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GSM NETWORK OVERVIEW
Slide 1
BSCBTS
BTS
Mobile
Station
Access Network:Base Station Subsystem
HLR VLR EIR AuC
MSCPSTN
Um Abis A
Core Network:GSM CS network
SS7
GSM NETWORK ARCHITECTURE
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
MOBILE STATION (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
THE BASE STATION SYSTEM (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
BSS (BASE STATION SUBSYSTEM)
BSC handles (through the Abis interface): Radio-channel setup Frequency hopping Handovers
BSC also connects MSto MSC usingA interface
Slide 5
BSS (BASE STATION SUBSYSTEM) Sometimes a Transcoder Rate Unit (TRAU) is placed on BTS
to perform transcoding between 64 KbpsA-law and 13 Kbps RPE/LTP(Regular Pulse Excited Long Term Prediction) speech channels
Slide 6
BSCBTS
BTS
HLR VLR EIR AuC
MSCPSTN
Um Abis A
SS7
TRAU
MOBILE SERVICES SWITCHING CENTER 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
MSC (MOBILE SWITCHING CENTER) (2)
Each MSC covers several cells (BSSs)
Slide 8MSC
BTS
BTS
BTS
BTS
BTS
BTS
BTS
BSC BSC
MSC (MOBILE SWITCHING CENTER) (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
OTHER GSM NETWORK ENTITIES
HLR: Home Location Register VLR: Visitor Location Register EIR: Equipment Identity Register AuC: Authentication Center
Slide 10
HOME LOCATION REGISTER
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
HLR (HOME LOCATION REGISTER)
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
Slide 13
HLR TYPES
Slide 14
HLR CONNECTIONS
Slide 15
GSM-GPRS
GPRS SERVICES ACCORDING TO THE BANDWIDTH AND BURSTINESS
Slide 16
GPRS(1)
Slide 17
GPRS(2)
Slide 18
GPRS(3)
Slide 19
GPRS(4)
Slide 20
GPRS(5)
Slide 21
GPRS-HLR
Slide 22
GPRS-MSC
Slide 23
GPRS-BSC
Slide 24
GPRS-BTS
Slide 25
GPRS PROTOCOL WITH BSS
Slide 26
VISITOR LOCATION REGISTER 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 setupDatabase 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
AUC (AUTHENTICATION CENTER)
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
Ki is stored in SIM card and AUC
Generate RANDAUC MS
RAND+Ki=SRES using A3AUC MS
If the SRES in MS is equal with SRES in AUC the subs is authenticated.
RAND+Ki=Kc using A8AUC 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
EIR (EQUIPMENT IDENTITY REGISTER)
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
OPERATION AND SUPPORT CENTER
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
OTHER FUNCTIONAL ELEMENTS
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
CALL ROUTING
Slide 33
7.When the new MSC/VLR receives the information from theHLR, it sends a location updating confirmation message to theMS.
6. The HLR also orders the old serving MSC/VLR to cancel allinformation for the subscriber because the mobile subscriber isnow served by another MSC/VLR.
5. The HLR sends the subscriber data to the new MSC/VLR
4 .The HLR stores the address of the new MSCIVLR
3. The new MSC/VLR requests the subscriber information for theMS from the HLR
2. In the new MSC/VLR, an analysis of the IMSI number is carriedout. The result of this analysis is a modification of the IMSI to aMobile Global Title (MGT) which is used to address the HLR.
1. The MS requests a location update to be carried out in thenew MSC/VLR. The IMSI is used to identify the MS.
Slide 34
NORMAL LOCATION UPDATE
3. In the VLR, an IMSI detach flag is set for the subscriber. This is used to reject incomingcalls to the MS.
4. The VLR returns an acknowledgment to the MS.
3. The MSC/VLR sets the IMSI attach in the VLR. The mobile is now ready fornormal call handling
2. The MSC/VLR receives the IMSI attach message from the MS.
IMSI attach is the respective to the IMSI detach procedure when turning on the mobile terminal.So it is used by the MS to inform the network that it has re-entered an active state and is still in thesame location area. If the MS changes location area while being switched off, a normal locationupdate takes place.The IMSI attach procedure is as follows :
1. The MS requests a signaling channel.
2. The MS uses this signaling channel to send the IMSI detach message to the MSC/VLR.
1. At power off or when the SIM card is taken out, the MS ask for a signaling channel.
In the system information broadcast on the control channel (BCCH), the MS receivesinformation on whether the IMSI attach/detach function is used or not. If it is used, the MSmust inform the network when it is entering an inactive state (detach)The following steps illustrate the whole procedure:
IMSI detach (switching off the terminal)
IMSI attach (swithing on the terminal)
Slide 35
GSM SPECIFICATIONS
Combination of FDMA and TDMA to send information
Frequencies: 800, 900, 1800, 1900 MHzFor 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
GSM SPECIFICATIONS
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
GSM Band Information
Slide 38
GSM NETWORK AREAS 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
Slide 40
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
Base Station Identity Code
Expressed as nccbcc where:
bcc BS colour code
ncc PLMN colour code
Slide 42
HANDOVER
Four types of handovers: Channels (time slots) in same cell Between cells within same BSC Between BSCs, within same MSC Between MSCs
Slide 43
PSTN
BTS
BTS
BTS
BTS
BTS
BTS
BTS
BSC BSC
MSC
BTS
BTS
BTS
BTS
BTS
BTS
BTS
BSC BSC
MSC
Slide 44
Slide 45
Slide 46
Addressing of managed objects
TS Time Slot RXOTS 0 <= ts <= 7
TX Transmitter RXOTX 0 <= trxc <= 15
RX Receiver RXORX 0 <= trxc <= 15
TRXC Transceiver Controller RXOTRX 0 <= trxc <= 15
DP Digital Path RXODP 0 <= dp <= 1
TF Timing Function RXOTF 0 <= tg <= 511
CF Central Function RXOCF 0 <= tg <= 511
TG Transceiver Group RXOTG 0 <= tg <= 511
MO MO class MO type Addressing Limit
IS Interworking Switch RXOIS 0 <= tg <= 511
Slide 47
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
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
Managed Object Hierarchy
Slide 50
GSM INTERFACES
Um InterfaceMobile station and base station subsystem communicate across Um interface, also known as air interface or radio link
Abis interfaceBase transceiver station (BTS) and base station controller (BSC) communicate across Abis interface
A interfaceBase station subsystem communicates with mobile service switching center across A interface Slide
51
Slide 52
Slide 53
SIGNALING SYSTEM #7
54
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
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
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
SS#7 NETWORKS
Slide 58
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
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
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
COMMON CHANNEL SIGNALING (1)
Slide 62
SwitchSP
SwitchSP
E1 31 voice channels using 31 time slots
2 separate signaling links using 2 time slots
COMMON CHANNEL SIGNALING (2)
Slide 63
SwitchSP
SwitchSP
E1 31 voice channels using 31 time slots
1 signaling link and 30 voice channels
SIGNALING 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 SPSP
STP
Voice trunks
Signaling Links
Associate Mode Quasi-Associate Mode
LINKSETS
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
ROUTES
Virtual path that a message takes to a destination node
Comprised of one or more linksets
Slide 66
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
SS#7 PROTOCOL STACK
Slide 68
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
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
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
SCCPSignaling 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
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
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
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
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
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
ISUP NORMAL CALL SCENARIO
Slide 78
Slide 79
Slide 80
Slide 81
Slide 82
GSM SUBSCRIBER SERVICES
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
SUPPLEMENTARY SERVICES
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