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Ps tr-o016-v2.1 tap 3

  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 1.2 1.3 1.4 Introduction ....................................................................................................... 1 What is the BSS? ............................................................................................... 2 What are BSS Parameters?................................................................................ 3 What is Optimisation? ....................................................................................... 5 2. The GSM Optimisation Process 2.1 2.2 2.3 2.4 2.5 Introduction ....................................................................................................... 7 The Purpose of Optimisation............................................................................. 8 The Reasons for Optimisation ........................................................................... 9 The Benefits of Optimisation .......................................................................... 10 Outline GSM Optimisation Process ................................................................ 11 3. BSS Parameter Review 3.1 3.2 3.3 3.4 3.5 3.6 Introduction ..................................................................................................... 15 BSS Parameter Review Process ...................................................................... 16 Database Consistency and Change Control..................................................... 17 BSS Configuration Parameter Sets.................................................................. 17 BSS Configuration Parameter Types............................................................... 18 Adjusting BSS Configuration Parameters ....................................................... 20 PART 2 – COMMON SITE/CELL PARAMETERS 4. Network Identifier Parameters 4.1 4.2 4.3 4.4 4.5 Introduction ..................................................................................................... 23 Subscriber Indentifiers..................................................................................... 24 Equipment Indentifiers .................................................................................... 26 Call Number Indentifiers................................................................................. 28 Call Routing Indentifiers ................................................................................. 29 5. Common Site and Cell Parameters 5.1 5.2 5.3 5.4 Introduction ..................................................................................................... 33 Cell Indentifiers ............................................................................................... 34 Cell Functions................................................................................................... 35 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 7.2 7.3 7.4 Introduction ..................................................................................................... 47 Generic Neighbour List Functionality............................................................. 48 Idle Mode Cell Measurements......................................................................... 51 Key Neighbour Relation Parameters ............................................................... 53 8. Cell Selection/Reselection 8.1 8.2 8.3 8.4 Introduction .................................................................................................... 55 Cell Selection Procedures................................................................................ 56 Cell Reselection Procedures ............................................................................ 57 Summary of Key Cell Selection/Reselection Parameters ............................... 62 Self Assessment Exercises .............................................................................. 65 9. Location Management and Paging Requirements 9.1 9.2 9.3 9.4 9.5 9.5 Introduction .................................................................................................... 67 Location Management Procedures .................................................................. 68 Paging Procedures ........................................................................................... 71 Calculations Using Paging Parameters............................................................ 73 Key Paging Parameters.................................................................................... 74 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 12.2 12.3 12.4 12.5 12.6 Introduction ................................................................................................... 99 Power Control Functions............................................................................. 100 Adaptive Power Control .............................................................................. 103 Discontinuous Transmission (DTX)............................................................ 107 Discontinuous Reception (DRX)................................................................. 109 Summary of Key Power Control Parameters .............................................. 111 Self Assessment Exercises ............................................................................ 15 13. Adaptive Frame Alignment 13.1 13.2 13.3 13.4 Introduction ................................................................................................. 117 Timing Advance Procedures ....................................................................... 118 Extended Cell Range ................................................................................... 119 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 Course Outline 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 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 v
