【正文】 vices。s identity before access is given to information systems. In addition, a fourth factor of authentication is now recognized: someone you know, where another person who knows you can provide a human element of authentication in situations where systems have been set up to allow for such scenarios. For example, a user may have their password, but have fotten their smart card. In such a scenario, if the user is known to designated cohorts, the cohorts may provide their smart card and password in bination with the extant factor of the user in question and thus provide two factors for the user with missing credential, and three factors overall to allow access. Now, as people39。 he now has access to the server room. To prevent this, twofactor authenticationcan be used. In a two factor transaction, the presented credential and a second factor are needed for access to be granted。 The Access control system Access control system is widely used in recent years and it is one of the hightech and safety equipment , has bee one of the intelligent signs of modern architecture. Access control , the export and the entrance control system, is to control the export and entrance channel system. Access control system is developed on the basis of the traditional locks (Entrance Guard/Access Control in English). Access control is, in reality, an everyday phenomenon. A lock on a car door is essentially a form of access control. A PIN on an ATM system at a bank is another means of access control. Bouncers standing in front of a night club is perhaps a more primitive mode of access control (given the evident lack of information technology involved). The possession of access control is of prime importance when persons seek to secure important, confidential, or sensitive information and equipment .Item control or electronic key management is an area within (and possibly integrated with) an access control system which concerns the managing of possession and location of small assets or physical (mechanical) keys. Physical access by a person may be allowed depending on payment, authorization, etc. Also there may be oneway traffic of people. These can be enforced by personnel such as a border guard, a doorman, a ticket checker, etc., or with a device such as a turnstile. There may be fences to avoid circumventing this access control. An alternative of access control in the strict sense (physically controlling access itself) is a system of checking authorized presence, see . Ticket controller (transportation). A variant is exit control, . of a shop (checkout) or a country. In physical security, the term access control refers to the practice of restricting entrance to a property, a building, or a room to authorized persons. Physical access control can be achieved by a human (a guard, bouncer, or receptionist), through mechanical means such as locks and keys, or through technological means such as access control systems like the Access control vestibule. Within these environments , physical key management may also be employed as a means of further managing and monitoring access to mechanically keyed areas or access to certain small assets . Physical access control is a matter of who , where, and when. An access control system determines who is allowed to enter or exit, where they are allowed to exit or enter, and when they are allowed to enter or exit. Historically this was partially acplished through keys and locks. When a door is locked only someone with a key can enter through the door depending on how the lock is configured. Mechanical locks and keys do not allow restriction of the key holder to specific times or dates. Mechanical locks and keys do not provide records of the key used on any specific door and the keys can be