Wray Castle - GSM System Overview

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(1)GSM System Overview. Wray Castle Limited Bridge Mills, Stramongate, Kendal, LA9 4UB, UK. [email protected] www.wraycastle.com © Wray Castle Limited all rights reserved.

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(3) GSM System Overview. GSM SYSTEM OVERVIEW. First published 1998 Last updated January 2006 WRAY CASTLE LIMITED BRIDGE MILLS STRAMONGATE KENDAL LA9 4UB UK. Yours to have and to hold but not to copy The manual you are reading is protected by copyright law. This means that Wray Castle Limited could take you and your employer to court and claim heavy legal damages. Apart from fair dealing for the purposes of research or private study, as permitted under the Copyright, Designs and Patents Act 1988, this manual may only be reproduced or transmitted in any form or by any means with the prior permission in writing of Wray Castle Limited. © Wray Castle Limited.

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(5) GSM System Overview. 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. © Wray Castle Limited. iii.

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(7) GSM System Overview. SECTION 1. AN INTRODUCTION TO GSM. © Wray Castle Limited. v.

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(9) GSM System Overview. 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. © Wray Castle Limited. vii.

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(11) GSM System Overview. 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. © Wray Castle Limited. ix.

(12) GSM System Overview. 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. © Wray Castle Limited. MB20/S1/v9.1.

(13) GSM System Overview. digital standardized confidentiality and security evolution to 3G. Figure 1 GSM Overview MB20/S1/v9.1. © Wray Castle Limited. 1.2.

(14) GSM System Overview. 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.. 1.3. © Wray Castle Limited. MB20/S1/v9.1.

(15) GSM System Overview. 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. © Wray Castle Limited. 1.4.

(16) GSM System Overview. 2. 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.. 1.5. © Wray Castle Limited. MB20/S1/v9.1.

(17) GSM System Overview. 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. © Wray Castle Limited. 1.6.

(18) GSM System Overview. 3. 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.. 1.7. © Wray Castle Limited. MB20/S1/v9.1.

(19) GSM System Overview. 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. © Wray Castle Limited. 1.8.

(20) GSM System Overview. 4. 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.. 1.9. © Wray Castle Limited. MB20/S1/v9.1.

(21) GSM System Overview. SECTION 2. GSM SERVICES. © Wray Castle Limited. i.


(23) GSM System Overview. 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. © Wray Castle Limited. iii.


(25) GSM System Overview. 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. © Wray Castle Limited. v.

(26) GSM System Overview. 1. 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. © Wray Castle Limited. MB20/S2/v9.1.

(27) GSM System Overview. 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. © Wray Castle Limited. 2.2.

(28) GSM System Overview. 2. 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.. 2.3. © Wray Castle Limited. MB20/S2/v9.1.

(29) GSM System Overview. Bearer Service Teleservice. Plus Supplementary Services. Teleservice Bearer Service. GSM Network. Transit Network. Terminating Network. Figure 2 Telecommunication Services MB20/S2/v9.1. © Wray Castle Limited. 2.4.

(30) GSM System Overview. 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.. 2.5. © Wray Castle Limited. MB20/S2/v9.1.

(31) GSM System Overview. 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. © Wray Castle Limited. 2.6.

(32) GSM System Overview. 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.. 2.7. © Wray Castle Limited. MB20/S2/v9.1.

(33) GSM System Overview. 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. © Wray Castle Limited. 2.8.

(34) GSM System Overview. 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.. 2.9. © Wray Castle Limited. MB20/S2/v9.1.

(35) GSM System Overview. 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. © Wray Castle Limited. 2.10.

(36) GSM System Overview. 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.. 2.11. © Wray Castle Limited. MB20/S2/v9.1.

(37) GSM System Overview. 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. © Wray Castle Limited. 2.12.

(38) GSM System Overview. 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.. 2.13. © Wray Castle Limited. MB20/S2/v9.1.

(39) GSM System Overview. 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. © Wray Castle Limited. 2.14.

(40) GSM System Overview. 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.. 2.15. © Wray Castle Limited. MB20/S2/v9.1.

(41) GSM System Overview. 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. © Wray Castle Limited. 2.16.

(42) GSM System Overview. 3. 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.. 2.17. © Wray Castle Limited. MB20/S2/v9.1.

(43) GSM System Overview. 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. © Wray Castle Limited. 2.18.

(44) GSM System Overview. 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.. 2.19. © Wray Castle Limited. MB20/S2/v9.1.

(45) GSM System Overview. 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. © Wray Castle Limited. 2.20.

(46) GSM System Overview. 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.. 2.21. © Wray Castle Limited. MB20/S2/v9.1.

(47) GSM System Overview. 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. © Wray Castle Limited. 2.22.

(48) GSM System Overview. 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.. 2.23. © Wray Castle Limited. MB20/S2/v9.1.

(49) GSM System Overview. 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. © Wray Castle Limited. 2.24.

(50) GSM System Overview. 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.. 2.25. © Wray Castle Limited. MB20/S2/v9.1.

(51) GSM System Overview. 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. © Wray Castle Limited. 2.26.

