10 Wireless LAN.rar (3,116Kb)

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1

Wireless LANs

EPL 657

Panayiotis Kolios

Contains slides and ideas from

Teknillinen Korkeakoulou, Finland: Wireless personal, local, metropolitan, and wide area networks, S-72.3240, and

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WIRELESS LAN (WLAN)

 _{Selected topics}_{Selected topics}

» Introduction_Introduction » WLAN aims_{WLAN aims}

» WLAN characteristics_{WLAN characteristics} » WLAN design goals_{WLAN design goals}

» Infrared vs radio transmission_{Infrared vs radio transmission}

» Infrastructure-based vs ad-hoc networks_{Infrastructure-based vs ad-hoc networks} » WLAN Standards_{WLAN Standards}

» IEEE 802.11_{IEEE 802.11} » WLAN Roaming_{WLAN Roaming} » WLAN Security_{WLAN Security}

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3

Why Wireless LANs (WLANs)



Mobility (portability)

_{Mobility (portability)}

and Flexibility



Places where there is no cabling infrastructure /

_{Places where there is no cabling infrastructure /}

Hard to wire areas



Reduced cost of wireless systems

_{Reduced cost of wireless systems}



Improved flexibility of wireless systems

_{Improved flexibility of wireless systems}



Cost

_Cost

–

Relatively low cost of deployment

_{Relatively low cost of deployment}

(4)

Wireless LAN Applications



LAN Extension

_{LAN Extension}



Cross building interconnection

_{Cross building interconnection}



Nomadic access

_{Nomadic access}

(5)

5

Vertical Markets



Factory foor



Home

networking



Hospital



Ofice workers



Retail stores



Warehouse



Stock market



Airport



Hotel



Starbuck



College

campus



Convention

Center

(6)

Example WLAN

deployment - Hotel



Competing Technologies



Wired Ethernet (802.3)



Phone Line



xDSL



Power Line



Proposed: Wireless LAN (802.11)



Why: Price/Performance and ease of

deployment

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7

Wireless LANs

(8)

Wireless LAN considerations



Throughput

_Throughput



Number of nodes

_{Number of nodes}



Connection to backbone

_{Connection to backbone}



Service area

_{Service area}



Battery power consumption

_{Battery power consumption}



Transmission robustness and security

_{Transmission robustness and security}



Collocated network operation

_{Collocated network operation}

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9

WLANs goal



A mature market introducing the flexibility of wireless

_{A mature market introducing the flexibility of wireless}

access into office, home, or production environments.



Typically restricted in their diameter to buildings, a

_{Typically restricted in their diameter to buildings, a}

campus, single rooms etc.



The global goal of WLANs is to

_{The global goal of WLANs is to}

replace office cabling,

_{replace office cabling,}

increase flexibility of connection especially for portable

devices and, additionally, to introduce a higher flexibility

for ad hoc communication in, e.g., group meetings

.

(10)

WLAN characteristics



Advantages:

_Advantages:

–

_very

_flexible

_{within radio coverage}

–

_{ad-hoc networks}

_without

_previous

_planning

_possible

–

_{wireless networks allow for the}

_design

_{of small,}

independent devices

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11

WLAN characteristics



Disadvantages:

_{Disadvantages:}

–

typically

_typically

lower bandwidth

compared to wired networks (~11

– 300 Mbit/s) due to limitations in radio transmission,

higher

error rates

due to interference, and

higher delay/delay

variation

due to extensive error correction and error detection

mechanisms

» offer lower QoS_{offer lower QoS}

–

a number of

_{a number of}

proprietary solutions

, especially for higher bit-

, especially for higher

bit-rates, standards take their time (e.g., IEEE 802.11n)

rates, standards take their time (e.g., IEEE 802.11n)

» standardized functionality plus many enhanced features_{standardized functionality plus many enhanced features}

» additional features only work in a homogeneous environment (i.e., _{additional features only work in a homogeneous environment (i.e.,}

when adapters from same vendors used)

(12)

WLAN design goals



global

_global

,

_,

seamless operation

_{seamless operation}

of WLAN products

_{of WLAN products}



low power

_{low power}

for battery use (special power saving modes and

_{for battery use (special power saving modes and}

power management functions)



no special permissions or licenses needed (

_{no special permissions or licenses needed (}

license-free

_license-free

band)

_band)



robust

_robust

transmission technology

_{transmission technology}



easy

_easy

to use for everyone, simple management

_{to use for everyone, simple management}



protection of investment

_{protection of investment}

in wired networks (support the same

_{in wired networks (support the same}

data types and services)



security

_security

– no one should be able to read other’s data,

_{– no one should be able to read other’s data,}

privacy

_privacy

–

_–

no one should be able to collect user profiles,

safety

– low

radiation

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13

Known problems with WLANs



_{Wireless link characteristics: media is error prone and the}

bit error rate (

BER

) is very

high

compared to the BER of

wired networks.



