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GSM N ETWORK O VERVIEW Slide 1 BSC BTS Mobile Station Access Network: Base Station Subsystem HLRVLREIRAuC MSC PSTN Um Abis A Core Network: GSM CS network.

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Presentation on theme: "GSM N ETWORK O VERVIEW Slide 1 BSC BTS Mobile Station Access Network: Base Station Subsystem HLRVLREIRAuC MSC PSTN Um Abis A Core Network: GSM CS network."— Presentation transcript:

1 GSM N ETWORK O VERVIEW Slide 1 BSC BTS Mobile Station Access Network: Base Station Subsystem HLRVLREIRAuC MSC PSTN Um Abis A Core Network: GSM CS network SS7

2 GSM N ETWORK A RCHITECTURE MS: Mobile Station BSS: Base Station Subsystem MSC: Mobile Switching Center O&M: Operations and Maintenance Center VLR, HLR, AuC, EiR … CGSN Slide 2 Prepared by E.Stambolliu, M.Koci & E.Kola

3 M OBILE S TATION (MS) Mobile Equipment (ME) SIM: Subscriber Identity Module While subscriber roams or is stationary, the MS transmits a radio signal to one of the many BTS using a radio-link protocol via the Um interface Slide 3

4 T HE B ASE S TATION S YSTEM (BSS) All radio-related functions performed in BSS The Base Station Controller (BSC) Is a high-capacity switch Provides all control functions and physical links between the MSC and the BTS A group of BSCs is served by an MSC The Base Transceiver Station (BTS) Handles the radio interface to the mobile unit Consists of transceivers and cell antennas A group of BTSs is controlled by a BSC Slide 4

5 BSS (B ASE S TATION S UBSYSTEM ) BSC handles (through the Abis interface): Radio-channel setup Frequency hopping Handovers BSC also connects MS to MSC using A interface Slide 5

6 BSS (B ASE S TATION S UBSYSTEM ) Sometimes a Transcoder Rate Unit (TRAU) is placed on BTS to perform transcoding between 64 Kbps A-law and 13 Kbps RPE/LTP(Regular Pulse Excited Long Term Prediction) speech channels Slide 6 BSC BTS HLRVLREIRAuC MSC PSTN Um Abis A SS7 TRAU

7 M OBILE S ERVICES S WITCHING C ENTER The MSC performs the telephony switching functions of the network Controls calls to and from other telephone and data systems Interface between radio system and fixed networks (PSTN and ISDN) Connected to BSS through A interface; usually an E- 1, either wireline or microwave Also performs functions such as: Toll ticketing Network interfacing Common channel signaling Slide 7

8 MSC (M OBILE S WITCHING C ENTER ) (2) Each MSC covers several cells (BSSs) Slide 8

9 MSC (M OBILE S WITCHING C ENTER ) (3) Also performs signaling between MSC and other functional entities using SS7: Registration Authentication Location updating Handovers Call routing to a roaming subscriber Slide 9

10 O THER GSM N ETWORK E NTITIES HLR: Home Location Register VLR: Visitor Location Register EIR: Equipment Identity Register AuC: Authentication Center Slide 10

11 H OME L OCATION R EGISTER The HLR is the most important database Storage and management of subscriptions Permanent data includes Subscriber’s service profile Subscriber’s location information Subscriber’s activity status Subscribing to a particular provider’s service registers you in the HLR of that provider Slide 11

12 HLR (H OME L OCATION R EGISTER ) Central database for all subscribers: Identity of the subscriber Services accessible to the subscriber Current location of the subscriber Given a Mobile Subscriber ISDN number (MS-ISDN), call is routed to IMSI number-VLR Each subscriber appears only once in database HLR might be physically distributed in several sites (e.g., using first two digits to identify physical HLR) Slide 12

