What are the options for remote maintenance of a WinCC flexible Runtime system (Panel/PC) via the Internet (WAN)?

Introduction: Using the options WinCC flexible / Sm@rtAccess and Sm@rtService you can also access a WinCC flexible Runtime system via the Internet. The connection can be made on the basis of an ISDN or DSL network. Sample application: Your customer wants you to set up external teleservicing of an operator panel in order to be able to access the operator panel via the Internet if there is a fault