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SYSTRA Training Material

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The information in this document is subject to change without notice and describes only the product defined in the introduction of this documentation. This document i s intended for the use of Nokia Telecommunications' customers only for the purposes of the agreement under which the document is submitted, and no part of it may be reproduced or transmitted in any form or means without the prior written permission of Noki a Telecommunications. The document has been prepared to be used by professional and properly trained personnel, and the customer assumes full responsibility when using it. Nokia Telecommunications welcomes customer comments as part of the process of continuous development and improvement of the documentation.

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Copyright © Nokia Telecommunications Oy 1998. All rights reserved.

Contents



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Contents

1 Introduction to GSM 7 1.1 Module Objectives 7 1.2 Introduction 8 1.2.1 Background and Requirements 8 1.2.2 Advantages of GSM 10 1.3 Evolution of GSM 10 1.4 Open Interfaces of GSM 13 1.5 GSM Technical Specifications 15 1.6 Introduction to GSM Review 16 1.6.1 Review Questions 16

2 Traffic Management 19 2.1 Module Objectives 19 2.2 Introduction 20 2.3 Mobility Functions 24 2.3.1 Registration and Database 24 2.3.2 Location Update 27 2.4 Call Set-Up in a GSM Network 29 2.4.1 Network Switching Subsystem (NSS) 37 2.4.2 Locating the Subscriber 39 2.4.3 Base Station Subsystem (BSS) 42 2.4.4 A Mobile Terminated Call and Paging 46 2.4.5 Mobile Originated Call 48 2.5 Location Update 50 2.5.1 Types of Location Update 50 2.5.2 Procedures 53 2.6 Handover 54 2.7 Charging 60 2.7.1 What to Charge? 60 2.7.2 Subscription Charge 60 2.7.3 Renting of Service 60 2.7.4 Charge for use of the network 61 2.7.5 Whom to Charge? 62 2.7.6 Charging Procedure in GSM 64 2.8 Network Architecture 68 2.8.1 NSS - Network Switching Subsystem 69 2.8.2 BSS - Base Station Subsystem 69 2.8.3 Network Management Subsystem 71 2.8.4 Authentication Principle 74 2.8.5 Security Algorithms 75 2.8.6 Ciphering/Speech Encryption 78 2.8.7 IMEI Checking 79 2.8.8 User Confidentiality 79 2.9 Services 80 2.9.1 What Are Services? 80 2.9.2 Classification Of Services 80

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2.9.3 Teleservices 82 2.9.4 Bearer Services 87 2.9.5 Supplementary Services 90 2.10 Summary of the Learning Points 91 2.11 Traffic Management Review 93 2.11.1 Review Questions 93

3 Signalling 99 3.1 Module Objectives 99 3.2 Introduction 100 3.2.1 Standard Messages 101 3.2.2 Implementation and Evolution 102 3.3 Common Channel Signalling System No.7 104 3.3.1 Message Transfer Part (MTP) 105 3.3.2 Telephone User Part (TUP) 106 3.3.3 Signalling Connection and Control Part (SCCP) 107 3.3.4 Summary 108 3.4 Other Applications of SS7 in GSM Networks 109 3.4.1 Base Station Subsystem Application Part (BSSAP) 110 3.4.2 Mobile Application Part 111 3.4.3 Transaction Capabilities Application Part (TCAP) 111 3.4.4 Summary 112 3.5 SS7 Layers in GSM Elements 113 3.5.1 Protocol Stack in MSC 113 3.5.2 Protocol Stack in HLR 114 3.5.3 Protocol Stack in BSC 115 3.5.4 Other Signalling Protocols in GSM 116 3.5.5 Summary 117 3.6 Summary of the Learning Points 118 3.7 Signalling Review 119 3.7.1 Review Questions 119

4 Transmission 123 4.1 Module Objectives 123 4.2 Introduction to Radio and Terrestrial Transmission 124 4.3 Transmission Through the Air Interface 129 4.3.1 Physical and Logical Channels 130 4.3.2 Logical channels 134 4.3.3 Time Slots And Bursts 139 4.4 Problems and Solutions of the Air Interface 141 4.4.1 Multipath propagation 141 4.4.2 Shadowing 146 4.4.3 Propagation Delay 147 4.5 Terrestrial Transmission 148 4.5.1 Base Transceiver Station 148 4.5.2 Transmission between BSC and BTS 149 4.5.3 The Concept of Multiplexing 150 4.6 Summary of the Learning Points 153 4.7 Transmission Review 154

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4.7.1 Review Questions 154

5 Network Planning 157 5.1 Module Objectives 157 5.2 Introduction 158 5.3 Radio Network 163 5.3.1 Dimensioning Cells 163 5.3.2 Frequency Reuse 165 5.4 Summary of the Learning Points 167 5.5 Network Planning Review 167 5.5.1 Review Questions 167

6 Nokia Implementation 169 6.1 Module Objectives 169 6.2 Introduction 170 6.3 Network Architecture 170 6.4 DX 200 Platform 172 6.5 DX 200 MSC/VLR Architecture 175 6.5.1 Functional Units in MSC/VLR 176 6.6 DX 200 HLR/AC/EIR 178 6.6.1 Functional Units in HLR 179 6.7 DX 200 BSC 179 6.7.1 Functional Units in DX 200 BSC 181 6.8 Nokia NMS/2000 182 6.8.1 Functional Units in NMS/2000 183 6.9 Nokia BTS 184 6.9.1 Nokia BTS Families 184 6.10 Summary of the Learning Points 190 6.11 Nokia Implementation Review 191 6.11.1 Review Questions 191

7 Next Step 195 7.1 Module Objectives 195 7.2 Introduction 196 7.2.1 High-Speed Circuit Switched Data (HSCSD) 198 7.2.2 General Packet Radio Service (GPRS) 199 7.2.3 Enhanced Data rates over GSM Evolution (EDGE) 201 7.2.4 The Wireless Application Protocol (WAP) 202 7.3 3 rd Generation Mobile Systems 203 7.3.1 Frequency Allocation for 3 rd Generation Systems 204 7.3.2 The Universal Mobile Telephone System (UMTS) 205 7.3.3 Code Division Multiple Access (CDMA) 206 7.3.4 W-CDMA (Wideband CDMA) 208 7.3.5 3G Network Architecture 209 7.3.6 3G Mobile Data Terminals 210 7.4 Summary of Learning Points 211 7.5 Next Step Review 212 7.5.1 Review Questions 212

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8 Intelligent Network 213 8.1 Module Objectives 213 8.2 Introduction 213 8.2.1 History 215 8.2.2 Basic telephone network 216 8.3 Intelligent Network Conceptual Models 219 8.3.1 Physical Entities 220 8.4 Nokia Implementation of IN 221 8.4.1 SSP and IP (Service Switching Point and Intelligent

Peripheral) in MSC 222 8.4.2 SCP - Service Control Point 223 8.4.3 SMP - Service Management Point 224 8.4.4 SCE - Service Creation Environment 225 8.5 IN Service Examples 226 8.5.1 New and existing services provided within IN 226 8.5.2 Flexible Billing 227 8.5.3 Reachability/Mobility 227 8.5.4 Customised User Groups 227 8.5.5 Customised services for mobile users 227 8.5.6 Final conclusive example: 228 8.6 Summary of the Learning Points 228 8.7 References 229 8.8 IN Glossary 230 8.9 Intelligent Network Review 232

9 SYSTRA Course Review 235 9.1 Review Questions 235 9.2 Acronyms 242

Introduction to GSM



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1 Introduction to GSM 1.1 Module Objectives

At the end of the module the student will be able to:

• Describe the evolution of the GSM network

• List four advantages of GSM over analogue networks

• Name and describe two open interfaces of GSM networks

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1.2 Introduction

1.2.1 Background and Requirements

At the beginning of the 1980s it was realised that the European countries were using many different, incompatible mobile phone systems. At the same time, the needs for telecommunication services were 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.

Figure 1.1 GSM – Global System for Mobile communications

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At the beginning of the 1990s, the lack of a common mobile system was seen to be a general, world-wide 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 had 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, it was foreseen that national telecommunication monopolies w ould be disbanded. This development set some requirements concerning the GSM system specifications and these requirements are built into the specifications as follows:

• There should be several network operators in each country. This would lead to competitio n in tariffs and service provisioning and it was assumed to be the best way to 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 mus t be built without causing any major changes to the already existing Public Switched Telephone Networks (PSTN).

In addition to the commercial demands above, some other main 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 compatible with the ISDN (Integrated

Services Digital Network). • The system must be compatible with other data communication

specifications. • The system must maintain good security concerning both

subscriber and transmitted information.

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1.2.2 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 quality of speech achieved is better than in analogue cellular systems.

• Data transmission is supported throughout the GSM system.

• Speech is encrypted and subscriber information security is guaranteed.

• Due to the ISDN compatibility, new services are offered compared to the analogue systems.

• 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.

1.3 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 will 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.

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

1985 CEPT made decision on time schedule and action plan.

1986 CEPT tested eight experimental systems in Paris.

1987 Memorandum of Understanding (MoU). Allocation of the frequencies.

- 890-915 uplink (from mobile to base station)

- 935-960 downlink (from base station to mobile)

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

1989 Final recommendations and specifications for GSM Phase 1.

1990 Validation systems implemented and the 1st GSM World congress in Rome with 650 participants.

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

1992 Worlds first GSM network launched in Finland. By December there were 13 networks operating in 7 areas. Australian operators were first non-European signatories of the GSM MoU. New frequency allocation for GSM 1800 (DCS 1800).

- 1710-1785MHz (uplink) - 1805-1880MHz (downlink)

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

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.

The GSM MoU is formally registered as an Association in Switzerland with 156 members from 86 areas. The GSM World Congress was held in Madrid with 1400 participants.

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

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for GSM 1900 (PCS 1900). The first GSM 1900 network is implemented in the USA. Telecom '95 is held in Geneva where Nokia demonstrates 33.6Kbits/s multimedia data via GSM.

1996 By December 1996 there were 120 networks operating. The 8K SIM was launched in addition to Pre-Paid GSM SIM Cards.

1998 Over 2million GSM 1900 users in the USA and a total of 120million GSM 900/1800/1900 users world-wide.

Figure 1.2 The Number of GSM Customers World-wide

1992 1994 1996 1998 2000 2002

50

100

150

200

250

300

350

Mill

ion

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1.4 Open Interfaces of GSM

The purpose behind the GSM specifications is to define several open interfaces, which then are limiting certain parts of the GSM system. Because of this interface openness, the operator maintaining the network may obtain different parts of the network from different GSM network suppliers. Also, when an interface is open it defines strictly what is happening through the interface and this in turn strictly defines what kind of actions/procedures/functions must be implemented between the interfaces.

Nowadays, GSM specifications define two truly open interfaces. The first one is between the Mobile Station and the Base Station. This open-air interface is appropriately named the “Air interface”. The second one is between the Mobile Services Switching Centre – MSC (which is the switching exchange in GSM) and the Base Station Controller (BSC). This interface is called the “A interface” . These two network elements will be discussed in greater detail in later chapters. The system includes more than the two defined interfaces but they are not totally open as the system specifications had not been completed when the commercial systems were launched.

When operating analogue mobile networks, experience has shown that centralised intelligence generated excessive load in the system, thus decreasing the capacity. For this reason, the GSM specification, in principle, provides the means to distribute intelligence throughout the network. Referring to the interfaces, the more complicated the interfaces in use, the more intelligence is required between the interfaces in order to implement all the functions required. In a GSM network, this decentralised intelligence is implemented by dividing the whole network into three separate subsystems:

• Network Sw itching Subsystem (NSS)

• Base Station Subsystem (BSS)

• Network Management Subsystem (NMS)

The actual network needed for establishing calls is composed of the NSS and the BSS. The BSS is responsible for radio path control and every call is connected through the BSS. The NSS takes care of call control functions. Calls are always connected by and through the NSS.

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The NMS is the operation and maintenance related part of the network and it is needed for the control of the whole GSM network. The network operator observes and maintains network quality and service offered through the NMS. The three subsystems in a GSM network are linked by the Air, A and O&M interfaces as shown.

Figure 1.3 The three Subsystems of GSM and their interfaces

The MS (Mobile Station) is a combination of terminal equipment and subscriber data. The terminal equipment as such is called ME (Mobile Equipment) and the subscriber's data is stored in a separate module called SIM (Subscriber Identity Module).

Therefore, ME + SIM = MS.

Figure 1.4 Inserting a SIM card in a mobile phone

A

NMSNMS

NSSNSSBSSBSS

O&M

Air

MS

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1.5 GSM Technical Specifications

From the specification point of view, the GSM system is divided into twelve different classes and these classes together are called GSM Technical Specifications. Nowadays, technical specifications are implemented by ETSI (European Telecommunication Standard Institute) in Subtechnical Committees and are referred to as Special Mobile Groups (SMG).

ETSI Subtechnical Committees:

SMG1: Services and Facilities SMG2: Radio Aspects SMG3: Network Aspects SMG4: Data Services SMG5: Closed SMG6: Operation & Maintenance SMG7: ME Testing SMG8: BSS Testing SMG9: SIM Aspects SMG10: Security SMG11: Speech SMG12: Architecture

GSM Technical Specifications:

01 General 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 08 BSS - MSC Interface 09 Network Inter Working

[10 Service Inter Working] - removed 11 Type Approval Procedures 12 Operation and Maintenance

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1.6 Introduction to GSM Review

1.6.1 Review Questions

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

1. Why do you think there should be several network operators in each country?

a) To help more people make money by operating networks

b) To help rapid expansion of GSM

c) To get more GSM subscribers

d) To help allocate the GSM frequency band easily

2. Which of the following is not a feature of GSM networks alone but is also a feature of analogue mobile communication networks?

a) Digital transmission of user data (speech and data) in the air interface

b) Possibility of full international roaming in any country which has GSM network

c) Better speech quality

d) A fully digitised switching exchange

3. Which of the following interfaces in not truly an open interface?

a) Between Network Switching Subsystem (NSS) and Network Management Subsystem (NMS)

b) Between Network Switching Subsystem (NSS) and Base Station Subsystem (BSS)

c) Between Network Switching Subsystem (NSS) and Public Switched Telephone Network (PSTN)

d) Between Base Station Subsystem (BSS) and Mobile Station

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4. Match the year on the left-hand column with the corresponding significant GSM event on the middle column.

Year Event Correct year

1982 Allocation of GSM frequencies 1987

1998 Experimental test in Paris 1986

1995 Frequency allocation for GSM 1800 1992

1989 First official GSM call in the world 1991

1991 Initiation of a new system 1982

1992 Final recommendations Phase 1 1989

1987 Phase 2 recommendations frozen 1995

1986 Total GSM subscribers reaches 100 million 1998

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Traffic Management



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2 Traffic Management 2.1 Module Objectives

At the end of the module the student is able to:

• Name the three subsystems of GSM.

• Explain the mobility concept (handover, location update, paging).

• Describe how mobile originated and mobile terminated calls are handled in GSM.

• Explain the concept of distributed charging.

• Explain the concept of security.

• List and explain the operation of at least four services offered by GSM networks.

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2.2 Introduction

A connection between two people - a caller and the called person - is the basic service of all telephone networks. To provide this service, the network must be able to set up and maintain a call, which involves a number of tasks: identifying the called person, determining his location, routing the call to him and ensuring that the connection is sustained as long as the conversation lasts. After the transaction, the connection is terminated and (normally) the calling user is charged for the service he has used.

In a fixed telephone network, providing and managing connections is a relatively easy process because telephones are connected by wires to the network and their location is permanent, at least from the networks’ viewpoint. In a mobile network however, the establishment of a call is a far more complex task as the wireless (radio) connection enables the users to move at their own free will - providing they stay within the service area of the network. In practice, the network has to find solutions to three problems before it can even set up a call:

•Where is thesubscriber

•Who is thesubscriber

•What does thesubscriber want

Information aboutthe subscriber

Figure 2.1 Information required by a mobile communications network

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In other words, the subscriber has to be located and identified to provide him with the requested services. Let’s take an example to demonstrate these processes.

A well-known professor is travelling around the world. He decides to spend the night in a hotel in, let’s say, Madrid. The first thing he will do is to contact the reception desk for registration. Basically, the reception desk is an office that supports registration. The receptionist records the registration in a database which we call the visitor’s register.

The receptionist carefully checks the passport of the professor. The passport is also a database - a small one, though - and the receptionist analyses the data recorded in it. She finds the basic facts, such as citizenship, identification and the name of the professor, and also the name of the authority that has released the document.

Figure 2.2 Registering into a hotel

The professor appears to have his visa expiring soon, so the receptionist decides to call the office that has released the passport (presumably an embassy). This is simple as she knows the nationality and identity of the professor, and the number of his passport. The receptionist talks with the embassy secretary who recognises the professor instantly and advises the receptionist that everything is okay. The professor is admitted into the hotel and the embassy of the professor’s home country, registers the latest information about his location.

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Figure 2.3 Updating the location data in the home country

In other words, this is a transaction between the two offices and, as a result, IDENTIFICATION and LOCALISATION of the customer takes place in both databases. In this example, the embassy maintains a database, which contains the basic data of all the citizens who are travelling around the world and a record of their movements.

When the registration is completed, the professor goes to his room. We can say that he is using a service provided by the hotel. As all the hotels in the world give this type of service, we can call it a basic service . In addition to the basic services (e.g. room and towels etc.), the hotel also provides additional services (e.g. restaurant, sauna, swimming pool etc.). These can be called supplementary services.

Figure 2.4 Services provided by a hotel

Embassy

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To sum up the operations of the hotel:

• It provides SERVICES.

• It maintains a VISITOR REGISTER.

• It informs the HOME REGISTER of a visitor.

The purpose of the two registers is to enable the Identification, Authorisation and Localisation of the customer.

Let’s assume that the professor checks out of Madrid and goes to Paris. He registers in another hotel and once again the receptionist informs the embassy in the home country.

The registration in Madrid is cancelled, registration in Paris is made, and the location data in the Home Register is brought up-to-date. We have thus made a successful location update.

Let us move on and take a closer look at the mobile network. The story of the professor visiting hotels bears a striking resemblance to the users and functions of a mobile GSM network. Within the mobile network there are subscribers who move around and register into the service areas of networks in order to use the services provided by them. The visitor register of the hotel and the permanent register of the embassy also have their counterparts within the GSM network. There are fixed databases that maintain basic information about their customers including data on their current location, and temporary databases storing information about the users who are currently located in their service area. Let’s start with the registration process and the various databases involved in it.

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2.3 Mobility Functions

2.3.1 Registration and Database

A new subscriber has just bought a mobile telephone and he switches it on for the first time. He can be practically anywhere in the world because, thanks to a network connection through a radio link , his telephone does not need wires.

Figure 2.5 Person about to use a mobile phone

On the other hand, a connection through the mobile network is possible only if there is a point to point connection between the caller and the person who is called. Therefore, it is absolutely necessary that the network knows the subscriber’s location. The network keeps track of the subscribers’ location with the help of various databases as in the hotel example.

BTS

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2.3.1.1 The Subscriber Identity Module From the user’s point of view, the first and most important database is inside the mobile phone: the Subscriber Identity Module (SIM). The SIM is a small memory device mounted on a card and contains user-specific identification. The SIM card can be taken out of one mobile equipment and inserted into another. In the GSM network, the SIM card identifies the user just like a traveller uses a passport to identify himself.

Figure 2.6 Example of a SIM card

The SIM card contains the identification numbers of the user, a list of the services that the user has subscribed to and a list of available networks. In addition, the SIM card contains tools needed for authentication and ciphering and, depending on the type of the card, there is also storage space for messages such as phone numbers, etc. A so-called “Home Operator” issues a SIM card when the user joins the network by making a service subscription. The Home Operator of the subscriber can be anywhere in the world, but for practical reasons the subscriber chooses one of the operators in the country where he spends most of his time.

Now, the new subscriber switches on his phone in an area where a local operator provides network service. The area is connected through an air interface to a database known as a Visitor Location Register (VLR). The VLR is integrated into a telephone exchange known as a Mobile Services Switching Centre (MSC).

The home operator of the subscriber also needs to know the location of the subscriber and so it maintains another register - just as the embassy did in our example - which is called a Home Location Register (HLR).

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Figure 2.7 Databases in a GSM Network

The HLR stores the basic data of the subscriber on a permanent basis. The only variable data in the HLR is the current location (VLR address) of the subscriber. However, in the VLR, the subscriber data is stored temporarily. When the subscriber moves to another VLR area, its data is erased from the old VLR and stored in the new VLR.

VLRMSC

GSM Network

SIM

HLR

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2.3.2 Location Update

As an owner of a mobile phone, the subscriber does not stay in one place but keeps moving from one place to another. No matter how often or how quickly he moves, the network must be able to locate him continuously in case somebody wants to call him. T he transaction that enables the network to keep track of the subscriber is called a Location Update and it happens in roughly the same way as in the example of the two hotels.

The mobile phone constantly receives information sent by the network. This information includes identification (ID) of the VLR area in which the mobile is currently located. In order to keep track of its location, the mobile stores the ID of the area in which it is currently registered. Every time the network broadcasts the ID of the area, the mobile compares this information to the area ID stored in its memory. When the two IDs are no longer the same, the mobile sends the network a request, i.e. a registration inquiry to the area it has just entered. The network receives the request and registers the mobile in the new VLR area. Simultaneously, the subscriber’s HLR is informed about the new VLR location and the data concerning the subscriber is cleared from the previous VLR.

Figure 2.8 Elements involved in location update

MSC(old)

VLRMSC(new)

HLR

SIM

LocationUpdate

VLR

Mobile moves

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The following figure gives a detailed description of the location update process.

Figure 2.9 Location update procedures

In this way, the network can keep track of the subscriber all the time, however, that is only a part of the job! Things become more complicated when it becomes necessary to set up a call. Let’s start with a call originating in a fixed telephone network, a Public Switched Telephone Network.

BSS MSC VLR HLR

REQUEST SUBSCRIBER INFO

ALL OK - HLR UPDATE

MS

LOCATION UPDATE REQUEST

SEND SUBSCRIBER ID

REQUEST SUBSCRIBER ID

SEND SUBSCRIBER INFO

AUTHENTICATION

AUTHENTICATION RESPONSE

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2.4 Call Set-Up in a GSM Network Let’s go through the main call set-up cases. The first is a call originating from the fixed network. Setting up a call appears to be a quick and simple operation, but if we study the process more closely, we discover that it consists of a considerable number of sub operations. These operations include signalling between switching centres, identifying and locating the subscriber who is being called, making routing decisions and traffic connections etc. This section contains a step by step analysis of setting up a connection between a telephone in a fixed network and a GSM mobile station (i.e. a mobile phone). Setting up a connection between two mobile stations is studied later. 1. A subscriber in a fixed network dials the number of a mobile

station. This can be either a national or an international number. An example of a national number is:

040 2207959

As you can see there is no country code in this number. The following is an example of an international number:

+358 40 2207959

The dialled number is called an MSISDN (Mobile Subscriber International ISDN Number) which contains the following elements:

MSISDN = CC + NDC + SN

• CC= Country code (33=France, 358=Finland, etc.)

• NDC= National Destination Code

• SN= Subscriber Number

Figure 2.10 PSTN originates the call

PSTNMSISDN

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2. The PSTN exchange analyses the dialled number. The result of the analysis is the routing information required for finding the mobile network (Public Land Mobile Network, PLMN) in which the called subscriber has made his subscription. The PSTN identifies the mobile network on the basis of the NDC, after which it accesses the mobile network via the nearest Gateway Mobile Services Switching Centre (GMSC).

Figure 2.11 Incoming Call from PSTN to GSM network

3. The GMSC analyses the MSISDN in the same way as the PSTN exchange did. As a result of the analysis, it obtains the HLR address in which the subscriber is permanently registered. Notice that the GMSC itself does not have any information about the location of the called subscriber. The subscriber’s location can only be determined by the two databases, the HLR and VLR. At this stage however, the GMSC only knows the HLR address and so it sends a message (containing the MSISDN) to the HLR. In practice this message is a request for locating the called subscriber in order to set up a call. This is called an “HLR Enquiry”.

VLR

GMSC

GSMNetwork

MSISDNPSTN

HLR

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4. The HLR analyses the message. It identifies the called subscriber on the basis of MSISDN and then checks its database to determine the subscribers location. As you remember, the HLR is informed every time the subscriber moves from one VLR area to another, i.e. the HLR knows in which VLR area the subscriber is currently registered.

It has to be pointed out that the HLR does not handle network traffic at all. A traffic connection requires two network elements that are able to provide speech connections. A speech connection is a network service and it can be handled only by an MSC. Therefore, to enable the traffic connection, maybe two MSC’s will have to be connected. The first MSC is the Gateway MSC which is contacted by the PSTN exchange. The HLR acts as a co-ordinator to set up the connection between the GMSC and the destination MSC (which could of course be the GMSC itself).

Figure 2.12 Routing the call inside the GSM Network

VLR

GMSC

GSMNetwork

MSISDNPSTN

HLR

VLR

MSC

HLREnquiry

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Let’s have a look at the contents of an HLR database to discover how it locates the called subscriber. We will use an Italian subscriber as an example:

HLR

As you can see the first field contains the identity numbers of the subscriber. The MSISDN has already been explained, but there is also another identification number involved in the process known as the International Mobile Subscriber Identity (IMSI). The purpose of IMSI is to identify the subscriber in the mobile network. The total length of the IMSI is 15 digits and it consists of the following elements:

IMSI = MCC + MNC + MSIN

• MCC = Mobile Country Code (three digits)

• MNC = Mobile Network Code (two digits)

• MSIN = Mobile Subscriber Identification Number (ten digits)

The IMSI number is used for registering a user in the Public Land Mobile Network (PLMN). To locate the subscriber and to enable the traffic connection, the HLR has to associate the MSISDN with the IMSI of the mobile subscriber. But why do we need the IMSI? Why not simply use the MSISDN both for network registration and for setting up a call? The reason for this can be explained with an example: Let’s suppose that three subscribers from three countries (Finland, Italy and the USA) are in the same location and their mobile stations try to register with the same VLR. Let’s also assume that they try to register using their MSISDN (which is actually not the case):

• John, from the USA, MSISDN = + 1 XYZ 1234567

• Ilkka, from Finland, MSISDN = + 358 AB 6543210

• Claudio, from Italy, MSISDN = + 39 GHI 1256890

MSISDN: 39 347 220759

IMSI: 222 10 1234567890

VLR address: xyz

Subscriber Data: services...

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Notice that the length of, for example, the country code is different for each number. If the MSISDN numbers were used in registering the subscribers, we would also need a length indicator for each field to prevent the various parts of the number from getting mixed up with each other - and that would be too complicated. If the length of the fields are the same for all countries, no extra information is needed and the identification process is relatively simple. Another reason for using the IMSI is that the MSISDN identifies the service used such as speech, data, fax, etc. Therefore one subscriber may need several MSISDNs depending on the type of services he uses, whereas he has only one IMSI.

To get back to the HLR database: one data field is reserved for the address of the MSC/VLR where the called subscriber is currently registered. (Normally the VLR and the MSC have the same address.) This is needed in the next phase of establishing the connection.

5. Now the HLR interrogates the MSC/VLR that is currently serving the called subscriber.

But why do we need to interrogate instead of connecting right away? First of all, the current status of the mobile station is stored in the VLR database and we need to know the status to avoid setting up a ca ll to a subscriber whose phone is switched off. Secondly, we need to have some sort of information that enables the GMSC to route the call to the target MSC, wherever in the world it may be.

