Gsm

Telecom Tutorials
Friday, June 21, 2013www.tempustelcosys.com
GSM Fundamentals & RF Survey

Basic Telecom concepts
Various Wireless Technologies
Cellular concepts & Principal of cellular Comm.
GSM Network Architecture
GSM channel Architecture
Call Flows in GSM
GSM Planning steps (Nominal Plan & RF surveys)

•Signaling
•Traffic
•Off Hook
•Dial Tone
•Dialing Digits
•RBT
•Conversation
SWITCH / EXCHANGE
•Ring
•Off Hook &
Conversation

Alternative means of wireless communication
Walkie - Talkie
Pagers
Trunked private radios
Mobile Phone - the magic technology that enables
everyone to communicate anywhere with anybody.

BTS BTS
MSC

Till 1982 Cellular Systems were exclusively Analog Radio Technology.
Advanced Mobile Phone Service (AMPS)
U.S. standard on the 800 MHz Band
Total Access Communication System (TACS)
U.K. standard on 900 MHz band
Nordic Mobile Telephone System (NMT)
Scandinavian standard on the 450 & 900 MHz band

GSM - 900
The term GSM-900 is used for any GSM system which operates in any 900 MHz band.
P-GSM - 900
P-GSM-900 band is the primary band for GSM-900 Frequency band for primary GSM-900 (P-GSM-900) : 2 x 25
MHz
890 – 915 MHz for MS to BTS (uplink)
935 – 960 MHz for BTS to MS (downlink)
E-GSM - 900
In some countries, GSM-900 is allowed to operate in part or in all of the following extension band. E-GSM-900
(Extended GSM-900) band includes the primary band (P-GSM-900) and the extension band :
R-GSM-900
R-GSM-900 (Railway GSM-900) band includes the primary band (P-GSM-900) and the following extension band:
GSM-1800
Frequency band: 2 x 75 MHz
1710 – 1785 MHz for MS to BTs (uplink)

Telecom Service Providers/Operators
GSM
CDMA
Basic-WLL
Internet Services
Long Distance
Vendor
Telecom Consultancy

End of 1980‟s Analog Systems unable to meet continuing demands
Severely confined spectrum allocations
Interference in multipath fading environment
Incompatibility among various analog systems
Inability to substantially reduce the cost of mobile terminals and
infrastructure required

Spectrum space - most limited and precious resource
Solution - further multiplex traffic (time domain)
Can be realized with Digital Techniques only

1979 Europe wide frequency band reserved
for Cellular
1982 “Groupe Speciale Mobile” created within CEPT
1986 GSM had full time in Paris
1988 ETSI takes over GSM Committee
1990 The phase 1 GSM Recommendations
frozen
1991 GSM Committee renamed “Special Mobile
Group” and GSM renamed as “Global
System for Mobile Communication”
1992 GSM launched for commercial
operations

A cellular system links Mobile subscribers to Public
Telephone System or to another Mobile subscribers.
It removes the fixed wiring used in a traditional
telephone installation.
Mobile subscriber is able to move around, perhaps can
travel
in a vehicle or on foot & still make & receive call.

CONSIDERATIONS -
 FREQUENCY
 SUBSCRIBER DENSITY
 COVERAGE
Base Station
Base Station
Base Station
Base Station
Base Station
Base Station

 Cellular Radio involves dividing a large service area into regions called
“cells.”
 Each cell has the equipment to switch, transmit and receive calls.
 Cells - Reduce the need of High powered transmission
 Cells - Conventionally regarded as being hexagonal, but in reality they are
irregularly shaped.
 Cell shape is determined by the nature of the surrounding
area e.g. Hills , tall building etc.

Large Cells
35 Km
Remote Areas
High Transmission Power
Few subscribers
 Small Cells
 Near about 1 KM
 Urban Areas
 Low Transmission Power
 Many Subscribers

 Coverage
› Percentage of the geographical area covered by
cellular service where mobile telephony is
available
 Capacity -
› Number of calls that can be handled in a certain
area within a certain period of time.
› Capacity can also refer to the probability that
users will be denied access to a system due to
the simple unavailability of radio channels.

