Introduction to GSM new

TC Finland Issue Dec 2002

© Nokia Networks Oy

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SYSTEM TRAINING

Introduction to GSM Training Document

Introduction to GSM

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Contents



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Contents

1 Module objectives ................................................................................5

2 Introduction ..........................................................................................6 2.1 Mobile communications: Basic concepts ................................................6 2.2 Channel organisation in GSM/GPRS....................................................11 2.2.1 Physical channel and TDMA frame.......................................................12 2.3 The Public Land Mobile Network ..........................................................13 2.4 GSM Specifications ..............................................................................14 2.5 GSM background and requirements .....................................................15 2.6 Advantages of GSM .............................................................................16

3 Evolution of GSM ...............................................................................18 3.1 The next step: UMTS............................................................................21

4 Review questions ...............................................................................23

Introduction to GSM



Preface

Already in the early days, GSM was superior to analogue mobile networks. When the standardisation work for GSM began in 1982, CEPT (Conférence Européenne des Postes et Télécommunications), could use experiences from analogue networks such as NMT (Nordic Mobile Telephone) and TACS (Total Access Communication System) to create a better digital network.

One main reason for the great success of GSM is that it was a European-wide project already from the very beginning. Furthermore, the digital mobile system enabled the supply of more sophisticated services, such as SMS (Short Message Service), and bearer services for data transmission.

Thanks to the global popularity of GSM, we may now use our mobile stations in more than 130 countries, a fact that has made both business and leisure travellers' lives much more convenient.

Still today, standardisation continues to specify new features for GSM networks. This ambitious work creates opportunities for the operators to differentiate themselves in a competitive environment. Today, we are more and more focusing on faster wireless transmission and the introduction of packet switched data.

The 3G (Third Generation) network technologies are also already specified and in many parts of the world operational. The 3G version in Europe, is named UMTS (Universal Mobile Telecommunications System). Its air interface will be based on WCDMA (Wideband Code Division Multiple Access) transmission. 3G networks provide substantially higher capacities than 2G.

This technically oriented material will help you to better understand the window of opportunities – today and in the near future. Your active participation will be an essential ingredient for a successful and fulfilling training.

Module objectives



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1 Module objectives

After completing this module, the participant should be able to:

• Name the key terms in mobile communications

• List the main improvements from 1G to 2G mobile communications systems

• Identify five events in the GSM evolution and connect each with the correct year

without using any references.

Introduction to GSM



2 Introduction

2.1 Mobile communications: Basic concepts

From ancient to modern times, mankind has been looking for means of long distance communications. For centuries, letters proofed to be the most reliable way to transmit information. Fire, flags, horns, etc. were used to transmit information faster. Technical improvements in the 19th century simplified long distance communications: Telegraphy, and later on telephony. Both techniques were wireline. In 1873, J. C. Maxwell laid the foundation of the electro-magnetic theory by summarising empirical results in four equations, which are still valid today. It would however be several decades before Marconi made economic use of this theory by developing devices for wireless transmission of Morse signals (about 1895). Already 6 years later, the first transatlantic wireless transmission of Morse signals took place. Voice was transmitted the first time in 1906 (R. Fesseden), and one of the first radio broadcast transmission 1909 in New York.

DuplexSimplex

one-way

two-way

Figure 1. Transmission

The economically most successful wireless application in the first half of the 20th century was radio broadcast. There is one transmitter, the so-called radio station. Information, such as news, music, etc. is transmitted from the radio station to the receiver equipment, the radio device. This type of one-way transmission is called simplex transmission. The transmission takes place only in one direction, from the transmitter to the receiver. When we take a human

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conversation, a technical solution is required, where the information flow can take place in two directions. This type of transmission is called duplex transmission. Walky-talky was already available the early 30ies. This system already allowed a transmission of user data in two directions, but there was a limitation: The users were not allowed to transmit at the same time. In other words, you could only receive or transmit user information. This type of transmission is therefore often called semi-duplex transmission. For telephony services, a technical solutions is required, where subscribers have the impression, that they can speak (transmit) and hear (receive) simultaneously. This type of transmission solution is regarded as full duplex transmission.

