The Mobile Environment GSM Organizations and Standards Bodies 3GPP Releases and Numbering Schemes...
GSM System Overview
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GSM System Overview
GSM SYSTEM OVERVIEW
First published 1998 Last updated January 2006 WRAY CASTLE LIMITED BRIDGE MILLS STRAMONGATE KENDAL LA9 4UB UK
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GSM SYSTEM OVERVIEW
CONTENTS Section 1 Section 2 Section 3 Section 4 Section 5 Section 6 Section 7 Section 8
An Introduction to GSM GSM Services Network Architecture Frequencies and Propagation GSM and GPRS Channels GSM Coverage Network Access Network Operation
Photographs courtesy of Mike Pratt. www.prattfamily.demon.co.uk
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SECTION 1
AN INTRODUCTION TO GSM
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CONTENTS 1
The Global System for Mobile Communications (GSM) 1.1 Introduction 1.2 The Mobile Environment
1.1 1.1 1.3
2
GSM Organizations and Standards Bodies 2.1 ETSI and 3GPP 2.2 The GSM MoU Association
1.5 1.5 1.5
3
3GPP Releases and Numbering Schemes 3.1 Releases 3.2 Numbering Schemes
1.7 1.7 1.7
4
Section 1 Questions
1.9
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OBJECTIVES At the end of this section you will be able to: • • • •
state how GSM differs from earlier mobile networks outline the principal advantages of using a digital mobile network name the organizations responsible for GSM standardization and regulation outline the provision and formats of the GSM specifications
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1
THE GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS (GSM) 1.1
Introduction
GSM is a digital mobile telephony system that operates in a cellular network environment. GSM is a second-generation, or 2G, system. The term 2G differentiates GSM from earlier, analogue systems known as firstgeneration systems. These were limited in the services they could offer, had poor security arrangements, and incompatibility between networks meant that subscribers could not visit, or ‘roam’, into other networks. Some of the features that differentiate GSM from earlier systems are: • it is digital • the GSM standards allow for interoperability between network operators • it uses Integrated Services Digital Network (ISDN)-based technologies and standards • it offers improved privacy and security • network performance has been enhanced • it is more spectrally efficient The GSM network is also a sound base upon which to build an even more sophisticated network, which for many network operators provides an evolution route to the third generation (3G).
1.1
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digital standardized confidentiality and security evolution to 3G
Figure 1 GSM Overview MB20/S1/v9.1
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1.2
The Mobile Environment
1.2.1
Advantages of Mobile Networks
In a fixed-line network, the user is connected to the network by an installed set of wires. In the mobile environment these wires between the user and the system do not exist in a permanent form – they have to be ‘created’, using radio, every time the user wants to make or receive a call. This requirement for a radio link, or ‘air interface’, offers many advantages to users, principally mobility: the user is free to move while using the phone. This means that an increasingly wide range of services is available wherever there is GSM coverage. 1.2.2
Disadvantages
Any radio environment is hostile, and the GSM air interface is no exception. It is susceptible to problems that face any radio system, such as interference and variations in signal strength. These matters require technical solutions and careful network planning to minimize their effects. Additionally, while mobility makes cellular networks attractive to subscribers, it creates problems for the networks themselves. As a mobile moves, or roams into other networks, its whereabouts need to be monitored for call routing purposes, and its power needs to be monitored to ensure it is transmitting neither too much nor too little. Far more signalling needs to take place in a mobile network than in the fixed network. The radio spectrum is a finite resource and its availability is problematic. The shortage of available spectrum increases its cost when auctions are held, and these costs are inevitably met by the end user. Finally, a huge infrastructure is required to provide good coverage. This includes the base station transmitters and receivers and all their associated equipment, as well as the acquisition of the base station site itself.
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Air Interface temporary ‘wires’ hostile environment
Roaming
Visited Network
Advantages: mobility roaming Home Network
Disadvantages: radio environment locating the mobile spectrum costs infrastructure
Figure 2 Mobility Considerations MB20/S1/v9.1
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GSM ORGANIZATIONS AND STANDARDS BODIES 2.1
ETSI and 3GPP
The European Telecommunications Standards Institute (ETSI) was founded in 1988. ETSI plays a comprehensive role in developing standards and other technical documentation for telecommunications, IT and broadcasting. ETSI has played a major part in the development of standards for 2G mobile phone systems. Until 2000, a number of working groups known as Special Mobile Groups (SMG) were responsible for separate aspects of GSM technology, but in July 2000 all GSM work was transferred to the 3rd Generation Partnership Project (3GPP). 3GPP was formed in 1998 to work towards standardization of 3G systems, and consists of standards bodies from around the world. 2.2
The GSM MoU Association
The GSM Memorandum of Understanding (MoU) was signed in 1987 by 15 signatories representing organizations from 13 countries. The aim of the MoU is to look after members’ interests. These include such issues as roaming, billing and accounting procedures, legal issues, and worldwide standardization issues. Originally, the MoU only comprised European members. In 1992, the Australian operator Telstra became the first non-European signatory, reflecting the worldwide appeal of GSM. The MoU was formally registered as an Association in 1995. It now has several hundred members from many countries including licensed network operators and regulatory bodies.
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Standards Bodies Service Providers Administrations Manufacturers Researchers Users
European Telecommunications Standards Insitute (ETSI) www.etsi.org
TETRA
DECT
Others
3rd Generation Partnership Project (3GPP) www.3gpp.org
GSM
EDGE
UMTS
Others
GPRS
Regulatory Bodies Interested Parties Operators
GSM Memorandum of Understanding (MoU) www.gsmworld.com
Roaming
Charging
Security
Others
Figure 3 GSM Organizations and Standards Bodies MB20/S1/v9.1
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3GPP RELEASES AND NUMBERING SCHEMES 3.1
Releases
The table in Figure 4 shows the 3G and GSM releases. The term Release 2000 was temporary and most of its constituent parts became Release 4, although some became Release 5. 3.2
Numbering Schemes
All 3G and GSM specifications have a numbering scheme comprising four or five digits, the first two digits of which define the series. In the former case using four digits the second two digits are used for the 01 to 13 series, while in the latter example, using five digits, the three further digits are for the 21 to 55 series.
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GSM/3G Releases GSM Release 3G Release Name
Frozen
Phase 1
Ph1
January 1990
Phase 2
Ph2
October 1995
Phase 2+ 1996
R96
February 1997
Phase 2+ 1997
R97
December 1997
Phase 2+ 1998
R98
February 1999
Phase 2+ 1999
R99
March 2000
R99
March 2000
R00
–
Release 2000
R00
–
Phase 2+ R4
Release 4
Rel-4
March 2001
Phase 2+ R5
Release 5
Rel-5
June 2002
Phase 2+ R6
Release 6
Rel-6
–
Release 1999 Phase 2+ 2000
Numbering Schemes GSM before Rel-4:
01 Series to 11 Series
GSM Rel-4 and after:
41 Series to 55 Series
3G/GSM R99 and later:
21 Series to 35 Series
Figure 4 3GPP Releases and Numbering Schemes MB20/S1/v9.1
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SECTION 1 QUESTIONS 1
Outline the principal advantages in the use of a digital, rather than analogue, mobile network.
2
List the disadvantages inherent in a mobile network compared to a fixed network.
3
List the organizations and standards bodies associated with GSM, and outline their functions.
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SECTION 2
GSM SERVICES
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CONTENTS 1
Network Service Provision 1.1 Operators 1.2 Service Providers 1.3 Virtual Operators
2.1 2.1 2.1 2.1
2
GSM Services 2.1 Introduction 2.2 Bearer Services and Data Rates 2.3 Teleservices 2.4 Supplementary Services 2.5 Value Added Services (VAS)
2.3 2.3 2.5 2.11 2.13 2.15
3
Messaging 3.1 Short Message Service (SMS) 3.2 Cell Broadcast Service (CBS) 3.3 Enhanced Messaging Service (EMS) 3.4 Multimedia Messaging Service (MMS) 3.5 Comparison Between MMS and SMS
2.17 2.17 2.19 2.21 2.23 2.25
4
Location Services (LCS) 4.1 Introduction to LCS 4.2 LCS Positioning Methods
2.27 2.27 2.27
5
Enabling Technologies 5.1 Service Enablers and Toolkits
2.29 2.29
6
Section 2 Questions
2.31
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OBJECTIVES At the end of this section you will be able to: • • • • • • • •
• • •
differentiate between network operators, service providers and virtual operators state what is meant by bearer services, teleservices, supplementary services and Value Added Services (VAS) explain the differences between circuit switching and packet switching state what data rates may be achieved in GSM and suggest how these may be increased describe the basic differences between HSCSD, GPRS and EDGE list the teleservices offered by a modern GSM network give examples of VAS describe the features of the messaging services used in GSM: Short Message Service (SMS), Enhanced Messaging Service (EMS) and Multimedia Messaging Service (MMS) explain how MMS differs from SMS in the basic nature of its delivery discuss the basic features of Location Services (LCS) state what toolkits and enablers are required to achieve effective service provision in GSM
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NETWORK SERVICE PROVISION 1.1
Operators
The operator is responsible for building and maintaining the network. A user can subscribe to an individual network either directly or via a service provider. 1.2
Service Providers
The service provider buys air time from the operator and then sells this air time to the users. Once subscribed to a network via a service provider, the user can make and receive calls. The bills for that subscription come from the service provider, not the operator. (Figure 1). 1.3
Virtual Operators
Virtual operators do not own their own infrastructure but utilize the infrastructure of existing network operators, thereby giving the illusion of having their own network. The virtual operator pays the network operators for use of their infrastructure. The bill for air time will be produced by the operator and passed to the virtual operator, who forwards it to the user. (Note, the network operator will take a percentage of the cost of a call).
2.1
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Virtual Operator Bill for air time
Payment for lease of £ infrastructure
Bill
Operator Payment £ for air time Bill for air time
Bill for air time
Service Provider
Bill for air time
Figure 1 Operators, Virtual Operators and Service Providers MB20/S2/v9.1
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GSM SERVICES 2.1
Introduction
All telecommunication networks offer services to their users. In GSM these services can be grouped into three main areas: • bearer services • teleservices • supplementary services Bearer service are the means by which the user information, or ‘traffic’, is transferred from source to destination. The bearer is analogous to a ‘pipe’ through which the information passes. A teleservice defines what the network allows the user to do with the bearer service: it provides end-to-end communication using the network’s bearer service. This provides the full capability for communication between users. To summarize, the teleservice is the information that is carried by the bearer service. Supplementary services complement or enhance the basic services. They are not offered to the subscriber as stand-alone services, but only as supplements to existing teleservices.
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Bearer Service Teleservice
Plus Supplementary Services
Teleservice Bearer Service
GSM Network
Transit Network
Terminating Network
Figure 2 Telecommunication Services MB20/S2/v9.1
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2.2
Bearer Services and Data Rates
2.2.1
Circuit-Switched and Packet-Switched Bearers
The type of traffic that a bearer carries may be speech, data, fax, or SMS. Data rates may vary considerably according to the technologies used. Two switching techniques are employed within most modern GSM networks: circuit switching and packet switching. In a circuit-switched connection, the circuit between users remains intact for the duration of a call. No one else is able to use this circuit while the connection is maintained. This is true whether or not data is being transmitted. With a packet-switched connection, data is broken down into ‘packets’ of data at one end of the circuit and reassembled at the other. Each packet is individually addressed and individually transits the network, so it is possible to have packets from different users utilizing the same links. In other words, while packets from one user are being transmitted, the circuit is still available to other users, and resources are not being utilized when there is no data being transmitted.
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a) Circuit Switching C A
Sorry all the lines are busy
D
B
b) Packet Switching
Figure 3 Circuit Switching and Packet Switching MB20/S2/v9.1
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2.2.2
Enhancements to GSM Bearer Rates
Basic circuit-switched data rates are slow in GSM, running at anything from below 2.4 kbit/s to around 9.6 kbit/s. However, the introduction of new technologies has increased this basic rate, and it is expected to increase still further as networks evolve towards 3G. These enhancements are: • 14.4 kbit/s data • High Speed Circuit Switched Data (HSCSD) • General Packet Radio Service (GPRS) • Enhanced Data rates for Global Evolution (EDGE) 2.2.3
14.4 kbit/s Data
By modifying the existing transmitted data, the 9.6 kbit/s rate can be increased to 14.4 kbit/s. Both of these rates are based on a single channel being allocated to a single user, using circuit switching. 2.2.4
High Speed Circuit Switched Data (HSCSD)
HSCSD offers the highest data rates achievable over the air interface in circuitswitched GSM. By granting more than one channel simultaneously to a single user (up to a maximum of four), bit rates can be increased from 14.4 kbit/s to a maximum of 57.4 kbit/s. The higher data rates offered by mobiles supporting HSCSD can potentially improve download times for Wireless Application Protocol (WAP)-enabled terminals, but this depends on the number of channels that can be allocated to a user. As shown in the diagram, 14.4 kbit/s offers the user a single channel. However, in HSCSD, multiple channels are allocated, though this will appear to the user as if a single channel with a higher data rate is being used. The disadvantage of HSCSD from the operator’s point of view is that if a large number of users wants higher data rates, hence more channels, the number of subscribers that can be supported at such times diminishes exponentially.
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a) 14.4 kbit/s 14.4 kbit/s 14.4 kbit/s
Data Call A
Cell Capacity
Data Call B
b) HSCSD 14.4 kbit/s (57.6 kbit/s) Higher Data Rate
Cell Capacity
Data Call
Figure 4 14.4 kbit/s and HSCSD MB20/S2/v9.1
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2.2.5
General Packet Radio Service (GPRS)
GPRS, known as a Generation 2.5 (2.5G) technology, is a packet-switched bearer service that provides mobile access to data applications, such as the Internet, intranets and X.25 networks, to users on demand. In GPRS, internet and GSM technologies are united, offering new applications and increased bit rates with an efficient use of network and radio resources. As a packet-switched technology, GPRS permits numerous users to exchange data over the same radio resource, and any new user will also be accepted to share that resource, which offers significant capacity benefits to network operators and subscribers. The only criterion is that all users do not try to send data at the same instant in time. GPRS operates a mechanism that ensures this concurrent sending does not occur. Sometimes referred to as an ‘always on’ service, GPRS allows subscribers to be constantly online while their phone is switched on, only being charged when they are transmitting or receiving data. GPRS is optimized for ‘bursty’ data transfer: that is, web browsing and information/application download, e-mails or multimedia messaging. It is not used for the transmission of speech. Theoretically, GPRS increases data rates to a maximum of 171.2 kbit/s. However, data rates of around 30 kbit/s are more usual. 2.2.6
Enhanced Data Rates for Global Evolution (EDGE)
EDGE is a 3GPP Release 4 technology that, by altering the way data is transmitted across circuit-switched networks, increases the data rate across the air interface to 384 kbit/s, significantly higher than HSCSD or GPRS.
