Gsm review note by zemaryali

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Gsm review note by zemaryali

  1. 1. Section 0 Review of GSM Principles
  2. 2. GSM Architecture Overview Air Interface Abis Interface A Interface (Um) OMC MS VLR BSS HLR MS TRX BTS MSC AuC BSC MS EIR NSS PSTN
  3. 3. GSM Mobile Terminal (MT) Reference Points R S Um A Interface Interface Base Station GSM Core TA Subsystem Network SIM ME TE MS Mobile Terminal (MT) TE - Terminal Equipment TA - Terminal Adaptor MS - Mobile Station ME - Mobile Equipment SIM - Subscriber Identity Module
  4. 4. The Mobile Station (MS) • The mobile station consists of: • mobile equipment (ME) • subscriber identity module (SIM) • The SIM stores permanent and temporary data about the mobile, the subscriber and the network, including: • The International Mobile Subscribers Identity (IMSI) • MS ISDN number of subscriber • Authentication key (Ki) and algorithms for authentication check • The mobile equipment has a unique International Mobile Equipment Identity (IMEI), which is used by the EIR
  5. 5. The Base Station Sub-System (BSS) • The BSS comprises: • Base Station Controller (BSC) • One or more Base Transceiver Stations (BTSs) BSS • The purpose of the BTS is to: • provide radio access to the mobile stations • manage the radio access aspects of the system • BTS contains: BTS • Radio Transmitter/Receiver (TRX) • Signal processing and control equipment • Antennas and feeder cables • The BSC: BSC • allocates a channel for the duration of a call BTS • maintains the call: – monitors quality – controls the power transmitted by the BTS or MS BTS – generates a handover to another cell when required • Siting of the BTS is crucial to the provision of BTS acceptable radio coverage
  6. 6. 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
  7. 7. Network Switching System (NSS) PSTN/ISDN • Key elements of the NSS: VLR MSC GMSC • Mobile Switching Centre (MSC) with: • Visitor Location Register (VLR) • Home Location Register (HLR) with: SS7 Network • Authentication Centre (AuC) • Equipment Identity Register (EIR) AuC • Gateway MSC (GMSC) EIR HLR • These elements are interconnected by means of an SS7 network
  8. 8. Mobile Switching Centre (MSC) Functions of the MSC: • Switching calls, controlling calls and logging calls • Interface with PSTN, ISDN, PSPDN • Mobility management over the radio network and other networks • Radio Resource management - handovers between BSCs • Billing Information VLR MSC
  9. 9. Visitor Location Register (VLR) • Each MSC has a VLR • VLR stores data temporarily for mobiles served by the MSC • Information stored includes: • IMSI • MSISDN VLR • MSRN • TMSI MSC • LAI • Supplementary service parameters
  10. 10. Home Location Register (HLR) • Stores details of all subscribers in the network , such as: • Subscription information • Location information: mobile station roaming number, VLR, MSC • International Mobile Subscriber Identity (IMSI) • MS ISDN number • Tele-service and bearer service subscription information AuC • Service restrictions • Supplementary services HLR • Together with the AuC, the HLR checks the validity and service profile of subscribers
  11. 11. HLR Implementation • One HLR in a network • May be split regionally • Stores details of several thousand subscribers • Stand alone computer - no switching capabilities • May be located anywhere on the SS7 network • Combined with AuC AuC HLR
  12. 12. Gateway Mobile Switching Centre (GMSC) • A Gateway Mobile Switching Centre (GMSC) is a device which routes traffic entering a mobile network to the correct destination • The GMSC accesses the network’s HLR to find the location of the required mobile subscriber • A particular MSC can be assigned to act as a GMSC • The operator may decide to assign more than one GMSC GMSC
  13. 13. Equipment Identity Register (EIR) • EIR is a database that stores a unique International Mobile Equipment Identity (IMEI) number for each EIR item of mobile equipment • The EIR controls access to the network by returning the status of a mobile in response to an IMEI query • Possible status levels are: • White-listed The terminal is allowed to connect to the network. • Grey-listed The terminal is under observation by the network for possible problems. • Black-listed The terminal has either been reported stolen, or is not a type approved for a GSM network. The terminal is not allowed to connect to the network.
