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:

= c/f

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

1 Mm300 Hz

10 km30 kHz

100 m3 MHz

1 m300 MHz

10 mm30 GHz

100 m3 THz

1 m300 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 Europe USA Japan

Mobile phones

Dig. Dividend 800MHz GSM 890-915 MHz, 935-960 MHz; 1710-1785 MHz, 1805-1880 MHz UMTS 1920-1980 MHz 2110-2170 MHz LTE 2600MHz

AMPS, TDMA, CDMA 824-849 MHz, 869-894 MHz; TDMA, CDMA, GSM 1850-1910 MHz, 1930-1990 MHz; UMTS 1850-1910 MHz 1930-1990 MHz

PDC 810-826 MHz, 940-956 MHz; 1429-1465 MHz, 1477-1513 MHz UMTS 1749.9-1784.9 1844.9-1879.9

Cordless telephones

CT1+ 885-887 MHz, 930-932 MHz; CT2 864-868 MHz DECT 1880-1900 MHz

PACS 1850-1910 MHz, 1930-1990 MHz PACS-UB 1910-1930 MHz

PHS 1895-1918 MHz JCT 254-380 MHz

Wireless LANs

IEEE 802.11 2400-2483 MHz 5725–5875 MHz

IEEE 802.11 2400-2483 MHz 5725–5875 MHz

IEEE 802.11 2471-2497 MHz 5725–5875 MHz

Note: in the coming years, frequencies will become technology-neutral

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 More difficult to secure

Data rate

Scope of Various WLAN and WPAN Standards

802.11n

Power consumption

Complexity

802.15.IBluetooth

802.11a

802.11g

802.11

WPAN

802.11b

WLAN

802.15.4

6WPAN: Wireless Personal Area Network

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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

8

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 materials 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 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

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Infrastructure vs. ad hoc networks

infrastructure network

Ad hoc network

APAP

AP

wired network

AP: Access Point

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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

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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

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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

13

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

PH

Y

Sta

tion

Man

agem

ent

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802.11b - 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 (rarely used in practice) 850-950 nm, diffuse light, around 10 m range carrier detection, energy detection, synchronization

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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 and rarely used in practice) access point polls terminals according to a list

DCF: Distributed Coordination FunctionPCF: Point Coordination Function

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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 busySIFS

PIFS

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

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t

medium busy

DIFSDIFS

next frame

contention window(randomized back-offmechanism)

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 slotdirect access if medium has been free for at least DIFS

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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 havethe same back-off time

=

Note: broadcast is not acknowledgedNote: broadcast is not acknowledged

(detection by upper layer)

(detection by upper layer)

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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

otherstations

receiver

senderdata

DIFS

Contentionwindow

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

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

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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

otherstations

receiver

senderdata

DIFS

Contentionwindow

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

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

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

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Fragmentation mode

t

SIFS

DIFS

data

ACK1

otherstations

receiver

senderfrag1

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 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

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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

FrameControl

DurationID

Address1

Address2

Address3

SequenceControl

Address4

Data CRC

2 2 6 6 6 62 40-2312bytes

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

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MAC address format

scenario to DS fromDS

address 1 address 2 address 3 address 4

ad-hoc network 0 0 DA SA BSSID -infrastructurenetwork, from AP

0 1 DA BSSID SA -

infrastructurenetwork, to AP

1 0 BSSID SA DA -

infrastructurenetwork, within DS

1 1 RA TA DA SA

DS: Distribution SystemAP: Access PointDA: Destination AddressSA: Source AddressBSSID: Basic Service Set Identifier - infrastructure BSS : MAC address of the Access Point - ad hoc BSS (IBSS): random numberRA: Receiver AddressTA: Transmitter Address

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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

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Synchronization (infrastructure case)

beacon interval

tmedium

accesspoint

busy

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

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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 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

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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?)

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Power saving (infrastructure case)

TIM interval

t

medium

accesspoint

busy

D

busy busy busy

T T D

T TIM D DTIM

DTIM interval

BB

B broadcast/multicast

station

awake

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

p

d

d

d data transmissionto/from the station

Here the access point announcesdata addressed to the station

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Power saving (ad-hoc case)

awake

A transmit ATIM D transmit data

t

station1

B1 B1

B beacon frame

station2

B2 B2

random delay

A

a

D

d

ATIMwindow 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)

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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

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Security of 802.11

WEP: Wired Equivalent Privacy Objectives:

Confidentiality Access control Data integrity

M

C(M)

Integritychecksum

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 =

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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 LANRADIUS: 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

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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 challenge

EAP-response/OTP,OTPpassword

EAP-success

Port authorizedAuthenticationsuccessfullycompleted

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)

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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 802.11ac Extension of IEEE 802.11n, under development

IEEE 802.11e Enhanced DCF: to support differentiated service

IEEE 802.11i Security, makes use of IEEE 802.1x

IEEE 802.11p For vehicular communications

IEEE 802.11s For mesh networks

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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?

36

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

37

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.

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AODV : Route discovery (1)

E G

M

H

R

FA

B

C

I

DS

K

N

L

PJ

Q

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AODV : Route discovery (2)

E G

M

H

R

FA

B

C

I

DS

K

N

L

PJ

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)

40

AODV : Route discovery (3)

E G

M

H

R

FA

B

C

I

DS

K

N

L

PJ

Q

: represents a link on the reverse path

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AODV : Route discovery (4)

E G

M

H

R

FA

B

C

I

DS

K

N

L

PJ

Q

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AODV : Route discovery (5)

E G

M

H

R

FA

B

C

I

DS

K

N

L

PJ

Q

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AODV : Route discovery (6)

M

D

K

L

PJ

E G

H

R

FA

B

C

I

S

N

Q

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AODV : Route discovery (7)

M

D

K

L

PJ

E G

H

R

FA

B

C

I

S

N

Q

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AODV : Route reply and setup of the forward path

M

D

K

L

PJ

E G

H

R

FA

B

C

I

S

N

Q

: Link over which the RREP is transmitted

: Forward path

46

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

47

AODV : Data delivery

M

D

K

L

PJ

E G

H

R

FA

B

C

I

S

N

Q

Data

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

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AODV : Route maintenance (1)

M

D

K

L

PJ

E G

H

R

FA

B

C

I

S

N

Q

Data

X

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AODV : Route maintenance (2)

M

D

K

L

PJ

E G

H

R

FA

B

C

I

S

N

Q

XRERR(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.

50

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

2011 Trial in MobNet with Nokiahttp://lca.epfl.ch/projects/lca1-nokia

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Adv

ersa

ry’s

AP

s

66 m

186 m