  7. 7. Course Outline PART 1 - Introduction PART 2 - Common Parameters 1. Course Introduction 4. Network Identifier Parameters 2. The GSM Optimisation Process 5. Common Site and Cell Parameters 3. BSS Parameter Review 6. Network Access Course Outline PART 3 – Idle Mode PART 4 – Dedicated Mode 7. 11. Dedicated Mode Cell Measurements 12. Power Control 13. Adaptive Frame Alignment 14. Handover Requirements BCCH Allocations Lists and Idle Mode Measurements 8. Cell Selection / Reselection 9. Location Management and Paging Requirements 10. vi Frequency Hopping GSM PRE LAUNCH - BSS Functions and Parameters © AIRCOM International 2002
  8. 8. 1. Introduction to BSS Parameter Optimisation 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 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 1
  9. 9. 1. Introduction to BSS Parameter Optimisation _____________________________________________________________________ 1.2 What is the BSS? 1.2.1 THE BSS COMPONENTS OF THE GSM NETWORK GSM Architecture Overview Air Interface (Um) A Interface Abis Interface OMC MS VLR BSS HLR TRX MS BTS MSC BSC AuC MS EIR NSS PSTN The Base Station Sub-System (BSS) • The BSS comprises: • • • • 2 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: • • • BSS The purpose of the BTS is to: • • • Base Station Controller (BSC) One or more Base Transceiver Stations (BTSs) 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 BTS BSC BTS BTS BTS GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  10. 10. 1. Introduction to BSS Parameter Optimisation 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 Topologies • • Chain: cheap, easy to implement • One link failure isolates several BTSs BSC Ring: Redundancy gives some protection if a link fails • More difficult to roll-out and extend • ring must be closed BSC • 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 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: GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 3
  11. 11. 1. Introduction to BSS Parameter Optimisation 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: • • Counting. An event has occurred that has caused the timer to start incrementing. • 4 Off/Reset. The timer is set to 0 and is inactive Expired. The timer has reached a pre-determined value and has expired, generally triggering a new event. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  12. 12. 1. Introduction to BSS Parameter Optimisation 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? • 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.’ GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 5
  13. 13. 1. Introduction to BSS Parameter Optimisation 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 1 - Summary • In this section the following topics have been covered: • • • • 6 What is the BSS in a GSN Network Basic BSS topologies What is meant by ‘Parameters’ What is meant by ‘Optimisation’ GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  14. 14. 2. The GSM Optimisation Process 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 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 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 7
  15. 15. 2. The GSM Optimisation Process _____________________________________________________________________ 2.2 The Purpose of Optimisation Purpose 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 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. • 8 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 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  16. 16. 2. The GSM Optimisation Process _____________________________________________________________________ 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 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 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 9
  17. 17. 2. The GSM Optimisation Process _____________________________________________________________________ 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 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. 10 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  18. 18. 2. The GSM Optimisation Process 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 Cycle Performance Management Initial Network Design and Implementation Optimisation Monitor Network Implement Changes Analyse Data Identify Problems Yes QoS Targets Met? No 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 11
  19. 19. 2. The GSM Optimisation Process 2.5.2 OUTLINE OPTIMISATION PROCESS Outline 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 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 12 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  20. 20. 2. The GSM Optimisation Process 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 13