  14. 14. GSM Network Interfaces VLR D HLR MS Um B C H MS TRX AuC BTS Abis BSC A MSC BSS F MS EIR NSS
  15. 15. P-GSM Spectrum (Primary GSM) 890 915 935 960 MHz Uplink Downlink Duplex spacing = 45 MHz Fu(n) Range of ARFCN: 1 - 124 1 2 3 4 n Guard Band 100 kHz wide Guard Band 100 kHz wide Channel Numbers (n) (ARFCN) 200 kHz spacing
  16. 16. E-GSM Spectrum (Extended GSM) 880 915 925 960 MHz Uplink Downlink Duplex spacing = 45 MHz Range of ARFCN: Fu(n) 1 – 124 975 - 1023 1 2 3 4 n Guard Band 100 kHz wide Guard Band 100 kHz wide Channel Numbers (n) (ARFCN) 200 kHz spacing
  17. 17. DCS - 1800 Spectrum 1710 1785 1805 1880 MHz Uplink Downlink Duplex spacing = 95 MHz Fu(n) Range of ARFCN: 512 - 885 1 2 3 4 n Guard Band 100 kHz wide Guard Band 100 kHz wide Channel Numbers (n) (ARFCN) 200 kHz spacing
  18. 18. 1800 MHz Utilization in UK The present distribution of frequencies among UK operator is: 1710 1721.5 1751.5 1781.5 1785 MHz Uplink DECT Vodafone/ One 2 One Orange Cellnet Downlink 1805 1816.5 1846.5 One 2 One 1876.5 1880 MHz DECT: Digital Enhanced Cordless Telecommunications
  19. 19. PCS - 1900 Spectrum 1850 1910 1930 1990 MHz Uplink Downlink Duplex spacing = 80 MHz Fu(n) Range of ARFCN: 512 - 810 1 2 3 4 n Guard Band 100 kHz wide Guard Band 100 kHz wide Channel Numbers (n) (ARFCN) 200 kHz spacing
  20. 20. Multiple Access Techniques • Purpose: to allow several users to share the resources of the air interface in one cell • Methods: • FDMA - Frequency Division Multiple Access • TDMA - Time Division Multiple Access • CDMA - Code Division Multiple Access
  21. 21. Frequency Division Multiple Access (FDMA) • Divide available frequency spectrum into channels each of the same bandwidth • Channel separation achieved by filters: • Good selectivity Frequency • Guard bands between channels User 1 • Signalling channel required to allocate a traffic User 2 channel to a user User 3 • Only one user per frequency channel at any time User 4 User 5 • Used in analog systems, such as AMPS, TACS Time • Limitations on: • frequency re-use • number of subscribers per area channel bandwidth
  22. 22. Time Division Multiple Access (TDMA) • Access to available spectrum is limited to timeslots • User is allocated the spectrum for the duration of one timeslot • Timeslots are repeated in frames Frequency Signalling Signalling User 6 User 3 User 7 User 1 User 2 User 4 User 5 User 6 User 7 User 1 User 2 User 3 User 4 User 5 Time Frame Timeslot
  23. 23. GSM Channels GSM defines two fundamental channel types: • Physical Channels: • the individual channels carried by a radio frequency carrier • Each carrier comprises 8 time-separated channels • Logical Channels: • time-dependant virtual channels carried on a single physical channel • one physical channel may support one or multiple logical channels
  24. 24. GSM Physical Channels • GSM employs both FDMA and TDMA at the Air Interface • Each BTS may comprise a number of TRXs, with the carrier of each TRX operating on a different frequency (FDM) • Each GSM carrier supports 8 time-separated physical channels (TDMA) • Each physical channel is allocated to a specific timeslot on the carrier • A group of 8 timeslots on a carrier is known as a TDMA frame 1 frame period 4.615 ms 0 1 2 3 4 5 6 7 timeslot = 0.577 ms
  25. 25. GSM Logical Channels • Two types of logical channel are defined; traffic and control channels • Each is further sub-divided as shown: Traffic Traffic Control Control TCH TCH BCH BCH CCCH CCCH DCCH DCCH FCCH FCCH PCH PCH SDCCH SDCCH TCH/F TCH/F SCH SCH RACH RACH SACCH SACCH TCH/H TCH/H BCCH BCCH AGCH AGCH FACCH FACCH CBCH CBCH
  26. 26. Traffic Channels (TCH) • One physical channel (1 timeslot) can support: • 1 TCH/F or 2 TCH/H • TCH/F: 13 kb/s voice or 9.6 kb/s data • TCH/H: 6.5 kb/s voice or 4.8 kb/s data Uplink / Downlink Synchronisation BTS transmits: The MS transmit burst is delayed by 3 timeslots after the BTS burst.. This delay allows enables: 0 1 2 3 4 5 6 7 • Use of the same UL and DL timeslot number in TDMA frame • Avoids simultaneous Tx/Rx requirement MS transmits: • Allows for timing advance (TA) 5 6 7 0 1 2 3 4 • Allows time to switch between Tx and Rx
  27. 27. Broadcast Channels (BCH) BCH channels are all downlink and are allocated to timeslot zero. BCH channels include: • FCCH: Frequency control channel sends the MS a burst of all ‘0’ bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time-synchronise. • SCH: Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN), together with the BTS’s BSIC • BCCH: Broadcast Control Channel sends network-specific information such as radio resource management and control messages, Location Area Code etc.
