Thursday, April 06, 2006

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Introduction to mobile telecommunication and GSM

Chapter 1 This chapter is designed to provide the student with an introduction to mobile telecommunications and an overview of the GSM standard. It introduces the main system components, the network structure and basic terminology used.

OBJECTIVES: Upon completion of this chapter the student will be able to:
· List 1 benefit of having a standard
· Describe the history of GSM development
· List 3 network components
· Describe the GSM geographical network structure
· List the GSM-900 frequency bands
· List 3 subscriberservices provided in the GSM network

Mobile telecommunications is one of the fastest growing and most demanding of all telecommunications technologies. Currently, it represents an increasingly high percentage of all new telephone subscriptions worldwide. In many cases, cellular solutions successfully compete with traditional wireline networks and cordless telephones. In the future, cellular systems employing digital technology will become the universal method of telecommunication.

The origins of mobile communications followed quickly behind the invention of radio in the late 1800s. The first applications of mobile radio were related to the navigation and safety of ships at sea. As radio concepts developed, so did it’s use as a communications tool.

The major milestones in the development of wireless communications are summarized in the following table:

Date Activity 1906 Reginald Fesseden successfully transmits human voice over radio. Up until that time, radio communications consisted of transmissions of Morse Code.

1915 J. A. Fleming invents the vacuum tube making it possible to build mobile radios.

1921 The Detroit police department used a 2 MHz frequency in the department's first vehicular mobile radio. The system was only one way and police had to find a wireline phone to respond to radio messages.

1930s Amplitude Modulation (AM) two-way mobile systems were in place in the U.S. that took advantage of newly developed mobile transmitters and utilized a "push-to-talk" or half-duplex transmission. By the end of the decade channel allocation grew from 11 to 40.

1935 Invention of Frequency Modulation (FM) improved audio quality. FM eliminated the need for large AM transmitters and resulted in radio equipment which required less power to operate. This made the use of transmitters in vehicles more practical.

1940s The Federal Communications Commission (FCC) recognized a communication service it classified as Domestic Public Land Mobile (DPLM) radio service. The first DPLM system was
established in St.

Louis in 1946 and it utilized the 150 MHz band. The following year, a "highway" system was developed along the New York - Boston corridor using the 35-40 MHz band.

1947 D.H. Ring, working at Bell Laboratories, envisions the cellular concept.

1948 Shockley, Bardeen and Brittain, at Bell Laboratories, invent the transistor which enables electronic equipment, including the radio to be miniaturized.

1949 Radio Common Carriers (RCCs) were recognized.

1949, 1958 Bell Systems made broadband proposals. 1964 AT&T introduces Improved Mobile Telephone System (IMTS).

1968 The FCC began to address issue of new US spectrum requirements.

1969 Nordic countries of Denmark, Finland, Iceland, Norway and Sweden agree to form a group to study and recommend areas of cooperation in telecommunication. This led to the standardization of telecommunications for all members of the Nordic Mobile Telephone (NMT) group, the first comprehensive international standardization group.

1973 The NMT group specifies a feature allowing mobile telephones to be located within and across networks. This feature would become the basis for roaming.

1979 The FCC authorized the installation and testing of the first developmental cellular system in the US (Illinois Bell Telephone Company).

1981 Ericsson launches the world’s first cellular system in Saudi Arabia based on the analog NMT 450 standard.

1991 The first digital cellular standard (GSM) is launched.

1998 The number of mobile subscribers world-wide has grown to over 200 million.


Standards play a major role in telecommunications by:
· Allowing products from diverse suppliers to be interconnected
· Facilitating innovation by creating large markets for common products The standards-making process is one of co-operation at many levels, both nationally and internationally and includes cooperation between:
· Industrial concerns within a country
· These industrial concerns and their governments
· National governments at an international level The primary purpose of a standard for mobile communications is to specify how mobile phone calls are to be handled by a mobile network. For example, this includes specification of the following:
· The signals to be transmitted and received by the mobile phone · The format of these signals · The interaction of network nodes
· The basic network services which should be available to mobile subscribers
· The basic network structure (i.e. cells, etc.)

Since the development of NMT 450 in 1981, many standards for mobile communication have been developed throughout the world. Each mobile standard has been developed to meet the particular requirements of the country or interest groups involved in its specification. For this reason, although a standard may be suitable for one country, it may not be suitable for another. The main standards and the main markets in which they are used are summarized in the following table.


This history of GSM is outlined in the following table:
Date Activity
1982- 1985 · Conférence Européenne des Postes et Télécommunications (CEPT) began specifying a European digital telecommunications standard in the 900 MHz frequency band. This standard later became known as Global System for Mobile communication (GSM).