Carrier Sensing/collision detection

is

difficult

in

wireless networks because a station is incapable of

listening to its own transmissions in order to detect a

collision

(more later)

.



The

Hidden Terminal

problem

also decreases the

performance of a WLAN

(more later)

.

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Wireless Link Characteristics

Differences from wired link ….

–

_{decreased signal strength:}

_{radio signal attenuates as}

it propagates through matter (path loss)

–

interference from other sources:

_{interference from other sources:}

standardized

wireless network frequencies (e.g., 2.4 GHz) shared

by other devices (e.g., phone); also devices (e.g.

motors) interfere as well (

noise

)

–

_{multipath propagation:}

_{radio signal reflects off}

objects, arriving at destination at slightly different

times (

channel quality varies over time

)

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15

Wireless LAN Radio Technology



Infrared (IR) LANs

_{Infrared (IR) LANs}



Spread spectrum LANs

_{Spread spectrum LANs}



Narrow band microwave

_{Narrow band microwave}

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ISM frequency bands

ISM (Industrial, Scientific and Medical) frequency bands:

• 900 MHz band (902 … 928 MHz)

•

2.4 GHz band (2.4 … 2.4835 GHz)

• 5.8 GHz band (5.725 … 5.850 GHz)

Anyone is allowed to use radio equipment for transmitting in these bands (provided specific

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17



Several WLAN standards:

–

IEEE 802.11b

_{IEEE 802.11b}

offering 11 Mbit/s at 2.4 GHz

–

The same radio spectrum is used by

_{The same radio spectrum is used by}

Bluetooth

»

A short-range technology to set-up wireless personal area

_{A short-range technology to set-up wireless personal area}

networks with gross data rates less than 1 Mbit/s

–

IEEE

_IEEE

802.11a

, operating at 5 GHz and offering gross

data rates of 54 Mbit/s

–

IEEE

_IEEE

802.11g

offering up to 54 Mbit/s at 2.4 GHz.

–

IEEE

_IEEE

802.11n

up and coming standard up to 300 Mbit/s

(two spatial streams; 600 Mbit/s with 4 spatial streams)

WLAN Standards

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19

IEEE 802 standardisation framework

802.1 802.2 Logical Link Control (LLC)

802.11 Medium Access Control (MAC)

CSMA/CD (Ethernet)

CSMA/CA

Token

Ring CSMA/CA (Wireless LAN) 802.11n

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IEEE 802 wireless network technology options

Network definition

Wireless personal area network (WPAN)

Low-rate WPAN (LR-WPAN)

Wireless local area network (WLAN)

Wireless metroplitan

IEEE standard

IEEE 802.15.1

IEEE 802.15.4

IEEE 802.11

IEEE 802.16

Known as

Bluetooth

ZigBee

WiFi

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21

IEEE 802.11 standard



As the

_{As the}

standards

_standards

number indicates, this standard belongs

_{number indicates, this standard belongs}

to the group of

802.x

LAN standards

.



This means that the standard specifies the

_{This means that the standard specifies the}

physical and

_{physical and}

medium access layer

adapted to the special requirements

of wireless LANs, but

offers

the

same interface as the

others to higher layers

to maintain interoperability.



The primary goal of the standard was the specification of a

_{The primary goal of the standard was the specification of a}

simple and robust WLAN

which offers time-bounded and

asynchronous services.

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IEEE 802.11 Wireless LAN

 _802.11b_802.11b

– 2.4-5 GHz unlicensed spectrum_{2.4-5 GHz unlicensed spectrum} – up to 11 Mbps_{up to 11 Mbps}

– direct sequence spread spectrum (DSSS) in _{direct sequence spread spectrum (DSSS) in}

physical layer

» all hosts use same chipping code_{all hosts use same chipping code}

 _802.11a_802.11a

– 5-6 GHz range_{5-6 GHz range} – up to 54 Mbps_{up to 54 Mbps}

» Shading is much more severe compared to _{Shading is much more severe compared to}

2.4 GHz

» Depending on the SNR, propagation _{Depending on the SNR, propagation}

conditions and distance between sender

and receiver, data rates may drop fast

 _802.11g_802.11g

– 2.4-5 GHz range_{2.4-5 GHz range} – up to 54 Mbps_{up to 54 Mbps}

– Benefits from the better _{Benefits from the better}

propagation characteristics at 2.4

GHz compared to 5 GHz

» Backward compatible to 802.11b_{Backward compatible to 802.11b}

 _802.11n:_802.11n:_{multiple antennae}_{multiple antennae}

– 2.4-5 GHz range_{2.4-5 GHz range} – typically 200 Mbps_{typically 200 Mbps}

 _{IEEE 802.11e}_{IEEE 802.11e}

– MAC enhancements for _{MAC enhancements for}

providing some QoS

» Some QoS guarantees can be given _{Some QoS guarantees can be given} only via polling using PCF

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23

Characteristics of selected wireless

link standards

Indoor

IS-95, CDMA, GSM 2G

UMTS/WCDMA, CDMA2000 _3G

802.15 802.11b 802.11a,g

UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 3G cellular

(24)

Infrastructure-based vs ad-hoc

wireless networks



Infrastructure networks provide access to other networks.

