DR i.e Direct retry is a feature in Nokia System.
With DR enabled if SD is allocated & all the TCH are busy then TCH is allocated from a different cell(neighboring cell).
As far as cell loading parameters is concerned there is a very good feature in Nokia Sys i.e TRHO
(Traffic Reason Handover) with this paramter enabled u can offload ur highly congested cells bt be aware of TGT value else there will be ping-pong handoveres.