ESS Design Considerations: A facility may require interior and exterior ESS elements, depending on the level of protection required. The applicable regulations, threat, and design criteria will define the ESS’s general requirements. For an existing ESS, hardware and software may need to be supple­mented, upgraded, or completely replaced. A site layout (in which all assets are identified and located) is required. It is a useful design tool for such tasks as configuring the DTM. The exterior and interior IDSs should be configured as layers of unbroken rings concentrically surrounding the asset. These rings should correspond to defensive layers that constitute the delay system. The first detection layer is located at the outermost defensive layer necessary to provide the required delay. Detec­tion layers can be on a defensive layer, in the area between two defensive layers, or on the asset itself, depending on the delay required. For example, if a wall of an interior room provides suf­ficient delay for effective response to aggression, detection layers could be between the facility exterior and interior-room wall or on the interior-room wall. They would detect the intruder before penetration of the interior wall is possible