GPRS in Practice · G:/PAGINATION/WILEY/GPRS/3B2/FINALS_12-07-04/PRELIMS.3D – 3 – [1–14/14] 12.7.2004 8:44PM GPRS in Practice A Companion to the Specifications

G:/PAGINATION/WILEY/GPRS/3B2/FINALS_12-07-04/PRELIMS.3D – 3 – [1–14/14] 12.7.2004 8:44PM

GPRS in PracticeA Companion to the Specifications

Peter McGuiggan

PMCG Consultancy, UK

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GPRS in Practice


GPRS in PracticeA Companion to the Specifications

Peter McGuiggan

PMCG Consultancy, UK


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To Angela

and Maximillian


Contents

Preface xiii

1 Introduction 11.1 The purpose of GPRS 11.2 So why GPRS? 11.3 Internet communication 31.4 Current Internet protocol – static addresses 51.5 Current Internet protocol – dynamic addresses 51.6 GPRS Internet addresses 51.7 Portable IP 71.8 The GPRS sub-network 101.9 Abbreviations used in this chapter 13

2 Radio Channels, Physical Channels and Logical Channels – theGSM/GPRS Air Interface 152.1 The radio channels (GSM 45.001) 152.2 Physical channels (GSM 45.001) 17

2.2.1 The characteristics of the GSM/GPRS physical channels(GSM 45.001) 19

2.3 Logical channels (GSM 45.001, 45.002, 43.064) 202.3.1 GSM logical channels 202.3.2 GPRS channels which are used with or without a PBCCH 222.3.3 GPRS logical channels which are used only in conjunction

with PBCCH 222.3.4 GPRS logical channels which are used in the absence

of a PBCCH 232.4 The BCCH radio carrier 242.5 The PBCCH 252.6 Abbreviations used in this chapter 25

3 Air Interface Frame and Multiframe Structures (GSM 45.002, 43.064) 273.1 The basic frame 293.2 The GPRS 52-frame multiframe and logical channel structures 30


3.3 The 52-frame multiframe uplink PRACH channel(GSM 45.002, 43.064) 34

3.4 The GSM 51-frame multiframe logical channel structures(non-combined configuration) 37

3.5 The GSM 51-frame multiframe and logical channel(combined configuration) 40

3.6 The GPRS 51-frame multiframe logical channel structures(GSM 45.002, 43.064) 40

3.7 Using the 51- and 52-frame logical channels 403.8 Abbreviations used in this chapter 45

4 The TBF and the MAC Layer (GSM 44.060, sections 5, 7, 8) 474.1 What is a TBF? An introduction to the temporary block flow 47

4.1.1 The radio link control/medium access control(RLC/MAC) block 47

4.1.2 Introduction to the MAC function (GSSM 44.060sections 5, 7, 8) 51

4.1.3 Combining the components of a TBF into a complete TBF 524.1.4 TBF arrow diagrams (GSM 44.060 sections 5, 7, 8, 9) 55

4.2 The MAC layer in action 584.2.1 Introduction: GPRS attach 58

4.3 ‘Attach’ MAC procedures (ETSI 123.060 section 6) 594.4 Packet data transfer – PDP context activation (ESTI 123.060

section 9) 654.5 GPRS sub-network originated TBFs 714.6 Alerting the mobile station for a DL TBF 754.7 Abbreviations used in this chapter 77

5 An Introduction to Protocol Layers Data Flow (ETSI 123.060) 795.1 The protocol stack 79

5.1.1 GPRS protocol layers – a brief description 825.2 GPRS signal flow – arrow diagrams (GSM 44.060 sections 5, 7, 8) 93

5.2.1 GPRS attach 935.2.2 Mobile originated PDP context activation and TBF

(ETSI 123.060 section 9) 975.2.3 Paging and MT PDP transfer (GSM 44.060 section 6) 100

5.3 Temporary block flow acknowledged 1015.4 Abbreviations used in this chapter 105

6 GPRS Mobile Station Characteristics (GSM 45.002) 1096.1 Mobile station types 1096.2 GPRS mobile equipment (ME) and subscriber profiles

(ETSI 123.060 section 15) 1096.2.1 Subscriber profile 109

6.3 Mobile equipment multislot capabilities (GSM 45.002 Annexe B) 1116.4 Abbreviations used in this chapter 119

viii Contents


7 Operations in the Physical Layers 1217.1 Physical layers 1227.2 PLMN selection (GSM 43.002 section 3) 1227.3 Initial cell selection (GSM 43.022 section 4) 1247.4 GPRS cell reselection when the GPRS sub-network is using

