gsm overview non technical module.pdf

GSM Overview

Non-Technical Staff

Agenda

• Introduction

• Basic GSM Network Structure

• Radio Coverage

• Traffic Cases

• GSM Services

Introduction

History of Wireless Communication

The First Digital Cellular Standard (GSM) is launchedEurope1991

Advanced Mobile Phone System. First Cellular Analog networkChicago1979

NMT specifies standards to allow mobiles phones to be located within or across their networks. Basis for roaming idea

Europe1973

Denmark, Finland, Iceland, Norway and Sweden form the first standardization group: Nordic Mobile Telephony (NMT)

Europe1969

FCC First Domestic Public Land Mobile service at 150 MHzSt. Louis1946

Invention of Frequency Modulation. Improved Audio QualityUS1935

Invention of Amplitude Modulation. Half Duplex transmissionUS1930s

2 MHz Vehicular Mobile Radio system for Police ApplicationsChicago1921

ActivityPlaceDate

Evolution of Cellular Communication


Early 1980s


Early 1990s

History of GSM

• CEPT began producing GSM specifications for phased implementation.

• Another five countries signed the MoU1988

• Hybrid TDMA/FDMA was selected as the transmission technology for GSM.

• September: 13 operators and administrators from 12 areas in the CEPT GSM advisory group

sign the charter GSM (Groupe Spéciale Mobile) MoU "Club" agreement, with a launch date of 1 July 1991.

• The original French name was later changed to Global System for Mobile Communications, but the original GSM acronym stuck.

• GSM spec drafted. (Digital Transmission, Time Multiplexing of order 8 and slow Hopping).

1987

• Field tests were held in Paris and a GSM permanent nucleus was created and comparative tests of 8 prototypes were performed.

• The choice was Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA).

1986

• Nordic Telecom and Netherlands PTT send a proposal to CEPT to specify a common European mobile telecommunication service.

• The European Commission (EC) issued a directive, which requires member states to reserve frequencies in the 900 MHz band for GSM.

1982

ActivityDate

History of GSM

• Australia became the first non-European country to sign the MoU. The MoU now had a total

of 70 signatories.

• GSM demonstrated for the first time in Africa at Telkom '93 in Cape Town

• GSM networks were launched in Norway, Austria, Ireland, Hong Kong and Australia.• The number of GSM subscribers reached one million.

• The first commercial DCS 1800 system was launched in the U.K.

• December 1993: 32 networks on air in 18 areas

1993

• Phase 1 specifications were completed.

• January - First commercial phase 1 GSM network operator is Oy Radiolinja Ab in Finland.• December 1992 - 13 networks on air in 7 areas.

• First International roaming agreement was signed between Telecom Finland and Vodafone in UK.

1992

• The GSM 1800 standard was released.

• An addendum was added to the MoU allowing countries outside CEPT to sign.1991

• Phase 1 specifications were frozen to allow manufacturers to develop network requirements and the first GSM prototype was brought to service.

1990

• The European Telecommunications Standards Institute (ETSI) defined GSM as the

internationally accepted digital cellular telephony standard and took over responsibility of GSM specifications.

1989

ActivityDate

History of GSM

• At the beginning of 1998 the MoU has a total of 253 members in over 100 countries and

there are over 70 million GSM subscribers worldwide. GSM subscribers account for 31% of the world’s mobile market.

• Vodacom Introduces Free Voice Mail.• GSM SIM Cracked in USA.

• 21 May 1998. Egypt privatizes its GSM operator.

• Iridium Live 11/98.

• 125 million GSM 900/1800/1900 users worldwide (12/98).

• 1 Dec 1998. Click GSM commercial launch.

1998

• First dual-band GSM 900-1900 phone launched by Bosch1997

• GSM MoU is formally registered as an Association registered in Switzerland

• December 1996: 120 networks on air in 84 areas

• 8000 SIM launched• Pre-Paid GSM SIM Cards launched

• Option International launches world's first GSM/Fixed-line modem

• Nov 1996 - Sole Governmental operator in Egypt goes online.

1996

ActivityDate

GSM Coverage Worldwide

Questions

GSM Network Elements

Basic GSM Nodes

BSC

MS

BTS

Other MSCs

HLR

Radio Interface

PABX PSTN

Gateways to Other Networks

MSC/VLR

AUC

Other Switching Nodes

Other Switching Nodes

Mobile Services Switching Center (MSC)

• Administers one or more Base Station Controllers (BSCs).

