How ciphering is done for different modes of RLC and Why TM mode ciphering is pushed to MAC layer?
Contributed by Senthil Kumar The ciphering function in UMTS is present in MAC or RLC in the UE and UTRAN The parameters to the ciphering algorithm includes, a counter called COUNT-C, the ciphering Key CK, the RB id and the direction(uplink or downlink). The UM and AM RLC mode ciphering uses the RLC sequence number(SN) which is in the header,since it keeps on changing for every RLC PDU. COUNT-C is a 32 bit counter derived from RLC Hyper Frame Number(HFN) RLC UM COUNT-C = RLC HFN(25 bits) + RLC SN(7 bits) RLC AM COUNT-C = RLC HFN(20 bits) + RLC SN(12 bits) The HFN is incremented once the RLC SN wraps around. Since for TM RLC, header is not present and hence there is no SN to be used as variable changing parameter. So the TM RLC ciphering is pushed to MAC layer where the CFN is used as a variable changing parameter. RLC TM COUNT-C = MAC HFN(25 bits) + CFN(7 bits) The HFN is incremented when the CFN wraps around. For all the three modes of RLC, the HFN value is initialized to a START value