_{Infrastructure networks provide access to other networks.}



Communication typically takes place only between the

_{Communication typically takes place only between the}

wireless nodes and the access point, but not directly

between the wireless nodes.

AP AP

AP

wired network

AP: Access Point

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25

Infrastructure-based vs ad-hoc

wireless networks



Several wireless networks may form one logical wireless network:

_{Several wireless networks may form one logical wireless network:}

– The access points together with the fixed network in between can connect The access points together with the fixed network in between can connect several wireless networks to form a larger network beyond actual radio several wireless networks to form a larger network beyond actual radio coverage.

coverage.



Network functionality lies within the access point (controls network

_{Network functionality lies within the access point (controls network}

flow), whereas the wireless clients can remain quite simple.



Can use different access schemes with or without collision.

_{Can use different access schemes with or without collision.}

– Collisions may occur if medium access of the wireless nodes and the access _{Collisions may occur if medium access of the wireless nodes and the access} point is not coordinated.

point is not coordinated.

» If only the access point controls medium access, no collisions are possible.If only the access point controls medium access, no collisions are possible.

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Infrastructure-based vs ad-hoc

wireless networks



Infrastructure-based wireless networks lose some of the

_{Infrastructure-based wireless networks lose some of the}

flexibility wireless networks can offer in general:

–

They cannot be used for disaster relief in cases where no

_{They cannot be used for disaster relief in cases where no}

infrastructure is left.

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27

Infrastructure-based vs ad-hoc

wireless networks



No need of any infrastructure to work

_{No need of any infrastructure to work}

–

_{greatest possible flexibility}



Each node communicate with other nodes, so no access

_{Each node communicate with other nodes, so no access}

point controlling medium access is necessary.

–

The complexity of each node is much higher

_{The complexity of each node is much higher}

» implement medium access mechanisms and forwarding data_{implement medium access mechanisms and forwarding data}

(28)

Infrastructure-based vs ad-hoc

wireless networks



Nodes within an ad-hoc network can only communicate if

_{Nodes within an ad-hoc network can only communicate if}

they can reach each other physically

–

if they are within each other’s radio range

_{if they are within each other’s radio range}

–

if other nodes can/want to forward the message

_{if other nodes can/want to forward the message}



IEEE 802.11 WLANs are typically infrastructure-based

_{IEEE 802.11 WLANs are typically infrastructure-based}

networks, which additionally support ad-hoc networking

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29

Elements of a wireless network

network infrastructure

wireless hosts

 _{laptop, PDA, IP phone}_{laptop, PDA, IP phone}  run applications_{run applications}

 may be stationary (non-_{may be stationary}

(non-mobile) or mobile mobile) or mobile

– wireless does wireless does notnot always always mean mobility

(30)

Elements of a wireless network

base stationbase station

 _{typically connected to}_{typically connected to}

wired network wired network

 relay - responsible for _{relay - responsible for}

sending packets between sending packets between

wired network and wired network and

wireless host(s) in its wireless host(s) in its

“area” “area”

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31

Elements of a wireless network

wireless linkwireless link

 _{typically used to connect}_{typically used to connect}

mobile(s) to base station mobile(s) to base station

 also can be used as _{also can be used as}

backbone links backbone links

 multiple access protocol _{multiple access protocol}

coordinates link access coordinates link access

 various data rates, _{various data rates,}

(32)

Elements of a wireless network

infrastructure mode

 _{base station connects}_{base station connects}

mobiles into wired mobiles into wired network

network

 handoff: mobile changes _{handoff: mobile changes}

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33

Elements of a wireless network

Ad hoc mode

 _{no base stations}_{no base stations}

 nodes can only transmit _{nodes can only transmit}

to other nodes within link to other nodes within link coverage

coverage

 nodes organize _{nodes organize}

themselves into a themselves into a

network: route among network: route among themselves

themselves

(34)

WLAN components

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35

IEEE 802.11 terminology

_{Basic Service Set (}_{Basic Service Set (}_BSS_BSS₎₎

– group of stations using the same _{group of stations using the same}

radio frequency

_{Access Point (}_{Access Point (}_AP_AP₎₎

– station integrated into the wireless _{station integrated into the wireless}

LAN and the distribution system

_{Station (STA}_{Station (}_STA₎₎

– terminal with access mechanisms to _{terminal with access mechanisms to}

the wireless medium and radio

contact to the access point

Portal_Portal

– bridge to other (wired) networks_{bridge to other (wired) networks}

_{Distribution System (}_{Distribution System (}_DS_DS₎₎

– interconnection network to form one _{interconnection network to form one}

logical network

_{Extended Service Set (}_{Extended Service Set (}_EES_EES₎₎

– based on several BSS_{based on several BSS}

Distribution System

System Architecture

(36)

IEEE 802.11 BSS



IEEE 802.11 allows the building of ad hoc

_{IEEE 802.11 allows the building of ad hoc}

networks between stations, thus forming one or

more BSSs.

