frame error rate (FER) has almost the same meaning as BER...but....the error rate calculation is between frame and not bit.....in other words, let say, you transmit 100 frames ....and out of them....only 3 frames having errors.......so......the FER is 3/100 ........
Pair-wise Error Probability is one of the term for union bound evaluation....have a look on error coding text book and you will find it.....
While BER has its theorectical importance, we can in no way measure this value in real world (if we know the transmit bits at the receiver, we in fact do not need to transmit at all).
When designing a code, the exact BER criteria might not be tractable. Therefore, PER is used instead.
In real system, we do not have access to BER or PER, but only FER through CRC.