Srb0 in RRC connection request and RRC connection setup

Hi Experts,

In below picture we have srb0 in tx and rx direction:

  • Srb0 generally refer to RRC connection setup message. Why it shown in picture separate for tx and rx?
  • Srb 1 refer to RRC connection setup complete, is this understanding correct?

So it is like srb0 referring to below both messages:

  • RRC connection request.
  • RRC connection setup.

Yes, correct.

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Ok, so it’s confirmed srb0 equal to RRC connection request and RRC connection setup

I didn’t find any NAS message carry on this srb0.
Like srb1 carry attach request mapped with rrc connection setup complete.
What use of srb0 then, if it not carry any NAS message?

To complete ra process, which is most important, and to get signalling established, for t300start.

Yes but no NAS message.

Srb0 does not carry NAS, it will never.
Srb1 is NAS start.

  • SRB0 is for RRC messages using the CCCH logical channel.

  • SRB1 is for RRC messages (which may include a piggybacked NAS message) as well as for NAS messages prior to the establishment of SRB2, all using DCCH logical channel.

  • SRB2 is for NAS messages, using DCCH logical channel. SRB2 has a lower-priority than SRB1 and is always configured by E-UTRAN after security activation.

So, purpose of Srb0 is to only carry RRC messages using CCCH.


Yes srb 0 is for RRC connection request and setup.
Srb1 for RRC connection setup complete.
Srb2 for first RRC connection reconfiguration.

Yes. I read that srb used to transfer NAS message that’s why put here. :wink:
Now clear, thanks!

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Not limited to these alone.

Nice complementar reading for this:


Slight correction:
SRB1 is for all RRC messages + NAS messages before security is activated.
SRB2 can be used only after security is activated.
SRB2 is used only for NAS messages, not RRC.

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Any Radio resource if present involves an RRC message, for SRB2 NAS msg is piggybagged in radio resource in air interface, right?

It’s mentioned that srb2 refer to dl/ul information transfer.

Yes, right.

I am finding RRC connection reconfiguration having srb to add list with id 2.
It’s piggybased with attach accept and activate default EPS bearer context request.

SRB 0:
UL: RRC Connection request & Re-establishment request
DL: RRC connection setup, RRC connection reject, RRC Re-establishment, RRC Re-establishment reject

SRB 2:
UL Info transfer & DL Info transfer (only after AS security is activated otherwise SRB 1)

SRB 1:
All other RRC messages