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GSM Introduction

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The global system for mobile communications (GSM) is a set of recommendations and specifications for a digital cellular telephone network (known as a Public Land Mobile Network, or PLMN). These recommendations ensure the compatibility of equipment from different GSM manufacturers, and interconnectivity between different administrations, including operations across international boundaries
The GSM network is comprised of the following components:
Network Elements
The GSM network incorporates a number of network elements to support mobile equipment. They are listed and described in the GSM network elements section of this chapter.
GSM subsystems
In addition, the network includes subsystems that are not formally recognized as network elements but are necessary for network operation. These are described in the GSM subsystems (non-network elements) section of this chapter.
Standardized Interfaces
GSM specifies standards for interfaces between network elements, which ensure the connectivity of GSM equipment from different manufacturers. These are listed in the Standardized interfaces section of this chapter.
Network Protocols
For most of the network communications on these interfaces, internationally recognized communications protocols have been used
These are identified in the Network protocols section of this chapter.

GSM Frequencies
The frequency allocations for GSM 900, Extended GSM and Digital Communications Systems are identified in the GSM frequencies section of this chapter.

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GSM Introduction

  1. 1. 05/31/13Tempus Telcosys 1 ADA CELLWORKS PVT LTD
  2. 2. INTRODUCTION  The global system for mobile communications (GSM) is a set of recommendations and specifications for a digital cellular telephone network (known as a Public Land Mobile Network, or PLMN). These recommendations ensure the compatibility of equipment from different GSM manufacturers, and interconnectivity between different administrations, including operations across international boundaries. 05/31/13Tempus Telcosys 2
  3. 3. THE GSM NETWORK  The GSM network is comprised of the following components:  Network Elements  The GSM network incorporates a number of network elements to support mobile equipment. They are listed and described in the GSM network elements section of this chapter.  GSM subsystems  In addition, the network includes subsystems that are not formally recognized as network elements but are necessary for network operation. These are described in the GSM subsystems (non-network elements) section of this chapter.  Standardized Interfaces  GSM specifies standards for interfaces between network elements, which ensure the connectivity of GSM equipment from different manufacturers. These are listed in the Standardized interfaces section of this chapter. 05/31/13Tempus Telcosys 3
  4. 4. THE GSM NETWORK - CONTINUED  Network Protocols  For most of the network communications on these interfaces, internationally recognized communications protocols have been used  These are identified in the Network protocols section of this chapter.  GSM Frequencies  The frequency allocations for GSM 900, Extended GSM and Digital Communications Systems are identified in the GSM frequencies section of this chapter. 05/31/13Tempus Telcosys 4
  5. 5. DIGITAL NETWORKS  GSM networks are digital and can cater for high system capacities. They are consistent with the world wide digitization of the telephone network, and are an extension of the Integrated Services Digital Network (ISDN), using a digital radio interface between the cellular network and the mobile subscriber equipment. 05/31/13Tempus Telcosys 5
  6. 6. INCREASED CAPACITY  The GSM system provides a greater subscriber capacity than analogue systems. GSM allows 25 kHz. Per user, that is, eight conversations per 200kHz. Channel pair (a pair comprising one transmit channel and one receive channel). Digital channel coding and the modulation used makes the signal resistant to interference from the cells where the same frequencies are re-used (co-channel interference); a Carrier to Interference Ratio (C/I) level of 9 dB is achieved, as opposed to the 18 dB typical with analogue cellular. This allows increased geographic reuse by permitting a reduction in the number of cells in the reuse pattern. Since this number is directly controlled by the amount of interference, the radio transmission design can deliver acceptable performance. 05/31/13Tempus Telcosys 6
  7. 7. CGI : CELL GLOBAL IDENTITY 05/31/13Tempus Telcosys 7 MCC MNC LAC CI LAI CGI MCC = Mobile Country Code MNC = Mobile Network Code LAC = Location Area Code CI = Cell Identity
  8. 8. MSISDN 05/31/13Tempus Telcosys 8 CC NDC SN 98 XXX 12345 CC = Country Code NDC = National Destination Code SN = Subscriber Number
  9. 9. MSISDN  The Mobile Subscriber ISDN (MSISDN) number is the telephone number of the MS. This is the number a calling party dials to reach the subscriber. It is used by the land network to route calls towards the MSC. 05/31/13Tempus Telcosys 9
  10. 10. IMSI  IMSI (International Mobile Subscriber Identity) Network Identity Unique To A Sim. 05/31/13Tempus Telcosys 10 MCC MNC MSIN 404 XX 12345..10 SIM = Subscriber Identity Module MCC = Mobile Country Code MNC = Mobile Network Code MSIN = Mobile Subscriber Identity Number
  11. 11. IMEI  IMEI : Serial number unique to each mobile 05/31/13Tempus Telcosys 11 TAC FAC SNR SP 6 2 6 1 IMEI = International Mobile Equipment Identity TAC = Type Approval Code FAC = Final Assembly Code SNR = Serial Number SP = Spare
  12. 12. SUBSCRIBER IDENTIFICATION  International Mobile Subscriber Identity (IMSI)  Just the IMEI identifies the mobile equipment, other numbers are used to identify the mobile subscriber. Different subscriber identities are used in different phases of call setup. The International Mobile Subscriber Identity (IMSI) is the primary identity of the subscriber within the mobile network and is permanently assigned to that subscriber.  Temporary Mobile Subscriber Identity (TMSI)  The GSM system can also assign a Temporary Mobile Subscriber Identity (TMSI). After the subscriber’s IMSI has been initialized on the system, the TMSI can be used for sending backward and forward across the network to identify the subscriber. The system automatically changes the TMSI at regular intervals, thus protecting the subscriber from being identified by someone attempting to monitor the radio channels. The TMSI is a local number and is always transmitted with the Local numbers and is always transmitted with the Location Area Identification (LAI) to avoid ambiguities. 05/31/13Tempus Telcosys 12
  13. 13. SUBSCRIBER IDENTIFICATION MODULE (SIM)  By making a distinction between the subscriber identity and the mobile equipment identity, a GSM PLMN can route calls and perform billing based on the identity of the subscriber rather than the mobile equipment being used. This can be done using a removable Subscriber Information Module (SIM). A ”smart card” is one possible implementation of a SIM module.  IMSI. This is transmitted at initialization of the mobile equipment.  TMSI This is updated periodically by the PLMN  MSISDN This is made up of a country code, a national code and a subscriber number.  Location Area Identity (LAI) This identified the current location of the subscriber.  Subscriber Authentication Key (KI) This is used to authenticate the SIM. 05/31/13Tempus Telcosys 13
  14. 14. EQUIPMENT IDENTITY NUMBER  International Mobile station Equipment Identity (IMEI)  Each MS is identified by an International Mobile station Equipment Identity (IMEI) number which is permanently stored in the mobile equipment. On request, the MS sends this number over the signalling channel to the MSC. The IMEI can be used to identify MS,s that are reported stolen or operating incorrectly.  Equipment Identity Register ( EIR )  A listing of the allowed IMEI is maintained by the PLMN’s in the Equipment Identity Register (EIR) to validate the mobile equipment. 05/31/13Tempus Telcosys 14
  15. 15. Frequency Bands Uplink 890 – 915 MHz 25 MHz 05/31/13Tempus Telcosys 15 Downlink 935 – 960 MHz 25 MHz 100 KHz 200 KHz 100 KHz 1 43 1242 ……………. A 200 KHz carrier spacing has been chosen. Excluding 2x100 KHz edges of the band, this gives 124 possible carriers for the uplink and downlink. The use of carrier 1 and 124 are optional for operators.
