Bss functions and paramaters

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Bss functions and paramaters

  1. 1. © Copyright 2002 AIRCOM International Ltd All rights reserved AIRCOM Training is committed to providing our customers with quality instructor led Telecommunications Training. This documentation is protected by copyright. No part of the contents of this documentation may be reproduced in any form, or by any means, without the prior written consent of AIRCOM International. Document Number: P/TR/003/P035/1.0a This manual prepared by: AIRCOM International Grosvenor House 65-71 London Road Redhill, Surrey RH1 1LQ ENGLAND Telephone: +44 (0) 1737 775700 Support Hotline: +44 (0) 1737 775777 Fax: +44 (0) 1737 775770 Web: http://www.aircom.co.uk GSM PRE LAUNCH BSS Functions and Parameters
  2. 2. Table of Contents PART 1 – OPTIMISATION PROCESSES 1. Introduction to BSS Parameter Optimisation 1.1 Introduction ....................................................................................................... 1 1.2 What is the BSS?............................................................................................... 2 1.3 What are BSS Parameters?................................................................................ 3 1.4 What is Optimisation? ....................................................................................... 5 2. The GSM Optimisation Process 2.1 Introduction ....................................................................................................... 7 2.2 The Purpose of Optimisation............................................................................. 8 2.3 The Reasons for Optimisation........................................................................... 9 2.4 The Benefits of Optimisation .......................................................................... 10 2.5 Outline GSM Optimisation Process ................................................................ 11 3. BSS Parameter Review 3.1 Introduction ..................................................................................................... 15 3.2 BSS Parameter Review Process ...................................................................... 16 3.3 Database Consistency and Change Control..................................................... 17 3.4 BSS Configuration Parameter Sets.................................................................. 17 3.5 BSS Configuration Parameter Types............................................................... 18 3.6 Adjusting BSS Configuration Parameters....................................................... 20 PART 2 – COMMON SITE/CELL PARAMETERS 4. Network Identifier Parameters 4.1 Introduction ..................................................................................................... 23 4.2 Subscriber Indentifiers..................................................................................... 24 4.3 Equipment Indentifiers.................................................................................... 26 4.4 Call Number Indentifiers................................................................................. 28 4.5 Call Routing Indentifiers................................................................................. 29 5. Common Site and Cell Parameters 5.1 Introduction ..................................................................................................... 33 5.2 Cell Indentifiers............................................................................................... 34 5.3 Cell Functions................................................................................................... 35 5.4 Cell Channel Configurations........................................................................... 36 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 i
  3. 3. PART 3 – IDLE MODE FUNCTIONS AND PARAMETERS 6. Network Access 6.1 Introduction .................................................................................................... 39 6.2 Network Access Procedures............................................................................ 40 6.3 Key Network Access Parameters .................................................................... 43 7. BCCH Allocation Lists and Idle Mode Measurements 7.1 Introduction ..................................................................................................... 47 7.2 Generic Neighbour List Functionality............................................................. 48 7.3 Idle Mode Cell Measurements......................................................................... 51 7.4 Key Neighbour Relation Parameters............................................................... 53 8. Cell Selection/Reselection 8.1 Introduction .................................................................................................... 55 8.2 Cell Selection Procedures................................................................................ 56 8.3 Cell Reselection Procedures............................................................................ 57 8.4 Summary of Key Cell Selection/Reselection Parameters ............................... 62 Self Assessment Exercises .............................................................................. 65 9. Location Management and Paging Requirements 9.1 Introduction .................................................................................................... 67 9.2 Location Management Procedures .................................................................. 68 9.3 Paging Procedures ........................................................................................... 71 9.4 Calculations Using Paging Parameters............................................................ 73 9.5 Key Paging Parameters.................................................................................... 74 9.5 Key Location Management Parameters........................................................... 77 Self Assessment Exercises .............................................................................. 79 10. Frequency Hopping 10.1 Introduction ................................................................................................... 81 10.2 Frequency Hopping Procedures .................................................................... 82 10.3 Key Frequency Hopping Parameters............................................................. 87 ii GSM PRE LAUNCH - BSS Functions and Parameters © AIRCOM International 2002
  4. 4. PART 4 – DEDICATED MODE FUNCTIONS AND PARAMETERS 11. Dedicated Mode Cell Measurements 11.1 Introduction .................................................................................................. 89 11.2 Dedicated Mode Cell Measurement Procedures and Parameters.................. 90 12. Power Control 12.1 Introduction ................................................................................................... 99 12.2 Power Control Functions............................................................................. 100 12.3 Adaptive Power Control.............................................................................. 103 12.4 Discontinuous Transmission (DTX)............................................................ 107 12.5 Discontinuous Reception (DRX)................................................................. 109 12.6 Summary of Key Power Control Parameters .............................................. 111 Self Assessment Exercises ............................................................................ 15 13. Adaptive Frame Alignment 13.1 Introduction ................................................................................................. 117 13.2 Timing Advance Procedures ....................................................................... 118 13.3 Extended Cell Range ................................................................................... 119 13.4 Key Timing Advance Parameters................................................................ 120 14. Handover 14.1 Introduction ................................................................................................. 123 14.2 Handover Procedures................................................................................... 125 14.3 Key Handover Parameters........................................................................... 129 Self Assessment Exercises .......................................................................... 133 Appendix A - Vendor Parameter Table Appendix B - Answers to Self-Assessment Exercises Appendix C - Glossary of Terms GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 iii
  5. 5. Intentional Blank Page iv GSM PRE LAUNCH - BSS Functions and Parameters © AIRCOM International 2002
  6. 6. Course Objectives and Structure The objectives of the BSS Functions and Parameter Optimisation Course are to enable the delegate to: • Understand the meaning of the terms ‘BSS’, ‘parameters’ and ‘optimisation’ in the context of GSM networks important for Network Pre launch Optimisation • Be able to describe the GSM Optimisation process and, in particular, the purpose of the BSS parameter review within this process. • Be familiar with the purpose and structure of network and cell identifier parameters and the parameters that control air-interface channel configurations • Be familiar with the functionality and parameters that control the following Idle Mode functions: • BSS mobile access functions and the key controlling parameters • Idle mode cell measurement functions and the key controlling parameters • Cell selection and reselection functions and the key controlling parameters • Paging mode functions and the key controlling parameters • Location management functions and the key controlling parameters • Be familiar with the functionality and parameters that control the following Dedicated Mode functions: • Dedicated mode cell measurement functions and the key controlling parameters • Power control functions and the key controlling parameters • Frequency hopping functions and the key controlling parameters • Handover functions and the key controlling parameters • Adaptive frame alignment functions and the key controlling parameters PART 1 - Introduction to BSS Parameter Optimisation PART 2 – Common Site / Cell Parameters PART 3 – Idle Mode Functions and Parameters PART 4 – Dedicated Mode Functions and Parameters Course OutlineCourse Outline GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 v
  7. 7. Course OutlineCourse Outline PART 2 - Common Parameters 4. Network Identifier Parameters 5. Common Site and Cell Parameters 6. Network Access PART 1 - Introduction 1. Course Introduction 2. The GSM Optimisation Process 3. BSS Parameter Review PART 3 – Idle Mode 7. BCCH Allocations Lists and Idle Mode Measurements 8. Cell Selection / Reselection 9. Location Management and Paging Requirements 10. Frequency Hopping Course OutlineCourse Outline PART 4 – Dedicated Mode 11. Dedicated Mode Cell Measurements 12. Power Control 13. Adaptive Frame Alignment 14. Handover Requirements vi © AIRCOM International 2002 GSM PRE LAUNCH - BSS Functions and Parameters
  8. 8. 1. Introduction to BSS Parameter Optimisation GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 1 1. Introduction to BSS Parameter Optimisation _____________________________________________________________________ 1.1 Introduction This section provides an introduction to the course by analysing the title of the course i.e. it describes what is meant, in the context of this course, by: • The BSS • Parameters • Optimisation
  9. 9. 1. Introduction to BSS Parameter Optimisation 2 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 _____________________________________________________________________ 1.2 What is the BSS? 1.2.1 THE BSS COMPONENTS OF THE GSM NETWORK GSM Architecture OverviewGSM Architecture Overview BTSBTS BSCBSC HLRHLR MSCMSC AuCAuC EIREIR PSTNPSTN TRXTRX Air InterfaceAir Interface (Um)(Um) BSSBSS OMCOMC A InterfaceA InterfaceAbisAbis InterfaceInterface MSMS MSMS MSMS VLRVLR NSSNSS The Base Station SubThe Base Station Sub--System (BSS)System (BSS) • The BSS comprises: • Base Station Controller (BSC) • One or more Base Transceiver Stations (BTSs) • The purpose of the BTS is to: • provide radio access to the mobile stations • manage the radio access aspects of the system • BTS contains: • Radio Transmitter/Receiver (TRX) • Signal processing and control equipment • Antennas and feeder cables • The BSC: • allocates a channel for the duration of a call • maintains the call: – monitors quality – controls the power transmitted by the BTS or MS – generates a handover to another cell when required • Siting of the BTS is crucial to the provision of acceptable radio coverage BSCBSC BTS BTS BTS BSSBSS BTS
  10. 10. 1. Introduction to BSS Parameter Optimisation GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 3 The Base Station System (BSS) is the system of base station equipments (transceivers, controllers, etc) which is viewed by the MSC through a single A-interface as being the entity responsible for communicating with Mobile Stations in a certain area. The radio equipment of a BSS may support one or more cells. The BSS consists of one Base Station Controller (BSC) and one or more Base Transceiver Station (BTS). Where multiple BTSs exist, an A-bis interface is implemented between the BSC and each BTS. BSS Network TopologiesBSS Network Topologies • Chain: cheap, easy to implement • One link failure isolates several BTSs • Ring: Redundancy gives some protection if a link fails • More difficult to roll-out and extend • ring must be closed • Star: most popular configuration for first GSM systems • Expensive as each BTS has its own link • One link failure always results in loss of BTS BSC BSC BSC Base stations are linked to the parent BSC in one of several standard network topologies. The actual physical link may be microwave, optical fibre or cable. Planning of these links may be done using a software application such as AIRCOM’s Connect planning tool. _____________________________________________________________________ 1.3 What are Parameters? Mobile network parameters are generally database settings or hardware switches within a network element that are used to control the functionality of that element. There are many types of parameters but can be grouped into the classes described in this section:
  11. 11. 1. Introduction to BSS Parameter Optimisation 4 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 What Are Parameters?What Are Parameters? • Identifier Parameters • Uniquely identifies network elements • Functional Parameters (Flags) • Enable/disable functions • Timer Parameters • Time-dependant counters • Counter Parameters • Event-dependant counters • Threshold Parameters • Define operating limits • Measurement Parameters • Stores measured values 1.3.1 IDENTIFIER PARAMETERS Identifier parameters are those that identify a specific element or functional area in the network. 1.3.2 FUNCTIONAL PARAMETERS Functional parameters are those that are used to turn on or off specific functions within a network entity. Hence there are generally in one of two states: • On. Function has been activated • Off. Function is available but inactive. 1.3.3 TIMER PARAMETERS Timers are used to control the time period for which a certain condition exists. They are always in one of three states: • Off/Reset. The timer is set to 0 and is inactive • Counting. An event has occurred that has caused the timer to start incrementing. • Expired. The timer has reached a pre-determined value and has expired, generally triggering a new event.
