We have all been using mobile phones for a long time, not only in the classic role of “buy-bread” and “will-have-drink”, but also for honing typing skills by typing SMS. These functions are gradually relegated to the background (well, except for “will you drink” :)). Telephones are increasingly being used in the work environment, and we work as luck would have it - both in offices and on the road. And although shaking in a train compartment and sitting in a chair are not exactly the same thing, access to information should sometimes be the same. It is as a means of quick access to the necessary information on a “now” basis that GPRS and EDGE technologies are being introduced into our daily lives. So, what kind of fruits are these, and let’s try to figure it out.
Where did GPRS come from in Rus'?
GPRS – stands for General Packet Radio Service, in our opinion – “wireless data transmission”. Now this technology has been implemented by all global cellular operators. Moreover, this was done abroad much earlier than in Russia (in general, the age of “bourgeois” GSM networks is 7-10 years older than ours).
Initially, already used GSM networks were “adjusted” for GPRS. What is the principle of operation? In order not to get too bogged down in technical terminology, let’s mention the data transmission rate in a timeslot (time interval) of a radio channel. There are only four of them - CS1, CS2, CS3, CS4.
When communicating with voice or data, the subscriber is allocated a part of the radio path at a speed of about 9.6 kbit/s. The dedicated radio channel is divided into time intervals (timeslots), their number varies depending on the capabilities of the phone and network congestion. GPRS transmission occurs precisely through currently free timeslots. The speed, as we see, is not so hot. This is due to the fact that initially GSM networks were conceived specifically for voice services, and when the need for data transmission struck out of the blue, it was networks of this type that first came under the watchful gaze of developers. So they forged GSM networks, squeezing the maximum out of them, while simultaneously realizing that this is only a temporary alternative, and that they need to develop networks according to their profile.
The introduction of GPRS “in Rus'” took place later, but under slightly better conditions, since foreign providers started from scratch, and after some time there was a need to modernize the equipment. Our networks are relatively young compared to foreign ones; our operators do not have to invest in upgrading outdated equipment - they follow the beaten path, purchasing GPRS-compatible equipment of the latest generation, which, moreover, already supports EDGE (this technology will be discussed Later).
In Russia, almost all federal operators offer GPRS-based services (Beeline, Megafon, MTS, regional companies). More and more territories of our vast homeland are covered by mobile Internet.
Providers provide different statistics on the use of GPRS - the numbers vary depending on the region, time of day, subscriber and operator equipment - from 6 to 45% of the subscriber base.
Phones with GPRS support are divided into 12 speed classes (MultySlot Class). Data transfer speed - up to 40 kbit/s. and more. Phones are also classified according to the way they work with data and voice (GPRS Class). Class A phones can simultaneously transmit data and voice. Class B does not allow you to do it at the same time. Class C supports one of the methods selectively.
Despite the positive changes, we are still far from Japan and the Philippines, recognized leaders in the distribution and use of GPRS.
Although our situation is gradually improving - operator income from the introduction of GPRS is gradually growing in total income.
According to experts, GPRS in Russia is becoming increasingly popular for the following reasons:
- The mobile content market is actively developing. Now, in RuNet there are several hundred WAP resources for which GPRS serves as a “vehicle”.
- The number of phones supporting GPRS is now in the absolute majority.
- Operators are gradually starting to introduce GPRS roaming.
But it is not without difficulties - technical and even strategic. One of the main disadvantages of GPRS in Russia today is its low speed. Theoretically, the maximum data transfer speed using GPRS technology reaches 171.3 kbit/s. In fact, it is much smaller and depends on many objective reasons, namely:
- To operate GPRS, equipment is used that can support either lower speed schemes (CS1–CS2) or higher speed schemes (CS4). Some older cellular base stations cannot work with the CS3–CS4 schemes. Of course, providers are well aware of the current situation and, if possible, replace the equipment with more modern ones.
- The number of requests from a subscriber’s phone and the number of free timeslots that the equipment can allocate may not always coincide, depending on the class of equipment, phone, and simply network congestion.
- Today you can invest money in GPRS-based services without fear, but still they are still in second or third place in importance for operators. If today we are attracted to something, it is tariffs with ridiculous prices for voice communications. As a result, we say, increase the load on the network and... completely forget about GPRS, which is almost impossible to use in such conditions. I think all residents of big cities will agree with me.