  16. 16. GSM Network Architecture 05/31/13Tempus Telcosys 16 BTS BTS BTS BTS BTS BSC BSC TRAU MSC HLR AUC VLR EIR PSTN SMSC
  17. 17. MS – Mobile Station  Mobile station provides user access to GSM network for voice and data  All GSM mobiles comply to GSM standards  Subscriber data is read from a SIM card that plugs into ME 05/31/13Tempus Telcosys 17 SIM ME MS
  18. 18. MS (cont..)  Each MS has a unique number called as IMEI number, which is stored in EIR for authentication purposes  Mobile camps on to the GSM network through the BTS serving the cell  Mobile also scans neighboring cells and reports signal strengths  Mobile transmits and receives voice at 13 kb/s over the air interface 05/31/13Tempus Telcosys 18
  19. 19. Mobile Station Output Power  CLASS 1 20 watts Vehicle and Portable  CLASS 2 8 watts Portable and Vehicle  CLASS 3 5 watts Hand-Held  CLASS 4 2 watts Hand-Held (GSM)  CLASS 5 0.8 watts Hand-Held (DCS 1800)  Output power determines:  Accessibility in areas of coverage  Talk Time and Standby time 05/31/13Tempus Telcosys 19
  20. 20. Mobile Station Identities  CC – Country Code  NDC – National Destination Code  SN – Serial Number 05/31/13Tempus Telcosys 20 MSISDN : Mobile Station ISDN Number It is the human identity used to call a Mobile Station CC SNNDC MSISDN 98 250 00134
  21. 21. IMSI (International Mobile Subscriber Identity)  MCC – Mobile Country Code  MNC – Mobile Network Code  MSIN – Mobile Subscriber Identity Number 05/31/13Tempus Telcosys 21 MCC MSINMNC IMSI 3 2 or 3 Not more than 15 NMSI
  22. 22. IMEI (International Mobile Equipment Identity)  TAC – Type Approval Code  FAC – Factory Assembly Code  SNR – Serial Number  SP – Spare digit (usually used to specify software version) 05/31/13Tempus Telcosys 22 TAC SPFAC IMEISNR 6 162 15
  23. 23. SIM ( Subscriber Identity Module)  Removable module inserted when the subscriber wants to use the ME  Two sizes: credit card size and stamp size  SIM features and contents are personalized by the Service Activator  ROM – 6kb to 16 kb  RAM – 128 bytes to 256 bytes  EEPROM – 3kb to 8 kb 05/31/13Tempus Telcosys 23 Space to insert SIM photo
  24. 24. Contents of SIM  Serial Number  IMSI, Subscriber Key Ki, Ciphering Key Kc  Algorithms for authentication and ciphering  Network Code  PIN, PUK  Charging Information  Abbreviated Dialling  Supplementary Features (e.g. Call barring) 05/31/13Tempus Telcosys 24
  25. 25. SIM Security  Two level protection  When mobile is turned on, it will ask for user to enter PIN (Personal Id Number)  3 tries for PIN, after that PIN locked  To unblock PIN, there is PUK (Pin Unblock Key)  10 attempts of PUK allowed  After that SIM is blocked 05/31/13Tempus Telcosys 25
  26. 26. BTS (Base Transceiver Station)  BTS has a set of Transceivers (TRXs) to communicate with mobiles in its area  One BTS covers one or more than one cell  The capacity of a cell depends on number of transceivers in the cell  BTS is connected to the BSC through Abis Interface which is 2Mbps  BTS transmits and receives voice at 13kbps over air interface to the mobiles.  BTS commands mobiles to set Tx. Power, timing advance and Handovers 05/31/13Tempus Telcosys 26
  27. 27. BTS 05/31/13Tempus Telcosys 27
  28. 28. BSC – Base Station Controller  Several BTSs are connected to the BSC  BSC Manages channel allocation, handovers and release of channels at connected BTSs  BSC connects to the BTS via the Abis interface and to the MSC on A interface  BSC has the entire database of cell parameters associated with the BTSs.  No mobile data is stored in the BSC  Less connections for MSC as intelligence is made common to all BTSs by the BSC 05/31/13Tempus Telcosys 28
  29. 29. BSC 05/31/13Tempus Telcosys 29
  30. 30. TRAU – Transcoder Rate Adaptation Unit 05/31/13Tempus Telcosys 30 BTS BSC PSTN 13 kbps 16 kbps 16 kbps 64 kbps MSC and TRAU
  31. 31. TRAU (cont..)  The MSC is based on ISDN switching. The Fixed Network is also ISDN based.  ISDN has speech rate of 64 kbps. Mobile communicates at 13 kbps.  TRAU converts the data rates between 13kbps GSM rate to 64kbps Standard ISDN rate  TRAU can be collocated with the BTS, BSC or MSC or it can be a separate unit. 05/31/13Tempus Telcosys 31
  32. 32. Location of Transcoder  Collocated with MSC, BSC, BTS  Separate Unit 05/31/13Tempus Telcosys 32 MSC Transco der BSC
  33. 33. MSC – Mobile Switching Centre 05/31/13Tempus Telcosys 33 BSC BSC BSC BTSs PSTN HLR VLR
  34. 34. MSC (cont..)  Exchange where calls are established, maintained and released  Database for all subscribers and their associated features.  Communicates with the BSCs on the A interface and with PSTN on fixed line.  MSC is weighted on the number of subscribers it can support. E.g. an MSC of 1 lac subscribers means one MSC is enough till subscriber base increases upto 1 lac, beyond which another MSC is required. 05/31/13Tempus Telcosys 34
  35. 35. Multiple MSCs  When there is more capacity, there are more than one MSCs.  All MSCs have to communicate with one another and to the outside world.  Very complicated to connect each MSC to each other and each MSC to PSTN  So there is a concept of GMSC (Gateway MSC) 05/31/13Tempus Telcosys 35 BSC BSC MSC MSC GMSC PSTN
  36. 36. HLR – Home Location Register  MSC has all subscriber database stored in HLR  HLR has all permanent subscriber database  HLR has a database which describes the subscriber’s profile i.e. basic features and supplementary services  MSC communicates with the HLR to get data for subscribers on call 05/31/13Tempus Telcosys 36
  37. 37. VLR – Visiting Location Register  A subscription when activated is registered in VLR  VLR has all the subscriber numbers which are active.  VLR has a temporary database of all active subscribers (on/off, location information) 05/31/13Tempus Telcosys 37 MSC VLRVLR HLR
  38. 38. VLR (cont..)  MSC communicates with HLR for subscribers coming from different MSCs. If the subscriber is found valid, then it registers the subscriber in the VLR 05/31/13Tempus Telcosys 38 MSC MSCVLRVLR HLR VLR
  39. 39. AUC – Authentication Centre  Authentication is a process by which a SIM is verified  Secret data and the verification process algorithm are stored in AUC  AUC is the element which carries out the verification of the SIM  AUC is associated with the HLR 05/31/13Tempus Telcosys 39 MS MSC HLR AUC
  40. 40. EIR (Equipment Identity Register)  EIR is the Mobile Equipment Database which has a series of IMEIs  MSC asks the Mobile to send its IMEI  MSC then checks the validity of IMEI with the EIR  All IMEIs are stored in EIR with relevant classifications 05/31/13Tempus Telcosys 40 EIR MSC
  41. 41. Classification of IMEIs 05/31/13Tempus Telcosys 41 White list: This contains the IMEI of type approved mobiles Black List: List of IMEIs which should be barred because either they are stolen or are not functioning properly Grey list: List of IMEIs which are to be evaluated before they are put in black list
  42. 42. Billing Centre (BC)  BC Generates the billing statement for each subscriber  BC may be directly connected to the MSC or through a mediation device  MSC sends CDRs (Call Detail Records) to the BC  According to the template of pulse rates and units set, BC creates a bill according to the destination called and the call duration 05/31/13Tempus Telcosys 42
  43. 43. Billing Centre (BC) (cont..) 05/31/13Tempus Telcosys 43 CDRs Templates for unit costs
  44. 44. OMC – Operations and Maintenance Centre  Also called the NOC (Network Operations centre)  It is the central monitoring and remote maintenance centre for all network elements  OMC has links to BSCs and MSCs 05/31/13Tempus Telcosys 44
  45. 45. OMC – Operations and Maintenance Centre  Also called the NOC (Network Operations centre)  It is the central monitoring and remote maintenance centre for all network elements  OMC has links to BSCs and MSCs 05/31/13Tempus Telcosys 45
  46. 46. OMC 05/31/13Tempus Telcosys 46 OMC System BSC BSC BSC BTSs BTSs BTSs OMC Terminals
  47. 47. 05/31/13Tempus Telcosys 47 GSM Channels
  48. 48. GSM Channels  Physical Channel  One time slot on one carrier is called physical channel.  Logical Channel  Information carried by physical channels is called logical Channels.  Logical channels are mapped on physical channels. 05/31/13Tempus Telcosys 48
  49. 49. Logical Channels  Traffic channels: Used for speech and data  Full Rate(TCH/F)  Half Rate(TCH/H)  Control channels: Used for signaling .i.e. setting up a radio connection, call or controlling an MS during conversation  BCH(Broadcast channels)  CCCH(common control channels)  DCCH(dedicated control channels) 05/31/13Tempus Telcosys 49
  50. 50. Traffic Channels(TCH) 05/31/13Tempus Telcosys 50 TCH/F (full Rate) TCH/H (half Rate) Traffic Channels(TCH)
  51. 51. Control Channels(CCH) 05/31/13Tempus Telcosys 51 CCH(Control Channel) BCH CCCH DCCH CCH RACH CBCH SDCCH ACCHSynch. Chanels SACCHFACCH PCH/ AGCHFCCHSCH
  52. 52. BCH(Broadcast Channels)  BCCH(Broadcast Control Channels)  Downlink Only.  Broadcast information of the serving cell (System Information).  Transmitted on timeslot zero of BCCH carrier.  Reads only by idle mobile at least once every 30 secs. 05/31/13Tempus Telcosys 52
