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GSM

Global System for Mobiles

Mrs. Shweta Shah

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TOPICS

GSM Network Components

GSM Terrestrial Interfaces

GSM CHANNELS

GSM RADIO LINK

CALL MANAGEMENT

Background to GSM

1G : Advanced Mobile Phone Service (AMPS) Analog, Circuit Switched, FDMA, FDD

2G : Digital Advanced Mobile Phone Service (D-AMPS) Digital, Circuit Switched, FDMA, FDD

2G : Global System for Mobile (GSM) Digital, Circuit Switched, FDMA and TDMA, FDD

2G : Code Division Multiple Access (CDMA) Digital, Circuit Switched, FDMA, SS, FDD

(provide the capacity necessary

to transmit appropriate data

signals between two access

points creating an interface to

the network.)

GSM SUBSCRIBER SERVICES

telephony (teleservices) data (bearer services).

(voice services that provide

subscribers with the

complete capability

(including necessary

terminal equipment) to

communicate with other

subscribers)

normal telephony and emergency calling

Wireless Data Technology Options

Circuit-Switched or Packet-Switched

Advantages of Cellular Communications

many advantages over the existing “land” telephone networks.

advantages for the network provider as well as the mobile subscriber.

Advantages to mobile subscriber

Mobility

Flexibility

Convenience

Advantages to network provider

Network expansion flexibility

Revenue/profit margins

Efficiency

Easier reconfiguration

Frequency Range

GSM 900 Receive (uplink) 890 – 915

MHz Transmit (downlink) 935 – 960

MHz 124 ARFCN

EGSM 900 Receive (uplink) 880– 915 MHz Transmit (downlink) 925 – 960

MHz 174 ARFCN

GSM 1800 Receive (uplink) 1710 – 1785

MHz Transmit (downlink) 1805 –

1880MHz 374 ARFCN

GSM 1900 Receive (uplink) 1850 – 1910 MHz Transmit (downlink) 1930 – 1990MHz 299 ARFCN

ARFCN Bandwidth = 200 kHz 8 TDMA timeslots

ARFCN (Absolute Radio Frequency Channel Number)

The ARFCN is a number that describes a pair of frequencies, one uplink and one downlink. The uplink and downlink frequencies each have a bandwidth of 200 kHz.

The uplink and downlink have a specific offset that varies for each band. The offset is the frequency separation of the uplink from the downlink.

Every time the ARFCN increases, the uplink will increase by 200 kHz and the downlink also increases by 200 kHz.

An ARFCN has an allowed bandwidth of 200 kHz, which corresponds exactly to the carrier separation.

GSM System specifications

GSM Concepts -

Cellular Structure

Cellular Networking technology

that breaks geographic area into cells shaped like honey comb

Cell is the radio coverage

area of one base transceiver station

Cell size

It is determined by the number of MS subscribers who will be operating in that area, and the geographic layout of the area (hills, lakes, buildings etc).

Two types

Large cells

Small cells

Large cells

The maximum cell size for GSM is approximately 70 km in diameter, dependent on the terrain, the cell is covering and the power class of the MS

In GSM, the MS can be transmitting anything up to 8 Watts;

the higher the power output of the MS the larger the cell size.

with no obstructions for miles, the radio waves will travel much further then described

Generally large cells are Remote areas

Areas with few subscribers.

Large areas which need to be covered with the minimum number of cell sites.

Small cells

Small cells are used where there is a requirement to support a large number of MSs, in a small geographic region, or where a low transmission power may be required to reduce the effects of interference.

Typical uses of small cells: Urban areas. Low transmission power required. High number of MSs.

Basic concept

Cellular system developed to provide mobile telephony: telephone access “anytime, anywhere.”

This was a simplified version of the system used today.

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System Architecture

A base station provides coverage (communication capabilities) to users on mobile phones within its coverage area.

Users outside the coverage area receive/transmit signals with too low amplitude for reliable communications.

Users within the coverage area transmit and receive signals from the base station.

The base station itself is connected to the wired telephone network.

First Mobile Telephone System

One and only one

high power base

station with which all

users communicate.

Entire Coverage

Area

Normal

Telephone

System

Wired connection

Problem with Original Design

Original mobile telephone system could only support a handful of users at a time…over an entire city!

With only one high power base station, users phones also needed to be able to transmit at high powers (to reliably transmit signals to the distant base station).

Car phones were therefore much more feasible than handheld phones, e.g., police car phones.

