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Overview of GSM Cellular Network and Operations

Overview of GSM Cellular Network and Operations. Network and switching subsystem. NSS is the main component of the public mobile network GSM switching, mobility management, interconnection to other networks, system control Components

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Overview of GSM Cellular Network and Operations

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  1. Overview of GSM Cellular Network and Operations

  2. Network and switching subsystem • NSS is the main component of the public mobile network GSM • switching, mobility management, interconnection to other networks, system control • Components • Mobile Services Switching Center (MSC)controls all connections via a separated network to/from a mobile terminal within the domain of the MSC - several BSC can belong to a MSC • Databases (important: scalability, high capacity, low delay) • Home Location Register (HLR)central master database containing user data, permanent and semi-permanent data of all subscribers assigned to the HLR (one provider can have several HLRs) • Visitor Location Register (VLR)local database for a subset of user data, including data about all user currently in the domain of the VLR

  3. Operation subsystem • The OSS (Operation Subsystem) enables centralized operation, management, and maintenance of all GSM subsystems • Components • Authentication Center (AUC) • generates user specific authentication parameters on request of a VLR • authentication parameters used for authentication of mobile terminals and encryption of user data on the air interface within the GSM system • Equipment Identity Register (EIR) • registers GSM mobile stations and user rights • stolen or malfunctioning mobile stations can be locked and sometimes even localized • Operation and Maintenance Center (OMC) • different control capabilities for the radio subsystem and the network subsystem

  4. Mobile Handset TEMPORARY DATA PERMANENT DATA - Temporary Subscriber Identity Permanent Subscriber Identity - Current Location Key/Algorithm for Authentication. - Ciphering Data Provides access to the GSM n/w Consists of Mobile equipment (ME) Subscriber Identity Module (SIM)

  5. The GSM Radio Interface

  6. The GSM Network Architecture • Time division multiple access-TDMA • 124 radio carriers, inter carrier spacing 200khz. • 890 to 915mhz mobile to base - UPLINK • 935 to 960mhz base to mobile - DOWNLINK • 8 channels/carrier

  7. GSM uses paired radio channels UPLINK DOWNLINK 890MHz 915MHz 935MHz 960MHz 0 124 0 124

  8. FDMA, TDMA, CDMA Access Mechanism

  9. Frequency multiplex • Separation of the whole spectrum into smaller frequency bands • A channel gets a certain band of the spectrum for the whole time • Advantages: • no dynamic coordination necessary • works also for analog signals • Disadvantages: • waste of bandwidth if the traffic is distributed unevenly • inflexible • guard spaces k1 k2 k3 k4 k5 k6 c f t

  10. c f t Time multiplex • A channel gets the whole spectrum for a certain amount of time • Advantages: • only one carrier in themedium at any time • throughput high even for many users • Disadvantages: • precise synchronization necessary k1 k2 k3 k4 k5 k6

  11. Time and Frequency Multiplex • Combination of both methods • A channel gets a certain frequency band for a certain amount of time k1 k2 k3 k4 k5 k6 c f t

  12. Time and Frequency Multiplex • Example: GSM • Advantages: • Better protection against tapping • Protection against frequency selective interference • Higher data rates compared tocode multiplex • But: precise coordinationrequired k1 k2 k3 k4 k5 k6 c f t

  13. GSM combines FDM and TDM: bandwidth is subdivided into channels of 200khz, shared by up to eight stations, assigning slots for transmission on demand.

  14. GSM uses paired radio channels UPLINK DOWNLINK 890MHz 915MHz 935MHz 960MHz 0 124 0 124

  15. Code Multiplex k1 k2 k3 k4 k5 k6 • Each channel has a unique code • All channels use the same spectrum at the same time • Advantages: • Bandwidth efficient • No coordination and synchronization necessary • Good protection against interference and tapping • Disadvantages: • Lower user data rates • More complex signal regeneration • Implemented using spread spectrum technology c f t

  16. Various Access Method

  17. Cells

  18. Network capacity at required QoS with conventional frequency plan Out of Capacity!!! Subscriber growth Time I wish I could increase capacity withoutadding NEW BTS! What can I do? Capacity & Spectrum Utilization Solution The need: • Optimum spectrum usage • More capacity • High quality of service • Low cost

  19. Representation of Cells Ideal cells Fictitious cells

  20. Cell size and capacity • Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users • Capacity within cell limited by available bandwidth and operational requirements • Each network operator has to size cells to handle expected traffic demand

  21. Cell structure • Implements space division multiplex: base station covers a certain transmission area (cell) • Mobile stations communicate only via the base station • Advantages of cell structures: • higher capacity, higher number of users • less transmission power needed • more robust, decentralized • base station deals with interference, transmission area etc. locally • Problems: • fixed network needed for the base stations • handover (changing from one cell to another) necessary • interference with other cells • Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