- The price of 1 MB of GPRS traffic in Russia is objectively less than abroad. This means that people tend to use the mobile Internet even more actively, thereby loading the network.
- The number of registered and potential MMS users is disproportionately less than it actually is, but MMS is also a GPRS-based service, and one that is actively advertised. There is not enough network capacity for it either.
- There are commercials on television every now and then - “send this, receive that.” Of course, all these pictures, melodies and games are also received via the mobile Internet.
As you can see, everything is not very rosy. And now the need to implement the next, 3rd generation (3G) networks is breathing down our necks, which already casts doubt on the further spread of GPRS networks. But while GSM communication is still alive, it is worth remembering another wonderful data transmission technology - EDGE. It is a necessary continuation of GPRS, as evidenced by the decoding of the name - Enhanced Data for Global Evolution.
EDGE vs GPRS
The information transfer speed using EDGE technology is 3 times higher than when using GPRS - up to 474.6 Kb/s (again, theoretically). EDGE allows you to transmit/receive data within the existing frequency range typical of GSM networks used today, but with capabilities characteristic of the 3G generation.
EDGE begins its history in the late 90s. Ericsson originally developed it for D-AMPS networks. But I also tried to implement it into the GSM network, not without some experience, since EDGE technology is a new modulation in the radio channel of the base station and mobile device. To further use this technology within existing networks, we need EDGE-compatible transmitters that convert the signal on the way to the base station, and phones that support EDGE (their number is constantly growing, but is still not sufficient). I would recommend that when buying a new phone, you should pay attention to whether it supports EDGE.
As already mentioned, Russian operators started their business by initially purchasing modern equipment, which was more “advanced” compared to the equipment of foreign operators. Moreover, the peak of popularity of mobile communications in Russia came just in time - at that time, EDGE had just begun to be introduced abroad. For Russian operators, a whole range of problems thereby disappeared - their new equipment was ready to work with EDGE. But other questions remain, namely: administrative permission to use this technology, since here we have a slightly different type of signal modulation (what if bourgeois machinations? :)). In addition, it is necessary to review all equipment for compatibility with EDGE, optimize it (while taking into account all existing problems with GPRS). It is simply necessary to expand the network capacity - after all, with the introduction of EDGE, the load on them will double or triple.
What do we have?
So, the only option for fast (or relatively fast) access to the Internet using a mobile phone is GPRS. Despite the shortcomings (low speed, the vagaries of the network), this is better than nothing - EDGE is coming, but has not yet arrived. Although if you are lucky and your city is already under the “EJ”, then you can safely experiment.
I immediately want to tone down the naive expectations of super-speeds a little. Considering the structural disorganization of GSM networks (this is not a sign of an original Russian “mess”, but a consequence of the fact that they have an “open architecture” topology and are constantly being overgrown with add-ons, and operators are experimenting with equipment and software), there will not be very fast data transfer. Get ready for speeds of 140-150 kbps. But that’s not bad, right? :)
Advice for GPRS and EDGE users - if you have to work leisurely with the Internet and you have everything for this (phone, cables, computer, software), then it is better to connect somewhere outside the city - in a village, in a country house. As a rule, if these places are within the access zone of the GSM network, then it is clearly not overloaded (there they still trust packet data transmission wirelessly according to the OBS standard - “one woman said” :)) You can surf the network faster than in the city, and better for health...
EDGE is a technology that allows you to transfer data over a mobile network at speeds of up to 200 Kbps.
This is on average four times faster than GPRS.
The main application of EDGE is high-speed Internet access; organizing a mobile office is an indispensable thing for business people.
And also, such opportunities as: exchanging pictures, photographs and other information via the same Internet, watching streaming video, Internet radio, sending faxes, mail, and many, many other interesting things.
Based on its advantages, we can say that EDGE technology is designed for 2 different classes of the population: businessmen, for whom it is important to always be up to date with the latest events, and teenagers for whom the Internet is a way of life.
To access modern services via EDGE, simply use a device that supports this technology, such as a Sony Ericsson GC85 or Sierra AirCard 775.