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Improved Design

Over the next few decades, researchers at AT&T Bell Labs developed the core ideas for today’s cellular systems.

Although these core ideas existed since the 60’s, it was not until the 80’s that electronic equipment became available to realize a cellular system.

In the mid 80’s the first generation of cellular systems was developed and deployed.

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The Core Idea: Cellular Concept

The core idea that led to today’s system was the cellular concept.

The cellular concept: multiple lower-power base stations that service mobile users within their coverage area and handoff users to neighboring base stations as users move. Together base stations construct the system coverage area.

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GSM: System Architecture

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GSM network

Divided into Mainly Four Parts:

The Mobile Station

The Base Station Subsystem (BSS)

The Network and Switching Subsystem (NSS)

The operation and Support Subsystem (OSS) or The Operations and Maintenance System (OMS)

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System Architecture

Mobile Station (MS)

Mobile Equipment (ME)

Subscriber Identity Module (SIM)

Base Station Subsystem (BSS)

Base Transceiver Station (BTS)

Base Station Controller (BSC)

Network Subsystem

Mobile Switching Center (MSC)

Home Location Register (HLR)

Visitor Location Register (VLR)

Authentication Center (AUC)

Equipment Identity Register (EIR) 24

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Mobile Station (MS = ME + SIM)

The MS (Mobile Station) is a combination of terminal equipment and subscriber data.

The terminal equipment as such is called ME (Mobile Equipment) and the subscriber's data is stored in a separate module called SIM (Subscriber Identity Module).

Therefore, ME + SIM = MS.

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MS

The hardware has an identity number associated with it, which is unique for that particular device and permanently stored in it.

This identity number is called the International Mobile Equipment Identity (IMEI)

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Mobile Equipment

There are three main types of ME: vehicle mounted equipment

Portable Mobile Phone

Handheld Unit

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Mobile Equipment

Different Terminals : differing principally by their power and application namely,

Fixed terminals : installed in cars. max allowed output power is 20 W.

Portable Terminals : allowed output power is 8W.

Handheld terminals: ~ 2 W. Advanced technologies can decrease it to 0.8 W

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RF power capabilities

Encryption capability

Frequency capability

Short message service capability

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Subscriber Identity Module (SIM)

It is a smart card which plug into the ME and contains information about the MS subscriber

Terminal can not be operated without SIM

SIM is protected by four digit personal identity Number (PIN)

SIM cards are designed to be difficult to duplicate.

SIM achieves separation of user mobility from equipment mobility. This makes international roaming independent of mobile equipment and network technologies.

The SIM also executes the Authentication Algorithm.

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The SIM contains several pieces of information:

International Mobile Subscriber Identity (IMSI) This number identifies the MS subscriber. It is only

transmitted over the air during initialization. Any alterations within the SIM is protected against

IMSI.

Temporary Mobile Subscriber Identity (TMSI) This number identifies the subscriber, it is

periodically changed by the system management to protect the subscriber from being identified by someone attempting to monitor the radio interface.

Location Area Identity (LAI) Identifies the current location of the subscriber.

Also updates the current location of subscriber.

Subscriber Authentication Key (Ki) This is used to authenticate the SIM card.

Most of the data contained within the SIM is protected against reading (Ki).

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Base Station System (BSS)

It comprises a combination of digital and RF equipment.

The BSS provides the link between the MS and the MSC.

The BSS communicates with the MS over the digital air interface and with the MSC via 2 Mbit/s links.

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three major hardware components:

The Base Transceiver Station – BTS

The Base Station Controller – BSC

The Transcoder – XCDR

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The Base Transceiver Station – BTS

It contains the RF components that provide the air interface for a particular cell.

It communicates with the MS.

The antenna is included as part of the BTS.

Each BTS will support 1 or more cells.

Consists of high speed transmitter and receivers (up to 16) depending upon density of the user.

It’s transmission power defines the size of the cell

Requirements for BTS:

reliability, portability & minimum costs Limited control functionality

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The Base Station Controller – BSC

Controls multiple BTS

Performs radio resource management

Assigns and releases frequencies and time slots for all the MS in its area

Reallocation of frequencies among cells

Hand over protocol

Time and frequency synchronization signals to BTS

Power Management of BTS and MS

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FUNCTION BTS BSC Mgmt of radio channels X

Freq. hopping X X

Mgmt of terrestrial channels X

Mapping of terrestrial onto radio channels

X

Channel coding & decoding X

Rate adaptation X

Encryption & decryption X X

Paging X X

U/L signal measurement X

Traffic measurement X

Authentication X

Location’s registry & update X

Handover mgmt X

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BSS Configurations

The BTSs and BSC may either be located at the same cell site “co-located”, or located at different sites “Remote”.