  22. Capacity of a Cellular System • Frequency Re-Use Distance • The K factor or the cluster size • Cellular coverage or Signal to interference ratio • Sectoring

  23. The K factor and Frequency Re-Use Distance 7 6 2 K = i2 + ij + j2 K= 22 + 2*1 + 12 K = 4 + 2 + 1 K = 7 1 5 3 j R 7 2 6 i 1 D 5 3 4 D = 3K * R D = 4.58R Frequency re-use distance is based on the cluster size K The cluster size is specified in terms of the offset of the center of a cluster from the center of the adjacent cluster

  24. D R i The Frequency Re-Use for K = 4 K = i2 + ij + j2 K= 22 + 2*0 + 02 K = 4 + 0 + 0 K = 4 D = 3K * R D = 3.46R

  25. 1 2 7 7 7 7 7 6 6 6 6 6 2 2 2 2 2 1 1 1 1 1 5 5 5 5 5 3 3 3 3 3 4 4 4 4 4 The Cell Structure for K = 7

  26. 1 1 2 4 1 4 2 3 4 2 1 3 3 4 2 1 1 3 4 2 1 4 2 3 3 4 2 3 Cell Structure for K = 4

  27. 9 9 8 10 8 10 2 11 2 11 7 3 7 3 1 12 1 12 6 4 6 4 9 5 9 5 8 10 8 10 2 11 2 11 7 3 7 3 1 12 1 12 6 4 6 4 5 5 Cell Structure for K = 12

  28. Increasing cellular system capacity • Cell sectoring • Directional antennas subdivide cell into 3 or 6 sectors • Might also increase cell capacity by factor of 3 or 6

  29. Increasing cellular system capacity • Cell splitting • Decrease transmission power in base and mobile • Results in more and smaller cells • Reuse frequencies in non-contiguous cell groups • Example: ½ cell radius leads 4 fold capacity increase

  30. Tri-Sector antenna for a cell

  31. Cell Distribution in a Network Rural Highway Town Suburb

  32. Optimum use of frequency spectrum • Operator bandwidth of 7.2MHz (36 freq of 200 kHz) • TDMA 8 traffic channels per carrier • K factor = 12 • What are the number of traffic channels available within its area for these three cases • Without cell splitting • With 72 cells • With 246 cells

  33. Re-use of the frequency One Cell = 288 traffic channels 8 X 36 = 288 72 Cell = 1728 traffic channels 8 X (72/12 X 36) = 1728 246 Cell = 5904 traffic channels

  34. c f t Concept of TDMA Frames and Channels • GSM combines FDM and TDM: bandwidth is subdivided into channels of 200khz, shared by up to eight stations, assigning slots for transmission on demand.

  35. GSM uses paired radio channels UPLINK DOWNLINK 890MHz 915MHz 935MHz 960MHz 0 124 0 124

  36. R1 R2 R3 R4 R5 R6 R7 R8 T1 T2 T3 T4 T5 T6 T7 T8 R T R T GSM delays uplink TDMA frames The start of the uplink TDMA is delayed of three time slots TDMA frame (4.615 ms) Downlink TDMA F1MHz Uplink TDMA Frame F1 + 45MHz Fixed transmit Delay of three time-slots

  37. higher GSM frame structures 5 7 8 1 2 4 6 3 4.615 ms S user data tail tail user data S Training 1 3 1 57 bits 3 bits 57 bits 26 bits 546.5 µs 577 µs GSM - TDMA/FDMA 935-960 MHz 124 channels (200 kHz) downlink frequency 890-915 MHz 124 channels (200 kHz) uplink time GSM TDMA frame GSM time-slot (normal burst) guard space guard space

  38. LOGICAL CHANNELS TRAFFIC SIGNALLING FULL RATE Bm 22.8 Kb/S HALF RATE Lm 11.4 Kb/S BROADCAST COMMON CONTROL DEDICATED CONTROL FCCH SCH BCCH RACH AGCH PCH FCCH -- FREQUENCY CORRECTION CHANNEL SCH -- SYNCHRONISATION CHANNEL BCCH -- BROADCAST CONTROL CHANNEL PCH -- PAGING CHANNEL RACH -- RANDOM ACCESS CHANNEL AGCH -- ACCESS GRANTED CHANNEL SDCCH -- STAND ALONE DEDICATED CONTROL CHANNEL SACCH -- SLOW ASSOCIATED CONTROL CHANNEL FACCH -- FAST ASSOCIATED CONTROL CHANNEL SDCCH SACCH FACCH DOWN LINK ONLY BOTH UP & DOWNLINKS UPLINK ONLY

