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Introduction to GSM

Introduction to GSM. GSM Architecture. GSM Architectures 3 broad parts. ME – Mobile Equipment BSS – Base Station Subsystem NSS – Network Switching Subsystem. PSTN. GSM System Architecture. UM. Handset. A-bis. A. SS7. BSC. MSC. BTS. HLR/ VLR. Data Terminal. SIM card. Mobile

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Introduction to GSM

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  1. Introduction to GSM

  2. GSM Architecture

  3. GSM Architectures 3 broad parts ME – Mobile Equipment BSS – Base Station Subsystem NSS – Network Switching Subsystem

  4. PSTN GSM System Architecture UM Handset A-bis A SS7 BSC MSC BTS HLR/ VLR Data Terminal SIM card Mobile Equipment Base Station Subsystem Network Switching Subsystem PSTN

  5. Mobile Equipment (Or Mobile Station)

  6. ME vs SIM • Mobile Equipment • The cellular telephone itself (or the vehicular telephone) • Address / identifier: IMEI • Subscriber Identity Module (SIM) • Fixed installed chip (plug-in SIM) or • Exchangeable card (SIM card) • Addresses / identifiers: IMSI / MSISDN

  7. International Mobile Equip. Identity Uniquely identifies the mobile equipment ( really ???) 15 digits hierarchical address

  8. Subscriber Identity Module • Uniquely associated to a user • Stores user addresses • IMSI / TMSI / MSISDN* • Authentication and encryption features • Ki / Authentication algorithm • Personalization • SIM stores user profile (subscribed services) • SMS, user’s directory, etc • Protection codes

  9. Temporary addresses TMSI – Temporary Mobile Subscriber Identity MSRN – Mobile Station Roaming Number

  10. Base Station Subsystem

  11. BSS Overview

  12. Base (Transceiver) Station Serves a single cell Usually placed in the center of a cell Responsible for communication to and from MSs via air interface BTS comprises radio transmission and reception devices Including the antennae and signaling processing specific to the radio interface BTS separates the speech and control signaling

  13. Base Station Controller (BSC) Switch calls from MSC to correct BTS Monitor and Control several base stations Frequency administration, control of BTSs Protocol and coding conversion voice & signaling (GSM to ISDN) 1 BSC may control up to 40 BTS

  14. Network Switching Subsystem

  15. Network Switching Subsystem • Functions: • Call control • User management • Elements: • MSC - Mobile Switching Center / GMSC - Gateway MSC • HLR/ VLR – Home / Visitor Location Register • AuC - Authentication Center • EIR - Equipment Identity Register • Inter-component communication • Via SS7 signalling network • With suitable extensions (e.g. MAP – Mobile Application Part)

  16. Mobile Switching Center • An ISDN switch (64 kbps channels) • switching and routing functions of a fixed network switching node • PLUS specific mobility-related functions: • Allocation and administration of radio resources • Management of mobile users • registration, authentication • Paging • A PLMN (operator network) has, in general, many MSC • Each MSC is responsible of a BSS

  17. Home Location Register 1 database per operator (PLMN) HLR entries: Every user / MSISDN that has subscribed to the operator

  18. HLR Stores - • Permanent information • IMSI, MSISDN • Services subscribed • Service restrictions (e.g. roaming restrictions) • Parameters for additional services • info about user equipment (IMEI) • Authentication data • Temporary information • Link to current location of the user: • Current VLR address (if avail) • Current MSC address (if avail) • MSRN (if user outside PLMN)

  19. Authentication Center • Associated to HLR • Eventually integrated with HLR • Search key: IMSI • Responsible of storing security-relevant subscriber data • Ki (for authentication) • Kc (for radio channel encryption) • Algorithms

  20. Gateway MSC Needed, as fixed network switches are not mobile capable!! GMSC task: query HLR for current MS location

  21. Gateway MSC

  22. Visitor Location Register • At most 1 database per MSC • Generally, joint MSC-VLR implementation (avoid signaling load ) • VLR entries: • Every user / MSISDN actually staying in the administrative area of the associated MSC • Stores: • Subscriber and subscription data • IMSI, MSISDN, IMEI • Tracking and routing information • MSRN / TMSI • Temporary Mobile Station Identity (TMSI) • Location Area Identity (LAI)

  23. PSTN GSM System Architecture UM Handset A-bis A SS7 BSC MSC BTS HLR/ VLR Data Terminal SIM card Mobile Equipment Base Station Subsystem Network Switching Subsystem PSTN

  24. Geographical Network Structure

  25. GSM – Frequencies • GSM-900: • Uplink: 890,2 MHz – 915 MHz (25 MHz) • Downlink: 935,2 MHz – 960 MHz (25 MHz) • Uplink-Downlink distance: 45 MHz • Frequency Division Multiple Access • Channels are 200 kHz wide. • 124 pairs of channels • Time Division Multiple Access • 8 connections each channel • Theoretical 124*8 = 992 channel to use.

