- SRB stands for Signaling Radio Bearer.”Signalling Radio Bearers” (SRBs) are defined as Radio Bearers (RBs) that are used only for the transmission of RRC and NAS messages.As defined per 3GPP specification TS38.331, there are four different types of SRB in 5G New Radio (NR) defined as follows.
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- SRB 0 is for RRC messages using the CCCH logical channel;
- SRB1 is for RRC messages as well as for NAS messages prior to the establishment of SRB2, all using DCCH logical channel;
- SRB2 is for NAS messages, all using DCCH logical channel. SRB2 has a lower-priority than SRB1
- SRB3 is for specific RRC messages when UE is in EN-DC, all using DCCH logical channel.
- In comparison to 4G, NR has one more SRB called SRB3.
- In case of EN-DC, SRB3 may be configured for the transfer of some NR RRC messages between UE and SgNB via the NR radio interface.
- Advantages of SRB 3:
- SRB3 is optional and provides a direct SRB between the Secondary RAN Node and the device, If SRB3 is not configured by network then LTE RRC (vai X2 interface) are able to pass the NR RRC messages between the UE and en-gNB and it’s utilizing SRB1.
- The advantages of introducing new SRB3:
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- Direct RRC communication between UE and SgNB
- Enable the 5G node to make intra-SgNB handover decisions.
- Reduced latency of the RRC signalling between UE and SgNB
- Direct Measurement Reports sent to SgNB which leads to faster and direct reports for HO purposes.
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