the ieee 802.11 standard - mobilityfirst fia...

The IEEE 802.11 standard

Imad Aad

INRIA, Planete team

IN’Tech, May 31st, 2002

IEEE 802.11 – p.1

Outline

WLANs vs. Wired LANs

History

Working modes

MAC sub-layer

The PHY layer (1997)

The PHY Extensions (1999)

Security

IEEE 802.11 – p.2


No wires � Mobility

Scarse bandwidth (?)

Multipath, pathloss, interference / noise � BER

Rx

s1

s2

s0

s

Tx

s0 + s1 + s2

Obstacle 2

Obstacle 1

IEEE 802.11 – p.3





Distance

Ave

rage

rec

eive

d po

wer

LOS

=2α

=4α

Distance

Ave

rage

rec

eive

d po

wer

No LOS

=2α

15−25 dB drop

=4−6α

IEEE 802.11 – p.3


IEEE 802.11 – p.3



The hidden node problem



Protection / Privacy

IEEE 802.11 – p.3


IEEE 802.11 – p.3


PHY layer

MAC sub−layer

LLC sub−layer

Network layer

Application layer

IEEE 802.11 − IEEE 802.3

IEEE 802.2

IEEE 802.11 – p.3

Outline


History

Working modes

MAC sub-layer



Security

IEEE 802.11 – p.4

History

1970s: ALOHA

1972: Slotted ALOHA

IEEE 802.11 – p.5

History

1970s: ALOHA

1972: Slotted ALOHA

1975: Carrier Sense Multiple Access (CSMA)non persistentp-persistent

IEEE 802.11 – p.6

History

1970s: ALOHA

1972: Slotted ALOHA

1975: Carrier Sense Multiple Access (CSMA)non persistentp-persistent

CSMA with collision detections (CD): Ethernet (1976)

CSMA w/ coll. avoidance (CA): IEEE 802.11 (1997)

IEEE 802.11 – p.7

Outline


History

Working modes

MAC sub-layer



Security

IEEE 802.11 – p.8

Working modes

Ad-hoc mode vs. Infrastructure mode (IS)

Independent BSS (IBSS), Basic Service Set (BSS),Extended Service Set (ESS)

IBSSIEEE 802.11 – p.9

Working modes



Acess Point (AP)

BSS

IEEE 802.11 – p.9

Working modes



Distribution System (DS)

AP1 AP2 AP3

ESS

Handoff on the MAC sub-layer

IEEE 802.11 – p.9

Outline


History

Working modes

MAC sub-layer



Security

IEEE 802.11 – p.10

MAC sub-layer

DCF: Distributed Coordination Function (ad-hoc, IS modes)PCF: Polling Coordination Function (in IS mode, optional)

IEEE 802.11 – p.11

MAC sub-layer

DCF: Distributed Coordination Function (ad-hoc, IS modes)- Basic machanism ( � ��

)

Source

Destination

Other

DIFSTime

(Tx)

(Tx)

Data

ACK

NAV

Contention Window

Backoff

DIFS

SIFS

Defer access = NAV+DIFS

CW

IEEE 802.11 – p.11

MAC sub-layer

DCF: Distributed Coordination Function (ad-hoc, IS modes)- The hidden node problem

A B C

IEEE 802.11 – p.11

MAC sub-layer

DCF: Distributed Coordination Function (ad-hoc, IS modes)- RTS/CTS mechanism ( � ��

)

Source

Destination

Other

DIFS Time

(Tx)

(Tx)

SIFS

RTS

CTS

SIFS

Data

SIFS

ACK

DIFS

NAV (RTS)

NAV (CTS)

NAV (data)

Defer access

CW

Backoff

IEEE 802.11 – p.11

MAC sub-layer

DCF: Distributed Coordination Function (ad-hoc, IS modes)- Fairness ? ... depends on scenario- QoS ? ... not yet ... wait for 802.11e

IEEE 802.11 – p.11

MAC sub-layer

DCF: Distributed Coordination Function (ad-hoc, IS modes)PCF: Polling Coordination Function (in IS mode, optional)

SIFS

D2+ACK+Poll

PIFS

D1+Poll

SIFS

CFP repetition interval

CFP CP

PCFB

CFP repetition interval

CFP CP

PCFB DCFDCF

B

U1+ACK

SIFS

SIFS

D3+ACK+Poll

PIFS

D4+Poll

U2+ACK

SIFS

U4+ACK

SIFS

SIFS

CF−End

CP

IEEE 802.11 – p.11

MAC sub-layer

Packet fragmentation

Fragment 0

ACK0

(Tx)

(Tx)

Src.