 Each Cell in the Cellular Network consists of one or more RF
carriers.
 An RF carrier is a pair of radio frequencies
› One used in upward direction by MS - Uplink
› Other used in downward direction by BTS - Downlink
› The transmit and receive frequencies are separated by a
gap of 45 MHz in GSM of 75 MHz in DCS.
 There are 124 carries in GSM Band. With each carrier carrying
7 timeslots, only 124 x 7 = 868 calls can be made!
 Frequency Reuse is the solution

Ful(n) = 890.0 + (0.2) *n MHz
Fdl(n) = Ful + 45 MHz
where n =ARFCN ; 1  n  124

1710 1785 1805 1880
duplex distance: 95 MHz
890 915 935 960
duplex distance: 45 MHz

Power
TDMA
Power
FDMA
Power
CDMA
FDMA: AMPS & NAMPS
•Each user occupies a private Frequency,
protected from interference through physical
separation from other users on the same
frequency
•TDMA: IS-136, GSM
•Each user occupies a specific frequency but
only during an assigned time slot. The
frequency is used by other users during
other time slots.
•CDMA
•Each user occupies a signal on a particular
frequency simultaneously with many other
users, but is uniquely distinguishable by
correlation with a special code used only by
this user

Three types of frequency reuse patterns
7 Cell reuse pattern
4 cell reuse pattern
3 cell reuse pattern

c2
c1
c3
a1
a3
b1
b2
b3
Cell Re-use

2
4
5
6
7
D

 Omni Directional Cells
 120 degree Sectors
 60 Degree sectors
› Each Sector in a Site has its own allocation of Radio
Carriers
 Advantage
› By frequent reuse of frequency more capacity can be
achieved

OMNI CELL
1 ANTENNA
b1
b2
b3
120O CELLS
3 ANTENNAS
60O CELLS
6 ANTENNAS
a1 a3
a4a6
a5
a2

Hard Handoff
Analog, TDMA and GSM
Soft Handoff
CDMA
Break before Make Make before Break

Compatibility
Noise Robust
Increased Capacity & Flexibility
Use of Standard Open Interfaces
Improved Security & Confidentiality
Cleaner Handovers
Subscriber Identification
ISDN Compatibility
Enhanced Range of Services

MSISDN : Human Identity used to call a Mobile Station
IMEI: Serial number unique to every Mobile Station
IMSI : Network Identity unique to a SIM
3
digits
2
digits
10
digits TMSI : Identity unique in a LAI
MSRN : Mobile Station Roaming
NoCC NDC SN
98 XXX 12345
MCC MNC MSIN
404 XX 12345
TAC FAC SNR S
6 digits 2 digits 6 digits 1 digit

BT
S
BT
S
BT
S
BT
S
BT
S
BT
S
BT
S
BT
S
BSC
VL
R
HL
R EI
R
OM
C
SMS
C
B
C
AU
C
VMS
C
MS
C
A
MS
BTS
BTS
BTS
BTS BTS
BTS
BTS
BTS
BSC
BSC
PSTN
VLR
TRAU
HLR
EIR
OMC
SMSC
BC
AUC
VMSC
MSC
A
OML

Mobile Station consists of two parts-
Mobile Equipment (ME)
Subscriber Identity Module (SIM)
ME
Hardware e.g. Telephone, Fax Machine, Computer.
SIM
Smart Card which plugs into the ME.

Class Power O/p
1 20 W
2 8 W
3 5 W
4 2 W
5 0.8 W
Typical
Settings

IMSI(International Mobile Subscriber Identity)
Transmitted over Air Interface on initialization
Permanently stored on SIM card
15 digit Decimal

MSISDN
10 digit number to which a subscriber is being called.
PIN (Personal Identification Number)
Four digit PIN
An internal security to Protect the SIM from illegal use.
Card blocks itself after three wrong entries

Temporary Mobile Subscriber Identity
Periodically changed by the System Management on
instances like location update etc.
Reason for use of TMSI
To prevent a possible intruder from identifying GSM users,
TMSI is used
Management
Assignment, Administration & Updating is performed by VLR.