1986 · Field tests were held in Paris to select which digital transmission technology to use. The choice was Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA).
1987 · A combination of TDMA and FDMA was selected as the transmission technology for GSM. · Operators from 12 countries signed a Memorandum of Understanding committing themselves to introducing GSM by 1991.
1988 · CEPT began producing GSM specifications for a phased implementation. · Another five countries signed the MoU.
1989 · European Telecommunication Standards Institute (ETSI) took over responsibility for GSM specification.
1990 · Phase 1 specifications were frozen to allow manufacturers to develop network equipment. 1991 · The GSM 1800 standard was released. · An addendum was added to the MoU allowing countries outside CEPT to sign.
1 Introduction to Mobile Telecommunication and GSM EN/LZT 123 3321 R3A – 7 – 1992 · Phase 1 specifications were completed. · First commercial Phase 1 GSM networks were launched. · The first international roaming agreement was established between Telecom Finland and Vodafone in UK.
1993 · Australia became the first non-European country to sign the MoU. · The MoU now had a total of 70 signatories. GSM networks were launched in Norway, Austria, Ireland, Hong Kong and Australia. · The number of GSM subscribers reached one million. · The first commercial DCS 1800 system was launched in the U.K.
1994 · The MoU now had over 100 signatories covering 60 countries. · More GSM networks were launched. · The total number of GSM subscribers exceeded 3 million.
1995 · The specification for the Personal Communications Services (PCS) was developed in the U.S.A. This version of GSM operates at 1900 MHz. · GSM growth trends continued steadily through 1995, with the number of GSM subscribers increasing at the rate of 10,000 per day and rising.
· In April 1995, there were 188 members of the MoU from 69 countries.
1996 · The first GSM 1900 systems became available. These comply with the PCS 1900 standard.
1998 · At the beginning of 1998 the MoU has a total of 253 members in over 100 countries and there are over 70 million GSM subscribers world-wide. GSM subscribers account for 31% of the world’s mobile market.

Did you know? The headquarters of the GSM MoU are in Dublin, Ireland.

Because GSM provides a common standard, cellular subscribers can use their telephones over the entire GSM service area which includes all the countries around the world where the GSM system is used. In addition, GSM provides user services such as high speed data communication, facsimile and a Short Message Service (SMS). The GSM technical specifications are also designed to work with other standards as it guarantees standard interfaces. Finally, a key aspect of GSM is that the specifications are openended and can be built upon to meet future requirements.

Did you know? The countries with the highest numbers of GSM subscribers are the United Kingdom and Italy.

Introduction to Mobile Telecommunication and GSM


GSM was designed to be platform-independent. The GSM specifications do not specify the actual hardware requirements, but instead specify the network functions and interfaces in detail. This allows hardware designers to be creative in how they provide the actual functionality, but at the same time makes it possible for operators to buy equipment from different suppliers. The GSM recommendations consist of twelve series which are listed in the table below. These series were written by different working parties and a number of expert groups. A permanent nucleus was established in order to coordinate the working parties and to manage the editing of the recommendations.

The GSM 1800 section is written as a delta part within the GSM recommendations, describing only those differences between GSM 900 and GSM 1800. GSM 1900 is based on GSM 1800 and has been adapted to meet the American National Standards Institute (ANSI) standard.


In the late 1980s, the groups involved in developing the GSM standard realized that within the given time-frame they could not complete the specifications for the entire range of GSM services and features as originally planned. Because of this, it was decided that GSM would be released in phases with phase 1 consisting of a limited set of services and features. Each new phase builds on the services offered by existing phases.

Phase 1
Phase 1 contains the most common services including:
· Voice telephony
· International roaming
· Basic fax/data services (up to 9.6 kbits/s)
· Call forwarding
· Call barring
· Short Message Service (SMS)
Phase 1 also incorporated features such as ciphering and Subscriber Identity Module (SIM) cards. Phase 1 specifications were then closed and cannot be modified.

Introduction to Mobile Telecommunication and GSM

Additional features were introduced in GSM phase 2 including:
· Advice of charge
· Calling line identification
· Call waiting
· Call hold
· Conference calling
· Closed user groups
· Additional data communications capabilities

Phase 2+

The standardization groups have already begun to define the next phase, 2+. The phase 2+ program will cover multiple subscriber numbers and a variety of business oriented features. Some of the enhancements offered by Phase 2+ include:

· Multiple service profiles
· Private numbering plans
· Access to Centrex services
· Interworking with GSM 1800, GSM 1900 and the Digital

Enhanced Cordless Telecommunications (DECT) standard Priorities and time schedules for new features and functions depend primarily on the interest shown by operating companies and manufacturers and technical developments in related areas.


