mobile network evolution – part 1 gsm and umts · 2g to 3g evolution: gsm - gprs - umts gsm ran...

Integrated Communication Systems Group

Ilmenau University of Technology

Mobile Network Evolution – Part 1

GSM and UMTS GSM

Cell layout Architecture Call setup Mobility management Security

GPRS Architecture Protocols QoS

EDGE

UMTS

Architecture

2


Mobile Communication Networks (RCSE)

2G to 3G Evolution: GSM - GPRS - UMTS

GSMRAN

Base station

Base stationcontroller

Base station

Base station

MSC

ISDN

GSM Core (Circuit switched)

HLRAuCEIR

GMSC

TransmissionATM based

GSM

3



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

GSM system comprises 3 subsystems

Radio Access Network: covers all radio aspectsMS (mobile station) BSS (base station subsystem) or RAN (radio access network)

BTS (base transeiver station) BSC (base station controller)

Core Network: call forwarding, handover, switchingMSC (mobile services switching center) LR (location register): HLR and VLROMC (operation and maintenance centre) AuC (authentication centre) EIR (equipment identity register)

4



possible radio coverage of the cell

idealized shape of the cellcell

segmentation of the area into cells

GSM: cellular network

use of several carrier frequencies not the same frequency in neighboring cells cell radius varies 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

5



Cellular systems: Frequency planning IFrequency reuse only with a certain distance between the base stations

Typical (hexagon) model:

reuse-3 cluster: reuse-7 cluster:

Other regular pattern: reuse-19 the frequency reuse pattern determines the experienced SIR 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 frequencies already used in

neighbor cellsFrequency Hopping (fixed or random sequence of frequencies)

Improves quality for slow moving or stationary users (frequency diversity) Reduces impact of intercell interference by statistical averaging

f4f5

f1f3

f2

f6

f7

f4f5

f1f3

f2

f6

f7

f4f5

f1f3

f2

f6

f7f2

f1f3

f2

f1f3

f2

f1f3

6



GSM: Air Interface

FDMA (Frequency Division Multiple Access) / FDD (Frequency Division Duplex)

1 2 3 123124. . .

890 MHz 915 MHz

1 2 3 123124. . .

935 MHz 960 MHz

200 kHz

Uplink Downlink

frequency

TDMA (Time Division Multiple Access)

time

Downlink

87654321

4,615 ms = 1250 bit

Uplink

87654321

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Framing Modulation(GMSK)

GSM: Voice Coding

Voice coding Channelcoding

Framing Modulation(GMSK)

114 bit/slot114 + 42 bit

Guard (8.25 bits): avoid overlap with other time slots (different time offset of neighboring slot)Training sequence: select the best radio path in the receiver and train equalizerTail: needed to enhance receiver performanceFlag S: indication for user data or control data

1 2 3 4 5 6 7 8

GSM TDMA frame

GSM time-slot (normal burst)

4.615 ms

546.5 µs577 µs

tail user data TrainingSguardspace S user data tail

guardspace

3 bits 57 bits 26 bits 57 bits1 1 3

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Mobile Terminated Call (MTC)

PSTNcallingstation GMSC

HLR VLR

BSSBSSBSS

MSC

MS

1 2

3

4

5

6

7

8 9

10

11 12

1316

10 10

11 11 11

14 15

17

1: calling a GSM subscriber2: forwarding call to GMSC3: signal call setup to HLR4, 5: request MSRN from VLR6: forward responsible

MSC to GMSC7: forward call to

current MSC8, 9: get current status of MS10, 11: paging of MS12, 13: MS answers14, 15: security checks16, 17: set up connection

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RA

RA

RA RA

RA

RA RA

RA

RA

LocationUpdate

LocationUpdate

LocationUpdate

LocationUpdate

LocationUpdate

Location Management / Mobility Management

The issue: Compromise between minimizing the area where

to search for a mobile minimizing the number of

location updates

Solution 1:Large paging area

Solution 2:Small paging area

PagingSignalling Cost

Paging Area UpdateSignalling Cost

TOTALSignalling Cost

+

=

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Handover

The problem:Change the cell whilecommunicating

Reasons for handover: Quality of radio link

deteriorates Communication in other cell

requires less radio resources Supported radius is

exceeded (e.g. Timing advance in GSM)

Overload in current cell Maintenance

Link

qua

lity

Link to cell 1 Link to cell 2 time

cell 1

cell 2

Handover margin (avoid ping-pong effect)

cell 1 cell 2

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Handover procedure (change of BSC)

HO access

BTSold BSCnew

measurementresult

BSCold

Link establishment

MSCMSmeasurementreport

HO decisionHO required

BTSnew

HO request

resource allocationch. activation

ch. activation ackHO request ackHO commandHO commandHO command

HO completeHO completeclear commandclear command

clear complete clear complete

„Make-before-break“ strategy

make

break

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GSM - authentication

A3

RANDKi

128 bit 128 bit

RAND

SRES* =? SRES

A3

RAND Ki

128 bit 128 bit

SRES 32 bit

SRES

Authentication Request (RAND)

