Monday, October 17, 2011

RRC Connection Reestablishment Request

The purpose of RRC CONNECTION RE-ESTABLISHMENT procedure is to re-establish the RRC connection, which involves the resumption of SRB1 operation and the re-activation of security (without changing algorithms). The UE shall only initiate this procedure when AS security has been activated.

 The connection re-establishment succeeds only if the concerned cell is prepared i.e. has a valid UE context. In case E-UTRAN accepts the re-establishment, SRB1 operation resumes while the operation of other radio bearers remains suspended. If AS security has not been activated, the UE doesn’t initiate this procedure, instead, it moves to RRC_IDLE directly

The UE initiates the RRC CONNECTION RE-ESTABLISHMENT procedure when one of the following conditions is met:
  • Upon detecting radio link failure; or
  • Upon handover failure; or
  • Upon mobility from E-UTRA failure; or
  • Upon integrity check failure indication from lower layers; or

RRC Connection Reestablishment Request Message

Direction: UE => E-UTRAN
Signalling Radio Bearer: SRB0
RLC Mode: TM
Logical Channel: CCCH
Transport Channel: UL-SCH


ue-Identity: UE identity is included to retrieve UE context and to facilitate contention resolution by lower layers. The UE Identity shall be set as follows:
  1. Set the C-RNTI to the C-RNTI used in the source PCell (In case of handover and mobility from E-UTRA failure) or used in the PCell in which the trigger for the re-establishment occurred (other cases);
  2. Set the physCellId to the physical cell identity of the source PCell (handover and mobility from E-UTRA failure) or of the PCell in which the trigger for the re-establishment occurred (other cases);
  3. Set the shortMAC-I to the 16 least significant bits of the calculated MAC-I
 reestablishmentCause: This IE indicates the failure cause that triggered the re-establishment procedure and shall be set as follows:
  1. If the re-establishment procedure was initiated due to reconfiguration failure (the UE is unable to comply with the reconfiguration sent in RRC CONNECTION RECONFIGURATION), then set the reestablishmentCause to the value 'reconfigurationFailure';
  2. If the re-establishment procedure was initiated due to handover failure (intra-LTE handover failure or inter-RAT mobility from EUTRA failure) then, set the reestablishmentCause to the value 'handoverFailure'
  3. Set the reestablishmentCause to the value 'otherFailure' if the re-establishment procedure was triggered due other causes than indicated in cases 1 and 2

Reference: 3GPP TS 36.331


  1. wht do u think about other failure?

  2. Hi

    'OtherFailure' could be because of radio link failure, integritycheck failure etc.. whichever doesn't fall in the category of 'handoverFailure' or 'reconfigurationFailure'

  3. Is it possible UE Can trigger re-estbalishment without initiative RACH procedure ?

  4. It is not possible to trigger re-establishment procedure without RACH procedure.

  5. Based on what condition eNB come to know UE has lost its Radio link between the UE and eNB?

  6. upon receiving N310 consecutive "out-of-sync" indications for the Cell from lower layers a timer T310 is started and when this timer expires. It is understood by UE that RLF has occured

  7. How many times a UE can try re-establishment?

  8. Once the RRC Connection Re-establishment procedure is triggered, the UE shall start the timer T311, and once the UE selects a suitable cell, this T311 timer is stopped, and RRC initiates the transmission of RRC Connection Re-establishment message and start the timer T301. This timer is stopped only when "Reception of RRC Connection Reestablishment or RRC Connection Reestablishment Reject message as well as when the selected cell becomes unsuitable"

    If either T311 or T301 expires, the UE goes to Idle mode and no more RRC Connection Re-establishments sent. This implies that the UE could try re-establishment only once after the procedure is triggered.

  9. Hi

    is it possible eNB detect the false RRC Connection Re-establishment request?

    1. Please specify what do you mean by False RRC connection re-est request

  10. Integrity Check Failure Case:

    It check fails at UE PDCP, it intimates to RRC about it and RRC triggers RRC Connection re-establishment.
    How about at eNodeB, What RRC will do once it got indication from PDCP of eNOdeB.


