B1 802 11 Presentation

8/6/2019 B1 802 11 Presentation

http://slidepdf.com/reader/full/b1-802-11-presentation 1/51

1

Module B

WLAN – Engineering Aspects

Prof. JP Hubaux

Mobile Networks

http://mobnet.epfl.ch



2

Reminder on frequencies and wavelenghts

VLF = Very Low Frequency UHF = Ultra High Frequency

LF = Low Frequency SHF = Super High Frequency

MF = Medium Frequency EHF = Extra High Frequency

HF = High Frequency UV = Ultraviolet Light

VHF = Very High Frequency

Frequency and wave length:

λ = c/f

wave length λ , speed of light c ≅ 3x108m/s, frequency f

1 Mm

300 Hz

10 km

30 kHz

100 m

3 MHz

1 m

300 MHz

10 mm

30 GHz

100

µ m

3 THz

1 µ m

300 THz

visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV

optical transmissioncoax cabletwisted

pair



3

Frequencies for mobile communication

VHF-/UHF-ranges for mobile radio

simple, small antenna for handset deterministic propagation characteristics, reliable connections

SHF and higher for directed radio links, satellite communication

small antenna

large bandwidth available

Wireless LANs use frequencies in UHF to SHF spectrum some systems planned up to EHF

limitations due to absorption by water and oxygen molecules

(resonance frequencies)

Weather-dependent fading, signal loss caused by heavy rainfall etc.



4

Frequency allocation

Mobile

phones



5

Characteristics of Wireless LANs

Advantages

flexibility (almost) no wiring difficulties (e.g., historic buildings)

more robust against disasters like, e.g., earthquakes, fire - or users

pulling a plug...

Disadvantages

lower bitrate compared to wired networks (1-100 Mbit/s) More difficult to secure



6

Design goals for wireless LANs

low power

no special permissions or licenses needed to use the LAN robust transmission technology

easy to use for everyone, simple management

protection of investment in wired networks (internetworking)

Security, privacy, safety (low radiation)

transparency concerning applications and higher layer protocols location awareness if necessary



7

Comparison: infrared vs. radio transmission

Infrared

uses IR diodes

Advantages simple, cheap, available in

many mobile devices

no licenses needed

simple shielding possible

Disadvantages interference by sunlight, heat

sources etc.

many things shield or absorb IR

light

low bandwidth

Example IrDA (Infrared Data Association)

interface used to be available

on many devices

Radio

typically using the license free

ISM band at 2.4 GHz and 5 GHzAdvantages

coverage of larger areas possible

(radio can penetrate walls,

furniture etc.)

Disadvantages

very limited license free

frequency bands

shielding more difficult,

interference with other electrical

devices

more difficult to secure

Examples IEEE 802.11, Bluetooth



8

Infrastructure vs. ad hoc networks

infrastructure

network

Ad hoc network

APAP

AP

wired network

AP: Access Point



9

Distribution System

Portal

802.x LAN

Access

Point

802.11 LAN

BSS2

802.11 LAN

BSS1

Access

Point

IEEE 802.11 - Architecture of an

infrastructure network

Station (STA)

terminal with access mechanisms

to the wireless medium and radio

contact to the access point

Basic Service Set (BSS)

group of stations using the same

radio frequency

Access Point station integrated into the wireless

LAN and the distribution system

Portal

bridge to other (wired) networks

Distribution System

interconnection network to form

one logical network (ESS:

Extended Service Set) based

on several BSS

STA1

STA2 STA3

ESS



10

802.11 - Architecture of an ad-hoc network

Direct communication within a

limited range Station (STA):

terminal with access

mechanisms to the wireless

medium

Basic Service Set (BSS):

group of stations using the

same radio frequency

802.11 LAN

BSS2

802.11 LAN

BSS1

STA1

STA4

STA5

STA2

STA3



11

Interconnection of IEEE 802.11 with Ethernet

mobile station

access point

server

fixed terminal

application

TCP

802.11 PHY

802.11 MAC

IP

802.3 MAC

802.3 PHY

application

TCP

802.3 PHY

802.3 MAC

IP

802.11 MAC

802.11 PHY

infrastructure network



12

802.11 - Layers and functions

PLCP (Physical Layer Convergence Protocol)