SYSTRA



6. In terms of routing the call, the serving MSC/VLR is the destination of the call. This means that we must direct the call to it by using the following procedure: after receiving the message from the HLR, the serving MSC/VLR generates a temporary Mobile Station Roaming Number (MSRN) and associates it with the IMSI. The roaming number is used in initiating the connection and it has the following structure:

MSRN = CC + NDC + SN

• CC = Country Code (of the visited country)

• NDC = National Destination Code (of the serving network)

• SN = Subscriber Number

Figure 2.13 MSRN request from HLR to the second MSC

VLR

VLR

MSC

MSC

Routinginformationrequest message

HLR

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If we compare the MSRN and the MSISDN, we notice that they have

the same structure, though they are used for different purposes. The MSISDN is used to interrogate the HLR, whereas the MSRN is the response given by the serving MSC/VLR and it is used for routing the call. The SN field of the MSRN is actually an internal number that is temporarily associated with the IMSI. The MSRN does not merely identify the subscriber, it also points to the exchange itself so that all intermediate exchanges, if there are any, know where the call is to be routed. Since the roaming number is temporary, it is available for establishing another traffic connections after the call has been set up. In essence, the SN field in the MSISDN points to a database entry in the HLR, and the SN field in the MSRN points to a database entry in the VLR.

Let’s take a look at an example with real numbers. This time the called subscriber is roaming in Finland.

VLR

Data: abc..

IMSI: 222 10 1234567890

MSRN: 358 50 456456

SYSTRA



7. The MSC/VLR sends the roaming number to the HLR.

The HLR does not analyse it because the MSRN is used for traffic transactions only and the HLR does not handle traffic, it is only a database that helps in locating subscribers and co-ordina tes call set-up. Therefore, the HLR simply sends the MSRN forward to the GMSC that originally initiated the process.

Figure 2.14 The HLR is giving the MSRN to the originating MSC.

8. When the GMSC receives the message containing the MSRN, it analyses the message. The roaming number identifies the location of the called subscriber, so the result of this analysis is a routing process which identifies the destination of the call - the serving MSC/VLR.

VLR

MSC

MSC

MSRN No.to HLR

VLR

HLR

358 50 456456

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9. The final phase of the routing process is taken care of by the serving MSC/VLR. In fact, the serving MSC/VLR also has to receive the roaming number so that it knows that this is not a new call, but one that is going to terminate here - i.e. a call to which it has already allocated an MSRN. By checking the VLR, it recognises the number and so it is able to trace the called subscriber.

At this point, we have to summarise what happened behind the scenes to be able to understand the rest of the process. We will take a closer look at two basic subsystems GSM network: The Network Switching Subsystem (NSS) and the Base Station Subsystem (BSS).

A ir A

O & M

VLRM SC

VL RM SC

HLR

Figure 2.15 GSM Subsystems

2.4.1 Network Switching Subsystem (NSS)

The GSM network is divided into three subsystems: Network Switching Subsystem (NSS), Base Station Subsystem (BSS), and Network Management Subsystem (NMS). The concept of the NSS is introduced in this section and the BSS and NMS are explained later.

The elements of Network Switching Subsystem that have been discussed so far are:

• MSC (Mobile Services Switching Centre)

• VLR (Visitor Location Register)

• HLR (Home Location Register)

The MSC is responsible for controlling calls in the mobile network. It identifies the origin and destination of a call (either a mobile station or

SYSTRA



a fixed telephone in both cases), as well as the type of a call. An MSC acting as a bridge between a mobile network and a fixed network is called a Gateway MSC. An MSC is normally integrated with a VLR, which maintains information related to the subscribers who are currently in the service area of the MSC. The VLR carries out location registrations and updates. The MSC associated with it initiates the paging process. A VLR database is always temporary (in the sense that the data is held as long as the subscriber is within its service area), whereas the HLR maintains a permanent register of the subscribers. In addition to the fixed data, the HLR also maintains a temporary database which contains the current location of its customers. This data is required for routing calls.

In addition, there are two more elements in the NSS: the Authentication Centre (AC) and the Equipment Identity Register (EIR). They are usually implemented as part of HLR and they deal with the security functions that will be discussed later.

To sum up, the main functions of NSS are:

Call Control This identifies the subscriber, establishes a call and clears the connection after the conversation is over.

Charging This collects the charging information about a call such as the numbers of the caller and the called subscriber, the time and type of the transaction, etc., and transfers it to the Billing Centre.

Mobility management This maintains information about the location of the subscriber.

Signalling with other networks and the BSS This applies to interfaces with the BSS and

PSTN.

Subscriber data handling This is the permanent data storage in the HLR and temporary storage of relevant data in the VLR.

Locating the subscriber This locates a subscriber before establishing a call.

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2.4.2 Locating the Subscriber

The GMSC/VLR and the MSC/VLR has now been connected via a traffic and signalling channel and the call set up has almost been completed. The caller is connected to the PSTN exchange, the PSTN is connected to the GMSC, the GMSC is connected to the MSC/VLR that is serving the called subscriber but we have not yet established a connection to the called subscriber. In order to set up the connection, we first have to understand how the subscriber is located.

As we do not know the exact location of the subscriber, it seems inevitable that we have to search for him in the entire VLR service area. This could be a wide geographical area and so finding the subscriber requires a lot of work for the MSC/VLR. Things will be easier to grasp if we go back for a moment and follow the famous professor in the hotel.

The hotel is an establishment that provides services. If we want to find our professor, we need to search the entire area of the hotel, which can be really frustrating. He might be in his room, in the sauna, in the swimming pool, in the restaurant, in fact practically anywhere. The only thing we know is that he is in the hotel, because he has not checked out. In order to simplify the search, we can register his movements by setting up a registration routine for the various parts of the hotel. In other words, the hotel service area is divided into location areas. This simplifies the search for the professor.

SYSTRA



Reception Restaurant Bar Pool

Figure 2.16 Location areas inside a hotel

Note that these location areas are only known within the area of the hotel, i.e. no information is given to the embassy (police department).

Something similar happens in the cellular network. When we want to find the subscriber, it would be necessary to conduct a search throughout the entire MSC/VLR area unless this area is divided into smaller areas. Therefore, the MSC/VLR area is divided into smaller areas. These are called Location Areas (LA) and they are managed by the MSC/VLR.

Figure 2.17 Location Areas under one MSC/VLR

LA 1

VLRMSC

LA 5

LA 4

LA 3 LA 2

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Each MSC/VLR contains several Location Areas. We can define a LA as an area in which we search for the subscriber in case there is a call addressed to a mobile station.

Let’s have a look at the data in the VLR and then get back to establishing the connection with the called subscriber:

VLR

Each LA is identified by a Location Area Identity (LAI). Its structure is as follows:

LAI = MCC + MNC + LAC

• MCC= Mobile Country Code (of the visited country)

• MNC= Mobile Network Code (of the serving PLMN)

• LAC= Location Area Code

10. Now that we know the LA of the subscriber, we can start searching for him. To locate the subscriber, a Paging process is initiated in the Location Area.

IMSI: 222 10 1234567890

LAC: 262 15 0987

Data: abc...

MSRN: 358 50 456456

SYSTRA



2.4.3 Base Station Subsystem (BSS)

To understand the paging process, we must analyse the functions of the BSS.

The Base Station Subsystem consists of the following elements:

• BSC Base Station Controller

• BTS Base Transceiver Station

• TC Transcoder

The Base Station Controller (BSC) is the central network element of the BSS and it controls the radio network. This means that the main responsibilities of the BSC are: Connection establishment between MS and NSS, Mobility management, Statistical raw data collection, Air and A interface signalling support.

The Base Transceiver Station (BTS) is a network element maintaining the Air interface. It takes care of Air interface signalling, Air interface ciphering and speech processing. In this context, speech processing refers to all the functions the BTS performs in order to guarantee an error-free connection between the MS and the BTS.

The TransCoder (TC) is a BSS element taking care of speech transcoding, i.e. it is capable of converting speech from one digital coding format to another and vice versa. We will describe more about the transcoder functions later.

Figure 2.18 The Base Station Subsystem (BSS)

BTS

TCBSC

BSC

TCBTS

BTS

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The BTS, BSC and TC together form the Base Station Subsystem (BSS) which is a part of the GSM network taking care of the following major functions:

Radio Path Control

In the GSM network, the Base Station Subsystem (BSS) is the part of the network taking care of Radio Resources, i.e. radio channel allocation and quality of the radio connection. For this purpose, the GSM Technical Specifications define about 120 different parameters for each BTS. These parameters define exactly what kind of BTS is in question and how MSs may "see" the network when moving in this BTS area. The BTS parameters handle the following major items: what kind of handovers (when and why), paging organisation, radio power level control and BTS identification.

BTS and TC Control

Inside the BSS, all the BTSs and TCs are connected to the BSC(s). The BSC maintains the BTSs. In other words, the BSC is capable of separating (barring) a BTS from the network and collecting alarm information. Transcoders are also maintained by the BSC, i.e. the BSC collects alarms related to the Transcoders.

Synchronisation

The BSS uses hierarchical synchronisation which means that the MSC synchronises the BSC and the BSC further synchronises the BTSs associated with that particular BSC. Inside the BSS, synchronisation is controlled by the BSC. Synchronisation is a critical issue in the GSM network due to the nature of the information transferred. If the synchronisation chain is not working correctly, calls may be cut or the call quality may not be the best possible. Ultimately, it may even be impossible to establish a call.

Air & A Interface Signalling:

In order to establish a call, the MS must have a connection through the BSS. This connection requires several signalling protocols that are explained in the Signalling Chapter.

Connection Establishment between MS and NSS

The BSS is located between two interfaces, the Air and the A interface. From the call establishment point of view, the MS must have a connection through these two interfaces before a call can be established. Generally speaking, this connection may be either a signalling type of connection or a traffic (speech, data) type of connection.

SYSTRA



Mobility Management and speech transcoding

BSS Mobility Management mainly covers the different cases of handovers. These handovers and speech transcoding are explained in later sections

Collection of Statistical Data

The BSS collects a lot of short-term statistical data that is further sent to the NMS for post processing purposes. By using the tools located in the NMS the operator is able to create statistical "views" and thus observe the network quality.

A Base Station Subsystem is controlled by an MSC. Typically, one MSC contains several BSSs. A BSS itself may cover a considerably large geographical area consisting of many cells. (A cell refers to an area covered by one or more frequency resources). Each cell is identified by an identification number called Cell Global Identity (CGI) which comprises the follow ing elements:

CGI = MCC + MNC + LAC + CI

• MCC Mobile Country Code

• MNC Mobile Network Code

• LAC Location Area Code

• CI Cell Identity

Let’s take an example of two adjacent BTSs. One serves an industrial area and the other a nightlife area.

Figure 2.19 Different types of areas

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It is obvious that traffic is not handled the same way and at the same time in these BTSs. One has traffic peaks during weekdays and especially during working hours, and the other during the evenings and weekends.

Figure 2.20 Traffic channel usage times for different areas

There is one 2Mbit/s PCM line reserved for each BTS to provide the connection to NSS. But as you can see, the BTS’s are used at different times and on different days. Why not use the same line for both of the two BTSs? It can be done, but in this case there has to be a concentrator between MSC and BTS. The BSC acts as a concentrator (in addition to being the radio network controller). One BSC is capable of serving several BTSs.

Figure 2.21 BTS-BSC- BTS connections

BTS

BTS

MSCBSC

Many partially used 2Mbit/s PCM lines

A few efficiently used 2Mbit/s PCM lines

BSC concentratesthe traffic to the MSC

SYSTRA



Note that there is no relation between a BSC area and Location Area and the serves as a concentrator in addition to its major role of radio network control. The purpose of the location area is to facilitate the paging process (searching for the subscriber), whereas a BSC area is related to traffic connections and radio resources.

2.4.4 A Mobile Terminated Call and Paging

Let us go back to our professor. We know that he is within the hotel area. Thanks to the registration sys tem of the hotel, we also know that he went to the restaurant and registered his presence there. Somebody calls him and the receptionist answers. The receptionist checks the registration system of the hotel and discovers that the professor is in the restaurant. A message about an incoming call is sent to the restaurant and one of the waiters starts looking for the professor. If the waiter does not know the right table, he uses the public address system and "pages" the professor as follows: “There is a telephone call for Mr. So and So. Could you please come forward?” Once the professor raises his hand, the search is complete and the call is set up.

Figure 2.22 The paging process

Again, a similar process is used in the cellular network. Paging is a signal that is transmitted by all the cells in the Location Area (LA). It contains the identification of the subscriber. All the mobile stations in the LA receive the paging signal, but only one of them recognises the identification and answers to it. As a consequence of this answer, a point to point connection is established. Now the two subscribers are connected, and traffic can be carried through the network. Let’s sum up the entire process:

BTS BTS

Paging

BTS

Mobile responds to paging

Location Area

Paging

Paging

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Figure 2.23 Simplified steps in setting up a call

1. A subscriber in a fixed network dials a number of a mobile phone. The dialled number is the MSISDN.

2. The Public Switched Telephone Network (PSTN) exchange analyses the number and contacts the Gateway Mobile Services Switching Centre (GMSC).

3. The Gateway MSC analyses the MSISDN and sends a message to the Home Location Register (HLR).

4. The HLR checks its database to determine the current location of the called subscriber.

5. The HLR interrogates the MSC/VLR (Visitor Location Register) that is currently serving the called subscriber.

6. The serving MSC/VLR generates a temporary MSRN (Mobile Subscriber Roaming Number).

7. MSC/VLR sends MSRN to HLR and the HLR forwards the MSRN to the GMSC.

8. The GMSC identifies the serving MSC/VLR as the destination for routing the call.

9. Destination MSC/VLR receives MSRN. It identifies the number that is called and traces the called subscriber.

PSTN GMSC HLR MSC/VLRA-Subscriber

CALL SETUP (MSISDN)

ANALYSE NUMBERCALL SETUP (MSISDN)

MSISDN

IMSI

MSRNMSRN

CALL SETUP (MSRN)

PAGING

SYSTRA



10. The destination MSC/VLR initiates a paging process in the Location Area to locate the called subscriber. The mobile phone of the called subscriber recognises the paging signal and answers it.

2.4.5 Mobile Originated Call

We have studied the phases of a PSTN originated call and traced the movements of the subscriber. We have examined the functions and architecture of the network elements.

Now it’s time to investigate another case: how is a connection established when the call is initiated by a mobile subscriber instead of a fixed one?

The mobile subscriber dials a number. In other words, the subscriber issues a service request to the network in which he is currently registered as a visitor. After receiving the request, the network analyses the data of the calling subscriber in order to do three things:

• Authorise or deny the use of the network.

• Activate the requested service.

• Route the call.

The call may have two types of destinations: a mobile station or a telephone in a fixed network. If the call is addressed to a telephone in a fixed telephone network, it is routed to the PSTN, which in turn routes it to the destination. If the called number is another mobile station in the same network, the MSC starts the HLR Enquiry procedure which is processed in the same way as in the example of a PSTN originated call.

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Figure 2.24 Mobile Originated Call procedure.

Identifying and locating the called subscriber are the two key preconditions of setting up a point to point connection. The MSISDN fulfils the purpose of identification, but locating requires a quick and comprehensive system for keeping track of the subscriber. If the network does not have up-to-date information about the subscriber’s current location, setting up a call would mean paging large network areas in order to find the subscriber and that would be a complex and time-consuming task. To avoid this, the GSM network monitors and records the movements of the subscribers all the time. This process is called Location Update . We have already discussed it briefly, but now we will analyse it in detail.

EXC GMSC HLR MSC VLR BSS MS

1. channel assignment

2. security procedures

3. call setup

4. check services etc.

5. all ok

6. call is proceeding

7. traffic channel allocated

8. set up the call

9. call set up complete

10. alert

11. B answers

SYSTRA



2.5 Location Update

2.5.1 Types of Location Update

In practice, there are three types of location updates:

• Location Registration (power on)

• Generic

• Periodic

Location registration takes place when a mobile station is turned on. This is also known as IMSI Attach because as soon as the mobile station is switched on it informs the Visitor Location Register (VLR) that it is now back in service and is able to receive calls. As a result of a successful registration, the network sends the mobile station two numbers that are stored in the SIM (Subscriber Identity Module) card of the mobile station.

These two numbers are the Location Area Identity (LAI) and the Temporary Mobile Subscriber Identity (TMSI). The network, via the control channels of the air interface, sends the LAI. The TMSI is used for security purposes, so that the IMSI of a subscriber does not have

to be transmitted over the air interface. The TMSI is a temporary identity, which regularly gets changed.

A Location Area Identity (LAI) is a globally unique number.

A Location Area Code (LAC) is only unique in a particular network.

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LA 2LA 1

VLR

MSCEvery time the mobile receives data through the control channels, it reads the LAI and compares it with the LAI stored in its SIM card. A generic location update is performed if they are different. The mobile starts a Location Update process by accessing the MSC/VLR that sent the location data.

A channel request message is sent that contains the subscriber identity (i.e. IMSI/TMSI) and the LAI stored in the SIM card. When the target MSC/VLR receives the request, it reads the old LAI which identifies the MSC/VLR that has served the mobile phone up to this point. A signalling connection is established between the two MSC/VLRs and the subscriber’s IMSI is transferred from the old MSC to the new MSC. Using this IMSI, the new MSC requests the subscriber data from the HLR and then updates the VLR and HLR after succes sful authentication.

Air A

O & M

VLRMSC

VLRMSC

Figure 2.25 Network Elements involved in location update

Periodic location update is carried out when the network does not receive any location update request from the mobile in a specified time. Such a situation is created when a mobile is switched on but no traffic

SYSTRA



is carried, in which case the mobile is only reading and measuring the information sent by the network. If the subscriber is moving within a single location area, there is no need to send a location update request.

Figure 2.26 Example of Periodic Location Update

A timer controls the periodic updates and the operator of the VLR sets the timer value. The network broadcasts this timer value so that a mobile station knows the periodic location update timer values. Therefore, when the set time is up, the mobile station initiates a registration process by sending a location update request signal. The VLR receives the request and confirms the registration of the mobile in the same location area. If the mobile station does not follow this procedure, it could be that the batteries of the mobile are exhausted or the subscriber is in an area where there is no network coverage. In such a case, the VLR changes the location data of the mobile station to “unknown” .

BTSBTS

9999 0000 Location Update Request

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2.5.2 Procedures

Figure 2.27 Location Update procedures

MS BSS MSC VLRnew VLRold HLR

1. channel assignment

3. request subscriber identity

4. request subscriber identity

5. request subscriber data

6. request subscriber data7. security procedures

2. location update request

8. update location

9. update HLR

10. update acknowledgement

11. cancel old location

12. location cancelling accepted

SYSTRA



2.6 Handover

In a mobile communications network, the subscriber can move around. How can we maintain the connection in such cases? To understand this, we must study the process of handing over the calls.

Maintaining the traffic connection with a moving subscriber is made possible with the help of the handover function. The basic concept is simple: when the subscriber moves from the coverage area of one cell to another, a new connection with the target cell has to be set up and the connection with the old cell has to be released. There are two reasons for performing a handover:

1. Handover due to measurements occurs when the quality or the strength of the radio signal falls below certain parameters specified in the BSC. The deterioration of the signal is detected by the constant signal measurements carried out by both the mobile station and the BTS. As a consequence, the connection is handed over to a cell with a stronger signal.

2. Handover due to traffic reasons occurs when the traffic capacity of a cell has reached its maximum or is approaching it. In such a case, the mobile stations near the edges of the cell may be handed over to neighbouring cells with less traffic load.

The decision to perform a handover is always made by the BSC that is currently serving the subscriber, except for the handover for traffic reasons. In the latter case the MSC makes the decision. There are four different types of handover and the best way to analyse them is to follow the subscriber as he moves:

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Intra cell - Intra BSC handover

The smallest of the handovers is the intra cell handover where the subscriber is handed over to another traffic channel (generally in another frequency) within the same cell. In this case the BSC controlling the cell makes the decision to perform handover.

Air A

TCBTS BSC

New Channel

Old Channel

Figure 2.28 Intra Cell - Intra BSC Handover

SYSTRA



Inter cell - Intra BSC handover

The subscriber moves from cell 1 to cell 2. In this case the handover process is controlled by BSC. The traffic connection with cell 1 is released when the connection with cell 2 is set up successfully.

Air A

TCBTS

BTS

BSC

Old Cell / BTS New Cell / BTS

Figure 2.29 Inter Cell - Intra BSC handover

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Inter cell - Inter BSC handover

The subscriber moves from cell 2 to cell 3, which is served by another BSC. In this case the handover process is carried out by the MSC, but, the decision to make the handover is still done by the first BSC. The connection with the first BSC (and BTS) is released when the connection with the new BSC (and BTS) is set up successfully.

Air A

BTS

Old Cell / BTS

New Cell / BTS

BTS

BSC TC

BSC TC

VLRMSC

Figure 2.30 Inter Cell - Inter BSC Handover

SYSTRA



Inter MSC handover

The subscriber moves from a cell controlled by one MSC/VLR to a cell in the domain of another MSC/VLR. This case is a bit more complicated. Considering that the first MSC/VLR is connected to the GMSC via a link that passes through PSTN lines, it is evident that the second MSC/VLR can not take over the first one just like that.

The MSC/VLR currently serving the subscriber (also known as the anchor MSC), contacts the target MSC/VLR and the traffic connection is transferred to the target MSC/VLR. As both MSCs are part of the same network, the connection is established smoothly. It is important to notice, however, that the target MSC and the source MSC are two telephone exchanges. The call can be transferred between two exchanges only if there is a telephone number identifying the target MSC.

Air A

BTS

Old Cell / BTS

New Cell / BTS

BTS

BSC TC

BSC TCVLRMSC

VLRMSC

Figure 2.31 Inter Cell - Inter MSC Handover

Such a situation makes it necessary to generate a new number, the Handover Number (HON). The generation and function of the HON are explained in the following text.

The anchor MSC/VLR receives the handover information from the BSS. It recognises that the destination is within the domain of anothe r MSC and sends a Handover Request to the target MSC via the signalling network. The target MSC answers by generating a HON and sends it to the anchor MSC/VLR, which performs a digit analysis in

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order to obtain the necessary routing information. This information allows the serving MSC/VLR to connect the target MSC/VLR. When the two MSCs are connected, the call is transferred to a new route.

In practice, the handover number is similar to the roaming number. Moreover, the roaming number and the handover numbe r have a similar purpose, that is connecting two MSCs. The structure of the handover number is shown below:

HON = CC + NDC + SN

• CC= Country Code

• NDC= National Destination Code (of the serving network)

• SN= Subscriber Number

The call will not last forever and the connection has to be released sooner or later. To understand the process of releasing the connection, we must consider a few things such as: Who pays for the call, which exchange takes care of the charging operation and where is the subscriber data stored. This will be discussed in the next section but before that, let us sum up the stages of Inter MSC handover.

Figure 2.32 Inter MSC handover procedure

MS BSSold MSCold MSCnew BSSnew MS (after HO)

1. measurement reports

3. request HON

5. radio resources reserved

6. provide HON and target cell info

7. set up speech connection (HON)

2. handover required

4. request for radio resources

8. handover command9. handover complete

10. handover complete11. connect

12. release old connections

SYSTRA



2.7 Charging

2.7.1 What to Charge?

Charging in GSM networks follows similar principles to that used in fixed telephone networks. In addition to a standard fee, subscribers have to pay for the calls they make and the services they use. However, there are a few differences in how the costs are calculated and who is liable to pay them.

Note that the information presented in this chapter outlines only the basic features of charging. The actual charging practices vary considerably from one network operator to another.

2.7.2 Subscription Charge

When a person joins a GSM network, he receives a personal SIM card from the network operator and his basic information (phone number, type of ordered services, etc.) is recorded in the network databases such as the HLR. To cover the costs of these operations, network operators often charge the subscriber an initial subscription charge.

2.7.3 Renting of Service

After the subscription has been made and the subscriber has become a customer of the particular network, he is usually charged for the availability of the network services and the right to use them. This is a regular fee which is c harged irrespective of whether the subscriber makes any calls or not. This kind of charge is also known as renting the service of the network.

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Figure 2.33 Different ways of charging a subscriber

2.7.4 Charge for use of the network

The third charging type is applied for the use of network on a call by call basis There are many factors that affect how the subscriber is charged for making (and, sometimes, receiving) a call. The following is a list of parameters that can be used as a basis for charging the subscribers.

• Type of the service, e.g. speech, short message service.

• Duration of the call.

• Time of day the call was made, e.g. working hours, evening.

• Destination of the call.

• Origin of the call, e.g., a certain cell.

• Use of networks, e.g. the PSTN.

• Use of supplementary services such as call forwarding and call barring.

• Use of radio resources.

• International Roaming leg (explained later)

• Installation fee

• Renting of the service

• Use of the network

SYSTRA



Call forwarding and roaming leg are factors that not only affect the amount of charge, but also bring up the issue of who is liable to pay. This is a major difference when compared to fixed telephone networks, which is why we have to take a closer look at it in the following chapter.

2.7.5 Whom to Charge?

Calling Party

Where is theCalled Party?

Bill to subscriber

PSTN

BC

Path ofthe call

MSC

BC

HLR

Charging

Transfer ofCharging data

Billing

MSC

BSC

PLMN 1

PLMN 2

Figure 2.34 PSTN - GSM Call Path

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Let’s take a closer look at the number dialled by the calling subscriber. The number includes the national destination code, which identifies the called subscriber’s home network. If the called subscriber is registered in a location area belonging to his home network, the connection is established as explained in the previous chapter and the calling subscriber pays for the call.

If however, the called subscriber is outside the service area of his home network (in a foreign country, for instance) and is connected to another network, then the call has to be routed to him using the services of one or more foreign networks. In such a case, we talk about international roaming leg, which refers to the connection between the home network and subscriber via a foreign network. In such a case, the charge for the call will be shared according to the following principle:

• The calling subscriber pays for the connection to the number he dialled (MSISDN).

• The called subscriber pays for the international roaming leg.

The same principle is applied when the mobile subscriber has forwarded incoming calls to another number. The calling subscriber is only responsible for the costs incurred by calling to the mobile station, and the mobile user pays for the forwarded call. This may seem strange and complicated, but the reason is quite clear: the calling subscriber does not necessarily know the location of the called subscriber or the services and connections that are required to access him. The calling subscriber only knows that he is dialling a number in a certain mobile network, and therefore he can only be charged for the services that he is aware of. The called subscriber knows - at least he should know - whether he is using the services of a foreign network or some chargeable supplementary services of his home network, and therefore he is liable to pay for them.

Collect call is the third case in which the called subscriber pays for the call. A collect call in a GSM network is similar to a collect call in a fixed network: first the called subscriber has to accept the call, after which he is responsible for all the costs.

SYSTRA



2.7.6 Charging Procedure in GSM

In the fixed network, the charging is normally determined by collecting metering pulses, by which the exchange can calculate the price of the call. This method is called time charging. The calling subscriber (subscriber A) is normally the one who pays for the call. In the mobile network the called subscriber is normally also charged for the so called roaming leg, because A does not necessarily know, where B is located.

In the GSM network there are so many different ways to define the charging that it is sensible to create a record in the MSC or/and HLR about every event which can be a basis for charging. These events can be the defined call cases or other possible chargeable eve nts, such as location updates. The record containing the information about one chargeable event is called the charging record . These records are stored primarily as charging files in the MSC or HLR and then transferred to a separate billing centre. The serving operator controls the entire charging process. The process begins when a call is set up and at the same time, a charging record is opened in the serving MSC/VLR. In general the first and the last MSC involved in a call set up, collect the charging record.

Figure 2.35 GMSC is responsible for creating charging records

BTS

BSC

HLR

PSTN

GMSC

Charging Record

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As the call continues, the subscriber moves in the service area of the operator and enters the service area of another MSC/VLR and thus an inter-MSC handover is performed. The charging record is not transferred to the new MSC during the handover. Instead, the first MSC keeps record of the call as long as it lasts.

