王志鹏的回答：

摘自xhy1331 的答案： 1、36.331 4.2.2 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. SRB1和SRB2都是用于信令的承载，区别在于SRB1用于承载RRC的信令（也可捎带NAS信令，但该NAS信令要和RRC信令相关），SRB2用于承载NAS信令。SRB1先于SRB2建立,其优先级也大于SRB2。SRB1优先级为1，SRB2优先级为3（36.331 9.2.1.1及9.2.1.2节）； 2、 两者是的区别在于，SRB1用于传送RRC信令，其中包括了信令流程所需要的NAS信息，例如RRC连接建立过程所需的UE的NAS信息，你也可以理解这是依存关系。在SRB1建立后，SRB2建立以传送NAS消息，这个NAS消息虽然是包括在RRC消息中传送的，但是它和RRC层信令流程无关。