RNTI(Radio Network Temporary Identities) are used as UE identities within the UTRAN and in signalling messages between UE and UTRAN.RNTI is used , so that UE original identity can be hide , so that no one can detect the UE , but as a identity UE can also send IMSI or IMEI , in which we try to avoid sending the IMEI number to the network, except in the case , when UE does not have the SIM or it is not properly camped to a cell and user wants to make the Emergency call.These RNTI are,SRNC RNTI (S-RNTI)Drift RNC RNTI(D-RNTI)Cell RNTI(C-RNTI)UTRAN RNTI(U-RNTI)Now let us explore briefly all these RNTI,S-RNTI is allocated in association with RRC connection setup by the SRNC to which the UE has the RRC connection, so that both can identify each other.D-RNTI is allocated by a DRNC in association with context establishment and is used to handle the UE connection and context over the Iur interface.C-RNTI is allocated when the UE accesses a new cell , so that we can say that it identifies the UE when UE is in common channels ( when UE is in cell FACH), i.e. with in a cell. By this CRNC and UE communicate each other.(in most cases CRNC and SRNC are incorporated in a single RNC )But, now the most important RNTI comes into the picture, and its the URNTI, it identifies the UE , uniquely within UTRAN. URNTI is used as a UE identifier for the first cell access , and it generated by the combination of SRNC identity and SRNTI. It is an unique one in UTRAN.Apart from them , we also have E-RNTI and H-RNTI , used in HSUPA and HSDPA respectively, which also have the same use as the above ones.we will explore all these more in a later post.