Figure 2.36 Elements involved in call handling

When the call has been released, the charging record is closed. When a sufficient number of charging records have been accumulated they are sent to a billing centre in one bulk via an X.25 or Ethernet connection.

BTS

BSC

HLR

GMSC

BSC

MSC

PSTN

SYSTRA



Figure 2.37 Transfer of charging data to Billing Centre

As the actual charging is affected by a variety of factors, the charging record contains all the events that can be used as a basis for determining the charge.

Information for one customer is collected from many MSCs and billing centres that the mobile has visited and is presented as one bill.

BTSBSC

MSC

HLR

X.25 orEthernet

Billing Centre (BC)

GMSC

PSTN

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2.7.6.1 Distributed Charging In order to produce bills for each subscriber, Billing Centres should collect detailed charging data from all the MSCs within the PLMN.

With International Roaming, this operation should be extended covering all the PLMNs where a Roaming Contract is signed. Charging information must be collected from the Billing Centres (BC) of all the networks that subscribers have been visited and passed to the Billing Centre of the home network.

Figure 2.38 Distributed Charging

When two GSM operators sign a “roaming contract”, they agree how often they will transfer charging data between each other. The home billing centre analyses the charging information collected from all the networks where a roaming contract exists, and produces a bill for each customer.

Home Billing Centre

SYSTRA



2.8 Network Architecture

After examining the course of a call from beginning to end it is useful to summarise the entire process by putting the pieces together and studying the network as a whole. Note that the network picture below contains equipment from a typical GSM network, some of which is not included in the GSM specifications.

Figure 2.39 GSM Network Architecture

If we take a look at the entire network, we can see that some Network Elements have not been discussed yet. These are:

• TransCoder (TC) - This belongs to the BSS.

• Equipment Identity Register (EIR) – This belongs to the NSS.

• Authentication Centre (AC) – This belongs to the NSS.

• The complete Network Management Subsystem (NMS)

Mobile StationsBase Station Subsystem Network Management Subsystem

BaseTransceiverStations

Base StationController

TranscoderSubmultiplexer

Digital CrossConnect

A-Interface Air Interface X.25 Interface Abis Interface

IN Service Control PointShort MessageService Centre

Voicemail

Mobile Switching Centre/Visitor Location Register

Home Location Register/Authentication Centre/Equipment IdentityRegister

Network Switching Subsystem

PSTN/ISDN

CommunicationsServer

DataCommunication

Network

Database Server

Workstations

NetworkPlanningSystemNetworkMeasurementSystem

TCP/IP

Data CommunicationsServer

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2.8.1 NSS - Network Switching Subsystem

The Network Elements we have seen so far are:

• The Mobile Services Switching Centre (MSC)/Visitor Location Register (VLR) one or all of which are also a Gateway MSC.

• The Home Location Register (HLR)

Also part of the Network Switching Subsystem are the Authentication Centre (AC) and Equipment Identity Register (EIR).

The Authentication Centre (AC) and Equipment Identity Register (EIR) are used to provide security. The subscriber and the mobile station have to be identified and authorised before accessing the network. These functions will be discussed later.

2.8.2 BSS - Base Station Subsystem

The Network Elements we have seen so far are:

• Base Station Controller (BSC)

• Base Transceiver Station (BTS)

Also part of the Base Station Subsystem is the Transcoder.

SYSTRA



Transcoder

In the case of the air interface, the media carrying the traffic is a radio frequency, but, as we saw in the example of a PSTN originated call, the traffic signal is also carried through fixed networks. To enable the efficient transmission of the digital speech information over the radio Air Interface the digital speech signal is compressed.

For transmission over the air interface, the speech signal is compressed by the mobile station to 13Kbits/s (Full Rate) or 5.6Kbits/s (Half Rate). This compression algorithm is known as "Regular Pulse Excitation with Long Term Prediction" (RPE-LTP). However, the standard bit rate for speech in the PSTN is 64Kbits/s Therefore, a converter has to be provided in the network to change the bit rate from one to another. This is called the Transcoder (TC). If the TC is located as close as possible to the MSC with standard PCM lines connecting the network elements, we can, in theory, multiplex four traffic channels in one PCM channel. This increases the efficiency of the PCM lines. But when connecting to the MSC, the multiplexed lines have to be de-multiplexed. In this case the unit is called Transcoder and Submultiplexer (TCSM).

Figure 2.40 Location of Transcoder and Submultiplexer

In the Nokia solution, the submultiplexing and the transcoding function is combined in one equipment which is called a TCSM2E (European version) or TCSM2A (American version).

A InterfaceA ter’ Interface

A ter Interface

MSC

SM2M

TC

TC

TC

TC

Transcoder andSubmultiplexer (TCSM)

BSC

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2.8.3 Network Management Subsystem

The Network Management Subsystem (NMS) is the third subsystem of the GSM network in addition to the Network Switching Subsystem (NSS) and Base Station Subsystem (BSS) which we have already discussed. The purpose of the NMS is to monitor various functions and elements of the network. These tasks are carried out by the NMS/2000 which consists of a number of Work Stations, Servers and a Router which connects to a Data Communications Network (DCN).

Figure 2.41 The NMS and the GSM Network

The operator workstations are connected to the database and communication servers via a Local Area Network (LAN). The database server stores the management information about the network. The communications server takes care of the data communications between the NMS and the equipment in the GSM network known as “Network Elements”. These communications are carried over a Data Communications Network (DCN) w hich connects to the NMS via a router. The DCN is normally implemented using an X.25 Packet Switching Network.

BSC

HLR/AC/EIR

TCSM

MSC /VLR

UnixWorkstations

Database andCommunications

Servers

NMS/200

GSM Network

Router

DataCommunicationNetwork

SYSTRA



The functions of the NMS can be divided into three categories:

• Fault Management

• Configuration Management

• Performance Management

These functions cover the whole of the GSM network elements from the level of individual BTSs, up to MSCs and HLRs.

Fault Management

The purpose of Fault Management is to ensure the smooth operation of the network and rapid correction of any kind of problems that are detected. Fault management provides the network operator with information about the current status of alarm events and maintains a history database of alarms.

The alarms are stored in the NMS database and this database can be searched according to criteria specified by the network operator.

Figure 2.42 Fault Management

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Configuration Management

The purpose of Configuration Management is to maintain up to date information about the operation and configuration status of network elements. Specific configuration functions include the management of the radio network, software and hardware management of the network elements, time synchronisation and security operations.

Figure 2.43 Configuration management

Performance Management

In performance management, the NMS collects measurement data from individual network elements and stores it in a database. On the basis of these data, the network operator is able to compare the actual performance of the network with the planned performance and detect both good and bad performance areas within the network.

Figure 2.44 Performance management

SYSTRA



2.8.4 Authentication Principle

Authentication is a procedure used in checking the validity and integrity of subscriber data. With the help of the authentication procedure the operator prevents the use of false SIM modules in the network. The authentication procedure is based on an identity key, Ki , that is issued to each subscriber when his data are established in the HLR. The authentication procedure verifies that the Ki is exactly the same on the subscriber side as on the network side.

Figure 2.45 Authentication

Authentication is performed by the VLR at the beginning of every call establishment, location update and call termination (at the called subscriber side). In order to perform the authentication, the VLR needs the basic authentication information. If the mobile station was asked to broadcast its K i, this would undermine the principle of authentication, because identification data would be sent across the air. The trick is to compare the K i stored in the mobile with the one stored in the network without actually having to transmit it over the radio air interface. The Ki is processed by a random number with a “one way” algorithm called A3 and the result of this processing is sent to the network. Due to the type of the algorithm A3, it is easy to get the result on the basis of Ki and a random number, but it is virtually impossible to get the Ki on the basis of the result and random number (hence the name “one way” algorithm).

Since the security issue concerns confidentiality as well, the network uses more than one algorithm. These are introduced in the following sections.

A

VLRMSC

Air

* IMSI

* Ki

* I MSI

* K i

ACSIMcard

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2.8.5 Security Algorithms

The GSM system uses three algorithms for the purposes of authentication and ciphering. These algorithms are A3, A5 and A8. A3 is used in authentication, A8 is used in generating a ciphering key and A5 is used in ciphering.

Figure 2.46 Security Algorithms

Algorithms A3 and A8 are located in the SIM module and in the Authentication Centre (AC). A5 is located in the mobile station and in the BTS.

Before an operator starts to use the security functions, the mobile subscriber is created in the Authentication Centre. The following information is required in creating the subscriber:

• IMSI of the Subscriber

• Ki of the subscriber

• Algorithm Version Used

The same information is also stored in the Mobile Subscriber's SIM. The basic principle of GSM security functions is to compare the data stored by the network to the data stored in the subscriber’s SIM. The IMSI number is the unique identification of the mobile subscriber. Ki is an authentication key with a length of 32 hexadecimal digits. The algorithms A3 and A8 use these digits as a basic value in authentication.

AAir

BTSTCBSC

ME + SIM

AC

A3 A8

A3 A8A5

A5

VLRMSC

SYSTRA



The Authentication Centre generates information that can be used for all the security purposes during one transaction. This information is called an Authentication Triplet.

The authentication triplet consists of three numbers:

• RAND

• SRES

• Kc.

RAND is a Random number, SRES (Signed Response) is a result that the algorithm A3 produces on the basis of certain source information and Kc is a ciphering key that A8 generates on the basis of certain source information.

Figure 2.47 Authentication Triplet

VLR

Random Number Generator Ki

RAND

Authentication Triplet


AC A8A3

SRES Kc

RAND SRES Kc

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All the values included in the authentication triplet depend on each other i.e. a certain RAND inserted to the algorithms with a certain Ki always produces a certain SRES and a certain Kc.

When the VLR has this kind of three-value combination and the Mobile Subscriber authentication procedure is initiated, the VLR sends the random number RAND through the BSS to the SIM in the mobile station. As the SIM has (or it should have) exactly the same algorithms as used in triplet generation on the network side, the RAND number that the SIM receives and inserts to the algorithm should produce exactly the same SRES value as the one generated on the network side.

RANDSRES Kc


BSC

BTS

A3

A8K i Kc

RAND

Kc

SRES

MS

SIM

VLRComparison

Figure 2.48 Authentication Procedure

If the SRES value in the authentication triplet is the same as the SRES calculated and sent by the mobile station, the authentication procedure is successful.

SYSTRA



2.8.6 Ciphering/Speech Encryption

Ciphering is used across the Air interface to provide speech and signalling encryption. When the authentication procedure has been completed successfully, the BTS and the mobile station are ready to start the ciphering procedure for further signalling and speech/data transmission.

The speech of the user and the ciphering key, Kc, are processed by the ciphering algorithm (A5) which produces the coded speech signal.

SPEECH/DATA

BTS

A5

A5

TDMA

Kc

A5

A5

TDMA

Kc

ENCRYPTEDSPEECH/DATA

SPEECH/DATA

Figure 2.49 Speech Encryption

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2.8.7 IMEI Checking

An option exists in GSM where the network may check the validity of the mobile station hardware. The mobile station is requested to provide the International Mobile Equipment Identity (IMEI) number. This number consists of type approval code, final assembly code and serial number of the mobile station. The network stores the IMEI numbers in the Equipment Identity Register (EIR).

2.8.8 User Confidentiality

User confidentiality in general refers to methods of ensuring that nobody calls at the expense of another person. From the end user's point of view, the mobile station secures itself against misuse by asking for the Personal Identification Number (PIN) when the station is turned on. If the PIN entered by the subscriber is correct, the phone is unlocked and it is ready for use. User identity is kept confidential by using Temporary Mobile Subscriber Identity (TMSI) numbers. After a successful first time location update, a mobile subscriber is allocated a Temporary Mobile Subscriber Identit y (TMSI). The next time a transaction between the GSM network and the MS is initiated, the subscriber is identified by the use of the TMSI. This is carried out in the case of both mobile originating transactions (such as channel request) in addition to network originating transactions (such as paging). TMSI is reallocated after every successful authentication verification. The format of a TMSI is operator dependent. It is a 32 bit binary number and these TMSI numbers are reallocated in a ciphered format. All relevant signalling information (IMEI, IMSI and directory numbers) is ciphered before transferring them over the radio Air Interface.

SYSTRA



2.9 Services

2.9.1 What Are Services?

In the broadest sense of the concept, any subscriber action that uses the facilities provided and supported by the GSM system, can be categorised as a service. Therefore, a person who has access to a GSM mobile phone and wishes to make a call, is trying to access the speech service provided by the system.

2.9.2 Classification Of Services

GSM is a multiservice system that allows various types of communication that can be distinguished by the nature of the transmitted information. Generally, based on the nature of the transmitted information, services can be grouped as speech services, where the transmitted data is speech and data services which covers the rest of the information types such as text, facsimile, etc.

However, if a person registers as a GSM subscriber and buys a mobile station, he takes it for granted that at least the speech service is guaranteed (after all, that is the reason why he bought the phone in the first place).

Services

Teleservices BearerServices

BasicServices

SupplementaryServices

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This raises another distinction in services:

• Basic Services which are individual functions and may be automatically available and included in the basic rights of the subscriber as soon as he registers.

• Supplementary Services which are extra services that are not included as basic features, but are associated with the basic services by enhancing and/or adding extra features to the basic services.

As an example, take a person who has a subscription for two basic services, Speech and Group 3 fax. In association with the fax service the subscriber may request a supplementary service of Call Forwarding On Mobile Subscriber Not Reachable, so that his data calls will be forwarded to another destination.

When a user subscribes for more than one basic service, he will have a different MSISDN for every basic service to which he subscribes. In the example above, the calling party has to dial a different number depending on whether he wants to talk or send a fax. When these services are provided as a basic or supplementary service, it is not only important to know what is transmitted. How the transmission is carried out plays a major role, too. In addition, there is the question of whether sufficient resources are available to support the service from end to end. It may be that the service is available at one end, but the target user may not have access to it and hence the use of this service is not possible. Basically, there are three essential points that need to be fulfilled before a service becomes available.

1. Whether the subscriber has access to the service or not?

2. Whether the GSM network from where the user is getting the service has the necessary resources or not?

3. Whether the equipment owned by the user is capable of supporting the service or not?

The second and third points raise an interesting scenario: using a service that is provided by a network which is independent of the end user equipment. An example of this is the use of facsimile in PSTN. It utilises the basic network resources, but the transmitted information is data which appears to be speech as far as the network is concerned.

SYSTRA



This brings us to the standard classification of services as described in the GSM 900 and GSM 1800 Technical Specifications . The first group is Teleservices , which provide the full communication capacity by means of terminals and network functions as well as those provided by dedicated centres. The other group is Bearer Services, which provide the capability of transmitting signals between a GSM network access point and an appropriate access point in the terminating network. The latter allows the use of the network resources only.

2.9.3 Teleservices

The various types of teleservices provided by GSM network can be summarised as shown in the following table.

Service Description GSM Spec. Code

Characteristics

Speech (Telephony) T11 The most important service for mobile systems, normal speech service, including emergency calls.

Speech, Emergency calls

T12 Emergency calls are possible automatically.

Short message Service (Mobile Terminated)

T21 For the reception of Short messages.

Short Message Service (Mobile Originated)

T22 For sending a short message to another GSM subscriber.

Short Message Service

(Cell broadcast)

T23 For the reception of broadcasted short messages.

Group 3 Facsimile transmission (with alternate speech)

T61 Presently not supported by NOKIA.

Automatic Group 3 Facsimile transmission

T62 For sending and receiving facsimile messages.

Table 2.1 Teleservices

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2.9.3.1 Speech (Telephony) and Emergency Calls These are the most common teleservices used in the GSM network. Speech is also the basic service that each subscriber is guaranteed to.

Telephony is a teleservice offering normal, traditional voice calls. The normal security procedures apply to all such calls except in the case of emergency calls which are processed regardless of possible security violations (up to the point that emergency calls can be made with a normal GSM mobile equipment that is not equipped with a SIM card). It is worth remembering that 112, the standard emergency number in most fixed networks is also used in GSM networks, even if 112 is not necessarily the standard emergency number in every country.

SYSTRA



2.9.3.2 Short Message Services: Mobile Originated, Mobile Terminated and Cell Broadcast

The Short Message Service (SMS) is a service enabling the mobile subscriber to receive and/or send short (max. 160 characters) messages in text format. These messages can be received at any time (also during a conversation).

This service requires a dedicated equipment called Short Message Service Centre (SMSC) which may be located in the Network Switching Subsystem or outside the GSM network, but it always has signalling connections to MSC. The SMSC acts as a temporary storing and forwarding centre if the Mobile Station is unreachable.

Figure 2.50 Short Message Service, MO and MT

Air A

SMS

Al fas ko p m 3 46 A lfa sk op m 34 6 A lfa sk op m 34 6

Short Message ServiceMobile Originated (MO)

Short Message ServiceMobile Terminated (MT)

VLR

MSC

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In the case of SMS-MO (Mobile Originated) the message sent by the mobile is stored in the SMSC, while in the case of SMS-MT (Mobile Terminated) the message stored in the SMSC is transmitted to the target mobile station. In a cell broadcast SMS, the information is sent to all the stations within a certain predefined geographical area. The services of SMSC are not required in cell broadcasting, as the BSC is equipped with the necessary SMSC functions. The maximum length of a cell broadcast SMS is 93 characters.

Figure 2.51 Short Message Service, Cell Broadcast

O & M

A lfa sk op m 3 46

Al fas k op m 34 6

A lfa sk o p m 3 46

Air A

BTS

BTS

BTS

BTS

BTS

BSC

Short Message ServiceCell Broadcast

SYSTRA



2.9.3.3 Facsimile Transmission (T61 and T62) Facsimile transmission is a teleservice that sets requirements for terminal equipment and their adaptation. There is one predefined case in which the Mobile Station needs to be interfaced with a computer equipped with a fax modem. However, because it is used for data transmission, there has to be a provision for the bearer service in order to define the characteristics of the bearer such as data transmission rate and Air Interface error correction protocol.

In the case of T61 Facsimile transmission, the receiver is either not aware that the incoming call is addressed to the fax and so he has to establish the nature of the call by talking with the calling party first, or the receiver knows that it is a facsimile call but still wants to talk with the calling party. In both cases, the nature of the transmitted information is data (group 3 facsimile) and speech alternately (during the same call). As shown in the table, this service is not currently supported by Nokia.

The T62 automatic facsimile is an automatic fax service where the receiver has a different MSISDN for the fax service and all calls to this number are purely data transmission calls.

Figure 2.52 Facsimile Transmission (T61/Transparent, T62/Non Transparent)

AAir

BTS

HLR EIRAC

BSC TC MSC VLR

IWF

Tr ansparent /Non Tr ansparent

Modems/Rate Adaptation

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2.9.4 Bearer Services

Bearer Services come into the picture when data transmission services are needed and there are a number of different types of data services available. The distinctions between these data services are based on the users (which can be connected to the PSTN, ISDN or a PSPDN network) and the mode of transmission (packet or circuit switched, whether end-to-end digital or not, synchronous or asynchronous).

Bearer Services (BS) supported by the GSM system only provide the capability of transmitting signals between the originating and terminating access points. There are no recommendations on the end user terminals. At the moment, the bearer services are divided into 10 categories, each of which describes the characteristics of the bearer. Four of the most essential categories are listed in the table below.

• Circuit Mode unstructured with Unrestricted Digital Capability Transparent.

• Circuit Mode unstructured with Unrestricted Digital Capability Non Transparent.

• PAD Service.

• Packet Service.

The first two types of bearer services are used for data communication in a similar fashion as in the PSTN and are essentially used for data communication between GSM networks and PSTN. Since the PSTN network is designed for voice communication with a bandwidth of 3.1Khz, digital data has to be modulated with an audio frequency carrier in order to enable transmission via the PSTN. This practice is still in extensive use throughout the world. Thus it is necessary for the GSM users to be able to use this function while communicating with the PSTN.

However, this is not easily realised in GSM networks because of the radio characteristics of the Air Interface. This interface is based on a special speech coding algorithm that ensures the best quality with the lowest possible bit rate e.g. 13 kbits /s which makes it incompatible for modem signals. Thus a GSM user will never need a modem for data communication. The connection between the mobile station and the GSM network is fully digital. Within the GSM network, the digital signal passes through a suitable modem to make the signal compatible for the addressed equipment in the PSTN. Alternatively, modem signals coming from the PSTN will be demodulated into digital signals in the

SYSTRA



GSM network and the unrestricted digital data will be forwarded to the mobile user. The modems are located in the MSC.

Figure 2.53 Data Services

Thus it becomes necessary to characterise the bearer (specifying the data rates and type of modem to be used). The terms transparent and non-transparent identify whether a second layer of error correction protocol is employed in the air interface or not. A non-transparent service employs re-transmission in case of errors whereas a transparent service does not employ it.

The last two types of data services refer to accessing a Packet Switched Public Data Network (PSPDN). These are “general purpose” data networks that use packet transmission techniques mostly between computers, as opposed to the conventional circuit switched techniques. Information is sent in packets along whichever route is available at the time. At the receiving end, these packets are reassembled in a sequential order and the original information is recreated.

AAir

BTS

HLR EIRAC

BSC TC MSC VLR

IWF

Transparent /Non Transparent

Modems /Rate Adaptation

Synchronous /Asynchronous

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A PSPDN can be accessed in a number of ways and some of the most common solutions are:

• A direct X.25 connection from the PSPDN to the user.

• Through PSTN or ISDN using a Packet Assembler/Dissembler (PAD) service in the network. The user is connected to the intermediate network (either PSTN or ISDN) and the PAD of this network will assemble/disassemble the user’s data to/from packets sent/received by/from the PSPDN.

• Through ISDN, which has the capability of sending and receiving data packets without the PAD service.

Thus a GSM user may use any of the methods available to him as shown in the figure below.

Figure 2.54 Packet Data Network Access methods

At the moment it is possible to achieve 14.4Kbits/s data connections thanks to the new radio access protocol. During 1999 it will be possible to use more than one traffic channel in the air interface to achieve up to 57.6Kbits/s data rates. We will discuss these new developments under the chapter “Next Step”.

Packet Assembler/Disassembler

Packet Interfaces

Non Packet Interfaces

1 Normal data communication access. PAD at PSPDN2 PAD service from GSM. PAD at GSM3 Packet Service from GSM. PAD at user terminal

PADPAD

GSM

GSM

GSM

PSTN/ISDN

PSPDN

1

2

3PADPAD PADPAD

PADPAD

SYSTRA



2.9.5 Supplementary Services

Supplementary services enhance or supplement the basic telecommunication services. The same supplementary services may or may not be employed by a number of different basic services such as basic telephony or T62 automatic facsimile service. The following list covers most of the common services, as well as the essential supplementary services.

• Advice of charge - AOC

• Alternate Line Service (ALS) – personal or business

• Barring of all incoming calls - BAIC

• Barring of all Incoming calls when roaming outside the HPLMN

• Barring of Incoming Calls when abroad

• Barring of outgoing calls - BOC

• Barring of outgoing International Calls - BOIC

• Barring of outgoing international calls excluding those directed to the HPLMN country

• Call forwarding on mobile subscriber busy - CFB

• Call forwarding on no answer - CFNA

• Call forwarding unconditional - CFU

• Call Hold

• Call Waiting - CW

• Calling line identification presentation - CLIP

• Calling line identification restriction – permanent or per-call - CLIR

• Centrex Services

• Closed user group - CUG

• Conference call - CONF

• Explicit Call Transfer

• Operator Determined Barring (ODB)

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2.10 Summary of the Learning Points

The GSM network is divided into three subsystems:

Network Switching Subsystem (NSS) - Contains the elements Mobile Services Switching Centre (MSC), Home Location Register (HLR), Visitor Location Register (VLR), Authentication Centre (AC), Equipment Identity Register (EIR). Its functions are:

• Call control. A mobile terminated call requires HLR enquiry to locate the called subscriber.

• Mobility Management.

– The HLR always knows in which MSC/VLR area a particular subscriber is located.

– An MSC/VLR knows in which Location Area a subscriber is located. This is enabled by a Location Update of which there are three types: Power On, Generic and Periodic.

– Mobility Management also helps in maintaining ongoing calls for a moving subscriber by a procedure known as Handover. There are four types of Handovers: Intra Cell, Inter Cell-Intra BSC, Inter Cell - Inter BSC and Inter MSC.

– Subscriber Data handling. A subscriber’s data is located in three places: the HLR, VLR and SIM card.

• Security Issues. Subscriber verification is performed in the VLR by an authentication process. Speech encryption is carried out between BTS and Mobile Station.

• Various types of numbers are used in the GSM network for different functions. The most important ones are: IMSI, MSISDN, MSRN, LAI, LAC, CGI, TMSI and HON.

• Charging. The MSC is responsible for c ollecting charging information. It is sent to the Billing Centre which creates bills for the subscriber.

• Signalling towards Base Station Subsystem and other networks.

SYSTRA



• The services offered by the GSM network are classified as

– Teleservices, and

– Bearer Services

– A teleservice or a bearer service is also a basic service which has an enhanced performance by the use of a number of supplementary services

Base Station Subsystem (BSS). The BSS consists of the following network elements: Base Station Controller (BSC), Base Transceiver Station (BTS) and Transcoder (TC). Its main functions are:

• Radio Network control and management. The BSS assigns, monitors and releases traffic and control connections on the radio interface. If necessary it performs handovers within one cell, between two cells under the same or different BSC and between cells connected to different MSCs.

• Speech transcoding The Transcoder is responsible for decreasing (towards the MS) respectively increasing (towards the MSC) the data rate for speech only (never for data or signalling) according to the transmission restrictions on the air interface.

• Air interface signalling and data processing

• Signalling towards the NSS and air interface

Network Management Subsystem (NMS). The main functions of the NMS are:

• Fault Management to detect and cancel faults as soon as possible to keep the network running correctly.

• Configuration Management to have centralised authority for managing hardware and software changes in addition to security operations.

• Performance Management on the one hand to avoid traffic overload and on the other hand to use the installed hardware in the most economic way.

