how gsm works

Prepare by: Sanjay Gohel 1

BTS 3

BTS 2

BTS 1


GSM-900 System Specifications

Ø Frequency Range : 890 MHz to 915 MHz for Uplink

935 MHz to 960 MHz for Downlink.

Ø Uses FDM / TDMA technology for Downlink / Uplink.

Ø 124 Reusable Spot frequencies of 200 kHz bandwidth each.

Ø Each Spot Frequency carries 8 Time slots for Traffic/Signalling.

Ø Separate Logical Signalling & Traffic channels.

Ø Compatible to ISDN & PSPDN.


GGlobal SSystem for MMobile

Ø New rapidly expanding & successful technology.

Ø Fully Digitized technology for better speech quality.

Ø Available in almost every part of the world.

Ø Fully compatible with existing Fixed Line Network.

Ø Single number operation with World wide Roaming.

Ø Very well defined interfaces makes truly open system

Ø Encryption of user information

Ø Available versions, GSM 900, 1800 & 1900


GSM Network Architecture

BTS 3

BTS 2

BTS 1

BSC

PSTN N/W

HLR

VLR

AUC

EIR

SMSC

VMSC

MSC

OMC

NSS

BSS


Mobile Station (MS)

Ø Mobile Station consist of two units

Mobile Hand set Subscriber Identity Module

Mobile Hand set is one of the most complicatedGSM device. It provides user the access to theNetwork. Each handset has unique identity no.called IMEI.

SIM is a removable module goes into the mobilehandset. Each SIM has unique number calledInternational Mobile Subscriber Identity (IMSI).It has built in Micro-computer & memory into it.It contains the ROM of 6 to 16KB,RAM of 128 to 256 bytes and EEPROM of 3 to 8KB


Base Transceiver Station (BTS)

BTSAntenna

ArrangementØ BTS has a set of Transceivers to talk to MS.

Ø One BTS covers one or more than one cell.

Ø Capacity of BTS depends on no of Transceivers.

Ø BTS is connected to BSC via A’bis interface.

Ø Transmission rate on A’bis is 2 Mbps (G.703).

Ø Interface between MS & BTS is called Air I/f.

Ø Transmission rate on Air interface is 13 Kbps.

Ø BTS controls RF parameters of MS.

Ø Each TRx has 8 TDMA channels to carry Voice & signalling.


Base Station Controller (BSC)

Ø BSC controls several BTSs.

Ø BSC manages channel allocation, & Handoverof calls from one BTS to another BTS.

Ø BSC is connected to MSC via A interface.

Ø Transmission rate on A I/f is 2 Mbps (G.703).

Ø Interface between BSC & BTS is called A’bis I/f.

Ø BSC has database for all of its BTS’s parameters.

Ø BSC provides path from MS to MSC.

BSC


Mobile Switching Centre (MSC)

Ø MSC is heart of the entire network connecting Fixed line network toMobile network.

Ø MSC manages all call related functions andBilling information.

Ø MSC is connected to HLR & VLR for subscriberidentification & routing incoming calls.

Ø MSC capacity is in terms of no of subscribers.

Ø MSC is connected to BSC at one end and FixedLine network on other end.

Ø Call Detail Record (CDR) is generated for each & every call in the MSC.

MSC


Visiting Location Register (VLR)

VLR

MSC

Ø Active Subscriber is registered in VLR.

Ø It has temporary data base of all the activesubscribers used for their call routing.

Ø HLR validates subscriber before registration.

Ø MSC ask VLR before routing incoming call.


Home Location Register (HLR)

HLR

MSC

Ø All Subscribers data is stored in HLR.

Ø It has permanent data base of all the registeredsubscribers.

Ø HLR has series of numbers for all subscribers.


Authentication Centre (AUC)

HLR

MSC

Ø Authentication is a process to verify the subscriber SIM.

Ø Secret data & verification algorithm are stored in to theAUC.

Ø AUC & HLR combined to authenticate the subscribers.

Ø Subscriber authentication can be done on every call, if required.

AUC


Equipment Identity Register (EIR)

EIR

MSC

Ø All subscriber's mobile handset data is stored in EIR.

Ø MSC asks mobile to send it IMEI & then checks it withdata available in EIR.

Ø EIR has different classification for mobile handsetslike, White list, Grey list & Black list.

Ø According to category the MS can make calls or can bestopped from making calls.


Operation & Maintenance Centre (OMC)

OMCØ All the network elements are connected to OMC.

Ø OMC monitors health of all network elements & carry out maintenance operation, if required.

Ø OMC link to BTSs are via parent BSC.

Ø OMC keeps records of all the faults occurred.

Ø OMC can also do Traffic analysis.

Ø OMC may prepares MIS Report for the network.


