2nd Generation (GERAN)

The capacity limitations, the quality issues and the limitations to rather national mobile communication standards of the first analogue mobile communication systems led to the development of a 2nd generation of digital cellular mobile communication systems.

ETSI SMG developed with GSM (Global System for Mobile Communications) a second-generation digital cellular radio access technology for Europe that became a worldwide success and that is still operating today.

Over time this system was further evolved under the umbrella acronym GERAN (GSM/EDGE Radio Access Network):

  • High Speed Circuit Switched Data (HSCSD)
  • General Packet Radio Service (GPRS)
  • Enhanced Data rates for Global Evolution (EDGE)

and integrated into 3GPPTM. GERAN was also the name of the 3GPP Technical Specification Group that was responsible for the development and maintenance of this system until June 2016. Afterwards, GERAN was and is further developed/maintained in 3GPP TSG RAN WG6 ("GERAN and UTRAN radio and protocol").

The Technical Specifications which together comprise a 3GPPTM system with a GSM/EDGE radio access network are listed in 3GPP TS 41.101.

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Global System for Mobile communication (GSM)

The technology behind the Global System for Mobile communication (GSMTM) uses Gaussian Minimum Shift Keying (GMSK) modulation a variant of Phase Shift Keying (PSK) with Time Division Multiple Access (TDMA) signalling over Frequency Division Duplex (FDD) carriers. The physical layer is specified in 3GPP TS 45.001 and the logical channels in 3GPP TS 45.002. The channel coding is specified in 3GPP TS 45.003 and the modulation is specified in 3GPP TS 45.004.

Although originally designed for operation in the 900 MHz band, it was soon adapted for 1800 MHz. The introduction of GSM into North America meant further adaptation to the 800 and 1900 MHz bands. Over the years, the versatility of GSM has resulted in the specifications being adapted to many more frequency bands to meet niche markets. A full list can be found in 3GPP TS 45.005.

GSM has a channel spacing of 200kHz and was designed principally for voice telephony, but a range of bearer services was defined (a subset of those available for fixed line Integrated Services Digital Networks, ISDN), allowing circuit-switched data connections at up to 9600 bits/s. At the time of the original system design, this rate compared favourably to those available over fixed connections. However, with the passage of time, fixed connection data rates increased dramatically. The GSM channel structure and modulation technique did not permit faster rates, and thus the High Speed Circuit-Switched Data (HSCSD) service was introduced in the GSM Phase 2+.

In the course of the next few years, the General Packet Radio Service (GPRS) was developed to allow aggregation of several carriers for higher speed, packet-switched applications such as always-on internet access. The first commercial GPRS offerings were introduced in the early 2000s.

Meanwhile, investigations had been continuing with a view to increasing the intrinsic bit rate of the GSM technology via novel modulation techniques. This resulted in Enhanced Data-rates for Global Evolution (EDGE), which offers an almost three-fold data rate increase in the same bandwidth.

The combination of GPRS and EDGE brings system capabilities into the range covered by the International Telecommunication Unions IMT-2000 (third generation) concept.

GSM radio technology is specified in the 3GPP TS 45.-series specifications. The overall GSM network architecture is described in 3GPP TS 23.002 and a complete list of Technical Specifications for GSM systems is given in 3GPP TS 41.101.

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High Speed Circuit Switched Data (HSCSD)

The standard GSMTM circuit-switched connection offers a data rate much too low for sophisticated web browsing and the transfer of large files, and as early as 3GPP Release 96 of the GSM specifications it was realized that a significant increase in speed could be obtained by aggregating two or more channels into a single, grouped, circuit-switched connection.

The specifications (3GPP TS 22.034 requirements, 3GPP TS 23.034 architecture) allow for up to four channels to be combined, giving 57.6 kbit/s (or 38.4 kbit/s in the USA). The service is known as High Speed Circuit Switched Data, HSCSD.

Since the four channels are tied up in a circuit-switched connection, the network operator was likely to charge an HSCSD call at a considerably higher rate than a simple, single channel call, and so the service was never enormously popular. HSCSD has been largely replaced by the General Packet Radio Service (GPRS) and Enhanced Data rates for Global Evolution (EDGE), which are more versatile, offer higher data rates, and are more economical.

