02 introduction gsm arch

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Introduction to Mobile communications



archaic mobile communication

• optical transmission

(smoke/light signals,..)

• acoustic transmission (drums,

alpine horns,...)

Early telecommunications

involved smoke, flags, drums,

and other such methods

to relay messages and

information.

History of communication



Basic Communications System Elements

● Source

● Destination

● Transmission Medium

Source Destination

Transmission Medium

What is communication

Networ

k



Transmission Medium

Wire Wireless

Transition medium

Twisted-pair cable Fiber-optic cableCoaxial cable LOS Non-LOS



What Wireless?



Why Wireless?



Drawbacks of wireless communication



Types of communication systems

TX RX

TX+RX TX +RX

TX+RX TX +RX



Duplex Transmission



Multiple-Access

• Three ways to separate signals.

– Frequency

– Time

– Code

Frequency

C o d e



Multiple Access

Most modern systems use combinations:

•TDMA/FDMA

•CDMA/FDMA



FDMA

– Frequency-division multiple-access.

– Each user is assigned one frequency

frequency

Channel 1 32 4

30 kHzguard

band



TDMA

• Time-division multiple-access

• All users transmit at same frequency.

• Each user transmits at a different time.

User 1

User 2

User 3

User 1

User 2

User 3

guard

time

20 msec

time slot

time



Wave Properties

Fadingdip

point

nλ

Lower frequencies, with longer

wavelengths, are better suited totransmission over large distances,

Higher frequencies, with shorter

wavelengths, are better suited totransmission over small distances,



Questions ?



GSM system overview



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

1

1

121

121

Downlink 935 – 960 MHz

Uplink 890 – 915 MHz

ARFCN Absolute Radio Frequency Channel Number



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

Spectrum Allocation (GSM 1800)



GSM 900 Band

960 MHz

VODAFONEMobinil

890 MHz 915 MHz

935 MHz

902.5 MHz

947.5 MHz

25 MHz

12.5 MHz 12.5 MHz



GSM Coverage

PlanTo provide coverage for a large service are of a mobilenetwork we have 2 Options:

(A) Install one transceiver with

high radio power at the center of

the service area

Drawbacks

• The mobile equipments used in this

network should 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 the service area into

smaller areas (cells)

Advantages

• Each cell as well as the mobile

handsets will have relatively small

power transceivers.

•The frequency spectrum might be

“reused” in two far separated cells,

Unlimited capacity of the system.

Good interference characteristics



Frequency Reuse

Total no of channels (frequencies) = 124

Every channel can be shared between 8 subscribers max.

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

Why do we need frequency reuse?

• The frequency reuse is performed by dividing the whole available

frequencies between a group of neighboring cells which is called frequency

reuse pattern or a “Cluster”, and then repeat this cluster over the whole

network on 2 conditions:

The group of frequencies allocated to a given cell must not be used in the adjacent

cells. Enough distance between the cells in which the same group of frequencies are

reused.



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

Frequency Reuse

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



Dead Spots

Cell Geometry

Problem of omni directional antennas



R R

To solve the dead spot problem

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

• The cell transceiver power.

Tradeoffs

R

Cell Geometrical

Shape



Transceiver

Antenna

Omni-Directional AntennaSectorial Antenna



The cells will take the form of overlapping circles.

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

cells would be Random.

Sectorial Antenna

Sectorial Antenna



cell sectorization



Sectorized Antennas

Further interference reduction by using sectorizedantennas.



Satellite

Global

Suburban& rural

Urban

Pico-Cell

Micro-CellMacro-Cell

In-building

Pico-Cell



Questions ?



GSM Architecture

GSM C l t M d l



GSM Complete Model

SS

BTS

O S S

ISDN

PLMN

PSTN M I N

S C

B G W

H L R

AUC

SMSC-GMSC

SMS-IWMSC

E I R

GWMSC

MSC/VLR

BSC

BSS



The Mobile Station MS

MS = ME + SIM

• Mobile Equipment (ME):

it is the terminal used by the user.

• Subscriber Identity Module (SIM)

It is an electronic microchip for storing

information about the subscription.



• Radio transmission termination.

• Radio channel management.

• Speech encoding / decoding.

• Flow control of data.

• Mobility management.

• Call control.

• Performance measurement of radio link.

Function of a Mobile Station



International Mobile Equipment Identity (IMEI)

SNR FAC

TAC

IMEI

TAC = Type Approval

CodeFAC = Final Assembly

CodeSNR = Serial Number

Identifies the Type

Identifies the manufacturer



ME is uniquely identified by the IMEI

TAC(6 digits)

FAC

(2 digits)

SNR

(6 digits)

SVN

(2 digits)

Final Assembly Codes (FAC)

01 ,02 AEG

07 ,40 Motorola

10 ,20 Nokia

40,41,44, Siemens

47 Optional International

51 Sony

51 Siemens

51 Ericsson

60 Alcatel

TAC: Type Approval Code

Places that is centrally assigned bya GSM body.