  28. 28. Common Control Channels (CCCH) CCCH contains all point to multi-point downlink channels (BTS to several MSs) and the uplink Random Access Channel: • RACH: Random Access Channel is sent by the MS to request a resources from the network e.g. an SDCCH channel for call setup. • AGCH: Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobile. • PCH: Paging Channel sends paging signal to inform mobile of a call. • CBCH: Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages, for example road traffic reports or network engineering messages.
  29. 29. Dedicated Control Channels (DCCH) DCCH comprise the following bi-directional (uplink / downlink) point to point control channels: • SDCCH: Standalone Dedicated Channel is used for call set up, location updating and also SMS • SACCH: Slow Associated Control Channel is used for link measurements and signalling during a call • FACCH: Fast Associated Control Channel is used (when needed) for signalling during a call, mainly for delivering handover messages and for acknowledgement when a TCH is assigned
  30. 30. Logical Channels • Multiframes provide a way of mapping the logical channels on to the physical channels (timeslots) • A logical channel is a series of consecutive instances of a particular timeslot Time TDMA Frame TDMA Frame TDMA Frame 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 Logical Channel 1 1 1 1 • A multiframe is a repeating combination of logical channels
  31. 31. Traffic Channel Multiframe • The TCH multiframe consists of 26 timeslots. • This multiframe maps the following logical channels: •TCH •SACCH • TCH Multiframe structure: •FACCH T T T T T T T T T T T T S T T T T T T T T T T T T I 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 T = TCH S = SACCH I = Idle FACCH is not allocated slots in the multiframe. It steals TCH slots when required.
  32. 32. Control Channel Multiframe • The control channel multiframe is formed of 51 timeslots • CCH multiframe maps the following logical channels: Downlink: Uplink: • FCCH • RACH • SCH • BCCH • CCCH (combination of PCH and AGCH) Downlink F = FCCH S = SCH I = Idle F S BCCH CCCH F S CCCH CCCH F S CCCH CCCH F S CCCH CCCH F S CCCH CCCH I 0 1 2-5 6-9 10 11 12-15 16-19 20 21 22-25 26-29 30 31 32-35 36-39 40 41 42-45 46-49 50 RACH Uplink
  33. 33. Multiple Signalling Channel Configurations • In a non combined multiframe, up to 7 of the 9 blocks may be reserved for AGCH: F S BCCH CCCH F S CCCH CCCH F S CCCH CCCH F S CCCH CCCH F S CCCH CCCH I • In a combined multiframe, up to 2 of the 3 blocks may be reserved for AGCH: SDCCH SDCCH SDCCH SDCCH SACCH SACCH F S BCCH CCCH F S CCCH CCCH F S 0 1 F S 2 3 F S 0 1 I • Additional CCCH capacity can be provided on other timeslots (TS 2,4 or 6) of the BCCH carrier if required • The number of AGCH blocks reserved is indicated to the MS in the system information messages that the MS reads on the BCCH
  34. 34. Frame Hierarchy 1 timeslot = 0.577 ms 0 1 2 3 4 5 6 7 1 frame = 8 timeslots = 4.615 ms Multiframe: = 26 TCH Frames (= 120 ms) or 51 BCCH Frames (= 235 ms) Superframe: = 26 BCCH Multiframes (= 6.12s) or 51 TCH Multiframes (= 6.12s) = 2048 Superframes Hyperframe: (= 3 hr 28 min 53.76 s)
  35. 35. TRAU Configurations Um Abis A BTS Site BSC Site MSC Site CCU TRAU A CCU 16kbps 64kbps 64kbps BTS Site BSC Site MSC Site CCU TRAU B CCU 16kbps 16kbps 64kbps BTS Site BSC Site MSC Site CCU TRAU C CCU 16kbps 16kbps 64kbps CCU Channel Coding Unit MSC Node BSC Node
  36. 36. Air Interface Layer Functions Speech and Data Speech and Data Layer 3 Signalling CC: Call Control Signalling MM: Mobility Management CC MM RR RR: Radio Resources CC MM RR Layer 2 Build frames Reconstruct frames Request Send acknowledgement acknowledgement Layer 1 Channel coding Error correction Error protection De - interleaving Interleaving Equalization RF modulation RF demodulation Radio waves
  37. 37. GSM Voice & Channel Coding Sequence Speech Coding 8000 Hz sampling 13-bit resolution Quantization 8000x13bits = 104 kbps 22.8 kbps 2080-bit (20ms) 456-bit blocks blocks (note 1) RPE-LTP Channel GMSK Speech Coder Coding Interleaving Modulation Channel Coding 156.25-bit bursts 260-bit blocks 13 kbps Radio Burst Encryption Multiplexing note 1: 160 samples of 13 bits per 20ms Radio Interface