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2.11 Traffic Management Review


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

1. Which of the following does not contain subscriber data?

a) Home Location Register (HLR)

b) Visitor Location Register (VLR)

c) Mobile Services Switching Centre (MSC)

d) Subscriber Identity Module (SIM)

2. Location Update procedure is initiated by

a) Mobile Station (MS)

b) Mobile Services Switching Centre (MSC)

c) Base Station Controller (BSC)

d) Home Location Register (HLR)

3. The format of International Mobile Subscriber Identity (IMSI) is

a) CC + NDC + SN

b) MCC + MNC + MSIN

c) MCC + MNC + LAC

d) Operator Specific 32 bit binary number

4. The three subsystems of GSM/DCS are

a) NMS, PSTN, MS

b) NMS, BSS, MS

c) NSS, BSS, MS

d) NSS, BSS, NMS

SYSTRA



5. Which of the following cases will result in an HLR Enquiry?

a) PSTN originated PSTN terminated call

b) Mobile originated PSTN terminated call

c) PSTN originated mobile terminated call

d) None of the above

6. Which of the following is not a task of the Network Switching Subsystem (NSS)?

a) Identifying the calling subscriber

b) Starting the location update procedure

c) Sending charging data to the billing centre

d) Paging a subscriber for mobile terminated calls

7. A Location Area

a) is the geographical area under one Base Station Controller (BSC)

b) is equal to one MSC area

c) is equal to one cell

d) is identified by a unique Location Area Identity

8. Which of the following combination best describes the Base Station Subsystem?

a) Base Station Controller, Transcoder, Base Transceiver Station

b) Mobile Station, Base Station Controller, Base Transceiver Station

c) Transcoder, Submultiplexer, Base Transceiver Station

d) Base Station Controller, Base Transceiver Station, Mobile Equipment

9. The Base Station Subsystem (BSS)

a) is responsible for radio network control

b) is located between Air and A interfaces

c) gets its synchronisation signal from the MSC

d) All of the above

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10. Initiation of the paging process for a mobile station is done by

a) MSC over a location area

b) all Base Transceiver Stations in a location area

c) a Base Station Controller over one BSC area

d) a Base Transceiver Station in one cell

11. In a mobile terminated call a traffic channel from the NSS in the terminating side to the called mobile station is reserved when

a) the user of the Mobile Station answers the ringing of the phone

b) B subscriber accepts the international roaming leg charging

c) MSRN analysis is done and the location area of the subscriber is known

d) the mobile station answers the paging signal

12. Why is it necessary to check the calling subscriber’s data before allowing the call to proceed in the originating NSS side?

a) To make sure that he has the correct IMSI

b) To make sure that he is provisioned the requested service

c) To make sure that he has paid the bill

d) To make sure that the NSS knows whom to charge for the call

13. If in a GSM/DCS network the periodic location update timer is set as 10 hours, then a periodic location update is done

a) 10 hours after the last periodic location update

b) 10 hours after the last power on location update

c) 10 hours after the last generic location update

d) 10 hours after the last transaction of any kind with the NSS

SYSTRA



14. In an inter VLR location update, the new VLR asks the old VLR for some information about the Mobile Subscriber. The old VLR responds to this query by providing which of the following information?

a) IMSI

b) IMSI, TMSI and Subscribed services

c) IMSI and last Location update time

d) IMSI and HLR address in case of a roaming subscriber

15. If a handover occurs during a call

a) the new frequency resource always belongs to a cell other than the current one

b) a handover number is always required

c) the initial speech path is not disconnected until a successful message comes from the mobile station on the new channel

d) the subscriber has to pay extra for the additional network operation required to maintain his call

16. A PSTN originated mobile terminated call has just had an inter MSC handover from GMSC A to MSC B. It has now become necessary to make another inter MSC handover to MSC C. After successful handover for the second time, the new call path will be:

a) PSTN - GMSC A - MSC C

b) PSTN - GMSC A - MSC B - MSC C

c) PSTN - MSC C

d) PSTN - GMSC A - MSC B - GMSC A -MSC C

17. Which of the following is not a possible way to charge the subscriber?

a) Subscription charge

b) Purchase of a Mobile Equipment

c) Purchase of a SIM card

d) Air time on a per call basis

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18. In which of the following situation the actual receiver of the call will also be responsible for paying part of the call charge?

a) Mobile A to mobile B of the same network but B roaming in another country

b) Mobile A to Mobile B of the same network, but B has unconditional call forwarding to mobile C of a different network

c) PSTN to mobile A who is wandering around in his own network

d) Mobile A to Mobile B of the same network, but B has unconditional call forwarding to PSTN C of a different network

19. Which network element creates bills for the subscriber?

a) HLR with information from MSC

b) MSC with information from Billing Centre

c) Billing Centre with information from MSC

d) Billing Centre with information from Transcoder

20. Authentication means

a) verifying the mobile phone user

b) verifying the mobile equipment

c) verifying the correct algorit hm

d) verifying the SIM card

21. The contents of the authentication triplet are

a) SRES, RAND, A3

b) SRES, RAND, Kc

c) A3, A5, A8

d) RAND, A3, A8

22. What is the result when you use Ki and RAND as inputs through A8?

a) coded speech

b) Signed response

c) Authentication triplet

d) Kc

SYSTRA



23. Encryption of user data is done between which network elements?

a) Base Transceiver Station and Mobile Station

b) Base Station Controller and Mobile Station

c) Mobile Services Switching Centre and Mobile Station

d) Mobile Station to Mobile Station - end to end

24. What is standard classification of services as mentioned in the specifications?

a) Basic services and Supplementary services

b) Speech service and data services

c) Teleservices and Bearer services

d) All of the above

25. Which of the following can not be a Basic Service of a mobile subscriber?

a) Emergency Calls

b) Unrestricted digital data at 9600 bps

c) Call forwarding unconditional

d) Short Message Service Mobile Originated Point to Point

26. Mobile Subscriber A has CLIP active, mobile subscriber B has CLIR active. A calls B, then

a) B can see A’s MSISDN

b) B cannot see A’s MSISDN

c) MSC prevents A’s MSISDN from going to B

d) None of the above.

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3 Signalling 3.1 Module Objectives


• Explain the needs of signalling

• Describe the C7 protocol stack and their functions in telephone exchanges and how they are different for GSM compared to PSTN.

• Identify the protocol stacks implemented in each GSM network element BSC, MSC and HLR

• Explain the specific needs for the GSM network and the additional protocol layers.

• Explain the SS7 requirements for individual GSM element (MSC/HLR/BSC).

SYSTRA



3.2 Introduction

Signalling in telecommunication networks has come a long way since the early days when a lady operator used to sit at the central exchange. Telecommunication networks were relatively simple and the general procedure of setting up a call would go something like this:

You would pick up the “handset” of your telephone, electrical current would flow to the exchange and a light would start blinking accompanied by a sound. This would let the lady know that you are requiring service. She would plug in one connector to your terminal and the other to her “headphone” and inquire about whom you wanted to talk to. After listening to your answer, she would then try to connect you to the person you want to ta lk with.

Then she would pull out the connector from your terminal and connect it to your intended party. He would then hear his phone ringing. After he answers, the lady will connect you to him. While you are talking, she will supervise the call, and once the conversation is over (which will be indicated by another light), she will “pull out the plugs.” That would be a typical scenario at a telephone exchange during the first half of this century.

Figure 3.1 Signalling in the old days.

%#!&?:^*(%&¤#”/=

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3.2.1 Standard Messages

Let us once again go over the actions and see what exactly is happening.

1. When you lift the receiver a light at the exchange starts to glow which is an Indication that you need service.

2 . The lady asks you who you want to talk with and you tell her the person’s name. An indication that service will be provided with a further request for address, which is provided.

3. She makes the connection to the called party and the phone starts ringing. Connection of the speech path is completed and the called party is alerted.

4 . The called party answers, and you are connected. Answer and conversation.

5 . The lady at the exchange monitors your call during the conversation. Supervision.

6 . When you hang up, a light glows to indicate this and she pulls out the plugs. An indication o f disconnection and clearing of the call.

Figure 3.2 Signalling Operations

This example highlights two important aspects. The conversation, which is the primary task, and what went on behind the scene to make that conversation successful. This has been the essence of Signalling in

Calling Party Exchange Called Party

Request for Service

Request Address

Provide Address

Process Informationand make connection

Alert Called party

Called Party Answer

Conversation

Disconnection

SYSTRA



telecommunications for a long time. The functions of signalling are as follows:

• To set up a call

• To supervise the call

• To clear a call.

There is a uniform set of standard messages in the signalling repertoire. Over the years, there has been a vast improvement in signalling technology and the lady operator has long since been replaced by automatic digital switching exchanges. They are still doing exactly the same job as the lady, but faster and more reliably over a global network.

3.2.2 Implementation and Evolution

As mentioned in the previous section, signalling in telecommunication systems is basically a set of messages used for setting up, supervising and clearing the call.

Many different factors have led to a variety of signalling systems being developed in telecommunications networks.

Different signalling standards were developed in different parts of the world. They were all doing the same task, but in a different way. This would obviously mean that when a call originates in one network with one type of signalling implementation and terminates in another network with another type of signalling system, some compromise, or adaptation would have to be used. Due to these kind of differences the then international governing body for te lecommunications, CCITT (now ITU), recommended the Channel Associated Signalling System (CAS) as the standard.

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Drawbacks of the CAS System

As a signalling system for setting up calls CAS was a very good system that performed quite well. A large number of telephone exchanges in the world are still using this system but its implementation is such, that it is only suitable for cases where traffic is low. Another problem with CAS is that it is not possible to send signalling messages in the absence of a call. This causes bottlenecks and wastes bandwidth.

Common Channel Signalling (CCS)

The CCITT (now the ITU) came up with a new recommendation for a signalling system which was the Common Channel Signalling System Number 7. One of the main advantages of this sys tem was that signalling did not have to go along the same path as the speech. It is abbreviated as CCS7, CCS#7, SS7 or simply C7 but they all refer to the same system.

SS7 was developed in the mid to late ‘80s and is a Common Channel Signalling system (CCS) with a signalling path bandwidth of 64Kbits/s. It is modular in design although the modules are not as clearly defined as is the case with the OSI 7-layer model, which it pre-dates.

Let us take a closer look at this system in the following sections.

SYSTRA



3.3 Common Channel Signalling System No.7

Originally, the Common Channel Signalling System No. 7 (hereafter referred to as SS7) consisted of two parts. The first part was responsible for transferring the message within a signalling network. The second part was the user of these messages.

As an analogy we can compare it to two managers with their own message runners. One manager writes a message, puts it in the envelope and gives it to the messenger. The messenger in turn looks at the address on the envelope, and gives it to the messenger of the other manager. The messenger of the receiving manager, looks at the address and gives it to his manager, who will then read and act as necessary.

Figure 3.3 Message bearers taking the message to their managers

Thus the initial phase of SS7 consisted of two parts

1. Message Transfer Part - MTP (responsible for transferring messages)

2. Telephone User Part - TUP (user of messages)

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3.3.1 Message Transfer Part (MTP)

We have so far established that signalling is used for setting up calls, and that there are standard sets of messages which are send back and forth to help facilitate this. The part which is responsible for taking these messages from one network element to other network element is known as the Message Transfer Part (MTP). The entire SS7 is built on the foundation of this MTP which consists of three sublayers as shown.

Figure 3.4 Message Transfer Part Layers

The lowest level, MTP layer 1 (Physical Connections), defines the physical and electrical characteristics. MTP layer 2 (Data link control) helps in error free transmission of the signalling messages between adjacent elements. MTP layer 3 (Network layer) is responsible for taking the message from any element in a signalling network to any other element within the same network.

Layer 1 Physical Connections

MessageTransfer

Part (MTP)

Signalling Message Handling

Layer 2 Data Link Control

Layer 3

SYSTRA



3.3.2 Telephone User Part (TUP)

The previous section explained the MTP. But who is the user who receives, sends and acts on these messages? The answer is the Telephone User Part (TUP). Those standard sets of messages that were mentioned previously are the standard TUP messages which help to set up the call, to supervise and clear it.

For many the SS7 in the fixed telephone network consisted of only two parts, the MTP and TUP. The CCITT (now the ITU) allowed for small variations in messages within one country alone. These variations were minor and very similar to the TUP, but they were called the National User Part (NUP).

Figure 3.5 Protocol stack of MTP and TUP/NUP/ISUP

With the introduction of the Integrated Services Digital Network (ISDN), which has a broader capability than the PSTN, some extra sets of messages were required. These became known as the ISDN User Part (ISUP). Whether it’s TUP, NUP or ISUP they are all doing the same job in helping to set up a call.

Physical Connections

Data Link Control

Transport of SignallingMessages within one network

Call Control Messages

MTPLayer 1

Layer 2

Layer 3

TUPNUPISUP

Layer 1

Layer 2

Layer 3

TUP

NUP

ISUP

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3.3.3 Signalling Connection and Control Part (SCCP)

The structure of SS7 with TUP/NUP/ISUP on top of MTP was quite satisfactory for call handling. However, with the passage of time and the development of newer and advanced technology, signalling requirements also started to become more stringe nt and demanding.

It was realised that the TUP/MTP combination alone was not sufficient when "virtual connections" became necessary. MTP guarantees the transfer of messages from any "signalling point" in the signalling network to any other "signalling point", safely and reliably. However, each message could reach the destination signalling point by using different paths. This may cause situations where the order of messages that are received, are different from the original sequence. When this order is important, there is need for establishing a "virtual connection".

Virtual Connections use a "Connection Oriented" protocol that will provide sequence numbers to enable the messages to be placed in the correct order at the distant end.

Another instance of when the TUP/MTP structure is inefficient, is when a signalling message has to be sent across multiple networks in the absence of a call. MTP is capable of routing a message within one network only. The case of setting up a call across multiple networks is not the same as signalling across the same network. The signalling goes leg by leg according to the call. But in the absence of a call, MTP cannot route a signalling message across multiple networks.

Figure 3.6 Virtual Connections

B

A

VirtualConnection using

“Connection Oriented”SCCP

SignallingPoint

SignallingPoint

SignallingPoint Destination

SignallingPoint

MTP

MTP

MTP

SYSTRA



The solution to these two problems was the creation of another protocol layer on top of MTP which was called the Signalling Connection and Control Part (SCCP). SCCP takes care of virtual connections and connectionless signalling. Note that the tasks of TUP and SCCP are different, and thus they are parallel to each other, but both use the services of MTP.

Figure 3.7 Location of SCCP

As far as the fixed telephone network (the Public Switched Telephone Network, PSTN) is concerned, this is all there is to SS7 and these protocol layers serve the purpose very well. At the moment there is no other protocol in SS7 for PSTN exchanges.

3.3.4 Summary

MTP is the message transfer part. It is responsible for transferring messages from one network element to another within the same network. It consists of three sublayers.

TUP is the user part of the messages transferred by MTP. These messages deal with setting up, supervising and clearing the call connections. It has two variations: NUP and ISUP.

SCCP is the signalling connection and control part. Its main function is to provide virtual connections and connectionless signalling.

MTP

TUPNUP ISUP

SCCP

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3.4 Other Applications of SS7 in GSM Networks

In GSM networks, signalling is not as simple as in the PSTN. There are extra signalling requirements in GSM due to the different architecture of the network which requires a large amount of non-call-related signalling. In the first instance the subscriber is mobile, unlike the PSTN telephone which is always in one place. Therefore, a continuous tracking of the mobile station is required which results in what is known as the Location Update procedure. This procedure is an example of non-call-related signalling, where the mobile phone and the network are communicating but no call is taking place. This requires additional sets of standard messages to fulfil the signalling requirements of GSM networks.

These additional protocol layers are

1. Base Station Subsystem Application Part (BSSAP)

2. Mobile Application Part (MAP)

3. Transaction Capabilities Application Part (TCAP)

SYSTRA



3.4.1 Base Station Subsystem Application Part (BSSAP)

The first of these additional protocol layers, which are specific to GSM networks, is the Base Station Subsystem Application Part (BSSAP). This layer is used when an MSC communicates with the BSC and the mobile station. Since the mobile station and MSC have to communicate via the BSC, there must be a virtual connection, therefore the service of SCCP is also needed.

The authentication verification procedure and assigning a new TMSI all take place with the standard sets of messages of BSSAP. Communication between MSC and BSC also uses the BSSAP protocol layer. Therefore, BSSAP serves two purposes:

• MSC-BSC signalling

• MSC-MS signalling.

Figure 3.8 Location of BSSAP in SS7

MTP

TUP

NUP

ISUPSCCP

BSSAP

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3.4.2 Mobile Application Part

The example of a location update procedure mentioned previously is not confined only to the MSC-BSC section, it spans multiple PLMNs. In case of a first time location update by an international roaming subscriber (where he is not in his home network), the VLR has to get the data from the subscriber’s HLR via the gateway MSC of the subscriber’s home network.

While a mobile terminated call is being handled, the MSRN has to be requested from the HLR without routing the call to HLR. Therefore, for these cases another protocol layer was adde d to the SS7 called Mobile Application Part (MAP). MAP is used for signalling communication between NSS elements.

NOTE: The MSC-MSC communication using MAP is used only in case of non-call-related signalling. For routing a call from one MSC to another MSC, TUP or ISUP is still used.

3.4.3 Transaction Capabilities Application Part (TCAP)

In MAP signalling, one MSC sends a message to an HLR, and that message requests (or invokes) a certain result. The HLR sends the result back, which may be the final result or some other messages might also follow (or it might not be the last result). These invocations and results that are sent back and forth between multiple elements using MAP need some sort of secretary to manage the transactions. This secretary is called the Transaction Capabilities Application Part (TCAP). This completes the SS7 protocol stack in the GSM network and their functions.

The SS7 picture is now complete.

SYSTRA



Figure 3.9 MAP and TCAP

3.4.4 Summary

Protocol Name Function

MTP Message Transfer Part Responsible for transferring an SS7 message from one network element to another within the same signalling network

TUP

NUP

ISUP

Telephone User Part

National User Part

ISDN User part

User parts of MTP. They send, receive, analyse and act on the messages delivered by MTP. All of these are Call Control Messages that help in setting up, supervising and clearing a call.

SCCP Signalling Connection and Control Part

Protocol layer responsible for making virtual connections and making connectionless signalling across multiple signalling networks.

BSSAP Base Station Subsystem Application Part

Protocol layer responsible for communicating GSM specific messages between MSC and BSC, and MSC and MS.

MAP Mobile Application Part A GSM specific protocol for non-call- related applica tions between NSS elements

TCAP Transaction Capabilities Application Part

Protocol layer responsible for providing service to MAP by handling the MAP transaction messages between multiple elements.

MTP

TUP NUP

ISUPSCCP

BSSAPMAP

TCAP

Signalling



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3.5 SS7 Layers in GSM Elements

In this section, the SS7 requirements for individual GSM elements will be shown. The previous sections explained why SS7 was needed in GSM and what are the protocol layers that are used. It is useful to note that not all the GSM elements have all the protocols in the SS7 stack. For example, a BSC would never need TUP because call control is not the task of the BSC.

3.5.1 Protocol Stack in MSC

Since the MTP is the foundation on which SS7 is built, this will be required in every element which is capable of processing SS7. The MSC is the element in GSM networks which is responsible for call control, therefore, TUP/ISUP sits on top of MTP for that purpose. The MSC/VLR is also responsible for location updates and communication with the BSC and the HLR. For this reason it also needs to have BSSAP and MAP which sit on top of SCCP. The MSC also has TCAP to provide services for MAP. It can be seen therefore, the MSC/VLR has all the SS7 protocol stacks implemented in it.

Figure 3.10 Protocol stack in MSC

MTP

TUP NUPISUP

SCCP

BSSAPMAP

TCAP

SYSTRA



3.5.2 Protocol Stack in HLR

The HLR is not respons ible for call control, therefore, TUP/ISUP is not necessary. In addition, the HLR does not communicate directly with the BSC; therefore, BSSAP is also not needed, which leaves MTP, SCCP, TCAP and MAP as the signalling protocols in the HLR.

Figure 3.11 Protocol stack in HLR

MTP

SCCP

TCAP

MAP

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3.5.3 Protocol Stack in BSC

The BSC only needs BSSAP, but since BSSAP needs the services of the SCCP which in turn needs the MTP, the BSC contains MTP, SCCP and BSSAP.

Figure 3.12 Protocol stack in BSC

Figure 3.13 SS7 Protocols in various network elements.

MAP

TCAP

SCCP

MTP

BSSAP MAPTCAP

SCCP

MTP

TUP NUPISUP

SCCP

MTP

BSSAP

SCCP

MTP

MSC

HLR

PSTN Exchange

BSC

TUP NUPISUP

MTP

SCCP

BSSAP

SYSTRA



3.5.4 Other Signalling Protocols in GSM

As we have already seen, the GSM core network elements use SS#7 (Signalling System No. 7) to pass signalling messages between them.

Figure 3.14 Signalling in GSM

Between the BSC and the BTS, a signalling protocol is used known as LAP-D (Link Access Procedure for the ISDN "D" channel). This is the same protocol that is used in ISDN networks between the customer and the network.

Between the mobile station and the BTS, the same signalling protocol is used with small modifications to cope with the characteristics of the radio transmission medium. This protocol is known as LAP-Dm where the "m" denotes modified.

The LAP-D message structure is similar to SS#7 but it does not support networking capabilities, therefore, it is used for point to point connections.

Protocols for Radio Resource (RR) management are passed using LAP-Dm and LAP-D. Other protocols for Mobility Management (MM) and Connection Management (CM) are passed between the Mobile Station and the MSC.

PSTN / HLRs /other MSCs

LAPDmSS#7BTSBTS

BSCBSC

LAPD

MSCMSC

Radio Resource Management

SS#7

Mobility and Connection Management

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3.5.5 Summary

The following table highlights the function of the SS7 protocol in every GSM network element capable of processing SS7.

MSC BSC HLR

MTP Transfer of SS7 messages between different network elements

Transfer of SS7 messages between different network elements

Transfer of SS7 messages between different network elements

TUP/ISUP

Setting up, supervising and clearing call connections

Unavailable

Unavailable

SCCP Connection-less signalling and virtual connections

Virtual Connection between MSC and MS

Connection-less signalling

BSSAP GSM signalling with BSC and MS

GSM Signalling with MSC

Unavailable

MAP GSM Specific signalling with HLR and other MSC

Unavailable GSM Specific signalling with MSCs and other HLRs

TCAP Service Provider to MAP

Unavailable Service Provider to MAP

A Virtual Connection uses packet type switching principles and the connection only exists when packets or messages are being transferred. In the simplest form of packet switching each packet is regarded as a complete transaction in itself. This is known as “Connectionless” mode as there is no sense of a connection being set up before communication begins and the network treats each packet independently. Some applications, however, involve the transfer of a sequence of packets, for which the “Connection-oriented” approach is more appropriate. In this case, a virtual connection is established by an initial exchange "set-up" packets between the communicating terminals. During the data transfer, each packet associated with a connection is passed over the same route through the network.

SYSTRA




• Signalling is the transfer of information between subscriber interface points and the network and between different network elements to help establish a call.

• Signalling Information is interchanged as standard sets of messages which was developed and standardised in to the present SS7 system.

• GSM networks need non-call related signalling which is possible with SS7.

• The SS7 used in PSTN networks is not sufficient to fulfil the signalling requirements of GSM networks, thus new protocols specific to GSM were developed.

• MTP is the basis of SS7, and it is responsible for transferring of signalling messages from one element to another within the same signalling network.

• TUP/ISUP are the user parts of MTP which handle call control.

• SCCP is needed for virtual connections and connectionless signalling.

• BSSAP is used for signalling between MSC-BSC and MSC-MS.

• MAP is needed for signalling between MSC-HLR, MSC-VLR, HLR-VLR (and MSC-MSC in the case of non-call related signalling).

• Link Access Protocol in D channel (LAP-D) provides a point-to-point signalling capability. It is used between the MS and the BTS and also between BTS and BSC.

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3.7 Signalling Review



1. Which of the following is not a signalling function?

a) To analyse the dialled digits

b) To digitise users speech before transmission

c) To make speech path connections

d) To inform the user of the progress of call

2. Which of the following is not a need for signalling?

a) The need to supervise a call

b) The need to make circuit reservations

c) The need to clear connections after call is over

d) The need to transfer charging information

3. Which of the following was a drawback of the CAS signalling?

a) It supported only call related signalling

b) It required to have one signalling channe l for every PCM line

c) It was not possible to have many different signalling messages

d) All of the above

SYSTRA



4. Which of the following is an advantage of SS7?

a) It can send call set-up messages

b) One signalling channel can support approximately ten thousand traffic channels

c) It can support non call related signalling

d) All of the above

5. Which of the following signalling requirements is specific to GSM networks only?

a) The ability to reserve circuits on the outgoing direction

b) The ability of one signalling channel to handle calls in other physically different cables

c) The ability to transport service dependent messages across switching exchanges

d) The ability to perform non call related signalling procedures

6. Which of the following combination of SS7 protocols is not present in PSTN exchanges?

a) MTP, SCCP

b) MTP, ISUP

c) MTP, TUP

d) MTP, SCCP, TCAP, MAP

7. Which of the following combination of SS7 protocols are specific to GSM networks only?

a) MAP, BSSAP

b) MAP, BSSAP, TUP

c) MAP, BSSAP, ISUP, SCCP

d) MAP, BSSAP, TCAP, SCCP, MTP

8. Which of following combination best contains the network elements in GSM network which do not have SS7?

a) MSC, HLR

b) BSC, BTS

c) MSC, OMC

d) BTS, TC

Signalling



121 (248)

9. Which of the following pictures is correct?

a)

MTP

SCCP

ISUPMAP

TCAP BSSAP

b)

MTP

TCAP

BSSAPMAP

SCCP TUPNUPISUP

c)

MTP

SCCP

BSSAPMAP

TCAP TUPNUPISUP

d)

MTP

SCCP

MAPBSSAPTCAP

TUPNUPISUP

SYSTRA



Transmission



123 (248)

4 Transmission 4.1 Module Objectives

At the end of the module the student will able to:

• Differentiate between physical and logical channels.

• List and describe the twelve different types of logical channels and their functions.

• Describe how the air interface properties affect the transmission of speech between the mobile station and the network and briefly explain the GSM solutions to these problems.

• Describe the main function of transcoder.

• List the three Base Station Controller (BSC)/Base Transceiver Station (BTS) connections.

SYSTRA



4.2 Introduction to Radio and Terrestrial Transmission

In a mobile communications network, part of the transmission connection uses a radio link and another part uses 2Mbit/s PCM links. Radio transmission is used between the Mobile Station and the Base Transceiver Station and the information must to be adapted to be carried over 2Mbit/s PCM transmission through the remainder of the network.

The radio link is the most vulnerable part of the connection and a great deal of work is needed to ensure its high quality and reliable operation. This will be analysed later in this chapter.

The frequency ranges of GSM 900 and GSM 1800 are indicated below:

Figure 4.1 Frequency Allocations for GSM

Note that the uplink refers to a signal flow from Mobile Station (MS) to Base Transceiver Station (BTS) and the downlink refers to a signal flow from Base Transceiver Station (BTS) to Mobile Station (MS). The simultaneous use of separate uplink and downlink frequencies enables communication in both the transmit (TX) and the receive (RX) directions. The radio carrier frequencies are arranged in pairs and the difference between these two frequencies (uplink-downlink) is called the Duplex Frequency.

GSM-900 Uplink Downlink

915MHz890MHz 935MHz 960MHz

GSM-1800 Uplink Downlink

1785MHz1710MHz 1805MHz 1880MHz

Transmission



125 (248)

The frequency ranges are divided into carrier frequencies spaced at 200kHz. As an example, the following table shows the distribution of frequencies in GSM 900:

Channel Uplink signal (MHz) Downlink signal (MHz)

1 890.1 – 890.3 (890.2 -centre freq.)

935.1 – 935.3 (935.2 -centre freq.)

2 890.4 (centre freq.) 935.4 (centre freq.)


… ... ...


In GSM 900 the duplex frequency (the difference between uplink and downlink frequencies) is 45 MHz. In GSM 1800 it is 95 MHz. The lowest and highest channels are not used to avoid interference with services using neighbouring frequencies, both in GSM 900 and GSM 1800.

The total number of carriers in GSM 900 is 124, whereas in GSM 1800 the number of carriers is 374.

The devices in the Base Transceiver Station (BTS) that transmit and receive the radio signals in each of the GSM channels (uplink and downlink together) are known as Transceivers (TRX).

The radio transmission in GSM networks is based on digital technology. Digital transmission in GSM is implemented using two methods known as Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA).

Frequency Division Multiple Access (FDMA) refers to the fact that each Base Transceiver Station is allocated different radio frequency channels. Mobile phones in adjacent cells (or in the same cell) can operate at the same time but are separated according to frequency. The FDMA method is employed by using multiple carrier frequencies, 124 in GSM 900 and 374 in GSM 1800.

SYSTRA



Time Division Multiple Access (TDMA), as the name suggests, is a method of sharing a resource (in this case a radio frequency) between multiple users, by allocating a specific time (known as a 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 however, some timeslots are used to provide signalling and location updates etc. between calls.

The figure below illustrates the TDMA principle.

Figure 4.2 Time Division Multiple Access principle

BTSBTS

TimeSLot 0

TSL 1TSL 2TSL 3TSL 4

TSL 5

TSL 6

TSL 7

Transmission



127 (248)

GSM uses digital techniques where the speech and control information are represented by 0s and 1s. How is it possible to transmit digital information over an analogue radio interface?

