Why RA_RNTI and T_CRNTI are different ?

Question

As i am seeing in Logs, RA-RNTI in Msg1 and T-CRNTI in MSg2 holding different values . Why these are differnt ? And if this values are different how UE and eNodeB identifies each other ?

Answer 1

RA-RNTI and C-RNTI both are different, RA-RNTI is calculated based on t_id and f_id while T-CRNTI is allocated by eNodeB MAC to UE so that once connection is successfully established between UE and eNodeB this T-CRNTI is used as C-RNTI to identify the UE at eNodeB MAC layer .
Formula to get the "RA-RNTI = 1 + t_id + 10 * f_id". In case of FDD cell, value of f_id is zero where 0 <= t_id < 9.
In short, RA-RNTI is generated at the UE while T-CRNTI is at the eNodeB.

Hope I answered your query.

Answer 2

Hi,

RA_RNIT is just to decode RAR from ENB, Because UE doesn't know about T-CRNTI so RA_RNTI is used. After receiving RAR UE will come to know T-CRNTI so it will use T-CRNTI for further communication.

RA_RNTI is (RACH Subframe + 1) . e.g If Rach subframe is 1 means RA_RNTI will be 2.

T_CRNIT will be from Enb .