OAM Connectivity Verification
OAM payload consists of loopback indication, a correlation identifier, loopback location identifier, and source identifier. Loopback indication indicates that the cell should either be looped back or discarded. If looped back, the value is decremented (to prevent the originator from interpreting it as a loopback from another node and sending it back).

Correlation ID allows the originating node to keep track of OAM responses when OAM cells are sent over the same virtual connetion. This value is used only by the originator of an OAM cell and is ignored by other nodes.

Loopback location ID is a 96-bit field identifying where the loopback should occur within a virtual connection. A value of all 1s indicates that the loopback should occur at the remote endpoint. Source ID allows the originator to identify the loopback instruction as one it has sent. No values are defined, and the standard allows any kind of identification.

Segment loopbacks take place between the subscriber equipment and public ATM switch on either end of the UNI. In other words, the segment is defined as the link between these two points. The loopback message never travels beyond these points.

End-to-end loopbacks can be generated by any node in the connection but can only be discarded or looped back by endpoints (defined by the VPC or VCC). These messages are identified as:

• Payload type = 101 for VCC (end to end)
• VCI = 4 for VPC (end to end)
• Payload type = 100 for VCC (segment)
• VCI = 3 for VPC (segment)

Loopbacks are used to ensure that a logical connection can be established. In analog facilities, a connection must be established over the facility before any connection requests can be made with the remote end. This is when analog facilities are tested for continuity. In digital facilities, there is no way of testing a connection prior to connection establishment because there is no physical connection, only virtual connections. Signaling is often sent over different channels. Loopbacks over digital facilities are sometimes called continuity checks.

NEXT PAGE: ATM Planes (con't)

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