gsm

Tempus Telcosys (P) Limited

Telecom Tutorials

Wednesday, April 12, 2023www.tempustelcosys.com

GSM Fundamentals & RF Survey

What you will learn here?

Basic Telecom conceptsVarious Wireless TechnologiesCellular concepts & Principal of cellular Comm.GSM Network ArchitectureGSM channel ArchitectureCall Flows in GSMGSM Planning steps (Nominal Plan & RF surveys)


BASIC Telephony


• Signaling• Traffic

• Off Hook

• Dial Tone

• Dialing Digits

• RBT

• Conversation

SWITCH / EXCHANGE

• Ring

• Off Hook & Conversation

Wireless Communication

Alternative means of wireless communicationWalkie - TalkiePagersTrunked private radios

Mobile Phone - the magic technology that enables everyone to communicate anywhere with anybody.


Wireless Telephony


BTS BTS

MSC

Different Standards WorldwideTill 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


Different Standards WorldwideGSM - 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 :

880 – 890 MHz for MS to BTS (uplink)

925 – 935 MHz for BTS to MS (downlink)R-GSM-900

R-GSM-900 (Railway GSM-900) band includes the primary band (P-GSM-900) and the following extension band:

876 – 890 MHz for MS to BTS (uplink)921 – 935 MHz for BTS to MS (downlink)

GSM-1800

Frequency band: 2 x 75 MHz

1710 – 1785 MHz for MS to BTs (uplink)1805 – 1880 MHz for BTS to MS (downlink)


Industry Vs Technology Spread

Telecom Service Providers/OperatorsGSM

CDMABasic-WLL

Internet ServicesLong Distance

VendorTelecom Consultancy


Analog Mobile Telephony

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


Digital Mobile Telephony

Spectrum space - most limited and precious

resource

Solution - further multiplex traffic (time domain)

Can be realized with Digital Techniques only


GSM History and Organization

1979 Europe wide frequency band reserved for Cellular

1982 “Groupe Speciale Mobile” created within CEPT1986 GSM had full time in Paris1988 ETSI takes over GSM Committee 1990 The phase 1 GSM Recommendations

frozen1991 GSM Committee renamed “Special Mobile Group” and GSM renamed as “Global

System for Mobile Communication”1992 GSM launched for commercial

operations


Cellular Communication

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.


Road Map of Mobile Communication


WHAT IS CELLULAR TELEPHONY ?

CONSIDERATIONS -

FREQUENCY

SUBSCRIBER DENSITY

COVERAGE


Base Station

Base Station

Base Station

Base Station

Base Station

Base Station

The Cell

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.


Cell Size


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 & Capacity

CoveragePercentage 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. Wednesday, April 12, 2023www.tempustelcosys.com

GSM - IN CELLULAR TELEPHONY

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 - UplinkOther used in downward direction by BTS - DownlinkThe 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


Frequency & ARFCN


ul=

890

to

915

MH

z

dl=

935

to

960

MH

z

Ful(n) = 890.0 + (0.2) *n MHz

Fdl(n) = Ful + 45 MHz

where n =ARFCN ; 1 n 124

GSM Bandwidth & Main Parameters900 MHz & 1800 MHz


1710 1785 1805 1880

duplex distance: 95 MHz

890 915 935 960

duplex distance: 45 MHz

Multiple Access Methods


Multiple Access Methods


FrequencyTime

Power

TDMA

Frequency

Time

Power

FDMA

FrequencyTime

Power

CDMA

CODE

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

Frequency Reuse Pattern

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

45

6

7D

Principal Of Sectorization

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

Handovers


Hard HandoffAnalog, TDMA and GSM

Soft HandoffCDMA

Break before Make Make before Break

Features of GSM

Compatibility

Noise Robust

Increased Capacity & Flexibility

Use of Standard Open Interfaces

Improved Security & Confidentiality

Cleaner Handovers

Subscriber Identification

ISDN Compatibility

Enhanced Range of ServicesWednesday, April 12, 2023www.tempustelcosys.com

Mobile Station Identities


GSM NETWORK ELEMENTS


BTS

BTS

BTS

BTS

BTS

BTS

BTS

BTS

BSC

VLR

HLR EI

R

OMC

SMSC

BC

AUC

VMSC

MSC

Abi

s

A

MS

BTS

BTS

BTS

BTS BTS

BTS

BTS

BTS

BSC

BSC

PSTN

VLR

TRAU HLREIR

OMC

SMSC

BC

AUC

VMSC

MSC

Abi

s

A

OML

GSM Network ComponentsMobile 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.