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Internet
GSM/GPRS Network
X.25
Intranet
GPRS Features: packet-switched network multiple users share resources ‘bursty’ traffic access to other packet-switched networks ‘always on’ connection billing based on throughput access via different terminal types – phone – laptop (via mobile or GPRS data card) – Personal Digital Assistant (PDA) Figure 5 GPRS MB20/S2/v9.1
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2.3
Teleservices
A teleservice is a service to which a user subscribes, and which the network carries across its bearers. GSM offers a range of teleservices. These are grouped into categories, as follows: • speech transmission • Short Message Service (SMS) • facsimile transmission • Voice Group Service (VGS) Speech transmission includes telephony and emergency calls. Transmissions are in the form of digitized speech and audio tones used for signalling. SMS includes point-to-point, which provides for the transmission of short messages from a service centre to the mobile. It also incudes the Cell Broadcast Service (CBS), which is used for the transmission of a short message from a service centre to all users in the area of the base station. Facsimile transmission facilitates alternate speech and fax, or automatic fax. Both support the use of ITU-T Group 3 fax, with automatic fax supporting the autocalling and answering mode only. VGS provides for group calls or a broadcast service. Group calls allow for transmissions to predefined groups of users; the broadcast service transmits to all users in a specific area. This service is a Release 4 enhancement.
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Short Message Service (SMS) 160 Characters
Fax
abc
1
def
2
ghi
3
Emergency Calls
Voice Group Service (VGS) Cell Broadcast Service (CBS) 93 Characters
1001011
Speech Figure 6 GSM Teleservices MB20/S2/v9.1
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2.4
Supplementary Services
While the most basic teleservices may include services such as speech, fax and data, the network may want to make its overall service appear more attractive to subscribers. Supplementary services, then, are an enhancement to basic teleservices. Common supplementary services include: • barring of outgoing calls • barring of incoming calls • calling line ID • call divert • call forwarding These and other services are achieved with a software platform within the network. It is usual for the networks to offer a wide range of additional services.
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Supplementary Services e.g. Call Divert Calling Line ID Call Forwarding
GSM Network Software Function
Additional features provided by the network
Figure 7 Supplementary Services MB20/S2/v9.1
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2.5
Value Added Services (VAS)
Value Added Services (VAS) are additional to basic telecommunication services. Some network operators may refer to a service as being value-added and so charge for it, while others will regard the same service as a supplementary service and not charge. VASs offered will differ from network to network and country to country, but typical services will fall into the following categories. A Call Answering Service Examples of a call answering service are GSM call back and voicemail. In the call back service a fixed line telephone is barred from calling a mobile. The fixed-line user may dial a predefined number to access the system, followed by their own fixed-line number for the mobile to call back. Voicemail enables the user to receive voice messages. A short message is normally sent to the mobile indicating the caller, date and time of the message along with the number to dial to retrieve the message. A Call Management Service Examples of this service are Calling Line Identification Presentation (CLIP), where the caller’s number is displayed along with their name (if already held by the mobile), and call waiting and holding, where the user is alerted to a second call when they are already using the service. Users may then either hold the present call and answer, or switch between the two. An Enhanced Communication Service This may be a speech/data SMS service whereby the caller dials the service number and leaves a spoken message, the message being delivered by SMS. Restrictive Call Service Some networks in some countries list these as VAS although no charge is made, while in the UK, for example, they are considered as supplementary services. Such services may include call barring of specific incoming or outgoing calls. Information Services Information services may include lottery results, horoscopes, and travel or restaurant guides.
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Call
Call Back
al
***
*
Inform
GSM Network
PSTN Di
Barred
um +M o bil e + H o m e N
C a ll B a c k
b er
Enhanced Communication Service Speech
SMS Delivery
Network
Information Services Lottery Results
Network
Horoscopes
Guides
Figure 8 Value Added Services (VAS) MB20/S2/v9.1
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MESSAGING 3.1
Short Message Service (SMS)
The GSM SMS allows for the transfer of short messages between a mobile and a Short Message Service Centre (SMSC) for onward transmission to a second mobile or a destination address on some other type of network. These other destinations may be telex, fax, e-mail or speech addresses. A digital satellite television unit may also send and receive SMSs. These messages can be up to 160 characters in length, although more than one message may be concatenated to form a longer message. The SMS point-to-point service can be mobile originated or mobile terminated. As part of the basic service an acknowledgement is provided to the sending terminal indicating that the message has been sent. This does not indicate that it has arrived. The sender may request notification of delivery (or not), but this does not mean that it has been read. The sender of the SMS may indicate an expiry time during which the SMSC will try to deliver the message, otherwise a default time of 24 hours will be used. Developments in SMS have resulted in a new range of applications such as televoting and group SMS messaging, which allows predefined groups of users to receive, for example, an invitation to a party, meeting or audioconference. In effect this is a voice group chat facility whereby the recipients of the SMS will receive the number of the sender but also a conference or chat number which, when dialled, connects all users within the group.
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Message store for John: Call me ASAP on mobile. Fred
SMSC 2 Submit
Message
1 Submit
Deliver 3 Message
GSM
Message
Deliver 4 Message
Inbox:
Write:
John, call me ASAP on mobile. Fred
John, call me ASAP on mobile. Fred
Figure 9 SMS Message MB20/S2/v9.1
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3.2
Cell Broadcast Service (CBS)
CBS is similar in concept to teletex in that it allows a number of unacknowledged general information messages to be broadcast to all receivers in a particular area. It is a point-to-multipoint service. Messages are broadcast in the downlink direction only and are addressed to all active mobiles within a cell or group of cells. CBS can be used to send information on services relating to specific geographical areas such as tourist information and local services. Within a GSM network, individual CBS messages may be addressed to one or more CBS areas.
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ST 016D 1
ST 016D 1
ST 016D 1
Cell Broadcast Provider
Cell Broadcast Centre
ST 016D 1
Figure 10 Cell Broadcast MB20/S2/v9.1
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3.3
Enhanced Messaging Service (EMS)
The Enhanced Messaging Service (EMS) is based upon SMS, but adds formatting to the text. This formatting allows messages to contain simple animations, pictures, short simple tunes, and formatting of the text itself. It is possible to download new sounds and images from WAP sites, or to receive them in a message. For example, text can be aligned left, centre or right. The font can be normal, large, or small, and the style can be normal, bold, italic, or underlined. Pictures will be only black and white (not greyscale), and will be 16 x 16 or 32 x 32 pixels (small or large), or variable. Animations may be predefined; however, they are not transmitted across the air interface. The transmission will indicate the animation to be used and its position in the short message. How this is achieved is manufacturer specific. User-defined animations will comprise four pictures, either small (8 x 8) or large (16 x 16). These pictures will be transmitted across the air interface. Short simple tunes can be predefined or user defined. In the former case there are 10 sounds stored, and therefore these are not transmitted across the air interface. The sender will select the sound to be played. In the latter case, the sender can compile a tune using an iMelody format. These are transmitted and comprise 128 bytes. EMS messages may be received or sent by any mobile that supports the EMS service. If EMS is not supported by a mobile that has received such a message, the pictures and sounds will be deleted, the formatting removed and the message displayed as a simple SMS.
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Aligned
Left
Centred
Right
Normal
Large
Small
BOLD
Italic
Underlined
16 x 16 or 32 x 32
Pictures
8x8
Animation
16 x 16
Animations
Tunes
Figure 11 Enhanced Messaging Service (EMS) MB20/S2/v9.1
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3.4
Multimedia Messaging Service (MMS)
MMS provides a messaging medium that includes still images, video, sound clips and text, or any combination of these. The messages are created by the sender according to predefined templates and sent to the recipient in the form of a ‘presentation’, which is like a slide show. When the recipient chooses to view the message the presentation is played back to them. Multimedia messages can be sent to: • other MMS-enabled terminals • terminals that do not support MMS – the message can be viewed at a website • e-mail addresses The network can determine from the recipient’s address whether the end terminal can support MMS. It will also know what content the terminal can support. Therefore, content can be formatted accordingly. Terminals that do not support MMS receive an SMS indicating that an MMS has been received for them, giving them a web site address where they can go to collect the message. Multimedia messages can be received from other MMS-enabled terminals or thirdparty service providers. Multimedia messages can be sent either to a single recipient or to multiple recipients. Multiple recipients could be different types of end terminal, e.g. e-mail, WAP-enabled terminal, and fully MMS-enabled terminal.
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Wray Castle
Other MMS-enabled mobile
You have received a multimedia message. To view the message go to web site address www.ournetwork.com
Wray Castle
Castle Wray
Originating Terminal
Non-MMS-capable mobile, via web site
E-mail Account Figure 12 Multimedia Messaging Service (MMS) MB20/S2/v9.1
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3.5
Comparison Between MMS and SMS
MMS can be compared to SMS in that it uses a store-and-forward mechanism; to the subscriber it appears to behave like SMS. However, the technology and delivery mechanisms are very different. Unlike SMS, where the SMSC residing in the sender’s network is responsible for delivery of the short message, in MMS the message is passed from the Multimedia Messaging Service Centre (MMSC) in the home network to a separate MMSC in the receiver’s network for onward transmission to the mobile. Only in the case of message delivery to an e-mail account is the MMS sent directly from the sending MMSC (via a number of servers/routers). MMS also differs from SMS in the way in which messages are sent to multiple recipients. With SMS, multiple SMSs are sent to the SMSC and then dispatched onwards to the recipients. In MMS, a single message with a number of addresses is sent to the MMSC. The MMSC then sends the messages on to the recipients.
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SMS
CU L8R
CU L8R
Sender’s mobile
Sender’s network
Receiver’s mobile
MMS
MMS mobile-enabled Receiver’s network
Sender’s mobile
Sender’s network
e-mail account Figure 13 SMS and MMS MB20/S2/v9.1
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LOCATION SERVICES (LCS) 4.1
Introduction to LCS
LCS facilitates the positioning of a mobile by the network in order that subscribers can receive information and services based on their location. There are four different categories of LCS client: VAS clients, which use LCS to support VAS; Public Land Mobile Network (PLMN) operators, who may use it to improve Operations and Maintenance (O&M) functions or supplementary services; the emergency services, to assist subscribers by responding quickly to emergency calls; and lawful intercept LCS clients, which may use LCS to perform services that are required or sanctioned by law. 4.2
LCS Positioning Methods
LCS provides a flexible and generic architecture that is capable of supporting all positioning methods. Specific support is provided for the following basic positioning methods: • Time Of Arrival (TOA) • Enhanced Observed Time Difference (E-OTD) • Global Positioning System (GPS) Using these methods it will be possible to identify and report the current location of the user’s terminal. It will do this using a standard format such as geographical coordinates.
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PLMN Clients VAS Clients enhance network operations location-assisted handover traffic engineering
list of restaurants places of interest navigation application
Positioning Methods: Time Of Arrival (TOA) Enhanced Observed Time Difference (E-OTD) Global Positioning System (GPS)
E.911 999, 112, 911 weather warnings
Police FBI
Emergency Services Clients
Lawful Intercept Clients
Figure 14 Users of LCS MB20/S2/v9.1
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5
ENABLING TECHNOLOGIES 5.1
Service Enablers and Toolkits
In order to offer VAS, GSM operators may employ a number of ‘enablers’, or ‘toolkits’. These provide means by which new services can be introduced into the network, and then delivered and presented to subscribers. Such technologies include: • Wireless Application Protocol (WAP) • Intelligent Networks (IN) • Customized Applications for Mobile network Enhanced Logic (CAMEL) • Mobile Execution Environment (MExE) • SIM Application Toolkit (SAT) WAP WAP is a protocol – a ‘language’ that enables devices to talk to one another and understand what each is saying. WAP can therefore be defined as a communication protocol and application environment that allows users with mobile devices to access information and services over wireless networks. IN and CAMEL INs are used by operators to control services, to facilitate the introduction of new services into the network, and to streamline them to a customer’s needs. When INs were first used in GSM, it was realized that there were problems associated with mobility and roaming. CAMEL – the term given to IN implementation in the mobile environment – was developed to solve this problem. CAMEL makes services and features available to subscribers while they are roaming. It defines the interactions between the home network and the visited network, specifying what information is exchanged and the procedures involved. MExE MExE is an application environment that resides within the mobile. It enables thirdparty software developers to create applications that can run on any terminal, in a standardized way, irrespective of the vendor or the network. It enables after-market applications such as games to be downloaded onto the mobile. MExE also provides sophisticated user menus that enable users to customize their mobile in terms of how they interact with it and what they can do with it. SIM Application Toolkit The SAT enables customer- or network-specific applications to run on the SIM card and interact with the capabilities of the mobile phone.
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GSM/UMTS Network
‘Toolkits’/‘Enablers’ MExE
WAP
SAT
CAMEL
Services MMS/EMS
Downloads
Entertainment
LCS
Figure 15 Toolkits and Enablers MB20/S2/v9.1
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6
SECTION 2 QUESTIONS 1
Outline in general terms the range of GSM bearer services.
2
State the advantages gained by the addition of GPRS as a bearer service.
3
List the GSM teleservices
4
Suggest some VAS that may be offered by network operators.