Initially, EDGE was intended as an extension of GPRS technology.
They first started talking about it back in 1997 at ESTI (European Telecommunication Standardization Institute).
At the same time, its first decoding was presented as Enhanced Data Rates for GSM Evolution.
EDGE uses eight-position phase shift keying (8-PSK), which provides approximately double the maximum speed compared to GPRS - it is 384 Kbps, while the maximum theoretical speed of GPRS is 171 Kbps.
Of course, the actual speed is much lower.
To transmit information, EDGE, like GPRS, uses timeslots (time segments of a frame).
There is a policy identical to GPRS for distributing timeslots between channels for reception and transmission.
Another advantage is that the maximum flow rate in one timeslot is 48 kbit/s (versus 9.6 kbit/s for GPRS).
Naturally, such a speed is achieved only with ideal reception; in reality, everything will be much worse.
Depending on the quality of communication, 9 coding algorithms are provided from MCS-1 to MCS-9 (the latter has the lowest coding redundancy, and therefore the fastest).
Subsequently, with the advent of the 3rd generation network specification, the name EDGE was rephrased and now it stands for Enhanced Data rates for Global Evolution.
So we can say that EDGE is a full-fledged transition link on the way to 3G or, as it is sometimes called, 2.5G.
EDGE, unlike GPRS, whose connection is very unstable, and the speed in rare cases rises above 56 Kbps, has two incomparable advantages: high speed and quality of communication.
Therefore, EDGE technology has every chance of replacing the obsolete GPRS technology.
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Users of SIM-enabled mobile phones or tablets may have noticed that the data icon next to the antenna may change to one of the following: G, E, 3G, 3.5G, 3G+, H, H+, 4G, L or LTE . Let's try to figure out what each of them means.
G (GPRS)
GPRS (General Packet Radio Service) is an add-on to GSM mobile communication technology that provides packet data transmission. It is one of the first implementations of the mobile Internet. Today, it is an outdated method of connecting to the World Wide Web. The theoretical maximum data transfer rate is 171.2 Kbps (depending on the GPRS class).
E (EDGE)
EDGE (Enhanced Data rates for GSM Evolution) or Enhanced GPRS is a digital technology for wireless data transmission for mobile communications, which is an add-on to 2G and 2.5G (GPRS) networks.
Connecting to the network via EDGE is approximately 3 times faster than via GPRS, namely the maximum data transfer speed can be 474 Kbps. In the picture above, the connection speed measured by the application has the dimension KB/s (kilobytes per second). To convert to kilobits per second, you need to multiply the displayed value by 8, that is, 17 Kbps x 8 = 136 Kbps.
3G
3G (from the English third generation - third generation) - 3rd generation mobile communication technologies - a set of services that combines both high-speed mobile access to the Internet and radio communication technology, which creates a data transmission channel (voice, messages, etc.) d.). Currently, this term most often refers to UMTS technology with an HSPA add-on (hence the “H” or “H+” icon on the phone).
Third generation 3G networks operate at frequencies slightly higher than traditional GSM (850 MHz, 900 MHz, 1800 MHz, 1900 MHz), namely 1900-2100 MHz, which, in addition to other serious differences from GSM and improvements, allows increasing frequency bandwidth and , respectively, the data transfer rate.
Varieties of 3G
HSPA
The maximum theoretical data transfer rate according to the HSPA standard is 14.4 Mbit/s (data transfer rate from the base station to all local subscribers) and up to 5.76 Mbit/s from the subscriber. The first stages of implementation of the standard had a speed of 3.6 Mbit/s to the HSDPA subscriber (D - downlink). After the introduction of the second stage of HSUPA (U-uplink, that is, acceleration of transmission from the subscriber), the entire technology was abbreviated as HSPA.
HSPA+
HSPA+ (English: Evolved High-Speed Packet Access, “developed high-speed packet access”) is a mobile communications standard, an upgrade of the third generation of mobile communications, with high speed comparable to 4G.
HSPA+ generally refers to technologies that allow packet data transfer with download speeds of up to 42.2 Mbit/s and upload speeds of up to 5.76 Mbit/s. In practice, the connection speed is lower and amounts to 10 - 20 Mbit/s (in the picture above 1.6 Mbit/s x 8 = 12.8 Mbit/s).
