The IEEE standard

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The IEEE 802.11 standard

Imad Aad

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Outline

WLANs vs. Wired LANs

History

Working modes MAC sub-layer

The PHY layer (1997)

The PHY Extensions (1999) Security

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WLANs vs. Wired LANs

No wires Mobility

Scarse bandwidth (?)

Multipath, pathloss, interference / noise BER

Rx s1 s2 s0 s Tx s0 + s1 + s2 Obstacle 2 Obstacle 1

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WLANs vs. Wired LANs

No wires Mobility

Scarse bandwidth (?)

Distance

Average received power

LOS =2 α =4 α Distance

Average received power

No LOS =2 α 15−25 dB drop =4−6 α

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WLANs vs. Wired LANs

No wires Mobility

The hidden node problem Scarse bandwidth (?)

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WLANs vs. Wired LANs

PHY layer MAC sub−layer LLC sub−layer Network layer Application layer IEEE 802.11 − IEEE 802.3 IEEE 802.2

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Outline

WLANs vs. Wired LANs

History

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History

1970s: ALOHA

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History

1970s: ALOHA

1972: Slotted ALOHA

1975: Carrier Sense Multiple Access (CSMA) non persistent

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History

1970s: ALOHA

1972: Slotted ALOHA

1975: Carrier Sense Multiple Access (CSMA) non persistent

p-persistent

CSMA with collision detections (CD): Ethernet (1976) CSMA w/ coll. avoidance (CA): IEEE 802.11 (1997)

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Outline

WLANs vs. Wired LANs History

Working modes

MAC sub-layer

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Working modes

Ad-hoc mode vs. Infrastructure mode (IS)

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

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Working modes

Acess Point (AP)

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Working modes

Distribution System (DS)

AP1 AP2 AP3

ESS

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Outline

Working modes

MAC sub-layer

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MAC sub-layer

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

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MAC sub-layer

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

- Basic machanism ( ) Source Destination Other DIFS Time (Tx) (Tx) Data ACK NAV Contention Window Backoff DIFS SIFS

Defer access = NAV+DIFS

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MAC sub-layer

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

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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) CW

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MAC sub-layer

DCF: Distributed Coordination Function (ad-hoc, IS modes) - Fairness ? ... depends on scenario

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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 PCF B CFP repetition interval CFP CP PCF B DCF DCF B U1+ACK SIFS SIFS D3+ACK+Poll PIFS D4+Poll U2+ACK SIFS U4+ACK SIFS SIFS CF−End CP

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MAC sub-layer

Packet fragmentation Fragment 0 ACK0 (Tx) (Tx) Src. Dest. Fragment 1 ACK1 Fragment 2 ACK2 CW Time Fragment burst SIFS

SIFS SIFS SIFS SIFS SIFS DIFS

NAV (CTS) NAV (fragment 0) NAV (fragment 1) NAV(fr.2) Other Other NAV (ACK1) NAV (ACK0)

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Outline

The PHY layer (1997)

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The PHY layer (1997)

PHY layer MAC sub−layer LLC sub−layer Network layer Application layer 3 PHY types: − IR (unknown products) − DSSS (most products) − FHSS (less products)

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The PHY layer (1997)

the EM spectrum allocation

!! " " " "## $ $ $ $%% & & & &'' ( ( ( ()) * * * *++ , , , ,--. . . .// 0 0 0 01 2 2 2 23 4 4 4 45 6 6 6 67 8 8 8 89 : : : :; 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

Infrared Visible UV X rays Gamma rays

3 GHz 30 GHz

HF VHF UHF SHF

(SW radio) (FM radio − TV) (TV − Cell.)

Freq.

