States in RRC
There are 2 states in RRC layer.
1. RRC_CONNECTED
2. RRC_IDLE
A UE is in RRC_CONNECTED when an RRC connection has been established.
A UE is in RRC_IDLE state if no RRC connection is established
RRC_IDLE:
A UE specific DRX [discontinuous reception] may be configured in upper layers.
UE controlled mobility
The UE will perform below tasks:
-> Monitor the paging channel to detect incoming calls.
-> Monitors system information change
-> ETWS capable UEs, ETWS notification
-> Performs neighboring cell measurement and cell re-selection
-> Acquires system information.
RRC_CONNECTED:
-> Transfer of unicast data to/from UE.
-> At lower layers, the UE may be configured with a UE specific DRX.
-> Network controlled mobility, i.e. handover and cell change order with optional network assistance (NACC) to GERAN;
The UE will perform below tasks:
-> Monitors a Paging channel and/ or System Information Block Type 1 contents to detect system information change, for ETWS capable UEs, ETWS notification, and for CMAS capable UEs, CMAS notification;
– Monitors control channels associated with the shared data channel to determine if data is scheduled for it;
-> Provides channel quality and feedback information;
-> Performs neighbouring cell measurements and measurement reporting;
-> Acquires system information.
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.
SRB0 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 RRC messages which include logged measurement information as well as for NAS messages, all using DCCH logical channel.
Functions of RRC Layer:
1. Broadcast of system information:
-> Including NAS common information;
-> Information applicable for UEs in RRC_IDLE, e.g. cell (re-)selection parameters, neighboring cell information and information (also) applicable for UEs in RRC_CONNECTED, e.g. common channel configuration information.
-> Including ETWS notification, CMAS notification;
2. RRC connection control:
-> Paging;
-> Establishment/ modification/ release of RRC connection, including e.g. assignment/ modification of UE identity (C-RNTI), establishment/ modification/ release of SRB1 and SRB2, access class barring;
-> Initial security activation, i.e. initial configuration of AS integrity protection (SRBs) and AS ciphering (SRBs, DRBs);
-> RRC connection mobility including e.g. intra-frequency and inter-frequency handover, associated security handling, i.e. key/ algorithm change, specification of RRC context information transferred between network nodes;
-> Establishment/ modification/ release of RBs carrying user data (DRBs);
-> Radio configuration control including e.g. assignment/ modification of ARQ configuration, HARQ configuration, DRX configuration;
-> QoS control including assignment/ modification of semi-persistent scheduling (SPS) configuration information for DL and UL, assignment/ modification of parameters for UL rate control in the UE, i.e. allocation of a priority and a prioritised bit rate (PBR) for each RB;
-> Recovery from radio link failure;
3. Inter-RAT mobility including e.g. security activation, transfer of RRC context information;
4. Generic protocol error handling;
Reference: 3GPP TS 36.331
