All door hardware is connected to sub-controllers. Sub-controllers usually do not make access decisions, and instead forward all requests to the main controllers. Main controllers usually support from 16 to 32 sub-controllers. TCP/IP Modules/interfaces are used as communication interface to the PC to transfer the logs of entry and exist. 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.