BSS (Base Station System)
BSC (Base Site Controller)
BTS (Base Transceiver Station)
XCDR (Transcoder)
Network
Switching
System
(NSS)
XCDR
BSC
BTS

Converts 64 Kbps PCM circuits from MSC to 16 Kbps BSS circuits.
Each 30 channel 2 Mbps PCM link can carry 120 GSM - specified voice
channels.

BSC
Conveys information to/from BTS
Connects terrestrial circuits & Air Interface Channels
Controls handovers between BTSs under itself
BTS
Contains RF Hardware
Limited control functionality
1 - 6 carriers in a BTS Cabinet
7 - 48 simultaneous calls per BTS

Collocated BTS
Remote BTS
Star Configuration
Daisy Chain BTS Loop Configuration
BSC
BTS
BTS
BTS
AllBTSon1E1
BSC
BTS
B
T
S
BTS
BTS

NSS (Network Switching System)
MSC (Mobile Switching Centre)
HLR (Home Location Register)
VLR (Visitor Location Register)
EIR (Equipment Identity Register)
AUC (Authentication Centre)
IWF (Interworking Function)
EC (Echo Canceller)

MSC
Call Switching
Operation & Management Support
Internetwork Interworking
Collects call billing data
Gateway MSC
MSC which provides interface between PSTN & BSS‟s in the GSM Network.

Reference database for the Subscriber profiles-
Subscriber ID (IMSI & MSISDN)
Current VLR Address
Supplementary Services subscribed
Supplementary Service Information
Subscriber Status (Registered/deregistered)
Authentication Key and AUC functionality
TMSI
MSRN

Temporary Data, which exists as long as the subscriber is
active in a particular Coverage area.
Contains the following-
Mobile Status (Busy/ Free/ No Answer/etc.)
Location Area Identity (LAI)
TMSI
MSRN (Mobile Station Roaming Number)

Um MS - BTS
Abis BTS - BSC
A BSC - MSC
B MSC - VLR
C MSC - HLR
D VLR - HLR
E MSC - MSC
F MSC - EIR
G VLR - VLR
H HLR - AUC

AUTHENTICATION
CIPHERING
REGISTRATION
CALL ESTABLISHMENT
HANDOVER / HANDOFF
ROAMING

Access Network
Microwave 15 /23 GHz
Backbone Network
Microwave 7 GHz
Optical Fibers
Leased Line( From Dot or any other service provider on any media)

Physical Channel
Logical Channel
Physical Channel
Physical channel is the medium over which the information is
carried.
Logical Channel
Logical channels consists of the information carried over the
Physical Channel.

0 1 2 3 4 5 6 7
3
57
encrypted
57
encrypted
26
training
1
S
1
S
3
T
8.25
GP
3
T
577S
TDMA Frame

DCCH(Dedicated Channels)
Downlink & Uplink
CCCH(Common Control Chan)
Downlink & Uplink
Synch.
Channels
RACH
Random
Access Channel
CBCH
Cell Broadcast
Channel
SDCCH
Standalone
dedicated
control channel
ACCH
Associated
Control Channels
SACCH
Slow associated
Control Channel
FACCH
Fast Associated
Control Channel
PCH/
AGCH
Paging/Access grant
FCCH
Frequency
Correction channel
Control Channels
BCH ( Broadcast channels )
Downlink only
BCCH
Broadcast
control channel
SCH
Synchronization
channel

