Example Applications - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 348

The central computer runs a database in which each terminal is assigned all the data
carriers authorised for access to that terminal. The authorisation data generated from
the database is loaded into the terminals (or into an intermediate door control unit) via
the network and saved there in a table.
Changes to an individual's access authorisation can be made by a single entry on
the central computer of the access control system. The data carrier itself does not need
to be present, since only an entry in the central database has to be edited. This is
advantageous, because it means that sensitive security areas can be protected against
unauthorised access even in the event of a data carrier being lost.
The data carriers of an online system only have to be able to store a small amount
of data, for example a unique pass number. The use of read-only transponders is also
possible. See Figure 13.16.
13.4.2 Offline systems
Offline systems have become prevalent primarily in situations where many individual
rooms, to which only a few people have access, are to be equipped with an electronic
access control system. Each terminal saves a list of key identifiers (e.g. general-key-3,
floor-waiter-7, guest-room-517), for which access to this terminal is to be authorised.
There is no network to other terminals or a central computer.
Information regarding the rooms to which the data carrier can provide access is
stored on the data carrier itself in the form of a table of key identifiers (e.g. 'guest-
room-517', 'sauna', 'fitness-room'). The terminal compares all the key identifiers stored
on a data carrier with those stored in its own list and permits access as soon as a
match is found. The transponder is programmed at a central programming station ,
for example at the reception of a hotel upon the arrival of the guest. In addition to
the authorised rooms, the transponder can also be programmed with the duration of
validity, so that hotel keys, for example, are automatically invalidated on the departure
date of the guest.
Figure 13.16 Access control and time keeping are combined in a single terminal. The watch
with an integral transponder performs the function of a contactless data carrier (reproduced by
permission of Legic -Installation, Kaba Security Locking Systems AG, CH-Wetzikon)