Authentication Response (SRES 32 bit)

mobile network

AuC

MSC

SIM

Ki: individual subscriber authentication key SRES: signed response

SRES* 32 bit

Challenge-Response:• Authentication center provides RAND to Mobile• AuC generates SRES using Ki of subscriber and

RAND via A3• Mobile (SIM) generates SRES using Ki and RAND• Mobile transmits SRES to network (MSC)• network (MSC) compares received SRES with one

generated by AuC

13



GSM - key generation and encryption

A8

RANDKi

128 bit 128 bit

Kc64 bit

A8

RAND Ki

128 bit 128 bit

SRES

RAND

encrypteddata

mobile network (BTS)

MS with SIM

AuC

BTS

SIM

A5

Kc64 bit

A5MS

data data

cipherkey

Ciphering:• Data sent on air interface ciphered for security• A8 algorithm used to generate cipher key• A5 algorithm used to cipher/decipher data• Ciphering Key is never transmitted on air

14



2G to 3G Evolution: GSM - GPRS - UMTS

GPRS Core (PacketSwitched)

SGSN

GGSN

Inter-net

GSMRAN

Base station


Base station

Base station

MSC

ISDN


HLRAuCEIR

GMSC

TransmissionATM based

GSM+GPRS

15



GPRS (General Packet Radio Service)

Introducing packet switching in the network Using shared radio channels for packet transmission over the air:

multiplexing multiple MS on one time slot flexible (also multiple) allocation of timeslots to MS

(scheduling by PCU Packet Control Unit in BSC or BTS) using free slots only if data packets are ready to send

(e.g., 115 kbit/s using 8 slots temporarily) standardization 1998, introduction 2001 advantage: first step towards UMTS, flexible data services

GPRS network elements GSN (GPRS Support Nodes): GGSN and SGSN GGSN (Gateway GSN)

interworking unit between GPRS and PDN (Packet Data Network) SGSN (Serving GSN)

supports the MS (location, billing, security) HLR (GPRS Register – GR)

maintains location and security information

16



carrierTS0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

Multiplexing

Multislot capability

GPRS: Multiplexing and multislot allocation

17



GPRS services

End-to-end packet switched traffic (peak channel rates) 28 kbps (full use of 3 time slots, CS-1: FEC) 171.2 kbps (full use of 8 time slots, CS-4: no FEC)

Average aggregate throughput of a cell (Source: H. Menkes, WirelessWeb, Aug.

2002) 95 kbps (for both up and downlink)Assumptions: 4/12 reuse, realistic RF conditions, random traffic Worse figures for individual TCP traffic

Adaptive Coding Schemes (adaptive Forward Error Control – FEC) CS 1: 9.05 Kbps/slot CS 2: 13.4 Kbps/slot CS 3: 15.6 Kbps/slot CS 4: 21.4 Kbps/slot (no FEC)

Problems and limits IP-based network => high latency, no guarantees Limited data rate: 28 kbps (3 slot/CS-1) - 64.2 kbps (3 slot/CS-4) Latency/flow control problems with TCP

18



EDGE (Enhanced Data Rates for GSM Evolution)Enhanced spectral efficiency depends on: Size of frequency band Duration of usage Level of interference with others (power)

EDGE Technology: EDGE can carry data speeds up to 236.8 kbit/s for 4

timeslots (theoretical maximum is 473.6 kbit/s for 8 timeslots)

Adaptation of modulation depending on quality of radio path GMSK (GSM standard – 1 bit per symbol) 8-PSK (3 bits per symbol)

Adaptation of coding scheme depending on quality of radio path (9 coding schemes)

Gain: data rate (gross) up to 69,2kbps (compare to 22.8kbps for GSM)

complex extension of GSM!

NodeB

UE 1

UE 2

Near-far problem

19



EDGE – Adaptive Modulation and Coding SchemesS c h e m e M o d u la t i o n M a x i m u m

ra te [k b /s ]C o d e R a te F a m i l y

M C S -9 5 9 .2 1 .0 AM C S -8 5 4 .4 0 .9 2 AM C S -7 4 4 .8 0 .7 6 BM C S -6 2 9 .6 / 2 7 .2 0 .4 9 AM C S -5

8 P S K

2 2 .4 0 .3 7 BM C S -4 1 7 .6 1 .0 CM C S -3 1 4 .8 / 1 3 .6 0 .8 0 AM C S -2 1 1 .2 0 .6 6 BM C S -1

G M S K

8 .8 0 .5 3 C

20



Payload for GPRS and EDGE

21



2G to 3G Evolution: GSM - GPRS – UMTS R99/R3


SGSN

GGSN

Inter-net

GSMRAN

Base station


Base station

Base station

UTRAN

Radio networkcontroller

Base station Base station

Base station

MSC

ISDN


HLRAuCEIR

GMSC

ATM based

GSM+GPRS+UMTS R99

22



2G to 3G Evolution: GSM - GPRS - UMTS R5 - IMS


SGSN

GGSN

Inter-net

GSMRAN

Base station


Base station

Base station

UTRAN

Radio networkcontroller

Base station Base station

Base station

IP based

3G Core

GERANGERAN + UMTS R5 + IMS

mobile network evolution – part 1 gsm and umts · 2g to 3g evolution: gsm - gprs - umts gsm ran...

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