9A. GSM - Architecture

7/28/2019 9A. GSM - Architecture

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GSM NETWORKARCHITECHTURE


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The Main Cellular Standard


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Frequencies for Major Mobile

Standards


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Frequency Specifications of

GSM Systems


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GSM Network Architecture


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GSM Geographical Network

Structure

Cell:

Basic unit of a cellular system

Defined as the area of radio coverage

Is assigned unique number called Cell Global Identity(CGI)


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Structure

Location Area (LA)

Is defined as a group of cells

Within the network, a subscribers location is linked to

the LA in which it is currently located

The identity of a LA (LAI) is store in the VLR

When the MS crosses the boundary between to cells

belongs to different LA, it must report its new location

area to the network


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Structure

MSC service Area

An MSC service area is made up of a number of LA

Represents the geographical part of the network

controlled by one MSC


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Structure

PLMN Service Area

Entire set of cells served by one network operator

There may be several PLMN service area, one for

each mobile operator


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Structure

GSM service area

Entire geographical area in

which a subscriber can gain

access to GSM network

GSM service area increases

as more operators signs

contract agreeing to work

together

International roaming is the

term applied when an MS

moves from one PLMN to

another when abroad


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GSM Network Architecture


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GSM Network Components


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GSM Subsystems

GSM system is composed of

Mobile station: Mobile equipment, including SIM card

Base station subsystem (BSS): including Base Station

(BTS, BS) and Base Station Controller (BSC)

Network Subsystem (NSS): being responsible for call

routing

Operation Subsystem (OSS/NMS): being responsible

for network management


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GSM Subsystems


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Mobile Station


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Mobile Station

Mobile equipment:

The cellular telephone or the vehicular telephone

Address / identifier

IMEI (international Mobile Equipment Identity)

Subscriber Identity Module (SIM) Fixed installed chip (plug-in SIM) or exchangeable card (SIM

card)

Address / identifier:

IMSI (International Mobile Subscriber Identity)

MSISDN (Mobile Subscriber ISDN number >> the telephone number)

TSMI (Temporary Mobile Subscriber Identity)

MSRN (Mobile Station Roaming Number)


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Mobile StationMobile Equipment Structure

Mobile termination

functions

Radio interface (Tx, Rx,

Signaling)

Terminal equipmentfunctions

User interface (microphone,

speaker, keypad)

Function specific of services

(fax, messaging, phone,

etc.)

Independent of GSM

Terminal adaptor functions


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Mobile StationFunctions

Transmission and

receipt

Measurements

Used in making decisionabout signal strength and

handovers

Power saving functions

Discontinuous

transmission & reception


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Mobile StationClasses


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Mobile StationIMEI

Uniquely identifies the mobile equipment

15 digits hierarchical address

Assigned to ME during manufacturing and type

approval testing Type approval procedure: guarantees that the MS

meets a minimum standard, regardless of the

manufacturer

IMEI structure: http://en.wikipedia.org/wiki/International_Mobile_Equipment_Identity


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Mobile StationIMEI

Example

Check IMEI: *#06#


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Mobile StationIMEI


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Mobile StationIMEI Management

Protection against stolen and malfunctioning terminals

Equipment Identity Register (EIR): 1 Database for each

operator; keeps:

WHITE list:

Valid IMEIs Corresponding MEs may be used in GSM network

BLACK list:

IMEIs of all MEs that must be barred from using the GSM network

Exception: emergency calls (to a set of emergency numbers)

Black list periodically exchanged among different operators

GRAY LIST: IMEIs that correspond to MEs that can be used, but that, for some reason

(malfunctioning, obsolete SW, evaluation terminals, etc), need to be tracked

by the operator

A call from a gray IMEI is reported to the operator personnel


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Mobile StationSIM Card

Subscriber Identification Module (SIM)

Smartcard single chip computer, containing OS, file

system, applications

Owned by operator


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Store user addresses

IMSI, MSISDN, TMSI, MSRN

Authentication and encryption features

All security features of GSM are stored in the SIM for maximum

protection Subscribers secret authentication key (Ki)

Authentication algorithm (secret algorithm - A3 not unique)

Cipher key generation algorithm (A8)

Personalization

SIM stores user profile (subscribed services) RAM available for SMS, short numbers, users directory, etc

Protection codes

PIN (Personal Identification Number, 4-8 digits)

PUK (PIN Unblocking Key, 8 digits)


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Mandatory storage in SIM:

Administrative information: describes the SIM mode of operation,

e.g., normal or type approval

IC card identification: unique information identifying the SIM and

the card issuer

SIM service table: indicates which optional services are provided

by the SIM (e.g. last number dialed, call length indication, etc.)

