A read-only value that reports the -on time- for a grant
burst in increments of Time Quanta (TQ) 16ns as defined
in [802.3ah] 60,64. The value returned shall be (on time
ns)/16. If this value exceeds (2^32-1) the value
(2^32-1) shall be returned.
This attribute is relevant for an OLT and an ONU.