RNTI(Radio Network Temporary Identities) are used as UE identities within the UTRAN and in signalling messages between UE and UTRAN/EUTRAN.
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”.
RNTI in UTRAN/UMTS
1) SRNC RNTI (S-RNTI) 2) Drift RNC RNTI(D-RNTI)
3 Cell RNTI(C-RNTI) 4) UTRAN RNTI(U-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 )
U-RNTI 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.
RNTI in EUTRAN/LTE
1) Paging RNTI (P-RNTI) 2) Random Access RNTI(RA-RNTI)
3 Cell RNTI(C-RNTI) 4) System Information RNTI(SI-RNTI)
P-RNTI identifies a group of mobiles in a paging message and derived from the IMSI of the user
RA-RNTI assigned in response to a random access preamble and UE will use it to initiate the RRC
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. It can be temp, semi persistent or permanent
SI-RNTI sent on PDCCH to specify the location of System Information Blocks on the PD-SCH.