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General Packet Radio Service (GPRS)

The General Packet Radio Service (GPRS), adds packet-switched functionality to GSMTM, which is essentially circuit switched. GPRS is the essential enabler for always-on data connection for applications such as "web browsing" and "Push-to-Talk over Cellular".

GPRS was introduced into the GSM specifications in 3GPP Release 97 and usability was further upgraded in Releases 98 and 99. It offers faster data rates than plain GSM by aggregating several GSM time slots into a single bearer, potentially up to eight, giving a theoretical data rate of 171 kbit/s. Most operators do not offer such high rates, because obviously if a slot is being used for a GPRS bearer, it is not available for other traffic. Also, not all mobiles are able to aggregate all combinations of slots.

The 'GPRS Class Number' indicates the maximum speed capability of a terminal, which might be typically 14 kbit/s in the uplink direction and 40 kbit/s in the downlink, comparable with the rates offered by wireline dial-up modems at that time.

Mobile terminals are further classified according to whether or not they can handle simultaneous GSM and GPRS connections: class A = both simultaneously, class B = GPRS connection interrupted during a GSM call, automatically resumed at end of call, class C = manual GSM / GPRS mode switching.

Further data rate increases have been achieved with the introduction of EDGE (Enhanced Data rates for Global Evolution).

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Enhanced Data rates for Global Evolution (EDGE)

As its name suggests, EDGE (Enhanced Data rates for Global Evolution) is an enhancement of the GSMTM radio access technology to provide faster bit rates for data applications, both circuit- and packet-switched. As an enhancement of the existing GSM physical layer, EDGE is realized via modifications of the existing layer 1 3GPP specifications TS 45.000 rather than by separate, stand-alone specifications.

The increased data rate is accomplished by a new modulation technique (8PSK as opposed to GSM/GPRS's GMSK, yielding a three-fold increase in bit rate for an identical symbol rate) coupled with new channel coding, resulting in improved spectral efficiency. This is important, because it allows EDGE to be introduced piecemeal into existing GSM networks without disrupting the frequency reuse plan of the existing deployment.

In fact, to cater for the potentially increased sensitivity to noise in marginal coverage areas, EDGE uses a combination of 8PSK and GMSK, to give a balanced improvement in bit rate under virtually all radio conditions. The four coding schemes of GPRS are increased to nine in EDGE, and new segmentation techniques can radically improve throughput by permitting the coding scheme to be changed on the fly in case of retransmission of a segment in rapidly changing radio conditions. In addition, the packet window size increased to 1024 compared with the 64 for GPRS, resulting in more robust transmission and reception.

The implementation of EDGE is summarized in 3GPPTM 3GPP TR 10.59  (later called 3GPP TR 50.059) - essentially a catalogue of Change Requests which introduced EDGE functionality into the existing specifications, for 3GPP Release 98. TR 10.59 / TR 50.059 has not been transposed into an official ETSI publication because it does not meet the necessary criteria. It remains, however, a very useful reference document.

EDGE is compatible with the North American cellular system, ANSI IS136.
A variant of EDGE, called 'EDGE Compact', would permit deployment in less than 1 MHz of spectrum; however, it was not felt interesting for US operators and was never implemented.

Other than providing improved data rates, EDGE is transparent to the service offering at the upper layers, so it is possible to apply EDGE on top of High Speed Circuit Switched Data (HSCSD) and also on top of GPRS (which is then called EGPRS for Enhanced GPRS).

By way of illustration, the General Packet Radio Service (GPRS) can offer a data rate of 115 (and a theoretical data rate of 171) kbit/s whereas EDGE on top of GPRS can increase this theoretical data rate to 384 kbit/s. This is comparable with the rate for early implementations of Wideband Code Division Multiple Access (W-CDMA) of the 3rd generation of UMTS and a reason why EDGE is considered to be the bridge between the 2nd and the 3rd generation of mobile communication systems.

The list of the published ETSI standards on EDGE includes the same specifications of GSM (TS 43-, 44- and 45-Series, TS 51.010 for and 51.021, etc.).

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