SVN: Software version Number

Refer to the version of software

SNR: Serial Number

Unique serial number assigned by

the manufacturer



Stores user addresses

IMSI,MSISDN,TMSI, rooming, etc

Personalization

SIM stores user profile (subscribed services)

RAM available for SMS, short numbers, user’sdirectory, etc

Protection codes PIN ,PUK

authentication and encryption features

subscriber’s secret authentication key (Ki)

Authentication algorithm (“secret” algorithm -

A3 – not unique)

Cipher key generation algorithm (A8)



The Mobile Subscriber ISDN Number (MSISDN)

CC(2-3 digits)

NDC

(2-3 digits)

SN

(max. 10 digits)

CC: Country Code

SN: Subscriber Number

NDC: National Destination Code



The International Mobile Subscriber Identity (IMSI)

MCC(3 digits)

MNC

(2 digits)

MSIN

(10 digits)

MCC: Mobile Country Code

International standardized

MSIN: Mobile Subscriber Identification Number

Identification of subscriber in his mobile home network

MNC: Mobile Network Code

Unique Identification of mobile networks within a country.



The Mobile Station Roaming Number (MSRN)

CC(2-3 digits)

NDC(2-3 digits)

SN(max. 10 digits)

CC: Country Code

Of the visited network

SN: Subscriber Number

Assigned by the current VLR and is unique in the current mobile

network

NDC: National Destination Code

Of the visited network

Used by the HLR for rerouting calls to the MS.

It is assigned upon demand by the HLR on a per call basis.



Temporary Mobile Subscriber Identity (TMSI)

Used in place of the IMSI for

define identification and

addressing of the MS.

The purpose is to keep

subscriber information confidentialon the air interface.

The TMSI is relevant on the local

MSC/VLR level only

Changed at new MSC/VLR areaor at time intervals (ID hopping)

B St ti S b t (BSS)



Base Station Subsystem (BSS)

MS

The Base Station Sub-system (BSS) is responsible for all theradio-related functions in the system.

BTS BSC

BSS

T

R

A

U



•Ciphering using the ciphering key.

•Channel coding and interleaving.

•Records and passes to the BSCthe Signal strength measurements.

•Modulation and demodulation

Function of BTS



• Paging

• Channel allocation

• Dynamic power control in MS and BTS

• Locating the MS

• Handover

• Frequency Hopping

Function of BSC

Network Switching Subsystem (NSS)



Network Switching Subsystem (NSS)

MSC/VLR

HLR

AUC

BSC

NSS

BTS

BTS



• Switching and call routing to or from MS.

• Charging.

• Service provisioning.

• Control of connected BSC’s.

• Access to PSTN.

• Provides the gateway functionality to

other networks.

• One MSC controls more than one BSC.

Function of MSC



• Its function is to connect the PLMN to the PSTN or to the other PLMN existingin the country.



The HLR is a centralized network database that stores and manages all

mobile subscriptions.

Permanent information

• IMSI, MSISDN

• Services subscribed

• Service restrictions (e.g. roaming restrictions)

• Parameters for additional services

• info about user equipment (IMEI)

• Authentication data

Temporary information

Link to current location of the user:

• Current VLR address (if avail)

• Current MSC address (if avail)

• MSRN (if user outside PLMN)



MSC/VLR



Eventually integrated with HLR

Triplets



Authentication Center

Triplet

RAN

D

SRE

S

Kc

Home Location Register

Triplet

RAN

D

SRE

S

Kc

Visitor Location Register

RAN

D

SRE

S

Kc

Triplets

The Authentication Procedure



Visi tor Locat ion RegisterRand SRES AUC Kc

SRES AUC SRES MS

Barred

Base Stat ion Subs ystem

SIM Card

A8

SRES MS

= Access

Kc

SRES MS

Rand

Rand

A3 KiRand

The Authentication Procedure

VLR

The Ciphering Procedure



The Ciphering Procedure

Base Transceiver StationDecryption

Algorithm A5

Encryption

Algorithm A5

From the BSC To th e BSC

Mobile Station

TDMA

Frame Number

DATA

E n c r y p t e d

D A T A

DATA

E n c r y p t e d

D A T A

Encryption

Algorithm A5

Decryption

Algorithm A5

Kc

TDMA

Frame Number

DATA DATA

Kc

Air Interface



Mobile Terminating Calls

Mobile Originating Calls

Mobile Calls

o e r g na ng



g gCalls

1. The mobile sends a call request along with its

IMSI to its serving MSC/VLR which will mark the

mobile as busy.

2. Authentication is performed by the MSC to

verify the mobile access to the network, and

then ciphering is initiated in order to protect the

mobile call on the radio path.

3. The mobile sends a call setup message to theMSC with information about the call type,

services required and the dialed number.

4. MSC checks the categories of the mobile

subscriber to verify that he is authorized to use

the required services, and then a link is

established between the MSC and BSC.

5. BSC checks the mobile serving BTS for an idletraffic channel and then orders that BTS to seize

this channel for a call.

6. The BSC informs the MSC when the traffic

channel assignment is complete, and then the

MSC/VLR starts to analyze the dialed number

and sets up a connection to the calledsubscriber.

BSC

PSTN

MSC

Mobile Terminating



PSTN

HLR

GMSC MSC BSC

1

3

2

5

4

7

6

9

8

gCalls



Transcoding and Rate Adaptation

We put TRAU at BSC side to save air resources

Questions ?



Questions ?

02 introduction gsm arch

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