LTE: RRC Connection Request

Direction: UE => E-UTRAN
Signalling Radio Bearer: SRB0
RLC Mode: TM
Logical Channel: CCCH
Transport Channel: UL-SCH
       RRC CONNECTION REQUEST message is used to request the E-UTRAN for the establishment of an RRC connection. It is sent as part of the Random Access procedure. It is transferred using SRB0 on the Common Control Channel (CCCH) because neither SRB1 nor a Dedicated Control Channel (DCCH) has been setup at this point.


      IEs in RRC CONNECTION REQUEST message are given below:

ue-Identity (Initial UE-Identity): This IE is included to facilitate contention resolution by lower layers. It can be either S-TMSI (40-bits) or a bit string of randomValue (40-bits). If the upper layers provide S-TMSI, then S-TMSI is used as the ue-Identity; else a random value in the range of 0 … 240-1 is drawn and used as the ue-Identity. Upper layers provide the S-TMSI if the UE is registered in the TA of the current cell
establishmentCause:The main cause values and the corresponding NAS procedure whichtriggers the RRC connection establishment are presented below:
emergency: This corresponds to NAS Procedure “MO-CS fallback Emergency call”
mt-Access: Corresponding NAS procedures are“Service Request” (paging response for PS domain) or “Extended Service Request”(MT-CS fallback)
mo-Signalling: Corresponding NAS procedures are Attach, Detach, and TAU
mo-Data: Corresponding NAS Procedures are“Service Request” and “Extended Service Request”


Example1: RRC CONNECTION REQUEST message with ue-Identity as a randomValue



Example2: RRC CONNECTION REQUEST message with ue-Identity as s-TMSI
Posted by KP
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47 comments:

  1. AnonymousMay 13, 2014 at 2:25 PM

    Upper layers provide the S-TMSI if the UE is registered in the TA of the current cell...

    maybe very silly doubt but what's TA in here..??

    ReplyDelete
  2. AnonymousJune 3, 2014 at 8:54 PM

    TA: Tracking Area

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  3. indu shekharAugust 25, 2014 at 3:43 PM

    Tracking Area is a logical concept of an area where a user can move around without updating the MME. The network allocates a list with one or more TAs to the user. In certain operation modes, the UE may move freely in all TAs of the list without updating the MME. You can think of 'Tracking Area' as 'Routing Area' in UMTS.

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  4. indu shekharSeptember 16, 2014 at 8:37 PM

    What is the use of establishment cause in RRC Connection set up from the eNodeB perspective as it is determined by NAS Procedure?

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    Replies
    1. KPSeptember 19, 2014 at 11:21 AM

      The RRC establishment cause can be used by the network to prioritise the connection establishment request from the UE at high load situations in the network

      Also see Table D.1.1 in 24.301 for priorities of different cause values

      Delete
  5. UnknownMarch 23, 2015 at 9:23 AM

    Hi, As for ue-Identity, shouldn't the random value actually the IMSI when UE attaches to the cell not for the response to PAGGING

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  6. AnonymousMarch 25, 2015 at 3:26 PM

    How do they decide if the access is High Priority?
    and can you explain mo signaling?
    Thanks

    ReplyDelete
  7. Suresh Kumar BansalJuly 24, 2015 at 1:34 PM

    can the mt- Access come as a response of paging for TAU or only as a response to a voice call Paging.

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    Replies
    1. KPJuly 27, 2015 at 9:19 AM

      Please refer to Annex D for specification 24.301 for various establishment causes. Answer to your question: If the UE does not have a PDN connection established for
      emergency bearer services and is not initiating a PDN
      CONNECTIVITY REQUEST that has request type set to
      "emergency, the RRC establishment cause shall be set to MO
      signalling

      Delete
  8. BALAJI KISHOREAugust 13, 2015 at 11:03 AM

    'what is the size of RRC Connection Request Message?

    ReplyDelete
  9. BALAJI KISHOREAugust 15, 2015 at 4:45 PM


    thank for the reply

    ReplyDelete
  10. BALAJI KISHOREAugust 15, 2015 at 4:47 PM

    What is the maximum retry limit for RRC connection request ?

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  11. KPAugust 17, 2015 at 10:34 AM

    There is no retry. At T300 expiry, inform upper layers about the failure to establish the RRC connection, upon which RRC connection establishment procedure ends.

    ReplyDelete
  12. Venkat NarayananOctober 14, 2015 at 5:08 PM

    This is useful.
    But I am really looking for MO and MT Message sequence chart.

    ReplyDelete
  13. UnknownDecember 23, 2015 at 11:45 AM

    ue-identiti takes 5 byte, and spare takes 1 bit, so wstablishmentCause takes 7 bits right??

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    Replies
    1. KPDecember 23, 2015 at 1:33 PM

      I guess EstablishmentCause takes only 3-bits as it is only representing 8 ENUM values (causes)

      Delete
    2. UnknownAugust 22, 2016 at 12:02 AM

      Hi Kumar,

      Is there a mapping table of establisment cause against 3bits
      for ex :

      000 mo-data
      100 mt-access
      110 mo-signalling

      Delete
    3. KPSeptember 21, 2016 at 12:31 PM

      Oh yes. Please refer to Annex D in 24.301

      Delete
  14. UnknownJanuary 26, 2016 at 1:38 PM

    Is the RRC Connection request message size related to the cause as well?