The digital values 0 and 1 are used to change one of t he characteristics of an analogue radio signal in a predetermined way. By altering the characteristic of a radio signal for every bit in the digital signal, we can "translate" an analogue signal into a bit stream in the frequency domain. This technique is called modulation. Analogue signals have three basic properties: Amplitude, Frequency, Phase. Therefore, there are basically three types of modulation process in common use:-

• Amplitude Modulation

• Frequency Modulation

• Phase Modulation

Figure 4.3 Examples of Frequency and Amplitude Modulation

GSM uses a phase modulation technique over the air interface known as Gaussian Minimum Shift Keying (GMSK). In order to understand how it works, let’s take a simple example.

Digital Signal

Amplitude Modulation

Frequency Modulation

0 1 0

SYSTRA



At the GSM air interface, the bit rate is approximately 270Kbits/s. (This will be explained later) At this bit rate, the duration of one bit is 3.69 µs , i.e. the value of the bit requires 3.69 µs of transmission time. GMSK changes the phase of the analogue radio signal depending on whether the bit to be transmitted is a 0 or a 1.

Figure 4.4 Example of Phase Modulation

The radio air interface has to cope with many problems such as variable signal strength due to the presence of obstacles along the way, radio frequencies reflecting from buildings, mountains etc. with different relative time delays and interference from other radio sources.

With such levels of interference, complex equalisation techniques are required with GMSK.

Digital Signal 0 1 01

Phase Modulation

00

00

900

900

3.69µs

Transmission



129 (248)

4.3 Transmission Through the Air Interface

To enable us to understand the principles of the air interface, let us imagine that an army has to be moved from one place to another, and a convoy of vehicles is set aside to do the job. The army consists of soldiers and officers.

Each vehicle has 8 seats and therefore only 8 people can be carried in each vehicle.

Figure 4.5 A small logistical problem

Obviously, the only solution is to divide the army into groups of eight people. One officer and seven soldiers are allocated to each vehicle. The officer sits in the front seat and seven soldiers sit in the others.

There are different types of people in the army, soldiers and officers. These could be referred to as "Logical" differences as they are all human beings but their functions are different. In addition, there can be many different ranks of officers, each one with different responsibilities.

To move them from one place to another, a "Physical" connection is employed i.e. the vehicles and the seats.

8 seats in each vehicle

SYSTRA



4.3.1 Physical and Logical Channels

Time Division Multiple Access (TDMA) divides one radio frequency channel into consecutive periods of time, each one called a "TDMA Frame". The TDMA frame can be compared to the vehicle in our example.

Each TDMA Frame contains eight shorter periods of time known as "Timeslots". These timeslots can be compared to the seats in the vehicle. The TDMA timeslots are called "Physical Channels" as they are used to physically move information from one place to another.

The radio carrier signal between the Mobile Station and the BTS is divided into a continuous stream of timeslots which in turn are transmitted in a continuous stream of TDMA frames - like a long line of vehicles with eight seats in each.

If the time slots of the TDMA frame represent the physical channels, what about the contents? The contents of the physical channels - i.e. the soldiers and officers travelling in the eight seats of the vehicle, according to their roles, are called "logical channels". In the example of the army, the soldiers are one type of logical channel and the officers are other types of logical channels and they exercise some kind of control depending on their responsibilities.

In GSM the logical channels can be divided into two types:

• Dedicated Channels

• Common Channels

Transmission



131 (248)

Let’s look at things in a more practical way: a subscriber switches on his mobile phone and receives a call. This simple act of switching on the phone involves the following steps:

1. The mobile scans all the radio frequencies and measures them.

2. It selects the frequency with the best quality and tunes to it.

3. With the help of a synchronisation signal in a TDMA frame, the mobile synchronises itself to the network.

Figure 4.6 Tuning into the Network

The synchronisation information required by this process is broadcast by the network and analysed by the mobile.

BTS

BTS

TDMA FrameTDMA Frame

Sync.Information Sync.Information

SYSTRA



Registration and authentication are the next steps and these consist of the following operations:

1. A point to point connection must be set up. The mobile station makes a request for a channel to establish the connection.

2. The network acknowledges the request and allocates a channel. The mobile receives and reads this information.

3. The mobile then moves to the allocated (dedicated) channel for further transactions with the network. The next steps are registration and authentication.

Figure 4.7 Initiation of a Call

BTS


RequestRequest

Channel allocationChannel allocation

Traffic Traffic

Transmission



133 (248)

Once the subscriber is registered in the network and the authentication is successful, calls can be set-up. In the case of a mobile terminated call, the subscriber has to be paged. This process is described below:

1. The network sends a Paging message to all the Base Transceiver Stations (BTS) within the Location Area (LA) where the subscriber is registe red.

2. The mobile station answers the paging message by sending a service/channel request.

3. The network acknowledges this request and again the authentication is needed. A dedicated signalling channel is assigned in order to transmit the data related to the call.

4. A Traffic Channel is assigned for the conversation.

During the conversation, the mobile measures the signal strength of adjacent carriers and sends measurement reports to the Base Station Controller (BSC). A channel must be dedicated also for this function.

Figure 4.8 Call completion from called side

This is a simplified description of the process, but it conveys the idea that there are many functions involved in the air interface to enable a mobile user to have conversation. Each one of these functions requires a separate "logical channel" as the data contents are different. Some of them are Uplink , others are Downlink and some are Bi-directional.


PagingPaging

AnswerAnswer

TrafficTrafficBTSBTS BTSBTS

SYSTRA



4.3.2 Logical channels

There are twelve different types of Logical Channels which are mapped into Physical Channels in the radio path. Logical channels comprise of Common Channels and Dedicated Channels . Common Channels are those which are used for broadcasting different information to mobile stations and setting up of signalling channels between the MSC/VLR and the mobile station.

Figure 4.9 Logical Channels

Over the radio path, different type of signalling channels are used to facilitate the discussions between the mobile station and the BTS, BSC and MSC/VLR. All these signalling channels are called Dedicated Control Channels.

Traffic channels are also Dedicated Channels as each channel is dedicated to only one user to carry speech or data.

COMMONCHANNELSCOMMON

CHANNELS

BROADCASTCHANNELS

BROADCASTCHANNELS

COMMONCONTROL

CHANNELS

COMMONCONTROL

CHANNELS

DEDICATEDCONTROL

CHANNELS

DEDICATEDCONTROL

CHANNELS

TRAFFICCHANNELSTRAFFIC

CHANNELS

FCCHFCCH SCHSCH BCCHBCCH SDCCHSDCCH SACCHSACCH FACCHFACCH

PCHPCH RACHRACH AGCHAGCH TCH/FTCH/F TCH/HTCH/H TCH/EFRTCH/EFR

DEDICATEDCHANNELS

DEDICATEDCHANNELS

LOGICALCHANNELSLOGICAL

CHANNELS

Transmission



135 (248)

Figure 4.10 TDMA frames with Common and Dedicated Channels

Dedicated Channels

26 Frame - Multiframe

SACCH Unused

TDMA Frame

0 1 2 3 4 5 6 7

Common Channels

51 Frame - Multiframe

0 1 2 3 49 50

TDMA Frame

0 1 2 3 4 5 6 7

0 1 2 3 12 2524

SYSTRA



4.3.2.1 Broadcast Channels Base Stations can use several TRXs but there is always only one TRX which can carry Common Channels. Broadcast channels are downlink point to multipoint channels. They contain general information about the network and the broadcasting cell. There are three types of broadcast channels:

1. Frequency Correction Channel (FCCH)

FCCH bursts consist of all "0"s which are transmitted as a pure sine wave. This acts like a flag for the mobile stations which enables them to find the TRX among several TRXs, which contains the Broadcast transmission. The MS scans for this signal after it has been switched on since it has no information as to which frequency to use.

2. Synchronisation Channel (SCH)

The SCH contains the Base Station Identity Code (BSIC) and a reduced TDMA frame number. The BSIC is needed to identify that the frequency strength being measured by the mobile station is coming from a particular base station. In some cases, a distant base station broadcasting the same frequency can also be detected by the mobile station. The TDMA frame number is required for speech encryption.

3. Broadcast Control Channel (BCCH)

The BCCH contains detailed network and cell specific information such as:

• Frequencies used in the particular cell and neighbouring cells.

• Channel combination. As we mentioned previously, there are a total of twelve logical channels. All the logical channels except Traffic Channels are mapped into Timeslot 0 or Timeslot 1 of the broadcasting TRX. Channel combination informs the mobile station about the mapping method used in the particular cell.

• Pag ing groups . Normally in one cell there is more than one paging channel (describer later). To prevent a mobile from listening to all the paging channels for a paging message, the paging channels are divided in such a way that only a group of mobile stations listen to a particular paging channel. These are referred to as paging groups.

• Information on surrounding cells . A mobile station has to know what are the cells surrounding the present cell and what frequencies are being broadcast on them. This is necessary if, for example, the user initiates a conversation in the current cell, and

Transmission



137 (248)

then decides to move on. The mobile station has to measure the signal strength and quality of the surrounding cells and report this information to the base station controller.

4.3.2.2 Common Control Channels

Common Control Channels comprise the second set of logical channels. They are used to set up a point to point connection. There are three types of common control channels:

1. Paging Channel (PCH)

The PCH is a downlink channel which is broadcast by all the BTSs of a Location Area in the case of a mobile terminated call.

2. Random Access Channel (RACH)

The RACH is the only uplink and the first point to point channel in the common control channels. It is used by the mobile station in order to initiate a transaction, or as a response to a PCH.

3. Access Grant Channel (AGCH)

The AGCH is the answer to the RACH. It is used to assign a mobile a Stand-alone Dedicated Control Channel (SDCCH). An additional information in the AGCH is the frequency hopping sequence. It is a downlink, point to point channel.

Note

Frequency hopping is designed to reduce the negative effects of the air interface, which sometimes results in the loss of information transmitted, the mobile station may transmit information on different frequencies within one cell. The order in which the mobile station should change the frequencies is called the "frequency hopping sequence". However, implementing Frequency Hopping in a cell is optional.

SYSTRA



4.3.2.3 Dedicated Control Channels Dedicated Control Channels compose the third group of channels. Once again, there are three dedicated channels. They are used for call set-up, sending measurement reports and handover. They are all bi-directional and point to point channels. There are three dedicated control channels:

1. Stand-alone Dedicated Control Channel (SDCCH)

The SDCCH is used for system signalling: call set-up, authentication, location update, assignment of traffic channels and transmission of short messages.

2. Slow Associated Control Channel (SACCH)

An SACCH is associated with each SDCCH and Traffic Channel (TCH). It transmits measurement reports and is also used for power control, time alignment and in some cases to transmit short messages.

3. Fast Associated Control Channel (FACCH)

The FACCH is used when a handover is required. It is mapped onto a TCH, and it replaces 20 ms of speech and therefore it is said to work in "stealing" mode.

4.3.2.4 Traffic Channels (TCH) Traffic Channels are logical channels that transfer user speech or data, which can be either in the form of Half rate traffic (5.6 kbits/s) or Full rate traffic (13 kbits/s). Another form of traffic channel is the Enhanced Full Rate (EFR) Traffic Channel. The speech coding in EFR is still done at 13Kbits/s, but the coding mechanism is different than that used for normal full rate traffic. EFR coding gives better speech quality at the same bit rate than normal full rate. Traffic channels can transmit both speech and data and are bi-directional channels.

Transmission



139 (248)

4.3.3 Time Slots And Bursts

We have already seen that the technique used in air interface is Time Division Multiple Access (TDMA) where one frequency is shared by, at the most, eight users. Consider the example of a 2Mbit/s PCM signal which can carry 30 speech channels with each channel occupying 64Kbits/s. The speech signals from the mobile stations must be placed into a 2Mbit/s signal that connects the BTS and the BSC. It is very important that all the mobile stations in the same cell send the digital information at the correct time to enable the BTS to place this information into the correct position in the 2Mbit/s signal. How do we manage the timing between multiple mobile stations in one cell? The aim is that each mobile sends its information at a precise time, so that when the information arrives at the Base Transceiver Station, it fits into the allocated time slot in the 2Mbit/s signal. Each Mobile Station must send a burst (a burst occupies one TDMA timeslot) of data at a different time to all the other Mobile Stations in the same cell. The mobile then falls silent for the next seven timeslots and then again sends the next burst and so on. It can be seen that the mobile station is sending information periodically. All the mobile stations send their information like this. If we go back to the analogy of the army, the road is the radio carrier frequency, the vehicle is the TDMA frame and the seats in each vehicle are the TDMA timeslots.

Figure 4.11 TDMA Bursts and Timeslots

... ...

Bursts from Mobile StationsBursts from Mobile Stations

BTSBTS

2Mbit/s to BSC2Mbit/s to BSC

TDMA Time SlotTDMA Time Slot


SYSTRA



In the air interface a TDMA timeslot is a time interval of approximately 576. 9 µs which corresponds to the duration of 156.25 bit times. All bursts occupy this period of time, but the actual arrangement of bits in the burst will depend on the burst type. Two examples of burst types are :

• Normal Burst is used to send the traffic channels, stand alone dedicated channels, broadcast control channel, paging channel, access grant channel, slow and fast associated control channels.

• Access Burst which is used to send information on the Random Access Channel (RACH). This burst contains the lowest number of bits. The purpose of this “extra free space” is to measure the distance between the Mobile Station to Base Transceiver Station at the beginning of a connection. This process determines a parameter called "timing advance" which ensures that the bursts from different mobile stations arrive at the correct time, even if the distances between the various MSs and the BTS are different. This process is carried out in connection with the first access request and after a handover. In GSM a maximum the oretical distance of about 35 km is allowed between the base transceiver station and mobile station.

Figure 4.12 Normal Bursts and Access Bursts

148 Bits148 Bits

Guard Time(8.25 Bits)

148 Bits

88 Bits 88 Bits 88 Bits

Guard Time(68.25 Bits)

576.9 micro secs(156.25 bit times)

NormalBursts

AccessBursts

Transmission



141 (248)

4.4 Problems and Solutions of the Air Interface

It has already been pointed out that the radio air interface link is the most vulnerable part of the GSM connection. In this section we will briefly discuss some of the problems that occur in air interface and some solutions. There are three major sources of problems in the air interface, which can lead to loss of data. These are:

• Multi path propagation

• Shadowing

• Propagation delay

4.4.1 Multipath propagation

Whenever a mobile station is in contact with the GSM network, it is quite rare that there is a direct "line of sight" transmission between the mobile station and the base transceiver station. In the majority of cases, the signals arriving at the mobile station have been reflected from various surfaces. Thus a mobile station (and the base transceiver station) receives the same signal more than once. Depending on the distance that the reflected signals have travelled, they may affect the same information bit or corrupt successive bits. In the worst case an entire burst might get lost.

Depending on whether the reflected signal comes from near or far, the effect is slightly different. A reflected signal that has travelled some distance causes "inter symbol interference" whereas near reflections cause "frequency dips". There are a number of solutions that have been designed to overcome these problems:

• Viterbi Equalisation

• Channe l Coding

• Interleaving

• Frequency Hopping

• Antenna Diversity

SYSTRA



Figure 4.13 The effects of Multipath Propagation

Viterbi Equalisation

This is generally applicable for signals that have been reflected from far away objects. When either the base transceiver station or mobile station transmits user information, the information contained in the burst is not all user data. There are 26 bits which are designated for a "training sequence" included in each TDMA burst transmitted. Both the mobile station and base transceiver station know these bits and by analysing the effect the radio propagation on these training bits, the air interface is mathematically modelled as a filter. Using this mathematical model, the transmitted bits are estimated based on the received bits. The mathematical algorithm used for this purpose is called "Viterbi equalisation".

Channel Coding

Channel coding (and the following solutions) is normally used for overcoming the problem caused by fading dips. In channel coding, the user data is coded using standard algorithms. This coding is not for encryption but for error detection and correction purposes and requires extra information to be added to the user data. In the case of speech, the amount of bits is increased from 260 per 20 ms to 456 bits per 20 ms. This gives the possibility to regenerate up to 12.5% of data loss.

Inter Symbol Interference

RX Sensitivity

BTSBTS

Fading dips caused by multipath propagation

Approx. 17cm

Fading Dips

Transmission



143 (248)

Interleaving

Interleaving is the spreading of the coded speech into many bursts. By spreading the information onto many bursts, we will be able to recover the data even if one burst is lost. (Ciphering is also carried out for security reasons).

Figure 4.14 Speech processing in the mobile station

SpeechDigitising and

Source CodingChannel Coding

Interleavingand Ciphering

TDMA Burst Formatting

GMSKModulation

22.8kbit/s

13kbit/s

33.8kbit/s

22.8kbit/sAirInterface

SYSTRA



Frequency Hopping

With Frequency Hopping, the frequency on which the information is transmitted is changed for every burst. Frequency hopping generally does not significantly improve the performance if there are less than four frequencies in the cell.

Figure 4.15 Example of Frequency Hopping

F2

F1

F3

F4

Time

Transmission



145 (248)

Antenna Receiver Diversity

In this case two physically separated antennas receive and pr ocess the same signal. This helps to eliminate fading dips. If a fading dip occurs at the position of one antenna, the other antenna will still be able to receive the signal. Since the distance between two antennas is a few metres, it can only be implemented at the Base Transceiver Station.

Figure 4.16 Antenna Receiver Diversity

Received Signal

RXRX

Signal Processing

Antennas

Approx. 6m (GSM-900)Approx. 3m (GSM-1800)

SYSTRA



4.4.2 Shadowing

Hills, buildings and other obstacles between antennas cause shadowing (also called Log Normal Fading). Instead of reflecting the signal these obstacles attenuate the signal.

Figure 4.17 Shadowing effect.

Shadowing is generally a problem in the uplink direction, because a Base Transceiver Station transmits information at a much higher power compared to that from the mobile station. The solution adopted to overcome this problem is known as adaptive power control. Based on quality and strength of the received signal, the base station informs the mobile station to increase or decrease the power as required. This information is sent in the Slow Associated Control Channel (SACCH).

BTSBTS

Transmission



147 (248)

4.4.3 Propagation Delay

As you remember, information is sent in bursts from the mobile station to the Base Transceiver Station (BTS). These bursts have to arrive at the base transceiver station such that they have to map exactly into their allocated time slots. However, the further away the mobile station is from the BTS then the longer it will take for the radio signal to travel over the air interface. This means that if the mobile station or base station transmits a burst only when the time slot appears, then when the burst arrives at the other end, it will cross onto the time domain of the next timeslot, thereby corrupting data from both sources.

The solution used to overcome this problem is called "adaptive frame alignment". The Base Transceiver Station measures the time delay from the received signal compared to the delay that would come from a mobile station that was transmitting at zero distance from the Base Transceiver Station. Based on this delay value, the Base Transceiver Station informs the mobile station to either advance or retard the time alignment by sending the burst slightly before the actual time slot. The base station also adopts this time alignment in the down link direction.

Figure 4.18 Propagation delay problem and solution

allocated time slot

BTS

allocated time slot

BTS

Effect Due to Propagation Delay Solution Using Adaptive Frame Alignment

SYSTRA



4.5 Terrestrial Transmission

So far, we have concentrated solely on the radio link between Base Transceiver Station (BTS) and the mobile station. Now we are going to follow the signal to the next phase and take a look at the transmission between the other network elements, in particular from Base Transceiver Station to Base Station Controller (BSC) and up to Mobile Services Switching Centre (MSC).

4.5.1 Base Transceiver Station

A base transceiver station is a physical site from where the radio transmission in both the downlink and uplink direction takes place. The radio resources are the frequencies allocated to the Base Station. The particular hardware element inside the Base Transceiver Station (BTS) responsible for transmitting and receiving these radio frequencies is appropriately named "Transceiver (TRX)". A Base Station site might have any number of TRXs from one to twelve. These TRXs are then configured into one, two or three cells. If a BTS is configured as one cell it is called an "Omnidirectional BTS" and if it is configured as either two or three cells it is called a "Sectorised BTS". In an omnidirectional BTS the maximum number of TRXs is ten, and in a sectorised BTS the maximum number of TRXs is four per sector.

Figure 4.19 Examples of BTS configurations

Omnidirectional BTSf1,f2, f3

3 sectorised BTS

2 sectorised BTS

f2f1, f2

f5, f6

f1

f3, f4

Transmission



149 (248)

4.5.2 Transmission between BSC and BTS

There are three alternative methods to provide the connections between a BSC and several BTSs. The method used will depend on a number of factors such as the distance between the Base Station Controller (BSC) and Base Transceiver Station (BTS), the number of TRXs used at a particular BTS site, the signalling channel rate between Base Station Controller (BSC) and Base Transceiver Station (BTS). There are three options available: point-to-point connection, multidrop chain and multidrop loop.

Figure 4.20 BTS - BSC connections

Point-to-point connection indicates that the Base Station Controller (BSC) is connected directly to every BTS with a 2Mbit/s PCM line. This is a simple and effective method particularly in cases when the distance between BSC and BTS is short. However, if the BSC -BTS distance is a few kilometres whereas the distance between a group of BTS’s is much shorter, it does not make sense to draw a point-to-point connection to every BTS. One PCM line has ample capacity to transfer data to several BTSs simultaneously. Therefore, it is possible to draw just one BSC - BTS connection and link the BTSs as a chain. This technique is called "multidrop chain". The BSC sends all the data in one 2Mbit/s PCM line and each BTS in turn analyses the signal, collects the data from the correct timeslots assigned for itself and passes the signal to the next BTS.

BSC

BTS BTS BTS

BTSBTSBTS

BTS BTS BTS

BTSPoint to Point Connection

Multi drop Chain

Multi drop loop

SYSTRA



But there is one problem with a multidrop chain. Consider what would happen if there is a malfunction somewhere along the line and the chain breaks. More BTSs are isolated and, if the BSC is not informed, it will continue to send data. The solution to this problem is called "multidrop loop" and instead of a chain we connect the BTSs in the form of a loop. Previously a dynamic node was needed to split the signal into the two directions around the loop, but later versions of BTS are capable of carrying out this function. The flow of the signal is similar to the signal flow in multidrop chain, except that a BTS will change the “listening” direction if the signal from one side fails. This ensures that the BTSs always receive information from the BSC even if the connection is cut off at some point in the loop.

4.5.3 The Concept of Multiplexing

According to GSM 900 and GSM 1800 specification, the bit rate in the air interface is 13 Kbits/s and the bit rate at the Mobile Services Switching Centre (MSC) and PSTN interface is 64 Kbits/s. This means that the bit rate has to be converted at some point after the signal has been received by the BTS and before it is sent to other networks. But the specifications do not put a constraint as to where exactly the conversion should take place. This brings up some interesting scenarios.

The actual hardware which does the conversion from 13 Kbits/s to 64 Kbits/s and vice versa is called a transcoder. In theory this piece of equipment belongs to the Base Transceiver Station. However, by putting the transcoder at a different place we can take some advantages in reducing the transmission costs.

If the transcoder is placed at the BTS site (in the BSC interface), then the user data rate from BTS to Base Station Controller (BSC) would be 64 Kbits/s. The transmission for this would be similar to standard PCM line transmission with 30 channels per PCM cable. The same would also apply between BSC and MSC.

If we put the transcoder somewhere else, say just after MSC, then also we can not get significant advantage. This is because although after transcoding the bit rate reduces to 13 kbit/s we still have to use the PCM structure to send the traffic channels, with 8 bits per time slot. However since after transcoding we have a bit rate of 13 Kbits/s and an additional 3 Kbits/s (making 16 Kbits/s) only two bits per time slot will be used. The other 6 bits are effectively wasted. The next figure shows these two types of connections.

Transmission



151 (248)

Independent from its actual position, the transcoder belongs to the BSS even if it is placed next to the MSC. (When the TC is placed away from the BTS it is called a Remote TC according to the GSM recommendations).

Figure 4.21 Implementation of Transcoder at different sites.

But the real advantage comes if we use the second configuration shown in the figure with another piece of hardware called submultiplexer. We saw that from the MSC data comes out at 64Kbits/s rate and from the Transcoder it comes out at 16Kbits/s. Each PCM channel (time slot) has 2 bits of information. It appears that we are able to put in data from other 3 PCM lines also here by multiplexing. However there are other issues as well such as Common Channel Signalling information, OMC data and some other network information which can not be transcoded. Thus we are able to multiplex 3 PCM lines and send 90 channels in one PCM line from MSC (transcoder) towards the BSC. The BSC is able to switch 2 bits per time slot (or 1 bit) to the correct direction. The next figure shows the configuration.

BSCMSC TC

64 kbps 64 kbps 13 kbps

Transcoder is at BTS site

BTS

64 kbps 16 (13+3) kbps

13 kbps

16 (13+3) kbps

Transcoder is at MSC site

BSCMSC TC BTS

SYSTRA



Figure 4.22 Transcoder and submultiplexer

TC

64 kbps

16 (13+3) kbps

13 kbps16 kbps (90 Channels)

16 kbps

TCSM (Transcoder/Submultiplexer)

MSC

BTS

BSC

SMUX

TC

TC

Transmission



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• GSM networks use Time Division Multiple Access (TDMA) technology in the air interface. By this method, one frequency resource can be shared by maximum eight mobile users.

• There are eight physical channels per frequency in the air interface.

• Logical channels are classified according to the type of information contained within each channel.

• There are eleven logical channels. Two of which are half and full rate traffic channels. The remaining nine are various control channels used to transfer information related to call set up.

• Information is sent from the mobile station to the Base Transceiver Station (BTS) in intermittent bursts.

• There are four primary types of bursts. Normal Burst, Access burst, synchronisation burst and frequency correction burst.

• There are primarily three sources of problems in the air interface. There is multipath propagation, shadowing and propagation delay. The methods adopted to overcome these problems are Viterbi Equalisation, Channel Coding, Frequency Hopping, interleaving, Antenna Receiver Diversity, Adaptive Power Control and Adaptive Frame Alignment.

• A Base Transceiver Station (BTS) site can be configure as an omni-directional BTS or a sectorised BTS.

• There are three different methods of connecting a Base Transceiver Station (BTS) to the Base Station Controller (BSC): Point to Point, Multi Drop Chain and Multi Drop Loop.

• Transcoder is placed at the Mobile Services Switching Centre (MSC). A transcoder used in conjunction with a submultiplexer makes it possible to multiplex traffic channels from three PCM lines, thereby reducing transmission costs.

SYSTRA



4.7 Transmission Review



1. Duplex frequency means

a) The difference between the uplink and downlink frequency pair

b) The uplink and downlink frequency pair

c) Twice the uplink or downlink frequency band

d) GSM 900 and GSM 1800 frequency bands

2. The modulation scheme used in GSM is

a) Frequency modulation

b) Amplitude modulation

c) Phase modulation


3. Which of the following are Dedicated Channels?

a) FCCH, SCH, AGCH

b) SDCCH, TCH, SACCH

c) RACH, FACCH, TCH

d) BCCH, SDCCH, SACCH

4. The function of AGCH is to

a) inform the mobile station of the frequency hopping sequence

b) provide the mobile station the handover information

c) inform the mobile station of a dedicated signalling channel

d) transmit adaptive frame alignment information to the mobile station

Transmission



155 (248)

5. Short message service is transmitted in

a) SDCCH

b) SACCH

c) Both of them

d) Neither of them

6. Information about frequency hopping sequence is in

a) BCCH

b) FCCH

c) RACH

d) AGCH

7. Inter symbol interference is caused by

a) Fading dips

b) Viterbi equaliser

c) Reflection

d) Interleaving

8. Frequency Hopping

a) Eliminates the problem of Fading dips

b) Eliminates the problem of inter symbol interference

c) is part of channel coding

d) spreads the problem of fading dips to many mobile stations

9. Speech transcoding from 13 to 64 kbits/s and vice versa is done by a transcoder between which two points?

a) BTS and BSC at BTS site

b) BTS and BSC at BSC site

c) BSC and MSC at MSC site

d) All above are possible

SYSTRA



Network Planning



157 (248)

5 Network Planning 5.1 Module Objectives

At the end of the module the student can:

• list the main steps of the radio network planning process

• define the main radio network parameters

• explain how the frequencies are reused

SYSTRA



5.2 Introduction

The geographical distribution of the subscribers poses a difficult problem for GSM networks. Without wire-connected telephones the subscribers can be virtually everywhere, but still the network must be able to provide a connection in spite of their movements. A good geographical coverage is the basis for providing network services. Careful network planning is thus a primary aspect of implementing GSM networks. Several requirements must be taken into consideration already in the early stages of the planning process:

• Costs of building the network

• Capacity of the network

• Coverage

• Maximum congestion allowed (grade of service)

• Quality of calls

• Further development of the network.

