Three GPRS classes have been defined: class A, class B, and class C.
The class A mobile can support simultaneously a communication in circuit-switched mode and another one in packet-switched mode. It is also capable of detecting in idle mode an incoming call in circuit or packet-switched mode.
The class B mobile can detect an incoming call in circuit-switched mode or in packet-switched mode during the idle mode but cannot support them simultaneously. The circuit and packet calls are performed sequentially. In some configurations desired by the user, a GPRS communication may be suspended in order to perform a communication in circuit-switched mode and then may be resumed after the communication release in circuit-switched mode.
The class C mobile supports either a communication in circuit-switched mode or in packet-switched mode but is not capable of simultaneously supporting communications in both modes. It is not capable of simultaneously detecting the incoming calls in circuit-switched and packet-switched mode during idle mode. Thus a class C mobile is configured either in circuit-switched mode or in packet-switched mode. The mode configuration is selected either manually by the user or automatically by an application.
A mobile defined in class A or class B is IMSI attached for GPRS services, and non-GPRS services while a mobile defined in class C is IMSI attached if it operates in circuit-switched mode or IMSI attached for GPRS services if it operates in packet-switched mode. (Note: An MS that is IMSI attached means that it is attached to the GSM network.)