gsm system essentials

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GSM System Essentials

Jouko Kurki, 14.3.2006

References: Jrg Erbespcher, Hans Jrg Vogel, Christian Bettstetter, GSM: Switching

services and protocols, Wiley 2001, ISBN 0-471-499903-X, 332 p.


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Development of mobiletelecommunication systems

1G 2G 3G2.5G

IS-95

cdmaOne

IS-136

TDMA

D-AMPS

GSM

PDC

GPRS

IMT-DS

UTRA FDD / W-CDMA

EDGE

IMT-TC

UTRA TDD / TD-CDMA

cdma2000 1X

1X EV-DV

(3X)

AMPS

NMT

IMT-SC

IS-136HS

UWC-136

IMT-TC

TD-SCDMA

CT0/1

CT2 IMT-FTDECT

CDMA

TDMA

F

DMA

IMT-MC

cdma2000 1X EV-DO


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Second Generation Mobile Network Architecture

PSTN / ISDN

MSC MSC

BSSBSS BSS BSS


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Architecture of the GSM system

GSM is a PLMN (Public Land Mobile Network)

several providers setup mobile networks following the GSM standard

within each country

components

MS (mobile station)

BS (base station) MSC (mobile switching center)

LR (location register)

subsystems

RSS (radio subsystem): covers all radio aspects

NSS (network and switching subsystem): call forwarding, handover,

switching OSS (operation subsystem): management of the network


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GSM: overview

fixed network

BSC

BSC

MSC MSC

GMSC

OMC, EIR,

AUC

VLR

HLR

NSS

with OSS

(Core Network)

RSS

(Radio

Sub-

System)

VLR


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System architecture: radio subsystem

Components

MS (Mobile Station)

BSS (Base Station Subsystem):

consisting of

BTS (Base Transceiver Station):

sender and receiver

BSC (Base Station Controller):controlling several transceivers

Interfaces

Um : radio interface

Abis : standardized, open interface with

16 kbit/s user channels

A: standardized, open interface with

64 kbit/s user channels

Um

Abis

A

BSS

radio

subsystem

network and switching

subsystem

MS MS

BTS

BSC MSCBTS

BTS

BSCBTS

MSC


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Basic GSM Architecture

BTS

MS

Um

BTS

BTS

Base St ati on Subsystem (BSS)

A-bisA (64 kb/ s)

BTS: Base Transceiver Stati onBSC: Base Stati on Cont roll er

HLR: Home Location Register

VLR: Visit ed Locat ion Register

OMC: Operat ion & Maint enanceCentreEI R: Equipment I dentity Regist er

AUC: Auth enticati on Centr e

MobileServices

SwitchingCentre

(MSC)

HLREI R VLR OMC

AUC

F

H G

BC

other MSCs

E

PSTN

I SDN

CSPDNPSPDN

other VLRs

ot her BSSs

D

TRAU

At er (16 kb/ s)

BSC


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GSM Interfaces

The component parts of the GSM system interconnectusing standard interfaces. These allows an operatorto purchase different parts of the systemcompetitively, I.e. from different manufacturers.

The more important interfaces are

Um the air interface, e.g. transfers GSM speechhaving bit rate 13 kb/s (bit rate at air interface about34 kb/s, includes data, signalling and error correctioncoding)

A interface between the BSC and MSC Abis Interface Between BSC and MSC, unoffcial,

some suppliers have proprietary solutions, i.e. BSCand BTS need to be from same vendor. However200..500 TRX / BSC, so not an issue.


