Abstract: A received PDU is sequentially identified by an n-bit frame number (FN) and an m-bit hyper frame number (HFN), which are synchronously maintained on first and second stations. The second station determines an activation time at which a ciphering key change is to occur, and composes a security mode command that includes an identifying FN corresponding to the activation time, and x least-significant bits (LSBs) from the HFN of the identifying FN. The second station transmits the security mode command to the first station. The x LSBs contained in the security mode command enable the first station to resolve cyclical ambiguities of the identifying FN to properly construct an application time. The first station uses a first ciphering key to decrypt PDUs with FNs sequentially prior to the application time, and uses a second ciphering key to decrypt PDUs with FNs sequentially on or after the application time.

Abstract: Within a move receiving window (MRW) super-field there is an initial sequence number that is arranged within the MRW super-field according to a set algorithm. The MRW super-field is transmitted to a receiver, and the receiver extracts the initial sequence number. The receiver then assumes that all other layer 2 PDU sequence numbers within the MRW super-field are sequentially after the initial sequence number to effect discarding of layer 2 SDUs. The initial sequence number is assumed to be either sequentially on or after a starting point sequence number, or sequentially before an end point sequence number of a receiving window of the receiver, depending on whether a special case value exists for the length field of the MRW super-field. The special case value indicates that a single SDU that extends beyond a transmitting window of the transmitter is to be discarded.