Description
Handover Resource Reservation (HRR) is a proactive resource management procedure used in cellular networks to prepare for an impending handover. The core principle involves the source network node (e.g., a base station or RNC) communicating with a potential target node to request the reservation of specific radio resources—such as time slots, codes, or bearers—for a particular User Equipment (UE) before the actual handover command is issued. This process is typically triggered based on measurement reports from the UE indicating deteriorating signal quality from the serving cell and improving quality from a neighboring cell.
The procedure operates within the radio resource control (RRC) and radio network layer signaling protocols. Upon deciding a handover is necessary, the source node sends a resource reservation request (e.g., a HANDOVER REQUEST message) to the target node. This request contains the UE's context and QoS requirements. The target node then performs admission control; if resources are available, it allocates and reserves them, sending a confirmation back to the source node. The source node then commands the UE to hand over to the target cell, which can immediately begin communication using the pre-allocated resources, avoiding contention or setup delays.
HRR's role is critical in managing mobility in congested networks or for services with stringent QoS demands, such as Voice over LTE (VoLTE) or real-time video. By ensuring resources are ready, it minimizes the time the UE is in a transient, unsynchronized state between cells, reducing packet loss and handover latency. The mechanism is a foundational part of network-controlled handovers in GSM, UMTS, and LTE, though its implementation specifics and messaging vary between the different radio access technologies (RATs).
Purpose & Motivation
HRR was developed to address the inherent unpredictability and potential failure points in mobile handovers, especially in early cellular systems. Without reservation, a handover attempt could fail if the target cell suddenly becomes congested between the measurement report and the execution, leading to dropped calls and poor user experience. The primary problem HRR solves is the guarantee of resource availability at the point of handover execution.
Its creation was motivated by the need for reliable, network-controlled mobility to support circuit-switched voice services, where even brief interruptions are unacceptable. By reserving resources beforehand, the network assumes control over the mobility process, optimizing resource utilization across cells and preventing scenarios where a UE successfully accesses a new cell but is immediately blocked due to lack of capacity. This proactive approach became a standard for managed mobility, forming a basis for more advanced techniques like seamless handovers in LTE and 5G.
Key Features
- Proactive reservation of radio resources in target cell prior to handover execution
- Reduces handover failure probability and call drop rates
- Minimizes service interruption time and packet loss during mobility
- Network-controlled procedure based on UE measurement reports
- Integrates with admission control in the target cell
- Fundamental to reliable handover in GSM, UMTS, and LTE
Evolution Across Releases
Initial specification of Handover Resource Reservation as a core network-controlled handover procedure within the UMTS framework. Defined the basic signaling flow between Radio Network Controllers (RNCs) for intra-system handovers, including resource reservation requests and confirmations as part of the Radio Access Network Application Part (RANAP) protocol.
Adaptation and re-specification of HRR principles for the LTE Evolved UTRAN (E-UTRAN) architecture. Defined the X2-based handover procedure where eNBs directly communicate for resource reservation, making the process faster and more efficient compared to the core-network-involved S1 handover.
Defining Specifications
| Specification | Title |
|---|---|
| TS 21.905 | 3GPP TS 21.905 |
| TS 22.960 | 3GPP TS 22.960 |