(52) GSM System Overview. 4. 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.. 2.27. © Wray Castle Limited. MB20/S2/v9.1.

(53) GSM System Overview. 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. © Wray Castle Limited. 2.28.

(54) GSM System Overview. 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.. 2.29. © Wray Castle Limited. MB20/S2/v9.1.

(55) GSM System Overview. GSM/UMTS Network. ‘Toolkits’/‘Enablers’ MExE. WAP. SAT. CAMEL. Services MMS/EMS. Downloads. Entertainment. LCS. Figure 15 Toolkits and Enablers MB20/S2/v9.1. © Wray Castle Limited. 2.30.

(56) GSM System Overview. 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. © Wray Castle Limited. MB20/S2/v9.1.

(57) GSM System Overview. SECTION 3. NETWORK ARCHITECTURE. © Wray Castle Limited. i.


(59) GSM System Overview. 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. © Wray Castle Limited. iii.


(61) GSM System Overview. 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. 3.45. © Wray Castle Limited. v.

(62) GSM System Overview. vi. © Wray Castle Limited.

(63) GSM System Overview. 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. © Wray Castle Limited. vii.

(64) GSM System Overview. 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.. 3.1. © Wray Castle Limited. MB20/S3/v9.1.

(65) GSM System Overview. 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. © Wray Castle Limited. 3.2.

(66) GSM System Overview. 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. © Wray Castle Limited. MB20/S3/v9.1.

(67) GSM System Overview. 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. © Wray Castle Limited. 3.4.

(68) GSM System Overview. 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. 3.5. © Wray Castle Limited. MB20/S3/v9.1.

(69) GSM System Overview. 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. © Wray Castle Limited. 3.6.

(70) GSM System Overview. 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. © Wray Castle Limited. MB20/S3/v9.1.

(71) GSM System Overview. 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. © Wray Castle Limited. 3.8.

(72) 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). 3.9. © Wray Castle Limited. MB20/S3/v9.1.

(73) GSM System Overview. 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. © Wray Castle Limited. 3.10.

(74) 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.. 3.11. © Wray Castle Limited. MB20/S3/v9.1.

(75) GSM System Overview. 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. © Wray Castle Limited. 3.12.

(76) GSM System Overview. 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. © Wray Castle Limited. MB20/S3/v9.1.

(77) GSM System Overview. 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. © Wray Castle Limited. 3.14.

(78) GSM System Overview. 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. © Wray Castle Limited. MB20/S3/v9.1.

(79) GSM System Overview. 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. © Wray Castle Limited. 3.16.

(80) GSM System Overview. 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. © Wray Castle Limited. MB20/S3/v9.1.

(81) GSM System Overview. 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. © Wray Castle Limited. 3.18.

(82) GSM System Overview. 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.. 3.19. © Wray Castle Limited. MB20/S3/v9.1.

(83) GSM System Overview. 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. © Wray Castle Limited. 3.20.

(84) GSM System Overview. 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.. 3.21. © Wray Castle Limited. MB20/S3/v9.1.

(85) GSM System Overview. 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. © Wray Castle Limited. 3.22.

(86) GSM System Overview. 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. © Wray Castle Limited. MB20/S3/v9.1.

(87) GSM System Overview. 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. © Wray Castle Limited. 3.24.

(88) GSM System Overview. 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.. 3.25. © Wray Castle Limited. MB20/S3/v9.1.

(89) GSM System Overview. 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. © Wray Castle Limited. 3.26.

(90) GSM System Overview. 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.. 3.27. © Wray Castle Limited. MB20/S3/v9.1.

(91) GSM System Overview. OM. C-. S. BSS. OMC-. R. OMC. events and alarms fault management performance management security management. Figure 14 OMC MB20/S3/v9.1. © Wray Castle Limited. 3.28.

(92) GSM System Overview. 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.. 3.29. © Wray Castle Limited. MB20/S3/v9.1.

(93) GSM System Overview. 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. © Wray Castle Limited. 3.30.

(94) GSM System Overview. 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.. 3.31. © Wray Castle Limited. MB20/S3/v9.1.

(95) GSM System Overview. 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. © Wray Castle Limited. 3.32.

(96) 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.. 3.33. © Wray Castle Limited. MB20/S3/v9.1.

(97) GSM System Overview. 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. © Wray Castle Limited. 3.34.

(98) 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.. 3.35. © Wray Castle Limited. MB20/S3/v9.1.

(99) GSM System Overview. 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. © Wray Castle Limited. 3.36.

(100) GSM System Overview. 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.. 3.37. © Wray Castle Limited. MB20/S3/v9.1.

(101) GSM System Overview. 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. © Wray Castle Limited. 3.38.

(102) GSM System Overview. 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.. 3.39. © Wray Castle Limited. MB20/S3/v9.1.

(103) GSM System Overview. GCR. group members priority entitlements network information. BSS. C Gs. HLR Gr. Figure 20 Group Call Register (GCR) MB20/S3/v9.1. © Wray Castle Limited. 3.40.

(104) 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.. 3.41. © Wray Castle Limited. MB20/S3/v9.1.

(105) GSM System Overview. 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. © Wray Castle Limited. 3.42.

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