call flow

2G/3G Mobile Communication Systems2G Review: GSM

ServicesArchitectureProtocolsCall setupMobility managementSecurity

HSCSD

GPRSArchitectureProtocolsQoS

EDGE

UMTS

Mobile Communication Networks 2Andreas Mitschele-Thiel Jan. 2008

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


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 subsystemsRSS (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 networkOMC (operation and maintenance centre)AuC (authentication centre)EIR (equipment identity register)


possible radio coverage of the cell

idealized shape of the cellcell

segmentation of the area into cells

GSM: cellular network

use of several carrier frequenciesnot the same frequency in neighboring cellscell 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 IFrequency reuse only with a certain distance between the base stations

Typical (hexagon) model:

reuse-3 cluster: reuse-7 cluster:

Other regular pattern: reuse-19the frequency reuse pattern determines the experienced CIR

Fixed frequency assignment:certain frequencies are assigned to a certain cellproblem: 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

f4f5

f1f3

f2

f6

f7

f4f5

f1f3

f2

f6

f7

f4f5

f1f3

f2

f6

f7f2

f1f3

f2

f1f3

f2

f1f3


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


Framing Modulation(GMSK)

GSM: Voice Coding

64-1

04 k

bps

Voice coding

9,6

kbps

Channelcoding

22,8

kbp

s

Framing

33,8

kbp

s

(270

,8 k

bps)

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


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


RA

RA

RA RA

RA

RA RA

RA

RA

LocationUpdate

LocationUpdate

LocationUpdate

LocationUpdate

LocationUpdate

The issue: Compromise betweenminimizing the area whereto search for a mobileminimizing the number of location updates

Location Management / Mobility Management

Solution 1:Large paging area

Solution 2:Small paging area

PagingSignalling Cost

Paging Area UpdateSignalling Cost

TOTALSignalling Cost

∑∑+

=


Handover

The problem:Change the cell whilecommunicating

Reasons for handover:Quality of radio link deterioratesCommunication in other cellrequires less radio resourcesSupported radius isexceeded (e.g. Timing advance in GSM)Overload in current cellMaintenance

Link

qua

lity

Link

qua

lity

Link to cell 1Link to cell 1 Link to cell 2Link to cell 2 timetime

cell 1cell 1

cell 2cell 2

Handover margin Handover margin (avoid ping(avoid ping--pong pong effect)effect)

cell 1cell 1 cell 2cell 2


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


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


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


GSM Evolution – Overview

Spectralefficency

Datatraffic

adaptivemodulation

EDGE

diversity

space

time

Macro diversity

Intelligent antennas

Equalizer

adaptiveredundancyinterference

Frequencyhopping

Dynamicchannel

allocation

GPRS

bursty

HSCSD

continuous

Interference cancelation(multi-userdetection)


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

71 2 3 84 5 6 1 2

71 2 3 84 5 6 1 2


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


GSM+GPRS


GPRS (General Packet Radio Service)

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

multiplexing multiple MS on one time slotflexible (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 2001advantage: first step towards UMTS, flexible data services

GPRS network elementsGSN (GPRS Support Nodes): GGSN and SGSNGGSN (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


carrierTS0 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

radioFR

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 UDPSNDCP 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/slotCS 2: 13.4 Kbps/slotCS 3: 15.6 Kbps/slotCS 4: 21.4 Kbps/slot (no Forward Error Correction)

Problems and limitsIP-based network => high latency, no guaranteesLimited 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 bandDuration of usageLevel 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 dependingon quality of radio path

GMSK (GSM standard – 1 bit per symbol)8-PSK (3 bits per symbol)

Adaptation of coding scheme dependingon 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!

NodeBNodeB

UE 1UE 1

UE 2UE 2

Near-far problem


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

0 5 10 15 20 25 300

1

2

3

4

5

6x 10

4

Tro

ughp

ut S

[kbp

s]

C/I [dB]

MCS−1 (R=1/2)MCS−2 (R=2/3)MCS−3 (R=6/7)MCS−4 (R=1)MCS−5 (R=3/8)MCS−6 (R=1/2)MCS−7 (R=3/4)MCS−8 (R=1)


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


SGSN

GGSN

Inter-net

GSMRAN

Base station


Base station

Base station

UTRAN

Radio networkcontroller

Base stationBase station

Base station

MSC

ISDN


HLRAuCEIR

GMSC


GSM+GPRS+UMTS R99


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


SGSN

GGSN

Inter-net

GSMRAN

Base station


Base station

Base station

UTRAN

Radio networkcontroller

Base stationBase station

Base station

TransmissionIP based

3G Core

GERANGERAN + UMTS R5 + IMS

call flow

Documents

gsm gprs

current cell maintenance

gsm overload

gsm standard

gsm systemgsm

gsm subscriber

cell cell idealized

cell use