Another BSS configuration is the daisy chain. A BTS need not communicate directly with the BSC which controls it, it can be connected to the BSC via a chain of BTSs.

Daisy chaining reduces the amount of cabling as a BTS can be connected to its nearest BTS rather than all the way to the BSC.

chaining BTSs adds transmission delay hence the length of the chain must be kept sufficiently short to prevent the round trip speech delay.

Other topologies are also permitted, including stars and loops. but they also adds redundancy into the network.

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Transcoder (XCDR)

The Transcoder is used to compact the signals from the MS so that they are more efficiently sent over the terrestrial interfaces.

This is an entity that performs a transcoding function for speech channels and RA (Rate Adaptation) for data channels.

Rate Adaptation is required when the equipment at one end is a TA (Terminal Adapter) connected to a synchronous or asynchronous device (or ISDN network) running at a speed lower than 64Kbps.

It is required to convert the speech or data output from the MSC (64 kbit/s PCM), into the form specified by GSM specifications for transmission over the air interface, that is, between the BSS and MS (16 kbps and vice versa).

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Network Switching System

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Network Switching System

Main role : to manage communications between mobile users and other users like another mobile user, ISDN user, PSTN user etc.

Includes Databases needed in order to store information about subscribers and to manage their mobility.

It manages communications between the GSM network and other Telecommunications networks.

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The components of the Network Switching System are listed below:

Mobile Services Switching Centre – MSC

Home Location Register – HLR

Visitor Location Register – VLR

Equipment Identity Register – EIR

Authentication Centre – AUC

InterWorking Function – IWF

Echo Canceller – EC

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Mobile Services Switching Centre (MSC)

Mainly do call switching like telephone exchange

It is the Central part of NSS.

When the MSC provides the interface between the PSTN and the BSSs in the GSM network it will be known as a Gateway MSC (GMSC).

Allocation of radio resource (RR)

Handover

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The functions carried out by the MSC are listed below: Call Processing

Includes control of data/voice call setup, inter-BSS and inter-MSC handovers and control of mobility management (subscriber validation and

location).

Operations and Maintenance Support Includes database management, traffic metering and measurement, and

Internetwork Interworking Manages the interface between the GSM network and

the PSTN.

Billing Collects call billing data.

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HLR (home location register)

Considered as very important data base that stores information of subscriber belongings to covering area of MSC.

Store the current location of subscriber and services to which they have access.

HLR maintain the following data on a permanent basis: Subscriber ID ( IMSI and MSISDN) Current location Supplementary Service subscription to Supplementary Service information and restriction. Mobile terminal characteristics. Billing/account information. Subscriber status (registered / deregistered) Authentication key and AUC functionality Mobile subscriber roaming number

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VLR (visitor location register)

The VLR contains a copy of most of the data stored at the HLR.

When subscriber enter the coverage area of new MSC, VLR associated to this new MSC and it will request the information about the new subscriber from corresponding HLR.

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Temporary subscriber information resides in VLR includes:

Mobile status (busy/free/no answer etc.).

Location Area Identity (LAI).

Temporary Mobile Subscriber Identity (TMSI).

Mobile Station Roaming Number (MSRN).

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Location Area Identity Cells within the Public Land Mobile Network

(PLMN) are grouped together into geographical areas.

Each area is assigned a Location Area Identity (LAI), a location area may typically contain 30 cells.

Each VLR controls several LAIs and as a subscriber moves from one LAI to another, the LAI is updated in the VLR.

As the subscriber moves from one VLR to another, the VLR address is updated at the HLR.

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•The location area is a group of cells.

•It is the area in which the subscriber is paged.

•Each LA is served by one or more base station controllers, yet only by a single MSC Each LA is assigned a location area identity (LAI) number.

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Temporary Mobile Subscriber Identity

The VLR controls the allocation of new Temporary Mobile Subscriber Identity (TMSI) numbers and notifies them to the HLR.

The TMSI will be updated frequently, this makes it very difficult for the call to be traced and therefore provides a high degree of security for the subscriber.

The TMSI may be updated in any of the following situations:

Call setup.

On entry to a new LAI.

On entry to a new VLR.

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Mobile Subscriber Roaming Number

As a subscriber may wish to operate outside its “home” system at some time, the VLR can also allocate a Mobile Station Roaming Number (MSRN).