5
Outline the service features applicable to: a b c d
2.31
CAMEL MMS MExE LCS
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SECTION 3
NETWORK ARCHITECTURE
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CONTENTS 1
GSM Network Overview
3.1
2
The Mobile Station (MS) 2.1 Introduction 2.2 The Subscriber Identity Module (SIM) Card 2.3 GSM/GPRS Identities 2.4 GPRS Mobile Classes
3.3 3.3 3.5 3.7 3.11
3
The Base Station System (BSS) 3.1 Introduction 3.2 The Base Transceiver Station (BTS) 3.3 The Base Station Controller (BSC) 3.4 Interfaces
3.13 3.13 3.13 3.15 3.15
4
The Network Switching System (NSS) 4.1 Introduction 4.2 Gateway MSC (GMSC) Functions 4.3 Transcoder and Rate Adaptation Unit (TRAU)
3.17 3.17 3.17 3.19
5
GSM Databases 5.1 The Home Location Register (HLR) 5.2 The Visitor Location Register (VLR) 5.3 The Authentication Centre (AuC) 5.4 The Equipment Identity Register (EIR)
3.23 3.23 3.23 3.25 3.25
6
Operations and Maintenance 6.1 Operations and Maintenance Centre (OMC) 6.2 Network Management Centre (NMC)
3.27 3.27 3.29
7
GPRS System Architecture 7.1 Introduction 7.2 Packet Control Unit (PCU) 7.3 Serving GPRS Support Node (SGSN) 7.4 Gateway GPRS Support Node (GGSN)
3.31 3.31 3.33 3.33 3.33
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CONTENTS 8
Messaging Architecture 8.1 Short Message Service (SMS) 8.2 Multimedia Messaging Service (MMS)
3.35 3.35 3.37
9
Group Calls and Broadcast Architecture 9.1 Group Call Register (GCR) 9.2 Cell Broadcast Centre (CBC)
3.39 3.39 3.41
10
CAMEL Architecture 10.1 GSM Service Control Function (gsmSCF) 10.2 GSM Service Switching Function (gsmSSF) 10.3 GPRS Service Switching Function (gprsSSF) 10.4 GSM Specialized Resource Function (gsmSRF)
3.43 3.43 3.43 3.43 3.43
11
Section 3 Questions
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OBJECTIVES At the end of this section you will be able to: • • • • • • • • • •
outline the general configuration of GSM network elements and interfaces describe the Mobile Station (MS) and state the GPRS mobile classes state the functions performed by the Subscriber Identity Module (SIM) list the identities used within GSM and GPRS describe the role of the Base Station System (BSS) and its constituent parts describe the role of the Network Switching System (NSS) and its constituent parts including the network databases state the overall functions of the operations and maintenance elements name and state the functions of the GPRS network elements list the architectural elements required to facilitate messaging services, broadcast services and group call services and state their basic functions name the CAMEL entities and describe their functions
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1
GSM NETWORK OVERVIEW The GSM network can be considered as comprising a number of distinct areas: • Mobile Station (MS) • Base Station System (BSS) • Network Switching System (NSS) The NSS consists of the Circuit-Switched (CS) and Packet-Switched (PS) domains. As shown in Figure 1, the CS domain connects to other PLMNs for circuit-switched services such as speech, and to the Public Switched Telephone Network (PSTN) and Integrated Services Digital Network (ISDN). The PS domain connects to Internet Protocol (IP) networks such as the Internet and internets, the packet-switched elements of other PLMNs, and VAS providers.
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Other PLMN (GSM)
PSTN/ ISDN
GSM (CS) elements
MS
Base Station System
Network Switching System GPRS (PS) elements
VAS Provider
Other PLMN (GPRS)
intranet
Internet
Figure 1 GSM Network Overview MB20/S3/v9.1
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2
THE MOBILE STATION (MS) 2.1
Introduction
The GSM mobile phone, known as the Mobile Station (MS), consists of two elements, each with its own functionality. These are the Mobile Equipment (ME), which incorporates hardware and software functions to allow it to operate over the air interface, and the Subscriber Identity Module (SIM) card. Neither on its own offers the user much in the way of a useful phone system, but bring them together and they operate as one to provide a basic telephone service, with supplementary services and, in the case of some phones, features such as a digital camera and colour display.
3.3
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Welcome to the Network
NETWORK
+ Mobile Equipment (ME)
= Subscriber Identity Module (SIM)
Mobile Station (MS)
Figure 2 The Mobile Station (MS) MB20/S3/v9.1
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2.2
The Subscriber Identity Module (SIM) Card
The SIM performs vital tasks in providing the user with access to the network. Possibly the most important is authentication, the process of validating the subscriber and, if necessary, the MS prior to use of the network. Authentication is done by means of what is known as a cryptographic challenge response mechanism. For security reasons, this procedure is carried out entirely on the SIM card. Other tasks performed by the SIM mainly involve assisting the ME in its operation. For example, it stores network parameters that the equipment refers to during the initial cell selection process when the mobile is turned on. The SIM card is removable and stores such details as: • phone book • International Mobile Subscriber Identity (IMSI) • Temporary Mobile Subscriber Identity (TMSI) • Cipher Key (Kc) (circuit-switched mode) • Cipher Key GPRS (KcGPRS) (packed-switched mode) • Authentication Key (Ki) • Location Area Identity (LAI) • list of carriers for cell selection • Packet TMSI (P-TMSI) • forbidden PLMNs • services available, e.g. GPRS
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IMSI International Mobile Subscriber Identity
TMSI Temporary Mobile Subscriber Identity
Phone Book Kc Cipher Key
SIM Card P-TMSI Packet TMSI KcGPRS Cipher Key GPRS Ki Authentication Key LAI Location Area Identity
List of Carriers for Cell Selection
Figure 3 The SIM Card MB20/S3/v9.1
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2.3
GSM/GPRS Identities
There are a number of identities associated with the subscriber and their terminal. These are described below. 2.3.1
Mobile Subscriber ISDN Number (MSISDN)
A user’s MSISDN is, in effect, their telephone number. It consists of the Country Code (CC), relating to the country in which the MS is registered; the national mobile number, detailing the National Destination Code (NDC), identifying the PLMN; and the Subscriber Number (SN). Every subscriber to a GSM network will be identified uniquely by this number. The MSISDN is a maximum of 15 digits in length. 2.3.2
International Mobile Subscriber Identity (IMSI)
Each subscriber is allocated a unique IMSI, which is held on the SIM card. All subscriber-related information is associated with the IMSI in the network databases; without it, the MS cannot operate. The IMSI cannot exceed 15 digits. It comprises: • Mobile Country Code (MCC) (3 digits) • Mobile Network Code (MNC) (2–3 digits) • Mobile Station Identification Number (MSIN) (remaining digits) The MCC details the subscriber’s country of residence; the MNC details their home PLMN; and the MSIN identifies the subscriber within that PLMN. 2.3.3
Temporary Mobile Subscriber Identity (TMSI)
To ensure that the subscriber’s identity remains secure, the IMSI may only be transmitted across the air interface in exceptional circumstances. Instead, a secure Temporary Mobile Subscriber Identity (TMSI) is allocated, which subsequently is used for identification. 2.3.4
Packet TMSI (P-TMSI)
The P-TMSI performs the same function as the TMSI, and has the same characteristics, but is used in packet-switched operation. 3.7
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a) Mobile Subscriber ISDN Number (MSISDN)
Country Code (CC) 1–3 digits
National Destination Code (NDC)
Subscriber Number (SN)
12–14 digits (15 – NCC) National Mobile Number (NMN)
b) International Mobile Subscriber Identity (IMSI)
Mobile Country Code (MCC) 3 digits
Mobile Network Code (MNC) 2–3 digits
Mobile Subscriber Identification Number (MSIN) remaining digits
c) Temporary Mobile Subscriber Identity (TMSI) and Packet TMSI (P-TMSI) * *
TMSI
1 1 P-TMSI
Structures may be agreed between manufacturer and operator 32 bits (4 octets) in length
Figure 4 GSM Identities MB20/S3/v9.1
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GSM System Overview
2.3.5
Mobile Station Roaming Number (MSRN)
The structure of the MSRN is the same as MSISDN, namely: MSRN = CC + NDC + SN Here, the SN does not identify a subscriber. Instead, it is used to address a Mobileservices Switching Centre/Visitor Location Register (MSC/VLR) in the network. 2.3.6
International Mobile Equipment Identity (IMEI)
The IMEI number is fixed to the ME and is also known to the Equipment Identity Register (EIR). The structure of the IMEI is as follows: IMEI = TAC + FAC + SNR + S/W Ver where: TAC = Type Approval Code (up to 6 digits) FAC = Final Assembly Code (2 digits) SNR = Serial Number (of up to 6 digits) S/W Ver = Software Version Number (1 digit)
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a) Mobile Station Roaming Number (MSRN)
Country Code (CC) 1–3 digits
National Destination Code (NDC)
Subscriber Number (SN)
12–14 digits (15 – NCC)
b) International Mobile Equipment Identity (IMEI)
Type Approval Code (TAC) 1–6 digits
Final Assembly Code (FAC) 2 digits
Serial Number (SNR) 1–6 digits
Software Version Number 1 digit
Figure 5 Other GSM Identities MB20/S3/v9.1
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GSM System Overview
2.4
GPRS Mobile Classes
GPRS mobile classes define the three modes of operation in which a GPRS MS can function. The mode of operation depends upon the services that the MS is attached to, i.e. only GPRS or both GSM and GPRS. The three mobile classes are defined as class-A, class-B and class-C. 2.4.1
Class-A
Class-A MSs support simultaneous attach (connection), activation, monitor, invocation, and traffic. Thus a subscriber using a class-A MS can make and/or receive calls on GSM and GPRS simultaneously, subject to Quality of Service (QoS) requirements. 2.4.2
Class-B
Class-B MSs support simultaneous attach, activation, and monitor. A class-B MS will, however, only support limited simultaneous invocation such that GPRS connections will not be cleared down due to the presence of circuit-switched traffic. Instead, the GPRS connection is ‘busy’ or ‘held’. Simultaneous traffic is not supported by a class-B MS. The subscriber can make or receive calls on either of the two services sequentially, but not simultaneously. The selection of the appropriate service is performed automatically, with GSM having priority. 2.4.3
Class-C
Class-C MSs only support alternate use. If both services are supported, then a classC MS can only make and/or receive calls from the manually or default-selected service. The status of the service not selected is ‘detached’ or ‘not reachable’. In addition, the transmission and reception of SMS messages by a class-C MS is optional. It should be noted that non-voice-only MSs do not have to (but may) support emergency calls.
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Class-A
–
Simultaneous attach Simultaneous GSM and GPRS operation CS
GSM/GPRS Network
and
PS
Class-B
–
Simultaneous attach GSM or GPRS operation CS
GSM/GPRS Network
or
PS
Class-C
–
Attach for GSM or GPRS
CS
GSM/GPRS Network
or
PS
Figure 6 GPRS Mobile Classes MB20/S3/v9.1
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3
THE BASE STATION SYSTEM (BSS) 3.1
Introduction
The BSS is the MS’s connection point with the rest of the world. A user’s perception of an MS is that of a fixed-network phone, where whenever the phone is picked up a connection to the local switch is obtained. In the mobile environment, however, these connections have to be established every time communication is required. This is the BSS’s responsibility. The BSS consists of: • Base Transceiver Station (BTS) • Base Station Controller (BSC) • Packet Control Unit (PCU) (for GPRS operation) The BSS’s area of responsibility is determined by the radio coverage achieved from the various BTS sites. These areas may contain many mobiles, so the BTSs may have to support a large number of wireless connections. 3.2
The Base Transceiver Station (BTS)
The BTS is the part of the BSS that is responsible for all of the radio functions associated with the connection (see Figure 7). Each BTS provides an area of radio coverage known as a cell. The BTS provides functions such as: • generation of Radio Frequency (RF) carriers • modulation and demodulation of RF carriers • baseband data processing • error coding • ciphering • channel mapping
3.13
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BTS
NSS MS
BSS
BTS Functions generation of RF carriers transmit/receive modulation and demodulation of RF carriers baseband data processing error coding ciphering channel mapping
Figure 7 Base Transceiver Station (BTS) Functions MB20/S3/v9.1
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3.3
The Base Station Controller (BSC)
The BSC controls a number of BTSs. It handles signalling, traffic and operations and maintenance messages to and from all BTSs under its control. Signalling messages for mobiles are also passed via the BSC. The BSC has some switching functionality, which enables it to establish terrestrial to radio channel connections for traffic and signalling to mobiles. The switching functionality also enables it to carry out intra- and inter-BTS handovers without the intervention of the MSC (see Figure 8). 3.4
Interfaces
The air interface is denoted Um. This is the wireless connection between the MS and BTS. The A-bis interface connects the BTS to the BSC. Its characteristics are similar to the A interface between the BSC and the MSC. The A interface exists between the MSC, generally referred to as the switch, and the BSC. The fundamental data rate for each channel is 64 kbit/s. This arises from the Pulse Code Modulation (PCM) process of sampling at a rate of 8000 samples per second and encoding each sample as an 8-bit word. The 64 kbit/s channels are then multiplexed with other channels to form the completed link frame. Note, however, that these 64 kbit/s channels may be split into 4 x 16 kbit/s subslots. Copper, fibre or point-to-point microwave links could be used for these links. Whatever the medium, the links are generally either European E1 bearers (2.048 Mbit/s) or North American T1 bearers (1.544 Mbit/s).
3.15
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CS
Um BTS
A E1/T1
NSS MS A-bis E1/T1 BSS
Gb
PS
BSC Functions establishes and clears radio connections power control timing control performs handovers
Figure 8 Base Station Controller (BSC) Functions MB20/S3/v9.1
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4
THE NETWORK SWITCHING SYSTEM (NSS) 4.1
Introduction
Where the BSC is connected to several BTSs on one side, it is connected to the NSS on the other. Its connection point with the NSS is the MSC. Between the BSC and the MSC, a Transcoder and Rate Adaptation Unit (TRAU) is used for converting GSM-encoded data into a suitable format for onward transmission, and vice versa. The MSC’s main function is switching: connecting mobile subscribers to other subscribers, fixed or mobile. Physically the switches may be no different to those of a digital telephone network system capable of switching many thousands of circuits. The MSC may have a large number of BSCs connected to it. There is, therefore, the potential for a very large number of subscribers to be within the MSC’s service area. A typical MSC will be able to cope with an area containing approximately 250,000 to 300,000 people. This could be a medium size city (of which not all will be subscribers). To successfully manage this potentially large number of subscribers the MSC must interface with a number of other devices, primarily the databases and other switching centres. 4.2
Gateway MSC (GMSC) Functions
So that GSM can support calls external to the home network it is important that there is some functionality within at least one of the switches that supports this. The GSM recommendations indicate that at least one of the switches must have this function. In reality, depending on the network size and configuration, all of the MSCs will have external connections. They are termed Gateway MSCs (GMSC).