  53. 53. BCH(Broadcast Channels) cont’d  SCH(Synchronisation Channels)  Downlink Only  Carries information for frame synchronisation.  Contains frame number and BSIC(Base Station Identity Code). 05/31/13Tempus Telcosys 53
  54. 54. BCH(Broadcast Channels) cont’d  FCCH(Frequency Correction Channels)  Downlink Only.  Enable MS to synchronies to the frequency. 05/31/13Tempus Telcosys 54
  55. 55. CCCH(Common Control Channel)  RACH(Random Access Channel)  Uplink only.  Used by the MS when making its first access to the Network.  The reason for access could be initiation of a call or a page response. 05/31/13Tempus Telcosys 55
  56. 56. CCCH(Common Control Channel) cont’d  AGCH(Assess Grant Channel)  Downlink only.  Used for acknowledgement of the access attempt sent on RACH.  Used by the network to assign a signaling cannel upon successful decoding of access bursts. 05/31/13Tempus Telcosys 56
  57. 57. CCCH(Common Control Channel) cont’d  PCH(Paging Channel)  Downlink only.  The network will page the MS ,if there is a incoming call or a short Message.  It contains the MS identity number, the IMSI or TMSI. 05/31/13Tempus Telcosys 57
  58. 58. DCCH(Dedicated Control Channel)  SDCCH (Stand-alone Dedicated Control Channel)  Uplink and Downlink.  Used for call setup, authentication, ciphering location update and SMS. 05/31/13Tempus Telcosys 58
  59. 59. DCCH(Dedicated Control Channel) cont’d  SACCH(Slow Associated Control Channel)  Downlink and Uplink.  Used to transfer signal while MS have ongoing conversation on traffic or while SDCCH is being used.  On the forward link, the SACCH is used to send slow but regularly changing control information to each mobile on that ARFCN, such as power control instructions and specific timing advance instructions 05/31/13Tempus Telcosys 59
  60. 60.  SACCH(Slow Associated Control Channel) cont’d  The reverse SACCH carries information about the received signal strength and quality of the TCH, as well as BCH measurement results from neighboring cells. 05/31/13Tempus Telcosys 60
  61. 61. DCCH(Dedicated Control Channel) cont’d  FACCH(Fast Associated Control Channel)  Downlink and uplink.  Associate with TCH only.  It is used to send fast message like hand over message.  Work by stealing traffic bursts. 05/31/13Tempus Telcosys 61
  62. 62. Mapping on Physical Channels  The Logical channels are mapped on the physical channels.  The TDMA frames are grouped together into multi-frame.  26 TDMA multi-frame for Traffic.  51 TDMA multi-frame for control signal. 05/31/13Tempus Telcosys 62
  63. 63. Channel Combination  Combined  All the controlling signals are in the time slot 0 of the Multi-frame.  Non Combined  Dedicated controlling signals are in time slot 1 of the Multi-frame. 05/31/13Tempus Telcosys 63
  64. 64. Combined  Cell with single carrier.  Timeslot 0 :BCCH+CCCH+SDCCH.  Timeslot 1-7 :TCH/FACCH+SACCH. 05/31/13Tempus Telcosys 64
  65. 65. Non Combined  Cell with Two carrier  Timeslot 0 (of carrier 1) BCCH+CCCH.  Timeslot 1 (of carrier1) SDCCH+SACCH.  Timeslot 2-7 & 0-7(of both carriers) TCH/FACCH+SACCH. 05/31/13Tempus Telcosys 65
  66. 66. BROADCAST MESSAGES  System information 5 and 6 sent on the SACCH immediately after Handover or whenever nothing else is being sent.  Downlink SACCH is used for system information messages while uplink SACCH is used for measurement reports.  System Information types 7 and 8 (optional) are an extension to type 4 and broadcast on the BCCH. 05/31/13Tempus Telcosys 66
  67. 67. SYSTEM INFORMATION 05/31/13Tempus Telcosys 67
  68. 68. SYSTEM INFORMATION 1  When frequency hopping is used in cell MS needs to know which frequency band to use and what frequency within the band it should use in hopping algorithm.  Cell channel description Cell Allocation Number(CANO)-Informs the band number of the frequency channels used. 00-Band 0(current GSM band) Cell Allocation ARFCN(CA ARFCN):- ARFCN’s used for hopping.It is coded in a bitmap of 124 bits. 05/31/13Tempus Telcosys 68
  69. 69. SYTEM INFORMATION 1 05/31/13Tempus Telcosys 69 124 123 122 121 024 023 022 021 020 019 018 017 016 015 014 013 012 011 010 009 008 007 006 005 004 003 002 001
  70. 70. SYSTEM INFORMATION 1  RACH Control Parameters Access Control Class(ACC) :-Bitmap with 16 bits. All MS spread out on class 0 –9 . Priority groups use class 11-15. A bit set to 1 barred access for that class. Bit 10 is used to tell the MS if emergency call is allowed or not. 0 – All MS can make emergency call. 1 - MS with class 11-15 only can make emergency calls. Cell barred for access(CB):- 0- Yes 1- No 05/31/13Tempus Telcosys 70
  71. 71. SYSTEM INFORMATION 1  RACH Control Parameters Re-establishment allowed(RE):- 0- Yes 1- No Max_retransmissions(MAXRET):-Number of times the MS attempts to access the Network [1,2,4 or 7]. Tx-integer(TX):- Number of slots to spread access retransmissions when a MS attempts to access the system. Emergency call allowed:- Yes/No. 05/31/13Tempus Telcosys 71
  72. 72. SYSTEM INFORMATION 2  System Information Type 2 message consists of the Double BA list which defines the BCCH frequencies used in the neighboring cells.  The Double BA list provides the MS with different frequencies on which to measure, depending on whether the MS is in idle or active mode.  In active mode, the MS should measure on a reduced number of frequencies in order to improve the accuracy of measurements. 05/31/13Tempus Telcosys 72
  73. 73. SYSTEM INFORMATION 2  In Idle mode,the MS should measure on larger number of frequencies, so that the time required for the MS to access the network after power on is reduced.  The MS is also informed which PLMN’s it may use.  As well as System Information Type 2,it is also possible to have System Information Type 2 Bis and System information Type 2 Ater, depending on the size of the BA List.  System Information Type 2 Bis/Ter are optional. 05/31/13Tempus Telcosys 73
  74. 74. SYSTEM INFORMATION 2  Neighbor Cell Description:- BA Indicator(BA IND):- Allows to differentiate measurement results related to different list of BCCH frequencies sent to MS. BCCH Allocation number(BANO):- Band 0 is used.  PLMN Permitted(NCCPERM):-This the PLMN color codes permitted and tells the MS which network color codes(NCC) on the BCCH carriers it is allowed to monitor when it is in this cell. . 05/31/13Tempus Telcosys 74
  75. 75. SYSTEM INFORMATION 2  RACH Control Parameters Access Control Class(ACC) :-Bitmap with 16 bits. All MS spread out on class 0 –9 . Priority groups use class 11-15. A bit set to 1 barred access for that class. Bit 10 is used to tell the MS if emergency call is allowed or not. 0 – All MS can make emergency call. 1 - MS with class 11-15 only can make emergency calls. Cell barred for access(CB):- 0- Yes 1- No 05/31/13Tempus Telcosys 75
  76. 76. SYSTEM INFORMATION 2 Re-establishment allowed(RE):- 0- Yes 1- No Max_retransmissions(MAXRET):-Number of times the MS attempts to access the Network [1,2,4 or 7]. Tx-integer(TX):- Number of slots to spread access retransmissions when a MS attempts to access the system. Emergency call allowed:- Yes/No. 05/31/13Tempus Telcosys 76
  77. 77. SYSTEM INFORMATION 2 BCCH ARFCN Number(BAIND):- ARFCN’s used for in a Bitmap of 124 bits 05/31/13Tempus Telcosys 77 124 123 122 121 024 023 022 021 020 019 018 017 016 015 014 013 012 011 010 009 008 007 006 005 004 003 002 001
  78. 78. SYSTEM INFORMATION 3  The System Information Type 3 contains information on the identity of the current LA and cell identity, because a change means that the MS must update the network.  System Information 3 also as Control Channel Description parameters used to calculate the Paging group.  When the MS is in idle mode it decides which cells to lock to. Information needed by the MS for cell selection is also broadcast in the Type 3 information. 05/31/13Tempus Telcosys 78
  79. 79. SYSTEM INFORMATION 3 8 7 6 5 4 3 2 1 1 1 1 1 LAC LOCATION AREA IDENTITTY(LAI) MCC DIG 1MCC DIG 2 MCC DIG 1 MNC DIG 1MNC DIG 2 CI CI CELL IDENTITY LAC 05/31/13Tempus Telcosys 79
  80. 80. SYSTEM INFORMATION 3  Control Channel Description Attach / Detach(ATT):- 0 = Allowed 1 = Not Allowed bs_agblk:-Number of block reserved for AGCH [0-7] Ba_pmfrms:-Number of 51 frame multi-frames between transmission of paging messages to MS of the same group T3212:- Periodic location update timer . [1-255 deci hours]. 05/31/13Tempus Telcosys 80
  81. 81. SYSTEM INFORMATION 3 cch_conf Physical channels combined No. of CCH 0 1 timeslot(0) No 9 1 1 timeslot(0) Yes 3 2 2 timeslot(0,2) No 18 4 3 timeslot(0,2,4) No 27 6 4 timeslot(0,2,4,6) No 36 05/31/13Tempus Telcosys 81
  82. 82. SYSTEM INFORMATION 3  Cell options DTX:-Whether Discontinuous Transmission used or not. PWRC:-Power control on the downlink. 0 = Not used. 1 = Used. Radio link timeout(RLINKT):- Radio link time-out is the time before an MS disconnects due to failure in decoding SACCH message. Sets the timer T100 in the MS. 05/31/13Tempus Telcosys 82