Dest.

Fragment 1

ACK1

Fragment 2

ACK2

CW

TimeFragment burst

SIFSSIFS SIFS SIFS SIFS SIFS DIFS

NAV (CTS)

NAV (fragment 0)

NAV (fragment 1)

NAV(fr.2)Other

OtherNAV (ACK1)

NAV (ACK0)

IEEE 802.11 – p.11

Outline


History

Working modes

MAC sub-layer



Security

IEEE 802.11 – p.12


PHY layer

MAC sub−layer

LLC sub−layer

Network layer

Application layer

3 PHY types:

− IR (unknown products)

− DSSS (most products)− FHSS (less products)

IEEE 802.11 – p.13


the EM spectrum allocation

� ��

�!! " "" "�## $ $$ $�%% & && &�'' ( (( (�)) * ** *�++, ,, ,�-- . .. .�//

0 00 0�1 2 22 2�3 4 44 4�56 66 6�7 8 88 8�9 : :: :�;300 KHz 3 MHz

LF MF(AM radio)

Freq.30 KHz 30 MHz 300 MHz

Freq.1 KHz 1 MHz 1 GHz 1 THz 1 PHz 1 EHz

Infr

ared

Vis

ible

UV

X r

ays

Gam

ma

rays

3 GHz 30 GHz

HF VHF UHF SHF(SW radio) (FM radio − TV) (TV − Cell.)

Freq.902 MHz 928 MHz

Cordless phonesBaby monitors

(old) Wireless LANs

5.785 GHz

IEEE 802.11aHiperlan II

2.4835 GHz

IEEE 802.11(b)Bluetooth

Microwave ovens

2.4 GHz 5.725 GHz

ISM U−NII

IEEE 802.11 – p.13


DSSS (Direct Sequence Spread Spectrum)

FHSS (Freq. Hopping Spread Spectrum)

IR (Infra Red)

IEEE 802.11 – p.13


DSSS: principle

1 0 Data

Periodic

Scrambled

11 BitBarker code

Carriermodulator

mod−2 adder

0 1 0 0 1 0 0 0 1 1 1

1 bit period

11 chips

1 0 1 1 0 1 1 1 0 0 0

Note:

single code (11-chips)

multiple access ? ... no

security ? ... noIEEE 802.11 – p.13


DSSS: principle

1 0 Data

Periodic

Scrambled

11 BitBarker code

Carriermodulator

mod−2 adder

0 1 0 0 1 0 0 0 1 1 1

1 bit period

11 chips

1 0 1 1 0 1 1 1 0 0 0

Transmitter baseband signalbefore spreading

Transmitter baseband signalafter spreading

IEEE 802.11 – p.13


DSSS: principle

after spreading before despreading after despreadingbefore spreading

@ Receiver@ Transmitter

narrowband

interference

IEEE 802.11 – p.13


PSK (Phase Shift Keying)

x

2πω ϕ

ϕ = 0

0

S

time

Data spreading code

S(t) = A sin ( t + (t))

0 0 1 01

IEEE 802.11 – p.13


DPSK (Differential PSK):no reference signal needed

x

2πω ϕ

0

S

time

Data spreading code

S(t) = A sin ( t + (t))

0 0 1 01

IEEE 802.11 – p.13


DSSS: modulation

<<>== ??>@@

A AA A>B BB BC CC CC CD DD DD D

EE>FF GG>HHDBPSK DQPSK

0 180 0 180

90

270

(1)(0) (00) (01)

(10)

(11)

1 Mbps 2Mbps

IEEE 802.11 – p.13


DSSS: Spectrum @ modulator output0dBr

−50dBr

−30dBr

fc

fc +

11M

Hz

fc +

22M

Hz

fc −

11M

Hz

fc −

22M

Hz

IEEE 802.11 – p.13


in France (few months ago): allowed channels

(ch1

3) 2

.472

MH

z

(ch.

11)

2.46

2 M

Hz

(ch.