  38. 38. Speech Coding • GSM transmits using digital modulation - speech must be converted to binary digits • Coder and decoder must work to the same standard • Simplest coding scheme is Pulse Code Modulation (PCM) • Sampling every 125 µs • Requires data rate of 64 kbps • This is too high for the bandwidth available on the radio channels 1.2 1 PCM 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 Sample analog signal at 8 kHZ Digital pulse train at 64 kbps
  39. 39. Advanced Speech Coding • We cannot send the 64 kbps required by PCM • We need alternative speech encoding techniques • Estimates are that speech only contains 50 bits per second of information • Compare time to speak a word or sentence with time to transmit corresponding text • Attempts to encode speech more efficiently: • speech consists of periodic waveforms - so just send the frequency and amplitude “yahoo” • model the vocal tract - phonemes, voiced and unvoiced speech • Vocoder - synthetic speech quality
  40. 40. GSM Voice Coding Sequence Speech Coding 8000 Hz sampling 13-bit resolution Quantization 8000x13bits = 104 kbps 22.8 kbps 2080-bit (20ms) 456-bit blocks blocks Radio Interface (note 1) RPE-LTP Channel GMSK Speech Coder Coding Interleaving Modulation 156.25-bit bursts 260-bit blocks 13 kbps Radio Burst Encryption Multiplexing Channel Coding note 1: 160 samples of 13 bits per 20ms
  41. 41. Speech Digitisation 8192 (213) quantisation levels 8000 samples per second 8000 samples per second x 13 bits per sample = 104kbps per second Divided into 20mS blocks = 2080 bits per block
  42. 42. GSM Channel Coding Speech Coding 8000 Hz sampling 13-bit resolution Quantization 8000x13bits = 104 kbps 22.8 kbps 2080-bit (20ms) 456-bit blocks blocks Radio Interface (note 1) RPE-LTP Channel GMSK Speech Coder Coding Interleaving Modulation 156.25-bit bursts 260-bit blocks 13 kbps Radio Burst Encryption Multiplexing note 1: 160 samples of 13 bits per 20ms
  43. 43. GSM (TCH/F) Channel Coding 260 bits 50 Class 1a 78 Class 2 bits 132 Class 1b bits bits (side information) Inc 3 4 189 bit block coding 53 bits parity 132 bits tail bits bits un co de d ½-rate x2 convolution encoder 78 non-encoded 378 convolution encoded bits + bits 456 bits
  44. 44. Block Interleaving Speech Coding 8000 Hz sampling 13-bit resolution Quantization 8000x13bits = 104 kbps 22.8 kbps 2080-bit (20ms) 456-bit blocks blocks Radio Interface (note 1) RPE-LTP Channel GMSK Speech Coder Coding Interleaving Modulation 156.25-bit bursts 260-bit blocks 13 kbps Radio Burst Encryption Multiplexing note 1: 160 samples of 13 bits per 20ms
  45. 45. Interleaving - Effects of ‘Burst’ Noise • Non – Interleaved Channels: 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 Noise burst 1 Channel 1 1 Channel 2 1 Channel 3 • Interleaved Channels: 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 7 7 7 8 8 8 Noise burst
  46. 46. Interleaving Channel Coder Channel Coder 456 bits 456 bits 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 (8 x 57 bit blocks) 1 11 2 22 3 33 4 44 5 55 6 66 7 77 8 8
  47. 47. GSM Burst Multiplexing Speech Coding 8000 Hz sampling 13-bit resolution Quantization 8000x13bits = 104 kbps 22.8 kbps 2080-bit (20ms) 456-bit blocks blocks Radio Interface (note 1) RPE-LTP Channel GMSK Speech Coder Coding Interleaving Modulation 156.25-bit bursts 260-bit blocks 13 kbps Radio Burst Encryption Multiplexing Channel Coding note 1: 160 samples of 13 bits per 20ms
  48. 48. Radio Burst Multiplexing 456 bits 456 bits 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 (8 x 57 bit blocks) 1 11 2 22 3 33 4 44 5 55 6 66 7 77 8 8 26 training 3 57 data bits 1 bits 1 57 data bits 3 8.25 1 burst = 156.25 bit periods (0.577mS)