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Base Station Controller, BSC


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Cell structure

Implements space division multiplex: base station covers a certaintransmission 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


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possible radio coverage of the cell

idealized shape of the cellcell

segmentation of the area into cellsGSM: cellular network

use of several carrier frequencies

not the same frequency in adjoining cells

cell sizes vary from some 100 m up to 35 km depending on user density,

geography, transceiver power etc.

hexagonal shape of cells is idealized (cells overlap, shapes depend on

geography)

if a mobile user changes cells handover of the connection to the neighbor cell


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Frequency planning I

Frequency reuse only with a certain distance between the base stations Standard model using 7 frequencies:

Fixed frequency assignment:

certain frequencies are assigned to a certain cell problem: different traffic load in different cells

Dynamic frequency assignment:

base station chooses frequencies depending on the frequenciesalready used in neighbor cells

more capacity in cells with more traffic

assignment can also be based on interference measurements

f4

f5

f1f3

f2

f6

f7

f3f2

f4

f5

f1


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Frequency Bands / Bandwidth

Uplink 890 915 MHz 25 MHz

Downlink 935 960 MHz 25 MHz

100 KHz 200 KHz 100 KHz

1 43 1242 .

A 200 kHz carrier spacing has been chosen. Excluding 2x100 kHz edges of

the band, this gives 124 possible carriers for the uplink and downlink. The

use of carrier 1 and 124 are optional for operators.

Each Frequency carrier provides 8 Timeslots for 8 users at a time.

Multiple Access Technique

FDMA/TDMA. The total band is divided into 124x200 kHz bands (FDMA).

Each group of 8 users transmit through a 200 kHz band sharing

transmission time (TDMA).


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GSM 900 and 1800 Characteristics

GSM operates at 900 and 1800 MHz frequency range. 1800 MHz frequency range was added later, basically

same functionality, only frequency is different. Morechannels -> more capacity !

In the USA frequency 1900 MHz used instead of 1800MHz


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Functions of the GSM

phone

Power amp

Modulation

Burst generation

Encryption

Interleaving

Channel Coder

Speech coder

sample

Output Power:

GSM900

Max. 2W in 10steps

GSM1800Max 1 W in 8 steps


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Basic GSM voice coder (RPE-LTP)

Incoming analog signal is divided in 20 ms long blocks for coding. Sampling rate is 8 kHz (160samples / 20 ms) and resolution 13 bits.

1. First step is short term analysis yielding parameters for short term analysis filter LPC . This

yields 36 bits for the 20 ms block.

2. LPC filter output works as excitation input for the RPE filter that makes high compression 4 x

47 b / 20 ms (9.4 kb/s)

3. Long term prediction LTP uses previous and current speech block and a calculates a RPE

prediction. This si subtracted before RPE analysis to make that data smaller. There are 4 x 9

bits / 20 ms from LTP analysis.

In total there are 260 bits / 20 ms resulting in bit rate of 13 kb/s


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Speech coding standards for mobileterminals


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GSM FDM/TDM structure

TIME

FREQUENCY

Allocated frequency range divided (Frequency DivisionMultiplexing, FDM) to Frequency channels, each 200 kHz

Within one frequency 8 Time Division Multiplexing (TDM) time

slots (=TS)

One user needs on time slot, so one frequency is adequate for 8

simultaneous calls


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GSM Time slots and burst

In the air interface user bits areinserted to one Time slot. The bits

are modulated by the Gaussian

Minimum Shift keying (G-MSK)

modulation which forms a burst

(duration one TS).

Total 2 x (57+1) user bits / burst;2 x 57 = 114 bits for data. 57 bits

for user, 1 bit for Stealing

bit/Control.

Data rata in air interface:

156.25 b/0.577 ms = 271 kb/s

33.8 kb/s per user (8 TS)

Data: 114/0.577 ms = 24.7 kb/s

/ user

Carrier frequency fc

8 TIME SLOTS / FRAME

NUMBER OF BITSP (dBm)

NORMAL BURST, 0.577 ms

FDMA FRAME DURATION 4,615 ms

TIME SLOT DURATION, 0.577 ms

Training

sequenceGuard


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GSM bit rates

26 TDMA FRAMES, (26 FRAME MULTIFRAME), 26 TDMA FRAMES

DURATION EXACTLY 120 MS. OTHER TIMES DERIVED FROM THIS.