  11. Most of such error handling is done by releasing the existing RRC connection with release cause "other"


  13. and please also explain short mac-i,pcell

  14. Usually, eNB maintains (at RRC) an UE context once the connection is established (also, a context between the MME and eNodeB)

    The IE ShortMAC-I is used to identify and verify the UE at RRC connection re-establishment. The 16 least significant bits of the MAC-I calculated using the security configuration of the source PCell

    PCell is the primary cell on which UE has initiated the connection establishment and attached to. This terminology is wrt to Carrier Aggregation where there could be more than one serving cell (aggregated) to increase the bandwidth. All additional serving cells apart from PCell are referred to as secondary cells (SCells)

  15. Hi,

    Since RRC Re-establishment occur after losing RRC connection so every rrc re-establishment request can consider as RRC drop irrespective of re-establishment success or failure. ??

    If re-establishment occur due to handover failure is it possible to get re-establishment success because handover means cell will be change and Ue context will not be available in new cell.

  16. The re-establishment (triggered due to HO failure) can also be successful as the UE context is not moved to new cell until HO success

    1. Means UE send RRC Re-Establishment Req to Old eNB rather than new eNB in case of HO?

  17. If the radio link is failed with current cell due to HO failure. UE tries to establish RRC connection again by sending RRC connection reestablishment request to eNB. Suppose in our scenario, by that time UE is moved to new cell (target eNB) where it is getting good signal, then,

    Will the same UE sends RRC connection re-establishment request or new RRC connection request to target eNB? (But still the existing UE context with source eNB has not been transferred to target eNB.)

    Can anyone clear this?

  18. The UE sends RRC Connection Re-establishment towards new eNB.

    The RLF Indication procedure may be initiated after a UE attempts to re-establish the radio link connection at eNB B after a failure at eNB A. The RLF INDICATION message sent from eNB B to eNB A shall contain the following
    information elements:
    - Failure Cell ID: PCI of the cell in which the UE was connected prior to the failure occurred;
    - C-RNTI: C-RNTI of the UE in the cell where UE was connected prior to the failure occurred;
    - shortMAC-I (optionally): the 16 least significant bits of the MAC-I calculated using the security configuration of the source cell and the re-establishment cell identity;
    - UE RLF Report Container (optionally): the RLF Report received from the UE

    eNB B may initiate RLF Indication towards multiple eNBs if they control cells which use the PCI signalled by the UE during the re-establishment procedure. The eNB A selects the UE context that matches the received Failure Cell ID and C-RNTI, and, if available, uses the shortMAC-I to confirm this identification, by calculating the shortMAC-I and comparing it to the received IE

    1. This comment has been removed by the author.

    2. This comment has been removed by the author.

    3. Very good reply KSP. Just a few updates

      -- shortMAC-I is signed with the source cell K_rrc_integrity (see 36.331, section

      There are two situations:

      -- if eNB B is already a "prepared" cell, i.e eNB B had received HO Request from eNB A which used the UE's old-CRNTI, then eNB B has UE context and will continue with successful RRC Re-establishment.

      -- Else, eNB B is not a prepared cell, i.e had not previously received HO Request from eNB A. From Rel-12 onwards, eNB-B will send a RLF indication message to eNB A. eNB A will verify the UE' CRNTI and then will proceed with Handover Request message to eNB B. (See 36.300 Section, If the previous serving eNB matches the correct context, it may also trigger the Handover Preparation procedure towards the eNB that initiated the Radio Link Failure Indication procedure.).
      This way eNB-B gets UE's context from eNB-A. But this is only supported from Rel-12 onwards.

      Once RRC Connection is Re-established the AS keys are changed to eNB-B.

  19. Hi,

    I have a particular problem was wondering if you could help. I have RRC connection recofiguration (HO information) then RRC Connection reconfiguration complete in target cell. X2 based so I can see pathswitch being acknowledged for erab id 5 but then the HO fails the UE reads MIB in the target cell and also does a RRC connection re-establishment with cause "HO Failure". Is this to do with lower layer sync issue or something else ?

  20. what is the total size of the message ?

