The mobile part of the lives of most people. At the time this text was developed, for example, Brazil registered around 255 million cell phone lines in use, many of which have Internet access. The problem is that behind this “communications revolution” there are so many technologies, that many people are not sure what the differences are between them.
If this is your case, don’t worry. In the next lines, we explain how cell phone works and then addresses the technologies that are known as 2G and 2.5G: TDMA, CDMA, GSM, GPRS, EDGE and HSCSD. The second part of this text shows the technologies that are part of the 3G and 4G categories: CMDA-2000, UMTS (W-CDMA), HSPA (HSDPA and HSUPA), HSPA + and LTE. Come on?
How does cell phone work?
In order for you to better understand the subject, it is convenient to have an idea of how cell phone works. Basically, it is a transmission system that uses radio waves, just like walkie-talkies do, for example.
But there are a number of differences. To begin with, a cell phone network divides a geographical area into segments called cells (hence the name “cell”). Each cell has a radio base station (called from this point only as a base ) formed by antennas with receivers and signal emitters, and connected to a telephone center.
Another difference is that cell phones are full-duplex systems, which means that when a call is established, the person can hear and speak at the same time. This is possible because a certain radio frequency is used for signal reception, while another is used for broadcasting. Walkie-talkies are half-duplex, that is, they use a single frequency, so only one person can speak at a time.
What essentially happens is that when a call is initiated, the cell phone establishes a connection with the base that covers the cell where the user is. This, in turn, communicates with a central that can locate the cell of the destination number and, thus, establish communication. If the called number belongs to fixed telephony, communication is made with the corresponding telephone exchange.
The interesting thing about the cell scheme is that several of them can use the same frequencies, as long as they are not adjacent. For example, imagine a network made up of fifteen cells, where cells 1, 7, 10 and 14 use the same frequencies. This is possible because neither is next to the other.
It is also interesting to note that each established call can have its cell switched during communication. That’s why you can use your cell phone while traveling by car or train, for example. In this case, when the telephone handset perceives that the signal from the current base is getting weak (that is, more distant), it looks for a closer base. If it does, the transfer begins.
Not infrequently, the limits of a cell are exceeded, but the device cannot find another base (a very common situation in rural areas, for example). The consequence could not be otherwise: the phone call is interrupted. The user will only be able to communicate again when the device finds another base.
Of course, each cell supports a maximum number of phones making calls. When the limit is reached, the surplus, obviously, is left without communication. That is why many people are unable to make calls right after the New Year’s turn, for example, since in events like this there is usually a significant increase in calls made simultaneously.
What determines the maximum capacity of each cell is, among other factors, the technology used, a subject that begins to be addressed from the next topic.
The first generation ( 1G ) of technologies for mobile telephony was marked by the air of novelty: until then, the maximum that most people had as a reference for mobile telephony were landlines with wireless devices. 1G gave rise to the first cell phones, with emphasis on AMPS ( Advanced Mobile Phone System ) technology, an analog standard adopted by several countries, including Brazil and the United States.
However, cell phone only started to be part of the life of the vast majority of people with the arrival of 2G standards, that is, the second generation of mobile technologies defined by AbbreviationFinder. This “revolution” was possible because this generation is made up of digital technologies, which are more stable, cover larger areas and have the capacity to support a large number of users.
Among the most used 2G technologies are TDMA, CDMA and GSM.
What is TDMA?
Acronym for Time Division Multiple Access (something like ” Time Division Multiple Access “), TDMA is a technology that works by causing a communication channel to be divided into slots, that is, at alternate time intervals.
Each established call is destined to two slots: one is used in the direction “telephone to base” and the other in reverse, that is, “base to telephone”. As the slots are switched, one call does not interfere with the other, although they all use the same channel.
TDMA can generally support up to three times more connections than analog technologies that use the same number of channels. This is possible because, as TDMA is a digital technology, the communication data is compressed, causing it to occupy only one third of the channel’s capacity. With that, the other two thirds can be used for other calls.
With regard to cellular telephony, TDMA technology was applied to three widely used systems: IS-54, IS-136 and GSM (this will be discussed later).