This technology is considered transitional between third (3G) and fourth (4G) generation networks. Sometimes it is also called "3.5G".
4G
If the L, LTE or 4G icon lights up on your phone, congratulations! Firstly, your device supports the LTE-A and WiMAX standard, and secondly, you are on the network of the newest and latest generation available in our country at the time of writing this article with data download speeds of up to 173 Mbit/s and upload speeds of up to 58 Mbps!
EDGE is an advanced technology replacing GPRS. It is also known as EGPRS (Enhanced GPRS). The technology allows you to transfer data over a mobile network at speeds of up to 200 kbit/sec, which has already been confirmed in practice. This is on average four times faster than GPRS. To access modern services via EDGE, it is enough to use a SonyEricsson GC85, Sierra AirCard 775, T-modem COM, T-modem USB, T-modem PCI card (or any other device supporting this technology) - in the test zone it automatically selects EDGE instead GPRS, no additional actions are required from the user.Nowadays, a professional needs more than just staying connected with his clients and colleagues while away from the office. For the vast majority of specialists who use mobile communications, the opportunity to work on the road is relevant. In this regard, technologies that enable connecting a laptop computer to an office network and communicating via e-mail with colleagues and clients in real time are in great demand. EDGE/GPRS cards, such as SonyEricsson GC85, Sierra AirCard 775, T-modem COM, T-modem USB, T-modem PCI, are the optimal solution for these purposes.
These devices will allow you to easily and quickly access the Internet, connect to your corporate network, and send E-Mail or SMS. Working in GSM/EDGE networks, such a card allows you to set the data transfer rate (247.4 Kbps!!!) three to four times higher than the speed of data transfer using GPRS technology. This means that the time spent receiving email or browsing the Internet will be significantly less.
EDGE is already in Russia. MegaFon became the first mobile operator to provide subscribers in Moscow and the Moscow region with access to high-speed data transmission using EDGE technology. In early January 2005, test operation of the latest Enhanced Data for Global Evolution technology began in a number of districts of the capital and Moscow region in the MegaFon-Moscow network.
2. Details about EDGE.
What is EDGE? Its advantages Lately, the mysterious word EDGE has been appearing more and more often on the shelves of our stores. What is this terrible beast, what advantages does this technology provide and what is its future in Russia?
Initially, EDGE was intended as an extension of GPRS technology. They first started talking about it back in 1997 at ESTI (European Telecommunication Standardization Institute). At the same time, its first decoding was presented as Enhanced Data Rates for GSM Evolution. EDGE uses eight-position phase shift keying (8-PSK), which provides approximately double the maximum speed compared to GPRS - it is 384 Kbps, while the maximum theoretical speed of GPRS is 171 Kbps. Of course, the actual speed is much lower. To transmit information, EDGE, like GPRS, uses timeslots (time segments of a frame). There is a policy identical to GPRS for distributing timeslots between channels for reception and transmission. Another advantage is that the maximum flow rate in one timeslot is 48 kbit/s (versus 9.6 kbit/s for GPRS). Naturally, such a speed is achieved only with ideal reception; in reality, everything will be much worse. Depending on the quality of communication, 9 coding algorithms are provided from MCS-1 to MCS-9 (the latter has the lowest coding redundancy, and therefore the fastest).
Subsequently, with the advent of the 3rd generation network specification, the name EDGE was rephrased and now it stands for Enhanced Data rates for Global Evolution. So we can say that EDGE is a full-fledged transition link on the way to 3G or, as it is sometimes called, 2.5G.
The main application of EDGE is high-speed Internet access; organizing a mobile office is an indispensable thing for business people. And also, such opportunities as: exchanging pictures, photographs and other information via the same Internet, watching streaming video, Internet radio, sending faxes, mail, and many, many other interesting things. Based on its advantages, we can say that EDGE technology is designed for 2 different classes of the population: businessmen, for whom it is important to always be up to date with the latest events, and teenagers for whom the Internet is a way of life.
