It is an overflow counter mechanism used in the eNB and UE in order to limit the actual number of sequence number bits that is needed to be sent over the radio. The HFN needs to be synchronized between the UE and eNB, i.e. when the PDCP SN has reach its maximum value (which in turn depends on number of bits used for PDCP SN (5, 7 or 12 bits), PDCP SN will be restarted from 0 AND HFN will be increased by one. COUNT (32 bits) is made up of PDCP SN together with HFN, and is used in the security process.