  83. 83. SYSTEM INFORMATION 3  Cell Selection Parameters Rxlev_access_min:- Minimum received signal level at the MS for which it is permitted to access the system. 0-63 = -100 dBm to –47 dBm. Mx_txpwr_cch:- Maximum power the MS will use when accessing the system. Cell_reselect_hysteresis:- Used for cell reselection.  RACH Control Parameters. 05/31/13Tempus Telcosys 83
  84. 84. SYSTEM INFORMATION 4  Location Area Identification.  Cell Selection Parameters Rxlev_access_min:- Minimum received signal level at the MS for which it is permitted to access the system. 0-63 = -100 dBm to –47 dBm. Mx_txpwr_cch:- Maximum power the MS will use when accessing the system. Cell_reselect_hysteresis:- Used for cell reselection. 05/31/13Tempus Telcosys 84
  85. 85. SYSTEM INFORMATION 4  RACH Control Parameters max_retransmissions(MAXRET) tx_integer(TX) Cell barred for access(CB). Re-establishment allowed(RE) Emergency Call Allowed Access Control Class (ACC) 05/31/13Tempus Telcosys 85
  86. 86. SYSTEM INFORMATION 4  CBCH Description(Optional) : CHN:- This is the channel number for CBCH. It is controlled internally in BSC. TSC:- Training Sequence Code. Base Station Color Code(BCC) part of BSIC is used. CBCHNO:- Absolute RF channel number of CBCH. MAC:- Mobile Allocation in the cell, describes the frequencies to be used in the hopping sequence if frequency hopping is used. 05/31/13Tempus Telcosys 86
  87. 87. SYSTEM INFORMATION 4 Hopping Channel(H):-Informs if CBCH Channel is hopping or single. ARFCN:- If H=0; MAIO:- If H=1, informs the MS where to start hopping. Values [0-63]. HSN:- If H=1, informs the MS in what order the hopping should take place. Values[0 –63]. HSN=0 Cyclic Hopping. MA:-Indicates which RF Channels are used for hopping. ARFCN numbers coded in bitmap. 05/31/13Tempus Telcosys 87
  88. 88. SYSTEM INFORMATION 5  Sent on the SACCH on the downlink to the MS in dedicated mode.  On SAACH, the MS also receives information about the BCCH carrier in each neighboring cell. This may differ from those sent in System information type 2.  It is also possible to have system Information Type 5 Bis and System Information Type 5Ter, depending on the size of the BA list. 05/31/13Tempus Telcosys 88
  89. 89. SYSTEM INFORMATION 5  Neighbor Cell Description:- BA-IND:-Used by the Network to discriminate measurements results related to different lists of BCCH carriers sent by the MS(Type 2 or 5). Values 0 or 1(different from type 2). BCCH Allocation number:-00-Band 0(current GSM band). 05/31/13Tempus Telcosys 89
  90. 90. SYSTEM INFORMATION 5 BCCH ARFCN:-Neighboring cells ARFCN’s. Sent as a bitmap. 0-Not used 1-Used. 124 123 122 121 024 023 022 021 020 019 018 017 016 015 014 013 012 011 010 009 008 007 006 005 004 003 002 001 05/31/13Tempus Telcosys 90
  91. 91. SYSTEM INFORMATION 6  Ms in dedicated mode needs to know if the LA has changed.If so, it must perform location updating when the call is released.  MS may change between cells with different Radio link timeout and DTX.  Cell Identity.  Location Area Identification.  PLMN permitted. 05/31/13Tempus Telcosys 91
  92. 92. SYSTEM INFORMATION 6  Cell options: DTX PWRC Radio Link timeout. 05/31/13Tempus Telcosys 92
  93. 93. SYSTEM INFORMATION 7/8  System Information Types 7 and 8 contain Cell Reselect parameters. Their function is to supplement System Information Type 4. 05/31/13Tempus Telcosys 93
  94. 94. GSM Interfaces  (Um) Air interface - MS to BTS  A bis interface - BTS to BSC  A Interface - BSC to MSC  B Interface - MSC to VLR  C interface - MSC to HLR 05/31/13Tempus Telcosys 94
  95. 95. MSC BSC VLRHLR AUC EIR GMSC MS A Interface A bis Interface Air Interface B Interface C Interface F Interface D Interface H Interface To other Networks 05/31/13Tempus Telcosys 95
  96. 96. GSM Interfaces  The interfaces between MSC and MS is called A, Abis and Um interfaces.  On these interfaces only three layers are defined.They are not corresponding to the OSI (Open System Interconnection) model. 05/31/13Tempus Telcosys 96
  97. 97. A Interface  A interface between the BSC and the MSC  The A interface provides two distinct types of information, signalling and traffic, between the MSC and the BSC.  The speech is transcoded in the TRC and the SS7 (Signalling system) signalling is transparently connected through the TRC or on a separate link to the BSC. 05/31/13Tempus Telcosys 97
  98. 98. Abis Interface  The A-bis interface responsible for transmitting traffic and signalling information between the BSC and the BTS.  The transmission protocol used for sending signalling information on the A-bis interface is Link Access Protocol on the D Channel (LAPD) 05/31/13Tempus Telcosys 98
  99. 99. (Um) Air Interface  This is the interface between the mobile station and the Base station.  The Air interface uses the Time Division Multiple Access (TDMA) technique to transmit and receive traffic and signalling information between the BTS and MS.  The TDMA technique is used to divide each carrier into eight time slots.These time slots are then assigned to specific users,allowing up to eight conversations to be handled Simultaneously by the same carrier. 05/31/13Tempus Telcosys 99
  100. 100. 7 56 34 12 0 1 2 43 5 76 Down Link Up Link 0 Time Slot 05/31/13Tempus Telcosys 100 • This interface is the radio interface between the mobile station and the network and uses layer Three messages. • On Layer three messages we have the division of message types into CM (communication Management), MM (Mobility Management), and RR (Radio Resource Management).
  101. 101. Connection Management (CM) There are three entities within CM:  Call Control(CC) – Which handles the procedures concerning call control. e.g. setup,Change of bearer service.  Supplementary Service (SS) – Which handles such as call bearing, call waiting , call forwarding etc.  Short Message Service (SMS) – Enables the MS to handle short message transfer to and from the network. 05/31/13Tempus Telcosys 101
  102. 102. Mobility Management (MM)  Mobility management handles functions for authentication, location updating, identification and others concerning the mobility of the mobile station. 05/31/13Tempus Telcosys 102
  103. 103. Radio Resource Management (RR)  It contains the functions concerning the radio link. Here we find the capability to establish,maintain and release the radio connection between the network and the mobile station, which includes the handover procedure. 05/31/13Tempus Telcosys 103
  104. 104. B Interface  The B interface between the MSC and the VLR uses the MAP/TCAP protocol.  Most MSCs are associated with a VLR, making the B interface "internal".  Whenever the MSC needs access to data regarding a MS located in its area, it interrogates the VLR using the MAP/B protocol over the B interface. 05/31/13Tempus Telcosys 104
  105. 105. C Interface  The C interface is between the HLR and a MSC.  Each call originating outside of GSM (i.e., a MS terminating call from the PSTN) has to go through a Gateway to obtain the routing information required to complete the call, and the MAP/TCAP protocol over the C interface is used for this purpose.  Also, the MSC may optionally forward billing information to the HLR after call clearing. 05/31/13Tempus Telcosys 105
  106. 106. D Interface  The D interface is between the VLR and HLR.  It uses the MAP/TCAP protocol to exchange the data related to the location of the MS and to the management of the subscriber. 05/31/13Tempus Telcosys 106
  107. 107. E Interface  The E interface interconnects two MSCs.  The E interface exchanges data related to handover between the anchor and relay MSCs using the -MAP/TCAP+ISUP/TUP protocol. 05/31/13Tempus Telcosys 107
  108. 108. F Interface  The F interface connects the MSC to the EIR.  It uses the MAP/TCAP protocol to verify the status of the IMEI that the MSC has retrieved from the MS. 05/31/13Tempus Telcosys 108
  109. 109. G Interface  The G interface interconnects two VLRs of different MSCs.  It uses the MAP/G protocol to transfer subscriber information, during e.g. a location update procedure. 05/31/13Tempus Telcosys 109
  110. 110. 05/31/13Tempus Telcosys 110
  111. 111. Topics for discussion  Speech Encoding  Data Encoding  Interleaving for Voice,Control and Data signals 05/31/13Tempus Telcosys 111
  112. 112. Speech Encoding  We shall start with a raw voice signal fed into the microphone, travel through the various stages involving vocoding, channel coding etc till it reaches the final burst format on the Air Interface. 05/31/13Tempus Telcosys 112
  113. 113. Speech Encoding ckt 05/31/13Tempus Telcosys 113 Voice Encoding Channel coding interleaving RF Modulation Raw Voice signal
  114. 114. Speech Encoding ckt  The voice is sampled at the rate of 50 samples per second.  This results in 20 msec blocks of speech  Each of this 20 msec block is passed on to the 13Kbps vocoder.  There are 260 information bits from the output of the vocoder for every 20 msec input i.e.; 13Kbps *20msec = 260 bits. 05/31/13Tempus Telcosys 114
  115. 115. Voice Encoding ckt 05/31/13Tempus Telcosys 115 Vocoder I/p 20 msec speech blocks 13Kbps Vocoder Vocoder O/p 260 bits
