RRC Connection Release in LTE

In LTE, the Radio Resource Control (RRC) protocol is responsible for managing the radio connection between the UE and eNodeB. Among the many procedures handled by RRC, one of the most crucial yet often overlooked is the RRC Connection Release.

This procedure ensures that once communication is no longer needed, or resources are required elsewhere, the UE is moved from the RRC_CONNECTED state to the RRC_IDLE state. By doing so, the network efficiently frees up valuable resources while the UE conserves power.

Unlike connection establishment or reconfiguration, which focus on creating or modifying resources, connection release is about proper dismantling of radio bearers and signaling connections in a way that ensures smooth idle mode mobility and potential redirection to other frequencies or RATs.

Procedure Overview

• The eNodeB sends the RRC CONNECTION RELEASE message to the UE.
• The message may include cause/reason for the release, such as load balancing or CS fallback scenarios.
• Redirection information can be present, which guides the UE to another frequency or RAT especially useful for inter-RAT mobility or MME load balancing.
• The UE immediately moves to the RRC_IDLE state without sending an acknowledgment, as there is no RRC CONNECTION RELEASE COMPLETE message defined in LTE.

Purpose of RRC Connection Release

The RRC Connection Release is not just a “disconnect” command but a carefully designed control mechanism within the LTE framework. It serves multiple purposes:

• Optimization of Radio Resources: Radio bearers and channel allocations are released, allowing the eNodeB to reassign them to other UEs that need service.
• Battery Conservation in UE: When in idle mode, the UE does not maintain continuous signaling or monitoring of data channels, which reduces power consumption significantly.
• Mobility Support: The release procedure may include redirection information or idle mode reselection priorities, guiding the UE to camp on specific frequencies or RATs, ensuring service continuity.
• Inter-RAT Operations: For example, during CS Fallback (CSFB), LTE releases the connection but provides the UE with redirection details toward 2G/3G for circuit-switched voice calls.

Thus, the release procedure is both a resource management tool for the operator and a battery/power management feature for the end-user.

When Does RRC Connection Release Occur?

The eNodeB initiates the release under several circumstances. These can be broadly categorized as follows:

• Inactivity Timer Expiry: When the UE has no active data or signaling exchange for a configured duration (e.g., 5–10 seconds), the eNodeB releases the connection.
• Load Balancing: To redistribute network load, the eNodeB may release a UE, prompting it to reconnect via another Tracking Area or MME.
• CS Fallback (CSFB): If the UE makes a voice call in LTE-only networks without VoLTE, the network releases the connection with redirection to 2G/3G.
• Service Restriction: In cases where certain services are barred (e.g., subscription restrictions), the network may terminate the session.
• Node or Link Failures: Handover failures, radio link issues, or node-specific errors may also lead to a release.

Each scenario reflects a strategic network decision to balance efficiency, mobility, and service continuity.

RRC Connection Release Message Structure

The RRCConnectionRelease message is defined in 3GPP TS 36.331. It is a downlink dedicated signaling message sent from the eNodeB to the UE. Its structure contains both mandatory and optional Information Elements (IEs):

• releaseCause (Mandatory)
  Specifies the reason behind the release (e.g., inactivity, load balancing, CS Fallback).
• redirectedCarrierInfo (Optional)
  Provides information on the new carrier frequency or RAT where the UE should camp.
• idleModeMobilityControlInfo (Optional)
  Supplies reselection priorities and parameters to guide the UE in idle mode mobility.
• cellInfoList (Optional)
  Contains system information of potential target cells for redirection.
• utra-BCCH-Container (Optional)
  Helps when redirecting to UMTS by providing required system information blocks.

This layered structure ensures that the UE is not left “blind” after the release but instead guided to the best available RAT or frequency.

The RRC Connection Release procedure is a fundamental part of LTE’s RRC protocol, ensuring that resources are released, UEs save power, and mobility is supported seamlessly. While it may appear as a simple “disconnect,” it carries strategic significance for network efficiency and service continuity.

In LTE, its role is critical for inactivity handling, CS Fallback, and load balancing. In 5G NR, the procedure evolves further with RRC_INACTIVE, bringing added flexibility for low-latency services.

References

• 3GPP TS 36.331 – Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification.
• 3GPP TS 36.304 – Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode.
• 3GPP TS 36.300 – Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description.
• 3GPP TS 23.272 – Circuit Switched (CS) fallback in Evolved Packet System (EPS); Stage 3.
• ETSI TS 136.331 V16.6.0 – ETSI adoption of 3GPP RRC specifications for LTE.
• Dahlman, E., Parkvall, S., & Sköld, J. (2016). 4G: LTE-Advanced Pro and The Road to 5G. Academic Press.
• Sesia, S., Toufik, I., & Baker, M. (2011). LTE – The UMTS Long Term Evolution: From Theory to Practice. Wiley, 2nd Edition.