r data. Introduction to 5G N2/N3 5G Core Network N2/N3 Figure 1.5 - NG-RAN architecture The gNB is responsible for radio resource management and a big part of that is scheduling uplink and downlink data onto the radio interface. The gNB handles numerous different devices and its cell or even cells. A single gNB may be in control of several different cells. It is responsible for scheduling user data correctly onto the downlink or telling the UE when to transmit data on the uplink. The gNBs are also responsible for handovers. The Xn reference point that sits between the gNBs allows them to coordinate handovers between themselves. Security is a key factor as well, SO the gNB will be involved in the security across the radio link as well as the UE. Finally, the gNBs are responsible for dual connectivity, which will be examined later in this chapter. The UE is responsible for bidirectional data transfer via the 5G NR, ensuring a high quality of service. So, the UE needs to be sure that the correct traffic is sent to the correct bearer. By being the other end of the radio connection, the device is also responsible for security. Finally, the UE needs to support dual connectivity if it is being used in the network architecture. Tracking areas Looking at the NG-RAN at a higher level, we can see that there are thousands of gNBs deployed to provide that RAN coverage. In any mobile communication system, the UE is not constantly connected to the network. The device will routinely be set to an IDLE state in order to preserve the battery life. In the IDLE state, the UE will be conducting operations such as cell reselection. But it will also periodically be listening to see whether it is being paged. Any data coming into the 5GC to be sent to the UE requires paging. The network needs to know where that device is. 5G NR and NG-RAN However, the problem is that the cell reselection that takes place in an IDLE state is done autonomously and the device does not keep the network updated as to which specific cell it's in. If we need to