  116. 116. Channel coding  Channel Coding is done to protect the logical channels from transmission errors introduced by the radio path.  The coding schemes depend on the type of the logical channels, hence the coding can differ from speech, control and data . 05/31/13Tempus Telcosys 116
  117. 117. Channel Coding for speech 05/31/13Tempus Telcosys 117 Class class 1b class 2 1a 50 3 132 4 tail Bits parity bits Convolutional coder ½ coder, k=5 456 bits=378 bits from Convolution coder + 78 class 2 bits 260 bits
  118. 118. Channel coding for Speech  The 260 bits of speech info from the vocoder is broken down into three parts.  Class 1a- 50 bits , these represent the filter coefficients of the speech and are the most important for proper detection of the speech at the receiver and hence are given maximum protection. 3 additional parity bits are derived from the class 1a bits for cyclic redundancy check (CRC). 05/31/13Tempus Telcosys 118
  119. 119. Channel coding for Speech cont’d  Class 1b - 132 bits are not parity checked but are fed into the convolutional coder along with 4 tail bits which are used to set the registers in the receiver to a known state for decoding purpose.  Class 1b- 78 bits, these are not so important and are not protected but are combined with the output of the convolution coder. 05/31/13Tempus Telcosys 119
  120. 120. Convolutional coder CC  The Convolutional coder is a series of shift registers implemented using logic gates, where for every one input bit we get 2 output bits. Hence it is called ½ coder.  Here k=5 is the constraint length, it means there are 5 shift register and each bit has memory depth of 4 , meaning it can influence the output of up to four next successive bits. This is useful during reception as bits can be derived even if a few consecutive bits are lost due to errors or corruption. 05/31/13Tempus Telcosys 120
  121. 121. ½ Convolutional coder 05/31/13Tempus Telcosys 121 R1 R2 R3 R5R4 + + C0 output C1 output 0110.. Input bits + EX-OR R=register
  122. 122. Convolutional coder cont’d  The output of the CC* is now 378 bits. (50+3+132+4)*2=378 The total number of bits now is 378+78=456 bits. *Note : The bit rate from the vocoder was 13Kbps for the 20 msec speech block, but after CC the bit rate increases to 22.8Kbps. 456 bits *20msecs=22.8Kbps * CC = Convolutional Coder. 05/31/13Tempus Telcosys 122
  123. 123. Control Channel Coding 05/31/13Tempus Telcosys 123 184 bits Control data 184 40 4 tail Fire coded parity bits ½ Convolutional Coder 456 bits output
  124. 124. Control Channel Coding  The control information is received in blocks of 184 bits.  These bits are first protected with a cyclic code called as Fire code, which is useful in correction and detection of burst errors.  40 Parity bits are added, along with 4 tail bits.  These 228 bits are given to the CC whose output is again 456 bits at a bitrate of 22.8Kbps.  The control channels include the RACH, PCH, AGCH etc. 05/31/13Tempus Telcosys 124
  125. 125. Data Channel Coding 05/31/13Tempus Telcosys 125 240 bits 4 tail Data bits ½ Convolutional Coder Output= 488 bits After Puncturing Output=456 bits
  126. 126. Data Channel Coding  The data bits are received in blocks of 240 bits. These are directly convolution coded after adding 4 tail bits.  The output of the CC is now 488 bits, which actually increases the bitrate to 24.4 Kbps.  To keep the bitrate constant on the air interface we need to puncture the output of the CC. Hence, we have a final bitrate of 22.8 Kbps again . 05/31/13Tempus Telcosys 126
  127. 127. Channel Coding cont’d  The above explanation was given keeping in view a full rate Traffic, Control, or Data channel.  For Half rate or Lesser rates the same principle of channel coding holds good, with slight differences in the encoding process. 05/31/13Tempus Telcosys 127
  128. 128. Interleaving  Having encoded the logical channel information, the next step is to build its bit stream into bursts that can be transmitted within the TDMA frame structure. This is the stage where the interleaving process is carried out.  Interleaving spreads the content of one information block across several TDMA timeslots or bursts. 05/31/13Tempus Telcosys 128
  129. 129. Interleaving cont’d  The following interleaving depths are used :  Speech – 8 blocks  Control – 4 blocks  Data – 22 blocks  The interleaving process for a speech block is shown wherein which a 456 bit speech block is divided into 8 blocks of 57 bits each and each of these odd and even 57 bit blocks are interleaved diagonally on to alternate bursts on the TDMA frame. 05/31/13Tempus Telcosys 129
  130. 130. Speech Interleaving 05/31/13Tempus Telcosys 130 8* 57 bits each = 456 bits Of Speech block N 57 Even Of N-1 57 Even Of N Speech block N-1 57 odd Of N-1 57 odd Of N The speech is spread over 8 such normal bursts Each normal burst consists of two blocks of 57 bit speech from different 20msec blocks (say N, N-1) along with 26 bit training sequence T and 2 flag F plus 6 start stop bits . T+FT+FT+F 456 bit speech data
  131. 131. Control Data Interleaving 05/31/13Tempus Telcosys 131 114 114 114 114 456 bits control data The control data is spread over 4 blocks using rectangular interleaving instead of diagonal interleaving as in speech the receiver will have to wait for at least 2 multiframes before being able to decode the control message TDMA Burst blocks
  132. 132. Data Interleaving 05/31/13Tempus Telcosys 132 114 114 114 114 Burst 1 Burst 22Burst 2 Burst 3 Burst 4 Burst 19 First 6 bits First 6 bits Last 6 bits Last 6 bits 456 bit data block
  133. 133. Data Interleaving cont’d  Here the data block of 456 bits is divided into 4 blocks of 114 bits each.  The first 6 bits from each of the 114 bit blocks is inserted in to each frame, the second 6 bits from each of the 114 bits into the next frame and so on spreading each 114 block over 19 TDMA bursts while the entire 456 bits is spread over 22 TDMA bursts.  Thus the data interleaving is said to have a depth of 22 bursts. 05/31/13Tempus Telcosys 133
  134. 134. Data Interleaving cont’d  The reason why data is spread over such along period of time is that if data burst is corrupted or lost, only a small part of it is lost which can be reproduced at the receiver.  This wide interleaving depth does produce a time delay during transmission but that is acceptable since it does not affect the data signal quality at the receiver, unlike speech where delay could result in bad quality of signal to the subscriber.  *Note – The interleaving used in data is diagonal interleaving. 05/31/13Tempus Telcosys 134
  135. 135. Before Deinterleaving 3 successive bursts corrupted After Deinterleaving The corrupted bursts are spread over a length equal to the interleaving depth so that the effect of the errors is minimized. 05/31/13Tempus Telcosys 135 Interleaving Advantage
  136. 136. Air Interface Bitrate  The information which is now coded and interleaved at 22.8 Kbps now has to be transmitted over the Air interface to the BTS.  The information burst is not sent directly , but is sent in ciphered form within a burst envelope. This ciphering is done using ciphering keys and algorithms known both by the mobile and the BSS. 05/31/13Tempus Telcosys 136
  137. 137. Air Interface Bitrate cont’d  The Kc is the ciphering key and A5 algorithm are applied to the information(speech or data) which increases the bitrate to a final rate of 33.8 Kbps from/to each mobile.  If we assume all 8 timeslots of the cell to be occupied then the bitrate of the Air interface comes to 33.8 * 8= 270.4 Kbps/channel. 05/31/13Tempus Telcosys 137
  138. 138. Air Interface Bitrate cont’d 05/31/13Tempus Telcosys 138 A5 Algorithm Kc Information Block 22.8 Kbps Sent on Air interface Ciphered information burst 33.8 Kbps
  139. 139. Air Interface Bitrate cont’d 05/31/13Tempus Telcosys 139 1 2 3 4 5 6 7 8 Mobile Tx’s at 33.8 Kbps Cell rx’s 8*33.8 KBps = 270.4 Kbps Per TDMA frame Cell coverage area TDMA Fn TDMA Fn+1
  140. 140. Decoding and Deinterleaving at the Receiver  At the receiver the reverse process of Deinterleaving and decoding have to take place respectively, so as to recover the information from the signal.  After Deinterleaving the signal will be decoded which is the reverse process of the Convolutional coding, using Viterbi decoders.  The decoder can recover lost or corrupted data up to 4 successive bits, because the memory depth of the CC is 4(for k=5). 05/31/13Tempus Telcosys 140
  141. 141. Channelization  Frequency band has several application segments  Certain blocks of the Band are reserved for certain applications by regulating authorities  Technologies have decided their frequency bands  E.g. AMPS/DAMPS: 824-894 MHz 05/31/13Tempus Telcosys 141
  142. 142. Channelization methods Channelization can be done primarily by three methods:  FDMA (Frequency Division Multiple Access)  TDMA (Time Division Multiple Access)  CDMA (Code Division Multiple Access) 05/31/13Tempus Telcosys 142
  143. 143. FDMA  E.g. AMPS band is divided into 30 KHz channels (1666 Freq. channels)  Television Channels (Star, Zee, Sony,..) 05/31/13Tempus Telcosys 143 Frequency Time Power