Various factors affecting the demand for network services must also be considered. These are mostly related to the inhabitants of the area, such as distribution of the population and vehicles, income of the population and statistics on telephone usage.

Network Planning



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The main steps of a Network Planning process are as follows:

• Collection of all relevant information such as topographical maps and statistical books

• Network Dimensioning based on coverage and capacity requirements

• Selection of Base Station sites

• Survey of intended sites

• Use of computer aided design system for coverage prediction, interference analysis and frequency planning

Figure 5.1 Simulated Cellular Radio Network Planning.

With these data it is possible to prepare a preliminary plan of site distribution, which enables the coverage prediction. During this phase the network is also dimensioned.

The survey of the installation sites refers to evaluating the intended location of each Base Transceiver Station (BTS), as well as its surroundings, possible structural and geographical obstacles and existing radio equipment. This determines whether the location is suitable for the installation.

SYSTRA



The type and location of the BTS depends on the characteristics of the surroundings. In sparsely populated areas we use powerful BTS’s which are usually mounted on high ground to provide maximum unobstructed coverage to all directions. This type of BTS is called Omnidirectional BTS. The maximum theoretical distance from an omnidirectional BTS to the edge of the cell is 35 kilometres and the number of available freque ncies depends on the traffic volume. In urban areas, where traffic volume is higher, the size of a cell is much smaller and the distance between BTS’s is shorter. The standard type of BTS is also different: the cell is divided into three sectors that have a few frequencies each. This is called Sectorised BTS. Another type of sectorised BTS is used to provide coverage for highways. Instead of three-sector BTS’s, a string of two-sector BTS’s is installed along the road.

Figure 5.2 BTS configurations.

After all the installation sites have been surveyed, a detailed network plan can be made. This includes the design of a transmission network which is usually supplied by existing operators (leased PCM lines), or by microwave links.

Omnidirectional BTSf1,f2, f3

3 sectorised BTS

2 sectorised BTS

f2f1, f2

f5, f6

f1

f3, f4

Network Planning



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After the installation work has been completed, the radio environment has to be measured and tested to ensure its proper operation and coverage before putting it into use. This is carried out in the surroundings of each individual site using portable test transmitters that are normally installed in a vehicle.

Nokia offers an integrated Radio Network Planning solution which brings flexibility for continuous improvement and adaptation to demand, more reliable and well managed mass parameterization and more streamlined operations.

Figure 5.3 Nokia’s integrated Network Planning solution

Parameters generated by the NPS tools can be converted to format which can be understood by the NMS/2000. Then, these parameters can be transferred to NMS/2000 to be used to configure/re-configure the base stations in question.

After the configuration/re-configuration of base stations, drive survey is done with Nokia’s NMS/X tool.

NMS/2000Planning

Tools

NetworkMeasurementSystem

BSC

BTS

BTS

BTSNMS/XNMS/X

NPS/XNPS/XNPS/iNPS/iNPS/ctNPS/ct

Data-base Data-

base File transfer

NetworkPlanningSystem

SYSTRA



Figure 5.4 Toolkit Drive survey with NMS/X

Network Planning



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5.3 Radio Network

5.3.1 Dimensioning Cells

A cell is the basic ‘construction block’ of a GSM network. One cell is the geographical area covered by one BTS. The actual size of a cell depends on several factors: the environment, number of users, etc. Cells are grouped under Base Station Controllers (BSC).

Dimensioning a cell means finding answers to two fundamental questions: How many traffic channels (TCH) does the cell need to handle and how many traffic channels are necessary? To solve these problems, i.e. to determine the traffic capacity, we have to calculate the number of Erlangs . Erlang is the measuring unit of network traffic. One Erlang equals the continuous use of a mobile device for one hour. The traffic is calculated using a simple formula:

xcalls per hour average conversation time

Erlangs Seconds

=×( ) ( )

3600

Amount of traffic is independent of the observation duration. For example, it is possible to make the observation for only 15 minutes and then, in the formula above calls per 15 minutes is taken and it is divided to 900 seconds.

The more traffic on available resources, the more chance that there will be congestion on these resources. Network planners carefully analyse the traffic volume on installed traffic channel capacity and according to quality limits in the network, decide if there is need to install more capacity.

Let’s take an example: if there are 400 calls per hour and the average conversation time is 100 seconds, the traffic capacity is approximately 11 Erlangs. After obtaining this value, we must take a look at to the Erlang Table .

SYSTRA



Chs 1% 2% 3% 5% Chs 1% 2% 3% 5% 1 0.01 0.02 0.03 0.05 21 12.80 14.00 14.90 16.20 2 0.15 0.22 0.28 0.38 22 13.70 14.90 15.80 17.10 3 0.46 0.60 0.72 0.90 23 14.50 15.80 16.70 18.10 4 0.87 1.09 1.26 1.52 24 15.30 16.60 17.60 19.00 5 1.36 1.66 1.88 2.22 25 16.10 17.50 18.50 20.00 6 1.91 2.28 2.54 2.96 26 17.00 18.40 19.40 20.90 7 2.50 2.94 3.25 3.75 27 17.80 19.30 20.30 21.90 8 3.13 3.63 3.99 4.54 28 18.60 20.20 21.20 22.90 9 3.78 4.34 4.75 5.37 29 19.50 21.00 22.10 23.80 10 4.46 5.08 5.53 6.22 30 20.30 21.90 23.10 24.80 11 5.16 5.84 6.33 7.08 31 21.20 22.80 24.00 25.80 12 5.88 6.61 7.14 7.95 32 22.00 23.70 24.90 26.70 13 6.61 7.40 7.97 8.83 33 22.90 24.60 25.80 27.70 14 7.35 8.20 8.80 9.73 34 23.80 25.50 26.80 28.70 15 8.11 9.01 9.65 10.60 35 24.60 26.40 27.70 29.70 16 8.88 9.83 10.50 11.50 36 25.50 27.30 28.60 30.70 17 9.65 10.70 11.40 12.50 37 26.40 28.30 29.60 31.60 18 10.40 11.50 12.20 13.40 38 27.30 29.20 30.50 32.60 19 11.20 12.30 13.10 14.30 39 28.10 30.10 31.50 33.60 20 12.00 13.20 14.00 15.20 40 29.00 31.00 32.40 34.60

Table 5.1 Erlang B table

As you can see, the table contains also the grade of service (GOS) figure which is the maximum congestion allowed. Supposing that GOS is 5 % - which means that during a certain observation period (usually 1 hour) 5 out of 100 calls fail due to lack of resources - the required number of channels is 16. Since each carrier supports eight channels, we can make a rough estimation that this cell must be equipped with two carriers, i.e. two Transceivers or TRX’s.

Network Planning



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5.3.2 Frequency Reuse

Now we have to resolve another problem. There is a limited number of frequencies available to each Base Station Subsystem and they must be distributed between the cells to ensure a balanced coverage throughout the BSS. Let’s take an exercise to illustrate the situation.

You are the network planner and the number of frequencies assigned to this project is 9. Your task is to distribute the frequencies in the network that is shown in the following figure with one frequency per cell.

•

•

•

•

• •

• •

•

•

•

•

•

• •

•

Figure 5.5 Frequency planning exercise

SYSTRA



As you can see, the frequencies have to be reused. If you do not distribute the frequencies properly throughout the network the result will be a high level of interference caused by overlapping frequencies. To avoid this, the GSM network includes a specification of the Frequency reuse patterns, one of which is presented in figure below.

1 234 5

7 8

9

6

•

1 23

4 5

7 8

9

6

•

1 234 5

7 8

9

6

•

1 234 5

7 8

9

6

•

1 234 5

7 8

9

6

• 1 234 5

7 8

9

6

•

• •

•

•

•

•

•

• •

•

Figure 5.6 Frequency reuse pattern example

The next step involves the dimensioning of the Location Areas. This is carried out according to the traffic characteristics of each area. The final phase is the dimensioning of the Fixed Network on the basis of the traffic requirements and dimensioning of the entire radio network.

Network Planning



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• Two of the most important factors in planning a GSM network are the coverage and capacity of the network.

• The demographic data of the intended coverage area plays a major role in network planning.

• Frequencies have to be reused in GSM network according to certain predefined methods.

• Nokia offers an integrated solution to network planning with Network Planning System (NPS) and Network Measurement System (NMS) tools.

5.5 Network Planning Review



1. Which of the following is not a factor in Network Planning?

a) Intended coverage area

b) Intended grade of service

c) Bit rate of the SS7 signalling links

d) cost of the network elements

2. Radio network Planning process starts with

a) Selection of Base station Sites

b) Survey of intended sites

c) Collection of all relevant information

d) Working out the frequency plan

SYSTRA



3. Frequency reuse is done in GSM Networks, because

a) A financially viable GSM network is not possible with available frequencies without reusing them

b) The spacing of 200 kHz between carriers instead of 25 kHz (like in analogue networks) reduces the number of frequencies

c) it helps in increasing the number of subscribers

d) None of above are quite correct

4. In a certain PLMN, an average subscriber makes 5 calls during office hours (8 AM - 6 PM). It is known that in a certain cell area, there are going be 1000 subscribers, at any given hour, during these office hours. Assuming that a subscriber’s conversation lasts for 100 seconds, how many TRXs are needed in this cell to provide a grade of service of 2%?

a) 2

b) 3

c) 4

d) There is not enough information given for an exact answer

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6 Nokia Implementation 6.1 Module Objectives


• Differentiate between the generic GSM network architecture and Nokia implementation of it.

• Describe the DX 200 platform’s modularity, distributed processing and network element architecture.

• Describe the NMS functions and architecture.

• List the different types of Nokia BTSs.

SYSTRA



6.2 Introduction

The preceding chapters have discussed the GSM network and it’s various concepts in considerable detail. These concepts and ideas are generic in nature and are based on the standard specifications of the GSM defined by ETSI (European Telecommunication Standard Institute). All the GSM networks operate in the same way as explained in the preceding chapters. However, the implementation of the network may differ between different organisations. The end result will of course be the same as specified in the standards, but the equipment and the implementation of it can vary. In this chapter we will see how Nokia has implemented the full GSM network with its various network elements.

6.3 Network Architecture

Consider the generic GSM network architecture shown below, which has all the elements discussed so far. In this diagram all the network elements and the interfaces are shown. Then compare this with the Nokia Implementation diagram of the same GSM network on figure 6.2 and note the differences.

Figure 6.1 Generic GSM Network Architecture

ACEIR

BTS

OMC

MSC

VLR

HLR

BSC

BTS

BTS

BTS

BTS

BSC

BSS - Base StationSubsystem

NSS - Network Subsystem

NMS - Network Management SystemMS

AbisAir A

IWF

SC

PSPDN

PSTNISDN

Transcoder




171 (248)

Figure 6.2 Nokia Implementation of GSM Network.

The differences are quite obvious. The main highlight seems to be a certain piece of equipment called DX 200 which has been implemented in BSC, MSC, TC, VLR, HLR, AC and EIR. We will discuss the DX 200 shortly, but let us first examine some other differences.

Integration of Elements

The picture shows that different GSM network elements are integrated into one DX 200 system. The first of these is the DX 200 MSC/VLR in which the MSC and the VLR are integrated. In the standard GSM specifications the MSC and VLR are two different network entities with functions of their own. However the amount of signalling between MSC and VLR is quite heavy and their tasks are so related with each other that it makes good sense to integrate them together.

The second integration is the DX 200 HLR/AC/EIR. All these three elements are basically databases which hold various types type of information. The HLR keeps the subscribers’ subscription data, AC holds the authentication data and the EIR, equipment data. In Nokia implementation these three elements are integrated and implemented as parts of the DX 200 HLR.

BTS

BTS

BTS

BTS

BTS

BSS - Base StationSubsystem

NSS - Network Subsystem

NMS / 2000Network

ManagementSystem

AbisAir A

PSPDN

PSTNISDN

SMSC

DX 200MSC/VLR

DX 200HLR/AC/EIR

DX 200BSC

DX 200TCSM

NSS Site

SYSTRA



6.4 DX 200 Platform

In the previous section we saw that the BSC, MSC/VLR, HLR/AC/EIR and TC are designed on the DX platform and this section will explain and show what it is.

In the early days of the telecommunication history, switching exchanges were huge, cumbersome and not so effective, requiring manual switching. As technology evolved, switches also started becoming automatic and more efficient. The cross bar exchange was considered one of the major developments in telecommunication, however, the development of the transistor after the Second World War has revolutionised telecommunications and other industries.

The computer industry changed dramatically and microprocessors started to evolve and develop. To make use of these developments, exchanges became heavily computerised. The various tasks required in a telephone exchange could be executed using the power of microprocessors.




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Figure 6.3 Centralised central processing Unit

We can see that there are different tasks for a CPU (Central Processing Unit) of a switching exchange. For one processor to carry out all the functions, a very powerful processor would be required. However, what if these tasks were to be distributed among many different computers? Would we still need very powerful processors? The answer is no. We could use commercially available processors and write software for it to execute. This is the principle on which Nokia has built switching equipment. All the various tasks of the exchange are distributed to different functional units which have a CPU of their own. Since these functional units have to communicate with each other, a message bus is used for communication. The next figure shows the distributed architecture of Nokia’s DX platform.

Signalling towards Subscribers

Collecting dialled numbers

Collecting Charging data

Hunting for a free circuit

Making Speech path connections

Signalling towards other exchanges

Analysing and subscriber data

Supervising the processes running

Collecting statistical data

SYSTRA



Figure 6.4 Distributed structure of Nokia’s DX platform

This kind of architecture has a number of advantages which make this a very reliable platform to implement high fault tolerant systems.

Distributed Processing is the first one. Since the tasks are distributed among different computer units, it makes it easier and faster to track down and solve problems. Modularity is another advantage as only those units which are necessary need to be installed in an exchange. If the units are required at a later date they can be installed later. One does not have to buy everything to have just a fraction of its resources. Alternatively, if a single unit performing certain task is not enough, then the same type of unit can be “added-on.” Reliability is another significant advantage. A functional unit, depending on its vulnerability to disturbances is either duplicated, or a spare unit for a number of units is installed. These are called the 2N redundancy and N+1 redundancy. In the former, there is always a unit in “hot stand-by” mode for each working one, in the latter there is one spare unit for N number of units. Based on this DX platform, Nokia has built different types of switching exchanges. DX210 and DX220 are the fixed telephone network (PSTN) switches. The DX 200 is the platform used for GSM elements BSC, MSC/VLR, HLR/AC/EIR

Exchange

Computer Units

Message bus

Signalling towards

Subscribers

Collecting dialled

numbers

Collecting Charging

data

Making Speech path

connections

Signalling towards

other exchanges

Collecting statistical data

Supervising the processes

running

Hunting for a free circuit




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6.5 DX 200 MSC/VLR Architecture

The DX 200 MSC/VLR consists of a number of functional units each with its own processor and back up facility carrying out a number of tasks. These functional units have independent tasks but communicate when and as necessary using a common message bus. The MSC is integrated with the VLR and communication between the two entities is entirely internal signalling. The MSC also has gateway functionality that is to say that it has interfaces to other networks outside the GSM PLMN.

Figure 6.5 Block diagram of the DX 200 MSC/VLR

The maximum number of visitor subscribers roaming under a DX 200 MSC is 150,000 and in the DX 200 MSC-i it is 400,000.

DX 200 MSC has full SSP functionality (refer to chapter “Intelligent Networks”).

2n 2n

CHUSTUVLRUCMCMUCCMU

TGFP

CDSU

CNFC

VANG

BSSBase StationSubsystem

CLS

HLRHome Location

Re gister

PSTNPubli c Swi tched

Telephone Network

GSWGroup Switch

OMU

I/O

2n

PAU CASU LSU IWCU BSU CCSU M BDCU

MB Me ssage Bus

ETET

ET

X.25 toAdC (Admini strative Centre)

ECU

SYSTRA



6.5.1 Functional Units in MSC/VLR

The DX 200 MSC/VLR is the biggest element (with respect to number of functional units) in the GSM network. It is not the intention of this section to give a complete explanation of all the units in this section, but a simple overview of them only to give an understanding. For the purpose of ease of understanding the units have been classified according their functions.

Signalling Units

There are six different types of signalling units. These are CCSU (Common Channel Signalling Unit) which handles trunk signalling (SS7) towards HLR, other MSC’s and PSTN exchanges and is responsible for call control for Trunk originated calls. The other one is BSU (Base Station Signalling Unit) which takes care of SS7 signalling towards BSC and call control for mobile originated calls. The CCMU (Common Channel Signalling Management Unit) handles the centralised functions of the SS7 signalling system. It is only needed in big exchanges. In smaller exchanges the CCMU's function are taken over by the CM and the STU. For MSC’s which are connected to networks which still use R2 signalling there is CASU (Channel Associated Signalling Unit) which performs R2 signalling. PAU (Primary Access Unit) handles DPNSS signalling towards PABXs and LSU (Line Signalling Unit) controls the announcement machine (ANM). One special unit is the IWCU (Inter Working Control Unit) which controls Compact Data Services Unit (CDSU), Echo Cancellers (EC) and any other interworking functional units.

Switching related units

In switching related units of MSC, GSW (Group Switch) is the switching matrix. The basic function of the MSC is switching of telephone calls. This is implemented by a group switch where each input being capable of being switched to any output. M (Marker) controls and supervises the GSW. There are three additional units which work together with GSW. The first of these is the DTMFG (Dual Tone Multi-Frequency Generator) for generating DTMF signals when required, such as when user is trying to transmit DTMF signals to an equipment at the other end. The next one is TG (Tone Generator) which is responsible for generating various types of tones such as dial tone, busy tone, information tone etc. The TG and DTMFG functions are combined in the TGFP (Tone Generator Field Programmable) unit




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in recent MSCs. The third is the CNFC (Conference Circuit), used for enabling multi-party conference.

Database and statistics related units

One of the most important unit in an MSC is the VLRU (Visitor Location Register Unit). This is the Nokia implementa tion of VLR as a functional unit of the MSC; thus it performs the VLR functions. CMU (Cellular Management Unit) controls and supervises the cellular network and handovers. Exchange specific statistical data is collected by STU (Statistical Unit). For a switch with relatively low traffic STU can also collect charging data, but for switches which handle high traffic CHU (Charging Unit) is needed to collect charging data.

External interface and data related units

The unit with which a person can do the normal operation and maintenance tasks is the OMU (Operation and Maintenance Unit). This is the link between the user and the MSC. It monitors the exchange continuously and starts recovery procedures if errors occur. The BDCU (Basic Data Communications Unit) contains all communication links to O&M network (terminals for X.25 packet network and/or for time slots of PCM link) and to the Billing Centre. For subscribers’ data (data calls, fax etc.) there are Data Service Pools. They contain modems for data communication services to the PSTN. ECU (Echo Canceller Unit) is needed in interworking to PSTN. It cancels the echo generated in the 2 wire subscriber cable in the PSTN. The ET (Exchange Terminal) is the unit which handles the external 2 Mb PCM circuits. There is no duplication of ETs since their reliability is much greater than that of the actual link, therefore in the event of failure, redundancy is taken care of by reorganising of the signalling and traffic.

Other Units

CM (Central Memory) is one of the most important units. It is RAM of the exchange, which holds the system software and also keeps a copy of all exchange specific software data. CLSU (Clock and Synchronisation Unit) is responsible for generating the synchronisation signals for different units as well as for other elements such as BSC, HLR. VANG (Verbal ANnouncement Generator) is used for playing recorded announcements. MB (Message Bus) is the parallel, duplicated message bus for sending and receiving DX messages between different functional units.

SYSTRA



6.6 DX 200 HLR/AC/EIR

Once the architecture of DX 200 MSC/VLR is understood, it becomes a matter of relative ease to understand all the other DX 200 elements, because of the same architecture and the presence of similar units in all the elements. HLR is no exception. Figure 6.6 shows the architecture DX 200 HLR and a brief explanation of HLR specific units follows thereafter.

Figure 6.6 Block Diagram of the DX 200 HLR

The maximum number of created subscribers in a DX 200 HLR is 300,000 and in a DX 200 HLR-i it is 1,200,000.

OMU

2n2n

STUACUHLRUCMEIRU

MB Message Bus

GSWGroup Swi tch

M BDCU

MSCMobile- services

X.25 toAdC

I/O

ET

CLS

CCSU




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6.6.1 Functional Units in HLR

It can be seen that there is only one signalling related unit in HLR compared to the many in MSC. This is because user traffic does not come to HLR. The rest of the units and their functions are exactly same as in MSC. However we see that there are three extra units in HLR which were not in MSC. These are the database related units. HLRU (Home Location Register Unit) is responsible for subscriber data management and mobility management. Authentication Unit (ACU) is responsible for the management of authentication data. It generates authentication triplets and sends them to the VLR and EIRU (Equipment Identification Register Unit) handles the equipment identity and its checking.

6.7 DX 200 BSC

Nokia’s DX 200 Base Station Controller (BSC) is also based on the same switching platform on which MSC and HLR are designed. However there is one major difference in the hardware of one particular unit. This is the group switch. The group switch used in MSC is capable of switching one PCM channel (8 bits) at one time only. That is a PCM channel as a whole (8 bits) can be switched from any cable to any channel. The group switch used in BSC, GSWB is capable of switching 1 bit from inside a PCM channel and switch to any other position in any other channel in any other PCM cable. This makes it very convenient to switch transcoded speech which is at the rate of 13 Kbits/s. This will also enable switching for half rate speech when it is implemented.

SYSTRA



CLS

MSC (TC)Mobi le Services Switching

(Transcoder)

BTSBase Transceiv er

S ta tion

GSWBG roup S witch

MB Message BusMB Message Bus

OMU

I/O

BCSU MCMU

X.25

ET ET

MB Message Bus

Figure 6.7 DX 200 BSC (second generation) architecture

The capacity of a DX 200 BSC is a maximum of 128 BTSs or 256 TRXs.




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6.7.1 Functional Units in DX 200 BSC

Apart from the operating difference of the group switch, as mentioned, other units work in the same manner as in MSC and HLR. Not all units which were present in MSC are present in BSC due to the functional difference of the network element. The function of the group switch here is also much reduced than in MSC. For a smaller group switch it is not necessary to have a marker dedicated to control the group switch. Thus in BSC the marker and the cellular management unit are combined to make one MCMU (Marker and Cellular Management Unit). The Marker part controls and supervises the GSW. Cellular Management Unit part is responsible of the cells and radio channels. It manages the configuration of the cellular network. There is also only one signalling unit. It is the BCSU (Base Station Controller Signalling Unit) which handles SS7 signalling between MSC and BSC and LAP-D signalling between BSC and BTS

SYSTRA



6.8 Nokia NMS/2000

The basics of Network Management Subsystem were already discussed in the Traffic Administration chapter. Here we see the actual implementation of it. The raw data from the GSM network is transferred to the NMS/2000 via a router and a Data Communications Network (DCN).

Figure 6.8 Nokia NMS/2000 Architecture

DataCommunications

Network

ServersRouter

Workstations

LAN




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6.8.1 Functional Units in NMS/2000

The standard Nokia NMS/2000 consists of servers and operator positions which can be either application workstations or X terminals. These components are connected to a Local Area network (LAN). Servers are also provided and consist of a communications server, a database server and a standby server or combinations of these. A router is provided to allow communication to the various elements in the GSM network which is connected to a Data Communication network (DCN).

Figure 6.9 NMS/2000 connections to GSM Network Elements.

Data CommunicationsNetwork

BTS

BTS

BTS

BTS

DN2

BSC

MSCVLR

HLRACEIR

SMSC

NMS/2000

SYSTRA



6.9 Nokia BTS

Base Transceiver Station, BTS, is a part of the Base Station sub-system, BSS. The BTS provides the radio part for the BSS and is located between the BSC and the Mobile Stations, MS. The Nokia BTS is not based on the DX switching platform. It has it’s own platform. BTS handles the signalling as well as traffic in the air interface. It also detects the MS.

Frequency Hopping is another function which is implemented using the Frequency Hopping Unit between the Frame Units and Carrier Units. Hopping can be cyclic or pseudo-random as defined in the GSM specifications. The BSC provides the Frequency Hopping parameters for the BTS that operates according to the parameters. Frequency hopping is optional. BTS is also responsible for power control in down link direction. LAP-D signalling is used between the BSC and BTS and LAP-Dm between MS and BTS.

Note that BTS is not based on the DX switching Platform.

6.9.1 Nokia BTS Families

The Base Stations manufactured by Nokia have gone through multiple generations. Each successive generation of BTSs have come up with new features. The following sections briefly give overview of different generations of BTSs.




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6.9.1.1 Nokia 2 nd generation BTS. The second generation BTSs had advanced system features with wide range of versions. They include flexible cell extension solutions, and an optimised transmission solutions. features also included Remote configuration management, Remote fault management, Downloadable SW from the OMC, Local MMI for monitoring and control, Non-volatile flash memory for SW storage Figure 6.11 shows some versions of the second generation BTS.

OUTDOORINDOOR

StreetlevelMiniSectorized Rooftop

Figure 6.10 Nokia 2nd generation BTS

SYSTRA



6.9.1.2 Nokia Talk Family BTSs The Nokia Talk family of BTSs has more versions than the second generation ones. Nokia Citytalk and Intratalk both have similar features and can be a one or two cabinet version. With the two cabinet version, it can ha ve a maximum sectorised capacity of 4+4+4. When used in Omnidirectional mode, up to 12 TRXs can be used. With a height of 136 cm, the Citytalk is also quite compact. The Intratalk is slightly bigger with 160 cm height.

The Flexitalk is a small and compact which finds most use while being wall mounted indoors and in shopping malls. It is also quite useful in special sites such as underground. Its capacity is 1 or 2 TRX in omni directional configuration. With all its dimensions around 50 cm, it can fit virtually anywhere. The Flexitalk+ is similar to Flexitalk in capacities and configurations, but is slightly bigger at 1m height. It is also suitable for rooftop mountings. Figure 6.12 shows the Nokia Talk Family of BTSs.

Nokia Flexitalk 2 TRX

Nokia Citytalk 6 TRX

Nokia Extratalk,Site Support System

Nokia Intratalk 6 TRX

Nokia Flexitalk+ 2 TRX

Figure 6.11 Nokia Talk Family of BTSs.




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6.9.1.3 Nokia PrimeSite The PrimeSite has a high level of hardware integration. It has a compact card-and-chassis construction. With a weight of less than 25 kg it is light and small in size. It has only one basic configuration. Large operational temperature range of -40...+50 degrees Celsius makes it suitable for various climates as well. The PrimeSite has only one BTS version with one TRX, suitable for outdoor and indoor applications. Although it includes only one TRX it can be configured to give sectored results as well, as the example diagram shows. Figure 6.13 shows the PrimeSite size in comparison to a mobile station. Figure 6.14 shows some typical PrimeSite Applications and figure 6.15 shows a 2+2+2 sectorised example using PrimeSite.