The GSM network is divided into two systems. Each of these systems are comprised of a number of functional units which are individual components of the mobile network. The two systems are:
· Switching System (SS)
· Base Station System (BSS)

In addition, as with all telecommunications networks, GSM networks are operated, maintained and managed from computerized centers.
NMC and OMC Switching System Signaling transmission Call connections and signaling transmission Base Station System Other networks

1 Introduction to Mobile Telecommunication and GSM EN/LZT 123 3321 R3A – 13 – Abbreviations: AUC AUthentication Center BSC Base Station Controller BTS Base Transceiver Station EIR Equipment Identity Register HLR Home Location Register MS Mobile Station MSC Mobile services Switching Center NMC Network Management Center OMC Operation and Maintenance Center VLR Visitor Location Register The SS is responsible for performing call processing and subscriber related functions. It includes the following functional units: · Mobile services Switching Center (MSC) · Home Location Register (HLR) · Visitor Location Register (VLR) · AUthentication Center (AUC) · Equipment Identity Register (EIR) The BSS performs all the radio-related functions. The BSS is comprised of the following functional units: · Base Station Controller (BSC) · Base Transceiver Station (BTS) The OMC performs all the operation and maintenance tasks for the network such as monitoring network traffic and network alarms. The OMC has access to both the SS and the BSS. MSs do not belong to any of these systems.

SWITCHING SYSTEM (SS) COMPONENTS Mobile services Switching Center (MSC) The MSC performs the telephony switching functions for the mobile network. It controls calls to and from other telephony and data systems, such as the Public Switched Telephone Network (PSTN), Integrated Services Digital Network (ISDN), public data networks, private networks and other mobile networks. Gateway Functionality Gateway functionality enables an MSC to interrogate a network’s HLR in order to route a call to a Mobile Station (MS). Such an MSC is called a Gateway MSC (GMSC). For example, if a person connected to the PSTN wants to make a call to a GSM mobile subscriber, then the PSTN exchange will access the GSM network by first connecting the call to a GMSC. The same is true of a call from an MS to another MS. Any MSC in the mobile network can function as a gateway by integration of the appropriate software. Home Location Register (HLR) The HLR is a centralized network database that stores and manages all mobile subscriptions belonging to a specific operator. It acts as a permanent store for a person’s subscription information until that subscription is canceled. The information stored includes: · Subscriber identity · Subscriber supplementary services · Subscriber location information · Subscriber authentication information The HLR can be implemented in the same network node as the MSC or as a stand-alone database. If the capacity of a HLR is exceeded by the number of subscribers, additional HLRs may be added.
1 Introduction to Mobile Telecommunication and GSM EN/LZT 123 3321 R3A – 15 – Visitor Location Register (VLR) The VLR database contains information about all the mobile subscribers currently located in an MSC service area. Thus, there is one VLR for each MSC in a network. The VLR temporarily stores subscription information so that the MSC can service all the subscribers currently visiting that MSC service area. The VLR can be regarded as a distributed HLR as it holds a copy of the HLR information stored about the subscriber. When a subscriber roams into a new MSC service area, the VLR connected to that MSC requests information about the subscriber from the subscriber’s HLR. The HLR sends a copy of the information to the VLR and updates its own location information. When the subscriber makes a call, the VLR will already have the information required for call set-up. AUthentication Center (AUC) The main function of the AUC is to authenticate the subscribers attempting to use a network. In this way, it is used to protect network operators against fraud. The AUC is a database connected to the HLR which provides it with the authentication parameters and ciphering keys used to ensure network security. Equipment Identity Register (EIR) The EIR is a database containing mobile equipment identity information which helps to block calls from stolen, unauthorized, or defective MSs. It should be noted that due to subscriber-equipment separation in GSM, the barring of MS equipment does not result in automatic barring of a subscriber. F Did you know? Although useful, the EIR is actually an optional component of a GSM network, and is therefore, often not used.

