Cellular Mobile Communications-III Introduction to GSM

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Cellular Mobile Communications-III Introduction to GSM

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Title: Cellular Mobile Communications-III Introduction to GSM

1
Cellular Mobile Communications-IIIIntroduction
to GSM

Dr. Nasir D. Gohar

2
AN INTRODUCTION TO GSM

Second Generation Cellular Mobile Radio
Systems-Digital Cellular Telephony
Main Advantages of Digital Cellular Networks
Higher Spectrum Efficiency
Speech Encoding
Better Access Technologies TDMA and CDMA
Higher System Capacity
Aggressive Frequency Reuse
Low Infra-Structure and Terminal Cost
Application of VLSI/VVLSI Chips
Many Traffic Channels per Carrier Channel
Transceivers are shared among several users)
Low S/I Ratio (SIR) Allowing Smarter Equipment
Better Integration with Digital PSTN ISDN
New Services such as Data Communications, Fax,
and SMS
Better Privacy due to Encryption

3
AN INTRODUCTION TO GSM

Digital Cellular Standards
Why So Many Standards?

4
AN INTRODUCTION TO GSM

Digital Cellular Standards

5
AN INTRODUCTION TO GSM

Overview of GSM Milestones
1982 GSM Group, set up by CEPT, started
Investigation to reserve a
Spectrum in 900 MHz band
for Pan-European Cellular Digital
Telephony
1986 Two 25 MHz frequency Blocks 890-915 MHz,
935-960 MHz
reserved by EC, July 01,
1991 set as Introduction date.
Choice of Digital and Narrow band Approx. 200
KHz TDMA made.
1987 13 Operators from 12 European Countries
signed MOU to Support GSM
1989 ETSI established which took over GSM from
CEPT
1992 Introduction of first Commercial GSM Network
1994 51 Commercial Networks Established
1996 More Than 20 Million Subscribers in 191
Networks, More than Double of all
Subscribers of other Digital Systems D-AMPS,
PDC, IS-95 CDMA
2006 More than 2 billion subscribers (30 of
world population and
82 of cellular market)

6
AN INTRODUCTION TO GSM

Implementation Phases of GSM
Phase-1 Most Essential Services Voice, Data,
SMS Implemented. Mostly all
Current GSM Systems belong to this Phase.
Phase-2 Half-Rate Speech Encoding, Doubling the
System Capacity, Several Encryption
Algorithms, Conference Call Facility up to 5
Participants ,
Call Waiting Facility,
Information on Call Charges, Calling or
Called Part etc.,
Cell-Broadcast Point to Multi-Point SMS.
Many of the Phase-2 Facilities have already been
Implemented by some Manufacturers.
Phase-2 GPRSGeneral Packet radio Service for
Packet Switched
Data Transmission to Support
LAN and Internet Traffic.
Considerably Delayed due to
Lack of Interest on part of Network
Operators due to Large
Investment required for Infra-Structure and MS
Modifications
GSM 3G..

7
AN INTRODUCTION TO GSM

2G EVOLUTION TO 3G

8
AN INTRODUCTION TO GSM

GSM EVOLUTION

9
AN INTRODUCTION TO GSM

GSM SYSTEM ARCHITECTURE

MS
SIM Card
EIN/ESN
Base Station Subsystem (BSS)
Several Dozens of BTS under one BSC
Each BTS has 3-5 Carrier Channels
Network Switching Subsystem
MTX up to 1 M users
Call set up , all Other Functions

GMTX An Interface to Other Networks
HLR Keeps Record of Systems Own Users
VLR Keeps Temporary Record of Visitors / Roamers
SID, ST,SVCES
Au C/EIR Manages the Sub. Authentication and
Encryption Data

10
AN INTRODUCTION TO GSM

GSM SYSTEM ARCHITECTURE-2

Management Subsystem Operation Subsystem (OSS)
Directly or Indirectly Connected to all Other
Subsystems
Fault Management
System Configuration
Performance Management
Cost Management
Security Management