10)

2.45

7 M

Hz

(ch1

2) 2

.467

MH

z

IEEE 802.11 – p.13


in France (few months ago): maximum channel separation

(ch1

3) 2

.472

MH

z

(ch.

10)

2.45

7 M

Hz

IEEE 802.11 – p.13


in Europe

(ch1

3) 2

.472

MH

z

(ch.

1) 2

.412

MH

z

IEEE 802.11 – p.13


Transmission power

GSM I wave IEEEoven 802.11

Typical 100 mW - 600 mW 0.2mW/ JK L

6.3 mWRegulations 1-5 mW/ JK L

100 mW@ 5cm (Eur.)

IEEE 802.11 – p.13



FHSS (Frequency Hopping Spread Spectrum)

IR (Infra Red)

IEEE 802.11 – p.13


FHSS

Modulation: GFSKbinary 0/1:

MON P QSR (for 1 Mbps)00, 01, 10, 11:

MON PT QSR (for 2 Mbps)MN sequence =

MOU V � WYX Z[ V � W]\ ^ _ K � � V`a W]\ bc

(France)[ V � W

: tables^: 3 sets

Fast-FH vs. Slow-FH: min 2.5 hops/s

Bluetooth interference ?... YES

IEEE 802.11 – p.13



FHSS (Freq. Hopping Spread Spectrum)

IR (Infra Red)

IEEE 802.11 – p.13


Infra Red (IR)Pulse Position Modulation (PPM)

1 Mbps: 4 data bits � 16-PPM symbol

2 Mbps: 2 data bits � 4-PPM symbol

ddddeeee ffffgggg00

01

10

11

0001

0010

0100

1000

Dat

a bi

ts

4−P

PM

sym

bol

1 0 1 1

1 0 0 0 0 1 0 0

Data

Txed Pulse

IEEE 802.11 – p.13

Outline


History

Working modes

MAC sub-layer



Security

IEEE 802.11 – p.14

PHY Extensions (1999)

IEEE 802.11b: 2.4 GHz. 1Mbps, 2Mbps, 5.5Mbps 11Mbps.

High Rate DSSS

Modulation: (backward compatible)DBPSK, DQPSKComplementary Code Keying (CCK) + DQPSK,(opt.) Packet Binary Convolutional Coding (PBCC) +(BPSK,QPSK)

Currently the most widely used one

IEEE 802.11 – p.15


IEEE 802.11a: 5.7 GHz, 6 Mbps � 54 Mbps!!

OFDM (Orthogonal Frequency Division Multiplexing)Principle:High-rate data is devided into several lower ratebinary signals.Each low-rate signal modulates a differentsub-carrier (48)Sub-carrier sets are orthogonal.Modulation: BPSK, QPSK, 16QAM and 64QAM

FEC: Convolutional encoding needed (Viterbi)

Close to Hiperlan 2 specs.

“coming soon”IEEE 802.11 – p.15


DataOut

Carriers FFT S/PRem Pre/

PostfixRem virtual

SignalMapper

DataIn

Carriers IFFT P/SS/PAdd Pre/Postfix

Add virtualp(t)

P/SEqualizer/Detector

MatchedFilter

IEEE 802.11 – p.15

Outline


History

Working modes

MAC sub-layer



Security

IEEE 802.11 – p.16

Security

WEP (Wired Equivalent Privacy)

EncryptionPlaintext

Key

CyphertextDecryption

Key

PlaintextOriginal

Eavesdropper

IEEE 802.11 – p.17

Security


Seed WEPKey

Sequence

PRNG

InitializationVector (IV)

Secret Key

Plaintext

Integrity Check

Value (ICV)

Integrity Algo.

XOR

Ciphertext

IV

Message

IEEE 802.11 – p.17

Security


default keys / established keys

40-128 bit key

Algorithm: RC4 (symmetric stream cypher)

Cracking tools: WEPcrack, AirSnort:if “100MB-1GB of data can be gathered” then one“can guess the encryption password in less than asecond”!!

Access control table ? ... inefficientNetwork ID ? ... inefficient

IEEE 802.11 – p.17

Conclusion

it works!

looks just like ethernet to higher layers

no QoS support... yet.

limited security management.

Planete team: http://www.inrialpes.fr/planeteImad AAD: [email protected]

IEEE 802.11 – p.18

the ieee 802.11 standard - mobilityfirst fia...

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