3.17
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Other PLMN
PSTN/ ISDN BSS NSS TRAU
A
Databases
MSC Functions connects to many BSCs performs circuit switching collects billing information interfaces with databases interfaces with external networks
Figure 9 Mobile-services Switching Centre (MSC) Functions MB20/S3/v9.1
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4.3
Transcoder and Rate Adaptation Unit (TRAU)
For bandwidth reasons, speech across the air interface in GSM is coded at a bit rate of 13 kbit/s and data at 2.4, 4.8 and 9.6 kbit/s (although this can be increased). Over the A-bis interface the E1 bearer traffic channels are subdivided into 4 x 16 kbit/s subrate channels. For voice the traffic channel comprises 13 kbit/s vocoded speech (received over the air interface) and 3 kbit/s overhead, whilst a data traffic channel (assuming 9.6 kbit/s data) comprises 9.6 kbit/s data plus 6.4 kbit/s overhead. Within a GSM network the MSC is intended to be a modified ISDN switch, operating on 64 kbit/s circuits. A conversion between the air interface rates/format and 64 kbit/s ISDN rate/format therefore needs to take place. This conversion is performed by the TRAU. The TRAU is split into two parts, as illustrated in Figure 10. The first part is the transcoder, which is concerned with voice traffic. The second part is the rate adaptation unit, which is concerned with data traffic to and from the mobile. The transcoder converts the 13 kbit/s vocoded GSM speech plus 3 kbit/s overhead (16 kbit/s) into 64 kbit/s PCM speech, and vice versa. The rate adaptation unit, as its name suggests, rate adapts, for instance 9.6 kbit/s GSM data plus 3.4 kbit/s overhead (16 kbit/s) into 64 kbit/s and vice versa.
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TRAU GSM encoded speech
13k
PCM encoded speech
Voice
Voice Transcoding
3k
64k PCM
Data
data (GSM rate adaptation)
Data
Rate 9.6k 6.4k Adaptation
16k
16k
BSS
16k
16k
64k TS2
TS3
9.6k 54.4k
data (ISDN standard rate adaptation)
64k TS4
TS17
64k TS18
TS19
TRAU
E1/T1 Link
E1/T1 Link
Figure 10 Transcoding and Rate Adaptation MB20/S3/v9.1
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4.3.1
TRAU Location
The TRAU can be situated at the BTS, BSC, or at the switch site (though not as part of the switch). For minimum transmission costs, the most efficient transcoder position is at the switch site. (Figure 11). If the TRAU is located at the BSC, each 2 Mbit/s link can carry 30 x 64 kbit/s traffic circuits. If the TRAU is located at the MSC, each 2 Mbit/s link can carry 120 x 16 kbit/s traffic circuits.
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TRAU at BSC NSS BSS TRAU
A E1/T1 30 x 64 kbit/s
TRAU at MSC NSS
x4 BSS
TRAU
A E1/T1 120 x 16 kbit/s
Figure 11 TRAU Location MB20/S3/v9.1
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5
GSM DATABASES Within the GSM network there are four main databases: • Home Location Register (HLR) • Visitor Location Register (VLR) • Authentication Centre (AuC) • Equipment Identity Register (EIR) 5.1
The Home Location Register (HLR)
The HLR is the main network database. There is (logically) only one of these in any network. The information stored relates to all of the subscribers registered with that network. The presence of the information is independent of the location of the subscribers. The type of information stored includes: • MSISDN(s) • IMSI • current location (MSC address) • subscription levels (relating to roaming authority and supplementary services) • security parameters 5.2
The Visitor Location Register (VLR)
The VLR holds similar information to the HLR. However, the VLR is diverse; there is one associated with each MSC. The information contained in the VLR is temporary and relates to all subscribers in the MSC area only. When a subscriber moves out of an MSC area the database entry will be deleted. The VLR will also contain details of subscribers roaming in its network. The VLR will contain the additional parameters: • security parameters • MSRN • TMSI The VLR plays an important part in the signalling to the mobile during the early stages of the set-up including authentication, enabling ciphering and initial service requests. 3.23
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Circuit-Switched Domain
VLR
security parameters LAI MSRN TMSI MSISDN(s) IMSI MSC address subscription details security parameters
AuC
HLR
EIR
Packet-Switched Domain
VLR
Figure 12 HLR and VLR MB20/S3/v9.1
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5.3
The Authentication Centre (AuC)
The AuC stores Authentication Keys (Ki) and IMSIs on its database. The AuC also contains a random number generator and two algorithms. Together they are responsible for generating the security parameters known as a triplet. These triplets are stored in the HLR and VLR for each subscriber. 5.4
The Equipment Identity Register (EIR)
The EIR records and monitors the IMEI. The rationale behind the EIR is to discourage the theft of GSM equipment. The EIR comprises three lists: Black, White, and Grey. An ME that is on the Black List may have been stolen, or it may be faulty to the extent that it is causing problems within the network. Any equipment on the Black List will not therefore be given access to the network. MEs on the Grey List may have some minor fault (such as not giving correct responses during signal sequences) or may be old equipment that will not respond to new services offered by the network. The Grey List could be used to generate a letter to the equipment user explaining the problems. The White List contains all mobiles that are functioning correctly and cause no problem to network operation. For this anti-theft concept to work on a global basis, a Global EIR would be required.
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Authentication Key (Ki) IMSI algorithms random number generator
Circuit-Switched Domain
VLR
AuC EIR HLR
IMEI Black List – stolen or faulty Grey List – minor fault or old equipment White List – acceptable
Packet-Switched Domain
VLR
Figure 13 AuC and EIR MB20/S3/v9.1
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6
OPERATIONS AND MAINTENANCE An essential element of any telecommunication network is the ability to manage the machines that constitute the network. In GSM, network management comprises a two-tier hierarchy consisting of Operations and Maintenance Centres (OMCs), which are regional centres, and a single Network Management Centre (NMC). 6.1
Operations and Maintenance Centre (OMC)
The OMC is a computer with associated database, which connects to the BSS it is managing. It provides network controllers with a graphical interface through which the network can be managed. As OMC devices tend to be highly proprietary, the network contains elements for radio management (OMC-R) and switch management (OMC-S). The functions performed by the OMC can be divided into the following categories. Fault Management Fault management can be considered as the complete process of detecting a fault and tracing all activities through to clearing the fault. A fault reported by a customer may trigger an alarm. Event Management This process collates events occurring within the network. An ‘event’ may be a switch between a primary unit and a standby unit. Event management logs all such events. Configuration Management Configuration management allows the hardware and software network configuration to be changed. Network elements may be configured via either remote access from the NMC or the Man–Machine Interfaces (MMI) associated with the relevant network element. Performance Management This collates statistics relating to network performance so that resources can be allocated to appropriate locations, for example to alleviate congestion at particular points. Performance management may also be used to detect ‘sleeping elements’. For example a BTS may have stopped processing calls but may have not reported an alarm to say why. The performance management application is able to detect this since the call rate will have dropped to zero, i.e. far below expected performance parameters. Security Management Security management controls data to and from the OMC and checks data validity. Operator access to the OMC, the network elements it supports and also OMC functional areas may be controlled. For example, operators may be granted ‘Read Only’ access, or they may granted ‘Read/Write’ access.
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OM
C-
S
BSS
OMC-
R
OMC
events and alarms fault management performance management security management
Figure 14 OMC MB20/S3/v9.1
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6.2
Network Management Centre (NMC)
OMCs provide a regional view of the network elements and their performance. The NMC allows the entire network to be managed from a central point. While the NMC may not necessarily be concerned with an individual alarm from a radio, for example, it will have a top-level view that will allow for long-term planning. The NMC will be connected to OMCs and other network elements.
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trunk route management high-level alarms OMC assistance OMC communication
OMC
NMC
OM
C-
S
BSS
OMC-
R
OMC
Figure 15 NMC MB20/S3/v9.1
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7
GPRS SYSTEM ARCHITECTURE 7.1
Introduction
The addition of GPRS has required significant modifications to the GSM network architecture to enable it to handle both packet- and circuit-switched connections. Three new entities have been added: • Gateway GPRS Support Node (GGSN) • Serving GPRS Support Node (SGSN) • Packet Control Unit (PCU) These elements are connected with each other, and with the GSM network elements, via a series of interfaces that all carry the prefix G. These interfaces handle both traffic connections and signalling connections. GPRS shares some network resources with the GSM network, including the HLR, VLR, AuC and EIR. The HLR and the VLR have been modified to cater for GPRS since it is important that both GSM and GPRS networks are able to keep track of the mobile. By allowing access to these common units the information is maintained from centralized resources, making the effective management and interworking of the two systems easier. In some networks the Gs interface will not be used, however, so a user register may be held at the SGSN and the GGSN. GPRS supports applications based on standard data protocols. Therefore interworking has been defined in the specifications for connections to external IP networks. Interfaces to external networks require firewall functionality to prevent unwanted access. This may occur at the GGSN. Figure 16 shows the GPRS network, and how it interfaces to circuit-switched GSM.
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F
EIR AuC
C
H HLR
A
Gf
Gs (optional) Gr
Internet/ Intranet
Gr
Um
Gi
A-bis
Gb
Gn Gi
BTS Third-Party Service Provider
MS
Figure 16 GPRS Network Architecture MB20/S3/v9.1
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GSM System Overview
7.2
Packet Control Unit (PCU)
The PCU provides radio access control. It allocates radio channels for data transfer, ensures packets are the correct size for transmission over the radio interface, and makes QoS measurements in respect of the radio link with the user’s mobile. Functionally, the PCU sits with the BSC. Because the BSC terminates transmission links from the BTS, communication must exist between the BSC and the PCU. 7.3
Serving GPRS Support Node (SGSN)
Like an MSC, an SGSN is responsible for a service area containing a number of mobiles. Within this service area the main functions of the SGSN include the authorization and authentication of mobiles; ciphering of packets across the air interface; the routing of data packets to and from mobiles; and location management, noting the location of mobiles new to the service area and tracking their subsequent position within the service area. The SGSN also has charging functionality. It gathers data relating to a subscriber’s use of the radio network and passes this to the Charging Gateway Function (CGF) for processing and onward transmission to the network billing system. 7.4
Gateway GPRS Support Node (GGSN)
The GGSN performs the functions necessary to allow mobiles to communicate with external networks. For incoming calls it contains routing tables so that incoming packets of data can be tunnelled to the SGSN that is supporting the destination mobile. In addition, the GGSN is responsible for encapsulation, message screening, and address translation. It can also act as a firewall to prevent unwanted access. In respect of charging, the GGSN is responsible for gathering data relating to packets entering and leaving the network and its onward passage to the CGF.
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CircuitSwitched Domain
BSS
Gs
Gb
Gn Gi
Internet/ Intranets
PCU Basic Functions • packet segmentation • radio channel allocation • QoS measurements
GGSN Basic Functions • can act as a firewall • routes PDUs to appropriate SGSN • data/packet counting (for charging) SGSN Basic Functions • serves attached mobiles • mobility management • session management • interacts with GSM part • data/packet counting (for charging)
Figure 17 GPRS Functional Elements MB20/S3/v9.1
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GSM System Overview
8
MESSAGING ARCHITECTURE 8.1
Short Message Service (SMS)
All SMS messages, whether Mobile-Originated (MO) or Mobile-Terminated (MT), must pass through a Short Message Service Centre (SMSC). This has the effect of splitting the delivery of the message into two point-to-point procedures. GSM does not specify the functionality of the SMSC or the transport protocols that connect it to the GSM network. It simply identifies the information elements that must be passed between the mobile station and the SMSC. An SMS gateway function is used to connect the SMSC to the network. For MT messages, this gateway is similar in function to a GMSC; for MO messages, the gateway provides the interworking between GSM and the SMSC, which is still essentially a gateway process. It is important to note that only one SMSC is involved in receiving and forwarding short messages to the final recipient. This SMSC will reside in the sender’s network. This is in contrast to MMS, in which more than one service centre is involved, one residing in the sender’s network and another in the receiver’s network.
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a) Intra-network SMS Delivery
‘Store and Forward’
SM
S
Radio Access Network
Core Network
SMS
b) Inter-network SMS Delivery
SM
S
Radio Access Network
Core Network
Other Licensed Operator
SMS
Figure 18 SMS Architecture MB20/S3/v9.1
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8.2
Multimedia Messaging Service (MMS)
The main network elements essential for the support of MMS are shown in Figure 19. The diagram also shows the generic interfaces defined by 3GPP. However, the architecture used to support MMS, and its functionality, is likely to be highly operator specific. The Multimedia Messaging Service Centre (MMSC) is the main element concerned with MMS handling.¹ Its principal functions are the handling and storage of multimedia messages and message transfer between systems. It holds a database of MMS subscribers, which contains user-related information such as subscription levels and user profiles. It also has a facility for storing messages for a predefined maximum period of time. The MMSC is regarded as a single entity by the outside world. From the network’s perspective, however, it may be viewed as a number of diverse elements comprising relays and servers. MMS uses the protocols Wireless Markup Language (WML) and Hypertext Transfer Protocol (HTTP). The WAP Gateway is used to translate between HTTP and WML and to provide the WAP services necessary to MMS, including WAP PUSH services, capability negotiations, and Over-The-Air (OTA) security. The SMSC may be used for SMS notifications to a client. An MMS user agent resides within the terminal. This provides for multimedia message presentation, notifications to the user, and message retrieval and delivery. It may also be responsible for composition and submission, signing of messages on an end-user to end-user basis, encryption and decryption of messages, message storage and user profile management. A number of firewalls provide for network security.