TDMA FRAME 120 MS/26 = 4.615 ms

TIMESLOT DURATION = 4.615 ms / 8 = 577 s

Normal burst: Total 156.25 bits / timeslot. User bits (gross) 2x57 = 114

bits / timeslot = 24.7 kb/s.

bit rate at TDMA frame level: 8 x timeslots 156.25 bits = 1250 bits

bit rate = 1250 /4.615 MS = 270.8 kb/s

GROSS bit rate / USER (NORMAL BURST) 156.25 BITS / 4.615 MS =33.8 kb/s

User data bit rate (Gross) 114 /4.615 ms = 24.7 kb/s. This still includes

error correction etc., so data rte for user traffic is normally less than 20

kb/s.


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GSM AND GPRS SERVICE Bit Rates

SPEECH 13 kb/s (FULL RATE) OR 6.5 kb/s (HALF RATE)

DATA 9.6 OR 14.4 kb/s

HSCSD (HIGH SPEED CIRCUIT SWITCHED DATA):

RESERVES ALL TIME SLOT FOR THE DURATION OF THE

CALL

NUMBER OF TIMESLOTS USED X SPEED / TS, E.G. 4 X 14.4kb/s = 57 kb/s.

GPRS (GENERAL PACKET RADIO SERVICE) PACKET DATA,

USES SEVERAL TIMESLOTS, BUT NOT WHEN VOICE CALLS

NEED THE CAPACITY: 9.05/13.4/15.6/21.4 kb/s PER TIMESLOT

(coding schemes CS-1/ CS-2/ CS-3 CS-4 ), up to 8 TS @21.4 kb/s =

171.2 kb/s. Typically 3-4 TS max, and often used CS-1..2 => 30..50

kb/s user data rate


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Overview of GSM transmission and bitrates


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GSM Air Interface - FDD/TDDGSM uses Frequency division duplexing: Transmission happens at different

frequency from receiving. The duplexing distance is 45 MHz in GSM900


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GSM FDMA / TDMA, Time Slot and burst


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guard period(8.25)

guard period(8.25)

data(57) data(57)

data(39) data(39)

data(36)

training(26)

fixed bits(142)

extended training(64)

mixed bits(58) mixed bits(58)training(26)

extended guard period(68.25)synch. seq.(41)

normal burst

frequency correction

burst

synchronization

burst

dummy burst

access burst

start(3)

start(3)

stop(3)

stop(3)

stop(3)extended start(8)

(1) (1)

In GSM data is transmitted as bursts occupying one time slot (0.577 ms)

of the TDMA frame

For synchronization and signaling other types of burst used

Bursts in GSM TDMA (1)

Voice and

data

Synchronization

and signaling


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Logical Channels in GSMLogical channels are bit pipes on which different kind of data is inserted.

They are formed of Physical Channels that actually transmit the bits usingfrequency bursts.

The key thing is multiplexing the desired data on successively repeated blocks of

data.

Naming conventions: at the end CH= Channel, C=Common, D=Dedicated, F=fast,

S=slow, C=Control, T=Traffic, not fully logical but may help


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Channel Types

1. Traffic channels (TCHs)

The traffic channels are intended to carry encoded speech or

user data.

2. Control Channels (CCHs)

The control channels are intended to carry signalling and

Synchronization data between the base station and theMobile station.

GSM Channels


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Traffic Channels

Traffic channels are intended to carry encoded speech and

user data. Full rate traffic channels at a raw net bit rate of 22.8 Kb/s (TCH/F)

Half rate traffic channels at a raw net bit rate of 11.4 Kb/s (TCH/H)

Data Channels

Speech Channels

Speech channels are defined for both full rate and half ratetraffic channels. The latter for the future system.