BCH channels are all downlink and are allocated to timeslot zero.
Channels are:
• FCCH: Frequency control channel sends the mobile a burst of all „0‟
bits which allows it to fine tune to the downlink frequency
• SCH: Synchronization channel sends the absolute value of the frame
number (FN), which is the internal clock of the BTS, together with the Base
Station Identity Code (BSIC)
• BCCH: Broadcast Control Channel sends radio resource
management and control messages, Location Area Code and so on.
Some messages go to all mobiles, others just to those that are in the
idle state

•CCCH contains all point to multi-point downlink channels (BTS to
several MSs) and the uplink Random Access Channel:
• CBCH: Cell Broadcast Channel is an optional channel for general
information such as road traffic reports sent in the form of SMS
• PCH: Paging Channel sends paging signal to inform mobile of a call
• RACH: Random Access Channel is sent by the MS to request a
channel from the BTS or accept a handover to another BTS.
A channel request is sent in response to a PCH message.
• AGCH: Access Grant Channel allocates a dedicated channel
(SDCCH) to the mobile
• NCH: Notification Channel informs MS about incoming group or
broadcast calls

SDCCH( Standalone Dedicated Control Channel )
Uplink and Downlink
Used for call setup, location update and SMS.
SACCH( Slow Associated Control Channel )
Used on Uplink and Downlink only in dedicated mode.
Uplink SACCH messages - Measurement reports.
Downlink SACCH messages - control info.
FACCH( Fast Associated Control Channel )
Uplink and Downlink.
Associated with TCH only.

The Time Slots are arranged in a sequence , conventionally numbered 0
to 7.
Each repetition of this sequence is called a TDMA Frame.
The information content carried in one time slot is called a “burst”.

0 1 2 3 4 5 6 7 2 4 5 6 730 1
FRAME 1 FRAME 2
Training SequenceInformation Information
GUARD
PERIOD
GUARD
PERIOD
TAIL BITS TAIL BITS

0 7
TDMA frame = 4.615 ms
= BURST PERIOD
0

1
3
CHANNEL REQUEST
DCCH ASSIGN
SIGNALLING LINK
ESTABLISHED
REQUEST FOR SERVICE
SET CIPHER MODE
SET-UP
EQUIPMENT ID
REQUEST
AUTHENTICATION
MS BSS MSC VLR HLR PSTNEIR
RACH
AGCH
SDCCH
SDCCH
Call
Info7
4
6
5
2
CR
CC

8
COMPLELTE CALL
CALL PROCEEDING
9 ASSIGNMENT COMMAND
INITIAL & FINAL
ADDRESS (IFAM)
ASSIGNMENT COMPLETE
(ACM)
10
ANSWER(ANS)
11
CONNECT ACKNOWLEDGE
SDCCH
SDCCH
ASSIGNMENT COMPLELTE
MS HEARS RINGTONE
FROM LAND PHONE
ALTERING
RING TONE
STOPS
CONNECT
(channel)
(TCH)
FACCH
FACCH
FACCH
TCH
(circuit)
FAACH
BILLING STARTS
Hello!
MS BSS MS
C
VLR HLR PSTN EIR

Various steps are listed below
CW survey
Model Tuning
Nominal Planning
RF site Surveys
Realized Planning
Frequency Planning
Implementation
Optimization
Drive Testing
Performance Analysis

It consists of planning a set of sites on planning tool so as to predict the
coverage of the target area
Tool needs to be made intelligent so as to predict the coverage as close as
possible to actual coverage
Coverage plots are based on customer intension of providing indoor and
outdoor coverage

Clutter types
Dense Urban
Urban
Sub Urban
Rural
Water
Vegetation
Industrial
Forest

Each nominal has a search ring defined by the RF Planner
Candidates needs to be identified as close as possible to the
nominal within the search ring
Height, orientations & antenna placement at site are the key RF
parameter which are based upon the coverage requirement in the
area
Major obstructions and clutter type in various directions to be
observed on RF survey

Equipment required for RF Survey
GPS
Digital Camera
Binoculars
Magnetic Compass
Measuring Tape
There might be 3 or more candidates surveys for one
site
Each candidate would have an RF survey form and
panoramic associated with it

Gsm

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