clear channel assessment

signal (carrier sense)PMD (Physical Medium Dependent)

modulation, coding

PHY Management channel selection, MIB

Station Management coordination of all management

functions

PMD

PLCP

MAC

IP

MAC Management

PHY Management

MAC

access mechanisms,

fragmentation, encryption

MAC Management synchronization, roaming, MIB,

power management

PHY

Statio

nMa



13

802.11 - Physical layer

3 versions: 2 radio: DSSS and FHSS (both typically at 2.4 GHz), 1 IR

data rates 1, 2, 5 or 11 Mbit/sDSSS (Direct Sequence Spread Spectrum)

DBPSK modulation (Differential Binary Phase Shift Keying) or DQPSK

(Differential Quadrature PSK)

chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code)

max. radiated power 1 W (USA), 100 mW (EU), min. 1mWFHSS (Frequency Hopping Spread Spectrum)

spreading, despreading, signal strength

min. 2.5 frequency hops/s, two-level GFSK modulation (Gaussian

Frequency Shift Keying)

Infrared 850-950 nm, diffuse light, around 10 m range

carrier detection, energy detection, synchronization



14

802.11 - MAC layer principles (1/2)

Traffic services Asynchronous Data Service (mandatory)

exchange of data packets based on “best-effort” support of broadcast and multicast

Time-Bounded Service (optional) implemented using PCF (Point Coordination Function)

Access methods (called DFWMAC: Distributed Foundation Wireless MAC) DCF CSMA/CA (mandatory)

collision avoidance via randomized „back-off“ mechanism minimum distance between consecutive packets

ACK packet for acknowledgements (not for broadcasts)

DCF with RTS/CTS (optional) avoids hidden terminal problem

PCF (optional)

access point polls terminals according to a list

DCF: Distributed Coordination Function

PCF: Point Coordination Function



15

802.11 - MAC layer principles (2/2)

Priorities defined through different inter frame spaces no guaranteed, hard priorities SIFS (Short Inter Frame Spacing)

highest priority, for ACK, CTS, polling response

PIFS (PCF IFS) medium priority, for time-bounded service using PCF

DIFS (DCF, Distributed Coordination Function IFS) lowest priority, for asynchronous data service

t

medium busySIFSPIFS

DIFSDIFS

next framecontention

direct access if

medium is free ≥ DIFS time slot

Note : IFS durations are specific to each PHYNote : IFS durations are specific to each PHY



16

t

medium busy

DIFSDIFS

next frame

contention window

(randomized back-off

mechanism)

802.11 - CSMA/CA principles

station ready to send starts sensing the medium (Carrier Sense

based on CCA, Clear Channel Assessment)

if the medium is free for the duration of an Inter-Frame Space (IFS),

the station can start sending (IFS depends on service type)

if the medium is busy, the station has to wait for a free IFS, then the

station must additionally wait a random back-off time (collision

avoidance, multiple of slot-time)

if another station occupies the medium during the back-off time of

the station, the back-off timer stops (to increase fairness)

time slot

direct access if

medium has been free

for at least DIFS



17

802.11 – CSMA/CA broadcast

t

busy

boe

station1

station2

station3

station4

station5

packet arrival at MAC

DIFSboe

boe

boe

busy

elapsed backoff time

bor residual backoff time

busy medium not idle (frame, ack etc.)

bor

bor

DIFS

boe

boe

boe bor

DIFS

busy

busy

DIFSboe busy

The size of the contention window can be adapted(if more collisions, then increase the size)

The size of the contention window can be adapted

(if more collisions, then increase the size)

Here St4 and St5 happen to have

the same back-off time

=

Note: broadcast is not acknowledgedNote: broadcast is not acknowledged

(detection by upper layer)

(detection by upper layer)



