introduction to gsm pwl

8/13/2019 Introduction to GSM PWL

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A Journey of Wireless Communication

RTT Pune presents


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V M DAPURKAR 223-Dec-13

Introduction to GSM Architecture

V.M.Dapurkar

SDE RTTC Pune

BRBRAITT JABALPUR


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Communication PSTN

Telephone Exchanges

Subscriber

Line(2W)

Inter-Exchange

Junction

Subscriber

Line(2W)


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Communication - PLMN

TelephoneExchange

Subscriber

Line(2W)

Inter-Exchange

Junction

Mobile SwitchingCentre (MSC)

BSC BTSMS


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Wired Vs Wireless N/W

Frequency/Time Division Multiplexing Limited RF spectrum and hence RF channels assigned on demand Dynamically changing teledensity in each area. Dynamic Network snapshots.

Mobile SwitchingCentre (MSC)

BSC BTS

MS

ONLY a windowof RF Spectrum is

available

RF Channels

WIRELESS NETWORK


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Mobility Issues

Demand on scarce Radio Resources.

Security.

Keep track of users. Maintain established communication.

Power Management of Mobile Phone.

Providing service across networks. Billing & Accounting


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What are the types inGSM Network?

GSM-900890-915MHz Uplink,935-960MHz Downlink,

Channels 124, carrier spacing 200khz, T/Rspacing 45Mhz, Max.cell size 35Km

GSM -1800

1710-1785 MHz Uplink,1805-1880 MHzDownlink,Channels 374, carrier spacing200khz, T/R spacing 95Mhz.Max Cell size4Km.



Spectral Allocation in GSM

935-960 MHz

890-915 MHz


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F1

F2F4

F5

F6

F7

F1

F2F4

F5

F6

F7

GSM - Cell layout

Non-interference Separation

F3

F3


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Security

Subscriber Identity Authentication To prevent unauthorised access to

GSM network & its services.

User data confidentiality To ensure privacy of the user data

Subscriber Identity Confidentiality To ensure identity of the user in the

radio path



Security

Registration The process of verifying the

authenticity of the user beforeallowing him to use the servicesof the network.

Use of Keys & Algorithms Authentication Ciphering.


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Keep Track of Users

Essential to know:Where the user is currently?Can he be reached.

Location UpdateDue to change in location.Due to entry into non-service

area.Power on/off


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Handover

Use of Cellular principle limits thearea of coverage of a single radio

path. Keep the communication path even

when the user crosses cellboundaries and enter into new cell.


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Service Across Networks

Users can move across ServiceArea and Geographical boundaries.Universal service coverage?Use of others Network to provide

service to visitorAgreement between Service

Providers. Billing and Revenue Sharing


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System Architecture

Functional Architecture broadly dividedinto six parts:

Mobile Station. Base Station Subsystem (BSS).

Network Switching Subsystem (NSS).

Operation and Maintenance Subsystem.


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GSM Network Overview

ME

SIM

BTS

BTS

BSC

BSC

MSC

HLR

EIR

VLR

AuC

MobileStation Base Station Subsystem Network Subsystem

PSTNISDN

PSPDNCSPDN

UmAbis

A

OMC

OMCR


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GSM Network Architecture

BSC

BSC

MSC

MSC

EIR

AUC

HLR

VLR

GPRS, SGSNand GGSNprovidepacket dataservices

BTS

BTS

Backhaul

Internet

SGSN

GGSN

TransitNet

Transit

Net

NSS

BSS

GSN GPRS Support Node

SGSN Serving GSNGGSN Gateway GSN


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Mobile Station

Mobile Station is really two distinct entities.

Mobile equipment or Mobile terminal.

SIM Card. Mobile equipment, which is the actual

Hardware, is almost anonymous.

SIM card, implemented as a smart cardholds the subscriber information.


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Mobile Terminal

Different terminals distinguishedprincipally by their power andapplication.

Fixed terminals20WPortable terminals for cars8WHandheld terminals2W

Sophisticated terminals0.8W Every terminal has an International

Mobile Equipment Identity (IMEI).


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The SIM

SIM provides Personal Mobility.

Holds a unique identifier calledInternational Mobile Subscriber Identity

(IMSI), Service profile, Secret key forauthentication, etc.

Provides user access to all subscribedservices. SIM makes the terminaloperational.

Can be password protected by a 4 to 8 digitPersonal Identification Number.


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The SIM

SIM is classified on the basis of:Operating voltage.

RAM capacity. Operating Voltages:

3V and 5V.