PBCCH (GSM 45.008 section 10) 1287.4.1 Conditions for GPRS cell reselection 1307.4.2 GPRS cell reselection parameters (GSM 45.008 section 10) 1307.4.3 GPRS cell reselection criteria (GSM 45.008 section 10) 1347.4.4 GPRS cell reselection when the GPRS sub-network is

not using PBCCH (GSM circuit switched cell reselection)(GSM 45.002 section 6) 138

7.5 Discontinuous reception (DRX) and paging in a cell with PBCCH 1437.5.1 Determining the PCCCH Group (deciding which physical

channel to use) 1447.5.2 Determining the paging group (GSM 45.002 section 6) 1447.5.3 Selection of paging group from PCCCHs 1487.5.4 Selection of paging group from CCCHs when there

is no PBCCH in a cell 1487.5.5 Monitoring PCCCHs and CCCHs for paging (the cell

has a PBCCH but no Gs interface) (GSM 45.002section 6) 154

7.5.6 Network mode of operation (NMO) and paging(GSM 44.060 section 6) 154

7.6 Neighbour cell measurements 1607.6.1 Measurements for cell reselection (GSM 45.008 section 6) 1627.6.2 Extended measurements (GSM 45.008 section 10) 1647.6.3 Interference measurements 1667.6.4 Measurement reports 1677.6.5 NC measurement reporting procedures 1677.6.6 EM reporting 1707.6.7 Interference reports 170

7.7 Mobile station transmitter power control (GSM 45.008section 10) 171

7.8 Timing advance (TA) (GSM 43.064 section 6) 1737.9 PRACH control parameters 1767.10 Contention resolution (GSM 43.064 section 6) 182

7.10.1 One-phase access contention resolution 1837.10.2 Two-phase access contention resolution 185

7.11 Channel encoding (GSM 45.003 and 43.064) 1867.12 Frequency hopping 190

7.12.1 Rayleigh fading 1917.12.2 Rician fading 1937.12.3 Interference spreading 1937.12.4 Frequency hopping 195

7.13 Abbreviations used in this chapter 207

Contents ix


8 RLC/MAC Layer Procedures (GSM 44.060) 2118.1 Introduction 2128.2 RLC procedures 212

8.2.1 Segmenting LLC PDUs into RLC data blocks(GSM 44.060 section 9) 217

8.3 RLC/MAC block headers and parameters (GSM 44.060section 10) 2208.3.1 RLC data block (uplink) parameters 2208.3.2 RLC/MAC parameters in action 2258.3.3 RLC control block (downlink) parameters 229

8.4 RLC unacknowledged mode 2358.5 RLC/MAC timers and counters (GSM 44.060 section 13) 235

8.5.1 Mobile station side RLC/MAC counters and timers 2368.5.2 Network side RLC/MAC counters and timers 244


9 LLC Layer Procedures (GSM 44.064) 2479.1 Function of the LLC layer 2489.2 LLC frames 252

9.2.1 Types of information/supervisory frame 2529.2.2 The unacknowledged information frame 2549.2.3 Types of unnumbered frame 2549.2.4 Flags and registers used for frame address fields 2549.2.5 Flags and registers used for control fields 2559.2.6 Registers used for variable arrays in the LLC 255

9.3 LLC operational parameters 2559.4 LLC data link flow – ABM establishment 256

9.4.1 Normal (successful) ABM establishment 2569.4.2 ABM establishment rejection (SAPI¼ 1) 2619.4.3 ABM establishment rejection, poor radio reception 261

9.5 Information/supervisory (IS) frame transfer 2619.5.1 IS frame transfer 262


10 GMM Layer Procedures (GSM 23.060 section 6 and 44.008sections 4 & 9) 26510.1 GMM states (GSM 23.060 section 6) 26710.2 GMM procedure attach (GSM 23.060 section 6) 269

10.2.1 Attach request primitives 27310.2.2 Attach accept primitives 27410.2.3 Attach complete 274

10.3 TLLI construction (GSM 23.060) 27510.4 Routing area update 275

10.4.1 Normal routing area update 27510.4.2 Periodic routing area update 279

10.5 Cell update 279

x Contents


10.6 Paging procedures 28110.7 Authentication and encryption procedures 28110.8 Identification 28610.9 Detach 286

10.9.1 Mobile originated detach 28610.9.2 GPRS sub-network originated detach 287

10.10 GPRS roaming 28810.11 Abbreviations used in this chapter 289

11 SM Layer Procedures (24.008 section 6) 29111.1 PDP context activation by the mobile station 29511.2 PDP context activation by the GPRS sub-network 30111.3 PDP context modification 30111.4 PDP context deactivation by the mobile station 30211.5 PDP context deactivation by the GPRS sub-network 30511.6 Negotiated QoS profiles and radio priority 305

