The technologies currently used for the transmission of information on networks are called multiple-access, because more than one user can use each of the information cells. There are currently three different ones, which differ in cell access methods:
* FDMA (Frequency Division Multiple Access): Access the cells depending on the frequencies. Basically, it separates the spectrum into different voice channels, dividing the bandwidth into several channels evenly depending on the transmission frequencies. Users share the communication channel, but each uses one of the different subchannels that are partitioned by the frequency. It is mostly used for analogue transmissions, even though it is capable of transmitting digital information (not recommended).
* TDMA (Multiple Time Division access): divides the transmission channel into time partitions. Compress the digital conversations and then send them using the radio signal for a period of time. In this case, different users share the same frequency channel, but they use it at different time intervals. Due to the compression of digital information, this technology allows three times the capacity of an analogue system using the same number of channels.
* CDMA (Multiple access by Code Division): This technology, after digitizing the information is transmitted through the entire bandwidth available, unlike TDMA and FDMA. Calls are superimposed on the transmission channel, differentiated by a single-sequence code. This allows users to share the channel and frequency. As it is a suitable method for the transmission of encrypted information, it was started to be used in the military area. This technology allows to compress from 8 to 10 digital calls so that they occupy the same thing that occupies an analogue call.
The following figure shows a graph of the operation of the aforementioned technologies.
It’s a global standard for cell phones. Called Global System for mobile communications (global mobile communications system), formally known as Group Special Mobile (GSM, Mobile Special group). It was created by CEPT (international body that brings together the entities responsible for the public administration of each European country of the policies and the regulation of the communications, both postal and telecommunications), and subsequently developed By ETSI (European telecommunications Standars Institute – Organization for the standardisation of the telecommunication industry of Europe with global projection) to standardize cellular telephony in Europe, then adopted by the rest of the world. In the year 2001, 70% of mobile phone users in the world used GSM. It is an open, non-proprietary standard and is in constant development.
GSM employs a combination of TDMA and FDMA between stations in a pair of duplex frequency radio channels, with low frequency hopping between channels. As explained above, TDMA is used for digital coded information, so GSM is a system designed to use digital signals, as well as digital voice channels, allowing for a moderate level of security.
There are four main versions, based on the band: GSM-850, GSM-900, GSM-1800 and GSM-1900, being each in the frequency of the bands.
In GSM, the connections can be used both to the voice and to data, which allowed the advance of the sending and consumption of data through the cellular ones. The most common cases are the images that can be sent and received, and the use of applications through mobile phones, such is the case of the Internet.
The fastest implementations of GSM are called GPRS and EDGE, also called intermediate generations, or 2.5 g, which lead to the third generation (3 G), or UMTS.
It’s basically a data packet-based communication. In GSM, time intervals are allocated via a switched connection, while GPRS is assigned by a system based on the need for packet connection. In other words, if no data is sent by the user, the frequencies are free to be used by other users. GPRS phones usually use a Bluetooth port for data transfer.
1-GPRS (General Packet Radio Service)
It is a GPRS update, which packs up to 69.2 Kbps in eight timeslots, considered a technology of 2.75 G, a little more evolved than GPRS. Geran (GPS/Edge Radio access Network) is the name given to the standards for GPS/edge access.