18

802.11 - CSMA/CA unicast

Sending unicast packets station has to wait for DIFS before sending data receiver acknowledges at once (after waiting for SIFS) if the packet

was received correctly (CRC) automatic retransmission of data packets in case of transmission

errors

t

SIFS

DIFS

data

ACK

waiting time

other

stations

receiver

sender data

DIFS

Contention

window

The ACK is sent right at the end of SIFS(no contention)

The ACK is sent right at the end of SIFS

(no contention)



19

802.11 – DCF with RTS/CTS

Sending unicast packets station can send RTS with reservation parameter after waiting for DIFS

(reservation determines amount of time the data packet needs the medium)

acknowledgement via CTS after SIFS by receiver (if ready to receive) sender can now send data at once, acknowledgement via ACK other stations store medium reservations distributed via RTS and CTS

t

SIFS

DIFS

data

ACK

defer access

other

stations

receiver

sender data

DIFS

Contention

window

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

NAV: Net Allocation Vector NAV: Net Allocation Vector RTS/CTS can be present for some packets and not for other

RTS/CTS can be present for some packets and not for other



20

Fragmentation mode

t

SIFS

DIFS

data

ACK1

other

stations

receiver

sender frag1

DIFS

contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

NAV (frag1)NAV (ACK1)

SIFSACK2

frag2

SIFS

•Fragmentation is used in case the size of the packets sent has to bereduced (e.g., to diminish the probability of erroneous frames)

• Each fragi (except the last one) also contains a duration (as RTS does),

which determines the duration of the NAV• By this mechanism, fragments are sent in a row• In this example, there are only 2 fragments



21

802.11 – Point Coordination Function (1/2)

PIFS

stations‘

NAV

wireless

stations

point

coordinator

D1

U1

SIFS

NAV

SIFSD2

U2

SIFS

SIFS

SuperFramet0

medium busy

t1

• Purpose: provide a time-bounded service

• Not usable for ad hoc networks

• Di represents the polling of station i

• Ui represents transmission of data from station i

contention free period



22

802.11 – Point Coordination Function (2/2)

tstations‘

NAV

wireless

stations

point

coordinator

D3

NAV

PIFSD4

U4SIFS

SIFSCFend

contention

period

contention free period

t2 t3 t4

• In this example, station 3 has no data to send



23

802.11 - MAC frame format

Types

control frames, management frames, data framesSequence numbers

important against duplicated frames due to lost ACKs

Addresses

receiver, transmitter (physical), BSS identifier, sender (logical)

Miscellaneous sending time, checksum, frame control, data

Frame

Control

Duration

ID

Address

1

Address

2

Address

3

Sequence

Control

Address

4 Data CRC

2 2 6 6 6 62 40-2312bytes

version, type, fragmentation, security, ... detection of duplication



24

MAC address format

scenario to DS from

DS

address 1 address 2 address 3 address 4

ad-hoc network 0 0 DA SA BSSID -

infrastructure

network, from AP

0 1 DA BSSID SA -

infrastructure

network, to AP

1 0 BSSID SA DA -

infrastructure

network, within DS

1 1 RA TA DA SA

DS: Distribution System

AP: Access Point

DA: Destination Address

SA: Source AddressBSSID: Basic Service Set Identifier

- infrastructure BSS : MAC address of the Access Point

- ad hoc BSS (IBSS): random number

RA: Receiver Address

TA: Transmitter Address



25

802.11 - MAC management

Synchronization

Purpose for the physical layer (e.g., maintaining in sync the frequency hop

sequence in the case of FHSS)

for power management

Principle: beacons with time stamps

Power management

sleep-mode without missing a message

periodic sleep, frame buffering, traffic measurements

Association/Reassociation

integration into a LAN

roaming, i.e. change networks by changing access points scanning, i.e. active search for a network

MIB - Management Information Base

managing, read, write



26

Synchronization (infrastructure case)

beacon interval

t

medium

access

pointbusy

B

busy busy busy

B B B

value of the timestamp B beacon frame

• The access point transmits the (quasi) periodic beacon signal

• The beacon contains a timestamp and other management information used for power management and roaming

• All other wireless nodes adjust their local timers to the timestamp



27

Synchronization (ad-hoc case)

tmedium

station1

busy

B1

beacon interval

busy busy busy

B1

value of the timestamp B beacon frame

station2

B2 B2

random delay (back-off)