Mobile Equipment

Class Power O/p 1 20 W

2 8 W

3 5 W

4 2 W

5 0.8 W


Typical Settings

SIM(IMSI)

IMSI(International Mobile Subscriber Identity)Transmitted over Air Interface on initialization

Permanently stored on SIM card

15 digit Decimal


SIM

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


SIM (TMSI)

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.


Base Station System (BSS)BSS (Base Station System)

BSC (Base Site Controller)

BTS (Base Transceiver Station)

XCDR (Transcoder)


Network Switching System (NSS)

XCDR

BSC

BTS

TranscoderConverts 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.

G e n e r a l l y ,Tr a n s c o d e r i s c o l l o c a t e d w i t h M S Cs o a s t o r e d u c e t h e n u m b e r o f2 M b p s A - l i n k st o e ffi c i e n t l y u s e t h e B W .


Base Station System (BSS)BSC

Conveys information to/from BTS

Connects terrestrial circuits & Air Interface Channels

Controls handovers between BTSs under itself

BTSContains RF Hardware

Limited control functionality

1 - 6 carriers in a BTS Cabinet

7 - 48 simultaneous calls per BTS


BSS ConfigurationCollocated BTS

Remote BTS

Star Configuration

Daisy Chain BTS Loop Configuration


BSC

BTS

BTS

BTS

All B

TS

on

1 E1

BSC

BTS

BTS

BTS

BTS

Network Switching System(NSS)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)


GSM Network ComponentMSC

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.


Home Location Register (HLR)

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


Visitor Location Register (VLR)

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)


Location Update


GSM Interfaces

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 - AUCWednesday, April 12, 2023www.tempustelcosys.com

Basic Processes

AUTHENTICATION

CIPHERING

REGISTRATION

CALL ESTABLISHMENT

HANDOVER / HANDOFF

ROAMING


Transmission Media

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)


Channels On Air InterfacePhysical 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.


LOGICAL CHANNELS


0 1 2 3 4 5 6 7

3

57 encrypted

57 encrypted

26 training

1S

1S

3T

8.25GP

3T

577S

TDMA Frame

GSM Channels


Control Channel


DCCH(Dedicated Channels)Downlink & Uplink

CCCH(Common Control Chan)Downlink & Uplink

Synch.Channels

RACHRandom Access Channel

CBCHCell Broadcast Channel

SDCCHStandalone dedicated control channel

ACCHAssociated Control Channels

SACCHSlow associated Control Channel

FACCHFast Associated Control Channel

PCH/AGCHPaging/Access grant

FCCHFrequency Correction channel

Control Channels

BCH ( Broadcast channels )Downlink only

BCCHBroadcast control channel

SCHSynchronization channel

Broadcast Channels (BCH)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


Common Control Channels (CCCH)

•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 callsWednesday, April 12, 2023www.tempustelcosys.com

Dedicated Control Channels (DCCH)

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.


BURST

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”.


GSM Burst & TDMA Frame


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

Mobile To Land Sequence


1

3

CHANNEL REQUEST

DCCH ASSIGN

SIGNALLING LINKESTABLISHED

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

2CR

CC

Call Contt.


8COMPLELTE CALL

CALL PROCEEDING

9 ASSIGNMENT COMMAND

INITIAL & FINALADDRESS (IFAM)

ASSIGNMENT COMPLETE (ACM)10

ANSWER(ANS)

11

CONNECT ACKNOWLEDGE

SDCCH

SDCCH

ASSIGNMENT COMPLELTE

MS HEARS RINGTONEFROM LAND PHONE

ALTERING

RING TONESTOPS

CONNECT

(channel)

(TCH)

FACCH

FACCH

FACCH

TCH

(circuit)

FAACH

BILLING STARTS

Hello!

MS BSS MSC

VLR HLR PSTN EIR

GSM Planning StepsVarious steps are listed below

CW surveyModel TuningNominal PlanningRF site SurveysRealized PlanningFrequency Planning

ImplementationOptimization

Drive TestingPerformance Analysis


Nominal PlanningIt consists of planning a set of sites on planning tool so as to predict

the coverage of the target areaTool needs to be made intelligent so as to predict the coverage as

close as possible to actual coverageCoverage plots are based on customer intension of providing indoor

and outdoor coverage


Clutter Types

Clutter typesDense Urban

UrbanSub Urban

RuralWater

VegetationIndustrial

Forest


RF surveysEach nominal has a search ring defined by the RF PlannerCandidates needs to be identified as close as possible to

the nominal within the search ringHeight, orientations & antenna placement at site are the key RF parameter which are based upon the coverage

requirement in the areaMajor obstructions and clutter type in various directions

to be observed on RF survey


RF surveysEquipment required for RF Survey

GPSDigital Camera

BinocularsMagnetic 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

Education

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primary gsm

cept1986 gsm

gsm committee1990

extended gsm

railway gsm

term gsm