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The physical placement represents where the providers should deploy their fog servers. Commonly, in the case of iFog, the provider may enable fog servers on all the possible nodes (e.g. cellular base stations) and rely on the underlying communication technologies (see Section 1.4) to support the accessibility. On the other hand, in case of mFog [24, 63], providers need to identify the best geo-location to place the mobile fog nodes in order to provide the best QoS to the end-devices and also to support the cost-efficiency of the operation. For example, in UAV-Fog, the provider may choose the locations for the mobile fog nodes based on the density of the end-devices, the signal coverage of the fog node, the distance between the fog server and the end-devices, and the other context factors described in the previous content related to context-awareness. In general, the primary goal of physical placement is to achieve the lowest latency in terms of request/response time, application service handover time, and application task migration time.