  21. 21. 2. The GSM Optimisation Process 2.5.4 ACTIVITY PHASE OF OPTIMISATION Activity Phase of Optimisation Feedback to Audit Procedures Network Audit Procedures Start Optimisation Activity Ongoing Performance Measurement Process Identify and Fix Hardware Problems BSS Database Parameters Review: Settings Consistency Change Control Identify and Fix Neighbour Problems Identify and Fix Frequency Plan Problems Review Process and Results Feedback to modify strategy decisions Design Review and Growth Plan Section 2 - Summary • In this section the following topics have been covered: • • • • • • • 14 The purpose of Optimisation The Network Audit phase of Optimisation The reasons for carrying out Optimisation The benefits of Optimisation The Performance Management Cycle An Optimisation Process The Activity Phase of Optimisation GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  22. 22. 3. BSS Parameter Review 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 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 15
  23. 23. 3. BSS Parameter Review _____________________________________________________________________ 3.2 BSS Parameter Review Process 3.2.1 REVIEW DATABASE SETTING BSS Configuration Parameter Review • • Review current settings as related to performance measurement results • Review includes: Recommend changes to improve/optimise performance of specific features. • • • • • • 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: • • 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. • 16 Handover parameters, timers, thresholds and margins (including inter-layer and inter-band handovers where applicable). Advanced traffic management algorithms (Congestion-based handovers, inter-band and inter-layer traffic distribution) GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  24. 24. 3. BSS Parameter Review _____________________________________________________________________ 3.3 Database Consistency and Change Control BSS 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 17
  25. 25. 3. BSS Parameter Review BSS 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: 18 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  26. 26. 3. BSS Parameter Review BSS 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 19
  27. 27. 3. BSS Parameter Review 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 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 • Only implement during low-traffic periods • • Use test BSS where available Be aware of hierarchical changes (MSC BSC BTS) 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. 20 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  28. 28. 3. BSS Parameter Review 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 21
  29. 29. 3. BSS Parameter Review Section 3 - Summary • In this section the following topics have been covered: • • • • • 22 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 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  30. 30. 4. GSM Network Identifiers 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 Parameters • Subscriber Identifier Parameters: • IMSI / TMSI • Equipment Identifiers: • IMEI • Call Number Identifiers: • MSISDN • Call Routing Identifiers: • LAC / LAI / MSRN GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 23
  31. 31. 4. GSM Network Identifiers _____________________________________________________________________ 4.2 Subscriber Identifiers 4.2.1 INTERNATIONAL MOBILE SUBSCRIBER IDENTITY (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) MCC MSIN 3 digits Example: MNC 2-3 digits 9-10 digits 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. 24 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  32. 32. 4. GSM Network Identifiers Temporary Mobile Subscriber Identity (TMSI) • • • • Replaces IMSI Unique only within a LA Issued on IMSI attach and LA change (minimum) Comprises 32-bits: Octet Octet Octet Octet 8 bits 8 bits 8 bits 8 bits TMSI Parameter Settings (VLR) PARAMETER REC Location Update New Visitor Y Location Update TMSI Y IMSI Attach Y Periodic Location Update N ALLOCATION Mobile Originating Call N COUNTERS Mobile Originating SMS N Mobile Terminating Call N Mobile Terminating SMS N Mobile Terminating USSD N Mobile SS Operation N GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 25
  33. 33. 4. GSM Network Identifiers IMSI/TMSI Authentication Parameter Settings PARAMETER Location Update New Visitor REC Y IMSI Attach Periodic Location Update N Mobile Originating Call N N Mobile Terminating Call N Mobile Terminating SMS N Mobile Terminating USSD N Mobile SS Operation COUNT N Mobile Originating SMS AUTHENTICATION Y Location Update N _____________________________________________________________________ 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: • • • • 26 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)) GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  34. 34. 4. GSM Network Identifiers 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) TAC SNR X 6 digits Example: FAC 2 digits 6 digits 1 digit 495020 10 123456 7 (Access: *# 92702689 #) IMEI Check Parameters (VLR) PARAMETER Location Update New Visitor IMSI Attach Location Update REC Y Y Y(10) Periodic Location Update N Mobile Terminating Call N Mobile Originating SMS N Mobile Terminating SMS N Mobile Terminating USSD IMEI CHECKING ON…. N Mobile Originating Call N Mobile SS Operation Black List Effect N BLOCK Grey List Effect GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 TRACE Unknown IMEI Effect BLOCK 27