11
AN INTRODUCTION TO GSM

GSM SYSTEM ARCHITECTURE-3

GSM Interfaces
Radio Interface Describes Data Interchange
between MS and BSS
Abis Interface Describes Data Communications
between BTS and BSC, allows Various Manufacturers
Equipment
A Interface Describes Data Interchange between
BSS and NSS

12
AN INTRODUCTION TO GSM

LOGICAL CHANNELS

13
AN INTRODUCTION TO GSM

Control Channels
Control channels fall into three categories
Broadcast BCCH, FCCH, SCH
One way, from base to mobile
Common Control RACH, AGCH, PCH
One way, some from base to mobile and some from
mobile to the base
Dedicated SDCCH, SACCG, FACCH
Two-way, stand-alone or embedded in the traffic
channels
All signaling channels share one carrier in a
cell
the dedicated control channels may be transmitted
on traffic carriers

14
AN INTRODUCTION TO GSM

Broadcast Channels
Frequency Correction Channel (FCCH)
Carries information for frequency correction
Synchronization Channel (SCH)
Carries information for frame synchronization and
for
identification of the BTS
Broadcast Control Channel (BCCH)
Broadcasts general information on the BTS
Broadcasts cell-specific information, e.g.
Control channel organization, frequency hopping
sequences, cell identification, etc.

15
AN INTRODUCTION TO GSM

Common Control Channels
Paging Channel (PCH) - downlink only
for paging purposes
Random Access Channel (RACH) - uplink only
used by any MS to request allocation of a
signaling channel (SDCCH)
a slotted Aloha protocol is used, so collisions
among MSs may happen
Access Grant Channel (AGCH) - downlink only
used to allocate a SDCCH or a TCH
Notification Channel (NCH) - downlink only
notify MS of voice group and voice broadcast call
(ASCI feature)

16
AN INTRODUCTION TO GSM

Dedicated Control Channels
Stand Alone Dedicated Control Channel (SDCCH)
used for call setup (authentication, signaling,,
traffic channel assignment), location updates and
SMS
Slow Associated Control Channel (SACCH)
always coupled with a SDCCH or TCH
for communicating measurement data and control
parameters
Fast Associated Control Channel (FACCH)
to respond to increased signaling demand, e.g.
during handover
bandwidth (bit slots) are stolen from the
associated TCH (traffic data are preempted)

17
AN INTRODUCTION TO GSM

Traffic Channels
GSM support two types of traffic channels
full rate (TCH/F) 22.8 kbps
half rate (TCH/H) 11.4 kbps
Mapping to physical channel
full rate traffic channel - 1 timeslot
half rate traffic channel - 1 timeslot in
alternating frames
Full rate channel may carry
13 kbps speech or data at 2.4, 4.8 or 9.6 kbps
Half rate channel may carry
6.5 kbps speech or data at 2.4 or 9.6 kbps

18
AN INTRODUCTION TO GSM

Channel Usage
MS Terminating Calls

19
AN INTRODUCTION TO GSM

GSM Call Setup Procedure
Locating the Subscriber
HLR keeps record of the MSC area which the
Subscriber last registered
VLR keeps record of the Location AreaLA in
which Subscriber last registered
Location Area A Group of Neighboring Cells
having the same LAC
LAC Each Cell in the Area Broadcasts this Code
Each Mobile itself periodically registers itself
with MSC
Paging and Random Access Procedure RAP
GMSC interrogates HLR for MSC the Subscriber last
registered
Call is Switched to that MSC
VLR tells about the LA the user last registered
MSC arranges a Paging in all the Cells in the LA

20
AN INTRODUCTION TO GSM

GSM Call Setup Procedure-2
Paging and Random Access ProcedureRAP-Contd
The Mobiles listen to the Paging Channels for
their Number
If received a Paging Message, the Mobile starts a
RAP
It sends a Random Number RN Brief Guide to
describe the Purpose
The System MSC responds to it by repeating the
RN and providing the decided Channel
The Mobile listens to this RN, if it is the same,
it gets hold of the allocated Channel
Reconfirmation is done by the System by sending
again the Mobile Number in case some other
Mobile has also sent the same RN at the same
time
If the Mobile hears its own number, it responds
to the System and thus it gets connected to the
incoming Call,
If it was a wrong number other than its own
number it must leave that Channel.
In case, a Mobile listens no same RN from the
System, it may repeat RAP at some time later
randomly.

