2G Mobile Communication Systems - Startseite TU Ilmenau

Integrated Communication Systems Group Ilmenau University of Technology

2G/3G Mobile Communication Systems

Winter 2012/13

Outline

2G Review: GSM

Services

Architecture

Protocols

Call setup

Mobility management

Security

HSCSD

GPRS

Architecture

Protocols

QoS

EDGE

UMTS

Mobile Communication Networks 3 Andreas Mitschele-Thiel, Florian Evers

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

GSM

RAN

Base station

Base station controller

Base station

Base station

MSC

ISDN

GSM Core (Circuit switched)

HLR AuC EIR

GMSC

TransmissionATM based

GSM


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

RSS (radio subsystem): covers all radio aspects

MS (mobile station)

BSS (base station subsystem) or RAN (radio access network)

BTS (base transeiver station)

BSC (base station controller)

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

MSC (mobile services switching center)

LR (location register): HLR and VLR

OSS (operation subsystem): management of the network

OMC (operation and maintenance center)

AuC (authentication center)

EIR (equipment identity register)


possible radio coverage of the cell

idealized shape of the cell cell

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


Cellular systems: Frequency planning I

Frequency 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 CIR 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 cells

Frequency 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

f4 f5

f1 f3

f2

f6

f7

f4 f5

f1 f3

f2

f6

f7

f4 f5

f1 f3

f2

f6

f7 f2

f1 f3

f2

f1 f3

f2

f1 f3


GSM: Air Interface

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

1 2 3 123 124 . . .

890 MHz 915 MHz

1 2 3 123 124 . . .

935 MHz 960 MHz

200 kHz

Uplink Downlink

frequency

TDMA (Time Division Multiple Access)

time

Downlink

8 7 6 5 4 3 2 1

4,615 ms

= 1250 bit

Uplink

8 7 6 5 4 3 2 1


Framing Modulation

(GMSK)

GSM: Voice Coding

Voice coding Channel coding

Framing Modulation

(GMSK)

114 bit/slot 114 + 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 equalizer

Tail: needed to enhance receiver performance

Flag 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 µs 577 µs

tail user data Training S guard

space S user data tail guard

space

3 bits 57 bits 26 bits 57 bits 1 1 3


Mobile Terminated Call (MTC)

PSTN calling

station GMSC

HLR VLR

BSS BSS BSS

MSC

MS

1 2

3

4

5

6

7

8 9

10

11 12

13

16 10 10

11 11 11

14 15

17

1: calling a GSM subscriber

2: forwarding call to GMSC

3: signal call setup to HLR

4, 5: request MSRN from VLR

6: forward responsible MSC to GMSC

7: forward call to

current MSC

8, 9: get current status of MS

10, 11: paging of MS

12, 13: MS answers

14, 15: security checks

16, 17: set up connection


RA

RA

RA RA

RA

RA RA

RA

RA

Location Update

Location Update

Location Update

Location Update

Location Update

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

Paging

Signalling Cost

Paging Area Update

Signalling Cost

TOTAL

Signalling Cost

+

=


Handover

The problem:

Change the cell while communicating

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

Lin

k q

ualit

y

Link to cell 1 Link to cell 2 time

cell 1

cell 2

Handover margin (avoid ping-pong effect)

cell 1 cell 2


Handover procedure (change of BSC)

HO access

BTSold BSCnew

measurement

result

BSCold

Link establishment

MSC MS

measurement

report

HO decision

HO required

BTSnew

HO request

resource allocation

ch. activation

ch. activation ack HO request ack

HO command HO command

HO command

HO complete HO complete

clear command clear command

clear complete clear complete

„Make-before-break“ strategy

make

break


GSM - authentication

A3

RAND Ki

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


GSM - key generation and encryption

A8

RAND Ki

128 bit 128 bit

Kc

64 bit

A8

RAND Ki

128 bit 128 bit

SRES

RAND

encrypted

data

mobile network (BTS)

MS with SIM

AuC

BTS

SIM

A5

Kc

64 bit

A5

MS

data data

cipher

key

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


GSM Evolution – Overview

Spectral

efficency

Data

traffic

adaptive

modulation EDGE

diversity

space

time

Macro diversity

Intelligent antennas

Equalizer

adaptive redundancy interference

Frequency hopping

Dynamic channel

allocation

GPRS

bursty

HSCSD

continuous

Interference cancelation (multi-user detection)


HSCSD (High-Speed Circuit Switched Data)

continuous use of multiple time slots for a single user

(on a single carrier frequency)

asynchronous allocation of time slots between DL and UL

gain: net data rate up to 115,2 kbps (allocation of all 8 traffic channels)

mainly software update

additional HW needed if more than 3 slots are used

Uplink

Downlink

7 1 2 3 8 4 5 6 1 2

7 1 2 3 8 4 5 6 1 2


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

GPRS Core (Packet Switched)

SGSN

GGSN

Inter-net

GSM

RAN

Base station


Base station

Base station

MSC

ISDN


HLR AuC EIR

GMSC


GSM+GPRS


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


carrier TS

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

Multiplexing

Multislot capability

GPRS: Multiplexing and multislot allocation


GPRS protocol architecture

appl.

IP/X.25

LLC

GTP

MAC

radio

MAC

radio FR

RLC BSSGP

IP/X.25

FR

Um Gb Gn

L1/L2 L1/L2

MS BSS SGSN GGSN

UDP

Gi

SNDCP

RLC BSSGP IP IP

LLC UDP

SNDCP GTP

BSSGP: Base Station Subsystem GPRS Protocol (control plane: routing & QoS)

SNDCP: Subnetwork-Dependent Convergence Protocol (mapping, segmentation,

header compression)


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 4: 21.4 Kbps/slot (no Forward Error Correction)

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


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


EDGE – Adaptive Modulation and Coding Schemes Scheme Modulation Maximum

rate [kb/s]

Code Rate Family

MCS-9 59.2 1.0 A

MCS-8 54.4 0.92 A

MCS-7 44.8 0.76 B

MCS-6 29.6 / 27.2 0.49 A

MCS-5

8PSK

22.4 0.37 B

MCS-4 17.6 1.0 C

MCS-3 14.8 / 13.6 0.80 A

MCS-2 11.2 0.66 B

MCS-1

GMSK

8.8 0.53 C


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


SGSN

GGSN

Inter-net

GSM

RAN

Base station


Base station

Base station

UTRAN

Radio network controller

Base station Base station

Base station

MSC

ISDN


HLR AuC EIR

GMSC


GSM+GPRS+UMTS R99


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


SGSN

GGSN

Inter-net

GSM

RAN

Base station


Base station

Base station

UTRAN

Radio network controller

Base station Base station

Base station

TransmissionIP based

3G Core

GERAN GERAN + UMTS R5 + IMS

2G Mobile Communication Systems - Startseite TU Ilmenau

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