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Mobile Networks Module B WLAN – Engineering Aspects Prof. JP Hubaux http://mobnet.epfl.ch Reminder on frequencies and wavelenghts twisted pair VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency

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Module b wlan engineering aspects l.jpg

Mobile Networks

Module BWLAN – Engineering Aspects

Prof. JP Hubaux

http://mobnet.epfl.ch


Reminder on frequencies and wavelenghts l.jpg
Reminder on frequencies and wavelenghts

twisted pair

  • 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:

  •  = c/f

  • wave length , speed of light c  3x108m/s, frequency f

coax cable

optical transmission

1 Mm

300 Hz

10 km

30 kHz

100 m

3 MHz

1 m

300 MHz

10 mm

30 GHz

100 m

3 THz

1 m

300 THz

VLF

LF

MF

HF

VHF

UHF

SHF

EHF

infrared

UV

visible light


Frequencies for mobile communication l.jpg
Frequencies for mobile communication

  • VHF-/UHF-ranges for mobile radio

    • simple, small antenna for cars

    • 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.



Characteristics of wireless lans l.jpg
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 (1-50 Mbit/s)

    • More difficult to secure


Design goals for wireless lans l.jpg
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 (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation)

  • transparency concerning applications and higher layer protocols, but also location awareness if necessary


Comparison infrared vs radio transmission l.jpg

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

Advantages

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

Comparison: infrared vs. radio transmission


Infrastructure vs ad hoc networks l.jpg
Infrastructure vs. ad hoc networks

infrastructure network

AP: Access Point

AP

AP

wired network

AP

Ad hoc network


Ieee 802 11 architecture of an infrastructure network l.jpg

Portal

Distribution System

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

802.11 LAN

802.x LAN

STA1

BSS1

Access

Point

Access

Point

ESS

BSS2

STA2

STA3

802.11 LAN


802 11 architecture of an ad hoc network l.jpg
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

STA3

STA1

BSS1

STA2

802.11 LAN

BSS2

STA5

STA4


Interconnection of ieee 802 11 with ethernet l.jpg
Interconnection of IEEE 802.11 with Ethernet

fixed terminal

mobile station

server

infrastructure network

access point

application

application

TCP

TCP

IP

IP

802.11 MAC

802.11 MAC

802.3 MAC

802.3 MAC

802.11 PHY

802.11 PHY

802.3 PHY

802.3 PHY


802 11 layers and functions l.jpg

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

MAC

access mechanisms, fragmentation, encryption

MAC Management

synchronization, roaming, MIB, power management

802.11 - Layers and functions

Station Management

IP

MAC

MAC Management

PLCP

PHY Management

PHY

PMD


802 11 physical layer l.jpg
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/s

  • DSSS (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. 1mW

  • FHSS (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


802 11 mac layer principles 1 2 l.jpg
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


802 11 mac layer principles 2 2 l.jpg
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

DIFS

DIFS

PIFS

SIFS

medium busy

contention

next frame

t

direct access if medium is free  DIFS

time slot

Note : IFS durations are specific to each PHY


802 11 csma ca principles l.jpg
802.11 - CSMA/CA principles

contention window

(randomized back-offmechanism)

  • 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)

DIFS

DIFS

medium busy

next frame

t

direct access if medium has been free for at least DIFS

time slot


802 11 csma ca broadcast l.jpg

=

802.11 – CSMA/CA broadcast

DIFS

DIFS

DIFS

DIFS

boe

bor

boe

bor

boe

busy

station1

boe

busy

station2

busy

station3

(detection by upper layer)

boe

busy

station4

boe

bor

boe

busy

(detection by upper layer)

station5

t

Here St4 and St5 happen to havethe same back-off time

medium not idle (frame, ack etc.)

busy

boe

elapsed backoff time

packet arrival at MAC

bor

residual backoff time

The size of the contention window can be adapted

(if more collisions, then increase the size)

Note: broadcast is not acknowledged


802 11 csma ca unicast l.jpg
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

DIFS

data

sender

SIFS

ACK

receiver

DIFS

data

other

stations

t

waiting time

Contentionwindow

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


802 11 dcf with rts cts l.jpg
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

DIFS

RTS

data

sender

SIFS

SIFS

SIFS

CTS

ACK

receiver

DIFS

NAV (RTS)

data

other

stations

NAV (CTS)

t

defer access

Contentionwindow

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

NAV: Net Allocation Vector


Fragmentation mode l.jpg
Fragmentation mode

DIFS

RTS

frag1

frag2

sender

SIFS

SIFS

SIFS

SIFS

SIFS

CTS

ACK1

ACK2

receiver

NAV (RTS)

NAV (CTS)

DIFS

NAV (frag1)

data

other

stations

NAV (ACK1)

t

contention

  • Fragmentation is used in case the size of the packets sent has to be reduced (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


