LTE: RRC Connection Reconfiguration

Direction: E-UTRAN => UE
Signalling Radio Bearer: SRB1
RLC Mode: AM
Logical Channel: DCCH
Transport Channel: DL-SCH

RRC CONNECTION RECONFIGURATION message is the command to modify an RRC connection. The purpose of this procedure is,
·            To establish/modify/release Radio Bearers
·            To perform Handover
·            To setup/modify/release Measurements
·            To add/modify/release SCells
·            Dedicated NAS Information might also be transferred from eNodeB to UE

Differences with 3G system's radio resource configuration procedure:
·         In LTE, RRC CONNECTION RECONFIGURATION is the only message used to perform all logical, transport, and physical channel configurations where as in case UMTS, there exists Transport Channel, Physical Channel and Radio Bearer Reconfiguration messages
·         In LTE, RRC CONNECTION RECONFIGURATION can also be used to send NASdedicated signalling to the UE to reduce the latency whereas this option is not available in 3G
·         LTE has only one RRC connected mode state (RRC_CONNECTED), so it has only one Temporary Radio Network Temporary Identity (RNTI) i.e. C-RNTI
·         In 3G, MEASUREMENT CONTROL is used to setup/modify/release measurements where as in LTE, RRC CONNECTION RECONFIGURATION message is the only message for this purpose

E-UTRAN includes the mobilityControlInfo (Handover) only when AS-security has been activated, and SRB2 with at least one DRB are setup but not suspended. The establishment of RBs (other than SRB1, which is established during RRC connection establishment) is included only when AS-security has been activated

          If the UE is unable to comply with (part of) the configuration included in the RRC CONNECTION RECONFIGURATION message, the UE shall continue using the configuration used prior to the reception of RRC CONNECTION RECONFIGURATION message. If the security has not been activated, UE should leave RRC_CONNECTED state, else, UE initiates the connection re-establishment procedure which is used to resume SRB1 operation and to reactivate security


Example2: RRC CONNECTION RECONFIGURATION - Measurement Configuration

Example3: RRC CONNECTION RECONFIGURATION - Mobility Config Information

Example4: RRC CONNECTION RECONFIGURATION - DedicatedInfoNAS (Other IEs are not shown)


  1. Hi..Very useful info in one place. Can you please check this sentence seems there is a typing error. "onnection re-establishment procedure which is used to resume SRB1 operation and to reactivate s640rity" - what is s640rity?

  2. I am not sure if you would be covering the IRAT aspects in another blog? I just wanted to know when and how does these IRAT related measurement procedure takes place? how are the measurement gaps handled in LTE?

  3. Hi RV,

    The E-UTRAN configures the UE with measurement gaps for IRAT using RRC Connection reconfiguration (for measurement configuration). The IE MeasGapConfig specifies the measurement gap configuration and controls setup/release of measurement gaps

    I have uploaded (to below location) an example of how measurement gaps are configured for CDMA2000

  4. Hi Kumar Swamy,

    very useful information!!! Thanks alot for such great effort.

  5. hi

    can you please explain the importance of each IE's in RRC CONNECTION RECONFIGURATION - Measurement Configuration

    thanks in advance

  6. Hi,

    If UE is unable to comply with any of the configuration which is received in RRC Connection Reconfiguration Message then UE will discard all this configuration received in RRC Connection Reconfiguration Message.
    If AS security is enable then UE will trigger RRC Connection Re-establishment Request with cause :: reconfiguration failure.
    Note:: Reconfiguration message contains NAS pdu "attach_accept" at the time initial attach. If reconfiguration is failed then UE will not get "attach_accept" message and in between re-establishment procedure , it will send attach_request message to eNodeB for "attach_accept" message.

    If this scenario is happens then how eNodeB handled it.

    Can anyone please tell me in brief ?

    1. Hello,

      Thanks for the details, I have a question abpout Reconfiguration failures. what can be the differnet causes? i fall in a case where the RRC Re establishement cause reconfiguration failure explosed in all the network; is this can be related to sequirity gateway issue? thanks in advance

  7. I've got a question for the meaning about both neighcellconfig and offsetfreq above Example 2
    with neighcellconfig = 01, or neighcellconfig = '00'B, offsetfreq = dB-24, etc. If carrier frequency equals 3100, does that configuration might apply for all the cells including serving cell and neighbor cell with carrier frequency 3100?
    And also what does it mean if neighborcellconfig has 01 or '00'B?
    Many thanks in advance.

    1. Hi, I know it has been a few years later, but what would be the cause if device receives RADIO_LINK_FAILURE on TAC 1043 freq38948. I am still new to this whole mobile network. We receive this one of our LTE modules, but when comparing toa different brand, they seem to work fine.

    2. RLF happens due to various reason like RACH failure, RLC MAX Retransmission, Physical layer out of sync.

  8. Hi,Is it RRC-reconfig is always from network to UE..?

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  10. which IE's contain by rrc rconfiguration message?

    1. RRC Connection Reconfiguration itself is a message and all the IEs mentioned about are part of this message.

  11. Really appreciate for your amazing article. Keep going on, good stuff. Thank you for this valuable information.

  12. Does we use RRC Reconfiguration for SCELL Configuration ?
    could you please explain the content of RRC Reconfig in case SCELL activation or configuration ?

    1. Yes. In fact RRC Connection Reconfiguration is the only way to provide the UE with sCell Configuration. Please go through the below link for other details you have asked for.

  13. Hi, I would like to know if this procedure about Handover InterBand in attached image is correct.

  14. Hi sir,
    In rrc reconfig we have
    Meas gapconfig release:null
    S measure 90
    Speed state parameters setup
    Mobility parameters
    T evaluation s240,
    T hyst normal s240
    N cell changemedium8
    N cell change high 16
    Time to trigger sf
    Sf medium 1dot0
    Sf high 1dot0
    Will explain y these parameters are used

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  17. This comment has been removed by the author.

  18. In LTE how UE know handover is X2HO or S1 based.

    Which message we check?

    1. I mean which component is change TenB, Tmme or both...?

  19. Hi
    Upon transmitting the RRC Connection Reconfiguration message to UE, at what point does the eNodeB apply these new configuration to the lower layers on the eNodeB side.
    Does eNB apply the new configuration to it's lower layers after the RLC ACK is received for this RRC Connection Reconfiguration from UE or is it applied after the RRC Connection Reconfiguration Complete is received from the UE?

    Please respond.

    Thanks & Regards