VMSC & SMSC

MSC

SMSC VMSC

Ø Voice Mail Service Centre : To provide Voice Mail service.

Ø It has database for all the VMS subscribers & also stores voice messages for them.

Ø Short Message Service Centre : To provide text message service.

Ø To send short messages from mobile to another mobile subscriber.Messages can also be sent by Manual Terminal connected to SMSC.


Call Processing in GSM … 1

BTSBSC PSTN N/WMSC

Mobile Originated Call

Ø Mobile originated calls goes to BTS first & then to BSC.

Ø BSC forwards this call to MSC.

Ø MSC does authentication & call routing as per dialed digits.

Ø If it the call to another mobile subscriber then the process for thatcall is same as Mobile Terminated Call as shown in next slide.


Call Processing in GSM … 2

Mobile Terminated Call

BTSBSC PSTN N/WMSC

Ø Mobile terminated calls comes to MSC first, where HLR/VLR inquiryis carried out and as per information the MS is paged in suitable BSC.

Ø BSC forwards this page to all children BTS where actual paging is done.

Ø After BTS gets response from Mobile it allocates channel for this call.

Ø After end of the call BTS informs BSC & MSC.


Road Map of Mobile Data Communications

9.6 Kb/sHSCSD

GPRS

EDGE

3GTECHNOLOGY

TIME 1999 - 2000 - 2001 - 2002

2G 2.5G 3G


Next Generation Services Requirements

ü High Speed Packet Switched Data Connectivity

ü Multimedia applications

ü High Speed Remote Corporate LAN Access

ü Web Browsing, E-mail, fax, wireless imaging

ü Video Telephoney and TV broadcasting

GSM Mobile service started in India with 2nd Generation serviceCapabilities, which supports voice & low speed circuit switcheddata.

To support ever increasing demand of the customers many moreadvanced services are getting developed on GSM or other digitalmobile platforms. A few of them are listed below :

ü Blue Tooth Services


Present Mobile Services & Technology

GSM (Phase 2) SERVICES :

u Voice Servicesu FAX & Data Services upto 9.6 Kbps , Short Message services

Limitations : Only circuit switching is possible, Spectral Efficiency is low Data Rate is limited to 9.6 Kbps

HSCSD (GSM Phase 2+) SERVICES :

Ø Normal Data rate enhanced upto 57.6 Kbps Ø Typical applications :Internet on mobile, Mobile LAN, Wireless real time Applications,File Transfer

Limitations : Only circuit switching is possible, Spectral Efficiency is low


Efficient Spectrum Utilization in GPRS

8

7 GPRS

6

5

4

3 Circuit Switched Traffic

2

1

Time

Res

cou

rce

usa

ge

in C

ell

One time slot statically assignedTime slot dynamically assigned


Next Generation Services & Technology …1GPRS SERVICES :

u Offer Data services upto 170 Kbpsu Optimized protocol for packet switched data. u Typical application :Internet on mobile, File transfer, Wireless LAN, Remote control &Monitoring of Real time application, etc.~ Good Spectral Efficiency & Low Delay

EDGE SERVICES :

Ø Offer Data services upto 400 Kbps. Ø Voice over Packet switched network. Ø Supports IP based application.Ø Typical application :Internet on mobile, Mobile LAN, Video Phone, Wireless real time Applications, File Transfer~ High Spectral Efficiency & Higher Data rates


Next Generation Services & Technology …23G (UMTS) SERVICES :

u Offer Data services upto 2 Mbpsu Voice over Packet switched IP based network. u It can offer data rates as per requirements from 64 Kbps to

2 Mbps in rural or urban environment.

u Typical application :Speech (symmetric) - teleconferencing & voice mailMessage (asymmetric) - SMS, E-mail, etc.Switched data (symmetric) - low speed LAN, Internet, etc.Medium multimedia (asymmetric) - E-commerce, LAN, Internet,

public messaging, etc.High multimedia (asymmetric) - Video clips, on-line shopping,

fast LAN & InternetHigh interactive multimedia (asymmetric) - Video Telephony,

Video conferencing

~ Good Spectral Efficiency & Low Delay,~ Complete mobility to the user.


Next Generation Services & Technology …3

The following is the comparison of Present & Next GenerationServices.

Parameter GSM EDGEModulation Gaussian MSK 3π /8-8PSKBit Rate 270.833 Kbps 812.499 KbpsChannel Bandwidth 200 KHz 200 KHz

Pulse shapingGaussian prefilter,

BT = 0.3Linearized

GMSK pulse

Modulation typeNon-linear,

Constant envelopeLinear

The next generations will offer more services on the MobileTerminals. However at the same time it requires more spectrum for its effective implementation & to support subscriber growth.

how gsm works

Documents