The IS-54 is, in a nutshell, the digital alternative to the AMPS (first generation) analog standard. Not least, both share some characteristics, including support for the same frequency range (in the 800 MHz range). For this reason, the IS-54 is also known as D-AMPS ( Digital AMPS ).
The IS-136 is an evolution of the IS-54, having among its main characteristics the support to the 800 MHz and 1,900 MHz bands, in addition to the use of a more advanced control channel.
Cellular telephone systems based on TDMA have been widely used for some time since the 1990s, but have lost space for CDMA and GSM technologies, seen below.
What is CDMA?
TDMA represented an important step for cellular telephony, but encountered limitations over time, especially with regard to the increase in the number of users, although networks based on technology have greater capacity than analog systems. Thus, alternatives had to be found, one of them being CDMA technology, which stands for Code Division Multiple Access (something like ” Code Division Multiple Access “).
Instead of using the slot division scheme, CDMA causes calls, after being digitized, to be “spread” across the frequency channel. In other words, there is no organization by time intervals, as all connections are made at the same time.
In order to distinguish between calls, the information for each call is given a unique encoding. This code is then used by the receiver: the data with the expected identification will be accepted; those that don’t have will be ignored.
Thanks to this method, CDMA is able to reduce interference situations between cells, since it allows the use of equal frequencies in adjacent cells, and allows each of them to support a greater number of users (up to ten times more than AMPS networks), once that the channels are better used.
In cellular telephony, the first widely used system based on CDMA technology is called IS-95, and is also referred to as CDMAOne (trade name used by the company Qualcomm).
There was also a revision called IS-95B, whose main differential is to allow data traffic at a maximum speed of 64 Kb / s ( kilobits per second ) – the original IS-95 only allows rates up to 14.4 Kb/s.
CDMA typically uses the 800 MHz and 1,900 MHz frequency bands.
In Brazil, CDMA technology had a lot of use thanks to the operator Vivo. However, the company gradually migrated its CDMA network to GSM technology, following a trend that was perceived in several countries.
What is GSM?
Acronym for Global System for Mobile Communications (something like ” Global System for Mobile Communications “), GSM is a technology developed and widely used in Europe, and it was not long before it arrived in countries on other continents, including Brazil. Not least, it is the most widespread mobile technology today.
GSM also performs transmissions based on the TDMA standard, although it does it a little differently compared to the IS-54 and IS-136 systems, since it uses up to eight slots on each channel.
It is worth noting, however, that GSM can also work with the FDMA standard ( Frequency Division Multiple Access – “Multiple Access by Frequency Division”) which, as the name implies, divides a frequency range and grants each part obtained to a connection. It is a method widely used in analog systems.
A very interesting feature of GSM technology is the use of a device called SIM ( Subscriber Identity Module – something like “Subscriber Identification Module”). Known in Brazil simply as “chip” or as “SIM card”, this tiny device stores information regarding the phone line and the user, such as number, operator, contact list, among others.
SIM cards are very popular because, thanks to them, users can change their cell phone while keeping the same number. To do this, simply transfer the card from the old device to the new one. CDMA networks do not require a SIM card, but the user needs to turn to their telephone operator to be able to change devices without changing numbers.
The use of SIM cards also helps to avoid a recurring problem on CDMA mobile devices: cloning the device. With the use of certain equipment and techniques, a malicious individual can cause a device to obtain the same number as another cell phone. With the popularization of GSM technology, this practice has been reduced dramatically, since the individual needs to clone the SIM card, an activity that is much more difficult.
Still in terms of security, GSM technology uses protection via encryption to prevent users’ data from being unduly intercepted.
Another advantage of GSM is that, because it is a technology used in several countries, it becomes easier to activate the roaming feature, which allows a line from a given location to work on networks from other places (cities or countries) from the infrastructure of the same operator or from companies associated with it.
GSM networks can operate on various frequencies, with the 900 MHz, 1,800 MHz and 1,900 MHz bands being the most common. The chosen range varies according to the country and the operator.