  144. 144. TDMA  E.g. AMPS has 3 timeslots on each 30 KHz channel 05/31/13Tempus Telcosys 144 Frequency Time Power
  145. 145. CDMA  Frequency channel is divided into code channels  E.g. in IS-95 CDMA, 1.228 MHz channel is divided into 64 Code Channels  Each user has a particular code  Codes are orthogonal to each other, do not interfere with each other 05/31/13Tempus Telcosys 145
  146. 146. Duplex Access Methods  Frequency Division Duplex (FDD)  Transmit on one frequency and receive on another frequency 05/31/13Tempus Telcosys 146 F1 F2 Frequency Amplitude Time Tx Rx
  147. 147. Time Division Duplex  Time division duplex  Tx and Rx is on the same frequency but on different times 05/31/13Tempus Telcosys 147 F1 Frequency Amplitude Time Tx Rx
  148. 148. GSM Air Interface  Separate Bands for Uplink and Downlink  Downlink: 935-960Mhz (EGSM: 925-960MHz)  Uplink: 890-915 MHz (EGSM: 880-915 MHz) 05/31/13Tempus Telcosys 148 • TDMA and TDMA Multiplex – 124 Frequency Channels (ARFCN) for GSM900 – 1 to 124 fro current band – 975 to 1023 for E-GSM – 200kHz Channels – 8 Mobiles share ARFCN by TDMA
  149. 149. GSM Air interface (1800)  1800: Downlink: 1805-1880 MHz  1800: Uplink: 1710-1785 MHx  374 ARFCNs  Separation of 95 MHz  ARFCNs are numbered from 512 to 885 inclusive 05/31/13Tempus Telcosys 149
  150. 150. The GSM Burst 05/31/13Tempus Telcosys 150 3 357 261 571 8.25 Tail Bits Data Control Bit Midamble Control Bit Data Tail Bits Guard Period
  151. 151. Speech Coder  RPE/LTP coder (Regular Pulse excitation/Long term Prediction)  Converts 64 kbps speech to 13 kbps  At the end we get 13kbps speech i.e. 260 bits in 20 ms 05/31/13Tempus Telcosys 151 20 ms blocks Speech Coder Bits Ordered 50 very important bits 132 important bits 78 other bits
  152. 152. Error Correction 05/31/13Tempus Telcosys 152 Type 1a 50 3(CRC)Type 1b 132 Type II 78 Reordering 25 66366 25 4 Type II 78 Type 1a Type 1b Type 1b Type 1a Tail Half rate convolutional code 378 Type II 78 456 bits from 20 ms of speech
  153. 153. Diagonal Interleaving  Traffic channel (TCH) bursts carry two 57 bit blocks (114)  Each 120 ms of speech = 456*6 = 2736 bits 2736/114 = 24 bursts i.3. 24 frames Multiframe has 26 frames in 120ms. There are 2 spare frames .. 1 SACCH, 1 Idle 05/31/13Tempus Telcosys 153 456 bits from 20ms of speech 456 bits from 20ms of speech 57 57575757575757 57 57575757575757 57 57 57 5757 5757 5757 5757 5757 5757 57
  154. 154. Convolutional Coding and Interleaving  Bits to be Tx ed: HELLO  Convolutionally encoded: HHEELLLLOO  Interleaved: EE HH LL LL OO  Bits Rx ed: EE HH LL LL OO  De-Interleaved: HHEELLLLOO  Viterbi Decoded: HELLO 05/31/13Tempus Telcosys 154
  155. 155. Speech Coding Process 05/31/13Tempus Telcosys 155 20 ms Speech Coder 260 bits 13 kbps 50 1a 132 1b 78 II Channel Coder 456 bits 22.8 kbps Transceiver (BTS) Transcoder Handler 260 bits 456 bits 16 kbps TRAU frame 260 + 60 = 320 bits Abis 13 kbps
  156. 156. TRAU frame  260 bits info + 60 TRAU bits = 320 bits/20ms = TRAU frame  60 bits contain frame Information data which indicates speech, data, O&M, full rate/half rate  60 bits = 35 synchronization + 21 control + 4 timing 05/31/13Tempus Telcosys 156
  157. 157. Midamble or Training Bits  8 midamble patterns (Colour codes) of 26 bits (BSIC)  RACH and SCH have longer 41 and 64 bit Midambles  Equalizer estimates channel impulse response from midamble  Mathematically construct inverse filter  Uses inverse to decode bits 05/31/13Tempus Telcosys 157 3 357 261 571 8.25 Tail Bits Data Control Bit Midamble Control Bit Data Tail Bits Guard Period
  158. 158. Downlink and Uplink  Uplink lags downlink by 3 timeslots  Uplink and downlink use same timeslot number  Uplink and downlink use same channel number (ARFCN)  Uplink and downlink use different bands (45 MHz apart for GSM 900) 05/31/13Tempus Telcosys 158
  159. 159. Measurements made by MS and BTS 05/31/13Tempus Telcosys 159  RxQual 0 < 0.2% 1 0.2 – 0.4 % 3 0.4 – 0.8 % 4 0.8 – 0.16 % 5 1.6 – 3.2 % 6 3.2 – 6.4 % 7 6.4 – 12.8 % Uplink RXLEV (-48 to -110 dbm) Uplink RXQUAL (0-7) Uplink RXLEV (-48 to -110 dbm) Uplink RXQUAL (0-7)
  160. 160. Mobile Power Control 05/31/13Tempus Telcosys 160  Mobile is commanded to change its Transmit Power  Change in Power is proportionate to the Path Loss  Change in Power is done in steps of 2 dbs Path Loss Power Command
  161. 161. Timing Advance  TDMA approach requires signals to arrive at BTS at the correct time  A mobile at 30 km will be late by 100micro seconds  Timing advance is in the range of 0-62  One unit is 550m  So maximum cell size is 63*0.55 = ~35 kms 05/31/13Tempus Telcosys 161
  162. 162. Concepts of Channels in GSM  A company vehicle is used for several purposes in a day  Similarly in GSM, the timeslots are used for different purposes at different times 05/31/13Tempus Telcosys 162
  163. 163. Frames and Multiframes 05/31/13Tempus Telcosys 163 0 654321 7 3 Data 1Midamble1 Data 3 8.25 bits 156.25 bits 576.92 micro sec 4.615 ms Time Slot Frame 0 50 0 25 Control Channel Multiframe Traffic Channel Multiframe
  164. 164. GSM Operations  Location Update  Mobile Originated Call  Mobile Terminated Call  Handover  Security Procedures  Cell Barring  DTX  Cell Broadcast  Short Message Service  Emergency calls  Supplementary Services  Roaming 05/31/13Tempus Telcosys 164
  165. 165. Mobile Turn On  Mobile Searches for Broadcast Channels (BCH)  Synchronizes Frequency and Timing  Decodes BCH sub-channels (BCCH)  Checks if Network Allowed by SIM  Location Update  Authentication 05/31/13Tempus Telcosys 165
  166. 166. Location Area 05/31/13Tempus Telcosys 166 Location Area 1Location Area 1 Location Area 2 Location Area 2 BTS BTS BTS BTS BTS BTS BTS BTS BSC BSC BSC MSC
  167. 167. Location Area Identity  Location area is the area covered by one or more BTSs where a mobile can move freely without updating the system  One Location area can be covered by one or more BSCs, but ony one MSC. 05/31/13Tempus Telcosys 167 MCC LACMNC
  168. 168. Importance of Location Area  Reduce Paging load  Resource Planning Smaller Location Areas – Location update increases Larger Location Areas – Paging load increases 05/31/13Tempus Telcosys 168
  169. 169. What is Location Update?  MSC should know the location of the Mobile for paging  Mobile is continuously changing location area  Mobile when changes Location Area informs the MSC about its new LA  Process of informing MSC about new Location area is Location Update 05/31/13Tempus Telcosys 169
  170. 170. Types of Location Updates 1. Normal Location Update 2. IMSI Attach 3. Periodic Location Update 05/31/13Tempus Telcosys 170 Hi, I am in Location area xxx
  171. 171. IMSI Attach  Mobile turns off and sends an IMSI Detach to MSC  Mobile turns on again and compares LAI  If same, sends an IMSI attach to MSC 05/31/13Tempus Telcosys 171 Is the received LAI same as before If same, Sends IMSI attach
  172. 172. Normal Location Update  Mobile Turns on Power  Reads the new LAI  If different, does a Location Update 05/31/13Tempus Telcosys 172 Is the received LAI same as before If different, does Location Update
  173. 173. Periodic Location Update  The periodic location Update time is set from OMC/MSC  After the periodic location update timer expires, the mobile has to do a location update 05/31/13Tempus Telcosys 173
  174. 174. What happens at Location Update?  Mobile changes location area  Reads the new Location Area from BCCH  Sends a RACH (request for channel)  Gets a SDCCH after AGCH  Sends its IMSI and new and old LAI in a Location Update request to MSC on SDCCH 05/31/13Tempus Telcosys 174
  175. 175. What happens at location update cont.. ….. . .  MSC starts Authentication  If successful, Updates the new Location area for the Mobile in the VLR  Sends a confirmation to the Mobile  Mobile leaves SDCCH, and comes to idle mode 05/31/13Tempus Telcosys 175
  176. 176. Mobile Originated Call 05/31/13Tempus Telcosys 176 Channel Request Immediate Assign Service Request Call Proceeding Set Up Ciphering Authentication Alerting Assignment Connection
  177. 177. Mobile Terminated Call 05/31/13Tempus Telcosys 177 Paging Channel Request Immediate Assign Set Up Ciphering Authentication Paging Response Assignment Call Confirmed Alerting Connection
  178. 178. Security Features  Authentication  Process to verify Authenticity of SIM  Mobile is asked to perform an operation using identity unique to SIM 05/31/13Tempus Telcosys 178 • Ciphering –Process of coding speech for secrecy –The speech bits are EXORed with bit stream unique to MS
  179. 179. Security Features (TMSI Reallocation) 05/31/13Tempus Telcosys 179 GSM Infrastructure Mobile Location Update TMSI Allocation Call Setup TMSI Reallocation TMSI- Temporary Mobile Subscriber Identity
  180. 180. Security Features (Identity Check) 05/31/13Tempus Telcosys 180 EIR Sends IMEI Identity Check White listed /Grey Listed/ Black Listed mobiles
  181. 181. Handover 05/31/13Tempus Telcosys 181 Cell 1 Cell 2 Handover is a GSM feature by which the control/communication of a Mobile is transferred from one cell to another if certain criteria’s are met. It is a network initiated process.