This number is assigned from a list of numbers held at the VLR (MSC).

The MSRN is then used to route the call to the MSC which controls the base station in the MSs current location.

The database in the VLR can be accessed by the IMSI, the TMSI or the MSRN.

Typically there will be one VLR per MSC.

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An MSC/VLR service area represents the part of the GSM network that is covered by one MSC and which is reachable, as it is registered in the VLR of the MSC.

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The PLMN service area is an area served by one network operator.

GSM identifiers

Identification of Mobile Subscriber International Mobile Subscriber Identity

(IMSI)/ Mobile Subscriber ISDN number (MSISDN)

Temporary IMSI (TMSI)

Identification of Mobile Equipment International Mobile Station Equipment

Identification (IMEI)

Mobile Station Roaming Number (MSRN)

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IMSI (International Mobile Subscriber Identity)

“real telephone number” of a MS

It is stored centrally in the HLR

MS can have several MSISDNs depending on SIM

It follows international ISDN numbering plan

Mobile Country Code (MCC): upto 3 decimal places

Mobile National Destination Code (MNC): 2-3 decimal places (CODE OF HLR)

Mobile Subscriber Identification Number (MSIN) : maximal 10 decimal places

IMSI= MCC + MNC + MSIN

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IMEI (International mobile station equipment identity )

Uniquely identifies mobile equipment internationally

unique 17 or 15 digits = type approval code + final assembly code + serial number + spare digit

• IMEI = TAC + FAC + SNR + SP

Type Allocation Code: 6 decimal places centrally assigned

Final Assembly Code: 2 decimal places manufacturer-specific code

Serial Number: 6 decimal places uniquely identifying a unit of this model

Spare : 1 decimal place

• Is registered by the Network operator and stored in Equipment Identity Register (EIR)

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TMSI and LMSI

Temporary mobile subscriber identity (TMSI):

32-bit number assigned by VLR to uniquely identify a mobile station within a VLR’s area

Has only local and temporal significance

Is used in place of IMSI for security reasons

Valid in the range of VLR.

Periodically changed by VLR

Local Mobile Subscriber Identity (LMSI)

Is an additional searching key given by VLR

It is also sent to HLR

Both are assigned in an operator specific way

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Location Area Identifier

LAI

Location Area Identifier of an LA of a PLMN

Based on international ISDN numbering plan

Country Code (CC): 3 decimal digits

Mobile Network Code (MNC): 2 decimal digits

Location Area Code (LAC) : maximum 5 decimal digits, or maximum twice 8 bits, coded in hexadecimal

Is broadcast regularly by the BTS on broadcast channel

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Cell Identifier (CI)

Within LA, individual cells are uniquely identified with Cell Identifier (CI).

It is maximum 2*8 bits

LAI + CI = Global Cell Identity

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•The cell is the area given radio coverage by one base transceiver station. • The GSM network identifies each cell via the global cell identity (GCI) number assigned to each cell.

MSRN (Mobile Subscriber Roaming Number)

Temporary location-dependent on ISDN number

Calls are routed to MS by using MSRN

Is assigned by locally responsible VLR to each MS in its area

Is done either at each registration or when HLR requests it for setting up a connection for incoming call

Is done in such a way that current MSC can be determined from it

Structure same as that of MSISDN

To hide exact identity of subscriber.

VCC : Visitor Country Code

VNDC : Visitor National Destination Code

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Gateway Mobile Switching Centre (GMSC)

Gateway is a node interconnecting two networks.

GMSC is the interface between cellular mobile network and PSTN.

GMSC is often implemented in the same machine.

International Switching Centre (ISC) : is used to connect to other mobile networks of international networks.

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Equipment Identity Register (EIR)

The EIR contains a centralized database for validating the IMEI

The EIR database consists of lists of IMEIs organized as :

White List

Contains those IMEIs which are known to have been assigned to valid MS equipment.

Black List

Contains IMEIs of MS which have been reported stolen or which are to be denied service for some other reason.

Grey List

Contains IMEIs of MS which have problems (for example, faulty software). These are not, however, sufficiently significant to warrant a ‘‘black listing”.

Call Processing Functions (EIR)

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Authentication Centre (AUC)

It will normally be co-located with the HLR as it will be required to continuously access and update as necessary the system subscriber records.

The AUC/HLR centre can be co-located with the MSC or located remote from the MSC.

The authentication process will usually take place each time the subscriber “initializes” on the system.