13 Slide 13 HLR TYPES


15 Slide 15 GSM-GPRS


17 GPRS(1) Slide 17

18 GPRS(2) Slide 18

19 GPRS(3) Slide 19

20 GPRS(4) Slide 20

21 GPRS(5) Slide 21

22 GPRS-HLR Slide 22

23 GPRS-MSC Slide 23

24 GPRS-BSC Slide 24

25 GPRS-BTS Slide 25


27 V ISITOR L OCATION R EGISTER The VLR contains temporary data about visiting (roaming) subscribers It’s always integrated with the MSC When a roamer enters the service area the VLR queries the appropriate HLR If a roamer makes a call the VLR will already have the information it needs for call setup Database with information on MS within area served by MSC: MS Roaming number TMSI if applicable Location area in which was last registered Supplementary services Used by an MSC to retrieve information for various purposes: Handling of calls to or from a roaming mobile station currently located in its area Typically part of MSC Slide 27

28 A U C (A UTHENTICATION C ENTER ) Entity associated to HLR for authentication: allow International Mobile Subscriber Identity (IMSI) to be authenticated Allows ciphering of communication over radio path between mobile station and network ciphered Transmits data needed for authentication and ciphering via HLR to VLR, MSC and SGSN which need to authenticate a mobile station (SIM validation) Slide 28

29 Ki is stored in SIM card and AUC Generate RAND AUC MS RAND+Ki=SRES using A3 AUC MS If the SRES in MS is equal with SRES in AUC the subs is authenticated. RAND+Ki=Kc using A8 AUC MS A3 This is an algorithm used to generate the Signed Response (SRES). A8 This is an algorithm used to generate the Ciphering Key (Kc). A3A8 This is an algorithm used to generate Signed Response (SRES) and Ciphering Key (Kc). A4 This is an algorithm used for encryption/decryption of Ki. Slide 29

30 EIR (E QUIPMENT I DENTITY R EGISTER ) Logical entity responsible for storing International Mobile Equipment Identities (IMEIs) in network used in GSM system Equipment classified as "white listed", "grey listed” and "black listed” Ensures that MEs being used are valid and authorized to function on the Public Land Mobile Network (PLMN) Slide 30

31 O PERATION AND S UPPORT C ENTER Operation and Maintenance Center (OMC) is connected with all the equipment in the switching center and to the BSC Network operation monitors and controls the system Provides centralized cost-effective support Provides a network overview at any moment Supports maintenance and operational activities for different organizations and groups Slide 31

32 O THER F UNCTIONAL E LEMENTS Message Center (MXE) – handles voice, fax, and data messaging Mobile Service Node (MSN) – handles mobile intelligent network (IN) services Gateway Mobile Services Switching Center (GMSC) – an MSC with a gateway that interconnects two networks GSM Interworking Unit (GIWU) – hardware and software that enables both voice and data Slide 32

33 C ALL R OUTING Slide 33


35 Slide 35

36 GSM S PECIFICATIONS Combination of FDMA and TDMA to send information Frequencies: 800, 900, 1800, 1900 MHz For example, GSM 900: Uplink = 890-915 MHz Downlink = 935-960 MHz Each 25 MHz bandwidth is divided into 124 carrier frequencies spaced 200 KHz with one or more frequencies allocated to each base station Transmission rate: 270 kbps over the air Speech coder: Linear Predictive Coding (LPC) at 13 kbps – filter reduces the bit rate Slide 36

37 GSM S PECIFICATIONS Frequency range: 1,850 to 1,990 MHz Duplex distance: 80 MHz Channel separation: 200 kHz Modulation: Gaussian minimum shift keying Transmission rate: 270 kbps over the air Access method: Time Division Multiple Access Speech coder: Linear Predictive Coding (LPC) at 13 kbps – filter reduces the bit rate Slide 37

38 GSM Band Information Slide 38

39 GSM N ETWORK A REAS In order of increasing geographic size: Cell – the area covered by one BTS – a number of these make up a: Location Area (LA) – a group of cells – a group of LAs makes up an: MSC/VLR service area – area covered by one MSC – a number of these make up the: Public Land Mobile Network (PLMN) service area– one operator’s network Slide 39

40 Slide 40

41 CGI- Cell Global Identification MCC-MNC-LAC-CI where: CI Cell identity LAC Location area code MNC Mobile network code MCC Mobile country code Slide 41

42 Base Station Identity Code Expressed as nccbcc where: bcc BS colour code ncc PLMN colour code Slide 42

43 H ANDOVER Four types of handovers: Channels (time slots) in same cell Between cells within same BSC Between BSCs, within same MSC Between MSCs Slide 43