902 MHz 928 MHz 2.4 GHz 2.4835 GHz 5.725 GHz 5.785 GHz

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The PHY layer (1997)

DSSS (Direct Sequence Spread Spectrum)

FHSS (Freq. Hopping Spread Spectrum) IR (Infra Red)

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The PHY layer (1997)

DSSS: principle 1 0 Data Periodic Scrambled 11 Bit Barker code Carrier modulator 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

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The PHY layer (1997)

DSSS: principle 1 0 Data Periodic Scrambled 11 Bit Barker code Carrier modulator 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 signal before spreading

Transmitter baseband signal after spreading

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The PHY layer (1997)

DSSS: principle

after spreading before despreading after despreading

before spreading

@ Receiver @ Transmitter

narrowband interference

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The PHY layer (1997)

PSK (Phase Shift Keying)

x 2πω ϕ ϕ = 0 0 S time

Data spreading code

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

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The PHY layer (1997)

DPSK (Differential PSK): no reference signal needed

x

2πω ϕ

0 S

time

Data spreading code

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The PHY layer (1997)

DSSS: modulation < <>== ? ?>@@ A A A A>B B B B C C C C C C D D D D D D E E>FF G G>HH DBPSK DQPSK 0 180 0 180 90 270 (1) (0) ₍₀₀₎ ₍₀₁₎ (10) (11) 1 Mbps 2Mbps

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The PHY layer (1997)

DSSS: Spectrum @ modulator output

0dBr −50dBr −30dBr fc fc + 11MHz fc + 22MHz fc − 11MHz fc − 22MHz

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The PHY layer (1997)

in France (few months ago): allowed channels

(ch13) 2.472 MHz

(ch.11) 2.462 MHz

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The PHY layer (1997)

in France (few months ago): maximum channel separation

(ch13) 2.472 MHz

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The PHY layer (1997)

in Europe

(ch13) 2.472 MHz

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The PHY layer (1997)

Transmission power GSM I wave IEEE oven 802.11 Typical 100 mW - 600 mW 0.2mW/ JK L 6.3 mW Regulations 1-5 mW/ JK L 100 mW @ 5cm (Eur.)

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The PHY layer (1997)

DSSS (Direct Sequence Spread Spectrum)

FHSS (Frequency Hopping Spread Spectrum)

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The PHY layer (1997)

FHSS Modulation: GFSK binary 0/1: MON P QSR (for 1 Mbps) 00, 01, 10, 11: MON PT QSR (for 2 Mbps) M N 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

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The PHY layer (1997)

DSSS (Direct Sequence Spread Spectrum) FHSS (Freq. Hopping Spread Spectrum)

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The PHY layer (1997)

Infra Red (IR)

Pulse Position Modulation (PPM)

1 Mbps: 4 data bits 16-PPM symbol

2 Mbps: 2 data bits 4-PPM symbol

00 01

0001 0010

Data bits 4−PPM symbol

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Outline

The PHY Extensions (1999)

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PHY Extensions (1999)

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

High Rate DSSS

Modulation: (backward compatible)DBPSK, DQPSK Complementary Code Keying (CCK) + DQPSK,

(opt.) Packet Binary Convolutional Coding (PBCC) + (BPSK,QPSK)

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PHY Extensions (1999)

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

OFDM (Orthogonal Frequency Division Multiplexing) Principle:

High-rate data is devided into several lower rate binary signals.

Each low-rate signal modulates a different sub-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”

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PHY Extensions (1999)

Data Out Carriers FFT S/P Rem Pre/ Postfix Rem virtual Signal Mapper Data In Carriers IFFT P/S

S/P Add virtual Add Pre/_Postfix p(t)

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Outline

The PHY Extensions (1999)

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Security

WEP (Wired Equivalent Privacy)

Encryption Plaintext Key Cyphertext Decryption Key Plaintext Original Eavesdropper

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Security

WEP (Wired Equivalent Privacy)

Seed WEP Key Sequence PRNG Initialization Vector (IV) Secret Key Plaintext Integrity Check Value (ICV) Integrity Algo. XOR Ciphertext IV Message

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Security

WEP (Wired Equivalent Privacy) 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 a second”!!

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Conclusion

it works!

looks just like ethernet to higher layers no QoS support... yet.

limited security management.

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