• Each node maintains its own synchronization timer and starts the transmissionof a beacon frame after the beacon interval

• Contention back-off mechanism only 1 beacon wins• All other stations adjust their internal clock according to the received beacon

and suppress their beacon for the current cycle



28

Power management

Idea: switch the transceiver off if not needed

States of a station: sleep and awakeTiming Synchronization Function (TSF)

stations wake up at the same time

Infrastructure case

Traffic Indication Map (TIM)

list of unicast receivers transmitted by AP Delivery Traffic Indication Map (DTIM)

list of broadcast/multicast receivers transmitted by AP

Ad-hoc case

Ad-hoc Traffic Indication Map (ATIM)

announcement of receivers by stations buffering frames more complicated - no central AP

collision of ATIMs possible (scalability?)



29

Power saving (infrastructure case)

TIM interval

t

medium

access

point

busy

D

busy busy busy

T T D

T TIM D DTIM

DTIM interval

BB

B broadcast/multicast

station

awake

pPower Saving poll: I am awake, please send the data

p

d

d

ddata transmission

to/from the station

Here the access point announces

data addressed to the station



30

Power saving (ad-hoc case)

awake

A transmit ATIM D transmit data

t

station1

B1 B1

B beacon frame

station2B2 B2

random delay

A

a

D

d

ATIM

window beacon interval

a acknowledge ATIM d acknowledge data

• ATIM: Ad hoc Traffic Indication Map (a station announces the list of buffered frames)• Potential problem: scalability (high number of collisions)



31

802.11 - Roaming

No or bad connection? Then perform:

Scanning scan the environment, i.e., listen into the medium for beacon

signals or send probes into the medium and wait for an answer

Reassociation Request

station sends a request to one or several AP(s)

Reassociation Response success: AP has answered, station can now participate

failure: continue scanning

AP accepts Reassociation Request

signal the new station to the distribution system

the distribution system updates its data base (i.e., locationinformation)

typically, the distribution system now informs the old AP so it can

release resources



32

Security of 802.11

WEP: Wired Equivalent Privacy

Objectives:

Confidentiality

Access control Data integrity

M

C(M)

Integrity

checksum

M C(M)P =

RC4

k

IV RC4

k

IV

Note: several security weaknesses have been identified and WEP should not be used

anymore.

M C(M)P =

The new solution for 802 11 security:



33

The new solution for 802.11 security:

standard 802.1x

Supplicant Authenticator Authentication Server

EAPOL

(over Ethernet or 802.11)

Encapsulated EAP,

Typically on RADIUS

EAP: Extensible Authentication Protocol (RFC 2284, 1998)

EAPOL: EAP over LAN

RADIUS: Remote authentication dial in user service (RFC 2138, 1997)

Features:

- Supports a wide range of authentication schemes, thanks to the usage of EAP- One-way authentication- Optional encryption and data integrity



34

More on IEEE 802.1xExample of authentication, using one-time passwords (OTP):

Supplicant Authenticator Authentication server

EAP-request/identity

EAP-response/identiy (MYID)

EAP-request/OTP,

OTP challengeEAP-response/OTP,

OTPpasswordEAP-success

Port authorizedAuthentication

successfully

completed

Notes :

1. Weaknesses have been found in 802.1x as well, but are corrected in the

various implementations.2. New standard in the making : IEEE 802.11i

Notes :

1. Weaknesses have been found in 802.1x as well, but are corrected in the

various implementations.

