There are four types of RNTI used to handle UTRAN's internal mobility
1. SRNC RNTI (S-RNTI)
S-RNTI is allocated at the same time as RRC connection setup. This is carried out by the SRNC with which the UE has an RRC connection.
2. Drift RNTI (D-RNTI)
D-RNTI is allocated by drift RNC at the same time as context establishment. It is used to handle UE connection and context over the Iur interface
3. Cell RNTI (C-RNTI)
C-RNTI is allocated when the UE access a new cell, is a CRNC-specific identifier that is used to identify the CRNC with which the UE has an RRC connection. By this means the UE can identify itself to the CRNC and also the CRNC can reach the UE. This identifier is used when the UE is in CELL_FACH state.
4. UTRAN RNTI (U-RNTI)
U-RNTI is, on the other hand, allocated to a UE having a RRC connection and identifies the UE within the UTRAN. It is used as a UE identifier for initial cell access (at cell change) when an RRC connection exists for this UE and for UTRAN-originated paging (including associated response messages).
U-RNTI is a concatenation of the SRNC identity and an S-RNTI.