Frame number identification and ciphering activation time synchronization for a wireless communications protocol

A wireless communications system comprising:

• a first station capable of receiving encrypted layer 2 protocol data units (PDUs), and capable of receiving a security mode command, the first station comprising:
• a receiving buffer for storing received PDUs, the first station associating a sequentially ordered n-bit frame number (FN) and an m-bit hyper frame number (HFN) with each received PDU;
• an extraction unit for obtaining an application time from a switching FN and x least significant bits (LSBs) of a second HFN, the switching FN and the x LSBs of the second HFN contained in the security mode command;
• a first ciphering key;
• a second ciphering key;
• and
• a decryption unit for decrypting the received PDUs, the decryption unit using the first ciphering key to decrypt any received PDU with an HFN/FN pair that is sequentially before the application time, and using the second ciphering key to decrypt any received PDU with an HFN/FN pair that is sequentially on or after the application time.

A data structure for use in a wireless communications system to synchronize a ciphering key change in the wireless communications system, the wireless communications system comprising:

• a first station capable of receiving encrypted layer 2 protocol data units (PDUs), assigning a first n-bit frame number (FN) and a first m-bit hyper frame number (HFN) to each received PDU to generate a first HFN/FN pair for each received PDU, and decrypting each received PDU according to the first HFN/FN pair, an application time, a first ciphering key, and a second ciphering key, the first station using the first ciphering key if the first HFN/FN pair is sequentially before the application time, and using the second ciphering key if the first HFN/FN pair is sequentially on or after the application time;
• and
• a second station capable of transmitting the encrypted PDUs, the second station assigning a second n-bit FN and a second m-bit HFN to each PDU to be transmitted to generate a second HFN/FN pair for each PDU to be transmitted, and encrypting each PDU to be transmitted according to the second HFN/FN pair, an activation time, a third ciphering key, and a fourth ciphering key, the second station using the third ciphering key if the second HFN/FN pair is sequentially before the activation time, and using the fourth ciphering key if the second HFN/FN pair is sequentially on or after the activation time;
• the data structure comprising:
• x least significant bits (LSBs) of a second HFN corresponding to the activation time;
• and
• a switching FN corresponding to the activation time;
• wherein the second station composes the data structure and transmits the data structure to the first station to enable the first station to synchronize the application time with the activation time.

A method for removing cyclical ambiguity of an n-bit identifying frame number (FN) transmitted in a signaling message from a first station to a second station in a wireless communications system, the identifying FN identifying a layer 2 protocol data unit (PDU) in a stream of PDUs, the first station comprising a first m-bit hyper frame number (HFN) that is incremented by a first value upon detection of roll-over of an FN in the stream of PDUs, each PDU in the stream of PDUs having an associated FN value and each FN value having an associated HFN value, the method comprising:

• the first station placing the identifying FN into a first field of a message;
• the first station placing x least significant bits (LSBs) from the HFN value associated with the identifying FN in a second field of the message;
• and
• the first station transmitting the message to the second station;
• wherein after reception of the message, the second station uses the x LSBs of the second field to determine a cyclical position of the identifying FN.