¹ The MMSC may also be known as the MMS Proxy-Relay.
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MMS User Database
Own Network
WAP Gateway
Firewall
Other Licensed Operator
Firewall
MMSC handling and temporary storage of messages
E-mail/web
subscriber database charging data records media conversion
WAP Gateway translation between WML and HTTP WAP PUSH services Over-The-Air security Figure 19 MMS Architecture MB20/S3/v9.1
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9
GROUP CALLS AND BROADCAST ARCHITECTURE 9.1
Group Call Register (GCR)
The Group Call Register (GCR) is a functional unit containing all the attributes needed for set-up and handling of voice group and broadcast calls. These include a group call membership list, priority entitlements and network information. When a group call is requested, the MSC will interrogate the GCR for the parameters needed to set up the call. There is one GCR per MSC if this functionality is used.
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GCR
group members priority entitlements network information
BSS
C Gs
HLR Gr
Figure 20 Group Call Register (GCR) MB20/S3/v9.1
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GSM System Overview
9.2
Cell Broadcast Centre (CBC)
CBS messages are collected from cell broadcast entities (network operator or outside organization) and passed to a central Cell Broadcast Centre (CBC). The CBC tables the messages and then passes them on for transmission from the appropriate BTS; alternatively the messages may be loaded manually at the BSS. Messages will then be transmitted cyclically by the BTS for a duration specified by the information provider. A CBS message can be up to 93 characters long. However, it is possible to join up to 15 of these messages together to form a macro-message. Each page of such a message will have the same message identifier and serial number, enabling the mobile to associate them. The repetition and information updating rate will depend on information type. For example, traffic news may change more quickly than weather news.
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Cell Broadcast Entity e.g. Tourist Information Road/Rail Information Local Services
Cell Broadcast Centre
GSM Network
BTS BSS
Configured Remotely
BTS
BSS
Configured Locally
Figure 21 Cell Broadcast Service (CBS) MB20/S3/v9.1
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10
CAMEL ARCHITECTURE CAMEL provides the mechanisms to support services, independent of the serving network. CAMEL facilitates service control of operator-specific services external from the serving PLMN. It is a tool to help the network operator to provide subscribers with such services even when they are roaming outside the Home PLMN (HPLMN). Because IN architecture is used in the fixed as well as the mobile network, the ‘gsm’ prefix is used to differentiate between IN fixed and IN mobile network elements. 10.1
GSM Service Control Function (gsmSCF)
The gsmSCF is a functional entity that contains the CAMEL service logic to implement operator-specific services. In other words, that is where the service is executed from. It interfaces with the gsmSSF, the gsmSRF and the HLR. 10.2
GSM Service Switching Function (gsmSSF)
The gsmSSF is a functional entity that interfaces the MSC/GMSC to the gsmSCF. Triggers within the MSC and gsmSSF can be set based on information defined in the user’s subscription sent from the HLR to the VLR. These triggers dictate when the gsmSSF will communicate with the gsmSCF. 10.3
GPRS Service Switching Function (gprsSSF)
The gprsSSF is a functional entity that interfaces with the gsmSCF to allow interaction between CAMEL and GPRS. The gprsSSF resides at the SGSN. 10.4
GSM Specialized Resource Function (gsmSRF)
The gsmSRF is a functional entity that provides various specialized resources like voice interaction, the playing of announcements, and decoding Dual Tone Multi Frequency (DTMF) digits. It interfaces with the gsmSCF and with the MSC. The concept of the gsmSSF and gprsSSF is derived from the IN SSF, but uses different triggering mechanisms because of the nature of the mobile network.
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gsmSCF
HLR
gsmSRF gsmSSF
gprsSSF
A
Gb
Figure 22 Basic CAMEL Architecture MB20/S3/v9.1
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11
SECTION 3 QUESTIONS 1
Identify the network elements in a GSM/GPRS system.
2
Outline the use of the following: a b c d e
MSISDN IMSI TMSI P-TMSI IMEI
3
List five parameters stored on the SIM card.
4
Which of the following is not a function associated with the BTS? a b c d
carrier generation coding/ciphering the establishment and clearing of the radio connection modulation
5
State the functions of the MSC.
6
Which GPRS element is concerned with mobility and session management?
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SECTION 4
FREQUENCIES AND PROPAGATION
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CONTENTS 1
GSM in the Radio Spectrum 1.1 Introduction 1.2 Radio Waves 1.3 The EM Spectrum
4.1 4.1 4.1 4.1
2
GSM Radio Frequencies 2.1 Primary GSM (P-GSM) 2.2 Extended GSM (E-GSM) 2.3 GSM Railway (GSM-R) 2.4 GSM 1800 2.5 GSM 1900 2.6 GSM 400 2.7 GSM 850
4.3 4.3 4.3 4.3 4.5 4.5 4.7 4.7
3
Radio Interference 3.1 Introduction 3.2 The Urban and Rural Environments 3.3 Multipath Propagation 3.4 Fast Fading 3.5 Rayleigh Fading 3.6 Rician Fading 3.7 Time Dispersion and Inter Symbol Interference (ISI) 3.8 Slow (Shadow) Fading 3.9 Co-channel Interference 3.10 Adjacent Channel Interference 3.11 Minimizing Interference 3.12 Diversity Reception 3.13 Frequency Hopping
4.9 4.9 4.11 4.13 4.15 4.17 4.19 4.21 4.23 4.25 4.25 4.27 4.29 4.33
4
Summary
4.35
5
Section 4 Questions
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OBJECTIVES At the end of this section you will be able to: • • •
identify where GSM is used in the electromagnetic spectrum state the frequencies used for GSM and identify how such frequencies propagate outline the causes and effects of interference on the GSM air interface and state how each can be minimized
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GSM System Overview
1
GSM IN THE RADIO SPECTRUM 1.1
Introduction
GSM is a digital mobile cellular system whereby a user is connected to a network via a radio link. Some fundamental radio theory is therefore included here so that the characteristics of radio links can be appreciated. 1.2
Radio Waves
A radio wave is an example of an electromagnetic (EM) wave. An EM wave is a combination of magnetic and electrostatic force fields that travel through free space at a speed of 300,000,000 metres per second. EM waves are sinusoidal (sine waves) and are described in terms of their amplitude and frequency. In particular, the frequency of a radio wave determines its propagation characteristics, and therefore the radio systems to which it is applicable. Any radio wave can be described in terms of its frequency or wavelength. Frequency and wavelength are inversely proportional in that as the frequency increases, the wavelength decreases.
λ=
c ƒ
where: c = speed of light (3 x 108 ms) ƒ = frequency in Hertz (Hz) λ = wavelength in metres (m) 1.3
The EM Spectrum
Figure 1 illustrates a section of the electromagnetic spectrum that shows radio and light waves. In addition, examples are given of specific applications that use each of the spectrum bands. It can be seen that mobile phones operate within the band known as Ultra High Frequency (UHF).
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3 MHz
30 MHz
300 MHz
3 GHz
30 GHz
300 GHz
Radio 5 909 kHz
PMR/ Paging
Micro -wave Links
760 nm
Laser for Fibre Optics
t
ht
rav iole
300 kHz
3 kHz
Navigation Submarine Comms
Ult
EHF
Lig
SHF
ible
UHF
400 nm
VHF
Vis
HF
red
MF
ra-
LF
Inf
VLF
30 kHz
GSM System Overview
Sunbeds!
TV
Radio 4 198 kHz
Maritime Ship-to-Ship
Mobile Phones
Satellite Links
Radar
Figure 1 The EM Spectrum MB20/S4/v9.1
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2
GSM RADIO FREQUENCIES The GSM bands have been allocated as follows: 2.1
Primary GSM (P-GSM)
The original Primary GSM (P-GSM) spectrum allocation was agreed in 1979. This consists of two sub-bands 25 MHz wide. With duplex spacing of 45 MHz, frequency spacing of 200 kHz and the inclusion of guard bands, this gives a total of 124 carriers. In accordance with the GSM MoU, this spectrum in the UK is split between at least two operators. 2.2
Extended GSM (E-GSM)
In response to a perceived future demand for more capacity, the P-GSM spectrum was extended to form Extended GSM (E-GSM). This represents an extension of the lower end of the two sub-blocks by 10 MHz, giving a further 50 carriers. 2.3
GSM Railway (GSM-R)
Two further GSM blocks of 4 MHz have been included at the lower end of the EGSM allocation for GSM Railway (GSM-R) applications. The above bands are collectively known as GSM 900.
4.3
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ARFCN
45 MHz Duplex Spacing
1023 975 0 1 2
1023 5
123 124
975
0 1
Uplink 876 880 890 MHz MHz MHz GSM-R E-GSM
ARFCN P-GSM E-GSM GSM-R
5
123 124
Downlink 915 MHz
P-GSM
2
921 925 935 MHz MHz MHz GSM-R E-GSM
960 MHz P-GSM
– Absolute Radio Frequency Channel Number – Primary GSM – Extended GSM – GSM Railway
Figure 2 GSM 900 Allocation MB20/S4/v9.1
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GSM System Overview
2.4
GSM 1800
At a later stage in GSM development the technology was modified to meet the need for further networks. This involved changes to the radio interface, which moved spectrum allocation up to around 1.8 GHz. More spectrum is available in this frequency range, for two sub-blocks of 75 MHz with a duplex spacing of 95 MHz, giving a total of 374 carriers. As with GSM 900, this spectrum is divided between two or more operators. 2.5
GSM 1900
GSM 1900, often known as Personal Communication Service (PCS) 1900 refers to the digital cellular phone technology used in the USA. The Federal Communications Commission (FCC) has split the designated spectrum into six duplex blocks. The USA has been divided into 51 Major Trading Areas (MTA) and 493 Basic Trading Areas (BTA). An MTA is broadly equivalent in size to a state, while a BTA approximates to a large city. Using a process of auctioning, the FCC has granted ten-year licences to two operators in every MTA, and four operators in every BTA. MTAs have access to 3 x 15 MHz duplex blocks, whilst BTAs have access to 3 x 5 MHz duplex blocks.
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GSM 1800 512 513
885
Uplink
1710
512 513
1785 1805
885
Downlink
1880
95 MHz
ARFCN
GSM 1900 512 513
1850
810
Uplink
512 513
1910 1930
810
Downlink
1990
80 MHz
ARFCN
All frequencies are quoted in MHz.
Figure 3 GSM 1800 and GSM 1900 Allocation MB20/S4/v9.1
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GSM System Overview
2.6
GSM 400
To provide a digital migration path for first-generation analogue services such as NMT450 (Europe) and C-NETZ (Germany), two sets of spectrum (GSM 450 and GSM 480) provide 34 + 34 = 68 carriers in total. 2.7
GSM 850
Mostly in the USA and Canada, GSM may be deployed in the 850 MHz band where 2 x 25 MHz blocks provide up to 124 radio channels or carriers.
4.7
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450.4 457.6 Uplink
GSM 450
460.4 467.6 Downlink
10 MHz Duplex Spacing GSM 400
GSM 480
478.8 486 Uplink
488.8 496 Downlink
10 MHz Duplex Spacing
GSM 750
747
762 Uplink
777 792 Downlink
30 MHz Duplex Spacing
GSM 850
824
849 Uplink
869 894 Downlink
45 MHz Duplex Spacing
All frequencies are quoted in MHz.
Figure 4 GSM 400, GSM 750, and GSM 850 MB20/S4/v9.1
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3
RADIO INTERFERENCE 3.1
Introduction
Any communication system that relies on a radio link has a large number of problems to overcome to make the system work reliably and with maximum quality. Typically, these problems are: • fading • multipath effects • delay spread • co-channel interference • adjacent channel interference • noise The GSM digital air interface employs a number of techniques to combat these problems and is, therefore, a very robust system.
4.9
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fading multipath effects delay spread co-channel interference adjacent channel interference noise
Figure 5 Problems of Mobile Radio Interference MB20/S4/v9.1
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GSM System Overview
3.2
The Urban and Rural Environments
At the frequencies used for mobile cellular radio, most objects in the path of the radio waves will have some effects on the behaviour of the wave. In general, the dimensions of the object have to be significant in relation to the operating wavelength. Since this is only a matter of centimetres at 900, 1800 and 1900 MHz, then people, buildings, vehicles, animals, street furniture, office fittings and furniture, etc. are all certain to have effects. These effects can be summarized as: • reflection • scattering • diffraction • attenuation • changes in the plane of polarization Reflection occurs when the wave is incident upon a relatively smooth surface. The wave is reflected with an angle of reflection equal to the angle of incidence and with a strength dependent upon the conductivity of the reflector. The greater the conductivity, the stronger the reflection. This type of reflection is often termed specular. See Figure 6a. Scattering occurs when a wave reflects off a rough surface, with the degree of scatter dependent on the roughness of the surface in relation to the operating wavelength. This is sometimes termed diffuse reflection. See Figure 6b. Diffraction occurs when a wave passes over an edge with the degree of refraction increasing with frequency. See Figure 6c. Attenuation will be caused by any object obstructing the path of the wave. Once again, this tends to increase with frequency and may be significant at the frequencies used for cellular radio. The value of attenuation in decibels (dB) will depend upon a number of factors including operating wavelength, the dimensions of the object and the materials from which it is made. See Figure 6d. Changes in the plane of polarization can accompany any, or all, of the above effects and can also be caused by atmospheric and geomagnetic effects.
4.11
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a) Reflected Wave
Incident Wave
α
Smooth reflecting surface
α
b) Incident Wave
Scatter Energy Rough reflecting surface
c)
Diffraction Edge Object Shadow area
d) Object Incoming Wave
Diffracted Waves
Wave is progressively attenuated by absorption as it passes through the object Attenuated Wave
Figure 6 The Urban and Rural Environments MB20/S4/v9.1
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3.3
Multipath Propagation
Radio waves propagating from transmitter to receiver do so over more than just the direct path. Indirect paths also exist due to reflections from the ground and other objects (both fixed and mobile) such as buildings, people and vehicles. Within buildings there will be similar reflections from walls, furniture, people, etc. 3.3.1
Phase Difference
Figure 7 illustrates the principle of multipath propagation. The level of signal received will be the phasor sum of all the individual signals arriving. Because they have travelled over different distances, the signals may not be in phase with each other and could therefore add together constructively (giving a relatively large total), destructively (giving a relatively small or even zero total), or in a manner somewhere in between these extremes. Taking a simple case where there is a direct path plus just one indirect path and the two signals arrive with equal amplitude, their sum could be twice the amplitude of one signal (in-phase arrival), or zero (anti-phase arrival), or somewhere in between.