• Switches calls to/from mobile subscribers.

• Records charging and accounting details.

• Provides the gateway functionality to other networks.

BSC

BTS

HLR

MSC/VLR

AUC

Home Location Register (HLR)

• Stores for each mobile subscriber:

– Basic subscriber categories.

– Supplementary services.

– Current location.

– Allowed/barred services.

– Authentication data.

• Helps the routing of mobile terminated calls and SMS.

BSC

BTS

HLR

MSC/VLR

AUC

Visitor Location Register (VLR)

• Stores a copy of the HLR Profile for all mobile subscribers who are covered

by cells belonging to the MSC coverage area.

• The VLR is always integrated with the MSC.

BSC

BTS

HLR

MSC/VLR

AUC

Base Station Controller (BSC)

• Manages the Radio Communication with the mobile station over the air

interface.

• Controls the handover of calls in progress Between BTSs.

• Supervises the transmission network and the operation of each BTS.

MSC/VLR

BTS

HLR

BSC

AUC

Base Transceiver Station (BTS)

• Consists of the radio transmitters, receivers and the antenna system

required to provide the coverage area for one cell.

• Converts the GSM radio signals into a format that can be recognized by the

BSC.

• Records and passes to the BSC the Signal strength measurements.

• Performs the network end of the ciphering/encryption process.

BSCBTS

HLR

MSC/VLR

AUC

Mobile Station (MS)

Handset

+Mobile Station =SIM Card

The SIM Card contains a processor and memory that stores:

• The international mobile subscriber Identity IMSI

• The authentication and ciphering keys.

GSM Identities

• Mobile Station ISDN Number (MSISDN)

CC : Country Code

NDC: Network Destination Code

SN : Subscriber Number

Vodafone Egypt MSISDN

20

CC

10

NDC

1100477

SN

Vodafone UK MSISDN

44

CC

385

NDC

196099

SN

GSM Identities

• International Mobile Subscriber Identity (IMSI)

MCC : Mobile Country Code

MNC : Mobile Network Code

MSIN : Mobile Station Identification Number

Vodafone UK IMSI

234

MCC

15

MNC

1234567890

MSIN

Vodafone Egypt IMSI

602

MCC

02

MNC

1234567890

MSIN

GSM Identities

• International Mobile Equipment Identity (IMEI)

TAC : Type Approval Codes

FAC : Factory Assembly Code

SN : Serial Number

IMEI

6 Digits

TAC

2 Digits

FAC

6 Digits

SN

Final Assembly Codes (FAC)

01,02 AEG07,40 Motorola10,20 Nokia30 Ericsson40,41,44 Siemens47 Optional International50 Bosch51 Sony51 Siemens51 Ericsson60 Alcatel70 Sagem75 Dancall80 Philips85 Panasonic

GSM Identities

• Temporary Mobile Subscriber Identity Number (TMSI)

– TMSI may be allocated to a mobile subscriber to be used instead of

IMSI during all radio communications in order to keep subscriber’s information confidential on the air interface.

– TMSI is relevant on the local MSC/VLR only and is changed at certain events or time intervals.

– Each operator can define its own TMSI structure.

GSM Identities

• Mobile Station Roaming Number (MSRN)

– During setup of a mobile-terminating call, the HLR of the called

subscriber requests the MSC/VLR to allocate an MSRN to the called subscriber.

– This MSRN is returned via the HLR to the GMSC.

– The GMSC routes the call to the MSC/VLR exchange where the called

subscriber is currently registered.

– The routing is done using the MSRN. When the routing is completed, the MSRN is released.

