In 5G dual connectivity, the determination of the master and secondary nodes is primarily based on the hierarchical relationship between the two nodes and their roles in providing connectivity to the User Equipment (UE). The master and secondary roles are typically assigned as follows:

1. gNodeB (gNB) as Master Node:
   • The gNB is the primary and more capable node in the dual connectivity setup.
   • It handles the control plane functions, including signaling, scheduling, and overall coordination of the network.
   • The gNB has greater control over radio resource management and scheduling decisions.
   • It manages mobility events, such as handovers and cell reselection, providing a centralized control mechanism.
   • The gNB decides when and how the secondary node (eNodeB) should transmit data to the UE.
2. eNodeB as Secondary Node:
   • The eNodeB takes on a secondary role in the dual connectivity configuration.
   • It primarily handles user plane functions, including forwarding data between the UE and the gNB.
   • The eNodeB assists in data transmission and reception but has a more limited role in control plane functions and resource management.
   • It may support the gNB in managing handover procedures, but the final decision is typically made by the gNB.

The decision of which node serves as the master and which serves as the secondary is generally determined by network planning and optimization considerations, taking into account factors such as:

• Network Architecture: The gNB is the central node in the 5G network architecture, designed to provide advanced features and capabilities compared to previous generations. It’s natural for the gNB to take the master role due to its superior functionalities.
• Capability and Resources: The gNB has more processing power, memory, and radio resource management capabilities, making it better suited to handle control plane functions and overall network coordination.
• Load and Traffic: The decision might also depend on the network load and traffic distribution. If a particular area experiences heavy traffic, the gNB could be assigned as the master to efficiently manage resources and maintain quality of service.
• Mobility Patterns: If a UE is experiencing frequent mobility events, having the gNB as the master allows for better handover management and control.

It’s important to note that this master and secondary node setup is part of the overall network design and can be optimized based on real-time network conditions. The arrangement ensures efficient resource utilization, improved mobility management, and seamless connectivity for UEs.