Need of saperate SRB-1 and SRB-2 in LTE

Question

Doubts:
1) Before creating SRB2 all messages goes on SRB1. So why we need SRB-2, still we can use SRB1 for RRC and NAS messages.
2) Why we use TM mode to send SRB0 data and paging messages. What is the intention behind not adding header to TM data.

Answer 1

1) AFAIK , this separation is also done for prioritization purposes as SRB1 may have more priority than SRB2. But I may be wrong.

2) For unidirectional messages like broadcast , paging etc TM mode is the most efficient and you do not need any initial handshaking for sync.

Answer 2

1) SRB2 is designated for the the NAS messages. SRB1 carries only the NAS message which are not security protected. Once the NAS and AS security is established SRB2 is configured and all NAS messages shall go on SRB2.

2)SRB0 is mapped to common control channel(CCCH) on which all the UE can listen and headers are not added as no sequence numbering/reordering is required for these messages and nor ACK/NACK is required for such messages. So TM mode doesn't add header.