The first commercial wireless car phone telephony service started in the late 1940 in St. Louise, Missouri (USA). It was a car phone service, because at that time, the mobile phone equipment was bulky and heavy. Actually, in the start-up, it filled the whole back of the car. But it was a real full duplex transmission solution. In the 50ies, several vehicle radio systems were also installed in Europe. These systems are nowadays called single cell systems. The user data transmission takes place between the mobile phone and the base station (BS). A base station transmits and receives user data. While a mobile phone is only responsible for its user’s data transmission and reception, a base station is capable to handle the calls of several subscribers simultaneously. The transmission of user data from the base station to the mobile phone is called downlink (DL), the transmission from the mobile phone to the base station uplink (UL) direction. The area, where the wireless transmission between mobile phones and the base station can take place, is the base stations supply area, called cell.

cell = supply area

Basestation

Downlink (DL)

Uplink(UL)

Figure 2. Single Cell System

Single cell systems are quite limited. The more and more distant the subscriber is from the base station, the lower the quality of the radio link. If the subscriber is leaving the supply area of the cell, no communication is possible any more. In

Introduction to GSM



other words, the mobile communication service was only available within the cell. In order to overcome this limitation, cellular systems were introduced. A cellular mobile communication system consists of several cells, which can overlap. By doing so, a whole geographical area can be supported with the mobile communication service.

Figure 3. Cellular System

But what happens, when a subscriber moves during a call from one cell to another cell? It would be very annoying, if the call is dropped. If the subscriber is leaving a cell, and in parallel is entering a new cell, then the system makes new radio resources available in the neighbouring cell, and then the call is handed over from on cell to the next one. By doing so, service continuation is guaranteed, even when the subscriber is moving. The process is called handover (HO).

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Mobile phone is active, e.g. a call takes place

Service continuationwithout interruption

Figure 4. Handover

A handover takes place during a call, i.e. when the mobile phone is in active (dedicated) mode. A mobile phone can also be in idle mode. In this case, the mobile phone is switched on, but no resources are allocated to it to allow user data transmission. In this mode, the mobile phone is still listening to information, broadcasted by the base station. Why? Imagine, there is a mobile terminated call. The mobile phone is then paged in the cell. This means the phone receives information that there is a mobile terminated call. A cellular system may consist of hundreds of cells. If the mobile network does not know, in which cell the mobile phone is located, it must be paged in all of them. To reduce load on networks, paging in is done in small parts of a mobile an operators network. Mobile network operators group cells in administrative units called location areas (LA). A mobile phone is paged in only one location area.

But how does the cellular system know, in which location area the mobile phone is located? And how does the mobile phone know? In every cell, system information is continuously transmitted. The system information includes the location area information. In the idle mode, the mobile phone is listening to this system information. If the subscriber moves hereby from one cell to the next cell, and the new cell belongs to the same location area, the mobile stays idle. If the new cell belongs to a new location area, then the mobile phone has to become active. It starts a communication with the network, informing it about it new location. This is stored in databases within the mobile network, and if there is a mobile terminated call, the network knows where to page the subscriber. The process, where the mobile phone informs the network about its new location is called Location Update Procedure (LUP).

Introduction to GSM



Location Area 1

Location Area 2

Location Area 3

no Location Update

Location Update

Location Area=

registrationarea of onesubscriber

Figure 5. Location Update and Paging

With the introduction of cellular mobile communication systems, we refer to generations. First generation mobile communication systems are

• TACS (Total Access Communications System)

• NMT (Nordic Mobile Telephony)

• AMPS (Advanced Mobile Phone Service)

• C450

• etc.

All of them were commercially launched in the 80s of the last century. The 1st generation mobile communication systems often offered national wide coverage. But there were limitations: Most of them did not support roaming. Roaming is the ability to use an other operator’s network infrastructure. International roaming is the ability to go even to another country and use the local operator’s infrastructure.

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Roaming=

ability to usedifferentoperator‘s

infrastructure

GSM subscriberfrom Spain

can use GSM networkin Australia, givenroaming agreement between home operatorand visited operator

Example:

Figure 6. Roaming

Most 1st generation mobile communication systems only support speech transmission, but not data transmission, such as fax. Supplementary services, well known from ISDN, were not available, such as number indication and call forwarding, when busy. The transmission takes place unprotected via the radio interface – as a consequence, eavesdropping is possible. Finally, mobile communication started to become a mass market. And the radio interface is the main bottleneck in terms of capacity. Improved solutions were urgently required. This led to the inauguration of the 2nd generation mobile communication systems, one of which is GSM.

