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Presentation Objectives
Introduction to GSM
Learning about GSM Network Architecture
Network components
Radio Interface
Cell structure
Frequency re-use and tri-sector antenna
Learning about GSM Operational Principles
Call delivery
Location Updating
Speech and Data transmission
Understanding of GSM Localization
Recognizing the Services provided by GSM
Understanding of GSM Security Issues
Conclusions
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Introduction to GSM
The most popular 2G cellular standard developed to cater
voice services and data delivery using digital
modulation.
Development background:
Developed by Grou pe Spéci ale Mob il e (founded
1982) which was an initiative of CEPT (Conference
of European Post and Telecommunication).
In 1989, ETSI (European Telecommunications
Standards Institute) took the control of GSM and
defined new acronym “Global System for Mobile
Communications.”
Commercial use has been started since 1991.
Features of GSM Standards:
Supports international roaming and
handheld terminals.
Good subjective speech quality and
wide-range of new services.
Low cost (?).
Compatible with other systems, say
ISDN and PSTN.
In March 2005, Around 1.3 billions subscribers in more than 135 countries(more than 70% world market) used GSM. Among them 43% users are
from Europe and 40% users are from Asia specific region.
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Network Components: GSM Architecture
Base TransceiverSystem (BTS)
Base StationController (BSC)
Base Station (BS)
VLR HLR
EIR AuC
Mobile Switching Centre
(MSC)
PSTN
Other
GSM
Mobile
Terminal (MT)
Wireless Connection
P2P Wireless or optic fiber connection
There are three main components of GSMnetworks:
Mobile Terminal (MT)
Mobile equipment for transmitting and receiving signals.
Subscriber Identity Module (SIM) for storing necessarypermanent and temporary data.
Base Station (BS)
Base Transceiver System (BTS) for
transmitting and receiving signals; and
manipulating signals such as encoding/decoding,encrypting, multiplexing and modulating.
Base station Controller (BSC) for assigning and managing resources, controlling handoff and power level, etc.
Mobile Switching Centre (MSC)
Manage communications, mobility, and billinginformation.
Home Location Register (HLR): central master database for users under the MSC.
Visitor location Register (VLR): Local database for the users currently under the domain of MSC.
Authentication Center (AuC): Authenticate mobile terminals and encrypt user data.
Equipment Identity Register (EIR): register MTs and locked stolen or malfunctioning MTs.
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Radio Interface: GSM Architecture
Most GSM Networks operate at 900 MHz and/or 1800MHz,
850 MHz and/or 1900MHz in Parts of Americas (including USA and Canada).
900 MHz Frequency Band
Uplink
Downlink
TDMA Frame:
Each frame contains 8 time slots.
Channel data rate: 270.833 kbps
Frame duration: 4.615 mS
There are two types of logicalchannels:
Traffic channels
Signaling Channels – for broadcasting,common control, and dedicated control.124 carriers
890MHz 915MHz 935MHz 960MHz
Downlink Uplink
124 carriers
Each Carrier has 200KHz Frequency band
TDMA divides each RF channel (i.e., carrier)to 8 voice Channel
t1 t2 t3 t4 t5 t6 t7 t8
Time-slots
Time slot duration 577 µs
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Cell Structure: GSM Architecture
Cell is a area covered by one BS.
GSM Network Areas
Public Land Mobile Network(one operator’s network)
MSC/VLR Service Area(Covered by one MSC)
Location Area
(one MSC covers severallocation areas)
Cell(Covered by one BS)
Cell Representation
Depending on the sizes, there are four types of cells:
Macro-cell: 5-35 km radius. For rural or suburban areas. Micro-cell: 1-5 km radius. Used in town or urban areas.
Pico-cell: radius <100 meters. For a campus area.
Umbrella-cell: Fill-up the gaps between two cells or covershadow areas of a cell.
Macro-cell Micro-cellUrban
Pico-cell
Suburban
In buildingcommunications
Cell radius varies depending on antenna height, antenna gain, and condition of propagation.
GSM supports the longest distance up to 35 km,
because, Timing limitations: Longer distance takes more propagation
time. So, utilization decreases with the increase is guard timebetween two time-slots.
Interference: More noises are added and signal is attenuatedwith the increase in distance, requiring more power level.
Limiting User: Longer cell radius reduces number of users.Ideal cells
R
Fictitious cells
R
R = Radius of cell
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Frequency re-use: GSM Architecture
For the purpose of filtering a given frequency used by a cell, a distance should bemaintained between two cells using same frequency.
For reusing frequency, cells are divided into clusters (each having K cells) so thata frequency band is used by only one cell of a cluster.
Tri-Sector Antenna
Used for 1 cell:
Cell SplittingUsed for 3 cells
Impacts of reducing cell-radius or
splitting cells: Increase cellular systems capacity.
Decrease transmission power in BS andMT
The Cell Structure for K = 7
1
2
3
4
5
6
7
1
2
3
4
5
6
7
1
2
3
4
5
6
7
4.5873
3Distance,
R R
R K D
D
R = Radius of the cell.
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Call delivery: GSM Operational Principles
MT BS1. Call Request
10. Reply
MSC
2 . C al l R e q u e s t
9 . R e p l y
HLR
VLR
3. LocationRequest
MSC
HLR
VLR
4. RouteRequest
4. RouteRequest5. Route
Request
7. Forwardinformation
8. Send Requestto called MSC
TLDN: Temporary localdirectory number.
