The Y.1564 Ethernet service availability definition is based on a model which uses two states to represent the ability or inability of the network to sustain the service in the available state. The transitions between the states are based on the occurrence of severely errored seconds (SES) in the Ethernet layer.

A severely errored second occurs for a block of frames observed during a one-second interval when the ratio of lost frames to the total frames in the block exceeds a certain pre-defined value. A provisional value of 0.5 was proposed by ITU-T in 2009, but Netrounds lets the user set any preferred value.

The Ethernet network is regarded to be in an unavailable state after 10 consecutive SES outcomes, with the time period for those first 10 consecutive SES outcomes being part of the period of unavailability. The network is regarded to be in an available state after 10 consecutive non-SES outcomes, with the time period for those 10 consecutive non-SES outcomes being part of the period of availability. For an illustration, see the picture from the ITU-T Recommendation Y.1563 below.

Netrounds will show the total number of UAS, the number of seconds that the network has been in the unavailable state during the current monitoring session. The number of SES can also be seen in the table view.
SES-UAS
Image source: http://www.itu.int/rec/T-REC-Y.1563-200901-I/en