  182. 182. Criteria for Handover  Receive Quality (RXQUAL) on uplink and downlink  Receive Signal Strength (RXLEV) on uplink and downlink  Distance (Timing Advance)  Interference Level  Power Budget 05/31/13Tempus Telcosys 182
  183. 183. Handover Decision  BSC process the measurements reported by Mobile and the BTS 05/31/13Tempus Telcosys 183 BTS BTS BTS BTS BTS BTS Mobile has measurements of six neighbors
  184. 184. Handover Decision (cont..)  BSS performs averaging function on these measurements every SACCH frame (480ms)  Handover Decision algorithm is activated after a set number of SACCH frame periods by comparison against thresholds 05/31/13Tempus Telcosys 184
  185. 185. Types of Handovers  INTRA-CELL HANDOVERS  INTER-CELL HANDOVERS  INTRA-BSC HANDOVERS  INTER-BSC HANDOVERS  INTER-MSC HANDOVERS 05/31/13Tempus Telcosys 185
  186. 186. INTRA-CELL HANDOVER 05/31/13Tempus Telcosys 186 C0 C1 Handover between timeslots of same frequency Handover between different frequencies of the same cell (to reduce interference) MSC is not aware about this
  187. 187. Inter-cell Handover 05/31/13Tempus Telcosys 187 Handover between cells of the same BTS BTS Cell 1 Cell 2
  188. 188. Inter-cell Handover (cont..)  MSC is told about HO  BTS -> BSC -> MSC  Why MSC is informed?  In case of change of LA, MSC may need LAC for paging. As MS is busy, a link already exists. So, MSC can send a tone in case of call waiting, and does not need to page again.  This is needed also for billing and call tracing 05/31/13Tempus Telcosys 188
  189. 189. INTRA-BSC Handover 05/31/13Tempus Telcosys 189 MSC BSC BTS BTS This HO takes place if the cell to which handover is to be done belongs to the same BSC
  190. 190. Inter BSC Handover 05/31/13Tempus Telcosys 190 MSC BSC BTS BTSBSC The MSC is completely involved in this Handover
  191. 191. Inter MSC Handover 05/31/13Tempus Telcosys 191 BSC BSC MSC MSC BTS BTS GMSC/ PSTN/ Backbone In this case the handover takes place through the interconnecting element which can be GMSC or PSTN or private Backbone between the MSCs
  192. 192. Cell Barring 05/31/13Tempus Telcosys 192 BTS Cell Barring is a GSM feature by which certain mobiles could be barred access to certain cells Cell barring is activated/deactivated at BTS level Cell barring is done for mobile categories and priorities
  193. 193. Cell Barring  Every mobile has an access class  The access class is stored in the SIM  Classes 0-9 are termed normal calsses  Classes 11-15 are emergency classes 05/31/13Tempus Telcosys 193 • Every cell has a set parameter which defines which access classes are barred for the particular cell. This parameter is broadcasted on the BCCH
  194. 194. What is DTX?  DTX (Discontinous Transmission)  Each direction of Transmission is only 50%  Transmitter is switched ON for useful information frames 05/31/13Tempus Telcosys 194 Need for DTX •To increase battery life •To reduce the average interference level DTX is done by DTX handlers which
  195. 195. VAD (Voice Activity Detector)  Senses for speech in 20ms blocks  Removes stationary noise  VAD is an energy detector  Compares Energy of filtered speech threshold  It determines which 20ms blocks contain speech and it only forwards those frames 05/31/13Tempus Telcosys 195
  196. 196. Evaluation of Background Noise  Background noise is always present with speech  DTX cuts off this noise with speech  Gives an uncomfortable feeling to the listener  VAD takes care of this by inserting comfort noise at the receiving end when speech discontinues. 05/31/13Tempus Telcosys 196
  197. 197. Emergency Calls  GSM specs define 112 as an emergency number  ‘112’ is accessible with or without SIM  Without SIM it is sent on the best channel  Mobile on sensing ‘112’ sets the establishment cause to emergency call in the RACH  Routing of this call be done to a desired location defined in the switch 05/31/13Tempus Telcosys 197
  198. 198. Cell (Re)selection  Cell reselection is done using C1 path loss criterion.  The purpose is to ensure that the MS is camped on to the cell with the best transmission quality.  The MS will camp on to the cell with the highest C1 value if C1 > 0. 05/31/13Tempus Telcosys 198
  199. 199. The following parameters are used to calculate the C1 criterion  The received signal at the MS side.  Rxlev_access_min - broadcast on the BCCH - The minimum received level at the MS required for access to the network.  Ms_txpwr_max_cch - the maximum power that an MS may use when initially accessing the network.  The maximum power of the MS 05/31/13Tempus Telcosys 199
  200. 200. C1 = A - Max(B,0)  A = Received level Average - Rxlev_access_min.  B = MS_txpwr_max_cch - maximum output power of the MS 05/31/13Tempus Telcosys 200
  201. 201. Cell Reselect Hysteresis  Cell reselection on the border of two location areas result in a location update. When an MS moves on the border of two location areas lots of location updates take place. To avoid these location updates, the reselect hysteresis is introduced.  A location update is performed only if:  The C1 value of the new location area is higher than the C1 value in the current location area and  The received signal strengths have at least a difference of the reselect hysteresis. 05/31/13Tempus Telcosys 201
  202. 202. Cellular concept 05/31/13Tempus Telcosys 202
  203. 203. Why to use the cellular concept ?  Solves the problem of Spectral congestion and user capacity by means of frequency reuse.  Offers high capacity in a limited spectrum allocation.  Offers system level approach, using low power transmitters instead of a single, high power transmitter (large cell) to cover larger area. 05/31/13Tempus Telcosys 203
  204. 204.  A portion of the total channels available is allocated to each base station.  Neighboring base stations are assigned different groups channels, in order to minimize interference. 05/31/13Tempus Telcosys 204
  205. 205. Cell shape 05/31/13Tempus Telcosys 205
  206. 206. 05/31/13Tempus Telcosys 206
  207. 207. 1-Omni-directional cell-site (Omni-directional antenna). 2-Rhombus-shaped sectors (Directive antenna). 3-Hexagonal shaped sectors (Directive antenna). 05/31/13Tempus Telcosys 207
  208. 208. Cell size Large cell : (up to 70km in diameter) It exists where : 1-Radio waves are unobstructed. 2-Transmission power can cover the area. 3-low subscriber density. Small cell : (up to 2km in diameter) It exists where : 1-Radio waves are obstructed. 2-Low transmission power to decrease interference. 3-High subscriber density. 05/31/13Tempus Telcosys 208
  209. 209. Types of cells 1-Macro-cells 2-Micro-cells. 3-Pico-cells. 4-Umbrella-cells. 05/31/13Tempus Telcosys 209
  210. 210. What is a cluster ?  A cluster is a group of cells.  No channels are reused within a cluster.  It is the unit of design. 05/31/13Tempus Telcosys 210
  211. 211. Cluster size  Definition : It is The number of cells per cluster N = i^2 + ij + j^2 Where : i = 0, 1, 2….& j = 0,1,2…. etc. N = 1 , 3 , 4 ,7, 9 , 12 ,…… 05/31/13Tempus Telcosys 211
  212. 212. Types of clusters 1-N=7 omni frequency plan (2-directional). 2-N=7 trapezoidal frequency plan (1-directional). 3-N=9 omni frequency plan. 4-Tricellular plans a) N=3 tricellular plan (3/9). b) N=4 tricellular plan (4/12). 05/31/13Tempus Telcosys 212
  213. 213. Channel assignment strategies  Considerations : 1) Max. capacity. 2) Min interference. 3) Perfect handover.  Types of assignment strategies : 1) Fixed :  Each cell has permanent predetermined set of voice channels.  New calls served by unused channels of this cell.  Borrowing strategy if all channels are occupied.  High probabiltity that call is Blocked if channels are occupied.( disadv.) 05/31/13Tempus Telcosys 213
  214. 214. 2) Dynamic :  Channels are not allocated to different cells permanently.  Each new call BTS requests new channel from MSC.  MSC allocate a channel, by using an algorithm that takes into account: 1- Frequency is not already in use. 2- Min. reuse distance to avoid co-channel interference. 05/31/13Tempus Telcosys 214
  215. 215.  Adv. of dynamic assignment strategy : 1) Increase channel utilization ( Increase trunking efficiency ). 2) Decrease probability of a blocked call. 05/31/13Tempus Telcosys 215
  216. 216. Frequency reuse Concept 05/31/13Tempus Telcosys 216
  217. 217. 05/31/13Tempus Telcosys 217
  218. 218. Reuse cluster 05/31/13Tempus Telcosys 218
  219. 219. Co-channel Reuse ratio (Q) :  R : cell radius.  D : reuse distance.  Q = D/R. = sqrt(3N). Where : N : cluster size 05/31/13Tempus Telcosys 219
