NB-IoT series protocol--3GPP--Release 16--TS 36.331--Radio Resource Control (RRC) protocol specification

1 Scope

This document specifies the radio resource control protocol for the radio interface between UE and E-UTRAN and the radio interface between RN and E-UTRAN. The scope of this document also includes:
a. When switching between eNBs, the radio-related information transmitted between the source eNB and the target eNB
in a transparent container ; b. In a transparent container, between one source or target eNB and another Radio-related information transmitted through inter RAT handover between a system.
The RRC protocol is also used to configure the radio interface between an IAB node and its parent node.

2. Overview

2.1 Introduction

In this specification, the (part of) procedures and messages specified for the UE are also applicable to the RN to obtain the functions required by the RN. There are also some (partial) procedures and messages that are only applicable to the communication between RN and E-UTRAN. In this case, the specification means RN instead of UE. This specification covers MR-DC, that is, the UE uses resources belonging to another node for configuration using NR RAT. According to TS 38.331, NR RRC is used for NR-related configuration.
NB-IoT is a non-backward compatible variant of E-UTRAN that supports a simplified feature set. In this specification, the (part of) procedures and messages specified for the UE are also applicable to the UE in NB-IoT. In NB-IoT, there are some functions and related programs and messages that are not supported by UEs.

2.2 Architecture

2.2.1 UE state and state transition, including inter RAT

When the RRC connection is established, the UE is in RRC_CONNECTED; when the RRC connection is suspended, the UE is in RRC_INACTIVE (if the UE is connected to 5GC). If this is not the case, that is, no RRC connection is established, the UE is in the RRC_IDLE state. Each state of RRC can be further described as: a.
RRC_IDLE –
UE controls movement;
– Monitors a paging channel to detect incoming calls (paging via CN), system information changes, for ETWS capability issues, ETWS notifications, and for CMAS notification for problems with CMAS capabilities;
– Perform neighboring unit measurement and unit (reselection) selection;
– Obtain system information;
– Perform log recording of available measurements, and record the location and time of measurement for configured problems;
– Possible execution EDT;-PUR
can be used to perform transmission.
b. RRC_INACTIVE – The
RRC layer is configured with a RAN-based notification area; –
UE stores inactive UEs as context;
– Applies the RRC_IDLE process unless otherwise specified;
– Uses 5G-S-TMSI to monitor CN paging channels and uses fullI-RNTI running paging; –Perform
RAN-based notification area update regularly ; –Perform RAN-based notification area update
when moving out of the configured RAN-based notification area;
c.RRC_CONNECTED
– transmit unicast data to/ Slave UE;
– For UEs supporting CA, use one or more SCells to aggregate with PCell to increase bandwidth;
– For UEs supporting DC, use one SCG to aggregate with MCG to increase bandwidth;
– For UEs supporting (NG) EN-DC, you can choose to configure an NR SCG and MCG for DRBs and SRBs to improve performance (SRBs) and increase bandwidth (DRBs);
– For solutions to problems that support NE-DC, An SCG can be configured as an NR MCG of DRBs and SRBs to improve performance (SRBs) and bandwidth (DRBs); -Monitor
the content of paging channels and/or system information block type 1 to detect changes in system information, for ETWS capabilities Issues, ETWS notifications, and CMAS notifications for CMAS capabilities issues (not applicable to BL issues, CE and NB-IoT issues);
-The monitor controls the channel associated with the shared data channel to determine whether the data is scheduled for it ;
- support for the CE in RRC_CONNECTED mode receives the ETWS / CMAS indicated by the UEs, and monitors the shared control channel data associated with the channel and to obtain the ETWS notification / CMAS notification or;
- acquiring system information (BL values do not apply to, CE Value and NB-IoT value), but applicable to ETWS/CMAS reception.

2.2.2 Signal broadcast holder

"Signalling Radio Bearers" (SRBs) are defined as radio bearers (RB) and are only used to transmit RRC and NAS messages. Specifically, we define the following SRBs:
–SRB0 is used for RRC messages using CCCH logical channels;
–SRB1 is used for RRC messages (which may include attached NAS messages) and NAS messages before the establishment of SRB2, all of which use DCCH Logical channel;
–For NB-IoT, the SRB1 bit is used for RRC messages (may include attached NAS messages) and NAS messages before security activation, all of which use the DCCH logical channel;
–SRB2 is used for RRC messages containing log measurement information And NAS messages, they both use DCCH logical channels. SRB2 has a lower priority than SRB1, and is always configured by E-UTRAN after security activation. SRB2 is not suitable for narrowband Internet of Things;
-SRB4 is an RRC message, which includes application layer measurement report information, and all use DCCH logical channels. SRB4 can only be configured by E-UTRAN after security activation. SRB4 is not suitable for narrowband IoT.

