【文章內(nèi)容簡(jiǎn)介】 es offered by GSM, which is the most important of the secondgeneration digital systems, are: Standardization。Capacity。Quality。 guarantees patibility among systems of different countries, allowing subscribers to use their own terminals in those countries that have adopted the digital lack of standardization in the firstgeneration system limited service to within the borders of a is improved, since roaming is no longer limited to areas covered by a certain can be charged and handled using the same personal number even when the subscriber moves from one country to 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 means that a mobile phone from any manufacturer is able to municate with any network, even if this network is built with entities from different leads to a large economy of scale and results in cost reduction for both the operator and the , the phone cost is also reduced, because GSM is an international standard, produced quantities are greater and the level of petition is respect to capacity, the use of the radio resource is much more efficient in a digital system such as GSM than in an analog means that more users can be allocated in the same frequency is possible with the use of advanced digital techniques, such as voice pression algorithms, channel coding, and multiple access that capacity gains are also achieved with radio frequency reuse, which had also used in analog reuse means that a given carrier can be employed in different 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 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 will be explained in greater detail in the next terms of security, powerful authentication and encryption techniques for voice and data munications are enabled with GSM, which guarantees protected access to the network, and Cellular Telephony In mobile radio systems, one of the most important factors is the frequency 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 cell has a Base Transceiver Station(BTS), which to avoid interference operates on a set of radio channels different from those of the adjacent division allows for the use of the same frequencies in nonadjacent group of cells that as a whole use the entire radio spectrum available to the operator is referred to as a 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 dense urban areas, for instance, where the mobile telephony traffic is important, the diameter of the cells is often reduced in order to increase is allowed since the same frequency channels are used in a smaller 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 order to minimize the level of interference, several techniques are used on the radio basic example of cluster organization is shown in Figure this example, we see a reuse at