Project PDA

GSM or Global System for Mobile Communications. GSM, a 2G technology, is the de facto European standard for digital cellular telephone service, and it is also available in the Americas. GSM is the most widely used of the three digital wireless telephone technologies (TDMA, GSM and CDMA), and it supports voice, data, text messaging and cross-border roaming. The SIM (Subscriber Identification Module), a removable plastic card that contains a users data, is an essential element in a GSM network.

The mobile phone platform used in Europe and much of the rest of the world- though it is not yet mainstream in North America. Dual band phones are capable of operating on other bandwidths abroad. GSM operates in multiple frequency bands, GSM 900, GSM 1800, GSM 1900. When GSM is working on a radio frequency of 1800 MHz, it is sometimes referred to as DCS 1800, GSM1800 or PCN.

GSM is a cellular network, which means that mobile phones connect to it by searching for cells in the immediate vicinity. GSM networks operate in four different frequency ranges. Most GSM networks operate in the 900 MHz or 1800 MHz bands. Some countries in the Americas (including the United States and Canada) use the 850 MHz and 1900 MHz bands because the 900 and 1800 MHz frequency bands were already allocated.

The rarer 400 and 450 MHz frequency bands are assigned in some countries, notably Scandinavia, where these frequencies were previously used for first-generation systems.

In the 900 MHz band the uplink frequency band is 890-915 MHz, and the downlink frequency band is 935-960 MHz. This 25 MHz bandwidth is subdivided into 124 carrier frequency channels, each spaced 200 kHz apart. Time division multiplexing is used to allow eight full-rate or sixteen half-rate speech channels per radio frequency channel. There are eight radio timeslots (giving eight burst periods) grouped into what is called a TDMA frame. Half rate channels use alternate frames in the same timeslot. The channel data rate is 270.833 kbit/s, and the frame duration is 4.615 ms.

The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.

GSM was designed with a moderate level of security. The system was designed to authenticate the subscriber using shared-secret cryptography. Communications between the subscriber and the base station can be encrypted. The development of UMTS introduces an optional USIM, that uses a longer authentication key to give greater security, as well as mutually authenticating the network and the user - whereas GSM only authenticated the user to the network (and not vice versa). The security model therefore offers confidentiality and authentication, but limited authorization capabilities, and no non-repudiation.