If the UE detects the change in cell identity, it performs a cell update procedure.

The UE is addressed by the C-RNTI(cell radio network temporary identifier) from the RNC that controls the cell which the UE is in.

is

one of states in RRC connected mode.

The UE is assigned to use common channels.

This state is triggered as either part of RRC connection request or some RB reconfiguration process.

The UE can move to any of other connected states as commanded by the network.

*Idle mode: CN-level

UE is identified with TMSI (temporary mobile subscriber identity)

*RRC connected mode: UTRAN-level

UE is identified with C-RNTI and U-RNTI

Cell_FACH and Cell_PCH: every cell change needs to be reported via cell update

URA_PCH: every URA change needs to be reported via cell update

(less signal overhead from UE perspective, but more paging messages need to be injected into multiple cells)

in idle mode, UE sends RRC connection establishment

in Cell_PCH and URA_PCH, the UE moves to Cell_FACH and sends Cell Update message with cause “paging response”

Thus, in such case, DCH channel needs to be  de-allocated and the UE should use RACH/FACH channel.

On the other hand, when the buffer of UE excceeds a certain amount of data the DCH channel needs to be allocated.

These changes between Cell_DCH and Cell_FACH can be done via Transport Channel Type Switching messages.