2. New standard in the making : IEEE 802.11i

: exchange of EAPOL frame

: exchange of EAP frames in a higher layer protocol (e.g., RADIUS)



35

IEEE 802.11 – Standardization effortsIEEE 802.11b

2.4 GHz band DSSS (Direct-sequence spread spectrum) Bitrates 1 – 11 Mbit/s

IEEE 802.11a 5 GHz band Based on OFDM (orthogonal frequency-division multiplexing) transmission rates up to 54 Mbit/s Coverage is not as good as in 802.11b

IEEE 802.11g 2.4 GHz band (same as 802.11b) Based on OFDM Bitrates up to 54Mb/s

IEEE 802.11n MIMO (multiple-input multiple-output) 40MHz channel (instead of 20MHz) Can operate in the 5GHz or 2.4Ghz (risk of interference with other systems, however) Bitrates up to 600Mb/s

IEEE 802.11i Security, makes use of IEEE 802.1x

IEEE 802.11p For vehicular communications

IEEE 802.11s For mesh networks



36

Conclusion of Wireless LANs

IEEE 802.11 Very widespread Often considered as the system underlying larger scale ad hoc

networks (although far from optimal, not designed for this purpose) Tremendous potential as a competitor of 3G cellular networks in hot

spots

Bluetooth

Security perceived as a major obstacle; initial solutions wereflawed in both IEEE 802.11 (WEP) and Bluetooth

Future developments Ultra Wide Band?



37

References

J. Schiller: Mobile Communications, Addison-Wesley, Second Edition,

2004

Leon-Garcia & Widjaja: Communication Networks, McGrawHill, 2000

IEEE 802.11 standards, available at www.ieee.org

www.bluetooth.com

J. Edney and W. Arbaugh: Real 802.11 Security, Addison-Wesley,

2003

http://www.bluetooth.com/

http://www.bluetooth.com/



38

Ad Hoc On-Demand Distance Vector Routing

(AODV)

Note: this and the following slides are provided here because

AODV is used in the hands-on exercises. We will come

back to this topic in a later module of the course.



39

AODV : Route discovery (1)

E G

M

H

R

F

A

B

C

I

DS

K

N

L

P

J

Q



40


E G

M

H

R

F

A

B

C

I

DS

K

N

L

P

J

Q

Note: if one of the intermediate nodes (e.g., A)

knows a route to D, it responds immediately to S

Note: if one of the intermediate nodes (e.g., A)

knows a route to D, it responds immediately to S: Route Request (RREQ)



41


E G

M

H

R

F

A

B

C

I

DS

K

N

L

P

J

Q

: represents a link on the reverse path



42


E G

M

H

R

F

A

B

C

I

DS

K

N

L

P

J

Q

AODV R di ( )



43


E G

M

H

R

F

A

B

C

I

DS

K

N

L

P

J

Q

AODV R t di (6)



44


M

D

K

L

P

J

E G

H

R

F

A

B

C

I

S

N

Q

AODV R t di (7)



45


M

D

K

L

P

J

E G

H

R

F

A

B

C

I

S

N

Q

AODV R t l d t f th f d



46

AODV : Route reply and setup of the forward

path

M

D

K

L

P

J

E G

H

R

F

A

B

C

I

S

N

Q

: Link over which the RREP is transmitted

: Forward path

R t l i AODV



47

Route reply in AODV

In case it knows a path more recent than the one previously known

to sender S, an intermediate node may also send a route reply

(RREP)

The freshness of a path is assessed by means of destination

sequence numbers

Both reverse and forward paths are purged at the expiration of

appropriately chosen timeout intervals

AODV D t d li



48

AODV : Data delivery

M

D

K

L

P

J

E G

H

R

F

A

B

C

I

S

N

Q

Data

The route is not included in the packet header The route is not included in the packet header

AODV R t i t (1)



49

AODV : Route maintenance (1)

M

D

K

L

P

J

E G

H

R

F

A

B

C

I

S

N

Q

Data

X

AODV R t i t (2)



50

AODV : Route maintenance (2)

M

D

K

L

P

J

E G

H

R

F

A

B

C

I

S

N

Q

X

RERR(G-J)

When receiving the Route Error message (RERR),

S removes the broken link from its cache.

It then initializes a new route discovery.

When receiving the Route Error message (RERR),

S removes the broken link from its cache.

It then initializes a new route discovery.

AODV (unicast) : Conclusion



51

AODV (unicast) : Conclusion

Nodes maintain routing information only for routes that are in active

use

Unused routes expire even when the topology does not change

Each node maintains at most one next-hop per destination

Documents

B1 802 11 Presentation