The Mean Holding Time for SDCCH is 3 to 4 sec. SDCCH HO is SDCCH to SDCCH HO during this time. It’s because of the radio conditions.
Directed Retry and Forced Directed Retry is SDCCH ( cell A ) to TCH ( cell B ) because of TCH ( cell A ) blocking. You are right – it’s powerful mechanism to fight blocking with.