  12. 12. 1. Introduction to BSS Parameter Optimisation GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 5 1.3.4 COUNTER PARAMETERS Counter parameters are similar to timers in that they can be in one of the same three states at any one time. However, whereas timers increment on a time basis (e.g. every second), counters increment on an event basis i.e. every time a specific event occurs. They can be linked to a threshold, beyond which a subsequent event can be triggered 1.3.5 THRESHOLD PARAMETERS These are values which, when exceeded above or below, will trigger and event. Threshold parameters can either be: • Fixed. For example, designed into the hardware • Variable. Set by operator to meet specific local operating conditions. 1.3.6 MEASUREMENT PARAMETERS These are parameters that stores measured values or averages of measured values. For example, power level measurements. _____________________________________________________________________ 1.4 What is Optimisation? The goal of optimisation is to ensure the network is operating at optimum efficiency within the defined quality of service constraints. What is Optimisation?What is Optimisation? • Dictionary Definition: ‘Determining the best compromise between potentially conflicting requirements in order to plan and implement an activity with maximum efficiency.’ • Mobile Radio Definition: ‘The identification and rectification of performance affecting problems within the constraints of an existing network infrastructure in order to maximise its efficiency.’
  13. 13. 1. Introduction to BSS Parameter Optimisation 6 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 This is achieved by implementing corrective action and procedures to rectify network problems identified though analysis of performance management monitoring parameters. The reason this optimisation process is carried out is to: • Maintain or improve quality of service • Reduce churn rate by retaining existing customers • Attract new customers. Section 1Section 1 -- SummarySummary • In this section the following topics have been covered: • What is the BSS in a GSN Network • Basic BSS topologies • What is meant by ‘Parameters’ • What is meant by ‘Optimisation’
  14. 14. 2. The GSM Optimisation Process GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 7 2. GSM Optimisation Process _____________________________________________________________________ 2.1 Introduction This section of the course reviews the purpose and goals of optimisation in a GSM network. It looks at the generic GSM optimisation process in order to identify the point in the process at which it is recommended that BSS parameter changes take place. It should be noted that the optimisation process is dependant upon a number variables and, as such the process may vary from organisation to organisation. GSM OptimisationGSM Optimisation No prescribed methodology Often network architecture dependant Often vendor equipment-dependant Often engineer-experience dependant Optimisation is an art as much as a science A ‘tool-box’ approach
  15. 15. 2. The GSM Optimisation Process 8 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 _____________________________________________________________________ 2.2 The Purpose of Optimisation Purpose of OptimisationPurpose of Optimisation • To correct identified performance shortfalls • To ensure network performance remains within QoS constraints • To make an existing network more efficient The essential purpose of optimising a network is to improve the current overall quality of a mobile network. This quality improvement can be achieved by addressing one or a combination of the following: • To correct identified performance shortfalls. These shortfalls are identified through continual monitoring of the defined network Key Performance Indicators (KPIs) or through customer complaints. • To ensure network performance remains within QoS constraints. When a network is initially designed, one of the key planning constraints is the Quality of Service (QoS) to be offered to customers. It is beholden upon the operator to maintain or improve the QoS levels advertised and offered to customers. • To make a network more efficient. It may become necessary to try and increase the revenue being generated by the network with minimum further investment. This can only be achieved by efficiency improvements within the existing network infrastructure.
  16. 16. 2. The GSM Optimisation Process GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 9 _____________________________________________________________________ 2.3 The Reasons for Optimisation There are a number of reasons for the instigation of an optimisation process: • After completion of network rollout where monitoring of KPIs indicates that problems are occurring due to incorrect initial planning assumptions. • On implementation of a new service (e.g. SMS/GPRS) in an attempt to introduce the new service with minimum impact on existing service levels and with minimum additional infrastructure investment. • As a result of problems identified through a Network Audit. • As a result of on-going performance monitoring where faults or performance degradation trends have been identified. • Where a new business case has been generated to increase the network performance above original targets without additional infrastructure investment in order to boost Return On Investment (ROI). • When it has been decided that certain operating parameters are to be changed e.g. a change to the offered Grade of Service from 2% to 1.5%. Reasons for OptimisationReasons for Optimisation • Correct identified post-roll-out inefficiencies • Preparation for new service implementation • Correct identified Network Audit performance deficiencies • Correct identified KPI performance monitoring degradation • Improve network efficiency to meet business requirements • Deliberate change in network operating parameters
  17. 17. 2. The GSM Optimisation Process 10 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 _____________________________________________________________________ 2.4 Benefits of Optimisation Successful network optimisation should accrue the following benefits: • Maintain or improve existing quality of service • Reduce churn rate by retaining existing customers • Attract new customers through offering better services and/or grade of service, achievable through efficient network performance. • Maximise revenue generating services by maximising efficiency of functional network elements. Benefits of OptimisationBenefits of Optimisation • Maintain/improve QoS • Reduce churn rate • Attract new customers • Maximise revenue-generating service • Maximize efficiency of network functional elements Vendors are continually seeking ways of maximising revenue generation with minimum additional investment. One way of achieving this is to identify areas where the network is not operating at peek efficiency and making adjustments for improvement. For example, over capacity may exist in certain areas allowing for a possible removal of TRXs. Alternatively, congestion may exist in certain areas and by prudent optimisation, additional capacity can be generated with no additional infrastructure investment. The Vendor may wish to add new services to the network (such as HSCSD/GPRS) in order to attract more customers and/or increase revenue generation. This may generate an increased requirement for network capacity either in terms of additional customers or an average increase in traffic per existing customer. Network optimisation may enable these services to be introduced with minimal additional infrastructure investment to meet the increased capacity demand.
  18. 18. 2. The GSM Optimisation Process GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 11 It may be that the original network design was based on flawed information and as a result the network is not performing as originally envisaged. Alternatively, information on which the network design was based has subsequently changed, requiring a change to the network configuration. For example, a new airport or shopping mall has been built creating unforeseen congestion in a particular part of the network. _____________________________________________________________________ 2.5 Outline Optimisation Process 2.5.1 PERFORMANCE MANAGEMENT AND OPTIMISATION Performance Management CyclePerformance Management Cycle Monitor Network Analyse Data Yes Identify Problems Implement Changes No Initial Network Design and Implementation Performance Management Optimisation QoS Targets Met? Optimisation can form part of the performance management process. The objective of the radio network optimisation is to extract the optimum performance from the cellular network, at any given phase of its lifecycle. All cellular systems will be associated with continuous change, with new radio sites being introduced, old sites being enhanced and assigned additional frequencies, omni-directional sites being sectorised, new frequency plans being implemented in different regions, etc. The initial step in performance management is to define a set of QoS (Quality of Service) parameters such as dropped call rates and call success rates. Key metrics are derived from data collected from sources such as drive tests, statistical data, customer complaints and field engineer reports and are used to measure the performance of the network. These metrics are analysed and compared to the QoS targets in order to identify any performance degradation in the network. If problematic areas are identified from analysis of the network performance parameters, corrective processes and/or procedures are implemented to rectify the situation using one or a combination of techniques. This process of corrective actions is known as optimisation.
  19. 19. 2. The GSM Optimisation Process 12 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 2.5.2 OUTLINE OPTIMISATION PROCESS Outline Optimisation ProcessOutline Optimisation Process Initial network design and roll-out phase Monitoring/ Network Audit Phase Optimisation Activity Phase Design Review and growth phase 2.5.3 NETWORK AUDIT PHASE OF OPTIMISATION Network Audit Phase of OptimisationNetwork Audit Phase of Optimisation Decide on KPIs, Measurement Strategy and Tools Measure Performance, Establish Benchmark Performance Review to Identify Major Performance Affecting Issues Decide on Strategy, Establish Action Plan Feedback from Optimisation Activities Start Optimisation Activity
  20. 20. 2. The GSM Optimisation Process GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 13 The network audit phase serves two primary purposes: • A review of existing network hardware and software (inc database) configurations to determine the both validity and consistency across the network. • Analysis of data gathered from performance-related network monitoring in order to identify weaknesses or sub-optimal operating performance levels. 2.5.3.1 Deciding on KPIs, Measurement Strategy and Tools This step in the process is normally only implemented when these elements do not exist within the network. A mature network should already have its KPIs and measurement strategy in place, together with tools to support this process. However, in such cases, part of the network audit process would be to validate these measurement counters and procedures. 2.5.3.2 Measuring Performance and Establishing Benchmarks In mature networks, benchmarks would have already been established. Measuring performance is an ongoing process. However, if a network audit is to carried out as an independent process, a review of existing performance parameters and benchmarks will be required to ensure their validity and applicability to the audit taking place. 2.5.3.3 Performance Review Having established benchmarks and validated performance parameters, a review of performance is carried out. A possible structure of such a review is as shown in the slide below. The performance Review is not intended to provide all the answers to all the problems, but to highlight the major issues and provide all the necessary background for further analysis, investigation and in-depth troubleshooting of the major performance-impacting problems in the network. It is important that any network performance audit should follow a methodical process and should be systematic in its approach to data collection.