– The MSRN is built up like an MSISDN

GSM Network Structure

SSSS

D T I

BTSBTS

O S S

ISDN

PLMN

PSTN

ISDNISDN

PLMNPLMN

PSTN M I N

S C

B G W

H L R

AUC

SMSC-GMSCSMS-IWMSC

E I R

GWMSCGWMSC

MSC/VLRMSC/VLR

MSMS

Air I/fGPRS

SGSN

BSC/TRCBSC/TRC

BSSBSS

Split Architecture

TSG

Server

Control Layer

Connectivity Layer

MGW

X

X

X

BSC

MGW X

MSC

Server

PSTN/ISDN/PLMN

GC

P

GC

P

BICC

ISUP

BSSAP

Vodafone Egypt Network Structure

• Live Nodes in December 2008:

16 MSC

33 MSC Server

42 Media Gateway

46 BSC

4 Transit Gateway

15 HLR

4 FNR

Questions

Radio Coverage

Cell Geometry

Dead Spots

Problem of omni directional antennas

Cell Geometry

• To solve the dead spot problem:

R R

Tradeoffs:

• The number of cells required to cover a given area.

• The cell transceiver power.

R

Transceiver Antenna

Omni-Directional AntennaSectorial Antenna

Sectorial Antenna

• The cells will take the form of overlapping circles.

• Due to the obstacles in the coverage area the actual shape of the

cells would be random.

Cell ClassificationMacrocell

Microcell

Slow moving subscribers

Fast moving subscribers

Picocell

In buildingcoverage

Cell Classification

Umbrella Cell

Overlaid &

Underlaid CellsNormal Cell Normal Cell

Spectrum Allocation: GSM 900

GSM 900 Frequency Allocation

F (MHz)915890

Uplink1 2 3 4 121 122 123 124

F (MHz)

Downlink

960935

1 2 3 4 121 122 123 124

890.2

890.4

890.6

935.2

935.4

935.6

200 KHz

Downlink 935 – 960 MHz

Uplink 890 – 915 MHz

ARFCN Absolute Radio Frequency Channel Number

Spectrum Allocation: GSM 1800

GSM 1800 Frequency Allocation

F (MHz)17851710

Uplink1 2 3 4 371 372 373 374

F (MHz)

Downlink

18801805

1 2 3 4 371 372 373 374

1710.2

1710.4

1710.6

1805.2

1805.4

1805.6

200 KHz

Downlink 1805 – 1880 MHz

Uplink 1710 – 1785 MHz

ARFCN Absolute Radio Frequency Channel Number

GSM Spectrum

890 915

935 960

GSM 900With 124 ARFCN

25 MHz

45

MH

z

Uplink

Downlink

1710 1785

1805 1880

GSM 1800

With 374 ARFCN

75 MHz

95

MH

z

Uplink

Downlink

GSM Coverage Plan

(A) One high power transceiver

• The mobile equipments will have high output power in order to be able to transmit signals across the coverage area.

• The usage of the radio resources would be limited.

(B) Divide area into cells

• Each cell will have relatively small power transceivers.

• The frequency may be “reused” in two far separated cells. This yields:

– Increased capacity of the system.

– Good interference characteristics

To provide coverage for a large area, we have two options:

Frequency Reuse

Why do we need frequency reuse?

Total no of channels (frequencies) = 124

Every channel can be shared between a maximum of 8 subscribers.

Maximum no of simultaneous calls = 8 X 124 = 992 !!

How is frequency reuse done?

The whole available frequencies is divided between a group of neighboring cells

which is called frequency reuse pattern or a “Cluster”, and this pattern is

repeated this over the whole network on 2 conditions:

• Frequencies must not be used in the adjacent cells.

• Enough distance between the cells where the same group of frequencies

are reused.

Frequency Reuse: 3/9 Cluster

A3

A2

A1

B3

B2

B1

C3

C2

C1

A3

A2

A1

B3

B2

B1

C3

C2

C1

A3

A2

A1

B3

B2

B1

C3

C2

C1

A3

A2

A1

B3

B2

B1

C3

C2

C1

A3

A2

A1

B3

B2

B1

C3

C2

C1

A3

A2

A1

B3

B2

B1

C3

C2

C1

A3

A2

A1

B3

B2

B1

C3

C2

C1

A3

A2

A1

B3

B2

B1

C3

C2

C1

Available frequencies are divided into 9 groups

and distributed between 3 sites (9 Sectors)

