5G-NR: SIGNALLING RADIO BEARERS:

5G-NR: SIGNALLING RADIO BEARERS:

Indroduction:

          The eNodeB is the Master Node so the majority of RRC signalling procedures terminate at the eNodeB rather than the gNodeB. Signalling Radio Bearer-0 (SRB0), SRB 1 and SRB2 terminate at the eNodeB. This means that the 4G RRC signalling protocol

specified in 3GPP TS 36.331 is applicable. SRB-I and SRB2 ,can be configured as 'split' SRB. This allows RRC messages to be transmitted and received by both the eNodeB and gNodeB.

      SRB3 can be setup at the request of the 5G Secondary Node. SRB3 terminates al the Secondary Node (gNode B) and so the 5G RRCsignalling protocol specified in 3GPP TS 38.331 is applicable. SRB3 is used for signalling procedures which are time sensitive with

respect to the gNode B, e.g. mobility procedures. SRB3 supports a limited number of signalling messages, i.e. RRC Reconjiguration, RRC Reconfiguration Complete and Measurement Report messages. 

SIGNALLING RADIO BEARERS:

=> The RRC signalling protocol operates between the UE and Base Station.

=> The Non Access Stratum (NAS) signalling protocol operates between the UE and AMF/SMF 5G NR in BULLETS.

=> Signalling Radio Bearers (SRB) are used to transfer RRC messages between the UE and Base Station. RRC messages can encapsulate NAS messages so SRB are also responsible for transferring NAS messages between the UE and Base Station. The NG Application
Protocol (NGAP) is used to transfer NAS messages between the Hase Station and AMF. NAS messages associated with Session Management terminate at the SMF rather than the AMF. The AMF acts a relay between the Base Station and SMF for Session Management NAS messages

=> below figure illustrates the protocol stacks used for both RRC and NAS signalling. The set of SRB provide a logical connection between the RRC layers within the UE and Base Station.

3GPP has specified 4 types ofSRB for New Radio (NR):

=> SRB-0 transfers RRC messages which use the Common Control Channel (CCCH) logical channel.

=>SRR-1, 2 and 3 transfer RRC messages which use the Dedicated Control Channel (DCCI-I) logical channel

=> SRB-1 supports RRC signalling between the UE and Base Station but can also encapsulate NAS messages prior to the setup of SRB2.

=> SRB-2 is always setup after security activation and is used to encapsulate NAS messages. SRB 2 messages arc handled with lower priority relative to SRB-1 messages.

=> SRB-3 is applicable when using the 'E-UTRAN New Radio Dual Connectivity' (EN-DC) configuration. In this case, SRIJ 0, 1 and 2 are managed by the E-UTRAN Master Node while SRB 3 is managed by the NR Secondary Node. SRB 3 allows RRC messages

to be transferred directly between the Secondary Node and the UE. SRB 3 is limited to transferring RRC Reconfiguration and Measurement Report messages. These messages arc a subset of those transferred by SRB-1

=> Below figure illustrates the concept of a 'Split SRB' which is applicalblc to SRB-1 and SRB 2 when using a Dual Connectivity configuration. A split SRB means that RRC messages can be transferred using the Master Node air-interface, the Secondary Node airinterface

or both air-interfaces. The use of both air-interfaces helps to improve reliability. The concept is applicable to both the uplink and downlink so the UE can be instructed to transmit uplink RRC messages on both air-interfaces

      Above  is based upon the Non-Standalone EN-DC configuration so SRB 3 is also shown within the Secondary Node. SRB O is only applicable to the Master Node. Splitting SRB I and SRB 2 creates SRB IS and SRB 2S which use the X2 Application Protocol (X2AP) to transfer RRC messages to and from the Secondary Node

The RRC messages associated with each SRB are presented in below Table

SRB-0 uses Transparent Mode (TM) RLC while SRB 1 and 2 use Acknowledged Mode (AM) RLC.

=>SRB-0 transfers messages associated with establishing, re-establishing and resuming a connection. The uplink messages are transmitted as MSG3 within the Random Access procedure, while the downllink messages can be transmitted as MSG4. The VE is allocated a DCCH logical channel once an RRC connection has been established so SR.B 1 and 2 are able to transfer subsequent messages.

=>The RRCResumeRequest message is an exception which has been specified to use the CCCH logical channel rather than the CCCH logical channel. The CCCHI logical channel is intended to transfer larger messages than the CCCH logical channel

=> After security activation, all messages transferred by SRB-I, 2 and 3 arc integrity protected and ciphered by the Packet Data Convergence Protocol (PDCP). In addition, NAS messages use integrity protection and ciphering between the UE and AMF.

=> The Uplink Information Transfer and Downlink Information Transfer messages are dedicated to sending NAS messages and do not include any RRC signalling content. These messages arc transfcned using SRB 2 unless SRB2 has not yet been configured

=> 3GPP References: TS 38.331, TS 37.340