RAM Capacity:8K, 16K, 32K and 64K


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Base Station Subsystem

BSS connects the MS and the NSS.

Composed of three parts:

The Base Transceiver Station (BTS) alsocalled Base Station (BS) Transcoder and. Base Station Controller (BSC).

BSS provides the radio interface functions.

Completely digital.


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Base Transceiver Station (BTS)

BTS houses the radio transceivers thatdefine a cell.

Handles the radio-link protocols withthe Mobile Station

Each BTS can support a number oftransceivers depending on the densityof subscribers in the cell.

Needs to be rugged, reliable, portable.


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BTS Functions

Synchronization of signals oftransmission

Radio resources management


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Base Station Controller

BSC manages the radio resourcesfor one or a group of BTSs.

Handles radio-channel setup,frequency hopping, handovers, andcontrol of the RF power levels.

Establishes connection betweenthe mobile station and the MSC.


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Network & Switching Subsystem

Manages the communication betweenthe mobile users and other users.

Provides all the functionality neededto handle a mobile subscriber such as:Registration.Authentication and security.

Location updating.Handovers andRouting to roaming subscribers.


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Central component is the MSC(Mobile Services Switching Centre).

It performs the switching functionsof the network.

Gateway Mobile Services Switching

Centre interfaces between themobile network and PSTN.


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Services provided in conjunctionwith four intelligent databases.

Home Location Register (HLR)

Visitor Location Register (VLR)

Authentication Centre (AuC) Equipment Identity Register (EIR)


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Home Location Register

Contains all administrativeinformation of each USER

subscribered in an GSM network. Logically there is one HLR per

GSM/PLMN network.


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HLR

Permanent Register of thesubscribers

maintains facilities given tosubscriber


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VLR

Maintains subscriber informationwho are currently in its service

area. Gives current location of the

subscriber

Temporary Database


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AUC

Verifies SIM cards

Authentication of SIM is done

using authentication triplets


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EIR

Used for security purposes

Checks IMEI

Contains 3 Lists ( White LISTGREY LIST

Black LIST )


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OMCR

The maintenance control point for allBTSs and BSCs in a GSM system.

Gives Fault status Of BTSs.

Operation and Administration.

Data and Software storage fornetwork elements.

Traffic reporting and analysis.


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OMCR

The maintenance control point for allBTSs and BSCs in a GSM system.

Gives Fault status Of BTSs.

Operation and Administration.

Data and Software storage fornetwork elements.

Traffic reporting and analysis.


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RADIO RESOURCE MANAGEMENT

Radio Resource management is a group offunctions concerned with themanagement of transmission resources onthe radio path (Um interface).

It must cope with limited radio resourcesand share them dynamically between alldemands.

The mobile station and the BSC mainlyperform the functions of the RadioResource management.

ROLE OF RADIO RESOURCE


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ROLE OF RADIO RESOURCEMANAGEMENT

Establish stable connectionsbetween the mobile stations andthe BSC

Maintain them despite usermovement for the duration of acall.

Release the connections betweenthe mobile stations and the BSC.


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Radio resources Management Functions

The Radio Resource management functionsare:

Power Control

Handover Discontinuous transmission

Call re-establishment

Frequency hopping.

POWER CONTROL


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POWER CONTROL

Power control enables the mobile stationand/or the BTS to increase or decreasethe transmission power on a per-radiolink basis.

Power Control is separately performed forthe uplink and downlink. In both casesthe BSC is responsible for initiating Power

Control; the mobile station and the BTSadopt transmit power according to theBSC Power Control commands.

MEASUREMENTS


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MEASUREMENTS

While a mobile station is active on a call, it has theresponsibility of providing measurement data aboutthe performance of the air-interface to its servingBTS so that the serving BSC can decide if a powercontrol should be performed. Also the serving BTS

measures the performance of the air-interface.Whereas the mobile station measures theperformance of the downlink, the BTS measures theperformance of the uplink.

These measurement includes Strength &Quality ofthe received signal


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The mobile station measures andreports the following measurements tothe BSC regarding the Performance ofthe downlink.

Strength of the signal being receiving

from its serving BTS (in dBm) Quality of the signal being received

from its serving BTS (in bit error rate).

Downlink measurements


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The BTS measures and reports thefollowing measurements to the BSCregarding the performance of theuplink:

Strength of the signal being received

from the mobile station. Quality of the signal being received

from the mobile station.