The question of which is better than GPRS or EDGE also cannot be given a definite answer, although at the moment the use of GPRS is more justified than the use of EDGE. This is mainly due to the fact that GPRS is widespread everywhere, and EDGE is just beginning to spread in Russia. But EDGE, unlike GPRS, whose connection is very unstable, and the speed in rare cases rises above 56 Kbps, has two incomparable advantages: high speed and quality of communication. Therefore, EDGE technology has every chance of replacing the obsolete GPRS technology.
EDGE technology: what is it and why is it needed?
The last 3GSM World Congress, and after it the CeBIT 2006 exhibition in Hannover, brought with them a lot of announcements of new cell phones supporting EDGE technology (Enhanced Data for Global Evolution or, as you sometimes hear, Enhanced Data rates for GSM Evolution). This is no coincidence - although mobile phone vendors are paying more and more attention to supporting third generation (3G) standards, such as CDMA2000 1x, W-CDMA and UMTS, the development of 3G networks is extremely slow, and interest in second generation (2G) and second generation networks and half (2.5G) is not weakening, but, on the contrary, growing, both in the markets of developing countries and in the markets of developed countries.
The evolution of cellular standards
In the name of “propaedeutics without bloodshed,” I’ll go back a little into history and talk about what generations of cellular communication standards are now known to science. Those of you who are already familiar with this issue can immediately skip to the next section, which is dedicated specifically to EDGE technology.
iSo, standards first generation cellular communications (1G), (developed in 1978, introduced in 1981) and (introduced in 1983), were analog: the low-frequency human voice was transmitted on a high-frequency carrier (~450 MHz in the case of NMT and 820-890 MHz in the case of AMPS) using an amplitude-frequency modulation scheme. In order to ensure communication between several people at the same time, in the AMPS standard, for example, frequency ranges were divided into 30 kHz wide channels; this approach was called FDMA (Frequency Division Multiple Access). The first generation standards were created for and provided exclusively voice communications.
Standards second generation(2G), such as (global system for mobile communications) and (Code Division Mutiple Access), brought with them several innovations. In addition to the frequency division of FDMA communication channels, a person’s voice was now digitized (coding), that is, a modulated carrier frequency was transmitted over the communication channel, as in the 1G standard, but no longer with an analog signal, but with a digital code. This is a common feature of all second generation standards. They differ in the methods of “compression” or division of channels: GSM uses the time division multiplex approach TDMA (Time Division Multiple Access), and CDMA uses code division of communication channels (Code Division Mutiple Access), which is why this standard is called that. The second generation standards were also created to provide voice communication, but due to their “digital nature” and in connection with the need that arose during the spread of the Global Wide Web to provide Internet access via mobile phones, they provided the ability to transmit digital data via a mobile phone, as through a regular wired modem. Initially, the second generation standards did not provide high throughput: GSM could provide only 9600 bps (exactly as much as is required to provide voice communication in one channel “densified” using TDMA), CDMA several tens of Kbps.
In standards third generation(3G), the main requirement for which, according to the specifications of the International Telecommunications Union (ITU) IMT-2000, was to provide video communication at least in QVGA resolution (320x240), it was necessary to achieve a digital data transmission capacity of at least 384 Kbps. To solve this problem, increased frequency bands (W-CDMA, Wideband CDMA) or a larger number of simultaneously used frequency channels (CDMA2000) are used. By the way, initially the CDMA2000 standard could not provide the required throughput (providing only 153 Kbps), however, with the introduction of new modulation schemes and multiplexing technologies using orthogonal carriers in the “add-ons” 1x RTT and EV-DO, the threshold was 384 Kbps s was successfully overcome. And a data transmission technology such as CDMA2000 1x EV-DV will have to provide a throughput of up to 2 Mbit/s, while the HSDPA (High-Speed Downlink Packet Access) technology currently being developed and promoted in W-CDMA networks up to 14.4 Mbit/s.
In addition, in Japan, South Korea and China, work is currently underway on the next, fourth generation standards, which will be able, in the future, to provide digital data transmission and reception speeds of over 20 Mbit/s, thus becoming an alternative to wired broadband networks.
