ECE Department Florida Institute of Technology Wireless Data Communication Networks Lecture 19:...

ECE DepartmentFlorida Institute of Technology

Wireless Data Communication Networks

Lecture 19: Introduction to GSM


Outline

Brief history

GSM network architecture

Elements of the air interface organization


Global System for Mobile Communication Most popular cellular standard

Started as Groupe Special Mobile

2nd generation global digital standard

Important time stamps:

o 1982 CEPT created Groupe Special Mobile

o 1987 MoU – 13 European countries

o 1989 Transferred to ETSI

o 1991 Publishing of Phase 1 specs

o 1991 Radiolinia Finland – first GSM network

o 1993 Telstra Australia – first non-European GSM network

o 1995 US implements GSM in PCS band

o 1996 Russia and China implement GSM

o Since 1996 tremendous growth all over the world

Official GSM logo

http://upload.wikimedia.org/wikipedia/en/5/5d/GSMLogo.svg


GSM in the World

GSM currently holds about 80% of the market share worldwide


GSM Network Layout

GSM architecture is first standardized cellular system architecture

MSCArea

H LR

MSCArea

VLR

MSCTRAUBSC

BTS

BTS

BSS

MSC Area

BSS

BSSBTS

PSTN

PLMN - Public Land Mobile Netw ork

Gatew ayMSC

NSS

GSM network can be divided into three main subsystems: Base Station Subsystem - BSS Mobile Station Subsystem - MSS Network Switching Subsystem - NSS


GSM Network Components

Network has many functional components

Components are integrated through a network protocol – MAP

Majority of GSM interfaces are standardized – allows interworking of equipment from different vendors

Major equipment vendors: Ericsson, Nokia-Siemens, Alcatel-Lucent, Huawei, Motorola, etc.

BSC

BSC

MSC

VLR

HLR

AUC

EIR

MSC VLR

BTS

BTS

BTS

BTS

BTS - Base StationBSC - Base Station ContollerMSC - Mobile Switching CenterVLR - Visitor Location RegisterHLR - Home Location RegisterAUC - Authentification CenterEIR - Equipment Identity Register

Um Interface

A-Bis Interface

AInterface

D

C

E

F

PSTN

B

B,C,D,E,F - MAP Interfaces


Overview of GSM - Mobile Station

Keyboard

Control

Display

Transmit AudioSignal

Processing

Receive AudioSignal

Processing

ChannelDecoding

DeinterleavingM essage

Regenerator

ChannelEncoding

InterleavingM essage

Generator

Ciphering

Ciphering

RFProcessing

RFProcessing

SIM

Duplexer

Antenna

ANTENNAASSEM BLY

TRANSM ITTER

RECEIVER

TRANSCEIVER UNITCONTROLSECTION

Offered as a phone for voice services

Data services will bring new devices to the market

Two functional parts:

HW/SW radio interface

SIM

• Two types of SIM (Subscriber Identity Module)– Smart Card– Plug-in

GSM Mobile Architectural Diagram


Overview of GSM - Base Transceiver Station

BTS is a set of transceivers (TX/RX).

GSM BTS can host up to 16 TX/RX.

In GSM one TX/RX is shared by 8 users.

The main role of TX/RX is to provide conversion between traffic data on the network side and RF communication on the MS side.

Depending on the application, it can be configured as megacell, macrocell, microcell, pico and femto cell, omni, sectored, etc.

BTS

Typical BTS installations


Overview of GSM - Base Station Controller

Provides a small digital exchange with some mobility tasks

Connects to one or several BTS on the Abis Interface

Connects to the MSC on the A Interface

Designed to offload most of the radio link related processes from the MSC

Provides clock distribution to BTS

Communicates with the OMC

Central Module

SwitchMatrix

Database

OMC

OMC - Operation and MaintenanceCenter

AInterfaceTowards

MSC

AbisInterfaceTowards

BTS

In GSM, BSC handles majority RRM functions• Handoff• Power control• Channel assignment


Overview of GSM - TRAU

TRAU is responsible for trans-coding the user data from 16Kb/sec to standard ISDN rates of 64Kb/sec.

It can physically reside on either BSC side or MSC side.

M S

M S

M S

TRAU

BSC MSCTRAU

BSC MSC

BSC MSCTRAU

BTS

BTS

64 kbps

64 kbps

64 kbps

64 kbps

16 kbps16 kbps

16 kbps

16 kbps

BTS

16 kbps

16 kbps

16 kbps

• If it resides on the MSC side, it provides substantial changes in the backhaul – 4 users over a single T-1/E-1 TDMA channel.

• TRAU, BSC and BTSs form Base Station Subsystem (BSS)

Location of TRAU in GSM


Overview of GSM - MSC

Responsible for connecting the mobile to the landline side

GSM MSC is commonly designed as a regular ISDN switch with some added functionality for mobility support

GSM Network can have more than one MSC

• One of the MSC has an added functionality for communication with public network – Gateway MSC (GMSC)

• All calls from the “outside networks” are routed through GMSC

GSM MSC and Gateway MSC

BTS

BSC MSC GMSC


Overview of GSM - HLR/AuC

Database for permanent or semi-permanent data associated with the user

Logically, there is only one HLR per network

Typical information stored in HLR: International Mobile Service Identification Number (IMSI), service subscription information, supplementary services, current location of the subscriber, etc.

HLR is usually implemented as an integral part of MSC

AUC is an integral part of HLR responsible for ciphering and encryption.

GSM specifies elaborate encryption schemes.

