@Hiteshwar: What you have written is right but in LTE as you know it is self optimized network. So the thing is UE has to send the measurements reports of serving cell as well as for neighbor cells, but when you are connected to one EARFCN how can you calculate the neighbor measurements. Therefore in LTE it is using as measurement gaps to do these measurements.
Measurement Gaps are periods where UE switches off its Rx and TX from the serving Cell and starts listening to other. This GAP period has to be in sync with the eNB because the eNB should know when the UE will enter the GAP state. To configure the GAP period in UE, eNB uses measGapConfig IE which is included in RRCConnectionReconfiguration message.
UE will use the Discontinuous reception for measurements. In this perios eNodeB will not grant any UL data and will not send any downlink data.
@Bheemesh: how it is useful that this is the only reason. DTx and DRx are measurement gaps used for the these reporting and all. Before LTE the purpose was only one and now it has more reasons.
and T321 timer is for UE to get the SIB1 information from neighbor cell and report it. It is also possible that if UE is not getting SIB info within timer expires eNodeB can relase the DRx mode.