UPLINK MEASUREMENTS


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POWER CONTROL

When the BSC notices that the signal strengthof a particular radio link measured on theuplink becomes below the lower pre-definedthreshold because the mobile station moves

away from the BTS, it sends a Power ControlCommand to the mobile station to increase itstransmits power (MS_TXPWR) by a pre-defined step (typically 2 dB). The transmit

power of the mobile station can be increaseduntil a maximum defined level is reached.


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POWER CONTROL

The BSC can also send a Power Controlcommand to the mobile station to reducetransmits power when it notices that the

signal strength measured becomes abovethe upper pre-defined threshold. The downlink Power Control process is

similar to the uplink Power Control

process.

REASONS FOR POWER CONTROL


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REASONS FOR POWER CONTROL

Power Control is a GSM feature that canbe enabled or disabled on a per cell basis

One reason to enable Power Control is to

save MS battery power. However, themain reason for Power Control is toimprove the carrier-to-interference ratio

within the cellular network, which helps tominimize interference caused on the othercalls in the surrounding area.

HANDOVER


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HANDOVER

Handover is the process of automaticallyswitching a call in progress from onetraffic channel to another to neutralize the

adverse effects of user movements. Theswitch can be made either to a TCH withinthe same cell or in another cell.

Note that the handover process willnormally be started if power control is nothelpful anymore.


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Why Handover

Use of Cellular principle limits thearea of coverage of a single radio

path. Keep the communication path even

when the user crosses cell

boundaries and enter into new cell.


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Handover Process

HANDOVER PROCESS


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HANDOVER PROCESS

As a mobile station moves away from itsserving BTS towards the coverage area ofneighboring BTSs, the mobile stationmeasurement reports will show a gradualdecrease in signal strength from itsserving BTS while showing an increase inmeasured signal strength from one or

more neighboring BTSs.

HANDOVER PROCESS


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HANDOVER PROCESS

It is the responsibility of the serving BSCto analyze the measurement reports fromthe mobile station and to decide when ahandover should be performed. If it isdetermined that there is a better BTS toserve the call, the serving BSC initiates

the handover procedure.


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Handover types

1)Internal or intra-BSS handover, whichcan be:

Intracell handover

Inter-cell handover. External or inter-BSS handover, which can

be:

Intra-MSC handover. Inter-MSC handover.


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Handover types

If the serving and candidate BTSsdo not reside within the same BSS,

then an inter-BSS handover isperformed, which requires the MSCto serving BTS and the candidate

BTS. This type of handover canalso be divided into intra-MSC andinter-MSC handovers.


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Inter-BSS, Intra-MSC handover PHASES

The MS is continuously listening to thebroadcast channel, measuring thestrength of the currently used radio

channel and the neighboring channels When the old BSS, currentlysupporting the MS, determines that theMS requires to be handed over it will

send a Handover required message tothe MSC with a list of cells to whichthe MS can be handed over.


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On receiving the Handover requiredmessage, the MSC begins the process ofhanding over the MS to the new BSS. TheMSC sends a Handover Request message to

the selected BSS (new BSS) The new BSS takes the necessary actions to

allow the MS to access the new radioresources and then sends the

acknowledgement (Handover Request-ACKmessage to the MSC containing theHandover Reference Number (hrn))


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The MSC informs the old BSS that thehandover can take place

The old BSS sends a command to the

MS to tune to the new radio channel The MS accesses the new radio

resource by sending the messageHandover Access containing the hrn.The hrn is checked by the new BSS toensure it is as expected and thecorrect MS has been captured.


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If this is the correct MS the newBSS sends a handover Detectmessage to the MSC

When the MS is successfullycommunicating with the new BSS itsends to it a Handover Completemessage.


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The new BSS forwards thismessage to the MSC.

The MS is now served by a new

BSS, the trunk reserved for thecommunication between the MSCand the old BSS can now bereleased.

Consequently the radio trafficchannel is released.


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Conditions that can trigger a handover

The BSC will perform a handover due to: Distance (or propagation delay) between the

mobile station and the BTS becomes too big Received signal level (or signal strength) on the

downlink or uplink becomes too bad Received signal quality on the downlink or uplink

becomes too bad Path loss situation for the mobile station to

another cell is better Duration that the mobile station stays in a cell


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Discontinuous transmission

Discontinuous Transmission (DTX) is amechanism that allows the radiotransmitter to be switched off most of

the time during speech pauses. DTX can be enabled or disabled for the

uplink and/or downlink mode on a per-

cell basis. The following figure showsDiscontinuous Transmission (DTX).