However, despite all the prospects that third generation networks promise, not many are in a hurry to switch to them. There are many reasons for this: the high cost of telephones, caused by the need to return funds invested in research and development; and the high cost of airtime associated with the high cost of licenses for frequency bands and the need to switch to equipment that is incompatible with the existing infrastructure; and short battery life due to excessively high (compared to second-generation devices) load when transferring large amounts of data. At the same time, the second generation GSM standard, due to the inherent possibility of global roaming and the lower cost of devices and airtime (here the licensing policy of the main supplier of CDMA technologies, Qualcomm, played a cruel joke on it), has become truly global, and Already last year the number of GSM subscribers exceeded 1 billion people. Not to take advantage of the situation would be wrong both from the point of view of operators who would like to increase average revenue per subscriber (ARPU) and ensure the provision of services competitive with those of 3G networks, and from the side of users who would like to have mobile access to Internet. What happened to this standard later can be called a small miracle: it was invented evolutionary approach, whose ultimate goal was to transform GSM into a third generation standard compatible with UMTS (Universal Mobile Telecommunications System).
Strictly speaking, mobile Internet access has been available for a long time: CSD (Circuit-Switched Data) technology made it possible to make a modem connection at a speed of 9600 bps, but, firstly, it was inconvenient due to the low speed, and secondly due to per minute billing. Therefore, first, data transmission technology (General Packet Radio Service) was invented and implemented, which marked the beginning of the transition to a packet approach, and then EDGE technology. By the way, there is also an alternative technology to GPRS, HSCSD (High-Speed Circuit Switched Data), but it is less common, since it also implies per-minute billing, while GPRS takes into account traffic packet forwarding. This is the main difference between GPRS and various technologies based on the CSD approach: in the first case, the subscriber terminal sends packets over the air that travel through arbitrary channels to the destination, in the second, a point-to-point connection is established between the terminal and the base station (working as a router). -point using a standard or extended communication channel. The GSM standard with GPRS technology occupies an intermediate position between the second and third generations of communication, therefore it is often called the second and a half generation (2.5G). It is also called that because GPRS marks the halfway point of GSM/GPRS networks towards compatibility with UMTS.
EDGE technology, as you might guess from its name (which can be translated as “improved data transfer rates for the evolution of the GSM standard”) plays two roles at once: firstly, it provides higher throughput for transmitting and receiving data, and secondly , serves as another step on the path from GSM to UMTS. The first step, the introduction of GPRS, has already been taken. The second step is just around the corner - the implementation of EDGE has already begun in the world and in our country.
Coverage map of the EDGE network of the Megafon operator in Moscow (at the end of February 2006)
EDGE what is it and what is it eaten with?
EDGE technology can be implemented in two different ways: as an extension of GPRS, in which case it should be called EGPRS (enhanced GPRS), or as an extension of CSD (ECSD). Considering that GPRS is much more widespread than HSCSD, let’s look at EGPRS.
1. EDGE is not a new cellular standard.
However, EDGE implies an additional physical layer that can be used to increase the throughput of GPRS or HSCSD services. At the same time, the services themselves are provided in exactly the same way as before. Theoretically, the GPRS service is capable of providing throughput up to 160 Kbps (at the physical level, in practice, devices supporting GPRS Class 10 or 4+1/3+2 provide only up to 38-42 Kbps and then, if the congestion of the cellular network allows), and EGPRS up to 384-473.6 Kbit/s. This requires the use of a new modulation scheme, new channel coding and error correction methods.
2. EDGE, in fact, is an “add-on” (or rather, an adjustment, if we assume that the physical layer is lower than the others) to GPRS and cannot exist separately from GPRS. EDGE, as mentioned above, involves the use of other modulation and code schemes, while maintaining compatibility with the CSD voice service.
Figure 1. Modified nodes are shown in yellow.
Thus, from the point of view of the client terminal, nothing should change with the introduction of EDGE. However, the base station infrastructure will undergo some changes (see Fig. 1), although not too serious. In addition to increasing data transmission capacity, the introduction of EDGE increases the capacity of the cellular network: more users can now be “packed” into the same time slot, accordingly, you can hopefully not receive a “network busy” message at the most inopportune moments.