11.6.1 QoS 30511.6.2 Radio priority 307


12 SNDCP Procedures (GSM 44.065) 31112.1 SNDCP operation overview 31312.2 Buffering, segmentation and acknowledged mode transmission

of network PDUs 31412.2.1 SN PDU headers 316

12.3 Acknowledged mode description of operation 31712.4 Buffering, segmentation and unacknowledged mode

transmission of network PDUs 31912.4.1 The unacknowledged SN-DATA PDU 319

12.5 Unacknowledged mode description of operation 32012.6 Management of multiple PDP contexts 32212.7 Abbreviations used in this chapter 324

Appendix 1: GMSK and EDGE 327A1.1 MSK 327A1.2 GMSK 329A1.3 EDGE – EGPRS 329A1.4 Abbreviations used in this appendix 331

Appendix 2: System Information and Packet System Information 333AS=appendix system informationAP=appendix packet system information

A2.1 Key 333AS.3 SI3 and SI4 messages on BCCH 335AS.13 SI13 messages on BCCH or extended BCCH 335AP.1 PSI1 messages on the PBCCH 339

Contents xi


AP.2 PSI2 messages on the PBCCH 342AP.3 PSI3 messages on the PBCCH 344AP.3 bis PSI3-bis messages on the PBCCH 347AP.4 PSI4 messages on the PBCCH and PACCH 347AP.5 PSI5 messages on the PBCCH 347AP.13 PSI13 messages on the PACCH 349A2.2 Abbreviations used in this appendix 349

Appendix 3: Inter-Layer Primitives 351A3.1 The interface user to mobility management – the

GMMREG-SAP and primitives 351A3.2 The interface user to SM – SMREG-SAP and primitives 351A3.3 The interface user to SNDCP – the SN-SAP and primitives 352A3.4 The interface SNDCP to SM – the SNSM-SAP and primitives 354A3.5 The interface SNDCP to LLC – the QoS-SAP and primitives 354A3.6 The interface SM to GMM – GMMSM-SAP and primitives 356A3.7 The interface GMM to LLC – the LLGMM-SAP and primitives 356A3.8 The interface GMM to RR (RLC/MAC) – the GMMRR-SAP

and primitives 361A3.9 The interface LLC to RR (RLC/MAC) – the GRR-SAP and

primitives 363A3.10 Abbreviations used in this appendix 363

Appendix 4: Mobile Station Uplink Power Control 365A4.1 Packet idle mode 365A4.2 Packet transfer mode 367A4.3 Abbreviations used in this appendix 368

Appendix 5: A Possible Problem with UL TBFs When the ApplicationLayer is Using TCP/IP 369

A5.1 Abbreviations used in this appendix 371

Recommended Reading 373

Index 375

xii Contents


Preface

The GSM system has now been in operation for 12 years and is a phenomenal success.This beautifully engineered system has passed all its tests with flying colours and is

probably the most successful self-contained technology ever!As they say, 1000 million (purported) customers cannot all be wrong!Is the GPRS system ‘bolted on’ to the GSM system as elegant in design and will it

become equally successful?The answer to the first part of this question is probably no, it is not such an elegant

design as it has to be adapted to a system which is designed for telephone conversations.The answer to the second part of the question is wait and see. It is becoming popular,but whether it will become so popular as to extend the life of GSM remains to be seen.EGPRS will assist in this direction, but only time will tell.

This book is constructed from four years of presenting the course ‘GPRS Operations’around the world. The course has been improved over the years as the GPRS system hasbecome operational and answers have been provided to ‘ . . . what will happen if’ andalso in no small part to the intense scrutiny to which the course notes have beensubjected and the ensuing lively discussions with engineers expert in their fields.

The book tries to present the subject matter in a simple way using simple English,whilst at the same time attempting to be thorough and rigorous.

Any engineer who has used the specifications to learn and understand the GSM andGPRS systems will know that the specifications ‘ . . . do what it says on the tin’; they givespecifications and not explanations. In general it is left to the reader to construct theunderlying concepts and puzzle out how the specifications will work in practice. Thereare some exceptions to this rule, but they are unfortunately thin on the ground.Learning from the specifications is very hard work.

This book attempts to give the engineer the concepts of GPRS and also to explainin detail how the GPRS air interface works. It is hoped that it will make a usefulcompanion to the specifications, with the specifications giving the word and this bookcasting some light upon those words.

There are many explanatory diagrams in the book. This is because I believe we tryto think in pictures as the best way of conceptualising quite complex ideas. Many ofthe diagrams are ‘busy’, but if the reader takes the time to examine the diagrams in


conjunction with the text, then I believe they will gain a good understanding of theGPRS concepts.