  21. How can I make Ue to trigger re-establishment request by integrity check failure. I mean after UE attached to Pcell and scell is configured I want UE to send re-establishment with reason as other failure. Can you please help?

  22. Try radio link failure instead of integrity check failure which must be easy in any test environment.

  23. Sorry, I have to use integrity check because it is a specific requirement. How to trigger this?
    Thanks in advance.

    1. Which Test environment is being used? after succesful security procedure, reconfigure integrity protection algorithm at the simulator side but don't signal to the UE. I think this will do the job, but I am not 100% sure

  24. Hello, Im noticing that all of the "LTE RRC Connection Reestablishment Requests" are rejected inmediatly by the network. Is it possible that the network im working on currently does not have active the Reestablishment feature? And if so What is it needed to activate it and what is the procedure? Thanks a lot.

    1. Hi,
      Is Reestablishment request is sent on the serving cell or any other cell? There may be a possibility that the UE is sending Reestablishment request message on some other cell, and that particular cell is not prepared for Reestablishment, meaning that it can't fetch UE's context from the source cell....

    2. Hi, In all of the cases the Reestablishment request is sent on the serving cell and on directed to the serving cell. Also in all of the drive tests i have seen there isn't a single successful "LTE RRC Connection Reestablishment".

  25. Could you please tell how rrc will get re-established if radio-link failure detect (due to RLC Max retransmission) at eNodeB side?

  26. Hi!

    I am trying to detect a radio link failure. In case the re establishment cause is 'otherFailure', but it is not specified exactly if it is RLF or something else, how can I track this using other messages? For example by receiving an RRCConnectionRequest, could it mean that the UE went into Idle mode due to a radio link failure and now it is trying to reconnect to the network?

    Thanks in advance :)

  27. Hi
    I want to reduce RRC reestablishment attempt for HO to Improve HO Performance so which NSN Parameter need to tune to Reduce RRC reestablishment attempt.

  28. Hi
    I want to reduce RRC reestablishment attempt for HO to Improve HO Performance so which NSN Parameter need to tune to Reduce RRC reestablishment attempt.


  29. This comment has been removed by the author.

  30. When Radio Link Failure occurs T311 timer starts along with Reestablishment. However receipt of which message or event on UE side will stop the T311 timer. Standard simply says "“selection of suitable E-UTRA cell” but doesn't explicitly say that on receipt of which message from eNB will UE stop running T311 Timer

    1. Hi Mohal,

      There is no need to receive any message from the eNodeB to for the "Selection of suitable E-UTRA Cell".

      The UE shall select a suitable cell based on idle mode measurements and cell selection criteria.

      A "suitable cell" is a cell on which the UE may camp on to obtain normal service. The UE shall have a valid USIM and such a cell shall fulfil all the following requirements.
      - The cell is part of either:
      - the selected PLMN, or:
      - the registered PLMN, or:
      - a PLMN of the Equivalent PLMN list
      - The cell is not barred, see subclause 5.3.1 in 36.304;
      - The cell is part of at least one TA that is not part of the list of "forbidden tracking areas for roaming" [4], which belongs to a PLMN that fulfils the first bullet above;
      - The cell selection criteria are fulfilled, see subclause in 36.304;

      If more than one PLMN identity is broadcast in the cell, the cell is considered to be part of all TAs with TAIs constructed from the PLMN identities and the TAC broadcast in the cell.

      For more information on Cell selection/reselection procedures, please look into 36.304

  31. Hi KSP,

    Which message EnodeB will communicate to MME after Receiving the Reestablishment Complete message from UE. what else messages will transverse between enodeb and mme, before sending Reconfiguration message to UE from EnodeB after Reestablishment complete message has been received at EnodeB.


  32. How UE sends Reestablishment request message to other cells?

    1. It is same way as RRC Connection Request (establishment)...

  33. Hi,
    Can anyone explain various reasons of HO fails in both preparation phase and execution phase in LTE?

  34. This comment has been removed by the author.

  35. Hi KMP,
    If you please tell me the initial trouble shooting for SINR. SRB1 success ratio can be degraded because of SINR?. or SINR is main cause of Re-establishment? thanks

  36. hi,
    interesting subject but there is a question i could not find answer for:
    why RRC re establishment on target cell after HO failure succeed if x2 interface exist and fails if there is no X2.

