Access Class Barring or ACB selectively limits the load offered by the public (not priority) UEs to the network under overload conditions. ACB is a highly configurable feature, but is generally configured such that it engages only during extreme overloads that traffic filtering cannot effectively process, it filters public traffic only, and the barring process more heavily affects data traffic vs. voice.
ACB is performed on cell basis in LTE and its parameters are broadcasted in SIB2. During RRC connection establishment UE applies ACB based on following -
1. one or more Access Class (AC) for which UE belongs
2. AC barring information shared on SIB2
3. Establishment cause
For a UE with high priority ACs network can allow selectively and in case of emergency call it is up to the SIB2 if or not ACB is applied. For mobile terminating call ACB is not applied.