–

In this case, a BSS comprises a group of stations using

_{In this case, a BSS comprises a group of stations using}

the same radio frequency.

–

_{Several BSSs can either be formed via the distance}

between the BSSs or by using different carrier

(37)

Distribution System (DS)



Used to interconnect wireless cells

_{Used to interconnect wireless cells}

(multiple BSS to form an ESS)



Allows multiple mobile stations to access

_{Allows multiple mobile stations to access}

fixed resources



Interconnects 802.11 technology

_{Interconnects 802.11 technology}

(38)

Access Points (AP)



Allows stations to associate with it

_{Allows stations to associate with it}



Supports Point Coordination Function (PCF)

_{Supports Point Coordination Function (PCF)}



Provides management features

_{Provides management features}

–

_{Join/Associate with BSS}

–

Time synchronisation (beaconing)

_{Time synchronisation (beaconing)}

–

_{Power management}

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39

IEEE standard 802.11

(40)

IEEE 802.11 protocol



Protocol architecture aims

–

Applications should not notice any difference apart

_{Applications should not notice any difference apart}

from the lower bandwidth and perhaps higher access

time from the wireless LAN.

»

WLAN behaves like, perhaps a ‘slower’, wired LAN.

_{WLAN behaves like, perhaps a ‘slower’, wired LAN.}

–

Consequently, the higher layers (application, TCP, IP)

_{Consequently, the higher layers (application, TCP, IP)}

look the same for the wireless node as for the wired

node.

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41

IEEE 802.11 protocol

–

The

_The

physical layer

provides a carrier sense signal, handles

modulation and encoding/decoding of signals.

–

The basic tasks of the MAC-medium

_{The basic tasks of the MAC-medium}

access control

protocol

comprise medium access, fragmentation of user data, and

encryption.



The standard also specifies

_{The standard also specifies}

management layers

_{management layers}

.

_.

–

The MAC management supports the

_{The MAC management supports the}

association

and re-

and

re-association of a station to an access point and

association of a station to an access point and

roaming

between

different APs.

–

Furthermore, it controls

_{Furthermore, it controls}

authentication

mechanisms, encryption,

synchronization

of a station with regard to an AP, and

power

management

(42)

IEEE 802.11



Physical layer

–

Includes the provision of the

_{Includes the provision of the}

Clear Channel Assessment-CCA

signal (energy detection).

–

This signal is needed for the MAC mechanisms controlling

_{This signal is needed for the MAC mechanisms controlling}

medium access and indicates if the medium is currently idle.

–

_{A number of physical channels}

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43

Physical layer

Wireless Transmission

Infrared

(IR)

Radio Frequency

(RF)

Spread

Spectrum

Frequency

Hopping

Direct

Sequence

(44)

Infrared vs radio transmission

Infrared light

 uses IR diodes, diffuse light reflected uses IR diodes, diffuse light reflected at walls, furniture etc, or directed light

at walls, furniture etc, or directed light

if a LOS exists btn sender and receiver

if a LOS exists btn sender and receiver  AdvantagesAdvantages

 simple, cheap, available in many mobile simple, cheap, available in many mobile

devices (PDAs, laptops, mobile phones)

 no licenses neededno licenses needed

 DisadvantagesDisadvantages

 interference by sunlight, heat sources interference by sunlight, heat sources

etc.

 many things shield or absorb IR lightmany things shield or absorb IR light  cannot penetrate obstacles (e.g., walls) cannot penetrate obstacles (e.g., walls)

low bandwidth (~115kbit/s, 4Mbit/s)

Radio

 _{typically using the license free frequency}

band at 2.4 GHz  _Advantages

– experience from wireless WAN

(microwave links) and mobile phones can be used

– coverage of larger areas possible (radio can penetrate (thinner) walls, furniture etc.)

– higher transmission rates (~11 – 54 Mbit/s)

 _{Disadvantages}

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45

Example WLAN physical

layer

802.11 Medium Access Control (MAC)

CSMA/CA

802.11g is the most popular physical layer, operating in the same band as 802.11b

ISM band: 2.4 … 2.4835 GHz The signal format is

OFDM (Orthogonal Frequency Division Multiplexing)

Data rates supported:

(46)

The ISM band at 2.4 GHz can be used by anyone as long as (in Europe...)