A3

A2

A1

B3

B2

B1

C3

C2

C1

A3

A2

A1

B3

B2

B1

C3

C2

C1




A3

A2

A1

B3

B2

B1

C3

C2

C1

D3

D2

D1

A3

A2

A1

B3

B2

B1

C3

C2

C1

D3

D2

D1

A3

A2

A1

B3

B2

B1

C3

C2

C1

D3

D2

D1

A3

A2

A1

B3

B2

B1

C3

C2

C1

D3

D2

D1

A3

A2

A1

B3

B2

B1

C3

C2

C1

D3

D2

D1

A3

A2

A1

B3

B2

B1

C3

C2

C1

D3

D2

D1

A3

A2

A1

B3

B2

B1

C3

C2

C1

D3

D2

D1

A3

A2

A1

B3

B2

B1

C3

C2

C1

D3

D2

D1

A3

A2

A1

B3

B2

B1

C3

C2

C1

D3

D2

D1

A3

A2

A1

B3

B2

B1

C3

C2

C1

D3

D2

D1

A3

A2

A1

B3

B2

B1

C3

C2

C1

D3

D2

D1

A3

A2

A1

B3

B2

B1

C3

C2

C1

D3

D2

D1




A3

A2

A1

C3

C2

C1

D3

D2

D1

B3

B2

B1

E3

E2

E1

F3

F2

F1

G3

G2

G1

A3

A2

A1

C3

C2

C1

D3

D2

D1

B3

B2

B1

E3

E2

E1

F3

F2

F1

G3

G2

G1

A3

A2

A1

C3

C2

C1

D3

D2

D1

B3

B2

B1

E3

E2

E1

F3

F2

F1

G3

G2

G1

A3

A2

A1

C3

C2

C1

D3

D2

D1

B3

B2

B1

E3

E2

E1

F3

F2

F1

G3

G2

G1

A3

A2

A1

C3

C2

C1

D3

D2

D1

B3

B2

B1

E3

E2

E1

F3

F2

F1

G3

G2

G1

Frequency Reuse: Which Cluster Size to use?

Carrier to Interference Ratio

It’s the difference in power level between the carrier in a given cell and the same carrier received from the nearest cell that reuses the same frequency.

HighLowLow7/21

MediumMediumMedium4/12

LowHighHigh3/9

C/I RatioTraffic ChannelsFrequencies per Site

Questions

Traffic Cases

Location Update

Why do we need to update our location data?

To identify subscribers’ locations within the network so that incoming calls could

be routed to the correct recipient.

Options for location update:

– Location update each time the subscriber changes his serving cell

→ huge amount of location update messages

– No location updates and paging subscriber in the whole network

→ huge amount of paging messages

Compromise

Location Update

• Location Area

– Location area is a part of the MSC/VLR coverage area.

– Each group of adjacent cells is assigned a universal unique Location

Area Identity (LAI).

– Mobile subscribers update the network with their new location every

time it changes its Location Area.

– The concept of “Location Area” enables us to make an approximate estimation of the subscriber’s location.

Location Update

• MSC Coverage Area (Service Area)

LA 1

LA 2

MSC

LA 3

LA4

Location Update

• MSC Coverage Area (Service Area)

Location Update



LAC : Location Area Code

Location Area Identifier (LAI)

234

MCC

15

MNC

1607

LAC



LAC : Location Area Code

CGI : Cell Global Identity

Cell Global Identity (CGI)

234

MCC

15

MNC

1607

LAC

781

CID

Location Update

• Types of Location Update

1. Normal location update within same MSC service area.

2. Normal location update between 2 different MSC service areas.

3. IMSI attach/detach

4. Periodic Location Update

Location Update

1. Normal location update within same MSC service area

BSC

1. The Mobile sends an allocation request message to the BTS

2. The BTS responds with the allocation message

3. The mobile sends a location update request message with its

IMSI to the MSC/VLR

4. The MSC/VLR updates the location information and sends a

Location Update confirmation message

MSC/VLR

UpdatesLA Record

Location Update

2. Normal location update between 2 different

MSC service areas

1. The mobile sends a location update request to the MSC.

2. The new MSC/VLR receives the IMSI and conclude the MGT.

3. The MSC/VLR sends a subscriber information request with

the IMSI to the proper HLR

4. The HLR stores the address of the new MSC/VLR

5. The HLR sends the data to the new MSC/VLR and it is kept

there

6. The HLR sends a location cancellation message to the oldMSC/VLR to remove the data

7. The new MSC/VLR sends a location updating confirmation message to the mobile

Old MSC/VLR New MSC/VLR

New BSCOld BSC

LA 1

LA 2

IMSI to MGTtranslation

VLR Address=

Old MSC

VLR Address=

New MSC

HLR

Location Update

3a. IMSI Detach

BSC

1. At power off, the MS asks for a signaling channel.

2. The MS uses this signaling channel to send the IMSI detach message to the MSC/VLR.

3. In the VLR, an IMSI detach flag is set for the subscriber. This is

used to reject incoming calls to the MS.