I would like to thank the many engineers around the world who have (in many casesunwittingly) helped me to write (and re-write!) this book. Your contributions to myunderstanding of the GPRS system have been invaluable. In particular I would like tothank Eric Mrzynski of Agere, Germany, a remarkable engineer who has contributedmuch through his discussions and correspondence. Also Dr Philip Williams of Sharp,UK who has always been open-minded in his discussions and constructive in hiscriticisms.

Finally I must thank my wife Angela; as she well knows, the writing of a book is amost solipsist, insufferable occupation, demanding patience, forbearance and tact fromthose closest to the author.

Peter McGuigannTamworth England

April 2004

xiv Preface

//INTEGRAS/KCG/PAGINATION/WILEY/GPRS/3B2/FINALS_12-07-04/CH001.3D – 1 – [1–14/14] 12.7.2004 4:27PM

1

Introduction

1.1 The purpose of GPRS

Because of the Internet, data communications over the fixed line telephone networknow exceed voice telephone calls in minutes and revenue. GPRS may achieve the sameend for mobile customers and, it could be said, has the business aim to provide wirelessInternet access to a mass market.

Wireless Internet access is also commonly available using standard GSM data circuitsand this capability is enhanced with the advent of HSCSD – high speed circuit switcheddata, which allows suitably equipped mobile stations to access multiple physical chan-nels, raising the access speed to multiples of 9.6 kb/s, the maximum rate for a singlephysical channel. In fact GPRS can do nothing that cannot already be done by HSCSD.CS means circuit switched, and HSCSD is high speed circuit switched data which usesmultiple physical channels to attain higher speeds.

1.2 So why GPRS?

A mass market probably emerges if the service provided is desirable and affordable.HSCSD is desirable, but whether it is affordable to a mass market remains to be seen.GPRS should be both desirable and affordable, as illustrated in Figure 1.1.

The available GSM radio bandwidth, the communication medium affected, is usedefficiently with packet transmission over the air interface, and this results in savings tothe customer. In addition, a customer uses the radio resources only when there is data tobe sent on the downlink or uplink and this should result in the customer being chargedonly for the packets which are transmitted or received. Compare this with HSCSD,where the radio resources are provided continuously until the call is closed down, andthe cost to the customer using GPRS should be reduced significantly.

GPRS in Practice: A Companion to the Specifications Peter McGuiggan� 2004 John Wiley & Sons, Ltd ISBN: 0-470-09507-5


A GPRS subscriber may also stay connected to the Internet all day, being chargedonly when the radio link is actually used to transfer data. This is largely true but over-simplified, as we shall see. (What this really means is the mobile station, once commu-nication is established with the Internet, is then in a position to receive data from theInternet – this condition can indeed last all day – or days if the GPRS sub-networkoperator tolerates it for so long!) The same capability should be possible with standardGSM data circuits but is currently not provided.

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In circuit switched communications a full duplex circuit is provided between theDTEs. As full duplex is rarely used and one direction, when in use is not used for100 % of the time, CS communication wastes resources and the user pays for thehire of the circuit irrespective of the percentage of the hired time that is used. Thismakes CS communications expensive, especially in Internet applications wherethere are long 'idle' periods in both directions.

Packet switched data communications uses the link much more efficiently andshould be cheaper to the user.

The sole purpose of GPRS is to condition standard IP 'packets' for transmissionacross the 'weak link' – the air interface Um, thus providing cheap mobile Internetaccess. It also provides an opaque vocabulary!

NSGS

NSGG

External FixedNetwork

SSB

Um

The weak link!

(a)

(b)

(c)

Figure 1.1 (a) Circuit switched data communications link; (b) Packet switched data com-munications link; (c) The purpose of GPRS

2 GPRS in Practice


This feature compares very favourably with current fixed network practices whereInternet data can only be sent to a customer after the customer has initiated a call to theInternet, and the communication is generally then closed down. However, at this time(January 2004) some Internet service providers (ISPs) and ISDN network providersare offering all-day Internet connection at very reasonable fixed monthly charges – insome cases even free connection – and of course broadband connections over landlinesalready do provide ‘all-day’ online services.

If GPRS over GSM does not become a mass-market service, then it could facea decline as has happened with other telecommunications services. If GPRS overGSM does become a mass-market service, its capabilities will be enhanced throughthe application of EDGE – enhanced data rates for GSM evolution – which intheory will raise the data rate of a GSM physical channel to 60 kb/s. These datarates could even delay the introduction of UMTS (the cellular technology thatwill eventually replace GSM), but in Europe enormous sums are committed alreadyto UMTS air-space, so it could be, because of this, that GSM GPRS services havea short lifespan.