2.2 Channel organisation in GSM/GPRS

In GSM 900, 25 MHz spectrum has been frequency divided into 124 bands, each having a bandwidth of 200 kHz. On each of the 200 kHz bands a carrier can be transmitted at the centre frequency of the band. The carriers are thus frequency division multiplexed.

Introduction to GSM



UPLINK DOWNLINK

GSM900: 890 MHz - 915 MHz 935 MHz - 960 MHz

GSM1800: 1710 MHz - 1785 MHz 1805 MHz - 1880 MHz

1 2 3 ...

Channel 1 - 1241 - 374

200 kHz

1 2 3 ...

Duplex frequency 45 MHz / 95 MHz

guard band

Figure 7. FDD and FDMA organisation in GSM

Each carrier is further time divided into timeslots (TSL) and each timeslot is referred to as a physical channel. It is possible to share a physical channel amongst many processes or users. This sharing is referred to as logical channels.

2.2.1 Physical channel and TDMA frame

TDMA frame= 8 timeslots

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76

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200 kHz

Physcial channel, e.g. allocated to one

subscriber with FR voice &no frequency hopping

frequency

time

TDMA frame

Figure 8. TDMA frame and physical channel

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Time Division Multiple Access (TDMA), is a method of sharing a resource (in this case a radio frequency) between multiple users, by allocating a specific time (known as the time slot) for each user. This is in contrast to the analogue mobile systems where one radio frequency is used by a single user for the duration of the conversation. In Time Division Multiple Access (TDMA) systems each user either receives or transmits bursts of information only in the allocated time slot. These time slots are allocated for speech only when a user has set up the call. Some timeslots are, however, used to provide signalling and location updates etc. between calls. The main benefit of the time based system is higher capacity than in earlier systems.

In GSM, a TDMA frame is defined as a grouping of TSLs which are numbered 0 to 7 as shown above. It has duration of 4.615ms (8 x 577µs).

TDMA frames are transmitted one after another. Every TDMA frame is allocated a frame number.

Um is the acronym for the GSM radio interface. It is an open interface, i.e. it is very accurately specified and thus vendor independent. A subscriber can use mobile phones from any manufacturer without bothering about the operator’s GSM infrastructure and supplier, as long as the network elements are compliant with the GSM specifications.

2.3 The Public Land Mobile Network

A mobile operator’s network is called Public Land Mobile Network. It is subdivided into three parts:

• Base Station Subsystem (BSS)

• Network Switching Subsystem (NSS)

• Network Management Subsystem (NMS)

The Network Switching Subsystem is responsible for switching, mobility management, and traffic element, i.e. that here network elements such as exchanges and data bases can be found. The exchanges are responsible for switching, while the data bases are used to keep track of the current location of the subscriber and his mobile phone. Mobile subscribers can be anywhere worldwide. This creates a challenge when there is a call since even in a mobile network a subscriber’s location must be known before a call can be set up. There are millions of cells, were the subscriber and his mobile phone can be located. The NSS databases serve to locate mobile subscribers when needed.

The Base Station Subsystem is responsible for the link between a mobile phone and a network exchange. The radio interface must contain many functions to enable mobile calls. For example user data must be protected by ciphering of user data in the base station and the mobile phone. The transmission must be reliable which invokes error protection methods. If a mobile subscriber wants to

Introduction to GSM



make a call, physical resources must be allocated to the user in a controlled manner. The tasks of the BSS can be summarised under the key terms Radio Resource Management (RRM) and Radio Link Management (RLM).

The Network Management System supports the operator in remote network supervision. Fault, configuration, and performance management are central tasks performed within the NMS.

Very important open interfaces are the Um interface and the A-interface. Open interfaces guarantee interworking of network elements from different vendors. An operator may select the NSS from one vendor, the BSS from a second vendor and the subscriber may use any vendors mobile phone.