Transmitters using FH (Frequency Hopping) technology:

• Total transmission power < 100 mW • Power density < 100 mW / 100 kHz

Transmitters using DSSS technology:

• Total transmission power < 100 mW • Power density < 10 mW / 1 MHz

ETSI

EN 300 328-1 requirements

ISM

(47)

47

802.11 spectrum

at 2.4 GHz

_{at 2.4 GHz}

Divided into

overlapping channels

.

For e.g. the 2.4000–2.4835 GHz band is divided into 13 channels each of width 22 MHz but spaced only 5 MHz apart, with channel 1 centred on 2.412 GHz and 13 on 2.472 GHz Availability of channels is regulated by country (e.g. Japan adds a 14th channel 12 MHz above channel 13).

Given the separation between channels 1, 6, and 11, the signal on any channel should be sufficiently attenuated to minimally interfere with a transmitter on any other

(48)

Recall: Free-space

loss is

_{loss is}

dependent on frequency

The free-space loss L of a radio signal is:

2 2

4 d

4 df

L

c



_



_



where d is the distance between transmitter and receiver,

 is the rf wavelength, f is the radio frequency, and c is

the speed of light. The formula is valid for d >> _, and does not take into account antenna gains (=> Friis

(49)

49

Free-space loss examples

For example, when d is 10 or 100 m, the free-space loss

values (in dB) for the different ISM bands are:

d = 10 m d = 100 m

f = 900 MHz

f = 2.4 GHz

f = 5.8 GHz

L = 51.5 dB L = 71.5 dB

L = 60.0 dB L = 80.0 dB

(50)

Network

IEEE 802.15.1 WPAN (Bluetooth)

IEEE 802.15.4 LR-WPAN (ZigBee)

IEEE 802.11 WLAN (WiFi)

IEEE 802.16 WMAN

Maximum data rate

1 Mbit/s (Bluetooth v. 1.2) 3 Mbit/s (Bluetooth v. 2.0) 250 kbit/s

11 Mbit/s (802.11b) 54 Mbit/s (802.11g) 134 Mbit/s

Maximum channel data rates

(51)

51

Network

IEEE 802.15.1 WPAN (Bluetooth)

IEEE 802.15.4 LR-WPAN (ZigBee)

IEEE 802.11 WLAN (WiFi)

IEEE 802.16 WMAN (WiMAX)

Modulation / spreading method

Gaussian FSK / FHSS

Offset-QPSK / DSSS

DQPSK / DSSS (802.11b)

64-QAM / OFDM (802.11g)

128-QAM / single carrier

64-QAM / OFDM

Modulation / Signal spreading

(52)

802.11: advanced capabilities

Rate Adaptation



base station and

_{base station and}

mobile dynamically

change transmission

rate (physical layer

modulation technique)

as mobile moves,

SNR varies

QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps)

10 20 30 40 base station SNR

(53)

53

IEEE 802.11: MAC

overview

_overview



Two basic access mechanisms have been defined for IEEE

_{Two basic access mechanisms have been defined for IEEE}

802.11

802.11 –

CSMA/CA

_CSMA/CA

(

mandatory

) summarized as

distributed

coordination function (

DCF

)

» Optional method (_{Optional method (}RTS/CTS)RTS/CTS) avoiding the hidden terminal problem avoiding the hidden terminal problem

–

A

_A

contention-free polling

method for time-bounded service

called p

oint coordination function

(

PCF

)

» access point polls terminals according to a listaccess point polls terminals according to a list

–

DCF

_DCF

only offers

asynchronous service

, while

PCF offers both

asynchronous and time-bounded service

, but needs the access

point to control medium access and to avoid contention.

(54)

IEEE 802.11: MAC

overview

_overview



Within the MAC layer,

Distributed Coordination

Function (DCF)

(asynchronous service) is used as a

fundamental access method, while

Point

Coordination Function (PCF)

(synchronous

service) is optional.

–

_DCF

_{is also known as Carrier Sense Multiple Access with}

Collision Avoidance (

CSMA/CA

) protocol. It is an

(55)

55 

_{most important differences between WLAN and LAN}

protocol design is the

impossibility to detect all collisions

.

– difficult to receive (sense collisions) when transmitting due to _{difficult to receive (sense collisions) when transmitting due to} weak received signals (fading)

weak received signals (fading)

» with receiving and sending antennas immediately next to each other, _{with receiving and sending antennas immediately next to each other,}

a station is unable to see any signal but its own.

» As a result, the complete packet will be sent before the incorrect _{As a result, the complete packet will be sent before the incorrect}

checksum reveals that a collision has happened.

