What is the difference between LTE UE timers t301 and t311?

Question

Both the timers t301 and t311 are related to RRC Connection ReEstablishment procedure, then what is the difference between them ?

Answer 1

Following are the difference between timer T301 and T311.

1. Both can not start at same time. If one of them is running then other timer will be stopped.
2. If ue detects ho failure, RLF etc..then it needs to perform the rrc reestablishment. For performing reestablishment first UE needs to select suitable cell so it starts t311 timer.
3. Before expiring t311 if UE finds suitable cell then it proceeds to rrc reestablishment procedure so it stop t311 and starts t301 timer.
4. If UE gets rrc reestablishment from eNB then UE stop the t301 time.
5. If any of the time expire before next step as explained above or ue gets rrc reestablishment reject then UE leaves the rrc connected mode with release cause as rrc connection failure.

Comments

RRC connection Re-establishment is a mechanism to avoid UE from moving to idle state and remain in RRC connected state then why we are saying that UE finds a suitable cell first and does cell selection and then send RRC conn rest request ?

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The UE will perform the RACH procedure after starting T311. Here, finding the cell means, getting the RBs from the same cell to send rrcConnectionRestablishmentRequest.