  21. 21. 2. The GSM Optimisation Process 14 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 2.5.4 ACTIVITY PHASE OF OPTIMISATION Activity Phase of OptimisationActivity Phase of Optimisation Start Optimisation Activity Identify and Fix Hardware ProblemsOngoing Performance Measurement Process Review Process and Results Feedback to modify strategy decisions BSS Database Parameters Review: Settings Consistency Change Control Design Review and Growth Plan Identify and Fix Neighbour Problems Identify and Fix Frequency Plan Problems Network Audit Procedures Feedback to Audit Procedures SectionSection 22 -- SummarySummary • In this section the following topics have been covered: • The purpose of Optimisation • The reasons for carrying out Optimisation • The benefits of Optimisation • The Performance Management Cycle • An Optimisation Process • The Network Audit phase of Optimisation • The Activity Phase of Optimisation
  22. 22. 3. BSS Parameter Review GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 15 3. BSS Parameter Review _____________________________________________________________________ 3.1 Introduction The BSS database parameter review is intended to review existing BSS database parameter settings in the light of performance measurement results, and recommend changes necessary to improve or optimise the performance of specific features
  23. 23. 3. BSS Parameter Review 16 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 _____________________________________________________________________ 3.2 BSS Parameter Review Process 3.2.1 REVIEW DATABASE SETTING BSS Configuration Parameter ReviewBSS Configuration Parameter Review • Review current settings as related to performance measurement results • Recommend changes to improve/optimise performance of specific features. • Review includes: • Handover parameters, timers, thresholds and margins • Power control thresholds • Voting and average mechanisms for handover and power control • Call setup parameters to maximise resource utilisation • C1/C2 cell reselection parameters • Any vendor-specific advance traffic management algorithms This review includes the following: • Handover parameters, timers, thresholds and margins (including inter-layer and inter-band handovers where applicable). • Power Control thresholds (power window settings, power up/down step size, adaptive power control parameters, and so on). • Voting and averaging mechanisms for handover and power control decisions. • Call setup and handover timers to maximise resource utilisation and availability • C1/C2 cell reselection parameters. • Advanced traffic management algorithms (Congestion-based handovers, inter-band and inter-layer traffic distribution)
  24. 24. 3. BSS Parameter Review GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 17 _____________________________________________________________________ 3.3 Database Consistency and Change Control BSS Database Parameter ConsistencyBSS Database Parameter Consistency • Within a network, different site types are defined (e.g. urban micro, rural macro etc) by a standard template • Each site type database will comprise a default parameter set • Each site may modify default set to suit local conditions • Consistency of the default parameter sets should be checked across BSS types • Change control management processes should be reviewed to ensure procedural consistency Review of all BSS databases to identify inconsistencies and discrepancies, and a review of change control, datafill and database management processes. For example: • Define pre-configured parameter templates for a variety of site types (micro, highway 2/2, urban 3/3/3, etc.). • Identify sets of parameters allowed for optimisation on permissions basis (fully configurable permissions per BSC, region, etc). • Check consistency of live network data against planned configuration. _____________________________________________________________________ 3.4 BSS Configuration Parameter Sets Each BSS performs in accordance with its software configuration. Generally, there will be a common set of default parameters for each BSS, for example, the handover algorithms. However, each BSS will be programmed with certain parameters which are tailored to suit specific actions, locations and/or applications. Examples of BTS-specific parameters include the cell ID and power output settings.
  25. 25. 3. BSS Parameter Review 18 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 BSS Configuration Parameter SetsBSS Configuration Parameter Sets • Each BSS operates in accordance with its software configuration • All BSS are configured with certain standard default parameters • Each BSS will have a subset of BSS-specific parameters • Potentially hundreds of configuration parameters • Many parameters are inter-dependant • Often vendor-specific abbreviations/acronyms used for same parameter The number of configurable BSS parameters run into their hundreds and are defined in two sources: • ETSI GSM Recommendations. ETSI has defined a primary set of parameters which are listed in the GSM 12.04) document. • Proprietary Parameters. These are additional parameters created by individual vendors to enhance the capabilities of their equipment when compared that of their competitors. This multitude of parameters allows for very sophisticated control of the BSS behaviour. However, the complexity can also lead to problems: • Many parameters are inter-related so changing one can have a corresponding effect on others. • Many equipment manufacturers use different abbreviations or acronyms for the same GSM-recommended parameter, leading to potential confusion when a network comprises equipment for more than one manufacturer. _____________________________________________________________________ 3.5 BSS Parameter Types As mentioned above, the number of performance configuration parameters in a BSS runs into three figures. Within the scope of this overview course, the following provides an overview of the type of parameters stored:
  26. 26. 3. BSS Parameter Review GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 19 BSS Configuration Parameter TypesBSS Configuration Parameter Types • Identifiers: • CI, LAI, GCI, BSIC etc • Channel Configuration: • TCH channels, Signalling channel configuration (e.g. CCCH) • Timers: • Location Updates, C2 calculations etc • Thresholds: • RxLev, RxQual for handover decisions etc • Offsets: • Hysteresis for handovers etc • Control Features: • SFH, DTX, DRX etc 3.5.1 IDENTIFIERS This includes such parameters as the Cell Identity (CI), Cell Global Identity (CGI) Location Area Identity (LAI) and Base Station Identity Code (BSIC) etc 3.5.2 CHANNEL CONFIGURATION PARAMETERS These parameters define the number of traffic channels and control channels. For example, the configuration of CCCH on the signalling channel (i.e. combined/non-combined multiframes). 3.5.3 TIMER PARAMETERS Timers are counters which are set at the start of a certain time period and count down. If an event has not happened by the time the counter reaches zero, an alternative action may be triggered. For example, the time periods between periodic location updates (T3212) or the penalty timer for C2 calculations (see Cell Reselection in Section 1 for details). 3.5.4 THRESHOLD PARAMETERS Thresholds are certain values which, when exceeded, trigger a certain event. For example, received signal strength or bit error rate thresholds which may trigger cell handovers. 3.5.5 OFFSET PARAMETERS Offsets are fixed values applied for the purposes of applying bias to certain actions. An example is the hysteresis bias value applied to BTS at location area boundaries.
  27. 27. 3. BSS Parameter Review 20 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 3.5.6 CONTROL FEATURE PARAMETERS A number of parameter settings exist to identify the implementation of certain functions and features. Such features include Frequency Hopping, DTX etc _____________________________________________________________________ 3.6 Adjusting BSS Configuration Parameters Adjusting BSS Configuration ParametersAdjusting BSS Configuration Parameters • Effected from: • PC connected directly to hardware • Remotely from OMC/NMC • Can be individually addressed or broadcast • May require hardware reset to effect change • Be aware of hierarchical changes (MSC BSC BTS) • Only implement during low-traffic periods • Use test BSS where available • Avoid simultaneous multiple parameter changes It is normally possible to carry out parameter changes from: • BSS – directly into the BSS database via a connected PC. • OMC/NMC – Many OMC/NMC systems allow parameters to be transmitted to the BSS remotely including a broadcast capability where a specific parameter change needs to be transmitted to several network entities simultaneously. Adjustment of parameters on live network elements should be deferred until low traffic periods in order to minimise any disruption to existing users. This is particularly important when a configuration change requires a hardware rest to become effective. A change BSC parameter is changes it may affect all BTSs associated with that BSC. Some network operators reserve a BSS for test purposes. This has the advantage of being able to assess the impact of a parameter change before making the adjustment to a live network. However, it is a non-revenue generating asset.
  28. 28. 3. BSS Parameter Review GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 21 Making simultaneous multiple parameter changes should also be avoided where possible for two reasons: • If an unexpected problem arises as a result of a multiple parameter change, it will be difficult to identify the specific parameter or parameter combination causing the problem. • Similarly if a performance improvement is observed, it may be difficult to identify which of the parameters are causing which part of the performance improvement.
  29. 29. 3. BSS Parameter Review 22 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 Section 3Section 3 -- SummarySummary • In this section the following topics have been covered: • The BSS Parameter review process • The importance of database consistency and control • BSS configuration parameter sets • BSS configuration parameter types • Practical aspects of changing parameter settings
  30. 30. 4. GSM Network Identifiers GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 23 4. GSM Network Identifiers _____________________________________________________________________ 4.1 Introduction There are many identifiers used within the GSM system for establishing a unique identity for each of the network elements, areas and subscribers. Many of these identities are used as parameters for controlling the performance of network elements. This section of the course details the main GSM network identifiers so that the reader will be aware of their meaning and composition when referred to later in the course. GSM Network Identifier ParametersGSM Network Identifier Parameters • Subscriber Identifier Parameters: • IMSI / TMSI • Equipment Identifiers: • IMEI • Call Number Identifiers: • MSISDN • Call Routing Identifiers: • LAC / LAI / MSRN
  31. 31. 4. GSM Network Identifiers 24 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 _____________________________________________________________________ 4.2 Subscriber Identifiers 4.2.1 INTERNATIONAL MOBILE SUBSCRIBER IDENTITY (IMSI) International Mobile Subscriber Identifier (IMSI)International Mobile Subscriber Identifier (IMSI) • Globally unique subscriber identity (15 digits max) • Comprises: • Mobile Country Code (MCC) • Mobile Network Code (MNC) (operator) • Mobile Subscriber Identification Number (MSIN) 3 digits MCC 9-10 digits MSINMNC 2-3 digits Example: 262 – Germany 01 – D1 Telekom 123456789 When a subscriber registers with a network operator, a unique subscriber IMSI identifier is issued and stored in the SIM of the MS. An MS can only function fully if it is operated with a valid SIM inserted into an <MS with a valid IMEI. 4.2.2 TEMPORARY MOBILE SUBSCRIBER IDENTITY (TMSI) A TMSI is used to protect the true identity (IMSI) of a subscriber. It is issued by and stored within a VLR (not in the HLR) when an IMSI attach takes place or a Location Area (LA) update takes place. At the MS it is stored in the MS’s SIM. The issued TMSI only has validity within a specific LA. Since the TMSI has only local significance (i.e. within the area controlled by a VLR), the structure and coding of it can be chosen by agreement between operator and manufacturer in order to meet local needs. The TMSI consists of 4 octets and is only be allocated in ciphered form. The network does not allocate a TMSI with all 32 bits equal to 1 as this indicates in the MS SIM that no valid TMSI is available.