DISCONTINUOUS TRANSMISSION PROCESS


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DISCONTINUOUS TRANSMISSION PROCESS

In the DTX mode, speech is encoded at13kbit/s when the user is effectivelyspeaking, but in a speech pause

information is transmitted at a bit ratearound 500 bit/s. this low rate flow issufficient to encode the background

noise, which is re-generated to ensurethat the listener does not think that theconnection is broken (comfort noise).


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Reasons for DTX

When DTX is applied, actual transmissionon the radio path is reduced. This will causea decrease of the interference level in co-channel cells (using the same frequency).

Another advantage will appear when usingDTX in the uplink mode: it saves batterypower for the mobile station. However, a

disadvantage of the DTX mode is that itslightly deteriorates the quality oftransmission.

CALL RE-ESTABLISHMENT


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CALL RE ESTABLISHMENT

Call re-establishment enables the mobilestation to resume the contact with thecellular network when the connection to aparticular BTS is suddenly broken. This mayhappen because of a brutal propagationloss, due to obstacles such as bridges andtunnels.

Call re-establishment is a GSM feature thatcan be enabled or disabled on a per cellbasis.

FREQUENCY HOPPING


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The Frequency Hopping functionpermits the dynamic switching of

radio links from one carrierfrequency to another. Frequencyhopping changes the frequency

used by a radio link every newTMDA frame in a regular pattern.

The reasons of using Frequency


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g q yHopping

Decreasing the probability ofinterference

Frequency Hopping will spread theannoyance of interference overdifferent mobile stations in a

particular cell. Suppressing the effect of fading


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Frequency hopping process

Mobility Management


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y g

1)Network attachment :It is the process ofselecting an appropriate cell (radio frequency) bythe mobile station to provide the availableservices, and making its location known to the

network. When a mobile station is switched on it Measure

the BCCH channels

Search for a suitable cell , it registers its presencein this cell (registration process) by means of alocation updating procedure.


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HLRSS 7

BSC

MSC 2

VLR 1

VLR 2

BSC

BSC

BSC

LA 1

LA 2

LA 3

BTS

BTS

BTS

BTS

BTS

BTS

Location Area

MSC 1

Location Related Identities


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LAI = MCC + MNC + LACMCC - Mobile Country Code.

MNC - Mobile Network Code.

LAC - Location Area Code .

Maximum length of LAC is 16 bits.

Thus there can be maximum of 65,536 Location Areas defined

in one GSM PLMN.

MCC MNC LAC

3 digits 2 digits 2 octets max

LAI

( 16 bits )


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Locating a mobile station

As an active Mobile Station (MS) movesaround in the coverage area of a PublicLand Mobile Network (PLMN), it reports itsmovements so that it can be located whenrequired using the Location Updateprocedure.

When a Mobile-services Switching Center

(MSC) in the network needs to establish acall to an MS operating in its area thefollowing happens:


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A page message is broadcast which containsthe identifications code of the MS Thebroadcast is limited to a cluster of radio cellsthat together form a location area. The last

reported position of the MS identifies thelocation area to be used for the broadcast. The MS monitors the page messages

transmitted by the radio cell in which it is

located and, on detecting its own identificationcode, & responds by transmitting a pageresponse message to the Base TransceiverStation (BTS).


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Communication is then establishedbetween the MSC and the MS via the BTSthat received the page response message.


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LOCATION AREA IDENTITY


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As the MS moves through the networkarea the signal received graduallydiminishes in strength. At this point theMS re-tunes to the channel that has

become dominant and examines the LAIcode that it is broadcasting. If thereceived LAI code differs from that storedon the SIM, then it confirms that MS has

entered another location area and initiatesa location update procedure to report thechange to the MSC. At the end of theprocedure the LAI code in the SIM is also


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Location Management By VLR

A VLR contains a data record for each of the MSthat are currently operating in its area. Eachrecord contains a set of subscriber identity codes,related subscription information, and a Location

Area Identity (LAI) code. This information is usedby the MSC when handling calls to or from an MSin the area.

When an MS moves from one area to another, theresponsibility for its supervision passes from one

VLR to another. A new data record is created bythe VLR that has adopted the MS, and the oldrecord is deleted.

L i M B HLR


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Location Management By HLR

HLR Stores the address of the VLR in the areawhere the subscribers MS is currently located,and the address of the associated MSC.

The location information enables incoming calls tobe routed to the MS. In The absence thisinformation indicates that the MS is inactive andcannot be reached.

When an MS moves from one VLR area toanother, the location information in the HLR is

updated with the new VLR and MSC addresses.The VLR then creates a new entry for the MS,using subscription data copied from the HLR.


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End

Thank you

introduction to gsm pwl

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