Not shown in Fig. 1 are some elements of the network that complicate security efforts. There may be a corporate firewall or network address translation (NAT) features that will require communication between the building services staff and IT staff, but will also provide greater protection against IT threats. There may be other external connections from a trusted network that bypass the SR, . a modem connection. Also not shown are physical security implementation details. Are the controllers in secure locations? Is there a security policy that governs operator access to the system (passwords, keys), mainte- nance procedures, presence of tamper.