44 Slide 44

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46 Slide 46

47 Addressing of managed objects TSTSTime SlotRXOTS 0 <= ts <= 7 TXTXTransmitterRXOTX0 <= trxc <= 15 RXRXReceiverRXORX0 <= trxc <= 15 TRXCTRXCTransceiver ControllerRXOTRX0 <= trxc <= 15 DPDPDigital PathRXODP0 <= dp <= 1 TFTFTiming FunctionRXOTF0 <= tg <= 511 CFCFCentral FunctionRXOCF0 <= tg <= 511 TGTGTransceiver GroupRXOTG0 <= tg <= 511 MOMO classMO type Addressing Limit ISISInterworking SwitchRXOIS0 <= tg <= 511 Slide 47

48 DXU - Distribution Switch Unit functions CF Central Function, is the control part of a TG. It is a SW function, handling common control functions within a TG. The BSC communicates with the CF using layer 2 LAPD, and is addressed by its TEI = 62. CON LAPD Concentrator, is used by the optional feature LAPD Concentration for RBS 2000. It is connected to DCP 64&&87. IS Interworking Switch, provides a system interface to the 2 Mbit/s link and cross connects individual time slots to certain transceivers. TF Timing Function, extracts synchronization information from the PCM link and generates a timing reference for the RBS. DP Digital Path, Layer 1 reception and transmission are not part of the BTS logical model. However, each of the PCM systems terminating in TG has an associated supervision object, the DP. Slide 48

49 TRU - Transceiver Unit functions TRXC The transceiver controller is controlling all the functions for Signal processing, Radio receiving and Radio Transmitting. Each TRX corresponds to one TRU unit. The BSC currently supports a maximum of 1020 TRXs. RX The receiver is an application object. It provides the radio frequency reception functionality for one transceiver. TX The transmitter is an application object. It provides the radio frequency transmission functionality on a time slot basis for eight TSs using different time slot numbers. Slide 49

50 Managed Object Hierarchy Slide 50

51 GSM I NTERFACES Um Interface Mobile station and base station subsystem communicate across Um interface, also known as air interface or radio link Abis interface Base transceiver station (BTS) and base station controller (BSC) communicate across Abis interface A interface Base station subsystem communicates with mobile service switching center across A interface Slide 51

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55 SS#7 SIGNALING MEETS THESE NEEDS Efficiency Out-of-band links at 64 Kbps Shorter information transfer time Ability to fall back to the originating end of the call (e.g., busy) Service enabling Free phone (0-800) services Automatic call back & calling number delivery Automatic calling card services Wireless services such as roaming Network reliability Carries extensive network management messages Network architecture Security Slide 55

56 SS#7 DEFINITION Common channel signaling system number 7 (SS#7) Out-of-band signaling system Facilitates exchange of call control information between network switching offices Voice and non-voice services Slide 56

57 SS#7 BASICS Voice and signaling are separated Control messages (packets) are routed through the network for call management Network elements are connected via signaling links Each element capable of SS#7 control messages, is called a signaling point (SP) All SPs in an SS7 network are identified by a unique code known as point code (PC) Slide 57

58 SS#7 NETWORKS Slide 58

59 SS#7 SIGNALING POINTS (1) Service switching point (SSP) Capable of controlling voice circuits via a voice switch The switch can originate, terminate, or tandem calls An SSP sends signaling messages to other SSPs to set up, manage, and release voice circuits required to complete a call An SSP can also send a query message to a centralized database (SCP) to determine how to route a call (e.g., toll free 1-800 number) Slide 59

60 SS#7 SIGNALING POINTS (2) Signaling transfer point (STP) Routes each incoming message to an outgoing signaling link, based on routing information contained in the SS#7 message and a pre-defined route table Does not offer termination services STPs are paired to ensure redundancy Slide 60

61 SS#7 SIGNALING POINTS (3) Service control point (SCP) Provides access to databases Accepts a query for information from a subsystem at another node Used by STP to perform a function called global title translation The database may not reside in the same location as the SCP Slide 61

62 C OMMON CHANNEL SIGNALING (1) Slide 62 Switch SP Switch SP E1 31 voice channels using 31 time slots 2 separate signaling links using 2 time slots

