The MS performs the call setup in the serving cell. During the SDCCH phase, after the MS is authentified, it is put in a waiting queue during as long as T11, at maximum.
Before expiry of T11, the BSC tries to find a TCH in a neighbour cell. The Ms is still waiting on its SDCCH.
If neighbour is well received by the MS : FDR is triggered.
If neighbour cell is well received by another MS in dedicated mode in the serving cell : FTH is triggered for that MS, then our MS can grab its old TCH.
If MS in waiting queue needs to do HO ’emergency or better cell): DR is triggered.