  32. 32. 4. GSM Network Identifiers GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 25 Temporary Mobile Subscriber Identity (TMSI)Temporary Mobile Subscriber Identity (TMSI) • Replaces IMSI • Unique only within a LA • Issued on IMSI attach and LA change (minimum) • Comprises 32-bits: 8 bits Octet 8 bits Octet 8 bits Octet 8 bits Octet TMSI Parameter Settings (VLR)TMSI Parameter Settings (VLR) NMobile SS Operation NMobile Terminating USSD NMobile Terminating SMS NMobile Terminating Call NMobile Originating SMSCOUNTERS NMobile Originating CallALLOCATION NPeriodic Location UpdateTMSI YLocation Update YIMSI Attach YLocation Update New Visitor RECPARAMETER
  33. 33. 4. GSM Network Identifiers 26 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 IMSI/TMSI Authentication Parameter SettingsIMSI/TMSI Authentication Parameter Settings NMobile SS Operation NMobile Terminating USSD NMobile Terminating SMS NMobile Terminating Call NMobile Originating SMS NMobile Originating CallCOUNT NPeriodic Location UpdateAUTHENTICATION NLocation Update YIMSI Attach YLocation Update New Visitor RECPARAMETER _____________________________________________________________________ 4.3 Equipment Identifiers 4.3.1 INTERNATIONAL MOBILE EQUIPMENT IDENTITY (IMEI) The IMEI has been implemented in order to identify the presence of a specific mobile station equipment in the network, irrespective of the owning subscriber. Its main purpose is to identify stolen or technically incompatible mobile equipments The IMEI is incorporated in an MS module (not the SIM) which is contained within the MS equipment. The IMEI should be fixed on completion equipment production process and is generally resistant to casual tampering The IMEI is an internationally-unique serial number allocated to the MS hardware at the time of manufacture. It is registered by the network operator and (optionally) stored in the AuC for validation purposes. The IMEI Software Version Number (IMEISV), is a 15 digit decimal number composed of four distinct elements: • a 6 digit Type Approval Code (TAC); • a 2 digit Final Assembly Code (FAC); • a 6 digit Serial Number (SNR); and • a spare digit (sometimes used as a check digit (CD))
  34. 34. 4. GSM Network Identifiers GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 27 International Mobile Equipment Identifier (IMEI)International Mobile Equipment Identifier (IMEI) • Globally unique MS equipment identity • Comprises: • Type Approval Code (TAC) • (2-digit (49 = Germany) country code + 4-digit approval code ) • Final Assembly Code (FAC) • (Manufacturer: e.g. 10 & 20 = Nokia) • Serial Number (SNR) • (unique 6-digit code) • Spare digit • (default to 0) 6 digits TAC 6 digits SNRFAC 2 digits Example: 495020 10 123456 7 (Access: *# 92702689 #) X 1 digit IMEI Check Parameters (VLR)IMEI Check Parameters (VLR) BLOCKUnknown IMEI Effect TRACEGrey List Effect BLOCKBlack List Effect NMobile SS Operation NMobile Terminating USSD NMobile Terminating SMS NMobile Originating SMS NMobile Terminating CallIMEI CHECKING ON…. NMobile Originating Call NPeriodic Location Update Y(10)Location Update YIMSI Attach YLocation Update New Visitor RECPARAMETER
  35. 35. 4. GSM Network Identifiers 28 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 _____________________________________________________________________ 4.4 Call Number Identifiers 4.4.1 MOBILE SUBSCRIBER ISDN NUMBER (MSISDN) The MS international ISDN numbers are allocated from the CCITT Recommendation E.164 numbering plan. The number consists of: • Country Code (CC) of the Country in which the MS is registered, followed by • National (significant) mobile number which consists of National Destination Code (NDC) • Subscriber Number (SN). For GSM applications, a National Destination Code is allocated to each GSM PLMN. In some countries more than one NDC may be required for each GSM PLMN. The composition of the MS international ISDN number should be such that it can be used as a global title address in the Signalling Connection Control Part (SCCP) of the SS7 protocol for routing messages to the MS’s HLR using the CC and NDC. If further routing information is required, it must be contained within the first few digits of the SN). A sub-address may be appended to an ISDN number for use in call set-up and in supplementary service operations The MSISDN represents the ‘true’ or ‘dialled’ number associated with the subscriber. It is assigned to the subscriber by the network operator at registration and is stored in the SIM. It is possible for an MS to hold multiple MSISDNs, each associated with a different service. Mobile Subscriber ISDN Number (MSISDN)Mobile Subscriber ISDN Number (MSISDN) • Identifies the global calling number • Comprises: • Country Code (CC) • National Destination (area) Code (NDC) • Subscriber Number (SN) 3 digits CC 9-10 digits SNNDC 2-3 digits Example: 262 – Germany 01 – D1 Telekom 123456789
  36. 36. 4. GSM Network Identifiers GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 29 _____________________________________________________________________ 4.5 Call Routing Identifiers 4.5.1 LOCATION AREA CODE (LAC) Location Area Code (LAC)Location Area Code (LAC) • Uniquely identifies a LA within a specific PLMN • Comprises 2 octets: 8 bits Octet 8 bits Octet 4.5.2 LOCATION AREA IDENTITY (LAI) Location Area Identifier (LAI)Location Area Identifier (LAI) • Globally unique Identity • Comprises: • Mobile Country Code (MCC) • Mobile Network Code (MNC) (operator) • Location Area Code (LAC) 3 digits MCC 2 octets LACMNC 2-3 digits Example: 262 – Germany 01 – D1 Telekom 54
  37. 37. 4. GSM Network Identifiers 30 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 Each Location Area within the PLMN has an associated internationally-unique identifier (LAI). The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH), thus uniquely identifying each cell with an associated LA. The purpose of LAs is covered later in this course. 4.5.3 MOBILE SUBSCRIBER ROAMING NUMBER (MSRN) The MSRN is used to route calls directed to an MS. It has the same structure as international ISDN numbers in the area in which the roaming number isallocated, i.e.: • the Country Code of the country in which the visitor location register is located; • the National Destination Code of the visitor GSM PLMN or numbering area; • a Subscriber Number with the appropriate structure for that numbering area. The MSRN is a temporary, location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility. It is stored in the VLR and associated HLR but not in the MS. The MSRN is used by the VLR-associated MSC for call routing within the MSC/VLR service area. On request from the GMSC via the HLR, the MSRN is temporarily allocated to an MS by the VLR with which the MS is registered. The MSRN is passed by the HLR to the GMSC for routing calls to the MS. Mobile Subscriber Roaming Number (MSRN)Mobile Subscriber Roaming Number (MSRN) • Unique routing identifier within a VLR area • Same structure as MSISDN for PSTN/ISDN routing • Comprises: • Country Code (CC) • National Destination (area) Code (NDC) • Subscriber Number (SN) 3 digits CC 9-10 digits SNNDC 2-3 digits Example: 262 – Germany 01 – D1 Telekom 123456789
  38. 38. 4. GSM Network Identifiers GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 31 Section 4Section 4 -- SummarySummary • Subscriber Identifiers: • IMSI / TMSI • Equipment Identifiers: • IMEI / IMEISV • Call Number Identifiers: • MSISDN • Call Routing Identifiers: • LAC / LAI / MSRN • In this section the following network identifiers were described:
  39. 39. 4. GSM Network Identifiers 32 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 Intentional Blank Page
  40. 40. 5.. Common Cell Parameters GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 33 5. Common Cell Parameters _____________________________________________________________________ 5.1 Introduction This section of the notes introduces parameters that have setting unique to a particular site or cell. These parameters include those relating to the cell identity, activated functions and channel configurations
  41. 41. 5.. Common Cell Parameters 34 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 _____________________________________________________________________ 5.2 Cell Identifiers 5.2.1 CELL IDENTITY (CI) AND GLOBAL CELL IDENTIFIER The CI an identifier assigned to each cell within a network. However, the CI is only unique within a specific Location Area. By adding the LAI to the CI, a globally-unique GCI is created. Cell Identifier (CI) and Global CI (GCI)Cell Identifier (CI) and Global CI (GCI) • CI: • Identity unique to a cell within a location area (LA) • Fixed Length of 2 octets: 8 bits Octet 8 bits Octet Cell Identity (CI) Location Area Identity (LAI) • GCI: • Globally unique cell identity • Comprises LAI +CI 5.2.2 BASE STATION IDENTITY CODE (BSIC) The BSIC is a local colour code that allows an MS to distinguish between different neighbouring base stations. BSIC is a 6-bit length code structured as shown in the next diagram. Each BTS is issued with a unique identity, the BSIC and is used to distinguish between the same frequency being received from a different neighbouring BTS. In the definition of the NCC, care must be taken to ensure that the same NCC is not used in adjacent PLMNs that may use the same BCCH carrier frequencies in neighbouring areas.
  42. 42. 5.. Common Cell Parameters GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 35 Base Station Identity Code (BSIC)Base Station Identity Code (BSIC) 3 bits Network Colour Code (NCC) • Identity that allows an MS to distinguish between different neighbouring BTSs transmitting on the same frequency • 6-bit code, therefore only 64 unique BSIC values • Structure: Base Station Colour Code (BCC) 3 bits • Countries/Operators may have different NCCs allocated (e.g. UK = 2, France = 0, Ireland = 3, Italy = 2) (ETSI GSM 03.03) • The BCC identifies the training sequence used by the BTS. • The NCC allows the MS to discriminate invalid BSICs _____________________________________________________________________ 5.3 Cell Functions Cell Function Implementation ExamplesCell Function Implementation Examples • SMS Cell Broadcast (CBCH) Usage: • SMSCBUSE (Y/N) {Ericsson} • HSCSD Usage • BTSHSCSD (Y/N) {Ericsson} • BSCHSCSD (Y/N) {Ericsson}
  43. 43. 5.. Common Cell Parameters 36 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 _____________________________________________________________________ 5.4 Cell Channel Configurations Control Channel ConfigurationsControl Channel Configurations • On the downlink, CCCH consists of paging (PCH) and access grant (AGCH) messages • A combined multiframe has only 3 CCCH blocks to allow for SDCCH and SACCH: • A non-combined multiframe has 9 CCCH blocks: • A complete paging or access grant message takes four bursts (timeslots), i.e. one CCCH block. S BCCHF CCCH S CCCHF CCCH S SDCCH 0F SF SF I SDCCH 1 SDCCH 2 SDCCH 3 SACCH 0 SACCH 1 S BCCHF CCCH S CCCHF CCCH S CCCHF CCCH S CCCHF CCCH S CCCHF CCCH I I SACCH 0 SACCH 1 SDCCH 0 SDCCH 1 SDCCH 2 SDCCH 3 SDCCH 4 SDCCH 5 SDCCH 6 SDCCH 7 SACCH 2 SACCH 3 I I Cell Channel Configuration ExamplesCell Channel Configuration Examples • BCCH Frequency in Use: • BCCHFREQ (0-1023) {Siemens} • BCCHNO (0-1023) {Ericsson} • BCCH Type: • BCCHTYPE (comb,ncomb) {Ericsson} • CCCH Block Allocation • CCCH_CONF • Reservation of CCCH blocks for AGCH • BS_AF_BLKS_RES
  44. 44. 5.. Common Cell Parameters GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 37 Control Channel ConfigurationControl Channel Configuration TS0 (non-combined),TS2,TS4,TS64110 TS0 (non-combined),TS2,TS43100 TS0 (non-combined),TS22010 TS0 (combined)1001 TS0 (non-combined)1000 Configuration Number of CCCH TimeslotsCCCH_CONF CCCH Configuration Parameter - CCCH_CONF BCCH contains a number of system information messages, BCCH/SYS_INFO n. BCCH/SYS_INFO 3 carries a parameter, CCCH_CONF, which informs the mobile of the CCCH configuration to be used, including number of timeslots, combined or non-combined multiframes, reservation of AGCH blocks etc.
