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  • Masters Thesis Information and Communication Technology

    GPRS for GSM Upgrade Strategies using Network Redundancy

    Erlend Larsen

    Grimstad, Norway Spring 2001

  • GPRS for GSM Upgrade Strategies using Network Redundancy

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    Abstract The Serving GPRS Support Node (SGSN) is the most fundamental node in GPRS. Ericsson produces and manages an increasing number of SGSN nodes in the world. SGSN nodes require upgrades on software and hardware regularly, forcing restarts, which gives service interrupts for subscribers. This report presents four different upgrade strategies intended to avoid or minimize service interrupts, while being cost-efficient: Hot standby, Subscriber based PCU routing, RA based PCU routing, and Redundant links solutions. The RA based PCU routing is considered the best, because it provides a smooth transfer of subscribers from one SGSN to another on a routing area basis, but not under all circumstances. It employs the Inter SGSN Routing Area Update (ISRAU) procedure to transfer subscribers. The routing mechanism is based on existing technology, namely Point-To-Point BVC connections bound to different Network Service Entities (NSEs) leading to different SGSN nodes from each cell through the Packet Control Unit (PCU).

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    Preface The thesis has been done on assignment from Ericsson, Grimstad. Though the title has changed since I got the assignment Network Redundancy Upgrade Strategies in GPRS, the idea has stayed the same, to find an efficient way to upgrade an SGSN node. The learning curve has been exponential, starting out with absolutely no knowledge about the GPRS system. Thanks to a two day GPRS introduction course in January I got an insight in the system that since then has grown bigger and bigger, as I have dug further into it. During three months from January to June I have been given invaluable help and feedback from Product Manager Steinar Strmsvg, my supervisor. Together we have developed the various solutions, and with help from other people at Ericsson, namely Ole Jonny Gangsy, Kjell Lindborg, Terje Urfjell and Bent Vale, we have made these solutions probable. I would also like to thank Stein Bergsmark, who was the contact at Ericsson giving me the assignment. He has also been a great help through his thesis seminars, held at HiA, and his review of my report. As this thesis marks the end of 17 years of school, I want to take the opportunity to thank some of my fellow students, especially Morten Thorsen, Arnfinn Andersen, Sigurd N. Daland and Stian Haugen. Finally, Kikki has been a great support to me, as she always is. Erlend Larsen Grimstad, May 28, 2001

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    Table of Contents ABSTRACT _____________________________________________________________________ 2

    PREFACE ______________________________________________________________________ 3

    TABLE OF FIGURES ____________________________________________________________ 5

    TABLE OF TABLES _____________________________________________________________ 5

    DEFINITIONS AND ABBREVIATIONS_____________________________________________ 6

    CHAPTER 1 INTRODUCTION ____________________________________________________ 7

    1.1 BACKGROUND _______________________________________________________________ 7 1.2 PROBLEM AND THESIS DESCRIPTION ______________________________________________ 7 1.3 GOALS AND LIMITATIONS ______________________________________________________ 8 1.4 METHODS___________________________________________________________________ 8 1.5 REPORT OUTLINE _____________________________________________________________ 9

    CHAPTER 2 GPRS OVERVIEW __________________________________________________ 11

    2.1 INTRODUCTION _____________________________________________________________ 11 2.2 CIRCUIT SWITCHED AND PACKET SWITCHED COMMUNICATION ________________________ 11 2.3 GSM______________________________________________________________________ 12 2.4 GPRS FOR GSM ____________________________________________________________ 13 2.5 SGSN _____________________________________________________________________ 14 2.6 GGSN ____________________________________________________________________ 15 2.7 BSS ______________________________________________________________________ 15 2.8 PCU ______________________________________________________________________ 15 2.9 HLR ______________________________________________________________________ 16 2.10 MS ______________________________________________________________________ 17 2.11 INTERFACES _______________________________________________________________ 18 2.12 IMPORTANT IDENTITIES ______________________________________________________ 21 2.13 ROUTING AREA UPDATE _____________________________________________________ 23 2.14 GPRS ATTACH_____________________________________________________________ 24

    CHAPTER 3 POSSIBLE UPGRADE STRATEGIES __________________________________ 26

    3.1 INTRODUCTION _____________________________________________________________ 26 3.2 THE PRESENT UPGRADE METHOD________________________________________________ 27 3.3 TRANSMISSION-BASED APPROACH ______________________________________________ 28 3.4 HOT STANDBY SGSN_________________________________________________________ 28 3.5 SUBSCRIBER-BASED PCU ROUTING______________________________________________ 29 3.6 ROUTING AREA-BASED PCU ROUTING ___________________________________________ 32 3.7 REDUNDANT LINKS __________________________________________________________ 33 3.8 COMPARISON OF THE SOLUTIONS _______________________________________________ 37

