and 59 are not used for payload, but designated as the fixed stuff columns, which leaves 84 columns $\times 9$ rows (or 756 bytes) for payload.

At a rate 8000 frames per second that works out to be at a rate of 51.84 Mbps at STS-1 data rate as shown below,

9 X (90 bytes/frame) X (8 bits/byte) X 8000 frame

= 51.84 Mbps date rate of STS-1

Section Overhead: The section overhead contains nine bytes that are accessed, generated, and processed by STE- such as a regenerator. This overhead supports functions of

• Performance monitoring
• Framing
• Messaging communication between STEs for control, monitoring, administration, and other communication needs
• Voice communication between STE

Line Overhead: The line overhead contains 18 bytes that are accessed, generated, and processed by LTE-such as multiplexers. This overhead supports functions of

• Locating the SPE in the frame
• Multiplexing or concatenating signals
• Performance monitoring
• Automatic protection switching
• Line maintenance

Path Overhead: The path overhead contains nine bytes, providing communication between the creation point of the SPE and the point where it is dissembled. This overhead supports functions of

• Performance monitoring of the SPE
• Path signal label, which indicates the content of the SPE
• Path status, which conveys status and performance back to the originating terminal
• Path trace, which allows verification of continued connection with the originating terminal