1-2. Send call request from calling MT to MSC
via BS.3. MSC determines the address of HLR of
the called MT and sends locationrequest message.
Call delivery Steps:
4-5. HLR determines the serving VLR of thecalled MT and send route request message. VLR then send the message
to the MSC serving the MT.6. MSC allocates a TLDN to the MT
and reply to HLR with TLDN.
7. HLR forward information to theMSC of the calling MT.
8. Calling MSC requests a call
set up to the called MSCthrough SS7 networks.
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Location Updating: GSM Operational Principles
MT BS1. Location Update
MSC
2 .L o c a t i on
U p d a t e
HLR
VLR
Location Updating Steps:
VLR
If VLR Changes.
3. Registrationquery
old
4 .L o c a t i on
R e gi s t r a t i on
5 . R e g i s t r a t i o n
A c
k n o w l e d g e m e n t
6. RegistrationCancellation
7. CancellationAcknowledgement
1-2: When the MT enters a new LA, Location updatemessage is sent to MSC via BS.
3: MSC launches a registration query to VLR andVLR updates the records.
4: If new LA belongs to a different VLR, the new VLR
determines the address of HLR of the MT frommobile identification number (MIN).
sends location registration message to HLR.
5: HLR authenticates the MT and records informationof new VLR of the MT.
6-7: HLR sends a registration cancellation message toold VLR. Old VLR then remove records andreturns a cancellation acknowledgementmessage to the HLR.
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Speech and Data Transmission: GSM Operation
Input SpeechSpeechencoder
Dataencryption
DataModulator
10101 10101
Speech
decoder
GSM to
PSTN
PSTN to
GSM
Speech
encoder
Data
demodulator
Data
decryption
Speech
Decoder
10101 10101Output Speech
Add-on module in standard GSM handset
Add-on module in standard GSM handset
Base station subsystemBase station subsystem64-bit PCMwaveform
GSM uses Gaussian-Filtered Minimum shift Keying (GMSK) Modulation.
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GSM Localization
GSM localization is the use of GSM
mobile phones to determine the locationof the user. Location of an MT can be determined using Time
of Arrival (TOA), Time difference of Arrival
(TDOA), and Angle of Arrival (AOA) measures.
Location measurement from TOA
R1
R2
R3 BS1
BS2
BS3
1. TOAs t1, t2, t3are measuredfrom signalstrength.
2. measure base station distances as R1= ct1, R2= ct2,and R3=ct3, where c is light speed.
3. Circles are formed with radius R1, R2, and R3.
4. Intersection point is the position of MT.
Location measurement from TDOA
R1
R2
R3
1. measure |R2-R1| = c(t2-t1) and |R3-R1| = c(t3-t1)and plot them.
2. Two hyperbolas will be found. Position of MT isthe crossing point of hyperbolas at R1 distancefrom BS1.
BS1
BS2
BS3
Location measurement from AOA
θ 1 θ 2
θ 1 θ 2 1. Angles can be measured
by MT and sent to therespective BS.
2. Crossing point of tangentbrown at BSs is theposition of the MT.
However, GPS is the most popular way ofgeolocating an MT precisely within 50 meters.
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GSM Services
GSM network Provides three types of services:
Tele Services: Normal voice calls.
Voice conversation
Emergency services
Bearer Services: data services.
Short message services (SMS), Videotex, teletex, and advanced messagehandling services.
Supplementary Services:
call forwarding and call barring,
Calling/connected line identification presentation and restriction
Malicious call identification
Multi-party services (i.e., tele-conferencing), etc.
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GSM Security IssuesGSM provides security in three areas:
Data and signaling confidentiality
Authentication of a user
Confidentiality of a user
Data and signaling confidentiality:transmit Cipher-text
A5/1, A5/2, and A5/3 ciphering algorithmsare used.
A8 algorithm is used to generate key forciphering algorithms.
User confidentiality is maintain by protecting SIM using a PIN and PUK (PIN unlock)code. If invalid PIN code is entered for a given time, PUK code should be collectedfrom the operator. Using invalid PUK several times damage the SIM permanently.
Authentication Procedure: when a connection is attempted.
Network MT SIM1. Identity (initial message) TMSI
2. Authentication Request (RAND) 3. Run GSM (A3) Algorithm (RAND)
4. Response (SRES)5. Authentication Response (SRES)
Network compares the SRES with its own SRES. If Authentication fails networkmay choose to repeat with International Mobile Subscriber Identity (IMSI).
TMSI: Temporary mobile subscriber identity SRES: Signed Response. SIM: Subscriber Identity Module.
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Conclusions
GSM extended circuit-switching through introducing TDMA and Digital
Technology. GSM introduces multimedia services for the first time.
Digital services of GSM networks enable tremendous growth of mobilecommunications.
People are now expecting wireless worldwide connectivity of their laptops and powerfulhandheld devices.
GSM provides moderate level of security.
Safe for general personal use, but not secure for top organizations like Government spyagencies.
Capacity is the main limitation of GSM networks. GSM cannot support thetoday’s demand of wireless multimedia services.
2.5G General Packet Radio Service (GPRS) and 3G CDMA2000 are now being used in someareas to get more capacity.
Indeed !
GSM makes the world a global village !
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