General Concepts

1.2 General Concepts

1.2.1 Analog Versus Digital Telephony Systems

First-generation systems were analog. During the early 1980s these systems underwent rapid development in Europe. Although the NMT system was used by all the Nordic countries, and the TACS system in the United Kingdom and Italy, there was a variety of systems and no compatibility among them. Compared with these systems, the main advantages offered by GSM, which is the most important of the second-generation digital systems, are:

  • Standardization;

  • Capacity;

  • Quality;

  • Security.

Standardization guarantees compatibility among systems of different countries, allowing subscribers to use their own terminals in those countries that have adopted the digital standard. The lack of standardization in the first-generation system limited service to within the borders of a country. Mobility is improved, since roaming is no longer limited to areas covered by a certain system (see Section 1.2.6). Calls can be charged and handled using the same personal number even when the subscriber moves from one country to another.

Standardization also allows the operator to buy entities of the network from different vendors, since the functional elements of the network and the interfaces between these elements are standardized. This means that a mobile phone from any manufacturer is able to communicate with any network, even if this network is built with entities from different vendors. This leads to a large economy of scale and results in cost reduction for both the operator and the subscriber. Furthermore, the phone cost is also reduced, because as GSM is an international standard, produced quantities are greater and the level of competition is high.

With respect to capacity, the use of the radio resource is much more efficient in a digital system such as GSM than in an analog system. This means that more users can be allocated in the same frequency bandwidth. This is possible with the use of advanced digital techniques, such as voice compression algorithms, channel coding, and multiple access techniques. Note that capacity gains are also achieved with radio frequency reuse, which had also been used in analog systems. Frequency reuse means that a given carrier can be employed in different areas, as explained in Section 1.2.2.

The quality in digital transmission systems is better, thanks to the channel coding schemes that increase the robustness in the face of noise and disturbances such as interference caused by other users or other systems. The quality improvement is also due to the improved control of the radio link, and adaptations to propagation conditions, with advanced techniques such as power control or frequency hopping. This will be explained in greater detail in Section

In terms of security, powerful authentication and encryption techniques for voice and data communications are enabled with GSM, which guarantees protected access to the network, and confidentiality.

1.2.2 Cellular Telephony

In mobile radio systems, one of the most important factors is the frequency spectrum. In order to make the best use of the bandwidth, the system is designed by means of the division of the service area into neighboring zones, or cells, which in theory have a hexagonal shape. Each cell has a base transceiver station (BTS), which to avoid interference operates on a set of radio channels different from those of the adjacent cells. This division allows for the use of the same frequencies in nonadjacent cells. A group of cells that as a whole use the entire radio spectrum available to the operator is referred to as a cluster. The shape of a cell is irregular, depending on the availability of a spot for the BTS, the geography of the terrain, the propagation of the radio signal in the presence of obstacles, and so on.

In dense urban areas, for instance, where the mobile telephony traffic is important, the diameter of the cells is often reduced in order to increase capacity. This is allowed since the same frequency channels are used in a smaller area. On the other hand, reducing the cell diameter leads to a decrease in the distance necessary to reuse the frequencies (that is, the distance between two cochannel cells), increasing cochannel interference. In order to minimize the level of interference, several techniques are used on the radio interface.

A basic example of cluster organization is shown in Figure 1.1. In this example, we see a reuse pattern for seven different frequencies, f1 to f7. These frequencies correspond to the beacon carrier of each cell, on which signaling information about the cell is broadcast (see Section 1.2.7). It can be seen from this figure that a given carrier can be reused in two separate geographical areas, as long as these areas are far enough from each other to reduce the effect of interference. With this technique of dividing the area in cells and clusters, the operator can increase the area it is able to cover with a limited frequency bandwidth.

Click To expand Figure 1.1: Example of a cell planning.

1.2.3 Public Land Mobile Network

A public land mobile network (PLMN) is a network established for the purpose of providing land mobile telecommunications services to the public. It may be considered as an extension of a fixed network, such as the Public Switched Telephone Network (PSTN), or as an integral part of the PSTN.