  45. 45. 5.. Common Cell Parameters 38 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 Section 5Section 5 -- SummarySummary • In this section the following topics have been covered: • Cell Identifier Parameters • Cell Function Parameters • Cell Channel Configuration Parameters
  46. 46. 6. Network Access GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 39 6. Network Access _____________________________________________________________________ 6.1 Introduction This section of the course looks at the functions and parameter settings required to provide and gain access to the GSM network including IMSI attach and detach and call connection.
  47. 47. 6. Network Access 40 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 _____________________________________________________________________ 6.2 Network Access Procedures 6.2.1 NETWORK CONNECTION SEQUENCE MS Network Connection SequenceMS Network Connection Sequence Power on Scan RF channels Select highest carrier level Scan for FCCH frequency correction burst Select next highest carrier level FCCH detected? Scan for SCH synchronisation burst SCH detected? ‘camp-on’ to BCCH and start decoding Monitor PCH and adjacent carriers YES YES NO NO When an MS powers on within network coverage, it starts by scanning all frequencies within its allocated band (e.g. 124 for standard GSM). It measures the received power on each of these frequencies and placed them in order. The MS then selects and listens out on the strongest RF level carrier for a frequency correction burst which is transmitted on the control channel of a BCCH carrier. This is to initially achieve frequency synchronisation with the transmitting BTS. Having achieved frequency synchronisation; the MS listens on the SCH for frame synchronisation information. The SCH channel provides frame timing, the current frame number and BSIC information. Once frame synchronisation is achieved, the MS starts to read and decode the additional information being transmitted on the BCCH. This includes the adjacent cell list, minimum received signal strength, the LAI and beacon frequencies from surrounding cells. The MS then continues to monitor the PCH for incoming call paging requests, sends periodic location updates and maintains a record of surround cell signal strengths. If the MS fails to detect either the FCCH or the SCH, it will reselect the next highest RF carrier level from its measured list and repeats the detection process.
  48. 48. 6. Network Access GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 41 6.2.2 IMSI ATTACH IMSI AttachIMSI Attach • Mobile camps on to best serving BTS • Mobile sends IMSI to MSC • MSC/VLR is updated in HLR • Subscriber data including current location area is added to local VLR • MSC and HLR carry out authentication check - challenge and response using Ki • Optionally EIR checks for status of mobile (white/grey/black) BSC EIR HLR AuC MSC VLR The IMSI attach procedure is used by the MS to indicate that it is has adopted the active (power-on) state within the network. The IMSI attach is also performed as part of the location updating procedure. The basic IMSI attach procedure is described below: Stage 1 The MS sends a message to BSS on the RACH requesting a channel allocation. The BSS responds with a ‘Immediate Assignment’ message on the AGCH. This message assigns a SDCCH channel to the MS. Stage 2 On assignment of the SDCCH channel, the MS sends and IMSI attach message over the SDCCH to the MSC/VLR relayed via the BSS. This informs the MSC/VLR of the MS’s IMSI. This information may also be updated in the HLR which provides subscriber profile data to the VLR is it does not already have it. Stage 3. Security procedures are activated including authentication and (optionally) and IMEI check with the EIR. Stage 4 Assuming successful authentication, the VLR responds to the MSC with an ‘IMSI Attach Acknowledge’ message which is forwarded by the MSC, via the BSS, to the MS. Stage 5 The MSC also sends a ‘Clear Command’ message to the MS over the SDCCH in order to release the dedicated resources used to effect the IMSI attach
  49. 49. 6. Network Access 42 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 Stage 6. The VLR assigns a Temporary Mobile Subscriber Identity (TMSI) to the MS and informs it that it is attached to the network. This function creates a ‘Mobility Management (MM) Context’. 6.2.3 IMSI ATTACH The IMSI detach procedure is invoked if the MS is deactivated either by powering down or if the SIM is removed or forced by the network. There are two causes of an IMSI detach: • Explicit: The MS informs the network that it is detaching. • Implicit: The VLR has not been able to contact (via the MSC) the MS for a pre- determined amount of time and forces an IMSI detach. IMSI DetachIMSI Detach • Explicit: • Mobile informs MSC it is switching off • HLR stores last location area for mobile • VLR records that mobile is no longer available on network • Mobile powers down • Implicit • VLR forces IMSI Detach due to no response BSC HLR AuC MSC VLR The detach procedure is essentially the reverse of the IMSI attach procedure and is as follows: Stage 1 The MS sends a ‘Channel Request’ message on the RACH to the BSS. The BSS assigns a an SDCCH channel, informing the MS over the AGCH. Stage 2 The MS sends a ‘IMSI Detach Indication’ message to the BSS which identifies the MS using its TMSI. The BSS forwards this message to the MSC which in turn updates its VLR using a ‘Detach IMSI’ signalling message.
  50. 50. 6. Network Access GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 43 Stage 3 The VLR informs the HLR of the change using a ‘Deregister Mobile Subscriber’ signalling message. The HLR responds to the VLR with a ‘Deregistration Accepted’ message. Stage 4 The VLR notifies the MSC of this acceptance with an ‘Acknowledge IMSI Detachment’ message. The MSC does not notify the MS as, by this stage, the MS may well be disconnected. Stage 5 The MSC sends a ‘Clear Command’ message to the BSS to release the assigned SDCCH resources. The BSS responds with a ‘Clear Complete’ message which completes the IMSI Detach procedure. ________________________________________________________________________________ 6.3 Key Network Access Parameters Network Access parameters refer to those parameters that can be used to modify the functions involved when an MS attempts to access and disconnect from the GSM network. Key Access ParametersKey Access Parameters • Minimum RXLEV necessary to access the network • Maximum allowable power level to access a cell’s control channel • If the cell is barred from access • Priority for access if contention exists • Access control class • Maximum number of Access retries • Time period between access attempts • Inactivity time period before being forced detached • Time interval before being marked for implicit detach • Time interval after lost contact before MS is deregistered 6.3.1 ACCESS PARAMETERS RXLEV_ACC_MIN (range 47 to 110) Receive Level Access Minimum. Defines the minimum dBm signal level required at the MS in order to permit access to the network 47 dBm = level 63, 110dBm = power level 0. See section relating to ‘Power Control’ for full details (default 110dBm).
  51. 51. 6. Network Access 44 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 Access ParametersAccess Parameters • RXLEV_ACC_MIN (range 47-110 dBm) – (default 110dBm): • Receive Level Access Minimum. Defines minimum signal level for MS to access the network • CCHPWR (range 13-43) – (no default): • Control Channel Power. Defines maximum power an MS can use to access a control channel • CB (range Y/N)– (default N): • Cell Barring. Bars the cell from being accessed by an MS • CBQ (range High/Low)– (default Low): • Cell Bar Qualifier. Used with CB to determine access priority by applying one of two levels CCHPWR (range 13-43/GSM900, 4-30/GSM1800) Control Channel Power. Defines the maximum power level an MS may used to access the network on a control channel (no default). CB (range YES,NO) Cell Barring. Defines whether or not an MS is barred from network access. It is possible to use CB to bar a cell from access for cell reselection purposes. However, it is still available for handovers. Can be used in conjunction with CBQ (default NO). YES = barred from access NO = access permitted. CBQ (range HIGH,LOW). Cell Bar Qualifier. Used in conjunction with CB to determine cell access priority. In idle mode, the MS looks for suitable cells to camp onto by checking cells in descending order of received signal strength. If a suitable cell is found, the MS camps onto it.. Cells can have 2 levels of priority. Suitable cells which are of low priority are only camped onto if there are no other suitable cells of normal priority. Interpretation of CB and CBQ are as follows: Phase 2 MS Phase 1 MS CBQ CB Cell Sel Cell Resel Cell Sel/Resel HIGH NO Normal Normal Normal HIGH YES Barred Barred Barred LOW NO Low Priority Normal Normal LOW YES Low Priority Normal Barred
  52. 52. 6. Network Access GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 45 6.3.2 ATTACH/CALL SETUP PARAMETERS Attach/Call Setup ParametersAttach/Call Setup Parameters • ACC (range 0-15, CLEAR) – (default CLEAR) • Access Control Class: • 0-9 = Reserved for Operator Use • 10 = Emergency calls not permitted in classes 0-9 • 11-15 = Specific Service uses • CLEAR = no access classes barred • MAXRET (range 1,2,47) – (default 1) • Maximum Retries. Defines maximum number of RACH access attempts • T3122 (0-255) – (default 5) • Time before access retry after resource allocation rejection ACC (0-15,CLEAR) Access Control Class. Up to 15 access classes can be defined. This parameter defines the classes that area barred (default CLEAR). 0-9 and 11-15 – Barred Access to specified Class 10 = Emergency calls not permitted to class 0-9 MSs CLEAR – NO access classes are barred Access Classes 0-9 = Reserved for Operator Use 10 = undefined 11 = PLMN use 12 = Security Services use 13 = Public Utilities 14 = Emergency Services 15 = PLMN Staff MAXRET. (range 1,2,4,7) Maximum Retransmissions. Defines the maximum number of Access retransmission by an MS when attempting on the RACH 6.3.3 DETACH/DISCONNECTION PARAMETERS BTDM (range 6 to 150 in steps of 6, OFF) (Ericsson MSC only) Base Time Detach Mobile. This timer determines the period before an MS is forced to detach by the MSC. Must be set longer than the GSM T3212 periodic LA update timer) (default OFF). GTDM (range 0-255) (Ericsson MSC only) Guard Time Detach Mobile. Guard time before marking an MS as implicitly detached (no default).