63 C OMMON CHANNEL SIGNALING (2) Slide 63 Switch SP Switch SP E1 31 voice channels using 31 time slots 1 signaling link and 30 voice channels

64 S IGNALING NETWORK TERMS Associate mode: signaling links follow the same path as the voice trunks Quasi-associate mode: signaling links follows a different route than the voice trunks Slide 64 SP STP Voice trunks Signaling Links Associate ModeQuasi-Associate Mode

65 L INKSETS Groups of links that connect two adjacent nodes Ensure traffic load sharing Combined linksets between STPs ensures load sharing Consist of up to 16 links in ANSI protocol and up to 8 in ITU protocol Signaling link code (SLC) is uniquely assigned to each link Slide 65

66 R OUTES Virtual path that a message takes to a destination node Comprised of one or more linksets Slide 66

67 MEATA SP=2-901 MEATR SP=2-903 AMC1 SP=2-272 AMC2 SP=2-256 First priority of signaling route LS=2-901 LS=2-256 LS=2-903 LS=2-272 Second priority of signaling route Slide 67


69 MTP Message transfer part: Reliably transfers messages over links or linksets For correct routing, the signaling point needs the signaling point code (SPC) of the node at the end of its links Receives the SPC by destination point code (DPC) in the messages it routes Needs information about other locations in the network, to select the best link set for routing the message to its destination Slide 69

70 MTP LEVEL 3 MANAGEMENT SERVICES (1) MTP level 3 provides signaling link selection (SLS) Rotates in each session A mechanism to assign traffic to a link in the linkset Results in load sharing of the links in the linkset SLC rotation stops for duration of message transfer Slide 70

71 MTP LEVEL 3 MANAGEMENT SERVICES (2) MTP restart Before returning to the network, a node can send TRW (traffic restart wait) to an adjacent node, indicating not to send traffic When restarting, if the node is satisfied that enough links are available, it is sends a TRA (traffic restart allowed) Optional signaling link test message (SLTM) and signaling test acknowledge (SLTA) Exchanged when a link is in service; ensures agreement on signaling link code Slide 71

72 SCCP Signaling connection control part: Provides connectionless and connection-oriented network services Provides global title translation (GTT) capabilities above MTP level 3; translates numbers to DPCs and subsystem numbers Provides more detailed addressing information than MTPs Used as transport layer for TCAP-based services Slide 72

73 TCAP Transaction capabilities applications part: Exchange of non-circuit related data Between applications across the SS#7 network Using the SCCP connectionless service Queries and responses sent between SSPs and SCPs Sends and receives database information Credit card validation Routing information Slide 73

74 TUP Telephone user part: Basic call setup and tear down Analog circuits only In many countries, ISUP has replaced TUP for call management Slide 74

75 ISUP ISDN user part: Necessary messaging for setup and tear down of all circuits (voice and digital) Messages follow the paths of voice circuits Messages are sent from a switch, to the switch where the next circuit connection is required Call circuits are identified using circuit identification code (CIC) Must be compatible on both sides Followed by each ISUP message Slide 75

76 ISUP MESSAGES (1) Initial address message (IAM): contains all necessary information for a switch to establish a connection Address complete message (ACM): acknowledge to IAM; the required circuit is reserved and the “phone is ringing” (ringback tone) Answer message (ANM): occurs when the called party picks up the phone Slide 76

77 ISUP MESSAGES (2) Release (REL): sent by the switch sensing that the phone hung up Release complete (RLC): each exchange that receives REL, sends an RLC message back (this acknowledges receipt of REL) Slide 77


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83 GSM S UBSCRIBER S ERVICES There are two basic types of service Telephony (teleservices): mainly voice services (including terminal equipment) for communicating with other subscribers – includes fax, paging, voice mail, and alphanumeric services Data (bearer services): capacity to transmit appropriate data signals between two access points creating an interface to the network Slide 83

84 S UPPLEMENTARY S ERVICES The following are the usual revenue generators Call forwarding Barring outgoing calls Advice of Charge (AoC) Call hold (for telephony only) Call waiting Multiparty service (for telephony only) Calling line identification presentation/restriction Closed user groups (CUGs) Slide 84

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