There are three levels of the encryption: A5/1 – Used by countries in Europe and USA

A5/2 – Used by countries and the so called COCOM list

No encryption – used by all other countries

Introduction to GPRS


GSM frequency bands

14 bands defined

Most dominant

o GSM-900 (P-GSM+ E-GSM)

o DCS-1800

o GSM-850 (US)

o PCS-1900 (US)

It is common that a phone is capable of multiband operation

o Dual band - 2 bands

o Quad band - 4 bands


GSM – Frequency <-> ARFCN calculations

ARFCN – Absolute Radio Frequency Channel Number

Example – US PCS allocation

2 2


Overview of GSM - Air Interface

GSM is a FDMA/TDMA based technology

Transmissions are discontinuous

Each user is assigned a timeslot

Each frequency is divided into eight timeslots

Each channel has a 200 kHz bandwidth

Overhead signaling is required for coordination and control

Information is sent in bursts

Several types of bursts

GSM as a FDMA/TDMA Interface

BTS

USER 1 USER 2 .... USER 8

USER 6 USER 7 USER 8 USER 1

USER 1,ARFCN 1

USER 2,ARFCN 1

USER 8,ARFCN 1

USER 9,ARFCN 2

USER 10,ARFCN 2

USER 16,ARFCN 2

ARFCN 1

ARFCN 2


Overview of GSM - Burst Types

Tail Traffic/Signaling Flag Training Sequence Flag Traffic/Signaling Tail

3 57 1 26 1 57 3

Tail Synchronization Training Sequence Synchronization Tail

3 33939 64

Used to carry information on both control and traffic channels

Mixture of data and overhead

GSM defines 8 training sequences assigned in color code mode

Both on the forward and reverse link

Facilitates the synchronization of the MS to the network at the base band

Commonly referred to as S-burst

Only on the forward link

The same sync sequence is used in all GSM networks

Synchronization Burst

Normal Burst



Used when the MS is accessing the system

Shorter in length – burst collision avoidance

Extended synchronization sequence

Used only on the reverse link

• Supports MAHO

• Used to ensure constant power level of the broadcast control channel

• Only on the forward link

Dummy Burst

Access Burst

Tail Predefined Bit Sequence Tail

3 3142

Tail Synchronization Access Bits Tail

8 41 36 3

• GSM mobiles use slotted ALOHA to access the system

• In the case of collision – a hashing algorithm is provided



Sometimes referred to as the F-burst

Provides mobile with precise reference to the frequency of the broadcast control channel

Inserting the F-bursts on the control channel produces spectral peak 67.7 KHz above the central frequency of the carrier

Only on the forward link• Spectral characteristics of the

control channel. • The peak in the spectrum allows

for easier MS network acquisition

Tail Fixed Bit Sequence (All zeros) Tail

3 3142

fc fc+67.7 KHz frequency

Power Spectrum Density

BW = 200KHz

Frequency synchronization burst


Overview of GSM - Physical Channels

Group Name Function Fwd. Link

Rev. Link

BCH BCCH Broadcast control Yes No FCCH Frequency correction Yes No SCH Synchronization Yes No

CCCH PCH Paging Yes No AGCH Access Grant Yes No RACH Random Access No Yes

DCCH SDCCH Standalone dedicated control Yes Yes SAACH Slow-associated control Yes Yes FACCH Fast-associated control Yes Yes

TCH TCH Traffic data and voice Yes Yes

A GSM physical channel can carry several different types of logical channels

Can be divided into two categories: traffic and signaling

Signaling channels can be further categorized as: Broadcast

Common Control

Dedicated Control


Overview of GSM - Frame Hierarchy

Different organization on the superframe level for different logical channels

0 1 2 3 4 5 6 7 21 22 23 24 25

1 TDM A Frame4.615 ms

26 M ultiframe120 ms

51 M ultiframe235.4 ms

51 x 26 Superframe or 26 x 51 Superframe6s 120 ms

Hyperframe3 h 28 min 53 s 760 ms

0 1 2 3 4 48 49 50

0 1 2 3 4 5 6 7 2043 2044 2045 2046 2047

0 1 2 3 4 5 6 7 46 47 48 49 50

0 1 2 3 4 23 24 25

0 1 2 3 4 5 6 7


GSM Time Division Duplex

Communication on the forward and reverse link does not happen simultaneously

Delay of three slots between TX and RX

Time division duplexing avoids RF duplexer at the RF stage

o Reduces the cost of mobile

o Saves battery

0

1 2 3 4 5 6 7 00

1 2 3 4 5765

F o rw a rd L ink - B T S T ra nsm its

R eve rse L in k - M S T ra nsm its


Review questions

What are the elements of GSM system?

What is a BTS?

What is a BSC?

What is a MSC?

What is a HLR?

What is a VLR?

What is TRAU?

What is a SIM card?

What is the Abis?

What are four most prevalent frequency bands for GSM deployment?

Explain GSM FDMA/TDMA access?

How many time slots are on one GSM ARFCN?

What is an ARFCN?

Name 2 basic channel types in GSM?

Name five burst types used in GSM?


Review questions

Draw GSM TDMA hierarchy for traffic channel.

Draw GSM TDMA hierarchy for control channel.

What is the bandwidth of a GSM channel?

What is the difference between DCS-1800 and PCS-1800?

What is a quad-band phone?

Do GSM mobiles use RF duplexers? Please explain.

ECE Department Florida Institute of Technology Wireless Data Communication Networks Lecture 19:...

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