Basic GSM Training Material

Basic GSM Training Material Introduction of GSM; GSM Network Architecture; Access Mode of GSM; Logical Channels in GSM; GSM Cell Structure; GSM Bands; Coding''s in GSM; Call Originating.

BASIC  GSM Training Material

INTRODUCTION  OF  GSM

There are TWO major parts of wireless communication:

TRANSMITTER (Tx) RECEIVER (Rx)

TRANSMITTER

BAND PASS 

LOCAL  OSCILLATO

R

IF OUT

Mobile - BTS - BSC - BTS - Mobile ( If it is same Network)

Mobile - BTS - BSC1 - PSTN - BSC2 - BTS - Mobile ( If it is different Network with same SP or different SP)

• GSM means Global System for Mobile

communications.

identified by different frequencies, with assigned time slots.

(850/900/1800/1900).

GSM

• 2G­2.5 Generation

• Example EDGE (Enhanced data rate for GSM evolution) Wireless data @  384Kbps 

GSM NETWORK ARCHIETECTURE

Base Station Subsystem

GMSC

Network Subsystem

AUC EIR HLR

MOBILE STATION

• Mobile Switching Center (MSC).

− Each MSC Will have a Visitors Location Register (VLR), Home Location Register (HLR), Authentication Center (AUC), 

Equipment Identity Register (EIR)

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• There are also facilities within the system for Operations and 

Maintenance (OMC) & Network Management (NMC) organizations

• The MSC also has the interface to other networks such as Private Land Mobile Networks (PLMN) and Public Switched Telephone Networks (PSTN) and ISDN

Base Station Subsystem ­ BSS

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• Calls are established, supervised and release by MSC

• All MO & MT  calls are routed & connected by MSC

• Charging information is generated and transferred to Billing Centre

• Communicates with BSCs on A interface

• Communicates with HLR, VLR, EIR & SMSC using MAP

6 digits 2 digits 6 digits 1digit

37

IMEI

SIM Card

• SIM Card comes in two Sizes :

− Standard (Credit Card Size)

− Micro (Postage Stamp Size)

• SIM’s (Subscriber Identification Modules) plug into the GSM Mobile

• The SIM Holds all the information related to a subscriber. For example:

− Unique Subscriber number or IMSI (International

Mobile Subscriber Identification)

− Any other user specific information like speed dial

numbers and memories

− K(i) – Secret key for authentication (Stores in SIM)

− Stores information on Last Location

− Stores user information : Speed Dial Lists, Memories etc

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GSM ACCESS MODES

Various Access Method

Frequency multiplex

• Separation of the whole spectrum into smaller frequency

bands

• A channel gets a certain band of the

spectrum for the whole time

f

t

c

t

c

k2 k3 k4 k5 k6 k1

Time multiplex

• A channel gets the whole spectrum for a certain amount of

time

• Advantages:

−only one carrier in the

medium at any time

−throughput high even

for many users

• Disadvantages:

−precise

synchronization

necessary

Time and Frequency Multiplex

• Combination of both methods

• A channel gets a certain frequency band for a certain amount

of time

t

c

k2 k3 k4 k5 k6 k1

• MS The 8 TS’s together are known as a Frame.

• Each one of the Traffic Channel uses a particular ARFCN and Timeslot

• The Combination of a TS number and ARFCN is called a

Physical

• Channel

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Code Multiplex

• Each channel has a unique code

• All channels use the same

spectrum at the same time

• Advantages:

−Bandwidth efficient

−No coordination and

synchronization necessary

−Good protection against

interference and tapping

• Disadvantages:

−Lower user data rates

−More complex signal

regeneration

• Implemented using spread

spectrum technology

k2 k3 k4 k5 k6 k1

f

t

c

Channel Utilization

Example of the TDMA being used in combination with CDMA

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• Uplink lags downlink by 3 Timeslot periods

• Uplink and Downlink use same Timeslot Number

• Uplink and Downlink use same channel number (ARFCN)

• Uplink and Downlink use different bands (45MHz apart for GSM900)

Time Division Multiple Access (TDMA)

POWER CONTROL

MS1

power control commands to the  mobiles

P1

P2

far

Power Control

59

Closed­Loop Power Control

LOGICAL  CHANNELS  IN  GSM

GSM network The network for a variety of user functions also uses the

BCH signal strength It’s a useful way of telling which is the closest BTS

to the mobile It also has information coded onto it, such as the identity

of the network, paging messages for any mobiles need bug to accept a

phone call, and a variety of other information Each mobile will monitor

the power of adjacent cell BCH’s to aid the network in making hand-off

decisions This channel contains system parameters needed to identify

the network and gain access These parameters include the

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channel in the downlink band only The corresponding channel

in the

uplink is effectively left clear The mobile can use this for

unscheduled

or Random Access Channels (RACH)

Traffic Channels are used to carry two types of information to and from the user:

Ø Encoded Speech

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There are two basic types of Encoded Speech channels

Encoded Speech - Encoded speech is voice audio that is converted into digital form and compressed

Full Rate Speech TCH (TCH/FS) - 13 kb/s

Half Rate Speech TCH (TCH/HS) - 5.6 kb/s

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67

Handover (handoff) is switching of an on-going call to a

different

channel or cell

There are four types of handovers

Ø Switching channels in the same cell

Ø Switching cells under control of the same Base Station

Controller (BSC)

Ø Switching cells under the control of different BSCs, but

belonging to the same Mobil service Switching Center (MSC)

Ø Switching cells under control of different MSCs

transmission power may cause interference with neighbor cell.

Ø Power budget Uses handover to improve transmission

GSM   CELL  STRUCTURE

2 6

7 1

2

Capacity & Spectrum

I wish I could increase capacity

without adding NEW BTS!

