A method of implementing security in a wireless communication device (108) comprises receiving (300), at the device (108), a security mode command for activating a security mode in the device and storing a sequence number of the received security mode command. A security mode complete or failure message is sent (302) based on whether a security mode is activated in the device. An acknowledgement of the security mode complete or failure message is received (304) and a timestamp of the acknowledgement is stored. On receiving a PDU, sequence numbers and timestamps of segments of the received PDU are compared (306) with the stored sequence number and timestamp of the acknowledgement. The received PDU segments are managed (308) in response to the comparisons, and the sending of the security mode complete or security mode failure message. A wireless communication device is also disclosed.