BASE STATION SYSTEM (BSS) COMPONENTS Base Station Controller (BSC) The BSC manages all the radio-related functions of a GSM network. It is a high capacity switch that provides functions such as MS handover, radio channel assignment and the collection of cell configuration data. A number of BSCs may be controlled by each MSC. Base Transceiver Station (BTS) The BTS controls the radio interface to the MS. The BTS comprises the radio equipment such as transceivers and antennas which are needed to serve each cell in the network. A group of BTSs are controlled by a BSC. NETWORK MONITORING CENTERS Operation and Maintenance Center (OMC) An OMC is a computerized monitoring center which is connected to other network components such as MSCs and BSCs via X.25 data network links. In the OMC, staff are presented with information about the status of the network and can monitor and control a variety of system parameters. There may be one or several OMCs within a network depending on the network size. Network Management Center (NMC) Centralized control of a network is done at a Network Management Center (NMC). Only one NMC is required for a network and this controls the subordinate OMCs. The advantage of this hierarchical approach is that staff at the NMC can concentrate on long term system-wide issues, whereas local personnel at each OMC can concentrate on short term, regional issues. OMC and NMC functionality can be combined in the same physical network node or implemented at different locations.
1 Introduction to Mobile Telecommunication and GSM EN/LZT 123 3321 R3A – 17 – MOBILE STATION (MS) An MS is used by a mobile subscriber to communicate with the mobile network. Several types of MSs exist, each allowing the subscriber to make and receive calls. Manufacturers of MSs offer a variety of designs and features to meet the needs of different markets. The range or coverage area of an MS depends on the output power of the MS. Different types of MSs have different output power capabilities and consequently different ranges. For example, hand-held MSs have a lower output power and shorter range than car-installed MSs with a roof mounted antenna. Figure 1-4 Ranges for different types of MSs GSM MSs consist of: · A mobile terminal · A Subscriber Identity Module (SIM) Unlike other standards, in GSM the subscriber is separated from the mobile terminal. Each subscriber’s information is stored as a "smart card" SIM. The SIM can be plugged into any GSM mobile terminal. This brings the advantages of security and portability for subscribers. For example, subscriber A’s mobile terminal may have been stolen. However, subscriber A’s own SIM can be used in another person’s mobile terminal and the calls will be charged to subscriber A.

