(in their Table 1) that "shared resources" lead to "more efficient use of radio resources" but require "more sophisticated techniques to ensure isolation of radio resources." In Li et al. [51] the authors state that "In general, the slice control function can be shared among slices," although they add the caveat that for some communications separate control functions are required. Cell 1 Cell 2 Cell 1 Cell 2 Inter-cell interference may be Slice 1 between users "belonging" to different slices Slice 2 Slice 3 Figure 8.15 Reduction in isolation between slices through inter-cell interference (a) Fixed sharing. (b) Dynamic sharing. 8.5 Isolation In fact, the concept of implicit isolation has been around for a very long time in the form of connection admission control (CAC), which is indeed a control-plane function. For example, quoting from Cuthbert and Sapanel [20] (for an ATM network), "A connection request is accepted only when sufficient resources are available to establish the call through the whole network at its required quality of service and to maintain the agreed quality of service of existing calls." It is important to note that it is the "maintain the agreed quality of service of existing calls" that is the essential factor to give implicit isolation. The problem becomes one of implementing a suitable CAC mechanism as a shared control-plane mechanism rather than finding a form of separation at the radio layer. Of course, the CAC mechanism has to be integrated with the resource allocation to ensure that is as efficient as possible. CAC has also been used in other areas of mobile networks, for instance in Canales et al. [52], Cho et al. [53], and Gelenbe et al. [54]. 8.5.1 Example Isolation Results Using CAC This work is based on that in Yang et al. [55]: this uses the game theory allocation approach described earlier but considers how to use CAC to force isolation. CAC is applied whenever a new user request is received to determine whether sufficient resources can be made available to make the new user qual