802 11 point coordination function 1 2 l.jpg
802.11 – Point Coordination Function (1/2)

t0

t1

SuperFrame

medium busy

PIFS

SIFS

SIFS

D1

D2

point

coordinator

SIFS

SIFS

U1

U2

wireless

stations

stations‘

NAV

NAV

contention free period

  • 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


802 11 point coordination function 2 2 l.jpg
802.11 – Point Coordination Function (2/2)

t2

t3

t4

PIFS

SIFS

D3

D4

CFend

point

coordinator

SIFS

U4

wireless

stations

stations‘

NAV

NAV

contention free period

t

contention

period

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


802 11 mac frame format l.jpg
802.11 - MAC frame format

  • Types

    • control frames, management frames, data frames

  • Sequence numbers

    • important against duplicated frames due to lost ACKs

  • Addresses

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

  • Miscellaneous

    • sending time, checksum, frame control, data

bytes

2

2

6

6

6

2

6

0-2312

4

Frame

Control

Duration

ID

Address

1

Address

2

Address

3

Sequence

Control

Address

4

Data

CRC

version, type, fragmentation, security, ...

detection of duplication


Mac address format l.jpg
MAC address format

DS: Distribution System

AP: Access Point

DA: Destination Address

SA: Source Address

BSSID: Basic Service Set Identifier

- infrastructure BSS : MAC address of the Access Point

- ad hoc BSS (IBSS): random number

RA: Receiver Address

TA: Transmitter Address


802 11 mac management l.jpg
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


Synchronization infrastructure case l.jpg
Synchronization (infrastructure case)

beacon interval

B

B

B

B

access

point

busy

busy

busy

busy

medium

t

B

value of the timestamp

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


Synchronization ad hoc case l.jpg
Synchronization (ad-hoc case)

beacon interval

B1

B1

station1

B2

B2

station2

busy

busy

busy

busy

medium

t

B

value of the timestamp

beacon frame

random delay

  • Each node maintains its own synchronization timer and starts the transmission of 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


Power management l.jpg
Power management

  • Idea: switch the transceiver off if not needed

  • States of a station: sleep and awake

  • Timing 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?)


Power saving infrastructure case l.jpg

T

D

awake

TIM

DTIM

data transmission

to/from the station

B

d

broadcast/multicast

Power saving (infrastructure case)

Here the access point announcesdata addressed to the station

TIM interval

DTIM interval

D

B

T

T

d

D

B

access

point

busy

busy

busy

busy

medium

p

d

station

t

p

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


Power saving ad hoc case l.jpg

A

transmit ATIM

Power saving (ad-hoc case)

ATIM

window

beacon interval

B1

A

D

B1

station1

B2

B2

a

d

station2

t

B

D

beacon frame

random delay

transmit data

a

d

awake

acknowledge ATIM

acknowledge data

  • ATIM: Ad hoc Traffic Indication Map (a station announces the list of buffered frames)

  • Potential problem: scalability (high number of collisions)


802 11 roaming l.jpg
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., location information)

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


Security of 802 11 l.jpg
Security of 802.11

  • WEP: Wired Equivalent Privacy

  • Objectives:

    • Confidentiality

    • Access control

    • Data integrity

k

k

M

Integritychecksum

RC4

IV

RC4

IV

C(M)

P =

M

C(M)

P =

M

C(M)

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


The new solution for 802 11 security standard 802 1x l.jpg
The new solution for 802.11 security: standard 802.1x

Encapsulated EAP,

Typically on RADIUS

EAPOL(over Ethernet or 802.11)

Authenticator

Authentication Server

Supplicant

  • 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


More on ieee 802 1x l.jpg
More on IEEE 802.1x

Example of authentication, using one-time passwords (OTP):

Supplicant

Authenticator

Authentication server

EAP-request/identity

EAP-response/identiy (MYID)

EAP-request/OTP,OTP challenge

EAP-response/OTP,

OTPpassword

EAP-success

Authenticationsuccessfully

completed

Port authorized

: exchange of EAPOL frame

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

  • Notes :

  • Weaknesses have been found in 802.1x as well, but are corrected in thevarious implementations.

  • New standard in the making : IEEE 802.11i


Ieee 802 11 standardization efforts l.jpg
IEEE 802.11 – Standardization efforts

  • IEEE 802.11b

    • 2.4 GHz band

    • Bitrates 1 – 11 Mbit/s

  • IEEE 802.11a

    • 5 GHz band

    • transmission rates up to 54 Mbit/s

    • close cooperation with BRAN (ETSI Broadband Radio Access Network)

    • Coverage is not as good as in 802.11b

  • IEEE 802.11g

    • Available since 2003, highly popular

    • 2.4 GHz band (same as 802.11b)

    • Bitrates up to 54Mb/s

  • IEEE 802.11i

    • Security, makes use of IEEE 802.1x

  • IEEE 802.11p

    • For vehicular communications

  • IEEE 802.11s

    • For mesh networks

  • + many other…


Conclusion of wireless lans l.jpg
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 were flawed in both IEEE 802.11 (WEP) and Bluetooth

  • Future developments

    • Ultra Wide Band?


References l.jpg
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


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