  39. Broadcast Channel - BCH • Broadcast control channel (BCCH) is a base to mobile channel which provides general information about the network, the cell in which the mobile is currently located and the adjacent cells • Frequency correction channel (FCCH) is a base to mobile channel which provides information for carrier synchronization • Synchronization channel (SCH) is a base to mobile channel which carries information for frame synchronization and identification of the base station transceiver

  40. Common Control Channel - CCH • Paging channel (PCH) is a base to mobile channel used to alert a mobile to a call originating from the network • Random access channel (RACH) is a mobile to base channel used to request for dedicated resources • Access grant channel (AGCH) is a base to mobile which is used to assign dedicated resources (SDCCH or TCH)

  41. Dedicated Control Channel - DCCH • Stand-alone dedicated control channel (SDCCH) is a bi-directional channel allocated to a specific mobile for exchange of location update information and call set up information

  42. Dedicated Control Channel - DCCH • Slow associated control channel (SACCH) is a bi-directional channel used for exchanging control information between base and a mobile during the progress of a call set up procedure. The SACCH is associated with a particular traffic channel or stand alone dedicated control channel • Fast associated control channel (FACCH) is a bi-directional channel which is used for exchange of time critical information between mobile and base station during the progress of a call. The FACCH transmits control information by stealing capacity from the associated TCH

  43. 57 57 26 3 1 1 3 8.25 NORMAL BURST - NB FREQUENCY CORRECTION BURST - FB 142 3 3 8.25 39 39 64 3 3 8.25 SYNCHRONISATION BURST - SB ACCESS BURST - AB 41 6 3 36 68.25 FIXED BITS SYNCHRONISATION BITS TAIL BIT GUARD PERIOD ENCRYPTION BIT TRAINING BITS FLAG BITS MIXED BITS DEFINITION OF TIME SLOT - 156.25 BITS 15/26ms = 0.577ms

  44. HIERARCHY OF FRAMES 0 1 2 3 4 48 49 50 0 1 2 3 4 48 49 50 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 HYPER FRAME = 2048 SUPERFRAMES = 2 715 648 TDMA FRAMES ( 3 H 28 MIN 53 S 760 MS ) 0 1 2 3 4 5 6 2043 2044 2045 2046 2047 1 SUPER FRAME = 1326 TDMA FRAMES ( 6.12 S ) LEFT (OR) RIGHT TRAFFIC CHANNELS 1 SUPER FRAME = 51 MULTI FRAMES SIGNALLING CHANNELS 1 SUPER FRAME = 26 MULTI FRAMES 0 1 2 24 25 1 MULTIFRAME = 26 TDMA FRAMES ( 120 ms ) 0 1 2 3 24 25 1 MULTI FRAME = 51 TDMA FRAMES (235 .4 ms ) (4.615ms) TDMA FRAME NO. 0 1 1 TIME SLOT = 156.25 BITS ( 0.577 ms) (4.615 ms) 1 2 3 4 155 156 0 1 1 bit =36.9 micro sec

  45. 0 1 2 12 24 25 0 1 2 3 4 5 6 7 57 57 26 3 1 1 3 8.25 GSM Frame Full rate channel is idle in 25 SACCH is transmitted in frame 12 0 to 11 and 13 to 24 Are used for traffic data Frame duration = 120ms Frame duration = 60/13ms Frame duration = 15/26ms

  46. 114 bits are available for data transmission. • The training sequence of 26 bits in the middle of the burst is used by the receiver to synchronize and compensate for time dispersion produced by multipath propagation. • 1 stealing bit for each information block (used for FACCH)

  47. LOGICAL CHANNELS TRAFFIC SIGNALLING FULL RATE Bm 22.8 Kb/S HALF RATE Lm 11.4 Kb/S BROADCAST COMMON CONTROL DEDICATED CONTROL FCCH SCH BCCH RACH AGCH PCH FCCH -- FREQUENCY CORRECTION CHANNEL SCH -- SYNCHRONISATION CHANNEL BCCH -- BROADCAST CONTROL CHANNEL PCH -- PAGING CHANNEL RACH -- RANDOM ACCESS CHANNEL AGCH -- ACCESS GRANTED CHANNEL SDCCH -- STAND ALONE DEDICATED CONTROL CHANNEL SACCH -- SLOW ASSOCIATED CONTROL CHANNEL FACCH -- FAST ASSOCIATED CONTROL CHANNEL SDCCH SACCH FACCH DOWN LINK ONLY BOTH UP & DOWNLINKS UPLINK ONLY

  48. Mobile looks for BCCH after switching on Location update from the mobile RACH send channel request AGCH receive SDCCH SDCCH request for location updating SDCCH authenticate SDCCH authenticate response SDCCH switch to cipher mode SDCCH cipher mode acknowledge SDCCH allocate TMSI SDCCH acknowledge new TMSI SDCCH switch idle update mode

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