1

WIRELESS LAN (WLAN)WIRELESS LAN (WLAN)

Selected topicsSelected topics» Introduction – WLAN StandardsIntroduction – WLAN Standards» WLAN definitionWLAN definition» WLAN characteristicsWLAN characteristics» WLAN design goalsWLAN design goals» Infrared vs radio transmissionInfrared vs radio transmission» Infrastructure-based vs ad-hoc networksInfrastructure-based vs ad-hoc networks» IEEE 802.11IEEE 802.11» WLAN RoamingWLAN Roaming» WLAN SecurityWLAN Security

» Other technologiesOther technologies

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IntroductionIntroduction

Several WLAN standards:Several WLAN standards:– IEEE 802.11bIEEE 802.11b offering 11 Mbit/s at 2.4 GHz offering 11 Mbit/s at 2.4 GHz– The same radio spectrum is used by The same radio spectrum is used by Bluetooth 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/snetworks with gross data rates less than 1 Mbit/s

– IEEE released a new WLAN standard, IEEE released a new WLAN standard, 802.11a802.11a, , operating at 5 GHz and offering gross data rates of 54 operating at 5 GHz and offering gross data rates of 54 Mbit/sMbit/s

» uses the same physical layer as uses the same physical layer as HiperLAN2HiperLAN2 does does tries to give QoS guarantees tries to give QoS guarantees

– IEEE IEEE 802.11g802.11g offering up to 54 Mbit/s at 2.4 GHz. offering up to 54 Mbit/s at 2.4 GHz.

3

WLAN StandardsWLAN StandardsWireless

LAN

2.4 GHz 5 GHz

802.11(2 Mbps)

802.11b(11 Mbps)

802.11g(22-54 Mbps)

HiSWANa(54 Mbps)

802.11a(54 Mbps)

HiperLAN2(54 Mbps)

HomeRF 2.0(10 Mbps)

Bluetooth(1 Mbps)

HomeRF 1.0(2 Mbps)

802.11e(QoS)

802.11i(Security)

802.11f(IAPP)

802.11h(TPC-DFS)

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WLAN definitionWLAN definition

A fast-growing market introducing the flexibility A fast-growing market introducing the flexibility of wireless access into office, home, or production of wireless access into office, home, or production environments.environments.

Typically restricted in their diameter to buildings, Typically restricted in their diameter to buildings, a campus, single rooms etc.a campus, single rooms etc.

The global goal of WLANs is toThe global goal of WLANs is to replace office cabling and, replace office cabling and, additionally, to introduce a higher additionally, to introduce a higher flexibility for ad hoc communication flexibility for ad hoc communication in, e.g., group meetingsin, e.g., group meetings..

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WLAN characteristicsWLAN characteristics

Advantages:Advantages:– very very flexibleflexible within radio coverage within radio coverage– ad-hoc networks without previous ad-hoc networks without previous planningplanning possible possible – wireless networks allow for the wireless networks allow for the designdesign of small, independent devices of small, independent devices – more more robustrobust against disasters (e.g., earthquakes, fire) against disasters (e.g., earthquakes, fire)

Disadvantages:Disadvantages:– typically very typically very low bandwidthlow bandwidth compared to wired networks (~11 – 54 Mbit/s) due to compared to wired networks (~11 – 54 Mbit/s) due to

limitations in radio transmission, limitations in radio transmission, higher error rates higher error rates due to interference, and due to interference, and higher higher delay/delay variation delay/delay variation due to extensive error correction and error detection mechanismsdue to extensive error correction and error detection mechanisms

» offer lower QoSoffer lower QoS– many proprietary solutions offered by companies, especially for higher bit-rates, many proprietary solutions offered by companies, especially for higher bit-rates,

standards take their time (e.g., IEEE 802.11) – slow standardization proceduresstandards take their time (e.g., IEEE 802.11) – slow standardization procedures» standardized functionality plus many enhanced featuresstandardized functionality plus many enhanced features» these additional features only work in a homogeneous environment (i.e., when adapters from these additional features only work in a homogeneous environment (i.e., when adapters from

the same vendors are used for all wireless nodes) the same vendors are used for all wireless nodes) – products have to follow many national products have to follow many national restrictionsrestrictions if working wireless, it takes a very if working wireless, it takes a very