Figure 6.12 Nokia PrimeSite BTS

SYSTRA



Figure 6.13 Nokia PrimeSite Applications

1 BTS1 TRX

1 BTS1 TRX

1 BTS1 TRX

1 BTS1 TRX

1 BTS1 TRX

1 BTS1 TRXA-bis

andClockSignal

one BTS isClock Master

BSCA-bis

Figure 6.14 A 2+2+2 sectorisation using Nokia PrimeSite




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6.9.1.4 Nokia MetroSite

Nokia MetroSite is a base station and transmission equipment combined solution. It is designed for micro-cellular applications.

Figure 6.15 Nokia MetroSite

A MetroSite base station is small (49 litres) and light–weighted (22 kg with 1 TRX and 35 kg with 4 TRXs installed) with a modular mechanics. It supports GSM 900, 1800 and 1900 frequency bands. It accommodates upto 4 TRX:s where a dual-band configuration with GSM 900 and 1800 frequencies can be implemented. It can be used both for indoors and outdoors purposes.

Simplicity of installation and integration brings shorter integration times. Having the possibility of different build-in transmission units, different transmission solutions can be easily applied.

SYSTRA




• Nokia’s implementation of the generic GSM network architecture is based on the DX 200 Switching platform.

• The DX 200 switching platforms based on the principles of distributed processing and modularity.

• The DX 200 based network elements from Nokia are:

• DX 200 Mobile Services Switching Centre (MSC) which includes the GSM network elements MSC and Visitor Location Register (VLR).

• DX 200 Home Location Register (HLR) which includes the GSM network elements HLR, Authentication Centre (AC) and Equipment Identity Register (EIR).

• DX 200 Base Station Controller (BSC).

• Nokia implementation of the Network Management Subsystem (NMS) consists of two parts.

• OMC Local Area Network consisting of Work Stations where various OMC applications are running.

• Nokia Base Transceivers Stations (BTS) is not based on the DX 200 platform.

• There have been four different generations of BTSs from Nokia.

• Each generation consists of different families of BTSs suitable for different environments including indoors and outdoors.




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6.11 Nokia Implementation Review



1. In Nokia’s implementation of the GSM network

a) The transcoder is included as part of MSC

b) The EIR is included as part of MSC

c) The VLR is included as par t of MSC

d) The HLR is included as part of MSC

2. The following subsystem is entirely built on the Nokia DX 200 switching platform:

a) Network Switching Subsystem (NSS)

b) Network Management Subsystem (NMS)

c) Base Station Subsystem (BSS)


3. Authentication Centre is implemented as part of

a) Billing Centre

b) Visitor Location Register

c) Short Message Service Centre

d) Home Location Register

4. Which of the following is a feature of the DX 200 Platform?

a) Distributed Processing

b) Ability to start with minimum number of units and “add on” more later if and when required

c) 2N and N+1 redundancies

d) All of the above

SYSTRA



5. Distributed processing in the DX 200 platform means

a) Sharing of tasks between different network elements like MSC, HLR and BSC

b) Sharing of tasks between different functional units within one network element such as BSC

c) Using parallel processing techniques within one computer unit

d) Using DX 200 platform

6. Which of the following is a task of NMS/2000?

a) Controlling the inter MSC handover

b) Routing of user’s calls

c) Counting the number of inter MSC handovers performed

d) Taking automatic recovery action after a fault occurs in MSC

7. How many bits of PCM time slot can the new GSWB in BSC switch at one time?

a) 1

b) 2

c) 4

d) 8

8. How many maximum frequencies can there be if an Intratalk BTS is configured as an omni-directional BTS?

a) 12

b) 6

c) 4

d) 2




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9. How many TRX does a Prime Site have?

a) one

b) two

c) one or two

d) two or more

10. How many TRX can a MetroSite base station have?

a) one

b) two

c) three

d) four

e) any of the above

SYSTRA



Next Step



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7 Next Step 7.1 Module Objectives

At the end of the module the student will be able to:

• Explain the principles of High Speed Circuit switched Data (HSCSD).

• Identify the facilities of the General Packet Radio Service (GPRS).

• Describe the capabilities of Enhanced Data rates over GSM Evolution (EDGE).

• Identify the facilities provided by the Wireless Application Protocol (WAP).

• Identify the 3rd Generation mobile systems and their facilities.

SYSTRA



7.2 Introduction

New demands will be made in the future on mobile cellular systems as individuals and businesses change the way they work.

Access to the Internet will become more important and executives will want to access corporate databases from virtually anywhere. New services will be required in addition to speech and data, therefore network operators will offer video and other multimedia applications. Advanced mobile handsets will be required to handle large amounts of high-speed data in what is known as the 3 rd Generation Mobile systems.

GSM networks will need to evolve to a point where these 3rd Generation technologies can be introduced more cost effectively. This GSM evolution will require network operators to have a well-identified, step-by-step approach to meeting future service and capacity requirements.

The European 3rd Generation system is known as UMTS (Universal Mobile Telecommunications System) and ETSI is promoting a smooth evolution from the present day GSM networks. The radio “air interface” will be based on W-CDMA (Wideband- Code Division Multiple Access) using different frequency bands for the uplink and downlink.

The ITU call the 3rd generation mobile system - IMT-2000 (International Mobile Telecommunications 2000). IMT-2000 refers not only to the approximate year when it is expected to be launched but also the frequency band in the region of 2000MHz.

IMT-2000 will provide a seamless, global communications service through small, lightweight terminals. The 1992 World Administrative Radio Conference (WARC) allocated the radio frequencies between 1885MHz and 2200MHz to be reserved for the IMT-2000 on a global basis.

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GSM systems will evolve towards the UMTS by progressively introducing new techniques to provide higher bandwidth. These steps are as follows:

• High Speed Circuit Switched Data (HSCSD)

• General Packet Radio Services (GPRS)

• Enhanced Data rates for GSM Evolution (EDGE)

Fig.7.1 Evolution with GSM

HSCSD GPRS

3rd GenerationUMTS

EDGE

SYSTRA



7.2.1 High-Speed Circuit Switched Data (HSCSD)

Traditionally TDMA timeslot provided a bit rate of 9.6 Kbits/s; however a new modified air interface brings the speed upto 14.4 Kbits/s.

It is possible to increase the speed in data calls further by using multiple timeslots (Multi timeslot usage can be done either with 9.6 Kbits/s or 14.4 Kbits/s timeslots). With HSCSD, a combination of upto four TDMA timeslots could be used to provide data transfer rate at 57.6 Kbits/s. During 1999, Nokia will offer data rates of upto 28.8 Kbits/s by using two timeslot and upto 57.6 Kbits/s with four timeslots. Using multiple timeslots probably costs more to the mobile subscribers for every minute; however, for example the time required to download a mail or a file will be significantly less.

Figure 7.2 Example of Multiple Bursts for HSCSD

In Nokia’s HSCSD solution, there is no need to make hardware changes in the network (if MSCs have CDSU).

... ...

Multiple Bursts from each Mobile StationMultiple Bursts from each Mobile Station

One Time SlotOne Time Slot

BTSBTS

Next Step



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7.2.2 General Packet Radio Service (GPRS)

The Internet has become part of everyday life, GPRS gives a direct link between the worlds of the Internet and mobile communications

GPRS is different from existing GSM data services. Firstly it allows users to have the same experience as if they were connected to their office LAN. The mobile user doesn't have to connect to the network each time he wants to transfer data, he can stay connected all day. Secondly GPRS allows users to be charged for the actual amount of data they transfer. This makes a w hole new area of mobile data applications possible.

With the higher transmission speeds provided by GPRS, end users will find that file downloads are faster, applications that were previously not possible now become possible and the overall attractiveness of the data services will increase.

For example a user browsing WWW pages will be able to download pages faster and also won't have to pay for the time in between each page download when he's reading the last page.

GPRS brings cost efficient use of existing resources to the operators. By allowing faster or slower data speeds dynamically according to the amount of radio resources will help to increase the average usage level of radio resources.

Fig.7.3 GPRS data traffic increases the efficiency of resource usage

load

time of day

Max capacity

Averageusedcapacitylevel

extra capacity canbe given to packetdata users

load

time of day

GPRS data traffic

SYSTRA



In order to offer package switched data service, there should be some modifications done in the GSM network architecture. Data packages are handled with the help of two new network elements:

• SGSN (Serving GPRS Support Node)

• GGSN (Gateway GPRS Support Node)

Fig. 7.4 GSM Network with GPRS Service

The Serving GPRS Support Node (SGSN) is a router that maintains the location information of the mobile station and the Gateway GPRS Support Node (GGSN) enables the data packets to be passed on to other packet switching networks.

MSCBSC

BSCGGSNSGSN

BS

BS

BS

BS

BS

A

A

ISUP

Gb

Gb

Mobile OperatorA

MSCBSC

BSCGGSN SGSN

BS

BS

BS

BS

BS

A

A

A

ISUP

Gb

Gb

MobileOperator B

PSTN

IP network

Gn

Gi

Gn

Gi

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MAP

SCP

CAP

HLR

MAP

SCP

CAP

Next Step



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7.2.3 Enhanced Data rates over GSM Evolution (EDGE)

EDGE will provide a bridge from GSM into the 3rd Generation mobile networks. It will use an advanced GSM modulation technique to provide data speeds of 384Kbits/s but still using the existing 200KHz GSM channel. (384Kbits/s is the H0 channel in ISDN which permits the transmission of 6 x 64Kbits/s signals).

The extra capacity is achieved by increasing the data capacity of a single GSM timeslot from 9.6Kbits/s to 48Kbits/s and possibly up to nearly 70Kbits/s under good radio conditions. These timeslots can be used flexibly to allow several simultaneous services: for example, a voice call on one timeslot, Internet browsing on two others, and video conferencing on the fourth.

EDGE will provide the present -day GSM network with the ability to handle wireless multimedia services such as Internet/intranet, videoconferencing, and fast electronic mail transfer. One of the attractions of EDGE technology is that it requires minor changes to network hardware and software, and can be introduced into an existing network using the current frequency bands.

EDGE uses the same TDMA frame structure and channel bandwidth of 200KHz as current GSM networks. Existing cell plans can remain intact which means that EDGE can be introduced gradually into a network, starting with high-capacity areas such as dense city areas, airports, etc.

SYSTRA



7.2.4 The Wireless Application Protocol (WAP)

The purpose of the Wireless Application Protocol is to bring Internet content and advanced services to digital cellular phones and other wireless terminals. The aim is to create a global wireless protocol specification to work across differing wireless network technologies such as GSM-900, GSM-1800, GSM-1900, CDMA and 3rd Generation systems.

The Wireless Application Protocol is under development and will include specifications for the transport and session layers of the OSI model as well as security features. The mobile terminals will use WML (Wireless Mark-up Language) which is a browsing language similar to HTML used on the Internet. WML is a tag-based display language providing navigational support, data input, hyperlinks in addition to text and image presentation.

A common standard means the potential for realising economies of scale, encouraging cellular phone manufacturers to invest in developing compatible products, and cellular network carriers to develop new differentiated service offerings as a way attracting new subscribers. Consumers benefit through more and varied choice in advanced mobile communications applications and services.

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7.3 3rd Generation Mobile Systems

The ITU has been developing the 3rd Generation mobile systems since 1985 and called it FPLMTS (Future Public Land Mobile Telecommunications System) but in 1996 it was re-named it as IMT-2000 (International Mobile Telecommunications 2000).

A summary of the main objectives for the IMT-2000 air interface are shown below:

• Full coverage and mobility for 144Kbits/s, preferably 384Kbits/s.

• Limited coverage and mobility for 2Mbit/s.

• Efficient use of radio spectrum compared with existing systems.

• Flexible architecture to allow introduction of new services.

Compromises must be made on the speed of data transmission compared with the distance and mobility. This is indicated in the diagram below.

Figure 7.5 Bit Rate versus Mobility

10Kbps

Wide Area / High MobilityShort Distance / Low Mobility

User Bit Rate

144Kbps

384Kbps

2Mbps

Evolved 2nd Generation Systems (GSM-HSCSD, GPRS,IS-95B)

2nd Generation Systems (GSM, IS-95, IS-136,PDC)

GSM-EDGE

IMT-2000

SYSTRA



7.3.1 Frequency Allocation for 3rd Generation Systems

The World Administrative Radio Conference (WARC'92) identified 230MHz of bandwidth for the IMT-2000. However, this will be allocated in different ways in different regions and countries.

Figure 7.6 IMT-2000 frequency allocations

In the USA, part of the IMT-2000 frequency allocation is already used for PCS systems (GSM-1900). In Europe and Japan, the frequency allocation is almost identical except that the PHS spectrum partially overlaps the UMTS TDD spectrum.

20001900 1950 2050 2100 21501850

JapanIMT-200PH

S

IMT-2000

ITU

Mo

bile

Sat

ellit

e

IMT-2000 IMT-2000

EuropeUMTS(FDD)D

EC

T

UM

TS (

TDD

)

GSM1800

UM

TS (

TDD

)

UMTS(FDD)

USA

PC

S

unlic

ense

d

PCSPCS

UM

TS (

TDD

)IM

T-20

00 (

TDD

)

Mo

bile

Sat

ellit

eM

ob

ileS

atel

lite

Mo

bile

Sat

ellit

eM

ob

ileS

atel

lite

Mo

bile

Sat

ellit

e

Mob

ileS

atel

lite

Mob

ileS

atel

lite

Next Step



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7.3.2 The Universal Mobile Telephone System (UMTS)

The success of GSM is having a strong impact on the standardisation of the 3rd Generation systems.

Within ETSI, the technical committee SMG has the responsibility of standardising the 3rd Generation system in Europe which is known as the Universal Mobile Telephone System (UMTS).

W-CDMA (Wideband-Code Division Multiple Access) will be employed on the air interface mainly for wide area applications and will use paired frequency bands, one for the uplink and one for the downlink. This is commonly referred to as Frequency Division Duplex (FDD).

UMTS will also employ TD-CDMA (Time Division-Code Division Multiple Access) for low mobility indoor applications using Time Division Duplex (TDD) similar to cordless technologies. Together, these two elements of the air interface (FDD and TDD) are known as UTRA (UMTS Terrestrial Radio Access).

UMTS has the objective of providing low cost mobile terminals which ensure compatibility with GSM and will facilitate FDD/TDD dual mode operation.

SYSTRA



7.3.3 Code Division Multiple Access (CDMA)

Code Division Multiple Access is a technique that allows many different mobile telephones to use the same frequency at the same time but with each phone assigned a unique code sequence known as a "spreading code".

CDMA is a form of "spread spectrum" where the information is spread across the available bandwidth of the radio channel.

The spreading code is used to encode an information bearing digital signal. The receiver uses the same code to decode the signal and recover the information data. As the bandwidth of the code signal is chosen to be much larger than the bandwidth of the information signal, the encoding process enlarges (spreads) the spe ctrum of the signal. This spectral spreading of the transmitted signal gives CDMA its multiple access capability.

CDMA is "direct sequence" spread spectrum technique as each information bit is replaced by a sequence of shorter "code bits" or "symbols" called "chips".

The basic operation of CDMA is as follows:

If the digital information signal was a “1” then the spreading code would be transmitted normally but if the digital signal was a “0” then the spreading code would be transmitted inverted. The resulting signal, phase modulates an analogue radio frequency but because the bit rate of the transmitted coded signal is very high, the bandwidth of the radio signal is spread right the designated radio spectrum. The signal transmitted has been “squashed” so that it is broad in frequency terms but flat in power terms. This is “spread spectrum” and the signal could be so low in power density that it cannot be detected above the normal radio noise level.

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Figure 7.7 Principle of CDMA

Code(pseudonoise)

Data xCode

Data

Code

Data

+1

+1

+1

+1

+1

Symbol

-1

-1

-1

-1

-1

ChipChip

DespreadingDespreading

SpectrumSymbol SpreadingSpreading

SYSTRA



7.3.4 W-CDMA (Wideband CDMA)

For the 3rd generation mobile systems, a high bit rate is required for multi-media data. Therefore, the spreading code must be of a higher bit rate. IS -95 CDMA uses a bandwidth of 1.25MHz but the W-CDMA systems for UMTS will occupy a bandwidth of approximately 5MHz.

In the W-CDMA system the spreading codes are used to spread out the data signal to cover the whole wideband spectrum which is allocated for the data transfer.

The ETSI proposals for W-CDMA uses direct spread with a chip rate of 4.096Mchips/s. The bandwidth of 5MHz was chosen for various reasons:

Firstly, the data rates of 144Kbits/s and 384Kbits/s are achievable within this bandwidth and can provide reasonable capacity 2Mbit/s peak rate under limited conditions.

Secondly, the large 5MHz bandwidth can resolve more multipaths than narrower bandwidths. This will increase diversity and improve performance. Wider bandwidths of 10, 15 and 20MHz may be proposed in the future to support high data rates more effectively.

Next Step



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7.3.5 3G Network Architecture

3G networks consists of Radio Access Network (RAN), Core Network and NMS. In the network solution, there can be GSM and 3G interworking possibility. Note that RAN represents BSS and Core Network represents NSS of a GSM network.

A mobile station can have possibility of handover between a GSM and WCDMA networks.

A pure WCDMA network would look like as below:

Co-sited GSM + WCDMABase Station Subsystem

Base StationController (GSM)

GSM Base Station

Radio NetworkController (WCDMA)

UMTS (WCDMA)Base Station

BSC

RNC

SIM Card

GSM mobile

UMTS mobile

GSM / UMTSmobile

Network Subsystem (GSM )

3G-IWU

Mobile Switching CentreHome Location Register(GSM)

Landline NW(PSTN/ISDN)Landline NW(PSTN/ISDN)

GGSN

SGSN

MSC

Packet Subsystem

HLR

Internet(TCP/IP)Internet(TCP/IP)

IN Service Control Point

Radio NetworkController (WCDMA)


RNC

SIM Card

UMTS mobile

GSM / UMTSmobile

WCDMA Network Subsystem

WCDMA Mobile Switching CentreHome Location RegisterWCDMA Packet Network

Landline NW(PSTN/ISDN)Landline NW(PSTN/ISDN)

HLR

Internet(TCP/IP)Internet(TCP/IP)

IN Service Control Point

3G-SGSN

WMSC


RNC

SYSTRA



7.3.6 3G Mobile Data Terminals

High bit rate capabilities will allow 3rd Generation mobile terminals to provide sophisticated data services and multimedia facilities.

Figure 7.7 Example of possible 3 rd Generation Data Terminal

Figure 7.8 Evolved SMS: Electronic Postcard

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Data terminals produced for the 3rd Generation systems will have electronic imaging, electronic postcards and general multimedia support. Voice calls will be combined with real-time images to provide mass market video telephony.

The arrival of Personal Mobile Multimedia (PMM) will have a significant impact on business and private individuals alike. Full scale web browsing will be available commonly from anywhere.

There will also be a variety of information and entertainment packages in addition to location based interactive services. Multiple 3rd Generation standards will be handled by multi-mode terminals that can communicate with different sys tems.

7.4 Summary of Learning Points

• ITU calls the 3rd Generation Mobile systems IMT-2000 (International Mobile Telecommunications 2000), Europe calls it UMTS (Universal Mobile Telecommunications System). UMTS will use W-CDMA (Wide band – Code Division Multiple Access) technique in the radio path.

• GSM is evolving with HSCSD (High Speed Circuit Switched Data), GPRS (General Packet Radio Service) and EDGE (Enhanced Data rates over GSM Evolution) to provide much higher data rates. This is opening a way of smooth transition to 3rd generation mobile services.

• Nokia provides flexible total solutions to HSCSD, GPRS and WAP (Wireless Application Protocol).

• 3G (3rd Generation) Mobile Data Terminals will be furnished with multimedia facilities.

SYSTRA



7.5 Next Step Review



1. At most, how many timeslot can be used with HSCSD?

a) 1

b) 4

c) 6

d) 8

2. What is the benefit of GPRS?

a) One does not pay for thinking when having Internet connection.

b) Faster data rates

c) Operator utilises the resources

d) All of the above

e) None of the above

3. Which is not part of 3G network?

a) Radio Access Network

b) Core Network

c) Base Station Subsystem

d) Network Management System

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8 Intelligent Network 8.1 Module Objectives


• explain the need for IN

• draw the IN architecture

• explain the Nokia implementation of IN

• describe how services are implemented using the IN architecture

8.2 Introduction

The public switched telecommunications network (PSTN) is a network which provides telephone and data services to individuals and businesses world-wide. The services provided by the PSTN are tending to become more sophisticated and driven by customer requirements. This is particularly evident in the last five years after the introduction of competition into many networks. Also, new technology is tending to emerge at faster rates. Once a technology has been deployed within a network, it is generally uneconomic and impractical to immediately replace it with the next generation. These trends have resulted in PSTN

SYSTRA



operating companies studying ways to make their networks more flexible in the sense that new emerging technology and services can be more rapidly and gracefully introduced into their network.

Then, let we try to image what really means, in the future, the introduction of IN. When we make a telephone call today, we want to either reach a particular person or perform a particular function, involving for example a bank, an insurance company or a pizzeria. When we make the call, we need to know the exact physical location we are calling so we reach the right telephone. In future, we want to be able to call a person or perform a function by using a unique number, no matter where that person or that function is physically located. We will just call the unique number and the network, not we, will know where to route the call.

The present and future needs of customers and operating companies are predicted to be:

• ability to adapt to rapidly changing technical, regulatory, and marketing environments;

• stimulating use (and hence revenue) of the network;

• ability to service niche markets and individual customers;

• improved customer control of their service, and

• universal accessibility to services;

• improved network operation, administration and management, then

• easier and more flexible service implementation;

• ability to deliver sophisticated services,

• lower cost,

• greater multivendor participation.

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8.2.1 History

During the 1970s AT&T implemented freephone and Line Information Database (LIDB) services using a centralised database. AT&T demonstrated that these services could be provided more flexibly using a centralised database compared to dedicated equipment in the PSTN.

Ameritech in 1984 proposed that Bell Communications Research (Bellcore) study a platform which centralises service logic. Bellcore developed this concept and prepared a Request for Information (RFI) for what was then called a Feature Node/Service Interface. It was at this seminar that the term `Intelligent Network' was first used. The previous concept of centralising only the service data became termed the Intelligent Network/1 (IN/1).

The proposed Featur e Node/Service Interface was an entity external to the PSTN which was to provide a software platform for the implementation of service logic and related service data. The motivation for the Feature Node idea was the fulfilment of the following objectives:

• the ability to rapidly introduce new services into the PSTN,

• the establishment of interface standards to facilitate equipment and software vendor independence.

These two objectives became the guiding principles of the Intelligent Network.

In 1987 the emphasis of Bellcore work on the Intelligent Network shifted in response to a request from the Regional Bell Operating Companies (RBOC) for an Intelligent Network architecture that could be implemented earlier than the IN/2. The result of this work was called the IN/1+. The objective of the IN/1+ was to develop an architecture that could be deployed by 1991.

During 1989 Bellcore realised that vendors could not supply the capability required for IN/1+ for 1991 deployment. In May 1989 Bellcore released a new implementation plan. The new plan replaced the IN/1+ and IN/2 architectures with architecture termed the Advanced Intelligent Network (AIN).

The ITU-T and ETSI (European Telecommunications Standards Institute) started work on defining an Intelligent Network arc hitecture in late 1990. Their IN architecture has been heavily influenced by Bellcore’s work. CCITT working party XI/4 have followed Bellcore

SYSTRA



and defined a three stage evolutionary approach. Each stage is defined by a capability set (CS). The CSs are planned to be fully specified at two yearly intervals starting from the end of 1991.

8.2.2 Basic telephone network

To explain the IN concept, it is useful to reflect on how a service is realised. A service is realised by using physical resources within the network. Physical resources include things such as: dial tone, circuits (bandwidth), collection of dialled digits. Service logic (i.e., service intelligence) controls these physical resources in such a way as to provide the service. In the existing networks (PSTN/GSM), the parts which provide service intelligence are intertwined and mixed with parts which provide the physical resources, there is no clear and standardised interface between the two.

8.2.2.1 Example of introducing new service in the network and problems it faces

The crucial difference between the existing network architecture and the IN architecture is that service logic is separated from the actual physical resources that provide the service. This seemingly small change is very significant because the service logic can now be managed and located independently of the physical resources.

Before the introduction of the IN features, the services were allowed by using a specialised logic implemented on each switch platform of the network (figure 7.1).

The traditional approach of implementing PSTN services is to implement the service at every (or most) exchange in the PSTN. This approach requires regular updating of exchange software and hardware. This traditional approach is now seen as inadequate to meet the present and future needs of customers and operating companies.

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This means that when a particular service is introduced all, the switches had to contain the relative software package; meaning all the switches were loaded with the same application.

TRADITIONAL WAY INTELLIGENT WAY

NEW FEATURESINTO EXCHANGES

FIXED

CELLULAR

SERVICESSCP

NEW SERVICE FEATURES

FIXED

CELLULAR

SSP-SWITCHES

Figure 8.1 Service logic implementation methods

8.2.2.2 Concept of a central service provider In the IN the service logic (and service data required to define the service) is centralised. This means the service can be completely managed from this one point within the network. This departs significantly from the existing PSTN where the service logic and service data is distributed throughout the network at each exchange. From this one centralised location, new services can be added without the need to modify the underlying network. This means services can be introduced quickly, and trial services can be evaluated and modified easily. The main idea of IN is to be service, switch and equipment-independent.

The logic required to control the IN services is removed from the switching element to a separate network element to the SCP (Service Control Point). The remaining IN service related functions in the switch (functions of the SSP, Service Switching Point) is limited to the reporting of call progress events to the SCP, and receiving instructions from the SCP for further actions. Thus introduction of a new service for network-wide utilisation requires only the provision of new service logic program to the SCP.

SYSTRA



8.2.2.3 Conceptual view of IN call Let us take an example of an IN call. Imagine that you need to make a flight reservation for your holiday. Which office are you calling? In which city? For you it is easy because the airline has advertised only one number. But in reality what happens is that once the call arrives at the MSC, it is detected that this is a call which requires special handling. So MSC sends the dialled numbers to the central service provider.

Depending on your town from where you have called, the call will be routed to the correct IN node that supports the call. Once the correct point is reached, the service control point will analyse which day of the week it is, the time of day and so on. Service Logic can choose, according to the zone/day/hour, to re-route the call to another point and/or play a courtesy announcement and/or collect digit further information (e.g. “please, press 5 for booking, 6 for flight information, 9 for .....).

NOKIA DX 200 MSC

SSP

IPSSP

IP

SCP1

6

2

7

3

4

58

9

digit “5”

digit “1234”B Numb. 1234

Translation: - location Milan= 9876 - location Rome= 6789

Request for annuncement

Annuncement:“please, dial the number..”

digit “5”

Translation: - Rome = 6789 +5 --> 6789555

Alert

“6789555“

HP 9000/800 - SCP

Figure 8.2 IN call example

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8.3 Intelligent Network Conceptual Models

The Intelligent Network has been made possible through developments in many fields including: signalling system no.7, stored program control exchanges, advanced software languages and powerful computers. The purpose of this section is to focus in on those parts of the network directly required for the Intelligent Network.

Signalling is required for exchanges, network data bases and other intelligent nodes in the network to exchange messages related to call set up, call supervision, call connection control information needed for distributed application processing and network management information.

Figure 8.3 SS7 stack for IN

The SS7 is a signalling system recommended by ITU-T and specifically designed for telephone networks. The SS7 is the generic name for a suite of protocols. These protocols are layered and closely match the Open System Interconnection (OSI) seven layer model. The layered structure of the SS7 protocol suite is shown in Fig. 5.

The lowest three layers are called the Message Transfer Part (MTP). Its function is to route a message efficiently and reliably between two signalling points in the network. Above MTP is the signalling connection control part (SCCP). The MTP together with SCCP provide most of the layer services specified in layers 1 through 3 of the OSI model.