  53. 53. 6. Network Access 46 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 Detach/DisconnectDetach/Disconnect ParametersParameters • BTMN (range 6-150, OFF) – (default OFF): • Base Time Detach Mobile. Defines the time before an MS is forced to Detach • GTMN (range 0-255) – (no default): • Guard Time Detach Mobile. Guard time before marking the MS as implicitly detached • TDD (range 0-255, OFF) – (default OFF): • Time Deregistration Default. Defines the time before an MS is automatically deregistered after loss of contact TDD (range 0-255, OFF) (Ericsson MSC only) Time Deregistration Default. Time period before loss of contact with an MS causes automatic deregistration (default OFF). Section 6Section 6 -- SummarySummary • This section has covered: • Network access procedures • IMSI attach and detach • Access parameters • Connection setup parameters • Detach and disconnection parameters
  54. 54. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 47 7. BCCH Allocation (BA) Lists and Idle Mode Measurements _____________________________________________________________________ 7.1 Introduction This section of the notes explains the structure and purpose of Broadcast Control Channel Allocation (BA) lists and how they are used in carrying out Idle mode radio path measurements on the serving and surrounding cells.
  55. 55. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements 48 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 ____________________________________________________________________ 7.2 Generic Neighbour List Functionality 7.2.1 PURPOSE OF BA LISTS A BCCH allocation (BA) is a list of BCCH carriers in use within a specific geographical area of a PLMN. It indicates the RF channels that the MS is required to monitor while camped on a cell of that PLMN. 7.2.2 TRANSMISSION OF BA LIST The BA List is broadcast in the system information messages on the BCCH, and is often referred to as the BA (BCCH). BCCH Allocation (BA) ListsBCCH Allocation (BA) Lists • Contains a list of BCCH carriers available in a certain PLMN region • Passed to MS and stored in the SIM during IMSI Attach procedure • MS monitors all carriers in BA list for signal strength. • In Idle mode this is used to reselect cell whilst moving • In dedicated mode, the measured values of the six best neighbouring cells are reported to the serving BSC for handover purposes 7.2.3 CAMPING ON WITHOUT A STORED BA LIST If an MS switches on in a PLMN where it has no knowledge of the PLMN’s BCCH carriers, it must scan the entire GSM band and prioritises the received RF signals in order of receive signal strength. It then checks each in turn for the presence of a BCCH channel to identify it as a BCCH carrier. Having detected a BCCH carrier and confirmed that it belongs to the expected PLMN, the MS reads the BCCH information to extract the BA information. It then knows which BCCH carriers to monitor for cell reselection purposes.
  56. 56. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 49 7.2.4 CAMPING ON WITH A STORED BA LIST Having switched off the MS off, when it is switched on again it will first refer to the existing stored BA list to scan for BCCH Carriers. This speeds up the processes of ‘camping-on’ when compared to the equivalent procedure without a BA list. However, a stored BA list is not necessarily valid when the MS is switched on again. For example, if the system operator has made a change in BA List or if the MS is switched on in a different geographical area. In these cases, the MS may not find a suitable cell for camping on using the stored BA list and reverts to the ‘no BA list’ procedure. 7.2.5 STORAGE OF BA LIST When a BA list has been received by a Mobile Station, the MS stored it in its non-volatile memory. This may or may not be the SIM depending of the form factor of the MS. When the MS switches off, it has the option of storing the last known BA(BCCH) in its SIM so that if it subsequently switches back on in the same PLMN, it does not need to search to find the BA(BCCH), and so can camp on a cell more quickly. If the BA(BCCH) is stored in the SIM, it is stored in the format specified in GSM 11.11. Any other information used by the MS in cell selection is also stored in the MS SIM. 7.2.6 THE BA(SACCH) LIST There is another BCCH carrier list, called BA(SACCH), which is sent on the SACCH when in dedicated (call connected) mode. This contains the list of BCCH carriers to be monitored by the MS for hand over purposes. The BA(BCCH) and BA(SACCH) may or may not be the same. For example, the BA(SACCH) might contain umbrella cells, or the BCCH carrier of the serving cell might be omitted. If the MS stores a BA list in the SIM, and there is a valid stored LAI, the BA list must be of the PLMN indicated by the stored LAI. 7.2.7 BA LIST INFORMATION BA list information is provide to the MS from two sources: • The BA Range List • The BA ARFCN list 7.2.8 THE BA RANGE LIST The BA(BCCH) may or may not be the total list of BCCH carriers in use throughout the PLMN coverage area. For example there may be differences in different geographical areas, and there may be "umbrella cells" which are only used for handover traffic and hence are not to be camped on. The BA Range list message provides ranges of frequencies through which the MS can search. Multiple ranges can be transmitted, each range determined by its upper and lower ARFCN:
  57. 57. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements 50 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 Parameters: • Number of Ranges: Number of ranges to be transmitted in the TE (min = 1) • RANGE_LOWER: The ARFCN at the bottom of the range of frequencies to be used by the MS in cell reselection • RANGE_HIGHER: The ARFCN at the top of the range of frequencies to be used by the MS in cell reselection BA Range ListBA Range List Octet n Octet 13RANGE4_HIGHER Octet 12RANGE4_HIGHERRANGE4_LOWER Octet 11RANGE4_LOWERRANGE3_HIGHER Octet 10RANGE3_HIGHERRANGE3_LOWER Octet 9RANGE3_LOWER Octet 8RANGE2_HIGHER Octet 7RANGE2_HIGHERRANGE2_LOWER Octet 6RANGE2_LOWERRANGE1_HIGHER Octet 5RANGE1_HIGHERRANGE1_LOWER Octet 4RANGE1_LOWER Octet 3Number of Ranges Octet 2Length of BA Range Contents Octet 1BA Range IEI 12345678 7.2.9 THE BA ARFCN LIST The BA message lists all 124 ARFCNs but indicates which ARFCN ranges are searchable. BA ARFCN ListBA ARFCN List 001002003004005006007008 ARFCNARFCNARFCNARFCNARFCNARFCNARFCNARFCN CACACACACACACACA 113114115116117118119120 ARFCNARFCNARFCNARFCNARFCNARFCNARFCNARFCN CACACACACACACACA 121122123124 ARFCNARFCNARFCNARFCNsparespareFormat ID CACACACA0000 Cell Channel Description IEI 12345678 CA = 0 – not part of BA list CA = 1 – included in BA list
  58. 58. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 51 7.2.10 EFFECT OF BA LIST LOADING ON MEASUREMENTS 12-138 10-1110 6-716 3-432 Number of samples per carrier in SACCH multiframe Number of BCCH carriers in BA List Sample RateSample Rate vsvs BA List LoadingBA List Loading _____________________________________________________________________ 7.3 Idle Mode Cell Measurements BCCH Carrier MeasurementBCCH Carrier Measurement –– Idle ModeIdle Mode • Neighbouring Cells: • MS scans all carriers listed in the BA list and identifies the 6 strongest • Signal level averaged over at least 5 measurements • Result stored in RXLEV (n) parameter • MS must attempt to decode the BCCH channel of 6 best neighbours at least every 30 seconds • Serving Cell • Measurements taken during allocated paging block • Measurements averaged over 5 consecutive paging blocks or 5 seconds (whichever greater)
  59. 59. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements 52 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 Measurement of channel levels must be made in both idle and dedicated mode. These measurements are critical to the following functions within a GSM Network: Idle M ode - cell selection/reselection Dedicated Mode - handovers - serving cell measurements - adaptive power control This section of the course looks specifically and cell measurement parameters in Idle mode. Dedicated mode cell measurements are covered later in the course Having camped onto a cell, part of the information the MS receives on the BCCH channel is the cell’s BCCH Allocation (BA) or Neighbour list. This list identifies the BCCH carriers in the surrounding cells that are potential candidates for cell reselection. The MS then scans all BCCH carriers listed in its BA list. For each BCCH carrier detected, the received signal level is measured. These measured values must be averaged over at least 5 measurements and the averaged result is stored in the MS for each BA entry where a carrier is detected. BCCH Carrier MeasurementsBCCH Carrier Measurements –– Idle ModeIdle Mode RxLev / BSICRxLev / BSIC RxLev / BSIC RxLev 110dBm 85Bm 112Bm 96dBm Upon completion of cell selection and when starting the cell reselection tasks, the MS shall synchronize to and read the BCCH information for the 6 strongest non-serving carriers in the BA list as quickly as possible. For multi- band mobile stations the strongest non-serving carriers may belong to different frequency bands. The MS must also attempt to decode the BCCH of the six best neighbouring cells at least once every 30 seconds to ensure that the BSIC values have not changed. The serving cell signal level is also measured. This measurement must take place at least once for every CCCH paging block assigned to the MS. The averaging takes place over 5 consecutive paging blocks or over a 5 second period, whichever the greater period.
  60. 60. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 53 These values are not communicated to the network as no uplink communication channel exists from the MS to the network in idle mode. However, they are critical to the cell reselection procedure. ____________________________________________________________________ 7.4 Key Neighbour Relation Parameters 7.4.1 NEIGHBOURING CELL RELATION PARAMETERS Neighbour Cell Relation ParametersNeighbour Cell Relation Parameters • Two parameters can be defined for each neighbour relation (for re- selection/handover purposes): • Hysteresis • Offset • Up to 64 neighbours can be defined for each cell in up to 2 lists each of 32 entries • As a BSC can accommodate up to 128 cells, 8192 cell relations can be defined per BSC There are two types of parameters that can be defined for each neighbouring cell relation: Hysteresis and Offset parameters. A BA List can comprise up to 32 neighbour relations. However, it is possible to implement 2 BA lists (one Idle mode and one Dedicated mode list) giving up to 64 neighbour relations per cell. As each BSC can accommodate up to 128 cells, up to 8192 cell relations can be defined per BSC. The following Ericsson parameters are defined for neighbour relations: CELLR (range: 1 to 7 characters) This is the identity of the neighbouring cell CTYPE (range: EXT, Omitted) If the neighbouring cell belongs to a different BSC, this parameter must be set to EXT RELATION (range: SINGLE, Omitted) If the relationship with the neighbouring cell is mutual (i.e. this cell is recorded as a neighbour in the neighbouring cell using same hysteresis and offset), this is omitted. If the relationship is not mutual, or the neighbour belongs to a different BSC (i.e. CTYPE= EXT), this is set to SINGLE.