» Furthermore, receiver and transmitter mostly not on at the same time_{Furthermore, receiver and transmitter mostly not on at the same time}

– can’t sense all collisions in any case: _{can’t sense all collisions in any case:}hidden terminal, fadinghidden terminal, fading

IEEE 802.11: MAC

overview

_overview

(56)

Hidden Station Problem

A _B _C

(57)

57

Utmost importance that number of collisions be

limited to the absolute minimum.

DCFs CSMA/CA

(CA-Collision Avoidance) is the

MAC method used in a WLAN.

(Wireless stations

cannot detect collisions, i.e. the whole packet will be transmitted anyway).

Basic CSMA/CA operation:

1) If medium is free, then

Wait a specified time (DIFS), Transmit frame

2) If medium busy, then backoff

CSMA/CA rule: backoff before

collision

IEEE 802.11: MAC

(58)

IEEE 802.11: MAC

overview

_overview



CSMA/CA protocol basics:

_{CSMA/CA protocol basics:}

–

_{medium can be}

_{busy or idle}

_{(detected by the}

_CCA

Clear

_Clear

Channel Assessment-CCA signal of the physical layer

)

»

_{If medium busy this can be due to data frames or other control}

frames

–

_{during a}

_{contention phase}

_{several nodes try to access}

medium

–

_optionally

_{, the standard allows for}

_{collision free}

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59 

_{Define (802.11b):}

– slot_slot = 20 = 20 _s (s (9 or 20 s for 802.11g))

– Short inter-frame spacing (SIFS) _{Short inter-frame spacing (SIFS)}= 10 _s (16 _s for 802.11a) » shortest waiting time for medium access_{shortest waiting time for medium access}

» defined for short _{defined for short}control messagescontrol messages (e.g., ACK of data packets) (e.g., ACK of data packets)

– DCF inter-frame spacing (DIFS)_{DCF inter-frame spacing (DIFS)} = 50 _s (28 _s for 802.11g)

» longest waiting time used for asynchronous data service within a contention period _{longest waiting time used for asynchronous data service within a contention period}

DIFS=SIFS + two slot times

– PCF inter-frame spacing (PIFS)_{PCF inter-frame spacing (PIFS)}

» an access point polling other nodes only has to wait PIFS for medium access (for a _{an access point polling other nodes only has to wait PIFS for medium access (for a}

time-bounded service) PIFS=SIFS + one slot time (30

time-bounded service) PIFS=SIFS + one slot time (30 s for 802.11for b)b)

 _{The standard defines also two control frames:}_{The standard defines also two control frames:}

– RTS: Request To Send_{RTS: Request To Send} – CTS: Clear To Send_{CTS: Clear To Send}

IEEE 802.11: MAC

(60)

Interframe Spacing (IFS) and

priorities



SIFS (Short IFS)

_{SIFS (Short IFS)}

– ACK, CTS, Poll Messages, Poll responses, CF-End_{ACK, CTS, Poll Messages, Poll responses, CF-End}



PIFS (PCF IFS)

_{PIFS (PCF IFS)}

– PCF operation mode, including Beacon, Retransmitted _{PCF operation mode, including Beacon, Retransmitted} poll messages

poll messages



DIFS (DCF IFS)

_{DIFS (DCF IFS)}

– DCF operation mode, including back-off, RTS_{DCF operation mode, including back-off, RTS}



_{EIFS (Extended IFS)}

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61



Collision Avoidance

_{Collision Avoidance}

–

_{idea is to}

_{prevent collisions}

_{at the moment they are}

most likely to occur , i.e. when the bus is released

(since many stations may compete then).

–

_{All clients are forced to}

_{wait for a random number}

of timeslots

and then sense the medium again,

before starting a transmission.

–

_{If the medium is sensed to be busy, the client}

freezes its timer until it becomes free again.

Thus, the chance of two clients starting to send

simultaneously is reduced.

IEEE 802.11: CSMA/CA

(62)

–

the

_the

overhead

introduced by the Collision Avoidance delays

should be as small as possible.

–

_{the protocol should keep the number of}

_collisions

_{to a}

minimum

, even under the highest possible load.

» To this end, the range of the random delay, or the _{To this end, the range of the random delay, or the}contention contention window

window, is set to , is set to vary with the loadvary with the load. .

» In the case of a collision, the delay window (CW) is doubled _{In the case of a collision, the delay window (CW) is doubled}

progressively: 15, 31, 63,...1023, until a successful transmission

occurs and the delay is reset to the minimal value.

» From the number _{From the number}CWCW (= 15 / 31 … 1023 slots) the random backoff (= 15 / 31 … 1023 slots) the random backoff bn

bn (in terms of slots) is chosen in such a way that (in terms of slots) is chosen in such a way that bnbn is is uniformly uniformly distributed between 0 … CW

distributed between 0 … CW..

» Since it is unlikely that several stations will choose the same value _{Since it is unlikely that several stations will choose the same value}

of

of bnbn, collisions are rare., collisions are rare.

