<Li> The overall cost of the system is lower, as sub-controllers are usually simple and inexpensive devices . </Li> <Li> All other advantages listed in the first paragraph apply . </Li> <Ul> <Li> Operation of the system is highly dependent on main controllers . In case one of the main controllers fails, events from its sub-controllers are not retrieved, and functions that require interaction between sub-controllers (i.e. anti-passback) stop working . </Li> <Li> Some models of sub-controllers (usually lower cost) do not have the memory or processing power to make access decisions independently . If the main controller fails, sub-controllers change to degraded mode in which doors are either completely locked or unlocked, and no events are recorded . Such sub-controllers should be avoided, or used only in areas that do not require high security . </Li> <Li> Main controllers tend to be expensive, therefore such a topology is not very well suited for systems with multiple remote locations that have only a few doors . </Li> <Li> All other RS - 485 - related disadvantages listed in the first paragraph apply . </Li> </Ul> <Li> Operation of the system is highly dependent on main controllers . In case one of the main controllers fails, events from its sub-controllers are not retrieved, and functions that require interaction between sub-controllers (i.e. anti-passback) stop working . </Li>

Access control is build of which of the following key principles