  49. 49. Types of Data Burst • The 156.25 bit periods of a timeslot can hold different types of data burst: Stealing flag bits Normal Burst 26 Training (Traffic and most control channels) 3 57 Data Bits 1 Bits 1 57 Data Bits 3 8.25 Frequency Correction Burst (FCCH) 3 142 fixed bits 3 8.25 Data and tail bits are all 0 Synchronisation Burst (SCH) 39 Data 64 Training Bits 39 Data 3 Sync Sequence 3 8.25 Data to synchronise MS with BTS Bits Bits Dummy Burst 26 Training Transmitted on BCCH carrier when there are no other 3 Bits 3 8.25 bursts - allows power level measurements Access Burst (RACH) 41 Training 8 36 Data Bits 3 68.25 Long guard period to avoid collisions Bits Tail bits Guard period
  50. 50. GSM Modulation Speech Coding 8000 Hz sampling 13-bit resolution Quantization 8000x13bits = 104 kbps 22.8 kbps 2080-bit (20ms) 456-bit blocks blocks Radio Interface (note 1) RPE-LTP Channel GMSK Speech Coder Coding Interleaving Modulation 156.25-bit bursts 260-bit blocks 13 kbps Radio Burst Encryption Multiplexing Channel Coding note 1: 160 samples of 13 bits per 20ms
  51. 51. GSM Voice/Channel Coding Summary Speech 20ms Block 20ms Block 20ms Block (2080 bits per block) Speech Coder Speech Coder RPE-LTP encoding 13kbps 260 bits 260 bits Channel Coder Channel Coder Block and convolution encoding 22.8kbps 456 bits 456 bits 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 8 x 57-bit blocks Interleaving 1 11 2 22 3 33 4 44 5 55 6 66 7 77 8 8 26 3 57 data bits 1 training bits 1 57 data bits 3 8.25 1 burst = 156.25 bit periods (0.577mS)
  52. 52. Mobile-Initiated RR Connection Setup Mobile BSS Channel Request RACH Channel Request RACH Channel Request RACH AGCH Immediate Assignment
  53. 53. Network-Initiated RR Connection Setup Mobile BSS PCH Paging Request Channel Request RACH AGCH Immediate Assignment
  54. 54. RR Connection Release • Initiated by network only • Reasons could include: • End of a call • Too many errors • Removal of channel in favour of higher priority call • MS waits for a short random period and returns to idle state Mobile BSS SDCCH Channel Release Short random delay Return to idle state
  55. 55. Handover Types There are four different types of handover in the GSM system, which involve transferring a call between: • Channels (time slots) in the same cell BSC Internal • Cells within the same BSS (same BSC) BSC • Cells in different BSSs (different BSCs) but under the control of the same MSC VLR External • Cells under the control of different MSCs MSC BSC GSM handovers are ‘hard’ – i.e. mobile only communicates VLR with one cell at a time MSC
  56. 56. Handover Causes • Handover can be initiated by either MS or MSC • Handover decision is based on the following parameters (in priority order): • Received signal quality • Received signal strength • Distance of MS from BTS • Drops below power budget margin • Each parameter has a operator-defined threshold and handover decisions can be based on one or a combination of the parameters
  57. 57. Handover Command Message Structure of the message sent to MS by original BSS: MS BSS Message Type Cell Description Handover Command Handover Reference Power Command Includes Frequency Hopping Channel Description information if required Frequency List Non - Frequency Hopping or Mobile Allocation Frequency Hopping
  58. 58. Handover Margin Handover to BTS 1 Handover to BTS 2 Mobile remains with BTS 1 BTS 1 or BTS 2 BTS 2 Nominal cell boundary Hysteresis due to handover margin
  59. 59. Example of Handover Signalling Signalling for a basic Inter-BSC handover involving only one MSC (Intra - MSC): MS BSS 1 BSS 2 MSC Measurement report Measurement report Handover Required Measurement report Handover Request Measurement report Acknowledgement Handover Command Handover Command Handover Access Handover Detection Physical Information Handover Complete Handover Complete Clear Command Measurement report Clear Complete Measurement report
  60. 60. 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
  61. 61. MS Mobility States A Mobile Station (MS) can be in one of three mobility states: • MS turned off • MS turned on in idle mode • MS turned on in dedicated mode
  62. 62. MS Network Connection Sequence Scan RF Select highest Scan for FCCH Power on channels carrier level frequency correction burst NO Select next highest FCCH carrier level detected? NO YES SCH Scan for SCH detected? synchronisation burst YES ‘camp-on’ to BCCH Monitor PCH and and start decoding adjacent carriers