IEEE 802.11: CSMA/CA

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63

IEEE 802.11: CSMA/CA

»

Broadcast data transfer (DCF)

_{Broadcast data transfer (DCF)}

t direct access if

medium is free  DIFS

– station ready to send starts sensing the medium (Carrier Sense based station ready to send starts sensing the medium (Carrier Sense based on CCA-Clear Channel Assessment)

on CCA-Clear Channel Assessment)

– if the medium is busy, the station has to wait for a free DIFS, then the if the medium is busy, the station has to wait for a free DIFS, then the station must additionally wait a random back-off time (

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

avoidance))

– if another station occupies the medium during the back-off time of the if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness – during the next phase this

station, the back-off timer stops (fairness – during the next phase this

node will continue its timer from where it stopped)

– if the medium is free for the duration of a if the medium is free for the duration of a Distributed Coordination Distributed Coordination Function Inter-Frame Space (DIFS)

(64)

IEEE 802.11 : CSMA/CA

»

E.g. Unicast data transfer

_{E.g. Unicast data transfer}

DIFS

data

ACK

other stations receiver sender

t

data

DIFS

waiting time _contention SIFS

– station has to wait for DIFS before sending datastation has to wait for DIFS before sending data

– receivers acknowledge after waiting for a duration of a Short receivers acknowledge after waiting for a duration of a Short

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65

EE802.11: Exponential backoff

mechanism

binary exponential backoff:

After k collisions, a random number of slot times between 15 and 2k+5_-1

is chosen. So, for the first collision, each sender might wait between 15 or 31 slot times. After the second collision, the senders might wait between 15 and 63 slot times, and so forth.

(66)

Contention window (CW) for 802.11b

If transmission of a frame was unsuccessful and the frame is allowed to be retransmitted, before each

retransmission the Contention Window (CW) from which

bn is chosen (at random, starting from 15 or 31) is 5th (and further)

retransmissions

:

…

CW 802.11b802.11b

EE802.11: Exponential backoff

mechanism

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67

Contention window (CW) for 802.11g

In the case of 802.11g operation, the initial CW length is

15 slots. The slot duration is 9 _s. The backoff operation of 802.11g is substantially faster than that of 802.11b.

DIFS … CW = 24-1 = 15 slots 6th (and further)

retransmissions

:

…

CW 802.11g802.11g

EE802.11: Exponential backoff

mechanism

(68)

Selection of random backoff

From the number CW (= 15 / 31 … 2k+5-1 slots) the

random backoff bn (in terms of slots) is chosen in such a way that bn is uniformly distributed between 0 … CW.

Since it is unlikely that several stations will choose the same value of bn, collisions are rare.

The next slides show wireless medium access in action. The example involves four stations: A, B, C and D.

”Sending a packet” means ”Data+SIFS+ACK” sequence. Note how the backoff time may be split into several parts.

EE802.11: Exponential backoff

mechanism

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69

Wireless medium access example

Station A sending a packet, stations B and C also wish to send packets, but have to wait (defer +

backoff)

2) Station C is

”winner” (backoff time expires first) and starts sending packet

2 1

ACK

Data+SIFS+ACK

EE802.11: Exponential backoff

mechanism

(70)

Wireless medium access example

wishes to send a packet 4) When medium

becomes idle plus DIFS elapses,

station B continues to count down and station C draws a CW number D(bn)

station B is ”winner”

3

4

ACK

EE802.11: Exponential backoff

mechanism

(71)

71

Wireless medium access

example

Station A counts down to 0 and then

starts sending packet. Now there is no competition.

DIFS

5 ACK

EE802.11: Exponential backoff

mechanism

(72)

No shortcuts for any station…

DIFS SIFS DIFS

ACK (B=>A) Transmitted

frame (A=>B)

Next frame (A=>B) Backoff

When a station wants to send more than one frame, it has to use the backoff mechanism like any other station (of

EE802.11: Exponential backoff

mechanism

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73

Avoiding collisions (using extra signalling). How?

idea:

allow sender to “ allow sender to “reservereserve” channel rather than random access of data ” channel rather than random access of data frames: avoid collisions of long data frames

frames: avoid collisions of long data frames

 _{sender first transmits}_{sender first transmits}_small_small_{request-to-send (}_{request-to-send (}_RTS_RTS_{) packets}_{) packets}_{to BS}_{to BS}_{using CSMA}_{using CSMA}

– RTS packets may still collide with each other (but they are very short)_{RTS packets may still collide with each other (but they are very short)}

 _{BS broadcasts clear-to-send}_{BS broadcasts clear-to-send}_CTS_CTS_{in response to RTS}_{in response to RTS}  _{CTS heard by all nodes}_{CTS heard by all nodes}

– sender transmits data frame_{sender transmits data frame}

– other stations defer transmissions. For how long? _{other stations defer transmissions. For how long?}

avoid data frame collisions completely

using small reservation packets!