21
AN INTRODUCTION TO GSM

CALL PROCESSING
MOBILE TERMINATING CALLS

22
AN INTRODUCTION TO GSM

CALL PROCESSING
MOBILE ORIGINATING

23
AN INTRODUCTION TO GSM

GSM Handover and Routing
Handover
The Mobile keeps updating MSC about the RSS
levels from the neighboring Cells.
MSC, while looking at the quality of the existing
link, decides whether a Handoff/ Handover is
necessary or not.
If yes, it asks the Candidate Cell to prepare the
Channel GSM uses Fixed Channel Assignment
When Channel is allocated, it tells the Mobile to
move to that Channel
When Mobile has moved, it deactivates the old
Channel
Routing
GMSC interrogates HLR to locate the Subscriber by
telling the MSC where the Mobile last registered.
PROBLEM Call originated from PSTN in CANADA for
a US Mobile roaming in CANADA will result in two
International Calls

TROMBONE PROBLEM ???? Dont Worry, Be Happy,
Other People will Take Care of this Problem
24
AN INTRODUCTION TO GSM

HANDOVER TYPES

Intra-BSC
Old and new BTSs are
controlled by the same BSC
The MSC is not involved
Intra-MSC
Old and new BTSs are
attached to different BSCs
The BSCs are attached to the
same MSC
Inter-MSC
Handover to a new MSC
Serving MSC becomes anchor
MSC
IMT (Inter Machine Trunk) is
required

25
AN INTRODUCTION TO GSM

GSM Numbering Scheme and Spectrum Efficiency
GSM Numbering Scheme
Each GSM Mobile has at least three Numbers
MSISDN Mobile Station Integrated Services
Digital Network stored in SIM Card
SIM Card holds Subscriber IDMSISDN, IMSI, some
Extra Memory to store phone numbers and
Encryption Algorithms
IMSIInternational Mobile Subscriber Identity
not known to the User, HLR does translation
between MSISDN to IMSI and vice versa.
ESN or IMEIPermanently stored/wired in the
Mobile Station
Spectrum Efficiency
TDMA, 200 KHz Channel BW, 8-Time Slots per
Carrier Channel
Radio Spectrum 2 x 25 MHz bands can support
125124 Duplex Carrier Channels and 1000 Traffic
Channels 1000 Simultaneous Calls.
A typical GSM System Cluster Size 12, so each
Cell have Approx. 10 Carrier Channels and a
Capacity of 80 Simultaneous Call Traffic
Channels
In case of half-rate Coder, Spectrum Efficiency
will be Doubled.

26
AN INTRODUCTION TO GSM

Technical Realization of GSM
GSM Speech Communications
User Speech is Digitized using Adaptive PCM
Speech Encoding using Linear Predictive Coding
LPC
User data flow 13 kbps 6.5 kbps in case of
half-rate coding
Adding an overhead of 9.8 kbpsfor error
detection, error correction, and
synchronization, we get an over all user data
flow of 22.8 kbps.
This user data flow is subdivided into short data
blocks each of 456 bits, which is divided into 8
sub-blocks, each of 57 bits, 2 such sub-blocks
are used to make a burst to fit into a time
slot of 0.58 ms. See frame structure.
These bursts from a user are interleaved over 8
time slots spread over 8 frames.
TDMA/FDMA Mechanism
Each Carrier Channels carries data burst of eight
(8) users, each in its allocated time slot in the
frame.
Time slots are transmitted on several Channels
Carrier Channel or Frequency
Each Carrier Channel transmits the data of 8
users at 271 kbps.

27
AN INTRODUCTION TO GSM

Technical Realization of GSM Contd
GSM Speech Communications-2
GSM Carrier Frequencies are numbered 1-124
ChannelsDuplex
Forward Channel 935.2 MHz (n-1) 200 KHz
Reverse Channel 890.2 MHz (n-1) 200 KHz
Due to slightly more than 200 KHz bandwidth of
the modulated signal, Consecutive Frequencies are
not used in the same System
Also, 1 and 124 Channels are generally not used.
The Spectrum of 124 Channels is generally not
allotted to one Service Provider.
Separation between two Duplex paired Channels is
45 MHz.
Each users bursts are transmitted every 4.6ms
apart.

