Overhead

From the 810 byte frame, overhead is enabled in several ways to handle the Operations, Administration, Maintenance, and Provisioning services (OAM&P).

The first part of the overhead is defined as the transport overhead. The transport overhead uses the first three columns and all nine rows. This creates 27 bytes of transport overhead. As shown in Figure 29-2, the transport overhead is divided into two pieces. The first three columns and the first three rows (9 bytes) are used for section overhead. The remaining six rows in the first three columns (18 bytes) are used for line overhead.

Figure 29-2: The transport overhead is divided into section and line overhead. The remaining 87 columns and nine rows (783 bytes) are designated as the Synchronous Payload Envelope (SPE). Inside the SPE, an additional one column, nine rows high (9 bytes) is set aside for Path Overhead (POH). This is shown in Figure 29-3. After the POH is set aside, the resulting payload is 774 bytes. In these 774 bytes, the services are then mapped into the frame. The STS-1 payload can carry the following:

Figure 29-2: The transport overhead is divided into section and line overhead. The remaining 87 columns and nine rows (783 bytes) are designated as the Synchronous Payload Envelope (SPE). Inside the SPE, an additional one column, nine rows high (9 bytes) is set aside for Path Overhead (POH). This is shown in Figure 29-3. After the POH is set aside, the resulting payload is 774 bytes. In these 774 bytes, the services are then mapped into the frame. The STS-1 payload can carry the following:

Figure 29-3: The SPE shown

Combinations of the previous payloads are also allowable. Two columns are reserved as fixed stuff columns; these are columns 30 and 59. The remaining 756 bytes carry the actual payload.

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