long time to establish global solutionslong time to establish global solutions

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WLAN design goalsWLAN design goals

global, seamless operation of WLAN productsglobal, seamless operation of WLAN products low power for battery use (special power saving modes and power low power for battery use (special power saving modes and power

management functions)management functions) no special permissions or licenses needed (license-free band)no special permissions or licenses needed (license-free band) robust transmission technologyrobust transmission technology easy to use for everyone, simple managementeasy to use for everyone, simple management protection of investment in wired networks (support the same data protection of investment in wired networks (support the same data

types and services)types and services) security – no one should be able to read other’s data, privacy – no security – no one should be able to read other’s data, privacy – no

one should be able to collect user profiles, safety – low radiationone should be able to collect user profiles, safety – low radiation

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Infrared vs radio transmissionInfrared vs radio transmissionInfrared lightInfrared light

uses IR diodes, diffuse light uses IR diodes, diffuse light reflected at walls, furniture etc, or reflected at walls, furniture etc, or directed light if a LOS exists btn directed light if a LOS exists btn sender and receiversender and receiver

AdvantagesAdvantages simple, cheap, available in many simple, cheap, available in many

mobile devices (PDAs, laptops, mobile devices (PDAs, laptops, mobile phones)mobile phones)

no licenses neededno licenses needed

DisadvantagesDisadvantages interference by sunlight, heat interference by sunlight, heat

sources etc.sources etc. many things shield or absorb IR many things shield or absorb IR

lightlight cannot penetrate obstacles (e.g., cannot penetrate obstacles (e.g.,

walls) walls) low bandwidth (~115kbit/s, low bandwidth (~115kbit/s,

4Mbit/s)4Mbit/s) ExampleExample

IrDA (Infrared Data Association) IrDA (Infrared Data Association) interface available everywhereinterface available everywhere

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– very limited license free frequency

bands – shielding more difficult, interference with

other senders, or electrical devices

Example– IEEE 802.11, HIPERLAN, Bluetooth

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Infrastructure-based vs ad-hoc Infrastructure-based vs ad-hoc wireless networks (I)wireless networks (I)

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 wireless nodes and the access point, but not directly between the wireless nodes.between the wireless nodes.

The access point does not just control medium access, but The access point does not just control medium access, but also acts as a bridge to other wireless or wired networks.also acts as a bridge to other wireless or wired networks.

APAP

AP

wired network

AP: Access PointInfrastructure-based wireless networks

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Infrastructure-based vs ad-hoc Infrastructure-based vs ad-hoc wireless networks (II)wireless networks (II)

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

Use different access schemes with or without collision.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.

Useful for quality of service guarantees (e.g., minimum bandwidth for certain nodes)Useful for quality of service guarantees (e.g., minimum bandwidth for certain nodes) The access point may poll the single wireless nodes to ensure the data rate.The access point may poll the single wireless nodes to ensure the data rate.

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Infrastructure-based vs ad-hoc Infrastructure-based vs ad-hoc wireless networks (III)wireless networks (III)

Infrastructure-based wireless networks lose some of the Infrastructure-based wireless networks lose some of the flexibility wireless networks can offer in general: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.infrastructure is left.

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Infrastructure-based vs ad-hoc Infrastructure-based vs ad-hoc wireless networks (III)wireless networks (III)

No need of any infrastructure to workNo need of any infrastructure to work– greatest possible flexibilitygreatest 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.point controlling medium access is necessary.– The complexity of each node is higherThe complexity of each node is higher

» implement medium access mechanisms, forwarding data implement medium access mechanisms, forwarding data

Ad-hoc wireless networks

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Infrastructure-based vs ad-hoc Infrastructure-based vs ad-hoc wireless networks (IV)wireless networks (IV)

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 physicallythey can reach each other physically– if they are within each other’s radio rangeif they are within each other’s radio range

– if other nodes can forward the message if other nodes can forward the message

IEEE 802.11 and HiperLAN2 are typically infrastructure-IEEE 802.11 and HiperLAN2 are typically infrastructure-based networks, which additionally support ad-hoc based networks, which additionally support ad-hoc networkingnetworking

Bluetooth is a typical wireless ad-hoc networkBluetooth is a typical wireless ad-hoc network

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IEEE 802.11 (I)IEEE 802.11 (I)

As the standards number indicates, this standard belongs As the standards number indicates, this standard belongs to the group of 802.x LAN standards.to the group of 802.x LAN standards.