28
AN INTRODUCTION TO GSM

SPEECH PROCESSING

29
AN INTRODUCTION TO GSM

SPEECH CODING

SPEECH ENCODER

30
AN INTRODUCTION TO GSM

SPEECH DATA PROCESSING

31
AN INTRODUCTION TO GSM

CHANNEL ENCODING

32
AN INTRODUCTION TO GSM

INTERLEAVING

33
AN INTRODUCTION TO GSM

VOICE TRANSMISSION PATH

34
AN INTRODUCTION TO GSM

Technical Realization of GSM Contd
GSM Framing Structure

35
AN INTRODUCTION TO GSM

TIME-SLOT STRUCTURE

36
AN INTRODUCTION TO GSM

FRAME HIERARCHY

37
AN INTRODUCTION TO GSM

BURST

38
AN INTRODUCTION TO GSM

TYPES OF BURST

Five different types of bursts
Normal burst
Traffic and control payload
Frequency correction burst
All zeroes sequence
Synchronization burst
A special fixed sequence
Random access burst
Extended guard period of 68.25 bitts (252 µs)
Dummy burst

39
AN INTRODUCTION TO GSM

BURST STRUCTURES

40
AN INTRODUCTION TO GSM

Technical Realization of GSM Contd
GSM Modulation Scheme
Gaussian Minimum Shift Keying GMSK (For Detail
see Ch-5 of the Text Book)
No Speech, No Transmission
Saves Energy in MS, Reduces Ave. Interference
Comfort Noise added for the Listeners Pleasure.
GSM MS Power Classes
Class 1 20 W Not (yet) Available
Class 2 8 W Car Phone
Class 3 5 W -
Class 4 2 W Normal Pocket Phone
Class 5 0.8 W Limited Coverage Phone Urban
Areas Only
Maximum Cell Size Depends on Max. Permitted
Delay, 35 km.
Maximum Mobile Speed 250 km/hr

41
AN INTRODUCTION TO GSM

Technical Realization of GSM Contd
GSM SMS
Individual Messages
up to 160 Characters long can be sent and
received by MS
SMS uses Signaling Channel, thus, it can be
received during current Communication Session
Max. rate 600 bps
Cell Broadcast
up to 93 Character long message can be sent to
all users in a given area Cells.
Uses Communication Channel, so, cant be received
during current Communication Session
Neither Addressed Nor Encrypted
Applications of SMS .Network Operator Messages,
Third Party Messages, Public Interest Information
Messages
SMS Cost Differs from Operator to Operator and
depends on type of Application

42
AN INTRODUCTION TO GSM

Technical Realization of GSM Contd
GSM Security Aspects
GSM provides security about the identity of MS
Guards against Eavesdropping
Implementation
User Authentication A3 Algorithm 128 bit
Secret Number, Ki, assigned to each User, stored
in SIM Card as well as AuC, is used to create
SRES using Ki and 128 bit RAND number from the
network, if SRES of MS matches with SRES of the
network, the MS is accepted.

43
AN INTRODUCTION TO GSM

Data Encryption A8A5 Algorithm
Frame (RANDKi(Alg-A8))(Alg-A5) -gt 144 bit
Code Train ? 144 bit user Data Train -gt Network
? Frame (RANDKi(Alg-A8))(Alg-A5) -gtOriginal
Message

44
AN INTRODUCTION TO GSM

Technical Realization of GSM Contd
GSM Inter-Connection with Other Networks
PSTN/ISDN for Voice
GSM Speech data is Digital and Compressed
Converted into audio of 3.1 KHz BW
PSTN/ISDN for Data
For Data Interconnection, Modem Pools are used in
GSM Network. Many Telephone Modems at GMSC/IWF
supporting all the important telephone modem
standards V.210.3 kbps, AS, V.221.2 kbps, AS
and S, V.32 4.8 kbps, S etc.
PSDN (Basic PAD or Dedicated PAD) for Data
300 bps to 9600 bps on AS/S links