IMSI

Location information: comprising LAI, current value of periodic

location updating timer and location update status

Ciphering key (Kc) and Ciphering key sequence number List of carrier frequencies to be used for cell selection

Forbidden PLMNs

Language preferred


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SIM must be able to manage and provide storage in

accordance with security requirements

PIN

PIN enable / disable indicator

PIN error counter PIN Unlock Key (PUK)

PUK error counter

Subscriber authentication key (Ki)


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Mobile StationIMSI

Uniquely identifies the user (SIM card)

GSM-specific address

Unlike MSISDN normal phone number

Assigned by operator to SIM card uponsubscription

ISMI permanently stored in SIM card

IMSI is permanently stored on the SIM card and

unknown by the subscriber In HLR, it is used as the storage address for the

subscriber data


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Mobile StationIMSI

15 digits hierarchical address

IMSI structure (http://en.wikipedia.org/wiki/IMSI )


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Mobile StationIMSI


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Mobile StationMSISDN

MSISDN: The usual telephone number

Follows International ISDN Numbering Plan (ITU-T

E.164)

Structure


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Mobile StationMSISDN

GSM is the first network to distinguish

The user identity, i.e., IMSI

The number to dial, i.e., MSISDN

Separation IMSI-MSISDN protects confidentiality

ISMI is real user address: never public Faking false identity: need signal ISMI to the network, but IMSI

hard to obtain

Separation IMSI-MSISDN allows

Easy modification of numbering and routing plans

Single IMSI may be associated with several MSISDNnumbers

e.g., different services (fax, voice, data, etc.) may be associated

with different MSISDN number


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Mobile StationAddress Temporary

TMSI

32 bits

Assigned by VLR within

an administrative area

Has significance only in

this area

Transmitted on the radio

interface instead of IMSI

to reduce problem of

eavesdropping

MSRN

An MSISDN number

CC, NDC of the visited

network SN assigned by VLR

Used to route calls to a

roaming MS

Subscriber number (SN)

assigned to providerouting information

towards actually

responsible MSC


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Mobile StationAddress Temporary


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Mobile StationSummary of GSM Numbers


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Fixed InfrastructureComponents and Interfaces


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Fixed InfrastructureComponents and Interfaces


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Fixed InfrastructureBase Station Subsystem (BSS)


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Fixed InfrastructureBase Station Subsystem (BSS)

Base Transceiver Station (BTS)

Transmitter & receiver devices, voice coding and

decoding, rate adaptation for data

Provides signaling channels on radio interface

Base Station Controller (BSC)

Performs most important radio interface management

functions:

Radio channel allocation / de-allocation

Handover management Transcoder Controller (TRC)

Provide BSS with rate adaptation capabilities


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Fixed InfrastructureBSS- BTS

TRX radio interface functions:

GMSK modulation-demodulation

Channel coding

Encryption/decryption

Burst formatting, interleaving

Signal strength measurements

Interference measurements

Fi d I f t t


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Fixed InfrastructureBSS - BTS

Fi d I f t t


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Fixed InfrastructureBSS - BSC

Functions

Switch calls from MSC

to correct BTS and

conversely

Protocol and coding

conversion: for traffic(voice) & signaling

(GSM-specific to ISDN

specific)

Manage MS mobility

Enforce power controlDB contains:

State information of all BTS

BTS software

Fi d I f t t


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Fixed InfrastructureBSS Transcoder Controller

BTS:

Collects speech traffic

Deciphers and removes error protection

Result: 13 kbps air-interface GSM speech-

coded signal

MSC:

A 64kp/s ISDN switch

Needs to receive ISDN-coded speech : 64

kbps PCM format (A-law)

Transcoding and

Rate Adaptation

Unit (TRAU)

Needed !

