system. Creates additional work for installers: usually terminal servers have to be configured independently, and not through the interface of the access control software. Serial communication link between the controller and the terminal server acts as a bottleneck: even though the data between the host PC and the terminal server travels at the 10/100/1000Mbit/sec network speed, it must slow down to the serial speed of 112.5 kbit/sec or less. There are also additional delays introduced in the process of conversion between serial and network data. All the RS-485-related advantages and disadvantages also apply. 5. Network-enabled main controllers. The topology is nearly the same as described in the second and third paragraphs. The same advantages and disadvantages apply, but the on-board network interface offers a couple of valuable improvements. Transmission of configuration and user data to the main controllers is faster, and may be done in parallel. This makes the system more responsive, and does not interrupt normal operations. No special hardware is required in order to achieve redundant host PC setup: in the case that the primary host PC fails, the secondary host PC may start polling network controllers. The disadvantages introduced by terminal servers (listed in the fourth paragraph) are also eliminated. 6. IP controllers. Controllers are connected to a host PC via Ethernet LAN or WAN. Advantages: An existing network infrastructure is fully utilized, and there is no need to install new communication lines. There are no limitations regarding the number of controllers (as the 32 per line in cases of RS-485). Special RS-485 installation, termination, grounding and troubleshooting knowledge is not required. Communication with the controllers may be done at the full network speed, which is important if transferring a lot of data (databases with thousands of users, possibly including biometric records). In case of an alarm, controllers may initiate connection to the host PC. This ability is important in large systems, because it serves to reduce network traffic caused by unnecessary polling. Simplifies installation of systems consisting of multiple sites that are separated by large distances. A basic Internet link is sufficient to establish connections to the remote locations. Wide selection of standard network equipment is available to provide connectivity in various situations (fiber, wireless, VPN, dual path, PoE) Disadvantages: The system becomes susceptible to network related problems, such as delays in case of heavy traffic and network equipment failures. Access controllers and workstations may become