5G NR RRC Timers, Counters and Constants

5G, 5G Network Architectures, 5G Network Planning, Future Network Optimization, New Radio, NR, Radio Resource Management, RRC, RRC Signalling, Signaling

5G NR radio resource control (RRC) has introduced following listed timers, counters and constant for reliable functioning of procedures like RRCsetuprequest, RRCreestablishment, RRCresume, RRCsuspend etc.

RRC timer indicates the timer duration and event when to start, event when to stop and actions to be taken if the timer is expired. These timers are denoted by  letter “T” followed by 3 digit number e.g. T300. Each timer start we zero value and  when the UE applies zero value for a timer, the timer shall be started and immediately expire unless explicitly stated otherwise.

RRC constant and counter defines the count or no. of attempts for particular event. Counter are denoted by letter “N” and followed by 3 digit number e.g. N310.

Following is the complete list and details about timers, constant and counter.

  • Timer: T300
    • Start Point:
      • Upon transmission of RRCSetupRequest
    • Stop Point:
      • Upon reception of RRCSetup or RRCReject message
      • Upon Cell re-selection
      • Upon abortion of connection establishment by upper layers
    • Action upon Timer Expiry:
  • Timer: T301
    • Start Point:
      • Upon transmission of RRCReestabilshmentRequest
    • Stop Point:
      • Upon reception of RRCReestablishment or RRCSetupmessage
      • when the selected cell becomes unsuitable
    • Action upon Timer Expiry:
      • Go to RRC-IDLE
  • Timer: T302
    • Start Point:
      • Upon reception of RRCReject while performing RRC connection establishment or resume,
      • Upon reception of RRCRelease with waitTime
    • Stop Point:
      • Upon entering RRC-CONNECTED or RRC-IDLE
      • Upon cell re-selection
      • Upon reception of RRCReject message
    • Action upon Timer Expiry:
      • Inform upper layers about barring alleviation
  • Timer: T304
    • Start Point:
      • Upon detecting physical layer problems for the SpCell i.e. upon receiving N310 consecutive out-of sync indications from lower layers
    • Stop Point:
      • Upon receiving N311 consecutive in-sync indications from lower layers for the SpCell,
      • Upon receiving RRCReconfiguration with reconfiguration With Sync for that cell group, and upon initiating the connection re-establishment procedure.
      • Upon SCG release, if the T310 is kept in SCG
    • Action upon Timer Expiry:
      • 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 of specification 38.331
  • Timer: T311
    • Start Point:
      • Upon initiating the RRC connection reestablishment procedure
    • Stop Point:
      • Upon selection of a suitable NR cell or a cell using another RAT
    • Action upon Timer Expiry:
      • Enter RRC_IDLE
  • Timer: T319
    • Start Point:
      • Upon transmission of RRCResumeRequest
    • Stop Point:
      • Upon reception of RRCResume, RRCSetup, RRCRelease, RRCRelease with suspendConfig or RRCReject message, cell re-selection
      • Upon abortion of connection establishment by upper layers
    • Action upon Timer Expiry:
      • Perform the actions as specified in 5.3.13.5 of 3GPP specification 38.331
  • Timer: T320
    • Start Point:
      • Upon reception of t320
      • upon cell (re)selection to NR from another RAT with validity time configured for dedicated priorities (in which case the remaining validity time is applied)
    • Stop Point:
      • Upon entering RRC-CONNECTED,
      • Upon reception of RRC Release, when PLMN selection is performed on request by NAS
      • Upon cell (re)selection to another RAT (in which case the timer is carried on to the other RAT)
    • Action upon Timer Expiry:
      • Discard the cell re-selection priority information provided by dedicated signalling
  • Timer: T321
    • Start Point:
      • Upon receiving measConfig including a reportConfig with the purpose set to report CGI
    • Stop Point:
      • Upon acquiring the information needed to set all fields of cgiinfo
      • Upon receiving measConfig that includes removal of the reportConfig with the purpose set to reportCGI
      • upon detecting that a cell is not broadcasting SIB1
    • Action upon Timer Expiry:
      • Initiate the measurement reporting procedure, stop performing the related measurements and remove the corresponding measId
  • Timer: T325
    • Start Point:
      • Upon reception of RRCRelease message with deprioritisationTimer
    • Action upon Timer Expiry:
      • Stop deprioritisation of all frequencies or NR signalled by RRCRelease
  • Timer: T380
    • Start Point:
      • Upon reception of RRCRelease including suspendConfig
    • Stop Point:
      • Upon reception of RRCResume, RRCSetup or RRCRelease
    • Action upon Timer Expiry:
      • Perform the actions as specified in 5.3.13 of 3GPP 38.331
  • Timer: T390
    • Start Point:
      • When access attempt is barred at access barring check for an Access Category
      • The UE shall maintain one instance of this timer per Access Category
    • Stop Point:
      • As specified in 5.3.14.3 of 3GPP 38.331
    • Action upon Timer Expiry:
      • Perform the actions as specified in 5.3.14.4  of 3GPP 38.331
  • Timer: T3xx
    • Start Point:
      • Upon transmitting UEAssistanceInformation message with DelayBudgetReport
    • Stop Point:
      • Upon initiating the connection re-establishment procedures
    • Action upon Timer Expiry:
      • No action.

Counter and Constant

  • N310: Maximum number of consecutive “out-of-sync” indications for the PCell received from lower layers
    • Reset
      • Upon reception of “insync” indication from lower layers
      • Upon receiving RRCReconfiguration with reconfigurationWithSync for that cell group
      • Upon initiating the connection reestablishment procedure
    • Incremented
      • Upon reception of “out-of-sync” from lower layer while the timer T310 is stopped
    • When reaching max value
      • Start timer T310
  • N311: Maximum number of consecutive “in-sync” indications for the PCell received from lower layers
    • Reset
      • Upon reception of “outof-sync” indication from lower layers
      • Upon receiving RRCReconfiguration with reconfigurationWithSync for that cell group
      • Upon initiating the connection reestablishment procedure
    • Incremented
      • Upon reception of the “in-sync” from lower layer while the timer T310 is running
    • When reaching max value
      • Stop the timer T310

Refereneces:

  • 3GPP TS 38.331 5G; NR; Radio Resource Control (RRC); Protocol specification