GSM Architecture


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GSM Architecture

  2. 2. GSM (Global System for Mobile Communications) , is a standard set developed by the European Telecommunications Standards Institute (ETSI) to describe protocols for second generation (2G) digital cellular networks used by mobile phones . It became the global standard for mobile communications with over 80% market share. The GSM standard was developed as a replacement for first generation (1G) analog cellular networks, and originally described a digital, circuit-switched network optimized for full duplex voice telephony
  3. 3. GSM Architecture Overview This includes a brief explanation of the different network subsystems and a description of the functionality of the elements within each of the subsystems. Topics include:  General architecture overview  The Mobile Station (MS) Subsystem and Elements  The Base Station Subsystem (BSS) and Elements  The Network Subsystem (NSS) and Elements
  4. 4. 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) • 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 Mobile Station
  5. 5.  The BSS comprises:  Base Station Controller (BSC) • One or more Base Transceiver Stations (BTSs)  BTS contains:  Radio Transmitter/Receiver (TRX) • Signal processing and control equipment  Antennas and feeder cables  The BSC:  allocates a channel for the duration of a call  maintains the call: • controls the power transmitted by the BTS or MS • generates a handover to another cell when required Base Station Subsystem (BSS)
  6. 6. Network Switching System (NSS) The NSS combines the call routing switches (MSCs and GMSC) with database registers required to keep track of subscribers’ movements and use of the system. Call routing between MSCs is taken via existing PSTN or ISDN networks. Signaling between the registers uses Signaling System No. 7 protocol. Section 1 – GSM Architecture Overview Key elements of the NSS:  Mobile Switching Center (MSC)  Visitor Location Register (VLR)  Home Location Register (HLR)  Authentication Center (AuC)  Equipment Identity Register (EIR)  Gateway MSC (GMSC) Section 3 – NSS Topology
  7. 7. Mobile Switching Center (MSC) The following are typical MSC functions in a cellular system:  Provide switched connections with PSTN  Provide switched connections between mobile subscribers  Coordinate the location and handover process  Provide custom services to mobile users  Collect billing data
  8. 8. Home Location Register HLR is a database that stores subscription and set of functions needed to manage subscriber data in one PLMN area. Any administrative action by the service provider or changes made by subscriber is first carried out on the HLR and then update the VLR. Section 3 – NSS Topology
  9. 9. Visitor Location Register It is a subscriber database containing the information about all the MS currently located in the MSC service area. VLR can be considered as a distributed HLR in the case of a roaming subscriber. If MS moves into a new service area (MSC), VLR requests the HLR to provide the relevant data and store it, for making the calls for that MS. It can also be viewed as a subset of a HLR. Section 3 – NSS Topology
  10. 10. Authentication Center (AUC) AUC is always integrated with HLR for the purpose of the authentication. At subscription time, the Subscriber Authentication Key (Ki) is allocated to the subscriber, together with the IMSI. AUC stores the following information for each subscriber 1. The IMSI number, 2. The individual authentication key Ki, Section 3 – NSS Topology
  11. 11. Equipment Identification Register (EIR) Purpose of this feature is to make sure that no stolen or unauthorized mobile equipment is used in the network. EIR is a database that stores a unique International Mobile Equipment Identity (IMEI) number for each item of mobile equipment. Procedure:  The MSC/VLR requests the IMEI from the MS and sends it to a EIR.  On request of IMEI, the EIR makes use of three possible defined lists: - A white list: containing all number of all equipment identities that have been allocated in the different participating countries. - A black list: containing all equipment identities that are considered to be barred. - A grey list: containing (operator’s decision) faulty or non- approved mobile equipment.  Result is sent to MSC/VLR and influences the decision about access to the system.
  12. 12. Operation And Maintenance Center (OMC) The OMC centralizes all operations and maintenance activities for the MSCs and BSSs using remote software control. It provides remote testing, operations, and maintenance capabilities for the entire system from one central location. Each BSS, MSC, HLR, VLR, EIR, and AUC can be monitored and controlled from the OMC.
  13. 13. GSM Call Setup
  14. 14. MSC EIR PSTN Fixed Network BSC PrePaid HLR MSC BSC
  15. 15. MSC EIR BSC PrePaid HLR MSC BSC PSTN Fixed Network
  16. 16. BSC Base Station Controller  The call request reaches the BSC from the BTS and is forwarded to MSC.  After call is established, the BSC will perform decoding of the call (in typical config.)
  17. 17. BSC HLR MSC EIR PrePaid MSC BSC PSTN Fixed Network
  18. 18. • Checks A number. • To whom is A subscriber calling? (Checks B number) • Example : A subscriber has prepaid number. MSC Mobile Switching Centre
  19. 19. BSC HLR MSC EIR PrePaid MSC BSC PSTN Fixed Network
  20. 20. • Does the A subscriber have money left on his/her account? • When the call is established and on-going the subscriber’s account is decremented accordingly. PrePaid Node
  21. 21. BSC HLR MSC EIR PrePaid MSC BSC PSTN Fixed Network
  22. 22. • Where is the B subscriber? • MSC interrogates in HLR. (HLR = Home Location Register) MSC Mobile Switching Centre
  23. 23. BSC HLR MSCPrePaid MSC BSC PSTN Fixed Network EIR
  24. 24. BSC HLR MSC MSC BSC PSTN Fixed Network PrePaid EIR
  25. 25. • HLR says ’I am the home of the B subscriber and I know where he/she is right now’ (i.e. which VLR) • Tells this VLR ’Give me a visitor address for this subscriber’ (Roaming Number) HLR Home Location Register
  26. 26. BSC HLR MSC MSC BSC PSTN Fixed Network PrePaid EIR
  27. 27. BSC HLR MSC MSC BSC PSTN Fixed Network PrePaid EIR
  28. 28. • Hosting MSC/VLR returns TEMPORARILY assigned visitor/roaming address for B- subscriber to HLR. MSC Mobile Switching Centre
  29. 29. BSC HLR MSC MSC BSC PSTN Fixed Network PrePaid EIR
  30. 30. BSC HLR MSC MSC BSC PSTN Fixed Network PrePaid EIR
  31. 31. • ’Thanks for the visitor address!’ • ’I will send this address to the MSC that requested for it.’ HLR Home Location Register
  32. 32. BSC HLR MSC MSC BSC PSTN Fixed Network PrePaid EIR
  33. 33. • ’Thank you, HLR!’ • Speech connection path will set up now towards the MSC/VLR where the B subscriber is presently located. MSC Mobile Switching Centre
  34. 34. BSC HLR MSC MSC BSC PSTN Fixed Network PrePaid EIR
  35. 35. • Sets up a speech connection towards BSC. • Delivers B-subscriber ID to BSC and asks to page for the subscriber using that ID. MSC Mobile Switching Centre
  36. 36. BSC HLR MSC MSC BSC PSTN Fixed Network PrePaid EIR
  37. 37. • BSC now sends a PAGING MESSAGE to all the BTSs that it covers/controls. • Carrys PAGING RESPONSE from called MS back to MSC. • Allocates a RADIO channel through a BTS with best available and allowable SIGNAL STRENGTH for called MS. • Performs SPEECH CODING when call is established. BSC Base Station Controller
  38. 38. BSC HLR MSC MSC BSC PSTN Fixed Network PrePaid EIR