IEEE 802.11: MAC

(74)

Network Allocation Vector

(NAV)



Each RTS frame includes the duration of the

time its needs to occupy the channel.



NAV

: a timer on other stations which have to

wait NAV before checking if the

channel/medium is free.



When a station (WS1) sends RTS (or CTS),

other stations on the system start NAV (WS2

and WS3 in example below)

(75)

75

Hidden Station Problem

(Solution)

B accepts RTS from A and rejects RTS from C.

CTS from B (actually BS) to A is also received on C which starts the NAV timer in CTS.

A and C want to send to B

B can hear A and C

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76

Busy Medium



Physically busy

_{Physically busy}

: a station senses the

wireless medium to determine if it is busy.



Virtually busy

_{Virtually busy}

: a station receives a control

message (RTS or CTS) which indicates the

wireless medium is busy for the duration

of the NAV timer.



All stations must monitor the headers

of all frames they receive and store the

NAV value in a counter.



The counter decrements in steps of one

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77

IEEE 802.11

» Sending _Sendingunicastunicast packets packets with RTS/CTS control frameswith RTS/CTS control frames

SIFS

defer access _contention

RTS

CTS

SIFS _SIFS

NAV (RTS)=3SIFS+CTS+data+ACK

NAV (CTS)=2SIFS+data+ACK

– station can send RTS with reservation parameter after waiting for DIFS station can send RTS with reservation parameter after waiting for DIFS

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

the ACK related to it).

– Every node receiving this RTS now has to set its net allocation vector – it specifies Every node receiving this RTS now has to set its net allocation vector – it specifies the earliest point at which the node can try to access the medium again

the earliest point at which the node can try to access the medium again – acknowledgement via CTS after SIFS by receiver (if ready to receive)acknowledgement via CTS after SIFS by receiver (if ready to receive)

– sender can now send data at once, acknowledgement via ACKsender can now send data at once, acknowledgement via ACK

(78)

Collision Avoidance: RTS-CTS exchange

AP

A B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

reservation collision

(79)

79

802.11 MAC Timing

(80)

Masters thesis

http://eeweb.poly.

edu/dgoodman/fai

nberg.pdf

Note that DIFS

Example

(81)

(82)

Point Coordination

Function (PCF)



Optional and implemented on top of DCF.

 Must be running in conjunction with DCF. Must be running in conjunction with DCF.



A single Access Point (AP) controls access to

the medium, and a Point Coordinator Agent

resides in the AP.



AP sends a beacon message and all stations

stop DCF.



AP polls each station for data, and after a

given time interval moves to the next station.

 Guaranteed maximum latency Guaranteed maximum latency



No station is allowed to transmit unless it is

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83

PCF (cont.)

B

PCF

DCF

busy B

PCF

NAV

B: beacon message

Contention

free period (CFP)

Contention

_{period (CP)}

repetition interval

(84)

Additional WLAN

Features



Positive Acknowledgement



Sequence Control



Fragmentation

(85)

85

IEEE 802.11 framing and

addressing

(86)

Internet router

AP

H1 _R1

AP MAC addr H1 MAC addr R1 MAC addr

R1 MAC addr H1 MAC addr

dest. address source address

802.3 frame

802.11 frame: addressing

(87)

87

3 payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seq control

802.11 frame: addressing

Address 2: MAC address of wireless host or AP transmitting this frame

Address 1: MAC address of wireless host or AP

to receive this frame Address 3:_{of router interface to} MAC address which AP is attached

(88)

Routing

in a (W)LAN

_{in a (W)LAN}

Recall: Routing in a (W)LAN is based on MAC addresses. A router performs mapping between these two address

types (IP-MAC):

IP network

(W)LAN

Router

Router Server_Server

(W)LAN device (W)LAN

device

00:90:4B:00:0C:72 124.2.10.57



00:90:4B:00:0C:72

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89

Address

allocation

_allocation

MAC addresses are associated with the hardware devices. IP addresses can be allocated to (W)LAN devices either on a permanent basis or dynamically from an address pool using the Dynamic Host Configuration Protocol (DHCP). The DHCP server may be a separate network element (or for example integrated into a RADIUS server that offers a set of additional features), or may be integrated with the address-mapping router and/or access point.

(90)

Network Address Translation

(NAT)

Recall:

On the (W)LAN side of the network address translator (NAT device), different (W)LAN users are identified using

private (reusable, globally not unique) IP addresses.

On the Internet side of the NAT device, only one (globally unique) IP address is used. Users are identified by means of different TCP/UDP port numbers.

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91

User 1 IP address

User 2 IP address _10.2.1.58

IP address for all users in (W)LAN:

124.0.6.12

14781

User 1 TCP port number