Technically, GPRS over GSM is designed for one purpose – to adapt standard datapacket communications (TCP/IP, and other Internet standards) to the communicationmedium of the GSM air interface.

1.3 Internet communication

Figure 1.2 shows the GPRS sub-network used for Internet communication. It is calleda sub-network because it is a conduit to another network, the Internet.

Note from this diagram that the SGSN (serving GPRS support node) has a splitpersonality. Facing downlink to the radio side it is a sub-network (the GPRS network iscalled the sub-network in the specifications), and facing uplink towards the Internet it islooking at the backbone network. The backbone network is almost identical to thestandard Internet transmission network protocol.

There are then three networks to consider:

1. The true GPRS network which is called the sub-network.2. The backbone network linking the SGSN with GGSN (gateway GPRS support

nodes).3. The network, which is the external network (most commonly the Internet) to which

the GPRS sub-network provides connection services to its customers.

It is interesting to note that the GPRS mobile station includes both network and sub-network layers. This book will take the liberty of referring to the mobile station networklayer interchangeably as the network, user or application layer.

The SGSN has the responsibility of setting up data calls when requested by a mobilestation. This is called PDP context activation. The SGSN checks with the HLR that therequested service is valid, and then asks the GGSN to set up the data call to the Internet.A part of this call set-up includes the allocation of an IP address to the call – if the callset-up is successful, the mobile station is informed of the allocated IP address. This isdynamic IP address allocation.

Introduction 3


The SGSN also manages the attachment of mobiles to the GPRS sub-network(exactly analogous to GSM IMSI attach), and the tracking of the location, (cell orrouteing area) for each attached mobile station.

Each GGSN has a specific IP address of the form gatewayxx. (mnc). (mcc). gprs andthe Internet regards the GGSN as a server or router in the same way as any otherInternet server or router.

The GPRS IP addresses allocated to mobile stations may be dynamic or static. TheGPRS sub-network allocates dynamic IP addresses to a GPRS subscriber when a call isset up (PDP context activation). The mobile station keeps this allocated dynamic IPaddress when a call is completed and the Internet may use this address to send data tothe mobile station. However, dynamic IP addresses are only allocated when a mobilestation initiates a call and the mobile station cannot receive incoming Internet initiatedcalls – the Internet cannot initiate calls to an unknown address! Static addresses arepermanent IP addresses. These can receive Internet initiated calls, but there are com-plications, as we shall see.

Figure 1.2 shows three subscribers time division multiplexed onto one physicalchannel (not, it must be added, in standard GPRS form!). In fact GPRS allows upto eight subscribers to be multiplexed on to one physical channel if dynamicallocation is used. There are three methods of allocating radio resources to a mobilestation:

. Dynamic allocation, where the GPRS sub-network gives a series of allocations duringa TBF (temporary block flow);

. Fixed allocation, where the GPRS sub-network gives just one allocation for a TBF; and

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(Server)

Source address

Destination (MS) address

–Fixed IP address or temporaryIP address supplied by server.

Static PLMN IP address, ordynamic PLMN IP addresssupplied by PLMN.

Source (MS) address

Destination address

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, supplied temporarilyby the HLR

Fixed IP address or temporaryIP address supplied by destinationserver.

Static

Dynamic

IP address

For mobility management (GMM),the SGSN uses IMSI and PTMSIto identify the MS.For data transfer managementthe SGSN uses TLLI and SAPIto route packets to (and from)the MS.

Figure 1.2 The GPRS network for Internet communications

4 GPRS in Practice


. Extended dynamic allocation, where, for a mobile station that is allocated more thanone physical channel, only one instruction is given to allow the mobile station to useall of the allocated channels.

1.4 Current Internet protocol – static addresses

With current Internet protocol (IP), the IP address may be allocated permanently. It isthen a static IP address; these are not in favour in IPv4 as the Internet addresses arenearing exhaustion. It is common for the ISP or corporate Intranet administrator toallocate IP addresses to the user dynamically on a per-communication basis (dynamic IPaddresses).

For security reasons, many corporate Intranet users do have static IP addresses. TheInternet will use the server as the primary address and the server will then distributemessages to the individual addresses on its network. This is shown in Figure 1.3.

However, if a user with a static IP address moves networks, then that user cannotreceive Internet communications.

It is evident that if the home server has the forwarding address of the server on theforeign network, then communications to the static IP address can be forwarded to thatforeign server. This forms the basis of mobile IP, which allows mobility of static IPaddresses between various networks. We will call it portable IP to distinguish it fromcellular radio mobility aspects.