4.13
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Refle c
ilding
t io n
Path
RX
Gr ou
nd
Re fl
ec
Direct
tion f rom B u
TX
Figure 7 Multipath Propagation MB20/S4/v9.1
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3.4
Fast Fading
Fading is an unwanted change in signal level at the receiver. This change may be so slow as to be unnoticeable, or it may be fast. When fast fading occurs, moving the receiver antenna just 8 cm (λ/2 at 1900 MHz) would be enough to move from a peak to a trough in signal strength. Thus, if the transmitter and/or receiver are mobile, the received signal strength will fluctuate rapidly, causing an effect variously known as flutter, short sector fading or, more commonly, fast fading. In practice there will still be a very large number of paths between transmitter and receiver and the signals arriving from different paths will be likely to differ not just in phase, but also in amplitude. This is because of the differing path lengths, and also because of the effects of reflection, where waves will be scattered and attenuated further. In addition, the waves encounter both fixed and moving reflectors, which further complicate the analysis. The typical overall effect on signal strength for a mobile receiver is summarized in Figure 8, which shows the effect of fast fading superimposed on the local mean signal level, which will vary slowly.
4.15
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Received Signal Strength
Fast Fading
Local Mean Signal Level
Log Distance
Figure 8 Fast Fading MB20/S4/v9.1
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3.5
Rayleigh Fading
In a mobile radio environment there is often no direct path or Line of Sight (LoS), and the total signal received is the phasor sum of many indirect signals. This leads to a fast fading envelope characterized by deep fades of 20–30 dB in some cases. The fading envelope has a Rayleigh distribution, and the mobile receiver is said to be suffering Rayleigh fading, or working in a Rayleigh environment. This is typified by an urban or in-building environment. See Figure 9.
4.17
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Obstructions e.g. buildings, hills
Figure 9 Rayleigh Fading MB20/S4/v9.1
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3.6
Rician Fading
If there is a relatively strong direct component of signal together with relatively few indirect signals, fast fading will still occur but fades will be less deep. The fading envelope has a Rician distribution. This type of fading is most likely to occur in rural environments. See Figure 10.
4.19
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RX TX
Figure 10 Rician Environment MB20/S4/v9.1
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3.7
Time Dispersion and Inter Symbol Interference (ISI)
The multipath environment causes another potential problem for digital systems like GSM: time dispersion. Many versions of the same signal arrive at the receiver, all having travelled different distances. The recovered data from each path will therefore be dispersed in time as illustrated in Figure 11, which shows a simple three-path case. The delay spread is the difference in time between the earliest and latest arriving signals. 3.7.1
Delay Spread for GSM
If the delay spread is significant in relation to one bit period (about 3.7 µs for GSM), then significant Inter-Symbol Interference (ISI) will occur and the receiving system will find unambiguous decoding of each symbol difficult because of interference from delayed data. For example, in Figure 11 correct decoding of the second bit (0) will prove difficult if seriously delayed versions of the first bit (1) are still arriving. Serious ISI will lead to an increased number of decoding errors and unacceptable Bit Error Rate (BER). The Viterbi channel equalizer used in the GSM receiver is designed to minimize the effects of time dispersion, and can compensate for delay spreads up to approximately 16 µs. 3.7.2
Assessment of Delay Spread
In general, the delay spread will depend on how open the environment is. To achieve a delay spread of 1 µs between two radio paths requires a path length difference of 300 m. A 30 m path difference will produce a 0.1 µs (100 ns) spread, and so on. Clearly, in an indoor environment, path length differences of hundreds of metres are likely only in very large open buildings such as factories and shopping areas.
4.21
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Objec t 1 ath P t irec Ind Indirect Path 2
TX
Object
Direct P
ath RX
Direct Path Received Baseband Data
1
Indirect Path 1 Indirect Path 2
0
1
0
1
1
1
0
1
Sum of All Three
Delay Spread
Figure 11 Time Dispersion MB20/S4/v9.1
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3.8
Slow (Shadow) Fading
This is caused by the effect of obstruction or shadowing caused by clutter such as buildings, vehicles, terrain, and trees. As shown in Figure 12, the effect manifests itself as relatively slow variation in received signal strength around a local mean. It can be shown that the signal strength values have a log-normal distribution about the local mean and this allows relatively simple statistical analysis of appropriate fade margins. For GSM a typical standard deviation of σs = 7 dB is quoted, and this can be used to compute an approximate fade margin based on a required probability of coverage. Dynamic power control helps to offset slow fading. As the mobile moves behind an obstruction, it will be commanded to power up, and vice versa. Similarly, receiver Automatic Gain Control (AGC) will help to compensate for slow fading by increasing sensitivity. Slow fading can also be termed macroscopic fading.
4.23
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Serving BTS
th
of
MS
Received Signal Strength
Pa
Mea n
Log Distance (Metres) • Note scale Figure 12 Slow Fading MB20/S4/v9.1
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GSM System Overview
3.9
Co-channel Interference
Co-channel interference results from radio transmissions on the same frequency as the serving cell. This type of interference is inevitable in a cellular system, where frequency reuse is essential to achieve required traffic capacity for the network. Since it is most likely to be caused by other cells in the same network, the level of co-channel interference can be controlled by careful frequency planning. The Carrier to Co-channel Interference ratio (C/I) for GSM should in theory be 9 dB or better; in practice, values of 12 dB or more should be the planning goal. 3.10
Adjacent Channel Interference
Adjacent channel interference is interference caused by radio transmissions using frequencies adjacent to that being used by the serving cell. In the case of GSM networks, this means frequencies at ƒc + 200 kHz and ƒc – 200 kHz, where ƒc is the frequency of the serving cell. Once again, this type of interference is almost certain to emanate from other cells in the same network and can therefore be controlled by careful frequency planning. Theoretically for GSM, the Carrier to Adjacent Channel Interference ratio (C/A) should be –9 dB or better; in practice, values of over 0 dB and possibly 3 dB should be the planning goal.
4.25
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a) Co-channel Interference
ƒ1
ƒ2
ƒ3
ƒ1
Server
Interferer
Wanted Signal (ƒ1)
Unwanted Signal (ƒ1)
b) Adjacent Channel Interference
ƒ1
ƒ2
Server
ƒ3
Unwanted Signal (ƒ2)
ƒ1 Interferer
Wanted Signal (ƒ1)
Figure 13 Co-channel and Adjacent Channel Interference MB20/S4/v9.1
© Wray Castle Limited
4.26
GSM System Overview
3.11
Minimizing Interference
In GSM 900 networks, operators are allocated relatively few frequencies and must therefore plan for frequent (or tight) reuse. Inevitably, this leads to relatively high interference and correspondingly low levels of C/I and C/A. In GSM 1800 networks, operators typically have three times more frequencies than their 900 MHz counterparts. Accordingly, frequency reuse can be less frequent and this leads to lower interference and thus higher C/I and C/A. For this reason, it should be easier to ensure overall higher quality in 1800 MHz networks. Interference and the effects of interference can also be minimized by employing techniques such as Discontinuous Transmission (DTX), dynamic power control of the BTS, and slow frequency hopping, ideally using synthesizer hopping over a relatively wide band and large number of frequencies. The use of some or all of these features is essential if an operator intends to plan for tight frequency reuse.
4.27
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GSM System Overview
Interference is limited by:
Power Control
F1 F7 F4 F2
Frequency Hopping
Speech
Speech
Speech
Discontinuous Transmission (DTX)
Figure 14 Minimizing Interference MB20/S4/v9.1
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4.28
GSM System Overview
3.12
Diversity Reception
To combat the effects of fading at a base station due to multipath effects, diversity reception is normally employed (although not usually in the case of micro cells). The object of diversity reception is to set up a second receiving antenna in such a way that the signal it receives is independent of the signal being received by the first antenna. In this case, it is statistically unlikely that both signals will experience a fade simultaneously. If both signals are combined in some way, the result will be a signal with fewer fades. There are two main methods of providing diversity reception in cellular systems: space diversity and polarization diversity. 3.12.1 Space Diversity Reception This is achieved by mounting two separate antennas several wavelengths apart (typically ten), as illustrated in Figure 15. The greater the separation, the more effective the system. At 900 MHz, ten wavelengths would be around 3 m; at 1800/1900 MHz it is around 1.5 m. These are practical distances in engineering terms. Space diversity reception is widely implemented in cellular systems and is effective in most macrocellular environments. Using a duplexer (to allow common antenna working) reduces the number of antennas per cell to two: one Transmit/Receive (TX/RX), one RX only.
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RX A
th Pa
RX B
1
h at
2
Base Station
P
Figure 15 Space Diversity MB20/S4/v9.1
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4.30
GSM System Overview
3.12.2 Polarization Diversity As radio waves travel between the base stations and mobile station antennas, they suffer multiple reflections, scattering, etc. These will cause random changes to the plane of polarization. As a result, signals exist at the antenna that have a wide variety of planes of polarization, each signal having arrived via an independent path. This can be compensated for using diversity reception by mounting co-located receiving antennas of different polarization. This type of cross-polar antenna has many advantages. Both antennas may be mounted in a single housing. Such an arrangement requires less space than a pair of antennas for space diversity, and this may permit the use of a mast instead of a wide-topped tower. The overall environmental impact is reduced. Polarization diversity is becoming increasingly popular. However, it is only effective in environments where a significant number of scatterers exist. Space diversity is therefore more effective in open/rural environments.
4.31
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GSM System Overview
Vertical Antenna
Path 1 Vertically Polarized
Ho
riz
on Pat ta h 2 lly Po la riz
ed
Horizontal Antenna
Figure 16 Polarization Diversity MB20/S4/v9.1
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4.32
GSM System Overview
3.13
Frequency Hopping
Traditionally, diversity on the downlink has been difficult to achieve by spatial means so a number of systems use frequency hopping to counter the effects of fading. 3.13.1 The Principle of Frequency Hopping The principle of frequency hopping is to change the phase of the multipath components seen at the receiver by changing the frequency and hence the wavelength of the carrier. Even over the same physical paths the relative path lengths in terms of wavelength will change when the frequency is changed, so a single receiver which is in a fade on one frequency at one point, should, without moving, not be faded on the next burst. The more frequencies hopped over, the better the effect. Frequency hopping is most applicable on the downlink to slow moving mobiles in a severe multipath environment (typified by the urban environment). 3.13.2 GSM Frequency Hopping Frequency hopping in GSM is defined as Slow Frequency Hopping (SFH), which means that multiple bits of information are sent before the frequency is changed. In GSM a hopping rate of approximately 271 hops or frequency changes take place per second.
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a) Non-Hopping Physical Channel
Frequency
TDMA Frame TS2
TS2 C1 0
7 0
TS2 7 0
7
Time
e.g. MS assigned to channel 1 and timeslot 2
b) Frequency-Hopping Physical Channel
Frequency
TDMA Frame C1 0
7 0
7 0
7
C2 0
7 0
7 0
7
C3 0
7 0
7 0
7
C4 0
7 0
7 0
7
C5 0
7 0
7 0
7
C6 0
7 0
7 0
7
e.g. MS assigned to timeslot 2 and [C1...C6]
Time
Figure 17 Frequency Hopping MB20/S4/v9.1
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GSM System Overview
4
SUMMARY Figure 18 summarizes the problems created in cellular networks by the radio environment, their causes, and the techniques that need to be applied to minimize these effects.
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Problem
Cause
Technique to Minimize Effects
Fast Fading
Multipath Propagation
Diversity Antennas Frequency Hopping
Slow Fading
Shadowing
Good Radio Network Planning Power Control
Time Dispersion
Multipath Propagation
Good Radio Network Planning Viterbi Equalizer
Interference
Co-channel and Adjacent Channel
Good Frequency Planning
Figure 18 Summary MB20/S4/v9.1
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GSM System Overview
5
SECTION 4 QUESTIONS 1
List the GSM frequency bands and state by what means such frequencies are propagated.
2
Outline the causes and effects of interference on the air interface and state how each is minimized.
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SECTION 5
GSM AND GPRS CHANNELS
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i
GSM System Overview
ii
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GSM System Overview
CONTENTS 1
Multiple Access Techniques 1.1 Frequency Division Multiple Access (FDMA) 1.2 Time Division Multiple Access (TDMA)
5.1 5.1 5.1
2
Physical Channels 2.1 Introduction 2.2 Physical Channel Offset 2.3 Implementation of Channels 2.4 Allocation of Resources to GPRS 2.5 Channel Functions
5.3 5.3 5.5 5.7 5.9 5.11
3
TDMA Considerations 3.1 Timing of Transmissions 3.2 Burst Mode Transmission 3.3 Tail Bits and Guard Period 3.4 Extended Range
5.13 5.13 5.15 5.17 5.19
4
Logical Channels 4.1 Introduction 4.2 Traffic Channels 4.3 Broadcast Channels (BCH) 4.4 Common Control Channels 4.5 Dedicated Control Channels (DCCH)
5.21 5.21 5.21 5.23 5.25 5.27
5
Section 5 Questions
5.29
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GSM System Overview
iv
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GSM System Overview
OBJECTIVES At the end of this section you will be able to: • • • • • • • •
describe the multiple access techniques FDMA and TDMA and state how they are used in GSM outline the meaning of the term Frequency Division Duplex (FDD) outline how physical resources can be implemented and allocated state how resources are allocated to GPRS, and how this differs from GSM resource allocation outline the types of bursts, stating their functions outline the relationship between the normal burst and the need for timing advance and state why timing advance is not used with the access burst state what a logical channel is and how it is implemented name the logical channels used in GSM and GPRS and describe their functions
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GSM System Overview
1
MULTIPLE ACCESS TECHNIQUES GSM is a telephone system. Therefore users need both to talk and to be heard. The spectrum allocation consists of two bands: the lower band is used by the mobiles to transmit to the network (uplink), and the upper band is used by the network to transmit to the mobiles (downlink). This use of separate frequencies in the uplink and downlink directions is a full duplex system. GSM and GPRS use a combination of two techniques to allow a number of users to access the system at a time. These techniques are Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA). 1.1
Frequency Division Multiple Access (FDMA)
A mobile is allocated a channel number, which refers to a pair of frequencies, one from the lower band and the other from the upper band. Each band contains carriers, spaced by 200 kHz. It is the allocation of a frequency from within the whole spectrum that provides the FDMA part of the GSM radio interface. 1.2
Time Division Multiple Access (TDMA)
When the network indicates a channel for the mobile to use, it not only indicates what frequencies to use, but also which timeslot. This is possible as each of the frequencies is further divided by time. Hence the concept of TDMA as well as FDMA is introduced to the GSM radio interface. The time structure on which the GSM air interface is based is a frame containing eight timeslots over a period of 4.62 ms.
