This attribute indicates the net change in the available link station
connections since the last Availability Message Packet (AMP) was
sent to the Hunt Group. Once the absolute value of this attribute
reaches the availabilityUpdateThreshold an AMP is sent to the host
and the availabilityDelta is reset to 0.
If this attribute is positive it means an increase of the number of
available link station connections. If it is negative it means a
decrease in the number of available link station connections.