1.2.4 Multiband Mobile Phones

Because of the increasing demand on the mobile networks, today the mobile stations (MSs) tend to be multiband. Indeed, to avoid network saturation in densely populated regions, mobile phones capable of supporting different frequency bands have been implemented, to allow for the user making communications in any area, at any time.

A dual-band phone can operate in two different frequency bands of the same technology, for instance in the 900-MHz and 1800-MHz frequency bands of the GSM system. Triple-band mobile phones have also come on the market, with the support of GSM-900 (900-MHz GSM band), DCS-1800 (1800-MHz GSM band), and PCS-1900 (1900-MHz GSM band), for example. Note that DCS-1800 and PCS-1900 are never deployed in the same country, and therefore this kind of phone can be used by travelers who want to have service coverage in a large number of countries.

1.2.5 SIM Card

One of the most interesting innovations of GSM is that the subscriber's data is not maintained in the mobile phone. Rather a "smart card," called a subscriber identity module (SIM) card, is used.

The SIM is inserted in the phone to allow the communications. A user may thus make telephone calls with a mobile phone that is not his own, or have several phones but only one contract. It is for example possible to use a SIM card in a different mobile when traveling to a country that has adopted the GSM on a different frequency band. A European can therefore rent a PCS1900 phone when traveling to the United States, while still using his own SIM card, and thus may receive or send calls. The SIM is used to keep names and phone numbers, in addition to those that are already kept in the phone's memory.

The card is also used for the protection of the subscriber, by means of a ciphering and authentication code.

1.2.6 Mobility

GSM is a cellular telephony system that supports mobility over a large area. Unlike cordless telephony systems, it provides location, roaming, and handover. Location Area

The ability to locate a user is not supported in first-generation cellular systems. This means that when a mobile is called, the network has to broadcast the notification of this call in all the radio coverage. In GSM, however, location areas (LAs), which are groups of cells, are defined by the operator. The system is able to identify the LA in which the subscriber is located. This way, when a user receives a call, the notification (or paging) is only transmitted in this area. This is far more efficient, since the physical resource use is limited. Roaming

In particular, the GSM system has the capability of international roaming, or the ability to make and receive phone calls to and from other nations as if one had never left home. This is possible because bilateral agreements have been signed between the different operators, to allow GSM mobile clients to take advantage of GSM services with the same subscription when traveling to different countries, as if they had a subscription to the local network. To allow this, the SIM card contains a list of the networks with which a roaming agreement exists.

When a user is "roaming" to a foreign country, the mobile phone automatically starts a search for a network stipulated on the SIM card list. The choice of a network is performed automatically, and if more than one network is given in the list, the choice is based on the order in which the operators appear. This order can be changed by the user. The home PLMN is the network in which the user has subscribed, while the visited PLMN often refers to the PLMN in which the user is roaming. When a user receives a call on a visited PLMN, the transfer of the call from the home PLMN to the visited PLMN is charged to the called user by his operator. Handover

When the user is moving from one cell to the other during a call, the radio link between BTS 1 and the MS can be replaced by another link, between BTS 2 and the MS. The continuity of the call can be performed in a seamless way for the user. This is called handover. With respect to dual-band telephones, one interesting feature is called the dual-band handover. It allows the user in an area covered both by the GSM-900 and by the DCS-1800 frequency bands, for instance, to be able to transfer automatically from one system to the other in the middle of a call.

1.2.7 Beacon Channel

For each BTS of a GSM network, one frequency channel is used to broadcast general signaling information about this cell. This particular carrier frequency is called a beacon channel, and it is transmitted by the BTS with the maximum power used in the cell, so that every MS in the cell is able to receive it.

1.2.8 MS Idle Mode

When it is not in communication, but still powered on, the MS is said to be in idle mode. This means that it is in a low consumption mode, but synchronized to the network and able to receive or initiate calls.