Complete Frame is discarded when CRC check fails over Class 1a 50 bits.So this is FER while BER is calculated over complete 378 bits of speech block as each 26 multifram contain 6 speech blocks and there are 4 26 multiframes ao there are total of 4*6*378 bits being tranmitted over 1 complete SACCH Multifram and these are used for BER calcualtion and BER is simply rate of erroneous bits out of these total 4*6*378 bits.
So hopefully this explains the BER and FER and their corelation.