DR is used when a MS is performing a call setup in a cell which has no more available TCH (= congestion case).
During the call setup, since the cell cannot grant a TCH to this MS, the MS is put in a queue. The MS is still connected, on a SDCCH.
During this queueing time (T11):
– if a TCH is made available in the serving cell, then the MS will get it and resume the call setup.
– if the BSC sees that the MS is experiencing “handover” needs (“emergency” or “better conditions”), and if DR is enabled in the cell, then it will attempt a “normal directed retry” : the MS will do a “SDCCH to TCH” handover, from the serving cell to the target cell.
– if the BSC detects the HO cause “forced directed retry” is fulfilled and the “FDR” is enabled in the cell, then the MS performs a HO from SDCCH to TCH, from serving cell to target cell.
So there you are… that’s the whole explanation 🙂