1.5 Current Internet protocol – dynamic addresses

Figure 1.4 shows a home network which has allocated a dynamic address to a user for acommunication. The dynamic address is unknown to other users until they are informedof it when the owner of the dynamic address initiates communication and all initialcommunication must be initiated by the dynamic address owner. This is commonplacefor home users of the Internet, where communication from the Internet cannot bereceived until the home user sets up a link to the Internet service provider; similarlywhen surfing the Internet, Internet sites are informed of the dynamic address which isaccessing them.

When the user moves to a foreign network, a new dynamic address may be allocatedto allow Internet communication to be initiated if the foreign network allows this.

1.6 GPRS Internet addresses

The mobile station on the left in Figure 1.5 is allocated a dynamic address (which isgiven when a mobile originated call is set up – PDP context activation), and canreceive communications from the external IP address that it has accessed. If thisaccessed address is, for example, the mobile station ISP then the mobile station andGPRS sub-network can leave the PDP context open all day (or a longer or shorterperiod contingent upon the QoS and the GPRS operator policy), and the ISP,

Introduction 5


knowing the mobile station dynamic address, can forward communication to thesubscriber.

For roaming, the IMSI is authenticated, as is normally the case for GSM operations(extracting the subscriber data and the authentication and ciphering triplets from thehome HLR). Chapter 10 covers roaming procedures in more detail.

Figure 1.6 further illustrates the working of dynamic IP addressing in GPRS sub-networks. The upper diagram shows a mobile station establishing a PDP contextactivation to its ISP. The GPRS sub-network supplies the IP address of the mobilestation dynamically. Establishing the call to the ISP, the network informs the ISP of themobile station’s dynamic address.

SERVER

INTERNET ACCESS

incoming calls from the Internetto the static address

133.4.16.XX

STATIC IP ADDRESS

'home'network

'foreign'network

133.4.16.XX

133.4.16.XX

133.4.16.

127.5.20.

Figure 1.3 Current IP – static addresses

6 GPRS in Practice


Upon completion of the communication the mobile station will go to GMM standbymode (a period exists after the TBF is completed, when the mobile station stays inGMM ready mode, before it changes to GMM standby), but the PDP context – inparticular the dynamic address – remains allocated to the mobile station. If the ISPsubsequently has a communication for that mobile station, this will be forwarded tothat GPRS IP dynamic address, and the GPRS sub-network will page the mobile stationwhich owns that address. The Internet message are then downloaded to the mobilestation. The lower diagram in Figure 1.6 shows this in operation.

1.7 Portable IP

Portable IP is a development of IP designed to allow a node or host with a fixed IPaddress to move from the home to a foreign network and yet receive communicationsfrom the Internet. A simplified principle of operation is shown in Figure 1.7.

SERVER

INTERNET ACCESS

no incoming calls from the Internetto dynamic addresses

DYNAMIC IP ADDRESS

'home'network

'foreign'network

133.4.16.YY

127.5.20.ZZ

133.4.16.

127.5.20.

allocated by network


Figure 1.4 Current IP – dynamic addresses

Introduction 7


The host with address 132.6.16.XX moves from its home network (server address132.6.16) to a foreign network (server address 126.7.13). The foreign network routerbroadcasts an agent advertisement and the visiting host receiving this, knows portable IPis used and registers with the foreign network. From that it gets the care-of address ofthe foreign network, and communicates that to the home network server.

Incoming data to the home network addressed to the absent host will be encapsulatedby the home server within the care-of address supplied and tunnelled down to the foreignnetwork. The foreign network receives this packet and removes the IP data with its‘absent host’ address from the tunnelling encapsulation and delivers it to the visiting host.

GGSN

INTERNET ACCESS

incoming calls from the Internetto static addresses

DYNAMIC IP ADDRESS

'home'GPRS

network

'foreignGPRS'network


dynamic IP addressallocated by networkwhen a PDP context

is established

no incoming callsfrom the Internet

to dynamicaddresses until

a PDP context isestablished

??

Data link forHLR-HLR

exchange oftriplets andsubscription

STATIC IP ADDRESS

Figure 1.5 GPRS Internet addresses

8 GPRS in Practice


When the visiting host establishes a communication with the Internet, the forwardrouteing from the visiting host can be very different from the return routeing which willalways be via the home network server. If the visiting mobile station is in a foreigncountry and the ISP (e.g. Intranet) is in the mobile’s home country, then all commu-nications to the Internet will be via the home country server.

Portable IP is not yet in general use.