Fi d I f t t


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Function

Perform transcoding and rate adaptation

Transcoding

Convert PCM coder information to GSM coder information

Rate adaptation Involves the conversion of information arriving from MSC/VLR at

rate of 64 Kbps to rate of 16 Kbps or transmission to a BSC

16 Kbps contains 13 Kbps of traffic and 3 Kbps of in-band signaling

information

Hardware unit: TRAU (Transcoder and Rate Adaptation

Unit)

Fi d I f t t


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Fixed Infrastructure


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Fixed InfrastructureNetwork Switching Sub-system



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Fixed InfrastructureNetwork Switching Sub-system

Elements

Mobile Switching Center (MSC) / Gateway MSC

Home Location Register (HLR) / Authentication

Center (AUC)

Visitor Location Center (VLR)

Equipment Identity Register (EIR)



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Fixed InfrastructureNSS Mobile Switching Center (MSC)

An ISDN switch (64 Kbps channel)

Perform all switching and routing functions of a

fixed network switching node

Plus specific mobility-related functions: Allocation and management of radio resources

Management of mobile users

Registration, authentication

Handover execution and control

Paging

A PLMN, in general, has many MSCs

Each MSC is responsible of a set of BSS



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Fixed InfrastructureNSS Home Location Register (HLR)

HLR:

Centralized network database that store and manage

all mobile subscriptions belonging to a specific

operator

Acts as a permanent store for a persons subscriptioninformation until that subscription is cancelled



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HLR:

The information stored includes:

Permanent information associated to a user

Subscriber identity (i.e., IMSI, MSISDN)

Subscriber supplementary service

Subscriber location information (i.e., MSC service area)

Subscriber authentication information

Temporary information associated to a user

Current VLR address (if available)

Current MSC address (if available)

MSRN (if user outside PLMN)



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Functions:

Subscription data management

Communication with MSCs

Communication with GMSCs

Communication with AUC

Communication with VLR/IRL



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Fixed InfrastructureNSS Visitor Location Register (VLR)

VLR acts as a temporary storage location for

subscription information for MSs which are

within a particular MSC service area

there is one VLR for each MSC service area

MSC does not have to contact the HLR every time

the subscriber uses a service or changes its status



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When MS moves to a new service area



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While a MS is within one MSC service area, the

VLR contains complete copy of necessary

subscription details:

Identity numbers of subscriber (IMSI, MSISDN)

Supplementary service information

Activity of MS (e.g. idle mode)

Current LA of MS



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Fixed InfrastructureNSS Authentication Center (AUC)

AUC is an intelligent database concerned with

the regulation of access to the network ensuring

that services can be used by those who are

entitled to do so and that the access is achieved

in a secure way AUC provides information which is then used by

MSC/VLR

To perform subscriber authentication and

To establish ciphering procedure on the radio linkbetween the network and MSs

The AUC is generally integrated with the HLR



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The information provided is called triplet and

consist of:

Non-predictable random number (RAND)

A Signed RESponse (SRES)

A ciphering key (Kc)



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The principle is that the AUC and the SIM have a unique key for

every subscriber (Ki ) which is used as the basis for generating a

response (SRES) to a random number (RAND) generated by the

AUC. Only the true SIM will be able to generate the correct

response and thus gain access to the network.



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Authentication can by operators choice be

performed during:

Each registration

Each call setup attempt

Location Updating

Before supplementary service activation and

deactivation

There can be exceptions for subscribers belonging to

other PLMN)



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NSS Equipment Identity Register

EIR: database that holds list of allowed

equipment identity, i.e. IMEI number: WHITE/

GREY/ BLACK list

EIR procedure:



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NSS Gateway MSC(GMSC)

GMSC: Interrogating Node

Gateway functionality enables an MSC to interrogate a

HLR in order to route a mobile terminating call.



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Operation & Maintenance Subsystem

Network measurement and control functions

Monitor and initiated from OMC (Operation and

Maintenance Center)

Basic functions:

Network Administration

Configuration, operation, performance management, statistic

collection and analysis, network maintenance

Commercial operation and charging

Accounting and billing

Security management

e.g. EIR management

Summary of User


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Identification

9A. GSM - Architecture

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