Why are different SRBs required while RRC setup?

Question

Can't it do only with one SRB? Pardon if the question is naive

Answer 1

In LTE, Complete connection is divided into two steps.
1. UE and ENB connection.
2. UE and MME connection.

1.While RRC connection setup procedure, SRB1 is established, which is also known as DCCH (Dedicated control channel). It is used to carry RRC messages. It might be used to carry NAS message as a piggy backing.
Please check RRC Connection Reconfiguration message which contains NAS info as a piggybacking.

1. While UE and MME signalling, which is known as NAS signaling. MME sends Initial conetext setup Req message to establish data radio bearer only. it is up to eNB that it will establish SRB2 and use to send NAS messages only or it will use SRB1.

SRB1 has higher priority over SRB2 if SRB2 is also established for an UE.

Answer 2

The approach used by LTE helps to reduce connection establishment delay and step activation. RRC connection establishment configures Signalling Radio Bearer (SRB) 1 and allows subsequent signalling to use the Dedicated Control Channel (DCCH) rather than the Common Control Channel (CCCH) used by SRB 0 similarly for the SRB 2 which is always configured after security activation.

Read the following paper which gives a feel about the whole stuff -
http://lte-bullets.com/LTE%20in%20Bullets%20-%20RRC%20Establishment.pdf