In the downlink, piggybacking of NAS messages is only used for one dependent (that is, association success/failure) process: bearer establishment/modification/release. In the uplink, NAS message piggybacking is only used to transmit the initial NAS message during connection setup. Once security is activated, all RRC messages on SRB1, SRB2 and SRB4, including those containing NAS or non-3gpp messages, will be integrity protected and encrypted by PDCP. NAS independently applies integrity protection and encryption to NAS messages.

For a UE configured with DC, all RRC messages, regardless of the SRB used and in downlink and uplink, are transmitted through MCG. For EN-DC, NR PDCP can be configured for SRB1 and SRB2 at the same time after the connection is established. If so, these SRBs can be configured as split SRBs. For NGEN-DC and NE-DC, always configure NR PDCP. For split SRB, the UE receives RRC messages through MCG and NR SCG, that is, processing failures and repeated pdus in the order specified in TS 38.323 [83]. For split SRB, the network configures which unit group UE is used to send the uplink RRC message.

2.3 Service

2.3.1 Services provided to the upper layer

The RRC protocol provides the following services to the upper layer:
a. Broadcasting of public control information;
b. Broadcasting positioning assistance data ; c.
Notification of UEs in RRC_IDLE and RRC_INACTIVE, such as termination calls about ETWS and CMAS;
d. Transmission of dedicated control information, That is, information for a specific UE.

2.3.2 Expect services from lower layers

The following are the main services that RRC expects from the lower layer: a.
PDCP: integrity protection and encryption
b. RLC: reliable and sequential information transmission, without introducing duplication, and supporting segmentation and connection.

2.4 Function

The RRC protocol includes the following main functions:
a. System information broadcasting
-including NAS public information;
-Information applicable to UEs in RRC_IDLE, such as cell (re-) selection parameters, neighboring cell information, and information applicable to UEs in RRC_CONNECTED, such as general Channel configuration information;
– including positioning assistance data.
b. RRC connection control
-paging;
-RRC connection establishment/modification/suspend/resume/release, including UE identity (C-RNTI) allocation/modification, SRB1, SRB1 1bis, SRB2 and SRB4 establishment/modification/suspend/ Recovery/release, access restrictions;
–Initial security activation, that is, as the initial configuration of integrity protection (SRBs) and encryption (SRBs, DRBs);
–For RNs, configuration as integrity protection of
DRBs ; –RRC connection transfer , Including, for example, intra-frequency and inter-frequency switching, related security processing, that is, key/algorithm changes, RRC context information transmitted between designated network nodes;
-establishment / modification / release of RBs (DRBs) carrying user data;
- Radio configuration control, including allocation/modification of ARQ configuration, HARQ configuration, and DRX configuration;
– For RNs, RN-specific radio configuration control for the radio interface between RN and E-UTRAN;
– In the case of CA, the unit Management includes changes to PCell, addition/modification/release of SCell(s) and addition/modification/release of STAG(s);
–In the case of DC, cell management includes changes to PSCell and addition/modification of SCG cell(s) /Release, add/modify/release of SCG TAG(s).
--For (NG)EN-DC, transparent transmission of NR RRC messages (e.g. DL: reconfiguration message used to add or modify NR SCG configuration or (re)configure measurement); configuration condition PSCell change; UL: measurement report and reconfiguration Configuration complete information) and the configuration of the radio bearer using NR PDCP. –Recover
from radio link failure;
–For LWA, RCLWI and LWIP, WLAN mobility set management, including adding/modifying/release WLAN from WLAN mobility set;

2.5 NB-IoT with data available for transmission

In order to achieve MAC data volume and power headspace reporting, NB-IoT UE should consider the following data for transmission at the RRC layer:
a. Submit
SDUs to lower layers – SDU itself, if the SDU has not been processed by RRC
– if the SDU has been processed RRC processing is called PDU
b. The data available for transmission at the upper layer is not submitted to the RRC layer.

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