45
NEW DEVELOPMENTS IN GSM

Frequency Shortage and Extended GSM Band
In some European countries, up to half GSM band
is occupied by Interim Systems NMT, ETACS etc.
Additional frequencies adjacent to GSM band is
recommended by ERO1996, new terminals for this
extended band are not available in bulk
New Encoding Techniques
Half-Rate Speech Encoding
Instead of 13 kbps only 6.5 kbps will be required
for the same speech quality
Standardized in early 1995, but, only few
manufacturers have implemented
lack of interest on part of system suppliers
fear of investment decrease in network expansion
uCell
Enhanced Full Rate EFRDeveloped by GSM and
DCS-1800 Suppliers
Same 13 kbps speed, but, considerable improvement
in speech quality
More Immunity to transmission errors

46
NEW DEVELOPMENTS IN GSM

Any Cellular Phone may have many Speech Coders,
including FR/HR/EFR, it must be able to switch,
and at least support FR for roaming purpose.
Data Communications over GSM
General Packet Radio ServiceGPRS offers
packet-switched data communications suitable for
LAN and Internet ApplicationsPVC
Requires GPRS enabled Handsets and Changes at
BSSaddition of Packet Control Unit, PCU
Option of Upgrading of BTS to support Enhanced
Data rate for GSM Evolution, EDGE 8-PSK system
Multi-band Terminals
Cellular phones to support more than frequency
bands GSM and DSC-1800 or GSM and PCS-1900 or
GSM/DCS/PCS or many more combinations
GSM Moving into 3G Mode
GSM extends itself into 3G as Universal Mobile
Telecomm. Services UMTS
Radio Interface will use WCDMA technology UMTS
Terrestrial Radio Access, UTRA in two different
modes FDD two different frequencies for uplink
and downlink or TDDsame frequency for both
uplink and downlink but time-shared

47
The Market Share of GSM Suppliers 1/97

Networking Switching Subsystems NSS
Ericsson 48 , Siemens 21 , Nokia 14 ,
and Alcatel 10 93
Others (Lucent, Motorola, Nortel, etc 7
Basestation Sub-System BSS
Nokia 22, Motorola 13, Alcatel 10 and
Ericsson 7 52
Others Italtel, Lucent, Matra, Philips etc
48
GSM / DCS-1800 / PCS-1900 Mobile Terminals
Ericsson 25 , Nokia 24, MOTOROLA 20 ,
SIEMENS 9 78
Others Alcatel, Panasonic, Nortel, etc. 22
Due to Licensing Problems, Unfair and Restricted
Competition in GSM Markets

48
GSM and Health Problems

GSM Terminals Cause Interference with
Hearing Aid Devices and
Pacemakers Instruments used to regulate the
heart functionality of Heart Patients, and
Some Research show that GSM phones cause Brain
Tumors
Inherent in GSM TDMA setup to generate strongly
pulsating transmission signals Continuously Tx
is switched on and off that generates LF signals
217, 434, 651 Hz
Extensive Research required to prove the
validity of GSM Cellular Phones being the cause
of Brain Tumors or Cancer.

49
GSM Derivative Systems DCS-1800 PCS-1900

Digital Cellular SystemDCS-1800
Originally started in 1990 as a separate
system, but, later on became just a GSM variant
Main modifications were made only in Air
Interface
Developed by ETSI particularly for densely
populated urban areas
1.7 - 2.3 GHZ band 2 x 75 MHz spectrum, 1710-
1785 1805-1888 MHz
Duplex separation is 95 MHz, Channel BW is 200
KHz, 374 duplex channels
Much smaller cells cells within a building,
lower power BTS and MS as compared to GSM
handoff problems are much cleverly settled using
hierarchical Cell structure
Max Cell size 8 km with Class 1 MS 1W
Max Cell size with Class II0.25W even smaller
0.5 -4/5 km
International as well as National roaming is
possible Country to Country, Network to Network,
and Intra-Network
Half-rate speech coding is possible/EFR is more
likely to prevail.