all requests to the main controllers. Main controllers usually support from 16 to 32 sub-controllers. Advantages: Work load on the host PC is significantly reduced, because it only needs to communicate with a few main controllers. The overall cost of the system is lower, as sub-controllers are usually simple and inexpensive devices. All other advantages listed in the first paragraph apply. Disadvantages: 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. 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. 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. All other RS-485-related disadvantages listed in the first paragraph apply. 3. Serial main controllers & intelligent readers. All door hardware is connected directly to intelligent or semi-intelligent readers. Readers usually do not make access decisions, and forward all requests to the main controller. Only if the connection to the main controller is unavailable, will the readers use their internal database to make access decisions and record events. Semi-intelligent reader that have no database and cannot function without the main controller should be used only in areas that do not require high security. Main controllers usually support from 16 to 64 readers. All advantages and disadvantages are the same as the ones listed in the second paragraph. 4. Serial controllers with terminal servers. In spite of the rapid development and increasing use of computer networks, access control manufacturers remained conservative, and did not rush to introduce network-enabled products. When pressed for solutions with network connectivity, many chose the option requiring less efforts: addition of a terminal server, a device that converts serial data for transmission via LAN or WAN. Advantages: Allows utilizing the existing network infrastructure for connecting separate segments of the system. Provides a convenient solution in cases when the installation of an RS-485 line would be difficult or impossible. Disadvantages: Increases complexity of the