NAS COUNT is one of the essential parameter, which is being used by NAS layer for ciphering and integrity protection as well as for verification. NAS COUNT is constructed by NAS Sequence Number and NAS overflow counter. NAS Sequence Number is 8 bits long string and NAS overflow counter is 16 bits long.
There are two separates NAS COUNT maintained at the NAS layer. One of uplink and otherone is used for downlink.
Bearer Id, direction and NAS Count is being used for ciphering and integrity.