This means that the standard specifies the physical and This means that the standard specifies the physical and medium access layer adapted to the special requirements medium access layer adapted to the special requirements of wireless LANs, but offers the same interface as the of wireless LANs, but offers the same interface as the others to higher layers to maintain interoperability.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 simple and robust WLAN which offers time-bounded and asynchronous services.asynchronous services.

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WLAN componentsWLAN components

Figure 2.11Figure 2.11 Photographs of popular 802.11b WLAN equipment. Access points and a client card are shown on left, and PCMCIA Photographs of popular 802.11b WLAN equipment. Access points and a client card are shown on left, and PCMCIA Client card is shown on right. (Courtesy of Cisco Systems, Inc.)Client card is shown on right. (Courtesy of Cisco Systems, Inc.)

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IEEE 802.11 (II)IEEE 802.11 (II)

Station (STA)Station (STA)– terminal with access mechanisms terminal with access mechanisms

to the wireless medium and radio to the wireless medium and radio contact to the access pointcontact to the access point

Basic Service Set (BSS)Basic Service Set (BSS)– group of stations using the same group of stations using the same

radio frequencyradio frequencyAccess PointAccess Point

– station integrated into the wireless station integrated into the wireless LAN and the distribution systemLAN and the distribution system

PortalPortal– bridge to other (wired) networksbridge to other (wired) networks

Distribution SystemDistribution System– interconnection network to form interconnection network to form

one logical network (EES: one logical network (EES: Extended Service Set) based Extended Service Set) based on several BSSon several BSS

Distribution System

Portal

802.x LAN

Access Point

802.11 LAN

BSS2

802.11 LAN

BSS1

Access Point

STA1

STA2 STA3

ESS

System Architecture of an infrastructure networkSystem Architecture of an infrastructure network

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IEEE 802.11 (III)IEEE 802.11 (III)

Stations can select an AP and associate with it.Stations can select an AP and associate with it. The APs support roaming (i.e., changing access The APs support roaming (i.e., changing access

points), the distribution system then handles data points), the distribution system then handles data transfer between the different APs. transfer between the different APs.

Furthermore, APs provide synchronization within Furthermore, APs provide synchronization within a BSS, support power management, and can a BSS, support power management, and can control medium access to support time-bounded control medium access to support time-bounded service.service.

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IEEE 802.11 (IV)IEEE 802.11 (IV)

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 networks between stations, thus forming one or more BSSs.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. the same radio frequency. – Several BSSs can either be formed via the distance Several BSSs can either be formed via the distance

between the BSSs or by using different carrier between the BSSs or by using different carrier frequencies.frequencies.

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IEEE 802.11 (V)IEEE 802.11 (V)

mobile terminal

access point

fixedterminal

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

LLC

infrastructurenetwork

LLC LLC

IEEE standard 802.11

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IEEE 802.11 (VI)IEEE 802.11 (VI)

Protocol architectureProtocol architecture– Applications should not notice any difference apart Applications should not notice any difference apart

from the lower bandwidth and perhaps higher access from the lower bandwidth and perhaps higher access time from the wireless LAN.time from the wireless LAN.

» WLAN behaves like a slow wired LAN.WLAN behaves like a slow 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 look the same for the wireless node as for the wired node.node.

– The differences are in physical and link layer The differences are in physical and link layer » different media and access control different media and access control

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IEEE 802.11 (VII)IEEE 802.11 (VII)

– The physical layer provides a carrier sense signal, handles The physical layer provides a carrier sense signal, handles modulation and encoding/decoding of signals.modulation and encoding/decoding of signals.

– The basic tasks of the MAC-medium access control protocol The basic tasks of the MAC-medium access control protocol comprise medium access, fragmentation of user data, and comprise medium access, fragmentation of user data, and encryption.encryption.

The standard also specifies management layers.The standard also specifies management layers.– The MAC management supports the association and re-The MAC management supports the association and re-

association of a station to an access point and roaming between association of a station to an access point and roaming between different APs.different APs.