  220. 220. Handover 05/31/13Tempus Telcosys 220
  221. 221. Definition : procedure that allows MS to change the cell or time-slot to keep as good link as possible during all the call. 05/31/13Tempus Telcosys 221
  222. 222. Types of handover  IntraCell : bet. 2 channels of same cell.  InterCell : bet. 2 channels of 2 different cell & same BTS.  InterBTS (intra BSC) : 2 cells of different BTS Same BSC.  InterBSC : bet. 2 cells of different BSC’s & same MSC. 05/31/13Tempus Telcosys 222
  223. 223. Measurements before handover 1- Measurements from MS to BSC : a) Strength of BTS signal. b) Quality of BTS signal. c) Signal strength of 6 neighbor BTS’s. 2-Measurements from BTS to BSC : a) Strength of MS signal. b) Quality of MS signal. c) Distance between serving BTS & MS. 05/31/13Tempus Telcosys 223
  224. 224. Different causes of handover 05/31/13Tempus Telcosys 224 Better cell HOEmergency HO Level Quality PBGT Traffic causes InterferenceDistance Different causes of Handover
  225. 225. Basic handover algorithms a)“Min. acceptable performance” algorithm: MS power is increased when quality deceases till handover is the only way. b) “Power budget “ algorithm: Prefer direct handover when quality deceases without increasing MS power first . 05/31/13Tempus Telcosys 225
  226. 226. Handover priority 1) UL quality cause (or interference). 2) DL quality cause (or interference). 3) UL level cause. 4) DL level cause. 5) Distance cause. 6) Better cell cause. 05/31/13Tempus Telcosys 226
  227. 227. Interference 05/31/13Tempus Telcosys 227
  228. 228. Sources of interference include: 1) Another mobile in the same cell. 2) A call in progress in the neighboring cell. 3) Other BTS’s operating in the same frequency band. 05/31/13Tempus Telcosys 228
  229. 229. Interference effects :  In voice channel causes crosstalk  In control channels it leads missed and blocked calls due to errors in the digital signaling. 05/31/13Tempus Telcosys 229
  230. 230. Main types of interference : 1) Co-channel interference. 2) Adjacent channel interference. 05/31/13Tempus Telcosys 230
  231. 231. 1) Co-channel interference  Source : Near cell using same frequency. It is a function of reuse distance(D/R).  General rule : io = No. of co-channel interfering cells. S = Signal power from a desired BS. Ii = interference power caused by the ith interfering co-channel cell BS. 05/31/13Tempus Telcosys 231
  232. 232.  Another form : C/I = 10 log {(1/n)(D/R)*m} Where : m = propagation constant (dep’s on nature of environment) n = number of co-channel interferers. Can be minimized by : Choosing minimum reuse distance = (2.5….3)(2R). 05/31/13Tempus Telcosys 232
  233. 233. 2) Adjacent channel interference  Source : A cell using a frequency adjacent to the one in another cell due to imperfect reciever’s filter. 05/31/13Tempus Telcosys 233
  234. 234. Can be minimized by : 1-careful filtering 2-careful channel assignments 3-Directional antenna.  General rule : ACI= -10 Log[(d1/d2)*m] – Adj ch isolation. Where : d1: distance between MS & proper BTs d2: dist. Bet MS & adj BTS causing interference. Adj ch isolation = Filter isolation = - 26db. 05/31/13Tempus Telcosys 234
  235. 235. Traffic engineering theory 05/31/13Tempus Telcosys 235
  236. 236. Why do we need to know traffic?  The amount of traffic during peak hours allows us to dimension our wireless system for a certain GOS.  GOS : probability of having a call blocked during busy hour (block rate). 05/31/13Tempus Telcosys 236
  237. 237. Traffic intensity (E)  Erlang : A unit of traffic intensity measure.  1 Erlang = 1 circuit in use for 1 hour.  T ( in Erlangs) = [No. of calls per hour*average call holding time(sec.)] / [3600] 05/31/13Tempus Telcosys 237
  238. 238. Typical traffic profile 05/31/13Tempus Telcosys 238
  239. 239. Traffic tables Erlang B Table Blocked calls are not held Erlang C Table Blocked calls are held in the queue indefinitely Poisson Table Blocked calls are held in the queue for a time = the mean holding time05/31/13Tempus Telcosys 239
  240. 240. Erlang – B table  P(N;T) = [ (T^N)*exp(-T) ] / N! N GOS 1% GOS 2% 2 0.153 0.223 4 0.869 1.093 10 4.46 5.084 20 12.0 13.182 40 29.0 30.99705/31/13Tempus Telcosys 240
  241. 241. Trunking  Sharing channel among several users.  Trunking efficiency (nT) : Measures the number of subscribers that each channel in every cell can accommodate. nT = (traffic in Erlangs / no. of channels)*100. 05/31/13Tempus Telcosys 241
  242. 242. Trunking efficiency in presence of one operator : N = 7 , 312 one direction voice channels No. of channels / cell = 312 / 7 = 44 ch./cell. From Erlang-B table @GOS 2%,this’s equivalent to 35 Erlangs nT = 35 / 44 = 79.55. Trunking efficiency in presence of two operators : N = 7 , 312 / 2 = 156 one direction voice channel for each operator. No. of channels / cell = 156 / 7 = 22 ch./cell. From Erlang-B table @GOS 2%,this’s equivalent to 15 Erlangs. nT = 15 / 22 = 68.18. 05/31/13Tempus Telcosys 242
  243. 243. System capacity 05/31/13Tempus Telcosys 243
  244. 244.  S : total duplex channels available for use = k*N Where: N : cluster size. k : No. of channels / cell.  C : total No. of duplex channels in system; C = M*k*N. Where : M : No. of times the cluster is repeated. 05/31/13Tempus Telcosys 244
  245. 245. Improving system capacity  Cell splitting.  Sectoring. 05/31/13Tempus Telcosys 245
  246. 246. Cell splitting 05/31/13Tempus Telcosys 246
  247. 247. Sectoring  We use directional antennas instead of being omnidirectional 05/31/13Tempus Telcosys 247
  248. 248. What does sectoring mean?  We can now assign frequency sets to sectors and decrease the re-use distance to fulfill : 1) More freq reuse. 2) Higher system capacity. 3) Improve S/I ratio ( better signal quality ).  How S/I ratio is improved? -e.g. In 120 degree sectoring there’s only 2 interferers instead of 6 incase of omnidirectional N=7 cluster. 05/31/13Tempus Telcosys 248
  249. 249. 05/31/13Tempus Telcosys 249
  250. 250. 05/31/13Tempus Telcosys 250
  251. 251. Directional frequency reuse  Here we use 7/21 pattern for frequency allocation. 05/31/13Tempus Telcosys 251
  252. 252. Comparison between various types of clusters 05/31/13Tempus Telcosys 252
  253. 253. N = 7 omni frequency plan :  n = 6 , m = 4.  D / R = 4.583.  1) Co-channel interference ratio : C / I = 18.6 dB.  2) Adjacent channel interference : ACI = -26 dB @ d1= d2. 05/31/13Tempus Telcosys 253
  254. 254. N = 7 trapezoidal frequency plan  n = 2 , m = 4.  D / R = 6.245.  1) Co-channel interference ratio : C / I = 28.8.  2) Adjacent channel interference : disappears because the channels are assigned alternatively to the cells. 05/31/13Tempus Telcosys 254
  255. 255.  Trunking efficiency :  312 one direction voice channels N = 7  312 / 7 = 44.57 ~ 44 ch./cell.  From Erlang-B table @ GOS = 2% T = 35 E.  nT = 35 / 44 = 79.55 %. 05/31/13Tempus Telcosys 255
  256. 256. N = 9 omni frequency plan  n = 4 , m = 4.  D / R = sqrt ( 3 * 9 ) = 5.2.  1) Co-channel interference : C / I = 22.6 dB.  2) Adjacent channel interference : ACI = -38 dB @ d2 = 2 (d1). 05/31/13Tempus Telcosys 256
  257. 257.  Trunking efficiency :  312 one direction voice channels N = 9  312 / 9 = 34.67 ~ 34 ch./cell.  From Erlang-B table @ GOS = 2% T = 25.529 E.  nT = 25.529 / 34 = 75.085 %. Conclusion : nT 7 > nT 9 But C/I 7 > C/I 9 ACI 7 > ACI 9 05/31/13Tempus Telcosys 257
  258. 258. 4 / 12 cell pattern  n = 1 , m = 4.  D / R = sqrt (3* 4) = 3.732.  C / I = 22.87 dB.  Trunking efficiency :  No. of channels/cell = 312 / 12 = 26 ch./cell.  From Erlang-B table @ GOS = 2 %.  T = 18.4 E/cell.  nT = 18.4 / 26= 70.77%. 05/31/13Tempus Telcosys 258
  259. 259. 3 / 9 cell pattern  n = 1 , m = 4.  D / R = sqrt (3* 3) = 3.  C / I = 19.1 dB.  Trunking efficiency :  No. of channels/cell =312 / 9 = 34 ch./cell.  From Erlang-B table @ GOS = 2 %.  T = 25.5 E/cell.  nT = 25.5 / 24 = 75 %. 05/31/13Tempus Telcosys 259
  260. 260. 120 degree cell sectoring  n = 2 , m = 4.  D / R = sqrt(3 * 7) = 4.583.  Co-channel interference : C / I = 23.436 + 6dB(due to isolation) = 29.436 dB.  Trunking efficiency :  No. of channels/cell = 312 / 21 = 14.857.  From Erlang-B @ GOS=2% T= 8.2003.  nT = 8.2003 / 14.857 =56.216%. 05/31/13Tempus Telcosys 260
  261. 261.  References :  Motorola CP02  NOKIA SYSTRA 05/31/13Tempus Telcosys 261
  262. 262. If any Query Contact 9903867731 TempusTelcosys@gmail.com 05/31/13Tempus Telcosys 262

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