2.9 Session Management from a User Point of View
GPRS can also be used to connect devices such as notebooks to the Internet. This can be done by connecting the PC or notebook to a mobile device over a USB cable, via infrared or Bluetooth. In all cases, the mobile device acts as a wireless ‘modem’ for the external device over a virtual serial interface. Physical serial interfaces are no longer used for this purpose, as the serial transmission speeds have become too slow, especially for multimode GPRS, UMTS and LTE devices.
Before we take a look at how the mobile device can be used as a wireless ‘modem’ to establish a GPRS connection to the Internet or company intranet, let us first look at the process that was and to some extent is still used to establish a circuit-switched connection to the Internet (see Figure 2.32). The process is the same for a fixed-line modem or mobile device acting as a wireless circuit-switched modem.
Modems communicate with external devices with the standardized AT command set, which is an ASCII command and response language. For example, to dial a telephone number, the notebook sends a dial command which includes the telephone number (e.g. ‘ATD 0399011782’) to the modem. The modem will then try to establish a circuit-switched data connection with the other end. If successful, the modem will return a connect ...