  35. 35. 4. GSM Network Identifiers _____________________________________________________________________ 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) • • Identifies the global calling number Comprises: • Country Code (CC) • National Destination (area) Code (NDC) • Subscriber Number (SN) CC 28 SN 3 digits Example: NDC 2-3 digits 9-10 digits 262 – Germany 01 – D1 Telekom 123456789 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  36. 36. 4. GSM Network Identifiers _____________________________________________________________________ 4.5 Call Routing Identifiers 4.5.1 LOCATION AREA CODE (LAC) Location Area Code (LAC) • • Uniquely identifies a LA within a specific PLMN Comprises 2 octets: Octet Octet 8 bits 8 bits 4.5.2 LOCATION AREA IDENTITY (LAI) Location Area Identifier (LAI) • • Globally unique Identity Comprises: • Mobile Country Code (MCC) • Mobile Network Code (MNC) (operator) • Location Area Code (LAC) MCC LAC 3 digits Example: MNC 2-3 digits 2 octets 262 – Germany GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 01 – D1 Telekom 54 29
  37. 37. 4. GSM Network Identifiers 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) • 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) CC 30 SN 3 digits Example: NDC 2-3 digits 9-10 digits 262 – Germany 01 – D1 Telekom 123456789 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  38. 38. 4. GSM Network Identifiers Section 4 - Summary • In this section the following network identifiers were described: • Subscriber Identifiers: • Equipment Identifiers: • IMSI / TMSI • IMEI / IMEISV • Call Number Identifiers: • Call Routing Identifiers: • MSISDN • LAC / LAI / MSRN GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 31
  39. 39. 4. GSM Network Identifiers Intentional Blank Page 32 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  40. 40. 5.. Common Cell Parameters 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 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 33
  41. 41. 5.. Common Cell Parameters _____________________________________________________________________ 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) • CI: • Identity unique to a cell within a location area (LA) • Fixed Length of 2 octets: Octet 8 bits • Octet 8 bits GCI: • Globally unique cell identity • Comprises LAI +CI Location Area Identity (LAI) Cell Identity (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. 34 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  42. 42. 5.. Common Cell Parameters Base Station Identity Code (BSIC) • 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: Network Colour Code (NCC) 3 bits Base Station Colour Code (BCC) 3 bits • Countries/Operators may have different NCCs allocated = 2, France = 0, Ireland = 3, Italy = 2) (ETSI GSM 03.03) • • The BCC identifies the training sequence used by the BTS. (e.g. UK The NCC allows the MS to discriminate invalid BSICs _____________________________________________________________________ 5.3 Cell Functions Cell Function Implementation Examples • SMS Cell Broadcast (CBCH) Usage: • SMSCBUSE (Y/N) {Ericsson} • HSCSD Usage • BTSHSCSD (Y/N) {Ericsson} • BSCHSCSD (Y/N) {Ericsson} GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 35
  43. 43. 5.. Common Cell Parameters _____________________________________________________________________ 5.4 Cell Channel Configurations Control 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: F S S SDCCH 0 • CCCH F S CCCH CCCH F S SDCCH 0 SDCCH 1 F S SDCCH 2 SDCCH 3 F S SACCH 0 SACCH 1 I F S CCCH CCCH F S CCCH CCCH I I I A non-combined multiframe has 9 CCCH blocks: • F BCCH BCCH CCCH SDCCH 1 F SDCCH 2 S CCCH SDCCH 3 CCCH SDCCH 4 F S SDCCH 5 CCCH SDCCH 6 CCCH SDCCH 7 SACCH 0 SACCH 1 SACCH 2 SACCH 3 I A complete paging or access grant message takes four bursts (timeslots), i.e. one CCCH block. Cell Channel Configuration Examples • • • • 36 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 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  44. 44. 5.. Common Cell Parameters Control Channel Configuration CCCH_CONF Number of CCCH Timeslots 000 1 TS0 (non-combined) 001 1 TS0 (combined) 010 2 TS0 (non-combined),TS2 100 3 TS0 (non-combined),TS2,TS4 110 4 TS0 (non-combined),TS2,TS4,TS6 Configuration 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 37
  45. 45. 5.. Common Cell Parameters Section 5 - Summary • In this section the following topics have been covered: • • • 38 Cell Identifier Parameters Cell Function Parameters Cell Channel Configuration Parameters GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  46. 46. 6. Network Access 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 39
  47. 47. 6. Network Access _____________________________________________________________________ 6.2 Network Access Procedures 6.2.1 NETWORK CONNECTION SEQUENCE MS Network Connection Sequence Power on Scan RF channels Select highest carrier level Scan for FCCH frequency correction burst Select next highest carrier level NO FCCH detected? NO YES SCH detected? Scan for SCH synchronisation burst YES ‘camp-on’ to BCCH and start decoding Monitor PCH and adjacent carriers 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. 40 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  48. 48. 6. Network Access 6.2.2 IMSI ATTACH IMSI Attach • • • • Mobile camps on to best serving BTS • MSC and HLR carry out authentication check challenge and response using Ki • Optionally EIR checks for status of mobile (white/grey/black) Mobile sends IMSI to MSC MSC/VLR is updated in HLR BSC Subscriber data including current location area is added to local VLR VLR MSC AuC HLR EIR 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 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 41
