Global System for Mobile Communications (GSM)
Before the Global System for Mobile Communications (GSM) was developed, the countries of Europe used a number of incompatible first-generation cellular phone technologies. GSM was developed to provide a common second-generation technology for Europe so that the same subscriber units could be used throughout the continent. The technology has been extremely successful and is probably the most popular standard worldwide for new implementations. GSM first appeared in 1991 in Europe. Similar systems have now been implemented in North and South America, Asia, North Africa, the Middle East, and Australia. The GSM Alliance claims 200 million customers worldwide and almost 5 million in the U.S. and Canada. Second-generation systems such as GSM are based on the first-generation systems and share many characteristics.
One attractive and novel feature of GSM is the Subscriber Identity Module (SIM). This portable device takes the form of a smart card or plug-in module that stores the subscriber's identification number, the networks that the subscriber is authorized to use, encryption keys, and other information specific to the subscriber. The GSM subscriber units are totally generic until an SIM is inserted. Therefore, a subscriber need only carry his or her SIM to use a wide variety of subscriber devices in many countries, simply by inserting the SIM in the device to be used. In fact, except for certain emergency communications, the subscriber units will not work without a SIM inserted. Thus, the SIMs roam, but not necessarily the subscriber devices.
GSM transmission is encrypted for privacy; in contrast, Advanced Mobile Phone System (AMPS) transmission can easily be monitored. A stream cipher, A5, is used to encrypt the transmission from subscriber to base transceiver. However, the conversation is in the clear in the landline network. Another cipher, A3, is used for authentication. In addition to classical telephony services, GSM has been designed from the ground up to support data and image services. These services are based on the ISDN model. User data rates up to 9.6 Kbps are supported.
Figure 1 indicates the major elements of GSM and a typical topology. The four basic elements are the subscriber, the base transceiver, the base station controllers, and the mobile services switching center (MSSC). We use the term subscriber rather than mobile for GSM because the SIM makes the subscriber the more relevant entity. The base station controllers may be collocated with the base transceivers or with the mobile services switching center. If base transceiver and base station controller are collocated, the pair is often called a base station subsystem. While the base station controller has much of the functionalitycontrolling handoffs, power levels, and frequency assignments for the subscribersthe mobile services switching center is the heart of the system. It provides the interface to the public wired network and carries out the data collection for billing. It's supported by four databases, which it controls.
Figure 1 GSM layout.
The home location register database (HLR) stores information, both permanent and temporary, about each subscriber that "belongs" to it (that is, for which the subscriber has its telephone number associated with the switching center). One important temporary piece of information is the location of the subscriber. The location is determined by the visitor location register (VLR) database, into which the subscriber is entered. The visitor location register maintains information about subscribers who are currently physically in the region covered by the switching center. It records whether the subscriber is active and other parameters associated with the subscriber.
For a call coming to the subscriber, the system uses the telephone number associated with the subscriber to identify the home switching center of the subscriber. This switching center can then find in its home location register the switching center in which the subscriber is currently physically located. For a call coming from the subscriber, the visitor location register is used to initiate the call. Even if the subscriber is in the area covered by its home switching center, it's also represented in the switching center's visitor location register, for consistency.
The authentication center database (AuC) is used for authentication activities of the system; for example, it holds the authentication and encryption keys for all the subscribers in both the home and visitor location registers. The center controls access to user data as well as being used for authentication when a subscriber joins a network. Finally, the equipment identity register (EIR) database keeps track of the type of equipment that exists at the mobile station. It also plays a role in security (blocking calls from stolen mobile stations and preventing use of the network by stations that have not been approved).
The GSM spectral allocation is 25 MHz for base transmission (935960 MHz) and 25 MHz for mobile transmission (890915 MHz). Users access the network using a combination of frequency-division multiple access (FDMA) and time-division multiple access (TDMA). There are radio-frequency carriers every 200 kHz, which provides for 124 full-duplex channels. The channels are modulated at a data rate of 270.833 Kbps. As with AMPS, there are two types of channels, traffic and control.
Traffic channels can carry digitized speech or user data. Each carrier can carry up to eight conversations using time-division multiple access. Each analog voice signal is converted using linear predictive coding into a 13 Kbps digital signal. (There are options at lower data rates as well.)
There are three general types of control channels: broadcast, common control, and dedicated control. Broadcast channels operate only from base transceivers to subscriber units. These channels act as a reference signal for mobiles; the channels identify the cell and network identity to mobiles, provide information on the status of the cell (for example, channel availability and congestion information), and provide synchronization. The common control channels are used to page subscribers, receive asynchronous from subscribers, and assign channels to subscribers. The dedicated control channels provide signaling services to subscribers.
Two virtually identical systems, DCS-1800 and DCS-1900, are based on GSM and differ mainly in using spectral allocations at 1800 and 1900 MHz, respectively. DCS-1900 is used primarily in the United States.