  26. Cell Patterns 4 1 5 3 2 1 5 3 6 2 1 7 K=3 K=7

  27. Location Areas

  28. GSM Service Area PLMN Service Area (one per operator) MSC/VLR Location Area cell GSM Service Area Hierarchy • The area in which a subscriber can access the network.

  29. Location Registry

  30. Responsibilities • Measurements • During : Call / Idle (parking) • Switching cells / LA (more?)

  31. Same MSC HLR VLR1 VLR2 3 2 MSC1 MSC2 4 1 LA1 LA2

  32. Different MSCs HLR 4 3 VLR2 VLR1 5 2 MSC1 MSC2 6 1 LA1 LA2

  33. Call Setup

  34. Call Delivery 2. 1. MSISDN MSISDN HLR GMSC PSTN 3. 5. MSISDN-->IMSI--> MSC/VLR Service Area MSRN 6. IMSI 4. MSC/VLR IMSI<-->MSRN 7. BSC 8. 9. BTS BTS BTS

  35. End of part I.

  36. Air Interface

  37. Air Interface Radio Transmission Techniques FDMA TDMA CDMA Channels Physical channels Logical channels

  38. FDMA Frequency Channel Time

  39. TDMA Time Slot Frequency Channel Time 0 1 2 3 4 5 6 7

  40. CDMA Frequency Code Time Code 1 Code 2 Code 3

  41. Channels Physical Channels Associated with frequency bands, time slots, codes Physical channels transfer bits from one network element to another Logical Channels Distinguished by the nature of carried information and the way to assemble bits into data units Three types one-to-one: traffic channels between a BTS and a MS one-to-many: synchronization signals from BTS to MSs in a cell many-to-one: from MSs to the same BTS

  42. Air Interface Layers Layer 3 Messages Messages Messages Layer 2 Packets Logical Channels Logical Channels Layer 1 Bits Radio Transmission Radio Transmission Base Station Terminal

  43. GSM Frame Structure Superframe 6.12 sec 51 Multiframe ... ... 120 msec 26 Frames 4.615 msec 8 Multiframe 0 7 Coded data Midamble 0.57692 msec 156.25 bits 3 57 1 57 26 1 3 8.25 Tail bit Stealing Flag Guard period

  44. Frame Types • There are two types of multiframe • 26 TDMA-frame multiframe is used to carry TCH, SACCH and FACCH • 51 TDMA-frame multiframe is used to carry BCCH, CCH, SDCCH and SACCH

  45. Burst and Frames • The information contained in one time slot on the TDMA frame is call a burst. • Five types of burst • Normal Burst (NB) • Frequency Correction Burst (FB) • Synchronization Burst (SB) • Access Burst (AB) • Dummy Burst

  46. Logical Channel List TCH/F: Full-rate Traffic Channel Traffic channels (TCH) Two-way TCH/H: Half-rate Traffic Channel FCCH: Frequency correction SCH: Synchronization BCH BCCH: Broadcast control Base-to- mobile PCH: Paging Signaling channel CCCH AGCH: Access grant RACH: Random access SDCCH: Stand-alone dedicated control DCCH SACCH: Slow associated control Two-way FACCH: Fast associated control

  47. Logical Channels • Control Channels • Broadcast Channels (BCH) • Common Control Channels (CCCH) • Dedicated Control Channels (DCCH) • Traffic Channels (TCH) • Full Rate (TCH/F) • Half Rate (TCH/H)

  48. UP / Down-Link Down-link: the transmission path from Base Station to Mobile Station Up-link: the transmission path from Mobile Station to Base Station

  49. Control Channels • Broadcast Channels (BCH) • Frequency Correction Channel (FCCH) • Synchronization Channel (SCH) • Broadcast Control Channel (BCCH) • Common Control Channels (CCCH) • Paging Channel (PCH) • Random Access Channel (RACH) • Access Grant Channel (AGCH) • Dedicated Control Channels (DCCH) • Stand alone Dedicated Control Channel (SDCCH) • Cell Broadcast Channel (CBCH) • Slow Associated Control Channel (SACCH) • Fast Associated Control Channel (FACCH)

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