| Table 1. Comparative characteristics of EDGE and GPRS | ||
| GPRS | EDGE | |
| Modulation scheme | GMSK | 8-PSK/GMSK |
| Symbol Rate | 270 thousand per second | 270 thousand per second |
| Bandwidth | 270 Kbps | 810 Kbps |
| Bandwidth per time slot | 22.8 Kbps | 69.2 Kbps |
| Data transfer rate per time slot | 20 Kbps (CS4) | 59.2 Kbps (MCS9) |
| Data transfer rate using 8 time slots | 160 (182.4) Kbps | 473.6 (553.6) Kbps |
Table 1 illustrates the different technical characteristics of EDGE and GPRS. Although both EDGE and GPRS send the same number of symbols per unit time, due to the use of a different modulation scheme, the number of data bits in EDGE is three times larger. Let’s immediately make a reservation here that the values of throughput and data transfer rates given in the table differ from each other due to the fact that the first also takes into account packet headers that are unnecessary for the user. Well, the maximum data transfer rate of 384 Kbps (required to comply with IMT-2000 specifications) is obtained if eight time slots are used, that is, 48 Kbps per time slot.
EDGE modulation scheme
The GSM standard uses the GMSK (Gaussian minimum shift keying) modulation scheme, which is a type of phase modulation of the signal. To explain the principle of the GMSK circuit, consider the phase diagram in Fig. 2, which shows the real (I) and imaginary (Q) parts of the complex signal. The phase of the transmitted logical “0” and “1” differs from each other by the phase p. Each character transmitted per unit of time corresponds to one bit.
Figure 2. Different modulation schemes in GPRS and EDGE.
EDGE technology uses an 8PSK modulation scheme (8-phase shift keying, the phase shift, as can be seen from the figure, is p / 4), using the same specifications of frequency channel structure, coding and bandwidth as in GSM/GPRS. Accordingly, adjacent frequency channels create exactly the same mutual interference as in GSM/GPRS. The smaller phase shift between symbols, which now encodes not one bit, but three (the symbols correspond to the combinations 000, 001, 010, 011, 100, 101, 110 and 111), makes the detection task more difficult, especially if the signal level is low. However, in conditions of good signal level and stable reception, it is not difficult to discriminate each character.
Coding
GPRS can use four different encoding schemes: CS1, CS2, CS3 and CS4, each of which uses its own error correction algorithm. Nine coding schemes have been developed for EGPRS, MCS1..MCS9, respectively, the purpose of which is also to provide error correction. Moreover, the “junior” MSC1..MSC4 uses the GMSK modulation scheme, and the “senior” MSC5..MSC9 uses the 8PSK modulation scheme. Figure 3 shows the dependence of the data transfer rate on the use of different modulation schemes coupled with different coding schemes (the data transfer rate varies depending on how much redundant information required for error correction algorithms to work is included in each encoded packet). It is not difficult to guess that the worse the reception conditions (signal-to-noise ratio), the more redundant information has to be included in each packet, which means the lower the data transfer speed. The slight difference in data rate observed between CS1 and MCS1, CS2 and MCS2, etc. is due to the difference in the size of the packet headers.
Figure 3. Different code schemes in GPRS and EDGE.
However, if the signal-to-noise ratio is small, not all is lost: in the older modulation code schemes EGPRS MCS7, MCS8, MCS9, an “overlay” procedure is provided: since the standard is capable of sending groups of packets on different carriers (within the frequency range), for each of where conditions (and above all “noise”) may be different, in this case retransmission of the entire block can be avoided if you know in which group the failure occurred and retransmit this particular group. Unlike the older GPRS CS4 code scheme, which does not use a similar error correction algorithm, in EGPRS MCS7, MCS8, MCS9 different data blocks are “overlaid” on each other, so if there is a failure in one of the groups (as shown in the figure), retransmission Only half of the packages are subject to (see Fig. 4).
Figure 4. Using Packet Group Overlay in EDGE.
Packet processing
If for some reason a packet sent using “higher” encoding schemes was not received correctly, EGPRS allows it to be retransmitted using a “lower” encoding scheme. In GPRS, such a feature, called “resegmentation,” was not provided: an incorrectly received packet is sent again using the same modulation-coding scheme as the previous time.