A

NMSNMS

NSSNSSBSSBSS

O&M

Air

MS

Network Switching Subsystem

• Switching• Mobility Management• Connection Management• Charging

Base Station Subsystem• Radio Resource Management• Radio Link Management Network Management System

• Fault Management• Configuration Management• Performance Management

Figure 9. The Public Land Mobile Network (PLMN)

2.4 GSM Specifications

One reason for the major success of GSM is, that it is very accurately standardised. The standard is open, i.e. it is available to everyone. The European Standards Telecommunications Institute (ETSI) is responsible for the GSM standards. The GSM technical specifications are grouped in this way:

01General Description of a GSM PLMN 02 Services 03 Network Functions 04 MS - BSS Interface 05 Radio Path 06 Speech Processing Functions 07 Terminal Adaptation Functions

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08 BSS - MSC Interface 09 Network Inter Working [10 Service Inter Working] 11 Type Approval Procedures 12 Operation and Maintenance

2.5 GSM background and requirements

At the beginning of the 1980s a problem was that the European countries were using many different, incompatible mobile phone systems. These systems are referred to as 1G (first generation) systems. In Europe, the most common 1G system was NMT (Nordic Mobile Telephone) and TACS (Total Access Communications System). In the United States, as well as in other American countries, AMPS (Advanced Mobile Phone System) was, and still is, a widely established system.

With the passage of time, the need for telecommunication services was remarkably increased. Due to this, CEPT (Conférence Européenne des Postes et Télécommunications) founded a group to specify a common mobile system for Western Europe. This group was named “Groupe Spéciale Mobile” and the system name GSM arose.

This abbreviation has since been interpreted in other ways, but the most common expression nowadays is Global System for Mobile communications. GSM is a 2G (second generation) system

Figure 10. GSM – Global System for Mobile communications

Introduction to GSM



At the beginning of the 1990s, the lack of a common mobile system was seen to be a general, worldwide problem. For this reason the GSM system has now spread also to the Eastern European countries, Africa, Asia and Australia. The USA, South America in general, and Japan has made a decision to adopt other types of mobile systems, which are not compatible with GSM. However, in the USA the Personal Communication System (PCS) has been adopted, which uses GSM technology with a few variations.

During the time the GSM system was being specified, national telecommunication markets were deregulated. Requirements for openness and competition were built into the specifications as follows:

• There should be several network operators in each country. This would lead to competition in tariffs and service provisioning and it would ensure the rapid expansion of the GSM system. The prices of the equipment would fall and the users would find the cost of calls reducing.

• The GSM system must be an open system, meaning that it should contain well-defined interfaces between different system parts. This enables the equipment from several manufacturers to coexist and hence improves the cost efficiency of the system from the operator's point of view.

• GSM networks must be built without causing any major changes to the already existing Public Switched Telephone Networks (PSTN).

In addition to the commercial demands, some other objectives were defined:

• The system must be Pan European.

• The system must maintain a good speech quality.

• The system must use radio frequencies as efficiently as possible.

• The system must have high/adequate capacity.

• The system must be ISDN compatible (Integrated Services Digital Network) and compatible with other data communication specifications.

• The system must maintain good security both for subscriber and transmitted information.

2.6 Advantages of GSM

Due to the requirements set for the GSM system, many advantages will be achieved. These advantages can be summarised as follows:

• GSM uses radio frequencies efficiently, and due to the digital radio path, the system tolerates more intercell disturbances.

• The average speech quality is better than in analogue systems.

• Data transmission is supported throughout the GSM system.

• Speech is encrypted and subscriber information security is guaranteed.

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• With ISDN compatibility, new services are offered.

• International roaming is technically possible within all countries using the GSM system.

• The large market increases competition and lowers the prices both for investments and usage.

Introduction to GSM



3 Evolution of GSM

One key factor for the success of GSM was that the standardisation work was not completed after 1989. It was initially decided that GSM would evolve over time. With improvements in computing and radio access technology, GSM will offer continuous improvement and more services. In 1995 the “Phase 2” recommendations were frozen. The GSM 900 and GSM 1800 specifications were merged and additional supplementary services were defined, the short message service was improved and improvements in radio access and SIM cards were introduced.