GSM GEOGRAPHICAL NETWORK STRUCTURE Every telephone network needs a specific structure to route incoming calls to the correct exchange and then on to the subscriber. In a mobile network, this structure is very important because the subscribers are mobile. As subscribers move through the network, these structures are used to monitor their location. CELL A cell is the basic unit of a cellular system and is defined as the area of radio coverage given by one BS antenna system. Each cell is assigned a unique number called Cell Global Identity (CGI). In a complete network covering an entire country, the number of cells can be quite high. Cell Figure 1-5 A cell LOCATION AREA (LA) A Location Area (LA) is defined as a group of cells. Within the network, a subscriber’s location is known by the LA which they are in. The identity of the LA in which an MS is currently located is stored in the VLR. When an MS crosses a boundary from a cell belonging to one LA into a cell belonging to another LA, it must report its new location to the network1. When an MS crosses a cell boundary within an LA, it does need to report its new location to the network. When there is call for an MS, a paging message is broadcast within all cells belonging to an LA. 1 Note: This only occurs when the MS is idle. When the MS is on a call, it location is not updated, even if it changes LAs.
1 Introduction to Mobile Telecommunication and GSM EN/LZT 123 3321 R3A – 19 – MSC SERVICE AREA An MSC service area is made up of a number of LAs and represents the geographical part of the network controlled by one MSC. In order to be able to route a call to an MS, the subscriber’s MSC service area is also recorded and monitored. The subscriber’s MSC service area is stored in the HLR. cell 6 cell 1 cell 5 cell 4 cell 2 cell 3 MSC LA1 LA3 LA5 LA4 VLR Figure 1-6 MSC service area PLMN SERVICE AREA A Public Land Mobile Network (PLMN) service area is the entire set of cells served by one network operator and is defined as the area in which an operator offers radio coverage and access to its network. In any one country there may be several PLMN service areas, one for each mobile operator’s network.
GSM System Survey – 20 – EN/LZT 123 3321 R3A GSM SERVICE AREA The GSM service area is the entire geographical area in which a subscriber can gain access to a GSM network. The GSM service area increases as more operators sign contracts agreeing to work together. Currently, the GSM service area spans dozens of countries across the world from Ireland to Australia and South Africa. International roaming is the term applied when an MS moves from one PLMN to another when abroad. GSM Service Area PLMN Service Area (one per operator) MSC Service Area Location Area Cell Figure 1-7 Relation between areas in GSM
1 Introduction to Mobile Telecommunication and GSM EN/LZT 123 3321 R3A – 21 – The figures below show alternative views of the same network: · The first figure shows the network nodes and their layout across the network. For simplicity, this may be referred to as the hardware view of the network. · The second figure shows the geographical network configuration. For simplicity, this may be referred to as the software view of the network. HLR EIR AUC GMSC ILR MSC/VLR 1 PSTN MSC Service Area 2 MSC Service Area 1 LEGEND MSC Boundary BSC Boundary PCM Links Base Station MSC/VLR 2 BSC 1B BSC 1C BSC 2B BSC 1A BSC 2A BSC 2C Figure 1-8 “Hardware” view of a sample network LA 1-B LA 1-A LA 2-D LA 2-A Cell 2-A-25 LA 2-B HLR EIR AUC GMSC ILR MSC/VLR 1 PSTN MSC Service Area 2 MSC Service Area 1 LEGEND MSC Boundary BSC Boundary PCM Links Base Station MSC/VLR 2LA 2-C Figure 1-9 “Software” view of a sample network
GSM System Survey – 22 – EN/LZT 123 3321 R3A GSM FREQUENCY BANDS As GSM has grown worldwide, it has expanded to operate at three frequency bands: 900, 1800 and 1900. 0.8 GHz 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 GSM 900 GSM 1800 GSM 1900 Figure 1-10 GSM frequency bands GSM 900 The original frequency band specified for GSM was 900 MHz. Most GSM networks worldwide use this band. In some countries and extended version of GSM 900 can be used, which provides extra network capacity. This extended version of GSM is called E-GSM, while the primary version is called P-GSM. GSM 1800 In 1990, in order to increase competition between operators, the United Kingdom requested the start of a new version of GSM adapted to the 1800 MHz frequency band. Licenses have been issued in several countries and networks are in full operation. By granting licenses for GSM 1800 in addition to GSM 900, a country can increase the number of operators. In this way, due to increased competition, the service to subscribers is improved. GSM 1900 In 1995, the Personal Communications Services (PCS) concept was specified in the United States. The basic idea is to enable "person-to-person" communication rather than "station-tostation". PCS does not require that such services be implemented using cellular technology, but this has proven to be the most effective method. The frequencies available for PCS are around 1900 MHz. As GSM 900 could not be used in North America due to prior allocation of the 900 MHz frequencies, GSM 1900 MHz is seen as an opportunity to bridge this gap. The main differences between the American GSM 1900 standard and GSM 900 is that it supports ANSI signaling. F Did you know? This was originally named Digital Cellular System (DCS) 1800 MHz. In 1997 it was renamed GSM 1800.
1 Introduction to Mobile Telecommunication and GSM EN/LZT 123 3321 R3A – 23 – GSM 400 Ericsson and Nokia support the work of ETSI on a global standard for GSM on the 450 MHz frequency band. Ericsson and Nokia are aiming to make GSM 450 products available for the market during 2001. The believe is that the introduction of GSM in the 450 MHz frequency band will further leverage the success of global GSM. GSM 400 also provides NMT system operators a logical way to introduce quality digital services and seamless international roaming possibilities.
GSM System Survey – 24 – EN/LZT 123 3321 R3A KEY TERMS During the development of mobile systems, many terms arose which are used to describe the call cases and situations involving MSs. The primary terms used are described below. An MS can have one of the following states: · Idle: the MS is ON but a call is not in progress · Active: the MS is ON and a call is in progress · Detached: the MS is OFF The following table defines the key terms used to describe GSM mobile traffic cases (there are no traffic cases in detached mode): Mode Term Description Idle Registration This is the process in which an MS informs a network that it is attached. Roaming When an MS moves around a network in idle mode, it is referred to as roaming. International Roaming When an MS moves into a network which is not its home network, it is referred to as international roaming. MSs can only roam into networks with which the home network has a roaming agreement. Location Updating An MS roaming around the network must inform the network when it enters a new LA. This is called location updating. Paging This is the process whereby a network attempts to contact a particular MS. This is achieved by broadcasting a paging message containing the identity of that MS. Active Handover This is the process in which control of a call is passed from one cell to another while the MS moves between cells. Table 1-6 Key terms
1 Introduction to Mobile Telecommunication and GSM EN/LZT 123 3321 R3A – 25 – MS Registration and Roaming When an MS is powered off it is detached from the network. When the subscriber switches power on, the MS scans the GSM frequencies for special channels called control channels. When it finds a control channel, the MS measures the signal strength it receives on that channel and records it. When all control channels have been measured, the MS tunes to the strongest one. When the MS has just been powered on, the MS must register with the network which will then update the MS’s status to idle. If the location of the MS is noticed to be different from the currently stored location then a location update will also take place. As the MS moves through the network, it continues to scan the control channels to ensure that it is tuned to the strongest possible channel. If the MS finds one which is stronger, then the MS retunes to this new control channel2. If the new control channel belongs to a new LA, the MS will also inform the network of its new location. MSC/VLR MSC/VLR LA 1 LA 2