 or during the reconfiguration of the radio link. The configuration is transmitted in the RRC Reconfiguration message, and the mobile is positioned on a single BWP band, called active BWP. In the connected state, at most four BWPs are configured at the mobile level in order to be able to switch from one part to another quickly. In the Release 15 and Release 16 specifications, the mobile is only allowed to position itself on a single BWP, the mobile is configured at a given time with a single numerology. 5G-NR Radio Interface - Bandwidth Part BWP switching is triggered by a control message transmitted from the gNB entity to the mobile or at the end of a timer: - a DCI format 1_1 or format 0_1 control message; - the expiration of the BWP-Inactivity Timer (between 2 ms and 2 s). The RRC message is used for the configuration of the radio link. The DCI message allows dynamic adaptation of the resource allocation by telling the mobile to switch to another BWP Uplink or Downlink part outside of the original BWP. The mobile switches to the initial BWP when the BWP-Inactivity Timer expires. In the event of loss of the radio link or of the beam (see Chapter 6), the mobile switches to the initial BWP to be able to start the random access procedure. BWP switching BWP switching DCI message Message DCI RRC Release RRC Reconfiguration BWP #1 BWP #1 5G-NR carrier BWP #2 BWP Initial Initial Synchronization Signal CORESET Figure 7.6. BWP switching. For a color version of this figure, see www.iste.co.uk/launay/5g.zip BWP switching is controlled by the physical PDCCH channel or by the MAC (Medium Access Control) layer during the random access procedure. In case of carrier aggregation, a BWP is activated by default when activating the SCell (secondary cell) in the RRC Reconfiguration message without requiring a DCI control message. NG-RAN and 5G-NR In case of dual DC-NR connectivity, a BWP is enabled by default when activating the secondary SpCell (Special Cell) in the RRC Reconfiguration message without requiring a DCI control messa