Fast Dormancy (FD) is a wireless technology feature introduced to improve battery life on the mobile devices.
Fast dormancy is a way by which UE informs the network to release the RRC connection. (Termination of RRC connection is always triggered by network)
UE consumes more battery when in connected state, so when there is no data to transfer UE informs to network to release its RRC connection to save the battery power.
However with bursty data traffic nature UE's may end up sending moreRRC connection request and SCRI(signalling connection release indications) to network.
From Rel-8 3GPP inroduced a timer T323 whose value is brodcasted in SIB1.
If network supports this rel-8 fast dormancy feature UE should not trigger fast dormancy request until the T323 timer expiry.