  49. 49. 6. Network Access 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 predetermined amount of time and forces an IMSI detach. IMSI Detach • Explicit: • • • • • Mobile informs MSC it is switching off BSC HLR stores last location area for mobile VLR records that mobile is no longer available on network Mobile powers down VLR Implicit • MSC VLR forces IMSI Detach due to no response AuC HLR 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. 42 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  50. 50. 6. Network Access 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 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). GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 43
  51. 51. 6. Network Access Access Parameters • RXLEV_ACC_MIN (range 47-110 dBm) – (default 110dBm): • • 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): • • Receive Level Access Minimum. Defines minimum signal level for MS to access the network 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: CBQ HIGH HIGH LOW LOW 44 CB NO YES NO YES Phase 2 MS Cell Sel Cell Resel Normal Normal Barred Barred Low Priority Normal Low Priority Normal Phase 1 MS Cell Sel/Resel Normal Barred Normal Barred GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  52. 52. 6. Network Access 6.3.2 ATTACH/CALL SETUP PARAMETERS Attach/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). GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 45
  53. 53. 6. Network Access Detach/Disconnect Parameters • BTMN (range 6-150, OFF) – (default OFF): • • GTMN (range 0-255) – (no default): • • Base Time Detach Mobile. Defines the time before an MS is forced to Detach 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 6 - Summary • This section has covered: • Network access procedures • IMSI attach and detach • Access parameters • Connection setup parameters • Detach and disconnection parameters 46 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  54. 54. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 47
  55. 55. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements ____________________________________________________________________ 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) 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. 48 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  56. 56. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements 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: GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 49
  57. 57. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements 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 List 8 7 6 5 4 3 2 1 BA Range IEI Octet 1 Length of BA Range Contents Octet 2 Number of Ranges Octet 3 RANGE1_LOWER Octet 4 RANGE1_LOWER RANGE1_HIGHER Octet 5 RANGE1_HIGHER RANGE2_LOWER Octet 6 RANGE2_LOWER RANGE2_HIGHER Octet 7 RANGE2_HIGHER Octet 8 RANGE3_LOWER Octet 9 RANGE3_LOWER RANGE3_HIGHER Octet 10 RANGE3_HIGHER RANGE4_LOWER Octet 11 RANGE4_LOWER RANGE4_HIGHER Octet 12 RANGE4_HIGHER Octet 13 Octet n 7.2.9 THE BA ARFCN LIST The BA message lists all 124 ARFCNs but indicates which ARFCN ranges are searchable. BA ARFCN List 8 7 6 5 4 3 2 1 Cell Channel Description IEI 0 0 0 0 CA CA CA CA spare spare ARFCN ARFCN ARFCN ARFCN 124 Format ID 123 122 121 CA CA CA CA CA CA CA CA ARFCN ARFCN ARFCN ARFCN ARFCN ARFCN ARFCN ARFCN 120 119 118 117 116 115 114 113 CA CA CA CA CA CA CA CA ARFCN ARFCN ARFCN ARFCN ARFCN ARFCN ARFCN ARFCN 008 007 006 005 004 003 002 001 CA = 0 – not part of BA list CA = 1 – included in BA list 50 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  58. 58. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements 7.2.10 EFFECT OF BA LIST LOADING ON MEASUREMENTS Sample Rate vs BA List Loading Number of BCCH carriers in BA List Number of samples per carrier in SACCH multiframe 32 3-4 16 6-7 10 10-11 8 12-13 _____________________________________________________________________ 7.3 Idle Mode Cell Measurements BCCH Carrier Measurement – Idle 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) GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 51
  59. 59. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements 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 Measurements – Idle Mode 85Bm RxLev m 110dB / BSIC xLev R 112Bm RxLe v / BS IC 96dBm RxLev / BSIC 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. 52 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  60. 60. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements 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 Parameters • Two parameters can be defined for each neighbour relation (for reselection/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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 53
  61. 61. 7. BCCH Allocation (BA) Lists and Idle Mode Measurements CS (range: YES, NO) If the current and neighbouring cell share the same location, this is set to YES, otherwise to NO. BA List-Related Parameters • Examples of Ericsson BA-related parameters: • CELLR (range: 1-7 characters) – (no default): • • 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): • • Neighbouring cell identifier 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 7 - Summary • In this section the following topics have been covered: • • • • • • 54 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 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  62. 62. 8. Cell Selection/Reselection 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 55