Among the security features of GSM technology is IMEI ( International Mobile Equipment Identify ). It is a unique numerical sequence for each device and defined by the manufacturer. Knowing the IMEI is important because, in case of loss or theft of the device, the operator will find it easier to block it remotely.
The IMEI consists of fifteen digits: the first eight have information regarding the manufacturer and model of the device; the other six digits correspond to the serial number; the last digit is a “checker” number, that is, it certifies the validity of the entire code.
To get your device’s IMEI, just type the sequence * # 06 # into it. The number can also be recorded on the back of the device, usually in the battery compartment.
For a while, cellular telephony served only its original purpose: to allow voice communication. However, as you know, mobile telephony also allows the exchange of short text messages via SMS ( Short Message Service ) and internet access.
These possibilities became reality based on the 2G standards, however they started to attract users in a wide way with the arrival of 2.5G technologies, a name that was never officially adopted, but that started to be used by manufacturers and commerce for convenience.
In fact, assigning 2.5G technologies to the “third generation” classification would be a fallout, since these standards use 2G systems as a basis – while they essentially work with circuit switching (making the channel remain busy for the entire duration). communication), 2.5G complements these networks with data packet switching capabilities (the channel remains busy only when data is transferred).
The 2.5G category is essentially formed by GPRS, EDGE and HSCSD technologies.
What is GPRS?
Acronym for General Packet Radio Service (something like ” Packet Radio Transmission Standard”), GRPS is based on GSM technology, functioning as an “extension” of this, roughly speaking. Its focus, of course, is data transfer, more precisely, communication with the internet, given its compatibility with the IP protocol. This activity can be performed simultaneously with voice calls.
GPRS also uses multiple slots (maximum of eight), but as it works with packet switching, they remain active only when sending or receiving data. That is why most operators do not charge for Internet usage time, but for the amount of data transferred.
Theoretically, the GRPS data transfer speed can reach 171.2 Kb / s (kilobits per second), but it usually does not exceed 80 Kb / s. It all depends on a number of factors, especially the number of slots in use, as each has a certain speed: if a channel encoding scheme called CS-1 is in use, each slot transmits up to 9.05 Kb / s; if CS-2 is used, the limit is 13.4 Kb / s; in the case of CS-3, the maximum rate is 15.6 Kb / s; finally, the slot reaches 21.4 Kb / s with CS-4.
In most cases, operators use five slots, one of which is for uploading. You can also find a scheme where three slots are for download and two for upload.
As you can see, these are low rates for current standards, however, they represent an evolution if we consider that GSM, by itself, achieves a maximum transfer of only 9.6 Kb / s.
It is worth noting that other systems based on TDMA technology, such as IS-136, are also compatible with GPRS.
What is EDGE?
Acronym for Enhanced Data Rates for GSM Evolution (something like “Improved Data Transfer for GSM Evolution”), EDGE is also based on GSM technology, but it is more sophisticated than the GPRS standard. There are even those who consider EDGE as an “improved” GPRS ( Enhanced GPRS ).
In fact, the characteristics of EDGE are very similar to the specifications of GPRS, including the use of multiple slots for connections, but the standard uses a more advanced modulation scheme ( 8-PSK ) and new types of channel encoding, making that data transfer rates increase considerably: the theoretical maximum speed of the technology is 473.6 Kb / s, although it hardly exceeds 384 Kb / s.
The better performance in EDGE’s data transfer opened space for an application hitherto virtually unprecedented in mobile internet access: streaming, that is, continuous data transmission, such as a live video show, for example.
Interestingly, there are those who consider EDGE technology as part of the 3G category because of its late appearance. However, as the standard is not officially recognized as such, EDGE is often referred to as a “2.75G” technology.
What is HSCSD?
Acronym for High Speed Circuit Switched Data (something like ” High Speed Circuit Data Switching”), HSCSD technology is also based on the GSM standard, actually appearing before GPRS. This is a specification that had little impact on mobile internet access, especially due to its low speed (maximum 57.6 Kb / s) and to use circuit switching, making its charging based on usage time. and not in the amount of data being transferred.