Internet address knowsthe MS IP address

INTERNET ACCESS

DYNAMIC IP ADDRESS

'home'GPRS

network

previously allocated by network

NSGS

SSB

GGSN

S

PDPcontext

dynamic address

GMM STANDBY

PAGE (PTMSI)

PDP contextremains activein GMM standby

PDP contextremains activein GMM standby

Internet address

INTERNET ACCESS

DYNAMIC IP ADDRESS

'home'GPRS

network


PDPcontext

Internet address

NSGS

SSB

GGSN

PDP CONTEXT ACTIVATION

Internet address knowsthe MS IP address

PDPcontext

S

PDPcontext

GMM READY

1. The MS establishes a PDPcontext and gets a dynamicIP address.

2. When the data transfer iscompleted, the radio resourcesare taken from the mobile stationbut the PDP context and its IPaddress remain and can be usedby the Internet to send data to theMS.

Figure 1.6 ‘Hanging on’ to the Internet all day?

Introduction 9


1.8 The GPRS sub-network

This book looks at the GPRS services primarily from a mobile station viewpoint, whilstconsidering the flow of messages across the GPRS sub-network. This section brieflyintroduces the GPRS sub-network. Figure 1.8 shows the components of a combinedGSM/GPRS network.

The GPRS sub-network is subsidiary to and dependent upon the GSM circuitswitched network and cannot exist without it. In particular, the BCCH (broadcastcontrol channel) carrier of the GSM network must be accessed so that the mobilestation knows that GPRS services are provided.

The additional components required for a GPRS sub-network are the SGSN (servingGPRS support node) which is the hierarchical equivalent of the GSM MSC (mobile

ROUTER

ROUTER

incoming call fromthe Internet

to static address132.6.16xx

STATIC IP ADDRESS132.6.16.xx

STATIC IPADDRESS132.6.16.xx

'home'network

'foreign'network

132.6.16

126.7.13

132.6.16xx126.7.13

132.6.16xx126.7.13

132.6.16xx

'agent advertisement'broadcast

portable node obtains'care-of' address fromforeign router

portable node registers'care-of' address 126.7.13

with home routerTUNNEL

encapsulated in'care-of' address

foreign routerstrips tunneladdress anddelivers visitoraddress on LAN

INTERNETACCESS

Figure 1.7 Portable IP

10 GPRS in Practice


switching centre) but in this case applied to packet switched services, and the GGSN(gateway GPRS support node) which has an hierarchical equivalence to the GMSC, butin this case applied to Internet working. The GGSN, from the Internet’s point of view, isthe server providing access to a private network.

In the configuration shown in Figure 1.8, there is no connection between the SGSNand the VLR; this is currently the norm and means that the SGSN must incorporate themobility management functions which, for GSM, are performed through the VLR.However, for NMO1 (network mode of operation), the Gs interface provides a connec-tion between the SGSN and MSC, allowing simultaneous IMSI and GPRS attach andauthentication. Paging for circuit switched operations via the GPRS paging channel isalso available if the Gs interface and a packet broadcast control channel (PBCCH) arefunctional.

An additional feature seen in Figure 1.8 is the routeing area. This is used for locationidentification when the mobile station is in GMM (GPRS mobility management) statesstandby and ready. The figure shows a multitude of routeing areas for one SGSN;whether the GPRS sub-network operators will follow this or ascribe just one routeingarea to one SGSN is left to the operators’ discretion, however as the paging load forpacket switched operations is far greater than that for circuit switched operations it isnecessary to provide more routeing areas than location areas in order to reduce theGPRS sub-network paging loading.

The names of the various interfaces are shown, for example to the left of Figure 1.8,the Gn interface is the link connecting all SGSNs within a PLMN together. This is

GSM PLMN

HLR

AUC

EIR

IWF

PDNSMS SC

TRAU

MSC VLR

MSC SERVICE AREA

GGSN

VLR

MSC/SGSN SERVICE AREA

GMSC

MSC

BTSBTSBTS

BTSBTSBTS

BTSBTSBTSBTSBTSBTSBTSBTSBTSBTSBTSBTSBTS

BTSBTSBTS

BTSBTSBTS


BTSBTSBTS

BTSBTSBTS


BSCBSCBSC

BTSBTSBTS

BTSBTSBTS


BTSBTSBTS

BTSBTSBTS


BTSBTSBTS

BTSBTSBTS


LA – LOCATIONAREA 1




LA–LOCATION

AREA 3


LA–LOCATION

AREA 4

BSC

RA – GPRSrouteing areas

Opt.

a-z

12

34

56

78

9 0

Um

A-bis

Gb

Gn

Gn

Gi

Gs

Gp

GrGfD

BE

C

R

EE

F

GcGd

A

SMS-GMSCSMS-IWMSC

to other SGSN insame PLMN

to GGSN inother PLMNs

TRAU

SGSN

BSC BSC

PSTN/ISDN

Local ServiceArea

Figure 1.8 Phase 2þ network configuration

Introduction 11


necessary for handovers (or cell change orders as they are called for GPRS operations)from a cell in one SGSN service area to a cell in a different SGSN service area. It is alsonecessary for identification of a mobile station, as these use TLLI to identify themselvesand TLLI is derived from a PTMSI allocated by a particular SGSN. Should an SGSNreceive a TLLI which is not from within the group of PTMSIs belonging to thatSGSN, then it will contact the SGSN to which the TLLI does belong and the detailsof the mobile station and its current data transfer status are transferred across the Gninterface.