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Authentication Process

The AUC is a processor system. It performs the "authentication " function.

In the authentication process, secure data stored on the SIM card is calculated and compared with the data held in the HLR database.

The Authentication Process is as follows:

1. A random number (RAND) is sent to the Mobile from the AUC.

2. This number is calculated together with Authentication Key (Ki) stored in the SIM card by authentication algorithms (A3, A8), which is held in the SIM card.

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3. The calculation of RAND and Ki will get two results. I. a response called as SRES, which are returned to the AUC.

II. Another is an Encryption Key (Kc) which is stored in the SIM card.

4. The Encryption key is used to encrypt data that is sent over the air interface in order to make the interface more secure.

5. While the mobile is carrying out these calculations, the AUC carries out exactly the same calculations using the RAND and ki stored in the HLR. Then AUC also gets a response.

6. The AUC compares it with the response from the subscriber. If the responses produced by the AUC and the subscriber are the same, the subscriber is permitted to access the network.

7. The Encryption Key produced by the AUC is stored and sent to the BTS to enable ciphering to take place.

Interworking Function (IWF)

The IWF provides the function to enable the GSM system to interface with the various forms of public and private data networks currently available.

The basic features of the IWF are listed below.

Data rate adaption.

Protocol conversion.

Some systems require more IWF capability than others, this depends upon the network to which it is being connected.

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Echo Canceller (EC)

An Echo Canceller is used between PSTN and MSC for all voice circuits.

Echo control is required at the MSC because the GSM system delay can cause an unacceptable echo condition even on a short distance with PSTN.

The GSM system delay may be caused by call processing, speech encoding and decoding etc.

The total round trip delay is approximately 180ms. This would not be apparent to the call between MS. But this case will be very different for the call between MS and land subscriber.

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Operations and Maintenance System

The operations and maintenance system provides the capability to manage the GSM network remotely.

This area of the GSM network is not currently tightly specified by the GSM specifications, it is left to the network provider to decide what capabilities they wish it to have.

The Operations and Maintenance System comprises of two parts.

Network Management Centre (NMC)

The Network Management Centre (NMC) has a view of the entire PLMN and is responsible for the management of the network as a whole.

The NMC resides at the top of the hierarchy and provides global network management.

Functionality of NMC

1. Monitors nodes on the network

2. Monitors GSM network element statistics

3. Passes on statistical information from one OMC region to another to improve problem solving strategies

4. Enables long term planning for the entire network

Operations and Maintenance Centre (OMC)

Operation and Maintenance Center is a centralized facility that supports daily management for the whole GSM system including MSC, HLR, BSC and BTS.

The OMC provides a central point from which to control and monitor the whole network entities as well as monitor the quality of service provided by the network.

An OMC manages a certain area of the PLMN thus giving regionalized network management.

OMC-R : OMC assigned specifically to BSS OMC-S : OMC assigned specifically to NSS

The OMC should support the following functions as per ITS–TS recommendations:

1. Event/Alarm Management.

2. Fault Management.

3. Performance Management.

4. Configuration Management.

5. Security Management.

OMC Functional Architecture

Enhanced Range Of Services

When services were specified on GSM, the current land PSTN and ISDN system had to be taken into consideration.

The services available to a subscriber will be determined by three factors:

The level of service provided by the network provider.

The level of service purchased by the subscriber.

The capabilities of the subscriber’s mobile equipment.

Telephony

Emergency Calls (with/without SIM Card inserted in MS)

Short Message Service Point To Point

Short Message Cell Broadcast (traffic information or advertising)

Advanced Message Handling Service (electronic mail)

Dual Personal and Business Numbers

Speech Services

Data Services

Text files, images, messages and fax may all be sent over the GSM network.

The data rates available are 2.4 kbps , 4.8 kbps and 9.6 kbps .

GSM also provides Fax transmission.

Supplementary Services

A supplementary service is a modification of, or a supplement to a basic telecommunication service. Number Identification

Receiving party requests calling number to be shown. Calling party requests calling number not to be shown.

Call Barring

block all incoming or all outgoing calls. block specific incoming or outgoing calls.

Call Forwarding

Forward all calls. Forward calls when subscriber is busy. Forward calls if subscriber does not answer. Forward calls if subscriber cannot be located.

Call Completion Enable incoming call to wait until subscriber completes

current call.

Enable subscriber to place incoming calls on hold.

Charging Display current cost of call.

Multi-party Three party service.

Conference calling.

infrastructure

Assignment

How is it possible to make emergency call without SIM card?

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