How To Understand The Gsm And Mts Mobile Network Evolution

Integrated Communication Systems Group

Mobile Network Evolution – Part 1

GSM and UMTS

GSM

Cell layout

Architecture

Call setup

Mobility management

Security

GPRS

Architecture

Protocols

QoS

EDGE

UMTS

Architecture

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

GSM

RAN

ISDN

GSM Core

(Circuit

switched)

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



Radio Access Network: covers all radio aspects

MS (mobile station)



BSS (base station subsystem)

or

RAN (radio access network)

BTS (base transeiver station)



BSC (base station controller)



Core Network: call forwarding, handover, switching

MSC (mobile services switching center)



LR (location register): HLR and VLR



OMC (operation and maintenance centre)

AuC (authentication centre)

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

)

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 SIR

 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 . . . 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 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 S Training guard

space S user data tail

guard space

Mobile Terminated Call (MTC)

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

Link quality

Link to cell 1

Link to cell 2

time

cell 1

cell 2

Handover margin

(avoid ping-pong

effect)

)

Handover procedure (change of BSC)

HO access BTS_old BSC_new measurement result BSC_old Link establishment MSC MS measurement report HO decision HO required BTS_new 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

GSM - authentication

Authentication Request (RAND)

Authentication Response (SRES 32 bit) mobile network

AuC

MSC

SIM

K_i: 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

)

GSM - key generation and encryption

A8 RAND K_i 128 bit 128 bit K_c 64 bit A8 RAND K_i 128 bit 128 bit SRES RAND encrypted data mobile network (BTS) MS with SIM AuC BTS SIM A5 K_c 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

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

GPRS Core

(Packet

Switched)

Inter-net

GSM

RAN

ISDN

GSM Core

(Circuit

switched)

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)

carrier TS 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Multiplexing

Multislot capability

)

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

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!

Near-far problem

)

EDGE – Adaptive Modulation and Coding Schemes

S c h e m e M o d u la t i o n M a x i m u m r a t e [ k b /s ] C o d e R a t e F a m i l y M C S - 9 5 9 . 2 1 . 0 A M C S - 8 5 4 . 4 0 . 9 2 A M C S - 7 4 4 . 8 0 . 7 6 B M C S - 6 2 9 . 6 / 2 7 . 2 0 . 4 9 A M C S - 5 8 P S K 2 2 . 4 0 . 3 7 B M C S - 4 1 7 . 6 1 . 0 C M C S - 3 1 4 . 8 / 1 3 . 6 0 . 8 0 A M C S - 2 1 1 . 2 0 . 6 6 B M C S - 1 G M S K 8 . 8 0 . 5 3 C

)

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

GPRS Core

(Packet

Switched)

Inter-net

GSM

RAN

UTRAN

Base station Base station

Base station MSC

ISDN

GSM Core

(Circuit

switched)

GSM+GPRS+UMTS R99

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

GPRS Core

(Packet

Switched)

Inter-net

GSM

RAN

UTRAN

Base station Base station

Base station

IP based

3G Core

GERAN