After the Phase 2 recommendations, GSM continues to evolve at full speed. Many new features are being introduced to GSM and the number of improvements is so large that together they are called "Phase 2+" features. These Phase 2+ features are frozen at regular intervals under what are known as "Releases".

The following list highlights some important years in the short history of GSM:

1982 CEPT initiated a new cellular system. The European Commission (EC) issued a directive that required member states to reserve frequencies in the 900 MHz band for GSM to allow for roaming.

1986 CEPT tested eight experimental systems in Paris.

1987 Memorandum of Understanding (MoU). Allocation of the frequencies: - 890 - 915 MHz uplink (from mobile to base station) - 935 - 960 MHz downlink (from base station to mobile)

1988 European Telecommunications Standard Institute (ETSI) was created. ETSI includes members from administrations, industry, and user groups.

1989 Final recommendations and specifications for GSM Phase 1.

1990 Validation systems implemented and the first GSM World Congress in Rome with 650 participants.

1991 First official call in the world with GSM on 1st July.

1992 World's first GSM network launched in Finland. The first roaming agreement was made. By December there were 13 networks operating in 7 areas. Australian operators were the first non-European signatories of the GSM MoU. New frequency allocation for GSM 1800 (DCS 1800). - 1710 - 1785 MHz (uplink) - 1805 - 1880 MHz (downlink)

1993 GSM demonstrated for the first time in Africa at Telkom '93 in Cape Town. Roaming agreements between several operators. By December 1993 there were 32 GSM networks operating in 18 areas.

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1994 The first GSM network in Africa was launched in South Africa. The GSM Phase 2 data/fax bearer services were launched. By December 1994 there were 69 GSM networks in operation.

1995 There were 117 GSM networks operating around the world. Fax, data, and SMS roaming was implemented. The GSM phase 2 standardisation was completed, including adaptation for GSM 1900 (PCS 1900). The first GSM 1900 network was implemented in the USA. Telecom '95 was held in Geneva where Nokia demonstrated 33.6 Kbits/s multimedia data via GSM.

1996 By December there were 120 networks operating. The 8K SIM was launched in addition to prepaid GSM SIM cards.

1998 HSCSD (High Speed Circuit Switched Data) trials in Singapore. Over 2 million GSM 1900 users in the USA and a total of 120 million GSM 900/1800/1900 users worldwide.

1999 The first mobile data call using GPRS (General Packet Radio Service) in a live network was made. The first HSCSD (High Speed Circuit Switched Data) networks are launched. In December, the 271 million subscribers sent over 3 billion short messages (G-mails) worldwide. In January 2000 there were 359 GSM networks operating in 132 different countries. GSM 850 support: UL: 824 – 849 MHz and DL: 869 – 894 MHz (for North America). Location Based Services (LBS) services standardised. LBS can be combined with GPS (Global Positioning System), so that a subscriber can determine his geographical location extremely accurately. Operator Specific Access (OSA) specified: with it, open application programming interfaces between an operator’s network infrastructure and external VASP are given. It allows service provision from external VASP without knowledge of the operator’s internal network infrastructure. The 3G mobile communications system UMTS was specified in December. It is based on the GSM standards to allow a smooth evolution from the 2nd generation to the 3rd generation. This ought to guarantee an investment protection for GSM operators.

2000 The first GPRS network is launched. Second release of UMTS delayed to year 2001

2001 The Multimedia Messaging Service (MMS) was standardised. GSM 700 supported; UL: 747 – 762 MHz and DL: 777 – 792 MHz UMTS and GSM standardised at 3GPP; UMTS/GSM Rel. 4 standardised

2002 UMTS/GSM Rel. 5 standardised (IMS) was standardised for GSM/GPRS and UMTS. More than 40 MMS services have been launched this year in countries such as Czech Republic, Hungary, Germany, and Hong Kong. MMS (Multimedia Messaging Service) enables personalised multimedia

Introduction to GSM



messages, which can include images, texts, photos, etc. It is forecasted by the analyst Foster, that in the year 2007, that MMS will account for more than 30% of the mobile messaging revues. MMS is especially flexible, when transmitted via the GPRS infrastructure. At the end of the year 2002, more that 120 operators are commercially offering GPRS, and more than 40 operators are testing GPRS or building up a GPRS infrastructure. Smartphones are under development for GSM/GPRS and UMTS – to allow a wide range of mobile services, such as mobile Internet, mobile gambling, enhanced LDAs, video messaging, agnostic services, etc.

