 that is above Thresh x,low" 4.5 Radio Resource Management in Multi-RAT Networks The drivers for inter-RAT radio resource management are captured in 3GPP TS 36.300 [14]. For idle mode, the methods and parameters are specified in 3GPP TS 36.304 [15]. The general working assumption in the 3GPP at the time of writing is that idle mode inter-RAT management is based on absolute priorities. For con- nected mode, the details of inter-RAT handover managements will be covered by 3GPP TS 36.331 [21], upon which the 3GPP is working. A schematic overview of inter-RAT RRM is provided in Figure 4.22. A general principle for inter-RAT (intra-3GPP) radio resource management is that the UE is not connected to multiple RATs simultaneously. In particular, when a UE is using a service that is best delivered over LTE, all other services that are used simulta- neously by this UE must also use LTE. Another general principle for inter-RAT handovers is that (1) triggering of inter-RAT measurements as well as the handover decision is made by the RAN that currently serves the UE, and (2) the target RAN gives guidance for the UE on how to make the radio access. The target RAN pro- vides information on the target cell, including radio resource configuration, neces- sary identities, etc., in a transparent container. Although the handover command must be compiled and sent to the UE by the serving RAN, the target RAN assists in this by providing information about the target cell. For inter-RAT handovers, the serving RAN needs to be able to (1) trigger inter-RAT measurements, (2) make comparison between different radio access technologies, and (3) make a handover decision and command the UE. The target RAN must be able to send information in a transparent container to the UE that guides the UE on how to make radio access in the target RAN. In the "from LTE to UTRAN/GERAN" direction, this is made possible by identifying the following events defined in terms of appropriate measurement quantities, associated threshold, offset, and timer values: The