 "The Packet Number (PN) associated with an SA is set to 1 when the SA is established and when a new TEK is installed. After each MAC PDU transmission, the PN is incremented by 1. On UL connections, the PN is XORed with <00000000 prior to encryption and transmission. On DL connections, the PN is used without such modification, which results in splitting of the PN space such that 0x00000001 0x7FFFFFFF values are used for the DL and 0x80000001 FFFFFFFF values are utilized in the UL, preventing a PN collision between the UL and the DL [1]. 8.4 Security association management Initial or Re-Authentication Key Agreement Message 1 (NONCE_BS) Create NONCE_MS Derive PMK, AK, CMAC keys Key Agreement Message 2 (MSID*,NONCE_BS,NONCE_MS)(CMAC) Derive PMK, AK, CMAC Keys CMAC Validation Key Agreement Message 3 (NONCE_BS,NONCE_MS)(CMAC) CMAC Validation Derive TEKs Derive TEKs FIGURE 8-3 Initial or reauthentication key derivation and exchange [2] The MS can request a key update when the packet number space of the TEKUL is exhausted, or the TEKUL is being used for downlink traffic, as well. For reauthentication, after key agreement, the AKold would be valid and only one new TEK would be derived by using AKnew; the TEKs are updated immediately after key agreement. The MS is then resyncronized with the new TEK or another new TEK is derived from AKnew [2]. 8.4 SECURITY ASSOCIATION MANAGEMENT The security association (SA) is defined as the set of information required for secure communication between the BS and MS. The SA is identified using an SA Identifier (SAID). The SA is applied to the service flows once it has been established. The IEEE 802.16m only supports unicast static SA, i.e., the SA is used to provide keying context to unicast transport connections. The SA is applied to all data exchanged over the connection with the MS. Multiple connections may be mapped to the same unicast SA. However, the SA is not equally applied to all the management messages over the same management connection. Depending on the extended header indica