Would Wi-Fi Benefit from the GSM Model? Tom Hewer CCS Seminar - 6th Feb 2008 with many thanks to Dr. Jochen Schiller, Free University of Berlin.

Would Wi-Fi Benefit from

the GSM Model?

Tom HewerCCS Seminar - 6th Feb 2008

with many thanks to Dr. Jochen Schiller, Free University of Berlin

Contents

• Intro to Radio

• GSM and WiFi

• Differences of Control

• Comparison of Implementation

• Cost Analysis

• What is Wi-MAX?

• Discussion

Introduction to Radio• A radiowave is an electromagnetic wave propagating through space

• A radiowave has a phase, wavelength and amplitude

• The wave carries signals for reception by a receiving node

• The frequencies discussed here fall between 900MHz and 5GHz

• Power restrictions are applied by the regulating bodies for transmission

• Coding methods exist to make more efficient use of the available spectrum

• Radio waves fall at the lower end of the EM spectrum

How to Transmit a Signal• Signals can be represented in three ways:

Frequency Amplitude Phase State

f [Hz]

A [V]

ϕ

I= M cos ϕ

Q = M sin ϕ

ϕ

A [V]

t[s]

• To carry data a wave must have a recognisable state that changes

• The reception of the state allows for a decision that translates to a result

• The simplest form of state is: ON or OFF

• The state can be changed over frequency, amplitude and phase and these can be combined to allow multiple access to the spectrum

OSI Model

DLL - Medium access control

and error handling

PHY - Puts the data on the

medium

Multiplexingk2 k3 k4 k5 k6k1

f

t

c

f

t

c

k2 k3 k4 k5 k6k1

f

t

c

k2 k3 k4 k5 k6k1

k2 k3 k4 k5 k6k1

f

c

Frequency

Time

Time and Frequency

Code

Propagation• EM waves propagate predictively in free space as per the extended Friis equation:

• Models exist to calculate propagation in ‘real’ environments

• The more complex the model, the more computationally expensive the calculation

• Propagation models can be ‘fitted’ by taking real readings of received signal strength and then converging the model until it shows the real data

• Massively complex propagation models are only suited to the environment for which they are designed and can take into account: Weather, Particulates, Urbanisation, Land Density, Living Matter and others

where Pr is the received power at distance dPt is the transmitted power, Gt and Gr are the antenna heightsλ is the wavelength of the transmitted signal and L is the systemloss

Development of GSM and WiFi

cellular phones satellites wireless LANcordlessphones

1992:GSM

1994:DCS 1800

2001:IMT-2000

1987:CT1+

1982:Inmarsat-A

1992:Inmarsat-BInmarsat-M

1998:Iridium

1989:CT 2

1991:DECT 199x:

proprietary

1997:IEEE 802.11

1999:802.11b, Bluetooth

1988:Inmarsat-C

analogue

digital

1991:D-AMPS

1991:CDMA

1981:NMT 450

1986:NMT 900

1980:CT0

1984:CT1

1983:AMPS

1993:PDC

2000:GPRS

2000:IEEE 802.11a

200?:Fourth Generation(Internet based)

2002:IEEE 802.11b2004:

3rd Generation

2002:UMTS/WCDMA

2004:IEEE 802.11g

2006:IEEE 802.11n

2006:IEEE 802.16

WiMAX

GSM System

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

radiosubsystem

network and switchingsubsystem

MS MS

BTSBSC MSC

BTS

BTSBSC

BTS

MSC

WiFi System

• One thing to note here is that we are discussing INFRASTRUCTURE based wireless rather than AD-HOC networks!

Distribution System

Portal

802.x LAN

Access Point

802.11 LAN

BSS2

802.11 LAN

BSS1

Access Point

STA1

STA2 STA3

ESS

Role of the access point: Time SyncPasses ALL data between network and nodesDecide coding techniqueDecide who can transmit and when

It is the BOSS of the network

STA - StationESS - Extended Service SetBSS - Basic Service Set

Hidden Node Problem

• Node 1 sends request to send (RTS) to router

• Router checks to see if anybody transmitting

• If not, router returns a clear to send (CTS) message

• This allows control when wireless nodes cannot see each other

BA C

Why is this problem less apparent in GSM

networks?

Control• GSM control comes from the Broadcast Control

Channel (BCCH) which is permanent and dedicated

• Nodes are assigned channels for transmission and programmed to hop frequencies every 0.2s

• Frequency hopping follows a pseudo-random sequence with a random seed

• This is secret and difficult to intercept (if you listen to GSM frequencies they sound like noise, unless you follow the same FHS)

• It is possible to leave one base station and join another. The handover is controlled by the BCCH measuring signal levels

• WiFi control comes from the router

• All nodes must use the same channel in the network

• Nodes must ‘join’ the network and provide any require credentials before transmission can begin

• Security is provided by WEP (weak) or WPA (hard) and keys can be session, server created or user created

• Once WEP/WPA key entered the data (not the packet headers) is encrypted as in a PKI system

• Handover to another base-station is currently not possible. The connection must be dropped and then reconnected to the new router (even if they are on the same network with the same settings)

Antennae• WiFi antennae

are usually omni-directional as :

• GSM antennae operate in clusters of shielded dipole arrangement:

No in UK: ??

No in UK: 350,000

Maximum separation of antennae in Manhattan: 30m!!

Cost

• Cost to erect one GSM antenna: approx. £75k

• Cost of standard wireless router: £40

• Approx number of antennae required to match GSM coverage by WiFi: 7,000,000 so cost (£2,800M)

• Antennae in UK split between operating companies, who cross-charge other networks for using their equipment

• Free use and no separation rules for WiFi so can put routers in any location

• Who would own the routers, as they would have be located on private premises?

• Free use and no separation rules for WiFi so can put routers in any location

WOULD IT BE WORTH IT?

WiMAX and the Last Mile

• Last mile is the term to describe the connection from private property to the nearest access point (usually a BT exchange)

• WiMAX is covered in IEEE802.16 and is a new(er) technology than WiFi (802.11), Ethernet (802.3) and Optic fibre

• Coverage 31 miles at 70Mbps

• 10-66GHz Burst Technology

• Efficient CDM

• High QoS and Guaranteed Frame Rate

• Includes protocols for Mesh networks and Synchronous data access, and DOES have handover between networks

http://www.ieee802.org/16/tutorial/index.html

Discussion:Over the past 5 years, the number of mobile phones and WiFi

networks have increased massively. The technology that runs WiFi networks has moved forward quickly

and efficiently (no 3G dead-ends!).Cellular systems require a great level of central control and inter-

device operability is limited (currently).Cellular systems offer more guaranteed quality and data transfer

(but 802.16 should improve this for WiFi).