    IF yes can you please provide a mapping?

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    Replies
    1. KPFebruary 2, 2016 at 9:51 AM

      The Size of the message is not related to the cause...

      Mapping is as follows:

      6 bytes => 12 nibbles
      1st Nibble (from left) identifies the type of RRC message, critical extentions, ue-Identity type (randomValue or s-TMSI).
      Next 10 Nibbles (40 bits) form the ue-Identity bit string.
      The last nibble consists of 3-bit establishmentCause and 1 spare bit

      Hope this helps...

      Delete
    2. UnknownFebruary 7, 2018 at 4:03 PM

      hii kumar,
      can u plz sujjest where can i get these message size details of rrc, paging,rach. what u explained above regarding size of rrc, where can i get to learn more details

      Delete
    3. KPMarch 4, 2018 at 9:39 AM

      Hi,

      I am sorry that I don't have a straight forward answer. The best way is that use one of the RRC message encoders (if you have access to R&S, Anritsu, Anite tools it should be easy)....

      Delete
  15. PatrickJuly 13, 2016 at 12:17 AM

    Why the ENodeB doesnt send Rrc Connection Setup?

    ReplyDelete
  16. UnknownJanuary 6, 2017 at 3:04 PM

    hi Kumar I am trying to calculated the signalling messages size so could you please refer me to 3GPP or can you clarify how to calculate them (RRCconnectionreconfiguration, measurement report, handover request and so on)

    ReplyDelete
    Replies
    1. KPJanuary 13, 2017 at 1:59 PM

      The easiest way could be to use a kind of message encoder and check the payload size. I know that R&S has such a tool. Since the size of the above mentioned messages varies based on the IEs included in the message, it is good to use such an encoder.

      Delete
  17. UnknownSeptember 27, 2017 at 8:15 PM

    Could you guys suggest me how to calculate the size of this message (in bytes)? What method should be applied to calculate every message in LTE (control messages over GTP-C, etc)?

    ReplyDelete
  18. MaheshNovember 4, 2017 at 7:14 AM

    what is spare in RRC_REQUEST_MESSAGE

    ReplyDelete
  19. vipinNovember 13, 2017 at 11:04 AM

    You have mentioned here that UE can share its S-TMSI also for the ue-identity if it is already registered with that cell. But Why the UE will send RRC Connection request if its already registered with the cell

    ReplyDelete
    Replies
    1. KPJanuary 17, 2018 at 11:48 AM

      There are several reasons for RRC Connection establishment on the same cell. One straight forward reason is that, due to inactivity the network releases RRC Connection. So whenever (for example) there is data to be transmitted or a need for MO call, the UE needs to establish the connection again

      Delete
    2. DhilipSeptember 9, 2021 at 2:17 AM

      Hi Kumar, Thanks for the explanation. So, RRC Connection establishment with cause "mo-Data" also happens when there is CSFB MT call to 4G UE?. Please confirm. Also, SGS-LU happens during this procedure? Thanks in Advance.

      Delete
  20. UnknownJanuary 10, 2018 at 9:59 AM

    Hi, In what cases UE will random value? Why cant it use IMSI instead?

    ReplyDelete
    Replies
    1. KPJanuary 17, 2018 at 11:52 AM

      If the upper layers provide S-TMSI, then S-TMSI is used as the ue-Identity; else a random value in the range of 0 … 240-1 is drawn and used as the ue-Identity. Upper layers provide the S-TMSI if the UE is registered in the TA of the current cell

      Regarding IMSI, sending the IMSI over the air before secure connection (authentication and security) is established is not a good option

      Delete
    2. sandeepMarch 19, 2021 at 6:25 PM

      Which layers are in the upper layers?

      Delete
  21. AnonymousNovember 30, 2018 at 10:06 AM

    how can Random values are identified uniquely??

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  22. SaatvikMay 16, 2019 at 1:46 PM

    On which Physical channel is msg3/RRC connection request mapped, PUCCH or PUCCH?

    ReplyDelete
  23. ArchieJune 11, 2019 at 2:27 PM

    Hi Kumar, I noticed you didnt discuss the Establishment cause: high priority access..

    ReplyDelete
    Replies
    1. UnknownSeptember 28, 2021 at 9:06 AM

      Yes we need rrc request with high priority access, especially what is the difference between highpriority access and emergency call

      Delete
  24. AkramAugust 23, 2019 at 5:10 PM

    Why RRC connection Request msg remain in UE buffer?

    ReplyDelete
  25. SudhirNovember 19, 2019 at 6:50 PM

    Which layer adds the random number in RRC connection request?

    ReplyDelete
  26. ArchieJanuary 6, 2020 at 3:57 PM

    Hi Kumar, how do you know if the establishment cause is related to a volte call?

    ReplyDelete
  27. AnonymousApril 26, 2023 at 1:16 PM

    Bmv dl g

    ReplyDelete

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