The SS7 application protocols sit immediately above MTP and SCCP. The Telephony User Part (TUP) is an application protocol for setting

MTP Layer 1

MTP Layer 2

MTP Layer 3

SCCP

ISUP TUPTCAP

Core INAP MAP

SYSTRA



up and clearing (non-ISDN) telephone calls. The ISDN User part (ISUP) protocol is an application protocol designed to support ISDN calling. The Transaction Capabilities and Application part (TCAP) is an application protocol for invoking applications on a remote network element. TCAP is used for signalling between the SSP and the SCP of the Intelligent Network. It is essential therefore distinguish between functions and products in the Intelligent Network concepts.

8.3.1 Physical Entities

The functional entities (FEs) are allocated to physical entities (PEs) or physical nodes. A FE must be found in at least one PE, but a PE can consist of more than one FE. However, there cannot be more than one “FE of the same type” in one PE.

The roles of the single element are:

• SSP. The Service Switching Point operates as call control “controlled” point. Sends service requests to IN platform and receives instructions. Requests are sent when triggers are invoked.

• SCP. The database that contains the subscribers data and subscriptions. The SCP contains also the software to manage the SSP requests. It’s responsible to manage calls and services. These platforms are based on HP computer.

• IP. This Intelligent Peripheral is designed to answer to the subscriber calls in order to furnish resources for particular specialised functions (mass calling/televoting). It’s controlled by SCP and has voice connected to SSP. The IP element is an intelligent answering machine that is able to play an announcement and wait both “digit” from the user and instruction from the SSP/SCP.

• SMS. Centralised or not, this entity is competence to provides the subscription provisioning, customisations, managing of the SCPs. Provides also interfaces towards external systems. These platforms are based on HP computer.

• SCE. On this host the service provider may define the related database schemas. Contains tools for service design, prototype and testing. Produces Service Logic Program (SLP) which is the SW “feature” used by the SCPs.

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8.4 Nokia Implementation of IN

NOKIA IN architecture is a platform for fast development and deployment of new value added services in a telecommunications network.

The Network Elements (figure 8.4) which make up the IN network are:

Figure 8.4 Nokia Intelligent Network Architecture

• DX 200 SSP is implemented on a DX 200 MSC. IP may be integrated to the DX 200 exchange.

• SS7 (Core INAP) network is used to connect SCP and SSP nodes.

• Service Control Point (SCP) is the source of call control within IN. It stores the service data, executes the service logic programs and communicates with the SSP-switches. Nokia SCP uses standard high availability HP-UNIX technology. For better availability a Mated Pair configuration is also possible.

• Service Management Processor (SMP) is the Nokia’s name for the SMS. It is used for managing, updating and backing up service data. It provides interfaces towards support systems like customer care and billing. Nokia SMP is based on high availability HP-UNIX technology. SMS manages also the Mated Pair requirements for SwitchOver and alarming.

LAN

CoreINA P

SCE

SM P

SC P

M SC/VL R/SSP

SMS

MSC/VL R

TUP /ISUP

Alfas

kop

m34

6

AC EIR HLR

MA PMAP

SS#7TUP/I SUP

SMS

PS TN/IS DN

V LR

MSC

M SC

V LR

Alfaskop m346

SSPIP

SS#7

SS#7

SYSTRA



• Service Creation Environment (SCE) is a resource for programming, simulating and testing new service solutions. It provides a user friendly graphical service logic editor. The SCE is running on a standard HP-UNIX platform.

Various services using voice processing, dialling control etc. can be produced with the Intelligent Peripheral (IP) that can be within DX 200 or as a separate unit connected to SCP and SSP via SS7 network.

8.4.1 SSP and IP (Service Switching Point and Intelligent Peripheral) in MSC

8.4.1.1 Software

Obviously the IN platform cannot be implemented until a basic telecommunication network (GSM or PSTN) exists. In the case of mobile application, IN platform is implemented on the “mobile network” and accessed through the MSC (with IN functionality), in such case SSP, with particular SS7 protocol for IN named Core INAP (Intelligent Network Application Part). Nokia application starts with the M7 release software on MSC which has the SSP functionality and equipment needed for the IN introduction.

8.4.1.2 Intelligent Peripheral

Currently the Intelligent Peripheral (IP) functionality is embedded into the DX 200 MSC/VLR/SSP. However in the near future a dedicated IP will be implemented as an independent network element. The current IP contains the implementation of the Specialised Resource Function (SRF) with the following two distinct requirements:

– capability to provide tailored announcements to the users, and

– capability to receive DTMF dialling for PSTN users of IN

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8.4.2 SCP - Service Control Point

The Service Control Function (SCF) represents the centralised authority in an Intelligent Network. The SCF contains the platform for service logic programs (SLP). A service logic program is executed when network exchange requests call handling instructions. The service logic programs basically determine the behaviour of the IN-based services.

8.4.2.1 Core INAP The communication between the functional entities of CS-1 is done using the Core INAP protocol. INAP is standardised by ETSI to support IN functionality (Capability Set 1 or CS-1) on basis of the corresponding work done internationally in ITU-T. The ETSI Technical Specifications define in detail the operations required to support Capability Set 1 functionality. The protocol contains a set of operations used between different functional entities. The operations obey the remote procedure call scheme. This means that they have specified arguments, results and error responses. Most of the operations are in fact simply messages passed from one entity or network element to another. Core INAP provides operations for various purposes: operations for detection point handling, routing, user interaction, charging, etc. The protocol interaction follows a simple state model. The detailed description of the Core INAP state models can be found in the ETSI recommendation.

However, these specifications are restricted to support basic telephony related information flow only, and do not fulfil the requireme nts set by a mobile environment. Therefore the Core INAP specifications of ETSI are supplemented by Nokia, using the standard extension possibility to allow the mobility aspect to be utilised to the fullest extent in the mobile networks.

SYSTRA



8.4.3 SMP - Service Management Point

The Nokia IN/SMP Platform provides a solid foundation for the environment needed to manage the services within an Intelligent Network. The role of the SMP in the Nokia IN architecture consists of:

• a management application for service and subscriber data provisioning;

• a custom -built interface between the operator’s administrative systems and the SMP platform;

• custom-built applications running on the SMP platform handling operator and service specific management activities.

Non real time database

The entire Service Management Point typically consists of the Nokia IN/SMP Platform which performs one or more of the following activities:

• to act as a safe repository for service and subscriber data (Oracle DB)

• to provide the administrative systems with an interface that hides the details of the SCPs and the distribution of services into different SCPs from the administrative systems

• to provision service and subscriber data updates into the relevant SCP to receive service and subscriber data updates from the SCPs and store them in the SMP.

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ServicePoint(SMP)

Service ControlService Control

Figure 8.5 SMP database

8.4.4 SCE - Service Creation Environment

In order to deploy intelligent services, Network Operators need easy-to-use software tools dedicated to the creation and customisation of these services. There are many views on service creation. The actual creation work is concentrated into two phases of the service creation life cycle:

• The service application programming and database definitions take place in the service implementation phase. The SCE-tool is used to program the logic and to define the SCP database schema. Commercially available development tools are used to create the service management applications and the SMP database.

• The service logic programs and the service database can be designed to include a certain degree of flexibility in the service itself allowing the operator to define in the service provisioning phase, which features and options are used in the final service that is offered to customers. Several different services can thus be created out of one generic service logic.

SYSTRA



8.5 IN Service Examples

8.5.1 New and existing services provided within IN

By the virtue of their nature IN services are almost unlimited in the functions they can provide. In the following sections few examples of IN services are briefly described, but the services will vary according to the needs of each operator.

Certain descriptions serve as full-bodied service concepts as such (e.g. Credit Card Calling), but in most cases by combining multiple individual IN services together to constitute a higher-level IN application one can gain better results.

There are many kinds of services designed by standardisation procedure:

Flexible Billing

Reachability/Mobility

Customized usergroups

Customized servicesfor mobile users

• Freephone• Premium Rate• Calling Card/Credit Card Calling• A-number validation

• Personal Number• UPT (Universal Personal Telecommunications)• Portable Number

• VPN (Virtual Private Networks)• Wide Area Centrex• Free Numbering Plan (mobile)

• Televoting

• Originating/terminating call screening• Originating/terminating location services• IN service control• Service override

Mass Calling Services

Figure 8.6 Possibilities for IN applications.

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8.5.2 Flexible Billing

Flexible Billing services are services provisioned for individual subscribers. As the name implies, the main functionality of these services is to modify the charging of the subscriber’s calls and add new charging possibilities in the network.

8.5.3 Reachability/Mobility

The Reachability/Mobility services provide a general access based on dialled digits for any caller. The main functionality of these services is the service number based routing of incoming/outgoing calls and associated charging control. Subscribers, with se veral telephone sets in different networks can define a “hunting order” for their calls. Additional control based on time schedules and caller’s location is also available.

8.5.4 Customised User Groups

In Group services, subscriber is a member of a group (a list of subscribers). Group services are applicable for both incoming and outgoing calls. Group service features include short numbers for group members (and other destination as well), parameterised routing for each member, special tariff for in-group calls and so on.

8.5.5 Customised services for mobile users

Terminating/Originating Call services provisioned for mobile subscribers and used for outgoing/incoming call cases. These services allow the SCP to control the routing of calls, apply special tariffs for these calls or screen the calls. Further service control may be based on time schedules; dialled digits or location of the caller and access by passwords are available.

SYSTRA



8.5.6 Final conclusive example:

All these services, mixed in different manner create special service. An example of using these services could be a “Mobile for Teenager” subscription option:

• Prepaid SIM and/or

• Alternate billing (the parent pays)

• Selective barring (special number like 144-00-...)

• Special Discount (frequently number called)

• Free short number (e.g. house)


Why

1. Marketing

The introduction of Intelligent Networks is in a way a revolutionary move. Switching and intelligence, from being separated in the early networks, and from being more and more closely integrated in the networks of yesterday and today, are once again being separated from each other.

2. Applicability

The ideal goal for continuous development of telecommunications networks is to serve all users at all times. In other words, this will mean providing mobility for the user, i.e. calling from any location, to provide information services to compete with the different media of today, and to provide customised value-added services for specific needs. In addition, it should also be possible for users with different means of access to reach each other, independent of what type of network is used for access, ISDN, PSTN or mobile networks.

3. Services

Networks will always differ, mostly of course because of size and the technology that is being used, but in the future it will be the services themselves that characterise the networks. And services, from the creation to deployment, are what Intelligent Networks are all about.

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Where

1. IN in GSM

How

1. Network Elements:

- SSP

- SCP

- SMP

- SCE

- IP

8.7 References

/1/ Guidelines for CS-1 Standards, ETSI, TCR-TR NA-602.04.

/2/ Intelligent Network Capability Set 1 (CS-1) Core INAP, ETSI, DRAFT pr ETS 300 374-1:1993.

/3/ Mobile IN Concept for GSM/DCS Networks, Nokia Cellular Systems Oy, E. Hirviniemi, v. 1.0, 1994.

/4/ NOKIA INTELLIGENT NETWORK PLATFORMS, General Description, NTCD IGD 015/0.2en

/5/ Intelligent Network, Artech House, London, Jan Thorner, 1994

SYSTRA



8.8 IN Glossary

INTELLIGENT NETWORKS TERMS

Abbreviation list

AC Authentication Centre

BSC Base Station Controller

BTS Base Transceiver Station

CCF Call Control Function

CCC Credit Card Calling, an IN service

CCS Common Channel Signalling

CCV Calling Card Validation

CLASS Custom Local Area Signalling Services

CS1 Capability Set 1

DTMF Dual Tone Multi Frequency

EIR Equipment Identity Register

ETSI European Telecommunications Standards Institute

FE Functional Entity

HLR Home Location Register

HP Hewlett Packard

IN Intelligent Network

INAP Intelligent Network Application Protocol

IP Intelligent Peripheral

ITU-T International Telecommunication Union

IVR Interactive Voice Response

LAN Local Area Network

MA Management Application

MSC Mobile Switching Centre

NP Number Portability (Portable Number)

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NTS Number Translation Services

OCS Originating Call Screening, an IN servic e

OLS Originating Location Services, an IN service

OSS Operation Support System

PBX Private Branch Exchange

PON Portable Number

PPC Pre-paid Contract

PSTN Public Switched Telephone Network

SCE Service Creation Environment

SCF Service Control Function

SCP Service Control Point

SIB Service Independent Building Block

SIM Subscriber Identity Module

SLEE Service Logic Execution Environment

SLP Service Logic Program

SMS Service Management System

SMP Service Management Point

SRF Specialised Resource Function

SSF Service Switching Function

SSP Service Switching Point

TCP/IP Transmission Control Protocol / Internet Protocol

TPS Transactions Per Second

UPT Universal Personal Telecommunications

VLR Visitor Location Register

VM Voice Mail

SYSTRA



8.9 Intelligent Network Review

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

1. Which of the following can not be said to be a reason for the evolution of Intelligent Network?

a) improved customer control of their services

b) easier and more flexible service implementation

c) ability to adapt to rapidly changing technical environments

d) none of the above

2. What is the main feature of the Intelligent Network Architecture?

a) Centralised Service logic

b) Distributed Service Switching points

c) use of Intelligent peripherals

d) Ability to span across multiple telecommunication networks

3. If a GSM - IN call is handled in a telecommunication network, which of the following will be the traffic path?

a) MS - BSC - MSC - SCP - MSC - route out

b) MS - BSC - MSC - SSP - route out

c) MS - BSC - MSC - route out

d) MS - BSC - MSC - SCP - (signalling with SMP) - route out

4. Which of the following is not an element of the IN architecture?

a) SMSC

b) SCP

c) SSP

d) IP

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5. In Nokia’s implementation of IN SSP is implemented in

a) BSC

b) MSC

c) HLR

d) HP work stations

6. Which of the following is the correct protocol stack, in ascending order, for signalling system in IN?

a) MTP - SCCP - TUP - Core INAP

b) MTP - SCCP - ISUP - Core INAP

c) MTP - SCCP - BSSAP - Core INAP

d) MTP - SCCP - TCAP - Core INAP

7. What is the signalling protocol used to communicate between SSP and SCP?

a) ISUP

b) Core INAP

c) MAP

d) BSSAP

8. During an IN call

a) the actual service logic is performed by MSC while the SCP simply routes the call

b) the subscriber database look up is performed on the SMS

c) The routing destination and associated parameters are informed by SCP to MSC

d) if the core INAP link is broken the call will clear

SYSTRA



SYSTRA Course Review



235 (248)

9 SYSTRA Course Review 9.1 Review Questions

1. Mark True or False to the following Statements.

a) The MSC in GSM is responsible for collecting charging data.

b) NMT and AMPS mobile networks are not digital at all, where as GSM is fully digital.

c) Frequency reuse can be better employed theoretically in GSM than in analogue mobile networks.

d) The level of subscriber security employed in GSM is the same as in other analogue networks.

2. The function of a VLR is:

a) To keep the database for all the subscribers of it’s PLMN.

b) To keep the database for all those subscribers which are being served by its MSC and was downloaded from it’s own HLR.

c) Keep a backup of the subscriber data stored in it’s own PLMNs HLR.

d) Keep the database for all those subscribers which are currently served by its MSC.

SYSTRA



3. A SIM card is:

a) A credit card used to pay GSM phone bills.

b) An accessory that is absolutely necessary for a GSM mobile phone to function.

c) A card issued by the operator to a subscriber containing his subscription information details.

d) a card which can be omitted in cases of international roaming.

4. Which of the following Network Entities do not contain subscriber data?

a) HLR

b) AC

c) VLR

d) BSC

5. CGI

a) Is different for all the cells in the world.

b) Is the total number of cells served by one BSC.

c) Gives information about base station and frequency.

d) Is one of the parameter in a SIM card.

6. Channel coding

a) is used for encrypting user data

b) to eliminate the problem of fading dips

c) is the voice coding mechanism used in the transcoder

d) is for error detecting and correcting purposes




237 (248)

7. The basic principle of speech coding in a GSM Mobile Station is

a) A-Law PCM with 8 bits per sample

b) µ-Law PCM at 104Kbits/s

c) A-Law PCM with special filtering at 13Kbits/s


8. Authentication verification is carried out in

a) HLR

b) MSC

c) VLR

d) Authentication Centre

9. Which of the following algorithms are Global, i.e., they have to be the same in all cases?

a) A5

b) A3 and A8

c) All three have to be global

d) None of the three have to be Global, they are operator dependent.

10. Which of the following is not included in the cost of a call?

a) Originating MSC to destination MSC, in MO-MT call

b) Use of the network resources

c) Location of the call terminating cell

d) Use of Fast Associated Logical Channel

11. Which of the following can not be true?

a) 892 MHz uplink corresponding to 937 MHz downlink

b) 950 MHz downlink corresponding to 905 MHz uplink

c) 1.780 GHz uplink corresponding to 1.875 GHz down link

d) 1810 MHz downlink corresponding to 1765 MHz uplink

SYSTRA



12. Two IMSI numbers are required of a subscriber if he/she

a) subscribes to any two different basic services

b) subscribes to speech and data services only

c) roams to another PLMN

d) none of the above

13. Frequency Hopping

a) does not produce optimal gain if there are less than 4 Frequencies for one cell

b) may or may not be employed by an operator

c) both are correct

d) neither of them are correct

14. Which of the following is true?

a) MAP stands for Mobile Access Part

b) LAP-D protocol is used to communicate between MSC and BSC

c) MAP is used for communication between MSC and HLR

d) BSSAP is used for communicating between BSC and MS

15. If an inter MSC handover occurs during a call, the decision to make a handover is done by

a) BSC controlling the target cell

b) MSC controlling the target cell

c) BSC controlling the current cell

d) MSC controlling the current cell

16. Which of the following is not an advantage of the GSM network compared to other networks which use the same frequency band?

a) Lower Carrier to Interference Ratio for signal reception

b) Use of MAP signalling

c) Frequency reuse is more efficient than in other networks

d) Lower bit rate for voice coding




239 (248)

17. Which of the following task is not performed on the SDCCH channel?

a) Assignment of Traffic Channel to the MS

b) Transmission of Short Messages

c) Adaptive Power Control information from BTS to MS only

d) Authentication

18. Which of the following in not true? Give reason.

a) GSM 1800 can support more traffic than GSM 900

b) Cell sizes are smaller in GSM 1800 than in GSM 900

c) Base Stations for GSM 900 and GSM 1800 are different

d) The cost of setting up a GSM 900 network is more than GSM 1800

Reason: GSM 1800 cell size is much smaller than 900, therefore, in order to cover the same cove rage we need more number of base station sites. More expensive

SYSTRA



19. The transcoder changes the 64Kbits/s data rate from MSC to 16Kbits/s. This 16Kbits/s data consists of 13 kbps speech plus 3 kbps additional information. Which of the following is included in this 3 kbps data stream?

a) Transcoder control information

b) LAP-D signalling

c) Instruction from Transcoder to SMUX about the multiplexing method used

d) SS7 signalling information

20. What will happen if the OMC is lost?

a) Local operation and maintenance will have to be used

b) Charging data will not be transferred to billing centre by FTAM protocol and magnetic tapes will have to be used

c) Remote sessions from MSC to HLR will not be possible

d) BTS will stop functioning

21. Which of the following is not a parameter affecting the ce ll size while planning the network?

a) Antenna Height

b) MS Power

c) BTS Power


22. BSSAP needs the services of SCCP to

a) analyse A subscriber data

b) To perform Connectionless signalling with the MSC

c) Send MAP messages to HLR via the MSC

d) To make a virtual connection between the MS and the MSC

23. Which of the following does not use the services of the MTP directly?

a) TUP

b) SCCP

c) Both of them use

d) Neither of them use




241 (248)

24. For a certain PLMN in a country, MCC=123, MNC=45, CC=321, NDC=54, SN=12xxxxx, MSIN=01xxxxx. Given this information match the numbers below on the left column, with the correct name for each number on the right column.

Number Answer Name

1 321541234567 1=MSISDN (C) A IMEI

2 123450123456789 2=IMSI (D) B CGI

3 1EF70A12 3=TMSI (E) C MSISDN

4 321546789000 4=MSRN (F) D IMSI

5 123456541234 5=CGI (B) E TMSI

6 490130204614290 6=IMEI (A) F MSRN

SYSTRA



9.2 Acronyms

A An interface that GSM recommendations define between Network Switching Subsystem and Base Station Subsystem

A3 Authentication algorithm

A5 Encryption Algorithm

A8 Authentication Algorithm

AB Access Burst

AC Authentication Centre

AGCH Access Grant Channel

ARFCN Absolute Radio Frequency Channel Number

ARQ Automatic Request for Retransmission

BC Billing Centre

BCC Base Station Colour Code

BCCH Broadcast Control Channel

BNHO Barring all outgoing calls except those to home PLMN

BS Base Station

BSC Base Station Controller

BSIC Base Transceiver Station Identity Code

BSIC-NCELL BSIC of an adjacent cell

BSS Base Station System

BSSAP Base Station Subsystem Application Part

BTS Base Transceiver Station

CBCH Cell Broadcast Channel (Not a standard logical channel)

CC Country Code, Call Control

CCS7, CCS#7 Common Channel Signalling System no. 7




243 (248)

CDSU Compact Data Services Unit

CEPT Conférence Européenne des Postes et Télécommunications

CGI Cell Global Identity

CI Cell Identity

CL(N)IP Calling Line (Number) Identification Presentation

CL(N)IR Calling Line (Number) Identification Restriction

CSPDN Circuit Switched Public Data Networks

DB Dummy Burst

DCCH Dedicated Control Channel

DCN Data Communication Networks

DCS Digital Cellular System (now replaced by GSM 1800)

DL Data Link (layer)

DLCI Data Link Connection Identifier

DLD Data Link Discriminator

Dm Control Channel (ISDN terminology applied to mobile service)

DRX Discontinuous Reception

DTAP Direct Transfer Application Part

DTE Data Terminal Equipment

DTMF Dual Tone Multi-Frequency (signalling)

DTX Discontinuous Transmission (Mechanism)

EIR Equipment Identity Register

ETSI European Telecommunication Standard Institute

FAC Final Assembly Code

FACCH Fast Associated Control Channel

FACCH/F Full rate Fast Associated Control Channel

FACCH/H Half rate Fast Associated Control Channel

FB Frequency correction Burst

GMSC Gateway Mobile Services Switching Centre

SYSTRA



GPA GSM PLMN Area

GSA GSM System Area

GSM Groupe Special Mobile/Global System for Mobile Communications

GSM PLMN GSM Public Land Mobile Network

HDLC High Level Data Link Control

HLR Home Location Register

HON Hand Over Number

HPLMN Home PLMN

HSN Hopping Sequence Number

IAM Init ial Address Message (ISUP message)

IDN Integrated Digital Networks

IMEI International Mobile (station) Equipment Identity

IMSI International Mobile Subscriber Identity

ISDN Integrated Services Digital Network

ISUP ISDN User Part

IWF Inter Working Function

Kc Cipher Key

Ki Identity key (Key used to calculate SRES)

L1 Layer 1 (OSI mode layer 1)

LAC Location Area Code

LAI Location Area Identity

LAP-D Link Access Protocol on the D channel

LAP-Dm Link Access Protocol on the Dm channel

Lm Traffic channel with capacity lower than Bm

LPLMN Local PLMN

MAP Mobile Application Part

MCC Mobile Country Code

MD Mediation Device

MM Mobility Management, Man Machine




245 (248)

MMI Man Machine Interface

MNC Mobile Network Code

MOC Mobile Originated Call

MS Mobile Station

MS ISDN Mobile Station ISDN Number

MS_PWR_CLASS MS Power Class. Parameter defining the power class of an MS expressed in the same way as the R parameters

MS_RANGE_MAX Mobile Station Range Maximum Handover criterion to determine serving cell

MS_RXLEV_L Lower Receiver Leve l. Threshold of RXLEV received from the serving BS below which either power control or handover must take place to improve the cell quality

MS_TXPWR_CONF MS Transmitted RF Power Confirmation Parameter sent by the MS to indicate its current transmitted RF power level

MS_TXPWR_MAX_CCH Maximum Allowed Transmitted RF power for MSs to Access the System until commanded otherwise

MS_TXPWR_REQUEST MS Transmitted RF Power Request. Parameter sent by the BSS that commands the required MS RF Power Level.

MSC Mobile-Services Switching Centre

MSCM Mobile Station Class Mark

MSIN Mobile Subscriber Identification Number

MSRN Mobile Station Roaming Number

MT Mobile Termination

MTC Mobile Terminated Call

MTP Message Transfer Part

MUMS Multi User Mobile Station

NB Normal Burst

SYSTRA



NDC National Destination Code

NE Network Element

NMC Network Management Centre

NMS Network Management Subsystem

NSS Network Switching Subsystem

O&M Operations &Maintenance

OMC Operations & Maintenance Centre

OSI Open System Interconnection

PAD Packet Assembler/Disassembler

PAGING GROUP The set of MSs monitoring a particular paging block

PCH Paging Channel

PDN Public Data Networks

PIN Personal Identification Number

PLMN Public Land Mobile Network

PSPDN Public Switched Public Data Network

PSTN Public Switched Telephone Network

QoS Quality of Service

RAND RANDom Number (authentication)

RFCH Radio Frequency Channel

RFN Reduced TDMA Frame Number

RLP Radio Link Protocol

RXLEV Received Signal Level.

RXLEV_ACCESS_MIN The minimum received signal level at a MS for access to a cell

RXLEV_MIN The minimum received signal level at a MS from a neighbouring cell for handover to be permitted.

RXLEV_NCELL Received signal level of neighbouring or current serving cell measured on the BCCH Carrier.




247 (248)

RXLEV_SERVING_CELLReceived signal level in the serving cell measured on the BCCH carrier

RXQUAL Received Signal Quality

RXQUAL-FULL Received signal quality assessed over the full set of TDMA Frames without a SACCH block

RXQUAL_SERVING_CELL Received signal quality of serving cell

RXQUAL_SUB Received signal quality assessed over a subset of 12 TDMA Frames

S/W Software

SABME Set Asynchronous Balanced Mode (L2 message)

SACCH Slow Associated Control Channel

SAPI Service Access Point Indicator

SB Synchronisation Burst

SCCP Signalling Connection Control Part

SCH Synchronisation Channel

SDCCH Stand alone Dedicated Control Channel

SIM Subscriber Identity Module

SMSCB Short Message Service Cell Broadcast

SN Subscriber Number

SP Signalling Point

SRES Signed Response (authentication)

SS Supplementary Service

STP Signalling Transfer Point

TA Terminal Adapter

TAC Type Approval Code

TC Transcoder

TCAP Transaction Capabilities Application Part

TCH Traffic Channel

TCH/EFR An Enhanced Full Rate TCH

SYSTRA



TCH/F A Full rate TCH

TCH/F2.4 A Full rate data TCH (<2.4Kbits/s).

TCH/F4.8 A Full rate data TCH (4.8Kbits/s)

TCH/F9.6 A Full rate data TCH (9.6Kbits/s).

TCH/H A Half rate TCH

TCH/H4.8 A Half rate data TCH (4.8Kbits/s)

TCSM Transcoder/Submultiplexer

TE Terminal Equipment

TEI Terminal Endpoint Identifier

TMN Telecommunications Management Network

TMSI Temporary Mobile Subscriber Identity

TRX Transceiver

TS Time Slot

TUP Telephony User Part

TXPRWR Transmit power

UI Unnumbered Information (Frame)

VAD Voice Activity Detection

VLR Visitor Location Register.

VMS Voice mail System

VPLMN Visited Public Land Mobile Network

2g systra

Engineering

nokia logo

gsm network

gsm review

evolution of gsm

advantages of gsm

en issue

gsm technical specifications

open interfaces of gsm