Addressing window
Before a sequence of encoded (i.e., "words" consisting of several bits are encoded) packets (frame) can be transmitted over the RF interface, the transmitter assigns the packets an identification number included in the header of each packet. Packet numbers in GPRS range from 1 to 128. After a sequence of packets (for example, 10 pieces) is sent to the recipient, the transmitter waits for confirmation from the receiver that they have been received. The report that the receiver sends back to the transmitter contains the numbers of packets that were successfully decoded and those that the receiver was unable to decode. An important nuance: packet numbers take values from 1 to 128, and the width of the address window is only 64, as a result of which a newly transmitted packet can receive the same number as in the previous frame. In this case, the protocol is forced to resend the entire current frame, which negatively affects the overall data transfer rate. To reduce the risk of such a situation occurring in EGPRS, the packet number can take values from 1 to 2048, and the address window is increased to 1024.
Measurement accuracy
To ensure the correct functioning of GPRS technology in a GSM environment, it is necessary to constantly measure radio conditions: signal/noise level in the channel, error rate, etc. These measurements do not in any way affect the quality of voice communication, where it is sufficient to constantly use the same coding scheme. When transmitting data to GPRS, measuring radio conditions is possible only in “pauses” twice in a period of 240 ms. In order not to wait every 120 ms, EGPRS determines a parameter such as the bit error probability (BEP) in each frame. The BEP value is affected by both the signal-to-noise ratio and the time dispersion of the signal and the speed of the terminal. BEP variation from frame to frame provides an estimate of terminal speed and frequency jitter, but for a more accurate estimate, the average bit error probability per four frames and its sample standard deviation are used. Thanks to this, EGPRS responds faster to changing conditions: it increases the data transfer rate when the BEP decreases and vice versa.
Controlling connection speed in EGPRS
EGPRS uses a combination of two approaches: link speed adjustment and incremental redundancy. Adjusting the connection speed, measured either by the mobile terminal by the amount of data received per unit of time, or by the base station by the amount, respectively, of transmitted data, allows you to select the optimal modulation-code scheme for subsequent volumes of data. Typically, the use of a new modulation code scheme can be assigned when transmitting a new block (of four groups) of data.
Incremental redundancy is initially applied to the most senior modulation code scheme, MCS9, with little attention to error correction and no consideration of radio conditions. If the information is not decoded correctly by the recipient, it is not the data itself that is transmitted over the communication channel, but a certain control code that is “added” (used for conversion) to the already downloaded data until the data is decoded successfully. Each such “incremental piece” of additional code increases the likelihood of successful decryption of the transmitted data; this is where redundancy lies. The main advantage of this approach is that there is no need to monitor the quality of radio communications, which is why incremental redundancy is mandatory in the EGPRS standard for mobile terminals.
Integration of EGPRS into existing GSM/GPRS networks UMTS is just around the corner!
As mentioned above, the main difference between GPRS and EGPRS is the use of a different modulation scheme at the physical level. Therefore, to support EGPRS, it is enough to install a transceiver and software for processing packets at the base station that supports new modulation schemes. To ensure compatibility with non-EDGE mobile phones, the standard states the following:
- EDGE and non-EDGE mobile terminals must be able to use the same time slot
- EDGE and non-EDGE transceivers must use the same frequency range
- Partial EDGE support possible
- Supporting 8PSK modulation scheme only in the receiving data stream (downlink) and
- Supports 8PSK in both the receiving and transmitting (uplink) data streams
The introduction of EGPRS, as mentioned above, allows you to achieve a throughput that is approximately three times greater than in GPRS technology. In this case, exactly the same QoS (quality of service) profiles are used as in GPRS, but taking into account the increased bandwidth. In addition to requiring a transceiver to be installed at the base station, EGPRS support requires a software update that will need to handle the changed packet protocol.
The next evolutionary step on the path of GSM/EDGE cellular communication systems to “full-fledged” third generation networks will be further improvement of packet (data) forwarding services to ensure their compatibility with UMTS/UTRAN (UMTS terrestrial radio access network). These improvements are currently under review and will most likely be included in a future version of the 3GPP (3G Partnership Project) specifications. The main difference between GERAN and the currently implemented EDGE technology will be QoS support for interactive, background, streaming and conversation classes. Support for these QoS classes is already available in UMTS, which makes it possible, for example, for video communication in UMTS networks (say, W-CDMA 2100 or 1900 MHz). In addition, in the future generation of EDGE it is planned to provide simultaneous parallel processing of data streams with different QoS priorities.