  63. 63. IMSI Attach • Mobile camps on to best serving BTS • Mobile sends IMSI to MSC • MSC/VLR is updated in HLR BSC • Subscriber data including current location area is added to local VLR • MSC and HLR carry out authentication check - VLR challenge and response using Ki MSC • Optionally EIR checks for status of mobile (white/grey/black) AuC EIR HLR
  64. 64. 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
  65. 65. Location Update Options • Send location update on every cell change • No paging requirement • Excessive signalling traffic load • Page every cell in network • No location update reuqirement • Excessive signalling traffic load • Subdivide network into paging areas • Requires paging procedure with reduced traffic load • Required location updating with reduced traffic load
  66. 66. Location Updates BSC • Location Area Change BSC • Periodic Location Update • IMSI Attach VLR MSC • Cell change during call BSC • TMSI update on LA change Au C HLR VLR MSC
  67. 67. TMSI Re-allocation • Used to protect a subscriber’s IMSI • TMSI only unique within a Location Area (LA) • Outside an LA, TMSI must be combined with LAI to remain unique • TMSI re-allocated on LA change (minimum) or as a result of an exceptional condition. • Normally takes place in encrypted mode • Normally tales place in conjunction with another procedure e.g. Location update, call setup etc
  68. 68. Mobile Originated Call • When the mobile requests access to the network to make a call: • BSS determines the nature of the call - e.g. regular voice call, emergency call, supplementary service • Allocates radio resources to the mobile for the call ? • NSS determines the destination of the call: •Mobile to mobile on same PLMN •Mobile to mobile on another PLMN •Mobile to fixed network (PSTN, ISDN) • MSC / GMSC routes the call appropriately and handles signalling
  69. 69. Mobile-Originated Call Setup Mobile BSS Channel Request RACH Channel Request RACH Radio Resource Connection Channel Request RACH AGCH Immediate Assignment LAPDm Connection Setup SDCCH Unnumbered Acknowledgement Service Request SDCCH
  70. 70. Mobile-Terminated (Network-Originated) Call VLR 3 HLR BSS 4 8 7 6 4 2 11 10 8 8 9 BSS 9 1 PSTN MSC GMSC 5 12 12 8 BSS
  71. 71. Network-Initiated Call Setup Mobile BSS PCH Paging Request Channel Request RACH Radio Resource Connection AGCH Immediate Assignment LAPDm Connection Setup Paging Response SDCCH SDCCH Unnumbered Acknowledgement
  72. 72. General Authentication Procedure AuC MS BSS MSC HLR Access Request MS HLR/AuC [IMSI] Ki RAND RAND Ki Send Authentication info [IMSI] A3 A3 Send Authentication info Ack SRES2 SRES1 [IMSI, Triplet (RAND SRES1 Kc)] Authentication & ciphering Request SRES1 SRES1/RAND [RAND] SRES2 = Authentication & ciphering Response [SRES2] MSC
  73. 73. User Data Encryption • Benefits of user data encryption include: • Provides confidentiality for user data across air interface • Selection from seven encryption algorithms • Capability is mandatory for MS and network • Implementation is optional • Does not provide for end-to-end encryption
  74. 74. General GSM Encryption Procedure MS BTS MSC AuC Ki RAND Ki A8 A8 Kc Kc Data Kc Data A5 A5 Encrypted Data
  75. 75. 2.5 Generation GSM • Evolution of GSM towards 3G systems • Main requirement is for increased data rates 3rd Generation • Mobile access to: n atio 384 kb/s • Internet er UMTS 2 Mb/s G en • E-mail .5 38.8 kb/s ECSD 2 • Corporate networks 69.2 kb/s EDGE EGPRS 14.4 HSCSD kb/s GPRS 21.4 kb/s 9.6 CSD kb/s SMS Circuit Switched 2nd Generation Packet Switched
  76. 76. HSCSD • Increases bit rate for GSM by a mainly software upgrade • Uses multiple GSM channel coding schemes to give 4.8 kb/s, 9.6 kb/s or 14.4 kb/s per timeslot Maximum data rate quoted as 115 kb/s = 14.4 x 8 • Multiple timeslots for a connection e.g. using two timeslots gives data rates up to 28.8 kb/s • Timeslots may be symmetrical or asymmetrical, e.g. two downlink, one uplink, giving 28.8 kb/s downloads but 14.4 kb/s uploads.
