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Reply To: Access class


GSM recognizes 16 different access classes. This parameter is
stored on the SIM module and allows the network operator to specifically bar
certain types of subscribers. A typical application is to set up an access class
exclusively for the operator personnel for test purposes during installation and
testing. In that case, the system can be on the air but ordinary users do not
recieve access. Another application is to define access classes for emergency personnel
only. This can prevent overload during an emergency and allows rescue
workers to be reachable via mobile phone.
The BTS broadcasts the admitted access classes within the RACH control
parameters, which are part of the information that the BTS permanently
broadcasts in its broadcast control channel (BCCH). The MS reads the information
and compares it with the access classes on the SIM. The MS attempts to
access the system only if it finds a matching access class. That prevents signaling
overload because an unauthorized MS does not even try to access the system.

The access classes in GSM use values from 0 to 15. The numbers do not
indicate any priority as such, that is, a higher number does not imply a higher
priority or vice versa. Table G.2 shows the use of the access classes. “Ordinary”
subscribers receive values from 0 through 9 on a random basis. Only the access
classes 11 through 15 were predefined. Note that one SIM module is capable of
storing several access classes, which allows one subscriber to belong to several
subscriber groups.