I heard that there is no much difference between idle and Cell_PCH from UE perspective.
In idle, UE is located within registration area.
In Cell_PCH, UE’s location is known in cell level.
Then, what about URA_PCH?
In URA_PCH, UE’s location is known in registration area level.
Then, is it true that there is no difference between idle and URA_PCH ?
It is said that the idle mode is characterized by the absense of an RRC connection.
That means in Cell_PCH and URA_PCH modes, UE still has RRC connection.
Recall that in idle, Cell_PCH, and URA_PCH, UE does not have any radio resources allocated and therefore it cannot start any type of user data transfer in the dedicated and common channels.
The difference among idle and other connected modes (Cell_PCH, URA_PCH) is happening when UE tries to talk to the network.
In idle, UE simply sends RRC connection Request via RACH.
In Cell_PCH and URA_PCH, UE moves to Cell_FACH sends some messages such as Cell Update message (probably not the RRC connection Request since it already has RRC connection).
So, UE doesn’t have to establish RRC connection and Signaling connection again.
Thus, UE can save some time to get service from the network in Cell_PCH and URA_PCH. (needs verification)
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Also, consider this from FACH perspective.
In idle mode, CCCH is mapped to FACH since there is no RRC connection.
However, in connected mode, DCCH can be mapped to FACH since there is RRC connection.
This complies the definition of CCCH and DCCH.
