page_title: Access control
section_title: Access control system topologies
hierarchical_section_title: Access control / Physical security / Access control system topologies
caption_attribution_description: English: Access control system diagrams, using terminal servers.
context_page_description: In the fields of physical security and information security, access control is the selective restriction of access to a place or other resource while access management describes the process. The act of accessing may mean consuming, entering, or using. Permission to access a resource is called authorization.
  Locks and login credentials are two analogous mechanisms of access control.
context_section_description: 1. Serial controllers. Controllers are connected to a host PC via a serial RS-485 communication line (or via 20mA current loop in some older systems). External RS-232/485 converters or internal RS-485 cards have to be installed, as standard PCs do not have RS-485 communication ports.
  Advantages:
  RS-485 standard allows long cable runs, up to 4000 feet (1200 m)
  Relatively short response time. The maximum number of devices on an RS-485 line is limited to 32, which means that the host can frequently request status updates from each device, and display events almost in real time.
  High reliability and security as the communication line is not shared with any other systems.
  Disadvantages: 
  RS-485 does not allow Star-type wiring unless splitters are used
  RS-485 is not well suited for transferring large amounts of data (i.e. configuration and users). The highest possible throughput is 115.2 kbit/sec, but in most system it is downgraded to 56.2 kbit/sec, or less, to increase reliability.
  RS-485 does not allow the host PC to communicate with several controllers connected to the same port simultaneously. Therefore, in large systems, transfers of configuration, and users to controllers may take a very long time, interfering with normal operations.
  Controllers cannot initiate communication in case of an alarm. The host PC acts as a master on the RS-485 communication line, and controllers have to wait until they are polled.
  Special serial switches are required, in order to build a redundant host PC setup.
  Separate RS-485 lines have to be installed, instead of using an already existing network infrastructure.
  Cable that meets RS-485 standards is significantly more expensive than regular Category 5 UTP network cable.
  Operation of the system is highly dependent on the host PC. In the case that the host PC fails, events from controllers are not retrieved, and functions that require interaction between controllers (i.e. anti-passback) stop working.
  2. Serial main and sub-controllers. All door hardware is connected to sub-controllers (a.k.a. door controllers or door interfaces). 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.
  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 sam
caption: Access control systems using serial controllers and terminal servers
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