  61. 61. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements 54 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 CS (range: YES, NO) If the current and neighbouring cell share the same location, this is set to YES, otherwise to NO. BA ListBA List--Related ParametersRelated Parameters • Examples of Ericsson BA-related parameters: • CELLR (range: 1-7 characters) – (no default): • Neighbouring cell identifier • CTYPE (range: EXT, omitted) – (default omitted): • Set to ‘EXT’ if neighbouring cell is in different BSC. Otherwise set to ‘omitted’ • RELATION (range: single, omitted) – (default single): • Set to ‘single’ if relationship is non-mutual or CTYPE= EXT. Otherwise set to ‘omitted’ • CS (range Yes/No)– (default No): • Set to ‘Yes’ if neighbour cell shares the same site location. Otherwise set to ‘No’. Section 7Section 7 -- SummarySummary • In this section the following topics have been covered: • What are BA lists? • BA range Lists • BA Neighbour Lists • Effect of BA List loading on measurements • Idle Mode BCCH carrier measurements • BA-related Parameters
  62. 62. 8. Cell Selection/Reselection GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 55 8. Cell Selection/Reselection _____________________________________________________________________ 8.1 Introduction During Idle mode, a mobile terminal must be capable of moving between cells within the network. This is achieved by periodically measuring the signal strength of neighbouring cells. The mobile terminal uses these measured values to determine the point at which it moves from its current (serving) cell to a new (neighbouring) cell. This section of the notes describes the procedures used in this Idle mode decision-making process and identifies the key parameters that control these procedures.
  63. 63. 8. Cell Selection/Reselection 56 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 _____________________________________________________________________ 8.2 Cell Selection Procedures Cell Selection ProcedureCell Selection Procedure • MS powers-up • MS starts measuring received power level of the BCCH carrier for all cells in range • MS calculates average power level received from each cell: • Stored in RXLEV(N) parameter • MS calculates a C1 parameter for each measured carrier based on the RXLEV(N) values • Mobile compares cells which give a positive value of C1 and ‘camps- on’ to the cell with the highest C1 value Cell selection (as opposed to cell reselection) implies that the MS is switching on. There are two methods by which a suitable cell is selected for ‘camping-on: No SIM-Stored BA List On first switching on in this scenario, the MS scans all GSM frequencies for signal level (124 for P-GSM, 174 for E-GSM and 374 for DCS 1800). It prioritises those signals received in order of signal strength and starting with the strongest checks each to identify it as a BCCH carriers, through detecting a FCCH burst. It is likely that the strongest received signals will be BCCH carriers since continuous transmission is required on them. Once it detects an FCCH burst on a suitable carrier, it synchronises at TDMA frame level using the TDMA frame number and BSIC from the SCH channel. It then carries out three functions: • Reading network information of the BCCH • Listening on its designated paging CCCH block for incoming pages • Monitors neighbouring cell BCCH carriers in preparation for cell reselection. SIM-Stored BA List On switch-on the MS scans all BCCH carriers listed in its BA list for suitable signal strength, rather than all GSM frequencies. This is clearly a quicker procedure than scanning all GSM frequencies. Once the most suitable BCCH carrier has been identified, the MS camps-on to that cell and follows the procedure described above.
  64. 64. 8. Cell Selection/Reselection GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 57 Cell SelectionCell Selection -- Parameter C1Parameter C1 • Mobile uses parameter C1 (the Path Loss Criterion) to select and camp-on to a cell when it is first switched on (idle mode) • For a particular cell (n): C1(n) = [ A – max (B,0) ] If A = RXLEV(n) - RXLEV_ACCESS_MIN and B = MS_TXPWR_MAX_CCH - P Then: C1(n) = [ RXLEV(n) - RXLEV_ACCESS_MIN - max((MS_TXPWR_MAX_CCH - P),0 ) ] where: RXLEV(n) = average received BCCH power level from cell n RXLEV_ACCESS_MIN = minimum received power level needed by the mobile to access the system MS_TXPWR_MAX_CCH = maximum transmit power mobile is allowed to use to access system P = maximum possible transmit power of the mobile max (0, x) = either x or 0 whichever is the greater • Mobile compares cells which give a positive value of C1 and selects the highest value The averaging is based on at least five measurement samples per RF carrier spread over 3 to 5 seconds, the measurement samples from the different RF carriers being spread evenly during this period. The averaged value is stored in the RxLev(n) parameter for each (n) cell. A multi band MS shall search all channels within its bands of operation as specified above. On switch-on, the MS scans all an MS periodically measures the received power level on each of the BCCH frequencies of all cells within range. From these periodic measurements the MS calculates the mean received level value from each cell, stored in the parameter RXLEV(n) where n=neighbouring cell number. _____________________________________________________________________ 8.3 Cell Reselection Procedures Having camped on to the serving cell, cell reselection is triggered by any one of the following causes: • The path loss criterion parameter C1 indicates that the path loss to the cell has become too high • There is a downlink signalling failure • The cell camped on (current serving cell) has become barred • There is a better cell (in terms of the path loss criterion C2) in the same LA, or a much better cell in another LA of the selected PLMN • A random access attempt is still unsuccessful after "Max retrans" repetitions "Max retrans" being a parameter broadcast on BCCH. The MS will then reselect a new cell based C1 (GSM Phase 1 and 2) and C2 (GSM phase 2 only) measurement parameters.
  65. 65. 8. Cell Selection/Reselection 58 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 Cell Reselection CausesCell Reselection Causes • C1 indicates that the path loss to the serving cell has become too high • Downlink signalling failure • Serving cell has become barred • Better cell (in terms C1/C2 values) • Random access attempts unsuccessful after a number of repeated attempts Having camped on to the serving cell, cell reselection is triggered by any one of the following causes: • The path loss criterion parameter C1 indicates that the path loss to the cell has • become too high • There is a downlink signalling failure • The cell camped on (current serving cell) has become barred • There is a better cell (in terms of the path loss criterion C2) in the same LA, or a much better cell in another LA of the selected PLMN • A random access attempt is still unsuccessful after "Max retrans" repetitions "Max retrans" being a parameter broadcast on BCCH. The MS will then reselect a new cell. 8.3.1 C1 CELL RESELECTION CRITERIA Based on these calculated values, the MS selects which cell to connect to. This connection process is referred top as ‘Camping-on’ to that cell. Once camped-on, a MS in idle mode must periodically measure the received power level on each of the BCCH frequencies of neighbouring cells and stores this measurement in the parameter RXLEV(N) where n=neighbouring cell number.
  66. 66. 8. Cell Selection/Reselection GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 59 Cell ReCell Re--selectionselection -- Phase 1 MobilesPhase 1 Mobiles • Once a mobile has camped on to a cell, it will continue to measure neighbouring BCCH carriers, looking for a better cell • Phase 1 mobiles use the C1 calculation, modified as follows: • Between cells within a location area, the criterion for selecting a new cell is: C1 (new) > C1 (old) for more than 5 seconds • Between cells on a location area boundary, the criterion is: C1 (new) > C1 (old) + CELL_RESELECTION_HYSTERESIS for more than 5 seconds • The hysteresis term prevents unnecessary re-selection on a location area boundary which would require extra signalling to perform the location update From these periodic measurements the MS calculates the mean received level value from each cell and stores the result of the calculation in the C1 parameter for the best 6 neighbouring cells. Any C1 values of 0 or below are discarded and the best 6 of the remainder are stored. 8.3.2 C2 CELL RESELECTION CRITERIA Cell ReCell Re--selectionselection –– Phase 2 MobilesPhase 2 Mobiles • GSM Phase 2 introduced a separate cell re-selection parameter, C2 • Intended to: • Prevent multiple handovers for fast-moving mobiles • Ensure MS camps on to cell with greatest chance of successful communications • The C2 calculated is: C2 = C1 + OFFSET – (TEMPORARY_OFFSET x H(PENALTY_TIME –T) where the function H is defined as: H(x) = 0 for x<0, H(x) = 1 for x ≥ 0
  67. 67. 8. Cell Selection/Reselection 60 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 The purpose of introducing the C2 parameter was primarily to control access to microcells. The CELL_RESELECT_OFFSET can be set to make a microcell more attractive than a surrounding macrocell, while the TEMPORARY_OFFSET controls access depending on the speed of the mobile. There is an example of this in the activities at the end of this section. C2 Cell Selection ParametersC2 Cell Selection Parameters C2 = C1 + OFFSET – (TEMPORARY_OFFSET x H(PENALTY_TIME –T) • Offset: • optional parameter to encourage or discourage cell selection • H(Penalty_Time-T): • when a cell is added to list of strongest cells, a negative ‘Temporary Offset’ offset is applied for a ‘Penalty time’: • If timer expires, offset is removed making cell more attractive • If cell is removed from list, timer is reset • Used to prevent fast-moving MSs from selecting the cell • Temporary Offset: • Value of the negative offset applied Cell ReCell Re--selectionselection -- C2 ParameterC2 Parameter • (CELL_RESELECT_OFFSET) effectively moves the boundary of the cell • (TEMPORARY_OFFSET) only applies while (T < PENALTY_TIME), where T is the time since the mobile first detected the cell with C1>0 • This introduces a time hysteresis to prevent fast moving mobiles from selecting cells for very short periods • To select a new cell using C2, either: • C2 > 0 within a location area or • C2 > CELL_RESELECT_HYSTERESIS on a location area boundary
  68. 68. 8. Cell Selection/Reselection GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 61 The hysteresis term prevents unnecessary re-selection on a location area boundary that require extra signalling to perform the location update. In order to optimise cell reselection, additional cell reselection parameters can be optionally broadcast on the BCCH of each cell. The cell reselection process can optionally employ a parameter C2, the value of which is determined by these parameters. The parameters used to calculate C2 are as follows: If PENALTY_TIME <> 11111 C2 = C1 + CELL_RESELECT_OFFSET - TEMPORARY OFFSET * H(PENALTY_TIME - T) If PENALTY_TIME = 11111 C2 = C1 - CELL_RESELECT_OFFSET Where: H(PENALTY_TIME-T) = 0 if x < 0 H(PENALTY_TIME-T) = 1 if x >= 0 Cell ReCell Re--selectionselection -- C2 ParameterC2 Parameter TEMPORARY_OFFSET C1 CELL_RESELECTION_OFFSET PENALTY_TIME C2 Cell Reselection Offset This optional parameter is a positive or negative offset applied to each cell to encourage or discourage MSs to reselect that cell. Penalty Time When the MS places the cell on the list of the strongest carriers (Neighbour list), it starts a timer which expires after the PENALTY_TIME. This timer will be reset when the cell is taken off the list. For the duration of this timer, C2 is given a negative offset. This will tend to prevent fast moving MSs from selecting the cell. Temporary Offset This is the amount of the negative offset described in the ‘Penalty Time’ above. An infinite value can be applied, but a number of finite values are also possible.