5.1
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GSM System Overview
a) FDMA – Frequency Division Multiple Access
f 1
MS1
2
MS2
3
MS3
4
MS4
t
b) TDMA – Time Division Multiple Access
f
MS8
MS3
1
MS4 MS1
t
MS2
Figure 1 FDMA and TDMA MB20/S5/v9.1
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5.2
GSM System Overview
2
PHYSICAL CHANNELS 2.1
Introduction
In GSM and GPRS, each carrier, or radio channel, is divided in the time domain to produce eight timeslots, each known as a ‘physical channel’. It is the physical channel that carries a logical channel. The P-GSM allocation supports 992 (8 x 124) physical channels. The 4.62 ms frame structure repeats, giving each mobile an opportunity to transmit and receive information for 577 µs every 4.62 ms.
5.3
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GSM System Overview
D Time
n
omai
20
3 1 2 0 7 5 6 4 s m 2 3 4.62 0 1 7 6 4 5 el 3 2 hann C 1 l a o i 0 ic Rad Phys 2 1 . 0 1 Ch 7 5 6 3 4 2 1 7 0 6 4 5 2 3 1 0 Radio Ch. 2 6 4 5 3 2 0 1 7 5 6 3 4 2 0 1 Radio Ch. 3 0k
cy
en
qu
Hz
Fre
20
Do
0k
in
Hz
ma
577 µ
s
Figure 2 Physical Channel MB20/S5/v9.1
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5.4
GSM System Overview
2.2
Physical Channel Offset
The physical channel is the channel in which an MS transmits and receives information. It consists of one timeslot on a downlink carrier, and the corresponding timeslot on the uplink carrier. This full-duplex method of operation is known as Frequency Division Duplex (FDD). The example in Figure 3 shows timeslot 2 being used. The MS in this case will only transmit and receive in timeslot 2. Note the three-timeslot offset in uplink and downlink. This is so the MS is not transmitting and receiving at the same time, allowing for simpler and cheaper equipment.
5.5
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GSM System Overview
Ch.n+1 Ch.n Ch.n+2
Ch.n+1 Ch.n+2 Ch.n
Ch.N
UPLINK
Ch.N
DOWNLINK
Duplex Separation
ARFCN
0 1
Dow
nlink
2 3 4 5 6 7
Ch.n 0 1
Ch.n + duplex separation
2 3 4 5 6 7 Upli n k
Physical Channel MS
Figure 3 Physical Channel Offset MB20/S5/v9.1
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5.6
GSM System Overview
2.3
Implementation of Channels
Each BTS may transmit and receive a number of carriers, each of which may carry up to eight simultaneous phone calls on its eight timeslots. However, not all timeslots are available for traffic as at least one of them (often several) is reserved exclusively for control channels. These control channels are normally transmitted on one of the cell’s carriers, known as the Broadcast Control Channel (BCCH). These channels provide facilities to the mobile phone which are necessary for it to establish telephone calls such as: • informing the MS of an incoming call (paging) • allowing the MS to contact the BTS requesting service • providing information, such as which channels are available • allowing the BTS and the MS to exchange information necessary to set up a call (e.g. phone number) 2.3.1
BCCH Allocation
Every GSM cell has one of its carriers designated as the BCCH carrier. Whereas all other carriers have eight timeslots available for telephone calls (although some may be allocated for GPRS use), the BCCH carrier has a reduced number (typically six) due to the presence of the control channels. Figure 4 shows two scenarios: a)
shows a cell with three radio channels and the standard control and signalling structure;
b)
shows a cell with only one radio channel where TS0 contains a combined control and signalling multiframe.
If more than three radio channels are used, it is possible to use extended control and signalling channels. The dedicated signalling channels are used for signalling between the BTS and the mobile after the initial communication on the BCCH.
5.7
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GSM System Overview
a) Control 8 x Dedicated Signalling Channels C0
0
C1 0
C2 0
Traffic Channels 1
2
3 4 5 6 BCC H Ca r rier 1 2 3 4 5 6 Traffi c Ch anne 1 2 ls 3 4 5 5 6 Traffic Chan nels
7
7
7
b)
Control + 4 x Dedicated Signalling Channels
C0
0
Traffic Channels 1 Com
2 bined
3
4
Mult
5
ifram
6
7
e
Figure 4 BCCH Allocation MB20/S5/v9.1
© Wray Castle Limited
5.8
GSM System Overview
2.4
Allocation of Resources to GPRS
In GSM, timeslots are allocated on a user-by-user basis. That is, when users tell the network that they want to make a call, they will be allocated a single timeslot (physical channel) or, if HSCSD is supported, up to four timeslots. They will use this timeslot or timeslots for the duration of their circuit-switched connection, and when the connection is closed the timeslots will be released for reallocation. The user may not have been continually sending information during the time they were connected, in which case resources may have been wasted. As a packet-switched bearer service, GPRS transmits user data in bursts. In other words, it seeks to optimize network usage by only granting resources to users when they have data to transmit. This means that physical channels are not allocated permanently, but on a ‘capacity on demand’ basis, and the number of channels allocated to GPRS will fluctuate according to need. The physical channel in GPRS is known as the Packet Data Channel (PDCH). GSM and GPRS share network resources; PDCHs are allocated to GPRS from the common pool of channels available on a cell. If the demand for the GPRS bearer is high and resources are available, then further PDCHs may be allocated. They may then be reallocated to circuit-switched services if required. Information indicating if adjacent cells support GPRS is transmitted on the Broadcast Control Channel (BCCH). The specifications do not require that PDCHs are permanently allocated. It remains an operator decision whether to grant resources permanently or temporarily. However, any allocation of PDCHs will always reduce the number of circuit-switched channels available in a cell. The number of timeslots allocated to a mobile will depend upon the following: • how much data the user has to send • the mobile’s multislot class • QoS parameters • resources available in the network The term multislot refers to multiple traffic channels and associated control channels allocated to the mobile. Different mobiles have different multislot classes, and allocation will be dependent on this.
5.9
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GSM System Overview
1) Multiple Timeslot Allocation MS ‘A’ TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 6 TS 7 MS ‘B’ TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 6 TS 7 MS ‘C’ TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 6 TS 7 MS ‘D’ TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 6 TS 7
2) Timeslot Sharing MS ‘A’ TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 6 TS 7 MS ‘B’ TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 6 TS 7 MS ‘C’ TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 6 TS 7 MS ‘D’ TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 6 TS 7 Figure 5 Timeslot Allocation in GPRS MB20/S5/v9.1
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5.10
GSM System Overview
2.5
Channel Functions
A cell supporting GSM and GPRS may share resources on one or several carriers. Figure 6 shows a single carrier with shared resources for GPRS and GSM. It shows timeslot 0 allocated as BCCH, which can be read by both GSM and GPRS mobiles. Timeslot 1 is allocated as a Dedicated Control Channel (DCCH) for GSM; timeslots 2–6 are allocated as Traffic Channels (TCH) and timeslot 7 is allocated as a PDCH for GPRS. A cell supporting GPRS may have GPRS radio resources allocated at any time. If no resources are allocated, a mobile can request allocation of such resources and then proceed to use them. The network may dynamically increase or decrease the radio resources allocated to GPRS from GSM to a predefined maximum or minimum. Information indicating if adjacent cells support GPRS is transmitted on the BCCH.
5.11
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GSM System Overview
Single Carrier
Broadcast Control Channel
Dedicated Common Control Channel
Traffic Channels (TCH)
Packet Data Channel (PDCH)
Only allocate PDCH when required to transfer GPRS data or signalling
Figure 6 Channel Functions MB20/S5/v9.1
© Wray Castle Limited
5.12
GSM System Overview
3
TDMA CONSIDERATIONS Since an MS (or BTS) must only transmit or receive for a short duration of time (577 µs every 4.62 ms), there are some technical considerations required to make the system operate. 3.1
Timing of Transmissions
The use of a timeslot multiple access system requires that the MS and BS transmit at the correct time. As far as the BTS is concerned, the timing of its transmissions is laid down by ETSI and its synchronization will be assured from a reference source either from an E1 link, or the Global Positioning System (GPS). The MS timing of transmissions must also conform to the ETSI standards, but its synchronization must come from the BTS. Having synchronized with a BTS, the MS will be in a position to transmit in an indicated timeslot. However, due to the mobile nature of the equipment, it is possible that during a call the MS will get closer to, or further from, the BTS. This effectively changes the transmission delay, and will affect the synchronization of the link. See Figure 7. The BTS can measure the transmission delay and will indicate to an MS a parameter called timing advance. This ensures that transmissions from an MS arrive at the BTS at the correct time. Timing advance is supplied to the mobile as x bit periods (1 bit period = 3.7 µs). One bit period equates to about 550 m of range. Maximum timing advance is 63 bit periods, equivalent to a maximum range of 35 km. For greater ranges, extended range techniques must be used.
5.13
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GSM System Overview
TS2
TS3
TS4
TS5
MS moves towards BTS TX delay reduces MS moves away from BTS TX delay increases
Burst arriving at BTS correctly in TS3
MS
BTS
BTS monitors position of burst in timeslot
Variation in transmission path length
Figure 7 Transmission Timing MB20/S5/v9.1
© Wray Castle Limited
5.14
GSM System Overview
3.2
Burst Mode Transmission
A user can transmit only for a short period of time, i.e. during the allocated timeslot of 577 µs, every frame period, which is every 4.62 ms. In order that the information can be transmitted coherently in such a short period of time, it is formatted into a suitable structure for burst transmission. There are four types of burst transmission used in GSM. These, shown in Figure 8, are: • Frequency Correction Burst (FCB) • Synchronization Burst (SB) • Access Burst (AB) • Normal Burst (NB) The FCB is transmitted downlink only in the main control channel. It provides the MS with two functions: it identifies the presence and position of the main control channel and it allows the MS to make any internal adjustments to its frequency source. The SB is transmitted downlink only in the main control channel, and carries information which allows the MS to become bit-, slot- and frame-synchronized with the network. The AB is transmitted uplink by an MS wishing to access the network. The significant part of this burst is its very long guard period. This allows the mobile to make unsynchronized access attempts from up to 35 km away, without causing interference. This is the most common structure used in both the uplink and downlink. It is used for carrying speech or data (traffic), or control information. The type of content is indicated by the Fast Associated Control Channel (FACCH) flags.
5.15
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GSM System Overview
a) Frequency Correction Burst (FCB) Tail Bits (3)
Tail Guard Bits Period (3) (8.25)
Fixed Bits (142)
b) Synchronization Burst (SB) Tail Bits (3)
Sync Bits (39)
Training Sequences (64)
Sync Bits (39)
Tail Guard Bits Period (3) (8.25)
c) Access Burst (AB) Tail Synchronization Sequence Bits (41) (8)
d) Normal Burst (NB) Tail Bits (3)
Coded Bits (57)
Coded Bits (36)
Tail Bits (3)
Guard Period (68.25)
FACCH Flags (1) Training Sequences (26)
Coded Bits (57)
Tail Guard Bits Period (3) (8.25)
Figure 8 TDMA Burst Structures MB20/S5/v9.1
© Wray Castle Limited
5.16
GSM System Overview
3.3
Tail Bits and Guard Period
If data is only transmitted in a designated timeslot, it is necessary to consider what the transmitter in the mobile equipment is doing. The mobile must not be radiating any power when it is not transmitting. When its designated timeslot arrives, it must increase its output power to the required level, and maintain that level for the duration of the ‘useful information’ in the burst. At the end of the ‘useful information’ it reduces its power to zero again. The tail bits are used to indicate start and stop points of the burst. The guard period is to allow for mobility and power ramping.
5.17
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GSM System Overview
a) Frequency Correction Burst (FCB) Tail Bits (3)
Tail Guard Bits Period (3) (8.25)
Fixed Bits (142)
b) Synchronization Burst (SB) Tail Bits (3)
Sync Bits (39)
Training Sequences (64)
Sync Bits (39)
Tail Guard Bits Period (3) (8.25)
c) Access Burst (AB) Tail Synchronization Sequence Bits (41) (8)
d) Normal Burst (NB) Tail Bits (3)
Coded Bits (57)
Coded Bits (36)
Tail Bits (3)
Guard Period (68.25)
FACCH Flags (1) Training Sequences (26)
Coded Bits (57)
Tail Guard Bits Period (3) (8.25)
Figure 8 (repeated) TDMA Burst Structures MB20/S5/v9.1
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5.18
GSM System Overview
3.4
Extended Range
The maximum timing advance of 63 bits imposes a theoretical limit on cell size of about 35 km from the base station. Mobiles at greater range would supply bursts which would arrive so late they encroached on the next timeslot. This theoretical limit can pose problems in providing coverage in specific areas, for example offshore coverage from a site on the coast. This can be overcome by allocating two or more successive timeslots to the mobile, thus breaking the theoretical 35 km limit. It is not actually possible to allocate more than one timeslot to a GSM mobile. However, leaving the next timeslot unallocated has the same effect in this case. Bursts from a mobile beyond the the 35 km limit will drift into the next timeslot. At a range of about 120 km, the burst would arrive such that it lies completely within the next timeslot. See Figure 9. This is the principle of extended-range cell operation. Note that traffic capacity is halved, because each channel uses two timeslots. A single carrier can thus handle four channels in an extended-range cell. In practice, the ability to achieve the theoretical maximum range will depend on power budget considerations as well as timing effects.