2003 First commercial starts of UMTS network operators in Europe.

2004 More than one billion people, almost one in six of the world’s population, are now using GSM mobile phones. More than 200 countries and territories has adopted GSM and it has become a truly global standard for mobile communications. As the choice of 80 per cent of all new mobile customers, GSM has driven wireless take-up to the extent that mobile phones now outnumber fixed telephone lines globally

0

200

400

600

800

1000

1200

1400

Dec 97 Dec 98 Dec 99 Dec 00 Dec 01 Dec 02 Dec 03 Dec 04 Dec 05

Figure 11. GSM customers worldwide and customer forecast (Dec 2002)

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0

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Jan 00

Apr 0

0Jul

00Okt

00Jan

01Ap

r 01

Jul 01

Okt 01

Jan 02

Apr 0

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02

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Jan 00 Jul 00 Jul 01 Jul 02Jan 00 Jan 02

Source: GSM Association

Figure 12. SMS growth during the last three years

In some European countries, SMS has reached saturation, e.g. it only slightly growing on a very high level. It is assumed, that MMS will substitute SMS over the next years.

3.1 The next step: UMTS

The third generation solution UMTS, is expected to complete the globalisation process of the mobile communication. UMTS will mostly be based on GSM technical solutions due to two reasons. Firstly, the GSM technology dominates the market, and secondly, great investments made to GSM should be utilised as much as possible. Based on this, the specification bodies created a vision about how mobile telecommunication will develop within the next decade. Through this vision, some requirements for 3G were short-listed as follows:

• The system to be developed must be fully specified (like GSM). The specifications generated should be valid worldwide.

• The system must bring clear added value when comparing to the GSM in all aspects. However, in the beginning phase(s) the 3G system must be backward compatible at least with GSM and ISDN.

• Multimedia and all of its components must be supported throughout the system.

• The radio access of the 3G must be generic. The services for the end-users must be independent of the access: radio access and the network infrastructure must not limit the services to be generated. That is, the

Introduction to GSM



technology platform is one issue and the services using the platform another issue.

The 3G system is expected to run a very high data rate that will allow us to have multimedia traffic through the wireless network. Enhanced packet switching and routing techniques are to be deployed to support video traffic and other real-time traffic in the 3G wireless network. The 3G system will use the Broadband Integrated Services Digital Network (B-ISDN) to provide data services between the existing data networks and the wireless network.

The Global 3G Partnership Project (3GPP) incorporating many organisations (ITU-T, ETSI, ARIB and ANSI) is developing standards for the 3G systems. These documents can be viewed at www.3gpp.org.

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4 Review questions

In the following questions, please select one alternative that you think is the best answer for the particular question. There may not be a perfect answer, select the one you think is the most correct.

1. Name advantages of cellular systems to single cell systems.

2. Explain the difference between handover and location update procedure.

3. Which of the following is a requirement for the GSM specifications?

a. The system must be compatible with existing mobile standards.

b. The system must be standardised globally.

c. The system must be built without causing any major changes in the existing Public Switched Telephone Networks (PSTN).

d. All of the above.

4. Which two statements in the following are generally seen as advantages of GSM over analogue networks?

a. Data transmission is supported in the whole GSM network.

b. It is only possible to use an analogue mobile telephone in the own network.

c. GSM mobile stations can be used in other digital mobile networks, for instance in NMT and TACS networks.

d. GSM is a more secure system than analogue systems with respect to subscriber information and transmission.

Introduction to GSM



5. Match the year in the left-hand column with the corresponding significant GSM event in the middle column.

Year Event Correct year

1982 Allocation of GSM frequencies

2000 Experimental test in Paris

1995 Frequency allocation for GSM 1800

1989 First official GSM call in the world

1991 Initiation of a new system

1987 ETSI begins the specification work for 3G/UMTS

1992 Final recommendations Phase 1

1999 Phase 2 recommendations frozen

1986 Total GSM subscribers exceeds 300 million

Introduction to GSM new

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