What can I do?

Network capacity at required QoS

with conventional frequency plan

Subscriber growth

Time

Out of Capacity!!!

Representation of Cells

Ideal cells Fictitious cells

Cell size and capacity

• Cell size determines number of cells available to cover

geographic area and (with frequency reuse) the total capacity available to all users

• Capacity within cell limited by available bandwidth and

operational requirements

• Each network operator has to size cells to handle expected traffic demand

Cell structure

• Implements space division multiplex: base station covers a

certain transmission area (cell)

• Mobile stations communicate only via the base station

• Advantages of cell structures:

−higher capacity, higher number of users

−less transmission power needed

−more robust, decentralized

−base station deals with interference, transmission area etc locally

• Problems:

−fixed network needed for the base stations

−handover (changing from one cell to another) necessary

−interference with other cells

• Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

GSM  FREQUENCY  BANDS

Each Base Transceiver Station (BTS) will be fitted with a number of TX/RX pairs or transceiver modules The number will determine how many frequency channels can

be used in the cell, and depends on the expected number of users The link between the Base Station (BS) and the Mobile Station (MS) is called the Downlink and the link between MS and BS is the uplink These are seperated by

Ø 45 Mhz in GSM

Ø 95 MHz in DCS

Ø 80 MHz in PCS

79

Frequency Bands ­Summary

The frequency range of the uplink and downlink show how the two bands are split into the two directions, rather than an uplink being followed by a downlink 200KHz

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battery will fun down too quickly.

All GSM mobiles are able to control their output power in 2dB steps

The base station commands the mobile to a particular MS TX Level

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GSM Power Steps

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Mobile communications – A full duplex system

Power Control Level Nominal output

Nominal output Power dBm

CODINGS  IN  GSM

GSM speech coding

AIR INTERFACE

MOBILE

BASE TRANSCEIVER STATION

GSM Speech Coding

• GSM is a digital system, so speech which is

inherently analog, has to be digitized.

• The method employed by current telephone

systems for multiplexing voice lines over high speed trunks and is pulse coded modulation (PCM) The

output stream from PCM is 64 kbps, too high a rate

to be feasible over a radio link.

CODING IN GSM

GMSK is a special type of digital FM modulation

Ones and zeroes are represented by shifting the RF carrier by plus or minus 67.708 kHz

Modulation techniques that use two frequencies to represent ones and zeroes are called frequency shift keying (FSK).

In the case of GSM, the data rate of 270.833 kbps is chosen to be exactly four times the RF frequency shift This has the effect of

minimizing the modulation spectrum and improving channel efficiency

FSK modulation where the bit rate is exactly four times the

frequency shift is called minimum shift keying (MSK). In GSM, the modulation spectrum is further reduced by applying a Gaussian pre- modulation filter This slows down the rapid frequency transitions,

which would otherwise spread energy into adjacent channels

GMSK –Gaussian Minimum shift keying

GSM MS Tester Close_Loop_Comand

Data Gen RF

Compare

BER

Speech Coder

A/D

Channel Coder

GMSK Modulator

GMSK Demod

Control

Channel Decoder

Speech Decoder

R F D/A

Mic

Speaker

BER < 2.5% at -102dBm "GO/NOGO"

TELECOM/MOBILE/MSS

103

GSM Receiver = Digital Data Receiver

Rx Performance = Correct receipt of DATA

Bit Error Rate (BER) % =

No of Errored Bits

Total No of Bits Sampled

CALL ORIGINATING IN GSM

105

Originated Call

BS pages MS using the CCCH MS sends RACH

BS ORIGINATED CALL

MS ORIGINATED CALL

MS responds with RACH

BS assigns

MS to SDCCH

or TCH

MS transmits and receives speech on TCH

TCH

Using the FACCH on TCH authentication & location update

Using the SDCCH, authentication, call setup, and TCH assignment

Using the SACCH, BS commands

MS to adjust power + timing,

MS reports power + Rx quality

During inactive timeslots, MS measures adjacent cells, BCCH

Call disconnected by MS or BS

Mobile looks for BCCH after switching on

RACH send channel request

AGCH receive SDCCH

SDCCH authenticate

SDCCH switch to cipher mode

SDCCH request for location updating

Mobile looks for BCCH after switching on

RACH send channel request

AGCH receive SDCCH

SDCCH do the authentication and TMSI allocation

SDCCH require traffic channel assignment

SDCCH send call establishment request

SDCCH send the setup message and desired number

FACCH switch to traffic channel and send ack (steal bits)

FACCH receive alert signal ringing sound

FACCH acknowledge connect message and use TCH

TCH conversation continues FACCH receive connect message

Call establishment from a mobile

Mobile looks for BCCH after switching on

Receive signaling channel SDCCH on AGCH

Receive alert signal and generate ringing on FACCH

Receive authentication request on SDCCH

Generate Channel Request on RACH

Answer paging message on SDCCH

Authenticate on SDCCH

Receive setup message on SDCCH

Receive traffic channel assignment on SDCCH

Mobile receives paging message on PCH

FACCH switch to traffic channel and send ack (steal bits)

Call establishment to a mobile

Performance characteristics of GSM

• Communication

−mobile, wireless communication; support for voice

and data services

−better frequency efficiency, smaller cells, more

customers per cell

• High transmission quality

−high audio quality and reliability for wireless,

uninterrupted phone calls at higher speeds (e.g., from cars, trains)

• Security functions

−access control, authentication via chip-card and PIN

• Abuse of private data possible

• High complexity of the system

• Several incompatibilities within the GSM standards

Thank You