    1. Broadly speaking, one of X2 control plane functions is to transfer the UE context from source eNB to the target eNB.

      After having detected handover failure, the UE attempts to resume the RRC connection either in the source eNB or in another eNB using the RRC re-establishment procedure. This connection resumption succeeds only if the accessed cell is prepared, i.e. concerns a cell of the source eNB or of another eNB towards which handover preparation has been performed.

  37. Hi KSP , First of all I want to mention that I have been reading your blog for some time now, I must appreciate your work and knowledge.I gained knowledge on many topics here.
    For this topic I have two queries :-
    1) You mentioned in one of your comments dated 26th August 2013 that "Once the RRC Connection Re-establishment procedure is triggered, the UE shall start the timer T311, and once the UE selects a suitable cell, this T311 timer is stopped, and RRC initiates the transmission of RRC Connection Re-establishment message and start the timer T301. This timer is stopped only when "Reception of RRC Connection Reestablishment or RRC Connection Reestablishment Reject message as well as when the selected cell becomes unsuitable".
    My doubt is that until the RRC connection re-establishment procedure is unsucessful the UE is not moved to RRC idle state and remains in RRC conencted state as per literatures.If for any reason RRC connection re-establishment is failed or rejected then only UE moves to idle state so why we are mentioning "suitable cell selection" here when a UE is already connected. This should be applicable only when UE attempts for new RRC connection request in a new cell , isn't it? As per my understanding timer T311 is started after sending RRC connection re-establishment request message and it covers all attempts of RRC connection re-establishments. Whereas timer T301 is also started after sending RRC connection re-extablishment request message but covers only duration waiting for a response (either accept or reject). I think timer T301 is subset of timer T311. T301 is for each attempt and there can be more than 1 attempt of RRC conn. restb and for covering all attempts we have timer T311.

    My understanding may be wrong , this is what I have read in Ericsson's literature.

    Please explain me properly if correction is required.

    Thanks & Regards,
    Ashutosh Kaushik

  38. My second question is that what contents are present in a "context fetch" message ie. what are things present in a "context" which is used between Target and source Enodeb during Multi-Target RRC connection re-estb. procedure?

    Many Thanks !!

  39. Hi Kumar Swamy , i need to know if the srvcc Get failed due to some reaon and then the reestablishment has been occured , how to know only from the UU TRace that QCI 1 get estalished after the reestablishment complete . i can see Invite message of QCI5 after the re-establishment and in the RRC Reconfiguration there is no information related to Dedicated bearer and RLC UM . would these hints can confirm that qci1 did nt get established after the reestablishment or to get a full pic we required S1 trace also . Please your help on this case

  40. In RRC connection reestablishment, Is the SN is required to resume SRB1 config in AM mode to resume configuration?

  41. Hi KSP,
    What is the significance of :
    A) ReestablishmentInfo &
    B) AdditionalReestabInfoList ::= SEQUENCE ( SIZE (1..maxReestabInfo) ) OF AdditionalReestabInfo

    in AS CONTEXT of HO PREP message which is sent by src ENB to target eNB.

    Thank You!!

    1. The ReestablishmentInfo IE contains information needed for a potential re-establishment to succeed in the target cell i.e., when the Handover fails and if the UE initiates Re-estatblishment procedure on the target cell.

  42. Are multiple RREs possible during T301?. My understanding it upon sending RRE to the eNB, UE will start T301 timer then it can expect the following 3 before going to idle mode as per 3GPP.
    a) RRE from eNB
    b) RRE Reject from eNB U
    c) Nothing from eNB
    T301 stops when a) or b) received by UE. In case of a) UE will carry on with RRE EST procedure. In case of b) Will the UE retry another RRE with the new T301???
    In case C) after T301 timer expiry UE will go to idle mode.
    Also I would like to know in what condition UE will send > 1 RRE to the enb??

    Thanks in Advance