  77. 77. HSCSD Mobile Equipment • HSCSD handsets are typically limited to 4 timeslots, allowing: • 2 up / 2 down (28.8 kb/s in both directions) • 3 down and 1 up (43.2 kb/s down 14.4 kb/s up) • This limitation arises because the handset operates in half duplex and needs time to change between transmit and receive modes • Nokia cardphone (PCMCIA card for laptops) uses HSCSD (Orange network) - quotes data downloads at 28.8 kb/s
  78. 78. GPRS • General Packet Radio Service • Packet switching: Data packet • Data divided into packets • Packets travel through network individually • Connection only exists while packet is transferred from one node to next • When packet has passed a node, the network resources become available for another packet • User sees an ‘always on’ virtual connection through the network
  79. 79. PCU Circuit/Packet Data Separation Visited Gateway PSTN MSC/VLR MSC Circuit Switched A BTS BSC PCU HLR Gb Packet Switched Serving Gateway GSN GSN PDN
  80. 80. GPRS Air Interface • New ‘Packet’ logical channels defined - PBCCH, PDTCH etc. • New multiframe structure based on ‘radio blocks’ of 4 timeslots • Allows up to 8 mobiles to share a timeslot • For high data rates, several physical channels may be allocated to one user • 4 levels of channel coding schemes (CS-1 to CS-4): • Decreasing level of error checking • Greater data throughput rates • Scheme selected according to CS-4 interference level (C/I) Data throughput CS-3 CS-2 CS-1 C/I
  81. 81. Using Spare GSM Capacity • GPRS can use traffic capacity on the GSM network away from the Maximum Capacity Available busy hour for non time critical Timeslot Usage Available for GPRS data transfers for GPRS • Even during the busy hour, there Circuit Switched Demand is spare capacity that GPRS can make use of: Time (hours) • Voice calls start and finish at 0 24 random times, leaving short periods when channels are unused Ti m es • Packets of data can be sent when Timeslots lot these channels become available s - dynamic allocation Time Time
  82. 82. Charging for GPRS Services • GPRS allows the user to be ‘always connected’ - charging by time is not appropriate • Some possible methods of charging are: • By volume of data transferred Internet • Flat rate for Internet access • By Quality of Service • For content - operator may provide own £ pages (value added services) • Quality of Service parameters: £ • Service Precedence (priority) £ • Reliability £ • Delay £ • Throughput
  83. 83. EDGE (0,1,0) (0,1,1) (1,1,0) • Enhanced Data rates for GSM Evolution • Use 8 Phase-Shift Keying (8PSK) modulation (0,0,1) (1,1,1) - 3 bits per symbol • Improved link control allows the system to adapt to variable channel quality - leads to slightly (0,0,0) (1,0,1) reduced coverage area (1,0,0) • Applied to GSM, EDGE allows a maximum data rate of 48 kb/s per timeslot, giving the quoted figure of 384 kb/s per carrier (8 timeslots) • EDGE can be applied to HSCSD (ECSD) and GPRS (EGPRS) • EDGE will be expensive for operators to implement: • Each base station will require a new EDGE transceiver • Abis interface between BTS and BSC must be upgraded

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