Signaling Radio Bearer basic Part-1

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In this section we will learn about SRB and its basics as well as interview questions.

Signaling Radio Bearer's:

ENDC (EUTRA-NR dual connectivity- 3gpp release-15 feature- deployment Option-3) is a feature that is introduced to support new radio NR (5G) data with existing LTE core and radio networks and without introducing 5G core network.  This ENDC (EUTRA New Radio dual connectivity) can be a useful feature for heterogeneous network where LTE provides reliable coverage and NR can be used for improving data rates.

In ENDC, the schedulers in the eNB and en-gNB are independent from each other and make their own decisions how much and when the data is to be send or receive from UE. This is also applicable to RRC, and the device now has one RRC context to the eNB and another separate and independent RRC context to the en-gNB.

Now the question is how RRC messages from th eNB and en-gNB are sent to and from UE?

There are two types of bearers SRB and DRB (Data radio bearer). SRB (Signaling Radio Bearer) is a type of radio bearer that carries signaling message i.e. RRC or NAS messages or both. DRB is used to carry the data associated with EPS bearer. There are four types of SRB in NR defined as follows:


SRB0 is for RRC messages using the CCCH logical channel (before UE attached with network, common signaling not assigned to UE) it will used RLC-TM mode. SRB0 is used During RRC Connection setup and re-establishment process.


SRB1 is for RRC (which may include a piggybacked NAS message) as well as for NAS (encapsulate) messages prior to the establishment of SRB2. All SRB1 msg are transmitting using DCCH logical channel and it will used RLC-AM mode. SRB1 priority is always higher than SRB2. It used for High priority DCCH messages.


SRB2 is for NAS messages only. All SRB2 msg are transmitting using DCCH logical channel and it will used RLC-AM mode. It has a lower-priority than SRB1 and is always configured by the network after security activation. It is used for Lower priority DCCH messages and established after security has been configured.


SRB3 is for specific RRC messages when UE is in EN-DC, All SRB3 msg are transmitting using DCCH logical channel and it will used RLC-AM mode. It is used for DCCH messages for secondary node in dual connectivity. It is established and changed by the secondary node. Also carries SN RRC reconfiguration and SN measurement report messages.

What is NAS?

In LTE it is a signaling protocol that operates between UE and MME whereas in 5G it operates between UE and AMF/SMF.

Which AP (Application Protocol) is used to transfer NAS messages between UE and AMF?

NGAP is used to transfer NAS msg between UE and AMF.

Note: in NR NAS message associated with session management terminate at SMF rather than at AMF. The AMF acts a relay between the BS and SMF for session management NAS messages.

Can we configure all SRB simultaneously?

Yes, in ENDC case SRB 0,1,2  is established between UE and Master node (eNB) and SRB 3 is established between UE and Secondary Node(en-gNB).

Is it mandatory to configure SRB3?

No, it is not mandatory. 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.  

In the next section we will learn more about SRB. So keep reading……. And Stay tune……………

Pinal Dobariya………..

3GPP TS 38.331
3GPP TS 37.340


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