    CHAPTER 4 IMPLEMENTING AN RA BASED PCU ROUTING SOLUTION ___________ 41

    4.1 INTRODUCTION _____________________________________________________________ 41 4.2 IMPLEMENTING _____________________________________________________________ 41 4.3 CONSEQUENCES _____________________________________________________________ 43

    CHAPTER 5 RESULTS __________________________________________________________ 44

    5.1 INTRODUCTION______________________________________________________________ 44 5.2 ROUTING AREA BASED PCU ROUTING ___________________________________________ 44 5.3 HOT STANDBY ______________________________________________________________ 46 5.4 SUBSCRIBER BASED PCU ROUTING ______________________________________________ 46 5.5 REDUNDANT LINKS __________________________________________________________ 46

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    CHAPTER 6 DISCUSSION _______________________________________________________ 48

    6.1 GENERAL __________________________________________________________________ 48 6.2 ROUTING AREA PCU ROUTING _________________________________________________ 48 6.3 HOT STANDBY SOLUTION______________________________________________________ 49 6.4 SUBSCRIBER BASED PCU ROUTING SOLUTION _____________________________________ 50 6.5 REDUNDANT LINKS SOLUTION __________________________________________________ 50 6.6 BEST RECOGNIZED SOLUTION __________________________________________________ 51

    CHAPTER 7 CONCLUSION______________________________________________________ 52

    7.1 INTRODUCTION _____________________________________________________________ 52 7.2 DIFFERENT STRATEGIES FOR UPGRADE ___________________________________________ 52 7.3 IMPLEMENTATION AND EFFECTS OF THE RAPR SOLUTION __________________________ 52

    CHAPTER 8 REFERENCES______________________________________________________ 54

    Table of figures FIGURE 2.1 CIRCUIT SWITCHED AND PACKET SWITCHED COMMUNICATION _________________ 12 FIGURE 2.2 GSM SYSTEM ARCHITECTURE ___________________________________________ 12 FIGURE 2.3 GPRS SYSTEM ARCHITECTURE __________________________________________ 14 FIGURE 2.4 THE PLACEMENT OF THE PCUS IN THE BSS ________________________________ 16 FIGURE 2.5 THE GPRS PROTOCOL STACK IN THE TRANSMISSION PLANE ___________________ 18 FIGURE 2.6 SGSN-BSS INTERFACE PROTOCOLS ______________________________________ 18 FIGURE 2.7 RELATIONSHIP BETWEEN NS-VCS AND NS-VLS ____________________________ 19 FIGURE 2.8 RELATIONSHIP BETWEEN BVCS AND NS-VCS ______________________________ 19 FIGURE 2.9 SGSN-MS INTERFACE PROTOCOLS_______________________________________ 20 FIGURE 2.10 INTER SGSN ROUTING AREA UPDATE PROCEDURE ________________________ 23 FIGURE 2.11 GPRS ATTACH _____________________________________________________ 25 FIGURE 3.1 SUBSCRIBER-BASED PCU ROUTING_______________________________________ 30 FIGURE 3.2 ROUTING AREA CHANGE FORCES AN INTER SGSN RA UPDATE_________________ 32 FIGURE 3.3 REDUNDANT LINKS ON THE PCU SGSN INTERFACE__________________________ 34 FIGURE 3.4 ATTACH CURVE FOR REATTACHING 100.000 SUBSCRIBERS ____________________ 35

    Table of tables TABLE 3.1 RELATIONS BETWEEN SGSN-ADDRESSES AND PCU-TECHNOLOGY ______________ 28 TABLE 3.2 THE VARIOUS SOLUTIONS WITH IMPORTANT CRITERIA ________________________ 37

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    Definitions and abbreviations See also GSM 01.04 Abbreviations and acronyms [1]. BSC Base Station Controller BSS Base Station System BSSGP Base Station System GPRS Protocol BTS Base Transceiver Station BVC BSSGP Virtual Connection BVCI BVC Identifier CN Core Network CS Circuit Switched ETSI European Telecommunication Standard Institute GGSN Gateway GPRS Support Node GPRS General Packet Radio Service GSM Global System for Mobile Communications GSN GPRS Support Node HLR Home Location Register IMSI International Mobile Subscriber Identity IP Internet Protocol ISRAU Inter SGSN Routing Area Update LCC Life Cycle Cost MSC Mobile-services Switc