MSC/VLR

Reject Incoming Calls

Switching Off

Location Update

3b. IMSI Attach

BSC

1. MS requests a signaling channel.

2. The MSC/VLR receives the IMSI attach message from the MS.

3. The MSC/VLR sets the MS status to IMSI attach in the VLR.

The mobile is now ready for normal call handling.

MSC/VLR

Reject Incoming Calls

Detached

Normal Call Handling

4. The VLR returns an acknowledgment to the MS.

Active

“Note: If the MS changed location area while detached, a

normal location update would take place”

Location Update

4. Periodic Location Update

– A routine task performed by the network if the MS doesn’t make any activity during a predefined period.

– If the MS doesn’t respond to this periodic location update, it will be

marked as implicitly detached (Temporarily out of service).

Handover

• Handover is keeping continuity of the call while the subscriber is roaming in

the network (moving from one cell to another).

• During a call, the MS is continuously measuring transmission quality of

neighboring cells and reports these results to the BSC through the BTS.

• The BSC is responsible of handover initiation.

• Good neighbor relations between cells is an important factor in keeping the

network performance within accepted levels.

Handover

• Types of Handover

1. Intra BSC Handover

Both serving and target cells belong to the same BSC.

2. Inter BSC / Intra MSC Handover

Target cell belongs to a different BSC, but to the same serving MSC.

3. Inter MSC Handover

Target cell belongs to a different BSC and a different MSC.

Handover

1. Intra BSC Handover

1. The BSC decides from the power measurement reports that the

call must be handed over to another cell.

2. The BSC checks for an vacant TCH in the new cell and orders

this cell to activate the TCH.

3. The BSC orders the serving cell to send a message to the MS

telling the information of new TCH.

4. The MS tunes to the new frequency and sends handover access

burst.

5. The new cell detects the handover burst and sends information

about the suitable timing advance to the MS.

6. The MS sends a HO complete message to the new cell.

7. The new cell sends a message to the BSC that the handover is successful.

BSC

Initiate Handover

New CellOld Cell

8. The BSC orders the old cell to release the TCH.

Handover

2. Inter BSC / Intra MSC Handover

Old BSC New BSC

MSC/VLR

Handover

3. Inter MSC Handover

Old BSC New BSC

Old MSC New MSC

Mobile Originated Call

1. MS sends a call request including its IMSI to its serving MSC.

2. MSC will mark the subscriber as busy, authenticate the user to verify

the mobile the right to access the network, then initiate ciphering in

order to protect the call on the radio path..

3. MS sends a call setup message to the MSC with information about

the call type, services required and dialed number.

4. MSC checks the subscriber profile to verify that he is authorized to

use the requested services, and then a establish a link with the BSC.

5. BSC checks the mobile serving BTS for an idle traffic channel and

then orders that BTS to seize this channel for the call.

6. The BSC informs the MSC when the traffic channel assignment is complete.

PSTN

BSC

MSC

7. The MSC starts to analyze the dialed number and sets up the

connection towards the called subscriber

Mobile Terminated Call

PSTN

HLR

GMSC MSC

1

3

2

5

4

7

6

8

9

BSC

Roaming: Location Update

HLR

Attached

VLR ADD=Egypt Airport

Roaming & Int. Allowed

Detached


MSC/VLR

Is a roaming agreement present ?IMSI

60202..

Is r

oa

min

g a

nd

In

t.

ca

lls

all

ow

ed

?

Attached

VLR ADD=Stock. Airport


Copy of the HLR Profile willbe stored in Stock. VLR

Roaming: Call to HPLMN

HLR

Attached

VLR ADD=xyz

GWMSC

MSC/VLR

Terminating Leg

Originating Leg

MSISDN

+2010….

Roaming: Call from HPLMN

HLR

Attached

VLR ADD=Stock. Airport

Roaming & Int. AllowedGWMSC

MSISDN

010…

MSC/VLR

Roaming Call Forwarding Leg

Originating Leg

Roaming: Call from another Roamer

HLR

GWMSC

MSC/VLR

AMSC/VLR

B

Attached

VLR ADD=Stock. B


MSISDN

+2010….

