Services Provided to Upper Layers
The RRC protocol offers the following services to upper layers (NAS and applications):
1. Broadcast of Common Control Information
What it means:
RRC broadcasts system information (MIB, SIBs) to all UEs in a cell. This includes cell access parameters, radio resource configuration, and mobility-related info.
Example:
The network broadcasts SIB1 containing cell identity, PLMN info, and scheduling of other SIBs. Every UE in RRC_IDLE/INACTIVE/CONNECTED reads this to know if the cell is suitable for camping.
2. Notification of UEs in RRC_IDLE (e.g., Mobile Terminating Call)
What it means:
RRC enables the network to page UEs that are in RRC_IDLE state to initiate a connection — typically for incoming calls, SMS, or downlink data.
Example:
A smartphone in idle mode receives a Paging message via P-RNTI over DCI. The UE responds with RRC Setup Request to establish a connection for an incoming voice call.
3. Notification of UEs about ETWS and/or CMAS
What it means:
RRC delivers Earthquake and Tsunami Warning System (ETWS) and Commercial Mobile Alert System (CMAS) notifications to UEs via Short Messages or dedicated RRC messages.
Example:
During an earthquake, the network sends ETWS primary notification via Paging message with warning type. The UE immediately displays the alert to the user, even in RRC_IDLE.
4. Transfer of Dedicated Signalling
What it means:
RRC supports UE-specific signalling for connection management, mobility (handover), configuration (radio bearers, measurement), and NAS message transfer.
Example:
A UE in RRC_CONNECTED receives an RRCReconfiguration message from the network to add a secondary cell (SCell) for Carrier Aggregation or to modify measurement reporting triggers.
5. Broadcast of Positioning Assistance Data
What it means:
RRC broadcasts assistance data to enable UE-based positioning (e.g., GPS/GNSS fine-tuning, OTDOA, or LTE/NR positioning) — especially useful for UEs in IDLE/INACTIVE.
Example:
The network broadcasts Positioning System Information (SIBpos) containing GNSS ephemeris data. A UE uses this to improve GPS fix accuracy without requesting dedicated signalling.
6. Transfer of Application Layer Measurement Configuration and Reporting
What it means:
RRC transports configuration for application-layer measurements (e.g., QoE/QoS metrics for video streaming, voice quality) and carries the measurement reports from UE to network.
Example:
The network sends an RRC message with application layer measurement configuration (e.g., video buffer delay reporting). The UE collects the data and sends it back via SRB4/SRB5 for network performance optimization.
Services Expected from Lower Layers:
RRC expects the following services from lower layers (PDCP, RLC, MAC, PHY) :
|
Service |
Description |
Example |
|
Integrity Protection |
Ensures RRC messages are not tampered with during transmission. |
PDCP applies integrity protection on RRCReconfiguration to prevent malicious modification. |
|
Ciphering (Encryption) |
Protects confidentiality of RRC/NAS messages over the air. |
PDCP ciphers RRC messages so that an eavesdropper cannot decode sensitive parameters like UE identity or security keys. |
|
Loss-less In-Sequence Delivery |
Guarantees that RRC messages are delivered in correct order without loss. |
RLC in Acknowledged Mode (AM) ensures RRCReconfiguration and RRCReconfigurationComplete reach the network in correct sequence, avoiding state mismatch. |
|
No Duplication |
Eliminates duplicate delivery of RRC messages to upper layers. |
PDCP discards duplicate PDUs, ensuring the RRC layer receives each message exactly once. |
Summary Table – RRC Services:
|
Service |
Direction |
Use Case |
|
Broadcast of common control info |
Network → UEs (broadcast) |
System information for cell access |
|
Notification of UEs in RRC_IDLE |
Network → UE (paging) |
Incoming call/SMS alert |
|
ETWS/CMAS notification |
Network → UE (paging/SI) |
Emergency alerts |
|
Transfer of dedicated signalling |
Network ↔ UE (dedicated) |
Handover, reconfiguration, NAS transport |
|
Broadcast of positioning assistance data |
Network → UEs (broadcast) |
Assisted GPS/OTDOA positioning |
|
Application layer measurement config/reporting |
Network ↔ UE (dedicated) |
QoE/QoS measurement collection |
Lower Layers Services Summary:
|
Service |
Layer |
Purpose |
|
Integrity Protection |
PDCP |
Prevents message tampering |
|
Ciphering |
PDCP |
Prevents eavesdropping |
|
Loss-less In-Sequence Delivery |
RLC (AM) |
Ensures correct order and no loss |
|
No Duplication |
PDCP |
Prevents duplicate message delivery |