The control plane establishes virtual connections and handles signaling traffic. The management plane is divided into two parts: plane management and layer management. The management plane provides alarm surveillance, connectivity verification, and verification of VPI/VCI. Operations, Administration and Management (OAM) cells are used to exchange alarm and connectivyt information between nodes. If an invalid VPI/VCI is received, the cell is discarded and layer management is informed.

OAM messages
The OAM cell uses a different format than other cells. There are two types of cells supported, F4 and F5. Figure 9.2 shows both formats.

There are two types of indications given through OAM alarming: Alarm Indication Signal (AIS) and Far End Receive Failure (FERF). Both types provide a failure type and failure location in the "function-specific" field of the OAM cell.

The AIS message is used to alert downstream nodes of an alarm condiction. Either an VPC/VCC failure or physical layer failure can cause AIS message generation. When a switch at the public UNI receives an AIS message, it returns an FERF message to alert the downstream nodes of the failure downstream. A virtual path AIS and virtual path FERF are always carried on VCI = 4. A virtual channel AIS and virtual channel FERF are always carried over cells with PT = 101.

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Copied with permission,McGraw-Hill Telecommunications from the book Telecommunications Protocols, by Travis Russell, 1997 McGraw-Hill Telecommunications, pages 372-386.