Roaming Call

Forwarding Leg

Originating Leg

Roaming: Call to The visited PLMN

STCK.

GWMSC

MSC/VLR

AMSC/VLR

B

MSISDN

+46…

GWMSC

HLR

Terminating LegOriginating Leg STCK.HLR

Questions

GSM Services

Introduction

• Examples of GSM non-Speech services

– Fax calls

– Data calls

– Short messages service (SMS)

• Conclusion: GSM is a telecommunication network rather than a telephony network.

Fax and data Calls

PSTN

Supported by :

The receiving node must know:

• What service is requested? e.g. speech, fax or data

• How will the service be performed? e.g. bit rate

ISDN GSM

Fax and data Calls

Why do we have additional MSISDN for the same subscriber to be able to receive fax and data calls?

– Information sent at call setup from ISDN or GSM on “how” the service will be

performed, is called Bearer Capabilities (BC).

– ISDN and GSM have different transmission requirements and different

coding schemes, so they have different bearer capabilities referred to as

ISDN-BC and GSM-BC.

– PSTN can not provide this type of information during call setup, so it can not

distinguish between a telephony call and a fax or data call.

– An Additional MSISDN (AMSISDN) will be allocated to a mobile subscriber

who has the service of receiving fax or data calls.

Fax and data Calls

For an MSC to handle fax or data calls, it needs a Data Transmission

Interface (DTI), which provides:

– Rate adaptation

– Protocol conversion

– Providing modems

Fax and data Calls

• Fax from PSTN

AMSISDN IMSI

AMSISDN BC

HLR

GWMSC MSC/VLRPSTN

AM

SIS

DN

AMSISDN

DTI

Fax and data Calls

• GSM fax call

MSC

DTI

PSTN

Short Message Service

• The Short Message Service (SMS) allows a mobile subscriber to send and

receive text messages composed of 160 characters at most.

• The short messages sent or received are handled by the Short Message

Service Center (SMSC), which consists of three parts :

– Service Center (SC)

– SMS Gateway MSC (SMS-GMSC)

– SMS Inter-Working MSC (SMS-IWMSC)

Short Message Service

• Service Center (SC)

– Handles the delivery of SMSs to/from Short Message Entities (SME), which can

be any originator or receiver of short messages such as mobile, fax, … etc.

– Stores the short messages.

– Creates billing files.

• SMS-GMSC

– Interrogates the HLR to determine the location of a mobile subscriber.

– Forwards the short message to a mobile subscriber via its serving MSC.

• SMS-IWMSC

– Receives the mobile originated short message from any MSC in the network.

– Receives alerts from the HLR to inform the SC that a mobile subscriber who

was absent during a previous SMS delivery attempt is reachable again.

Mobile Originated SMS

MO

MSC1 MSC2

MT

SMS - IWMSC SMS - GMSC

HLR

SC

Unsuccessful Message Transfer

MO

MSC1 MSC2

MT


HLR

SC

Pending Message FlagW

aitin

g Mes

sage

Dat

a

The Mobile is present once more

MO

MSC1 MSC2

MT


HLR

SC

Alert: The M

S is

present

Attach

Alert: The MS is

present

Other GSM Services

SS Supplementary Services

IN Intelligent Networks

CAMEL Customized Application of Mobile Enhanced Logic

MMS Multimedia Message Service

WAP Wireless Applications Protocol

Evolution of GSM

HSCSD High Speed Circuit Switched Data

GPRS General Packet Radio Services

EDGE Enhanced Data Rates for GSM Evolution

UMTS Universal Mobile Telecommunication System

HSDPA High Speed Downlink Packet Access

HSUPA High Speed Uplink Packet Access

IMS IP Multimedia Subsystem

Evolution of GSM

Functionality

& capabilities

Speech

Circuit data

<9.6 kbps

HSCSD

57.6 kbpsCircuit Switched

UMTS capable systems

GPRS

115 kbpsPacket Switched

EDGE/IS-136

384 kbps

WCDMA2 Mbps

1998 1999 2000 2001 20021997Time

Questions

Thank You

gsm overview non technical module.pdf

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