if SRB1 can do transferring RRC and NAS Messages why they introduced SRB2?

Question

For SRB's mentioned default parameter values :

RLC configuration CHOICE
ul-RLC-Config
>t-PollRetransmit
>pollPDU
>pollByte
>maxRetxThreshold
dl-RLC-Config
>t-Reordering
>t-StatusProhibit
Logical channel configuration
priority
prioritisedBitRate
bucketSizeDuration
logicalChannelGroup

So is their any default parameters value for DRB for RLC AM mode in LTE for UL & DL?

Answer 1

As per 36.331 4.2.2 below is mention for SRB.
Signalling radio bearers
"Signalling Radio Bearers" (SRBs) are defined as Radio Bearers (RB) that are used only for the transmission of RRC and NAS messages. More specifically, the following three SRBs are defined:

• 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.

Answer 2

On behalf of Rakesh Kumar Suman

SRB1 is for RRC messages as well as for NAS messages
SRB2 is used for NAS messages ony using DCCH channel.
SRB1 has more priority than SRB2.
SRB2 is configured by eNodeB after security activation.