5.19
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GSM System Overview
TS0
TS1
TS2
TS3
TS4
TS5
TS6
TS7
Time at BTS
Burst arriving from a mobile at 35 km using maximum timing advance (63 bits)
Burst arriving from a mobile at > 35 km using maximum timing advance
Burst arriving from a mobile at ≈ 120 km using maximum timing advance
Note:
The MS is assigned to TS0
Figure 9 Extended Range Principle MB20/S5/v9.1
© Wray Castle Limited
5.20
GSM System Overview
4
LOGICAL CHANNELS 4.1
Introduction
The logical channel describes the current utilization of a physical channel for either GSM or GPRS operation. A physical channel is used for transmitting and receiving traffic information (speech or user data), or control information (e.g. call set-up information). If it is currently transmitting or receiving traffic information, then ‘logically’ the physical channel is a traffic channel. However, the same physical channel (timeslot) can be used to transmit or receive control information. In that case the physical channel becomes a ‘logical’ control channel. 4.2
Traffic Channels
The Full-rate Traffic Channel (TCH/F) supports encoded/protected speech at a gross rate of 22.8 kbit/s, or Forward Error Correction (FEC) coded data at 9.6, 4.8 or 2.4 kbit/s (TCH/F9.6/F4.8 or /F2.4). The Half-rate Traffic Channel (TCH/H) will support second-generation speech vocoders at a gross rate of 11.4 kbit/s, or FECcoded data at 4.8 or 2.4 kbit/s. A full-rate channel requires one physical channel, whereas two half-rate channels can be supported by a single physical channel. 4.2.1
Packet Data Traffic Channel (PDTCH)
The PDTCH is allocated for data transfer. It is temporarily dedicated to one MS or a group of MSs in the Point-to-Multipoint – Multicast (PTM-M) case. In multislot operation, one MS may use multiple PDTCHs in parallel for individual packet transfer. The PDTCH can be found on both uplink and downlink.
5.21
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GSM System Overview
UL TCH/F TCH/H
DL
PDTCH
UL
DL
TCH/F (Full Rate Traffic) Speech 22.8 kbit/s Data 14.4 kbit/s 9.6 kbit/s 4.8 kbit/s c. 2.4 kbit/s
TCH/H (Half Rate Traffic) Speech 11.4 kbit/s Data 4.8 kbit/s c. 2.4 kbit/s
PDTCH – Packet Data Traffic Channel
Figure 10 Logical Traffic Channels MB20/S5/v9.1
© Wray Castle Limited
5.22
GSM System Overview
4.3
Broadcast Channels (BCH)
The BCH has three functions: frequency control of the MS in respect of the BTS, frame synchronization of the MS, and general broadcast functions. The BCH functional channel types are downlink only and are: • Frequency Correction Channel (FCCH) • Synchronization Channel (SCH) • Broadcast Control Channel (BCCH) • Packet Broadcast Control Channel (PBCCH) The FCCH transmits a frequency correction data burst comprising all zeros. This produces a constant frequency shift of the carrier, which is used by the mobile for frequency correction purposes. The SCH is used to time-synchronize the mobile. The information transmitted includes the TDMA frame number and the Base Station Identity Code (BSIC). The BCCH, which is always transmitted on timeslot 0, transmits cell identities and information about the common control channels and the services available in the form of system information messages. Included in this information is an indication as to whether packet-switched traffic is supported, i.e. GPRS. The PBCCH broadcasts packet data system information. If this channel is not allocated the information is available on the BCCH. Note that everything transmitted on the BCH carrier is always at full power.
5.23
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FCCH Frequency Correction
SCH
Full Power
Synchronization
BCCH Broadcast Channel
PBCCH Packet Broadcast Channel
Figure 11 Logical Broadcast Channels MB20/S5/v9.1
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4.4
Common Control Channels
There are five GSM Common Control Channels (CCCHs): • Paging Channel (PCH) • Random Access Channel (RACH) • Access Grant Channel (AGCH) • Notification Channel (NCH) • Cell Broadcast Channel (CBCH) The PCH is used downlink to inform mobiles of incoming traffic. The RACH is used uplink to initiate a call, respond to a page, or perform a location update. In each case, the mobile is requesting a Dedicated Control Channel (DCCH). The AGCH is used downlink to allocate the DCCH. The NCH is used downlink to notify mobiles of a voice group call. The CBCH is used downlink to carry short messages, local postcodes, advertisements, etc. Four of these channels have GPRS equivalents: • Packet Paging Channel (PPCH) • Packet Random Access Channel (PRACH) • Packet Access Grant Channel (PAGCH) • Packet Notification Channel (PNCH) In addition to the functions outlined above, the PRACH is used to request timing advance information, the PAGCH sends resource assignment messages to the mobile prior to packet transfer, and the PNCH sends a Point-to-Point – Multicast (PTP-M) message to the MS, also prior to packet transfer.
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PCH/PPCH Paging
RACH/PRACH Random Access
AGCH/PAGCH Access Grant
NCH/PNCH Notification
CBCH Cell Broadcast
Figure 12 Logical Common Control Channels MB20/S5/v9.1
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4.5
Dedicated Control Channels (DCCH)
DCCHs are used as two-way links for specific network-to-MS signalling (and vice versa). They consist of: • Stand-Alone Dedicated Control Channels (SDCCH) • Associated Control Channels (ACCH) • Packet Associated Control Channel (PACCH) Stand-Alone Dedicated Control Channels (SDCCH) There are two types of SDCCH, one with four sub-channels (SDCCH/4) and one with eight sub-channels (SDCCH/8). The SDCCH is a bidirectional channel used to convey the signalling messages between the mobile and the network at call set-up and for activities such as location updating, supplementary service control and SMS traffic. Associated Control Channels (ACCH) There are two types of ACCH, Slow Associated Control Channel (SACCH) and Fast Associated Control Channel (FACCH). The ACCHs are always allocated with either a traffic channel or an SDCCH. The ACCHs are bidirectional and support the transfer of information such as signal measurements, power adjustment commands and handover instructions. The FACCH is associated with a traffic channel (either full-rate or half-rate) and facilitates the rapid transfer of data, such as handover commands, by means of a bit stealing technique. The FACCH is supported by stealing bits from the traffic channel, which leads to a reduction in traffic quality. The SACCH is associated with either traffic channels or SDCCHs. The low data rate of the SACCH is sufficient to support power control commands in the DL, for example, or signal level measurement of adjacent cells in the uplink. Packet Associated Control Channel (PACCH) This conveys signalling information related to a given MS. Such information will include acknowledgments, power control information and resource reassignments. One PACCH is associated to one or several PDTCHs that are concurrently assigned to one MS. The PACCH is found on both uplink and downlink.
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SDCCH/4 SDCCH/8 Stand-alone Dedicated
SACCH FACCH Associated Control
PACCH Packet Associated
Figure 13 Logical Dedicated Control Channel MB20/S5/v9.1
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5
SECTION 5 QUESTIONS 1
State the data rates on full-rate traffic channels.
2
State the functions associated with the BCH.
3
List the uplink and downlink logical common control channels.
4
By what means is the FACCH implemented?
5
Outline the need for timing advance and state why this is not required when using the access burst.
6
List the GSM TDMA burst structures and outline their use.
7
State the function of: a tail bits b guard periods c training sequences
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SECTION 6
GSM COVERAGE
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CONTENTS 1
Cellular Networks 1.1 The Cellular Philosophy 1.2 Frequency Reuse 1.3 Reuse Distance 1.4 System Growth
6.1 6.1 6.3 6.5 6.7
2
Cell Types 2.1 Macro Cells 2.2 Micro Cells 2.3 Pico Cells
6.9 6.9 6.9 6.9
3
In-Building Coverage
6.11
4
Geographical Areas 4.1 Introduction 4.2 Network Area 4.3 Location Area (LA) 4.4 Routing Area (RA)
6.13 6.13 6.13 6.15 6.17
5
Section 6 Questions
6.19
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OBJECTIVES At the end of this section you will be able to: • • • •
explain the importance of frequency reuse and reuse distance in cellular systems outline the use of cell splitting in the planning of a cellular network in terms of traffic volumes outline the need for, and the use of, macro cells, micro cells, and pico cells define network areas, location areas and routing areas and state what identities are associated with each
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1
CELLULAR NETWORKS 1.1
The Cellular Philosophy
Unidirectional broadcasting systems such as television and radio can be accessed by many users, all of whom may wish to receive the same channel at the same time. This is achieved by large antennas covering a wide geographical area. In a mobile telecommunication system, although many users will need to access the network simultaneously, each user will require a bidirectional private channel upon which to transmit and receive information. In order to maximize capacity, the geographical area covered by a base station has to be much smaller. These areas are termed ‘cells’. The maximum radius of a cell is 35 km, but this may vary according to terrain and the practicalities of cell planning.
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u
i nid
rec
na t io
un
l
idir ec
t io na
l
Television/Radio c. 120 km coverage area
up to 35 km cell radius
ect bidir
iona
l
ec bidir
tiona
l
Mobile Communications Figure 1 Cellular Network MB20/S6/v9.1
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1.2
Frequency Reuse
In a mobile telecommunication network, the priority is to maximize the amount of traffic that can be supported by the network rather than to increase the coverage area of a single cell. In a cellular system, therefore, one large cell will often be substituted by several smaller cells, especially in urban areas where a lot of capacity is required. The EM spectrum is a finite resource, which means that it has to be used as efficiently as possible. If the maximum capacity is to be achieved in a network, the same frequencies will have to be reused many times. In GSM, one frequency can support eight users. If there is only one frequency on a cell, only eight users can be supported simultaneously. As can be seen in Figure 2a, where there is a large cell area covered by one BTS, the number of users able to access the system will be very small. However, looking at Figure 2b, the large cell has been replaced with several smaller cells, operating a four-cell frequency reuse pattern. This reuse pattern can be repeated, or ‘tessellated’, to cover the same area as the single, large cell. The equivalent coverage has been achieved and there is also much greater capacity. As well as providing more capacity there are some other benefits that arise from this strategy: • reduced base station transmit power • reduced mobile phone transmit power • reduced visual impact • consistent and reliable coverage • competitive pricing • advanced services When cells are smaller, the mobile phones tend to be closer to the base stations, therefore less transmit power is required. This significantly extends battery life and further reduces any possible health risks. Base stations can also be physically smaller, making it much easier to reduce their visual impact. The service level to the user is improved in terms of reliability of coverage and competitive pricing. The extra capacity not only enables more users to be supported, but also makes it easier to introduce advanced services.
6.3
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a) Single Frequency
ƒ1
Total number of simultaneous users = 1 x 8 = 8
b) Frequency Reuse 2
4
3
3
ƒ1
ƒ4
ƒ2
ƒ3
1
ƒ4
ƒ1
3 ƒ2
ƒ4
ƒ2 2
1
Total number of simultaneous users = 27 x 8 = 216 Figure 2 Frequency Reuse MB20/S6/v9.1
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1.3
Reuse Distance
While the same frequencies can be reused many times, they cannot be used indiscriminately. Two adjacent cells cannot be on the same frequency or interference will occur. There has to be a minimum distance between cells on the same frequency. This is termed the minimum reuse distance. The reuse distance depends on two factors: the cell radius (defined by the coverage provided by the BTS) and the number of cells in the repeat pattern. Figure 3 shows the relationship between cluster size, distance and cell radius. Cluster sizes can be of 1, 2, 3, 4, 7, 9, 12 or 21 cells, the important point being that they tessellate over a large area.
6.5
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A
R
D
A'
D = Reuse Distance R = Cell Radius N = No. of cells in cluster In general D = R 3N
Figure 3 Reuse Distance (Seven-Cell) MB20/S6/v9.1
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1.4
System Growth
To allow growth in cellular systems it is essential that increased traffic volumes are accommodated without compromising the Grade of Service (GoS) offered to network subscribers. When the initial cell areas in a system can no longer accommodate the traffic levels, the cells can be either split or sectorized. 1.4.1
Cell Splitting
Cell splitting is based on the concept of dividing a cell into a number of smaller cells (see Figure 4a). To facilitate cell splitting an additional number of base stations must be provided, and the channels reallocated. This technique is expensive because of the additional hardware requirements, and is limited by co-channel interference as the reuse distance is dramatically reduced. 1.4.2
Cell Sectorization
Sectorization can increase the traffic handling capability of a cell, at the same time minimizing co-channel interference. Sectorization is achieved using directional antennas to divide a site into three sectors (see Figure 4b). It is this method that is used in practice.
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Coverage area showing progressive cell splitting
BTS
Figure 4a Cell Splitting
1 BTS 2 3
Three-Cell Site
Figure 4b Cell Sectorization MB20/S6/v9.1
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2
CELL TYPES In general terms, three types of cell exist: • macro cells • micro cells • pico cells 2.1
Macro Cells
Macro cells may be classed as either small or large. Large macro cells are most appropriate for use in rural areas where traffic is relatively low and cells large. The maximum cell range is up to 35 km in theory, although in practice the range may be much less in order to minimize co-channel interference. In this respect, the siting of the antenna is also important. Small macro cells may also be used in urban areas to provide coverage, while smaller micro cells or pico cells are being established to provide capacity. Alternatively, they may form part of a hierarchical cellular layer designed to manage handovers between smaller macro cells and micro cells. 2.2
Micro Cells
A micro cell serves a small area (dimensions of hundreds of metres being normal) and employs antennas usually mounted below rooftop height in the street canyon. This tends to contain the energy within the streets, thereby minimizing interference to neighbour cells. Because micro cells use low power levels, less interference is produced, allowing tighter frequency reuse and thus greater traffic capacity in the network as a whole. They are generally simpler to set up than large macro cells and can thus offer a rapid means of capacity increase. 2.3
Pico Cells
Pico cells provide public or private coverage to smaller, indoor areas such as homes or offices, where mobility is low. Base stations are situated at street level.
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Cell Type
Characteristic
Range
Large Macro
Antennas above all rooftops
