hi krish,
SD blocking is when the BSC is not able to allocate a SDCCH channel to the MS. In other words, it is when the BSC doesn’t send an “immediate assignment” message back to the MS.
Since the MS sends the RACH twice only, instead of three times, then there are only 2 blocking counted, instead of 3.
The best feature to reduce congestion is to use the Immediate Assignment Reject, and set the WI_OC to > 2s.
Cheers,
Pix