  69. 69. 8. Cell Selection/Reselection 62 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 ________________________________________________________________________________ 8.4 Summary of Key Cell Selection/Reselection Parameters This section defines those configurable parameters that can be used to modify the c ell selection/reselection process. Cell Selection/ReCell Selection/Re--selection Related Parametersselection Related Parameters (*) Siemens DB Field Names 2dB0…63CRESOFFCELL_RESELECT_OFFSET 10dB0…6TEMPOFFTEMPORARY OFFSET 20sec0…30PENTIMEPENALTY_TIME 0…1CRESPARICELL_RESELECT_PARAM_IND 0…1CQBCELL_BAR_QUALIFY 2dB0…7CELLRESHCELL_RESELECT_HYSTERESIS 1dB0…63RXLEVAMIRXLEV_ACCESS_MIN 2dB0…31MTPWRCCHMS_TXPWR_MAX_CCH T/FCELLBARCELL_BAR_ACCESS GSMR PCS1900 GSM1800 EXT900 BB900 SYSIDSYS_ID 0…1023BCCHFREQBA Step SizeRangeDB Name (*)Parameter 8.4.1 CELL GSM OPERATING SYSTEM/FREQUENCY BA (range 0 to 1023) Defines the ARFCN on which the cell is operating. SYS_ID (range: system type – see above) Defines the GSM system being operated from the cell. The MS will only be able to camp-on to a cell operating on a compatible GSM system. 8.4.2 CELL ACCESS CELLBAR (range YES,NO) Cell Barring. Defines whether or not an MS is barred from network access (see ‘Network Access’ section for more details). CBQ (range HIGH,LOW). Cell Bar Qualifier. Used in conjunction with CB to determine cell access priority. (see ‘Network Access’ section for more details).
  70. 70. 8. Cell Selection/Reselection GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 63 8.4.3 HYSTERESIS CELLRESH (range 0 to 14 dB in steps of 2) Cell Reselection Hysteresis. Defines the amount of hysteresis added to the nominal cell boundary threshold to prevent ping-pong effects for cell reselection on cell border areas (default 4). 8.4.4 RESELECTION THRESHOLDS CELL_RESELECT_PARAM_IND (range 0 or 1) The parameter is broadcast to the MS on the BCCH and indicates which cell reselection criterion are to be used in the cell. CELL_RESELECT_OFFSET (range 0…63 dB) This optional parameter is a positive or negative offset applied to each cell to encourage or discourage MSs to reselect that cell. PENALTY_TIME (range (0…30) x 20s) When the MS places the cell on the list of the strongest carriers (Neighbour list), it starts a timer which expires after the PENALTY_TIME. This timer will be reset when the cell is taken off the list. For the duration of this timer, C2 is given a negative offset. This will tend to prevent fast moving MSs from selecting the cell. TEMPORARY_OFFSET (range (0…6) x 10dB) This is the amount of the negative offset described in the ‘Penalty Time’ above. An infinite value can be applied, but a number of finite values are also possible.
  71. 71. 8. Cell Selection/Reselection 64 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 Section 8Section 8 -- SummarySummary • In this section the following topics have been covered: • Cell selection procedures • Cell reselection procedures • Summary of key related parameters
  72. 72. 8. Cell Selection/Reselection GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 65 Section 8 Self-Assessment Exercises Exercise 8.1 BSS parameters have been configures as follows: RXLEV_ACCESS_MIN = -90 dBm MS_TXPWR_MAX_CCH = 37 dBm Mobile Characteristics: Class 4 GSM 900 mobile RXLEV (n) measured by mobile: = -85 dBm Calculate the C1 value for the cell being measured Exercise 8.2 When a Class 4 GSM 900 mobile is switched on, it measures the RXLEV for two cells (1 and 2) as: RXLEV(1) = -90 dBm RXLEV(2) = -93 dBm The network parameters are set as: RXLEV_ACCESS_MIN = -95 dBm MS_TXPWR_MAX_CCH = 35 dBm Decide which cell the mobile will camp on to based on C1 values.
  73. 73. 8. Cell Selection/Reselection 66 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 Exercise 8.3 Macrocell Microcell A mobile is switched on within the coverage area of a microcell and a macrocell as shown. The set value of RXLEV_ACCESSA_MIN is –90 dBm The mobile measures the BCCH power levels for each cell as: RXLEV(macrocell) = -70 dBm RXLEV(microcell) = -80 dBm The network has set MS_TXPWR_MAX_CCH as 35 dBm. The mobile is a Class 4 GSM 900 handset with maximum power output of 33 dBm. 1. Compare C1 for the two cells and state which one the mobile will camp on to. 2. The network operator wants to attract slow moving mobiles in this location into the microcell. Suggest a possible value for CELL_RESELECT_OFFSET in the C2 re-selection criterion of the microcell, which will achieve this. Note that values for CELL_RESELECT_OFFSET are from 0 to 126 dB in steps of 2 dB. 3. Mobiles should only re-select to the microcell if they remain in the area for more than 1 minute. Show how the rest of the C2 calculation could be used to achieve this. Note that TEMPORARY_OFFSET values range from 0 to 60 dB in steps of 10 dB and PENALTY_TIME ranges from 20 to 620 seconds in steps of 20 seconds.
  74. 74. 9. Location Management and Paging Requirements GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 67 9. Location Management and Paging Requirements _____________________________________________________________________ 9.1 Introduction Location management is the process by which the GSM network tracks the movement of mobile users throughout the PLMN down to Location Area (LA). Paging is process of locating the mobile user down to cell level for the purpose of establishing a Mobile- Terminated (MT) connection. This section of the course describes these tow procedures and identifies the key parameters that control their associated functions
  75. 75. 9. Location Management and Paging Requirements 68 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 _____________________________________________________________________ 9.2 Location Management Procedures 9.2.1 LOCATION MANAGEMENT OPTIONS Location Management OptionsLocation Management Options • Send location update on every cell change • No paging requirement • Excessive signalling traffic load • Page every cell in network • No location update requirement • Excessive signalling traffic load • Subdivide network into paging areas • Requires paging procedure with reduced traffic load • Required location updating with reduced traffic load In order to ensure the correct routing of an incoming call to a MS, the network needs to know the current location of that MS down to cell level. This can be achieved by one of three methods: • Location Update on Every Cell Change Every time an MS moves into a different cell area, it sends a location update to the network. This has the advantage that no paging is required to establish the cell location of an MS for each incoming call. However, it imposes a significant load on the network signalling channels. • Paging All Cells Every time an incoming call is to be routed to an MS, all cells in the network are paged to identify the cell owning the MS. This has the advantage that no location updating is required to maintain a current MS location log. However, it imposes a significant load on the network signalling channels. • Subdivide Network into Paging Sub-Regions Every time an MS moves in a new paging sub-region it informs the network of that sub-region identity. Every time an incoming call is to be routed to an MS, only the cells in its current paging sub-region are checked. This provides a comprise between the two above options and has been proved to reduce signalling channel loads significantly.
  76. 76. 9. Location Management and Paging Requirements GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 69 9.2.2 GSM LOCATION AREAS (LAs) Within GSM networks these paging sub-regions are known as a Location Areas (LAs) and comprise a number of cells. All cells within the LA must be under the control of a single MSC and within the same PLMN. Each LA within the PLMN is uniquely identified by a Location Area Identifier (LAI). Network AreasNetwork Areas • Cell: radio coverage area of one base station (BTS) • GSM assigns a cell global identity number to each cell • Location Area: Group of cells served by one or more BSCs. • When there is an incoming call, the mobile is paged throughout its location area. A unique Location Area Identity (LAI) is assigned to each LA. • MSC Service Area: part of network covered by one MSC. • All mobiles in this area will be registered in the VLR associated with the MSC. • PLMN Service Area: public land mobile network area - the area served by one network operator 9.2.3 LOCATION UPDATE REQUIREMENTS Location Update RequirementsLocation Update Requirements • Location Area Change • Periodic Location Update • IMSI Attach • Cell change during call • TMSI update on LA change
  77. 77. 9. Location Management and Paging Requirements 70 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 Location Updates occur under one of the following conditions: • On change of LA • Periodic Updates • On MS switch on (IMSI attach) • When changing cells during a call MSC 2 Location UpdatingLocation Updating • The PLMN is divided into a number of location areas • Each location area consists of several cells controlled by the same MSC, but not necessarily the same BSC • The main use of location areas is in paging a mobile • Location area is stored in the VLR of the MSC • The HLR stores the current MSC the mobile is registered with and is only updated if the mobile moves to new MSC • LAI - Location Area Identity - is transmitted on BCCH MSC 1 Location Areas VLR VLR Every time a moving MS enters a new LA (identified by the information transmitted on the BCCH) it initiates a location update. Automatic Location UpdatingAutomatic Location Updating • Mobile receives LAI from BCCH transmission • When it detects a new LAI it automatically requests an update • Within same MSC: Location Update Request Acknowledgement New Location Area stored in VLR • Change of MSC: MSC2 VLR HLR Location Update Request / Acknowledgement New VLR Request / Acknowledgement Cancel old VLR Location / Acknowledgement MSC1 VLR MSC1 VLR

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