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5G NR Timers: Understanding T304, T310, and T311

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This page delves into the 5G NR timers, specifically T304, T310, and T311, which are crucial for various functionalities within the 5G New Radio (NR) ecosystem. We’ll also touch upon the constants N310 and N311 and their roles.

5G NR Timers Overview

The following table summarizes the behavior of these key timers:

TimerStartStop atExpiry
T304Reception of RRCReconfiguration message including reconfigurationWithSyncSuccessful completion of random access on the corresponding SpCellFor T304 of SCG, upon SCG release. For T304 of SCG, inform the network about the reconfiguration with sync failure by initiating the SCG failure information procedure as specified in 5.7.3 of TS 38.331 (3GPP).
T310Upon detecting physical layer problems for the SpCell (i.e., upon receiving N310 consecutive out-of-sync indications from lower layers).Upon receiving N311 consecutive in-sync indications from lower layers for the SpCell, upon receiving RRCReconfiguration with reconfigurationWithSync for that cell group, and upon initiating the connection re-establishment procedure.Upon SCG release, if the T310 is kept in SCG. If the T310 is kept in MCG: If security is not activated: go to RRC_IDLE else: initiate the connection re-establishment procedure. If the T310 is kept in SCG, Inform E-UTRAN/NR about the SCG radio link failure by initiating the SCG failure information procedure as specified in 5.7.3.
T311Upon initiating the RRC connection re-establishment procedureSelection of a suitable NR cell or a cell using another RAT.Enter RRC_IDLE

Important Note: When the 5G NR UE applies a zero value for the timer, the timer shall be started and immediately expire unless explicitly stated otherwise.

5G NR Constants: N310 and N311

The following table outlines the usage of the constants N310 and N311 in 5G NR:

  • N310: Maximum number of consecutive “out-of-sync” indications for the PCell received from lower layers.
  • N311: Maximum number of consecutive “in-sync” indications for the PCell received from lower layers.