Data channels support a variety of data rates (2.4, 4.8 and

9.6 Kb/s) on both half and full rate traffic channels. The 9.6Kb/s data rate is only defined for full rate application.


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Channel bit rates

Traffic Channels

Control Channels


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Security in GSM

Security services access control/authentication

user SIM (Subscriber Identity Module): secret PIN (personal

identification number)

SIM network: challenge response method

confidentiality

voice and signaling encrypted on the wireless link (after successful

authentication) anonymity

temporary identity TMSI

(Temporary Mobile Subscriber Identity)

newly assigned at each new location update (LUP)

encrypted transmission

3 algorithms specified in GSM

A3 for authentication (secret, open interface)

A5 for encryption (standardized)

A8 for key generation (secret, open interface)

secret:

A3 and A8

available via the

Internet network providers

can use stronger

mechanisms


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GSM - authentication

A3

RANDKi

128 bit 128 bit

SRES* 32 bit

A3

RAND Ki

128 bit 128 bit

SRES 32 bit

SRES* =? SRES SRES

RAND

SRES

32 bit

mobile network SIM

AC

MSC

SIM

Ki: individual subscriber authentication key SRES: signed response


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GSM frame structure

Traffic and

SACCH

Non-Associated

Control Channels

6.12 seconds

3 h 28 min 53.76 s

Used to multiplex traffic and control data on time slots repeated at

predetermined intervals


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GSM system features

Adaptive time alignment / Timing Advance (TA)

BS is initially calculated the timing advance of MSs on the basis of

the received access burst on the RACH

The required timing advance for each MS is calculated in terms ofthe number of bit periods and sent to the MS as a 6 bit number.

Timing advances from 0 to 63 bit periods can therefore be

accommodated, giving a maximum BS MS separation of 35 Km


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Timing Advance (TA)


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Power control

RF power control will be used in the GSM MS and BS to reducethe transmit power to the minimum required to achieve the

minimum quality objective and hence reduce the level of co-

channel interference

The MS will be capable of varying its transmit power form itsmaximum output down to 20 mW in steps of nominally 2 dB.

Frequency of power control approximately 2 Hz.

The BS calculates the RF Power level to be used by the MS and

sends a 4 bit number instruction to the corresponding MS


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Transmission withing GSM Network

Some GSM transmission options


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CAPACITY PLANNING

What is Erlang ?


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Capacity and Erlang

To calculate the capacity of the radio cell, we introduce a concept of traffic

that measures the usage of resource. The traffic produced by one user In

Erlangs (x Erl) is calculated as follows:

Typically one person produces a traffic amount of:

20 mErl for normal users; e.g. 1 call / hour, duration 72 s ->

x = 1 x 72 s /3600 s = 0.02 Erl = 20 mErl

60 mErl for business users

Total traffic is the sum of the calls from all users, this is statistically distributed

so statistical methods can be used for capacity planning.


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Trunking gain


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Erlang B-table


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GSM capacity dimensioning examplesCapacity offered by the network has to be large enough to cope with the offered

traffic. Erlang B-table gives the capacity of the network from the number of available

channels (circuits) in Erlangs. E.g. one GSM frequency has 8 time slots, so 8

different connections can exist simultaneously, and the capacity from Erlang Table

with the normally used 2 % call blocking probability) is 3.63 Erl.

The amount of users that can be served is:Available capacity / traffic proced by one user.

E.g. one GSM frequency of 200 kHz offers 8 TDMA channels. Thus from erlang

table the capacity is 3.63 Erl and it can serve 3.63 Erl /20 mErl = 181 normal

users.

Another example is a busy place in city center and during busy hour we need to

support 10.000 people (50 %normal users, 50 % business users) with base

stations having 3 frequencies. How many base stations do we need (no coverage

problems)?

Capacity needed: Average traffic /user = 40 mErl, so total traffic is 40 mErl x

10.000 = 400 Erl. Amount of channels in base station is 8 x 3 = 24. One is needed

for BCCH (Broadcast Control Channel) so there are 23 Ch for traffic. Capacity

from Erlang B-table is the 15.8 Erl. So we need 400 /15.8 = 26 base stations with 3

frequencies.

gsm system essentials

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