  63. 63. 8. Cell Selection/Reselection _____________________________________________________________________ 8.2 Cell Selection Procedures Cell 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 ‘campson’ 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. 56 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  64. 64. 8. Cell Selection/Reselection Cell Selection - Parameter 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 57
  65. 65. 8. Cell Selection/Reselection Cell 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. 58 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  66. 66. 8. Cell Selection/Reselection Cell Re-selection - Phase 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 Re-selection – Phase 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 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 59
  67. 67. 8. Cell Selection/Reselection 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 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 Re-selection - C2 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 • 60 C2 > CELL_RESELECT_HYSTERESIS on a location area boundary GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  68. 68. 8. Cell Selection/Reselection 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 Re-selection - C2 Parameter C2 CELL_RESELECTION_OFFSET C1 TEMPORARY_OFFSET PENALTY_TIME 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 61
  69. 69. 8. Cell Selection/Reselection ________________________________________________________________________________ 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/Re-selection Related Parameters Parameter DB Name (*) Range BA BCCHFREQ 0…1023 Step Size BB900 EXT900 SYS_ID SYSID GSM1800 PCS1900 GSMR CELL_BAR_ACCESS CELLBAR T/F MTPWRCCH 0…31 2dB RXLEV_ACCESS_MIN RXLEVAMI 0…63 1dB CELL_RESELECT_HYSTERESIS CELLRESH 0…7 2dB CQB 0…1 MS_TXPWR_MAX_CCH CELL_BAR_QUALIFY CELL_RESELECT_PARAM_IND CRESPARI 0…1 PENALTY_TIME PENTIME 0…30 20sec TEMPORARY OFFSET TEMPOFF 0…6 10dB CELL_RESELECT_OFFSET CRESOFF 0…63 2dB (*) Siemens DB Field Names 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). 62 GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  70. 70. 8. Cell Selection/Reselection 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 63
  71. 71. 8. Cell Selection/Reselection Section 8 - Summary • In this section the following topics have been covered: • • • 64 Cell selection procedures Cell reselection procedures Summary of key related parameters GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  72. 72. 8. Cell Selection/Reselection 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 65
  73. 73. 8. Cell Selection/Reselection 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. 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. 66 Compare C1 for the two cells and state which one the mobile will camp on to. 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  74. 74. 9. Location Management and Paging Requirements 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 MobileTerminated (MT) connection. This section of the course describes these tow procedures and identifies the key parameters that control their associated functions GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 67
  75. 75. 9. Location Management and Paging Requirements _____________________________________________________________________ 9.2 Location Management Procedures 9.2.1 LOCATION MANAGEMENT OPTIONS Location 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: • • 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. • 68 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. 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. GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002
  76. 76. 9. Location Management and Paging Requirements 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 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 Requirements • Location Area Change • Periodic Location Update • IMSI Attach • Cell change during call • TMSI update on LA change GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002 69
  77. 77. 9. Location Management and Paging Requirements 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 Location 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 • Location area is stored in the VLR of the MSC • MSC 1 The main use of location areas is in paging a mobile • • VLR LAI - Location Area Identity - is transmitted on BCCH The HLR stores the current MSC the mobile is registered with and is only updated if the mobile moves to new MSC VLR Location Areas MSC 2 Every time a moving MS enters a new LA (identified by the information transmitted on the BCCH) it initiates a location update. Automatic 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 New Location Area stored in VLR VLR MSC1 • Acknowledgement Change of MSC: Locati on Up date R / Ackn eques owled t geme nt VLR VLR MSC1 70 Cancel old VLR Location / Acknowledgement HLR MSC2 New VLR Request / Acknowledgement GSM BSS Functions and Parameter Optimisation © AIRCOM International 2002

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