The Gp interface is used for PLMN roaming – it is expected that this will connect toan international switching centre so that each PLMN has only one roaming contactpoint rather than a connection to each of its roaming partners.

Finally, for this brief introduction, GPRS QoS (Quality of Service) parameters aremeasured between the R interface to the bottom of the diagram and the Gi interface tothe Internet. The interactions between the elements of the GPRS sub-network andmobile stations are covered in later chapters.

Table 1.1 summarises some of the interfaces, abbreviations and other characteristicsof the GPRS system.

Table 1.1

External protocols used IPMultiplexing FDD radio channels

TDM physical channelsTDM logical channels

Location management Routeing area and SoLSAInterfaces A-bis BTS-BSC

Gb BSC-SGSNGc GGSN-HLRGd SGSN-SMS-GMSCGf SGSN-EIRGl GGSN-PDNGn SGSN-GGSN/SGSNGp SGSN-GGSN & other PLMNGr SGSN-HLRGs MSC-SGSN

Internal protocolsGPRS tunnelling protocol (GTP) SGSN-GGSN/SGSNTransmission control protocol (TCP) SGSN-GGSN/SGSNUser datagram protocol (UDP) SGSN-GGSN/SGSNSub-network dependent convergence

protocol (SNDCP)MS-SGSN

Logical link control (LLC) MS-SGSNRadio link control/medium access

control (RLC/MAC)BSS-MS

Relay Transfers from BSS Um side to BSS NSS sideNetwork service SGSN-BSS

12 GPRS in Practice


1.9 Abbreviations used in this chapter

BCCH Broadcast control channelCS Circuit switchedEDGE Enhanced data rate for GSM evolutionGGSN Gateway GPRS support nodeGMM GPRS mobility managementGPRS General packet radio serviceGSM Global system for mobile communication

Network mode of operationNMO1 Gs interface operational and PBCCH

CS Paging on both physical channels CCCHand PCCCH

NMO2 No PBCCHAll paging on CCCHs

NMO3 PBCCH operational, no Gs interfacePaging separately on PCCCH and CCCHs

MS typesA Simultaneous CS and packet traffic operationB Packet traffic suspended to take CS callsC Only CS or packet trafficType 1 Non-simultaneous Tx and RxType 2 Simultaneous Tx and RxMultislot class Determines uplink and downlink physical

channelsAir Interface Logical Channels Packet broadcast control channel (PBCCH)

Packet common control channel (PCCCH)Packet access grant channel (DL PAGCH)Packet paging channel (DL PPCH)Packet random access channel (UL PRACH)Packet associated control channel (PACCH)Packet data traffic channel (PDTCH)

Air Interface Physical Channels Packet data channel (PDCH)

Maximum data rates per physical channelGPRS 9.05 kb/s GMSKGPRS 13.4 kb/s GMSKGPRS 15.6 kb/s GMSKGPRS 21.4 kb/s GMSKEGPRS 8.8 kb/s GMSKEGPRS 11.2 kb/s GMSKEGPRS 14.8 kb/s GMSKEGPRS 17.6 kb/s GMSKEGPRS 22.4 kb/s 8-PSKEGPRS 29.6 kb/s 8-PSKEGPRS 44.8 kb/s 8-PSKEGPRS 54.4 kb/s 8-PSKEGPRS 59.2 kb/s 8-PSK

Introduction 13


HLR Home location registerHSCSD High speed circuit switched dataIMSI International mobile subscriber identityIP Internet protocolISDN Integrated services digital networkISP Internet service providerMSC Mobile switching centreNMO Network mode of operationPBCCH Packet broadcast control channelPDP Packet data protocolPLMN Public land mobile networkPTMSI Packet temporary mobile subscriber identityQoS Quality of serviceSGSN Serving GPRS support nodeTBF Temporary block flowTCP/IP Transmission control protocol/Internet protocolTLLI Temporary logical link identifierUMTS Universal mobile telecommunications systemVLR Visitor location register

14 GPRS in Practice

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GPRS in Practice · G:/PAGINATION/WILEY/GPRS/3B2/FINALS_12-07-04/PRELIMS.3D – 3 – [1–14/14] 12.7.2004 8:44PM GPRS in Practice A Companion to the Specifications