– Furthermore, it controls authentication mechanisms, encryption, Furthermore, it controls authentication mechanisms, encryption, synchronization of a station with regard to an AP, and power synchronization of a station with regard to an AP, and power management to save battery power.management to save battery power.

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IEEE 802.11 (VIII)IEEE 802.11 (VIII) Physical layerPhysical layer

– Includes the provision of the Clear Channel Includes the provision of the Clear Channel Assessment-CCA signal (energy detection).Assessment-CCA signal (energy detection).

– This signal is needed for the MAC mechanisms This signal is needed for the MAC mechanisms controlling medium access and indicates if the medium controlling medium access and indicates if the medium is currently idle. is currently idle.

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IEEE 802.11 (IX)IEEE 802.11 (IX) Medium Access ControlMedium Access Control

– The basic services provided by the MAC layer are the mandatory The basic services provided by the MAC layer are the mandatory asynchronous data service asynchronous data service and an optional and an optional time-bounded servicetime-bounded service..

– IEEE 802.11 offers only the asynchronous data service in ad-hoc IEEE 802.11 offers only the asynchronous data service in ad-hoc network modenetwork mode

– Both service types can be offered using an infrastructure-based Both service types can be offered using an infrastructure-based network together with the access point coordinating medium access.network together with the access point coordinating medium access.

– The asynchronous service supports broadcast and multicast packets, The asynchronous service supports broadcast and multicast packets, and packet exchange is based on a “best-effort” model and packet exchange is based on a “best-effort” model

» no delay bounds can be given for transmissionno delay bounds can be given for transmission» cannot guarantee a maximum access delay or minimum transmission cannot guarantee a maximum access delay or minimum transmission

bandwidthbandwidth Need for a time-bounded service provision Need for a time-bounded service provision

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IEEE 802.11 (X)IEEE 802.11 (X) Medium Access Control (cnt’d)Medium Access Control (cnt’d)

– Three basic access mechanisms have been defined for IEEE Three basic access mechanisms have been defined for IEEE 802.11802.11

» CSMA/CA (mandatory)CSMA/CA (mandatory)» Optional method avoiding the hidden terminal problemOptional method avoiding the hidden terminal problem» A contention-free polling method for time-bounded serviceA contention-free polling method for time-bounded service

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

– The first two methods are also summarized as The first two methods are also summarized as distributed distributed coordination function (DCF)coordination function (DCF)

– The third method is called pThe third method is called point coordination function oint coordination function ((PCF)PCF) – DCF only offers asynchronous service, while PCF offers both DCF only offers asynchronous service, while PCF offers both

asynchronous and time-bounded service, but needs an access asynchronous and time-bounded service, but needs an access point to control medium access and to avoid contention.point to control medium access and to avoid contention.

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IEEE 802.11 (XI)IEEE 802.11 (XI) Medium Access Control (cnt’d)Medium Access Control (cnt’d)

– The medium can be The medium can be busy or idlebusy or idle (detected by the CCA) (detected by the CCA)– If the medium is busy this can be due to data frames or other control framesIf the medium is busy this can be due to data frames or other control frames– During a During a contention phasecontention phase several nodes try to access the medium several nodes try to access the medium

» Short inter-frame spacing (SIFS)Short inter-frame spacing (SIFS) the shortest waiting time for medium accessthe 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) the longest waiting time used for asynchronous data service within a contention periodthe longest waiting time used for asynchronous data service within a contention period SIFS + two slot timesSIFS + 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)time-bounded service) SIFS + one slot timeSIFS + one slot time

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IEEE 802.11 (XII)IEEE 802.11 (XII)– Medium Access Control (cnt’d)Medium Access Control (cnt’d)

» The mandatory access mechanism of IEEE 802.11 is based The mandatory access mechanism of IEEE 802.11 is based on carrier sense multiple access with collision avoidance on carrier sense multiple access with collision avoidance ((CSMA/CACSMA/CA).).

a random access scheme with a random access scheme with carrier sensecarrier sense (with the help of the Clear (with the help of the Clear Channel Assessment-CCA signal of the physical layer) and Channel Assessment-CCA signal of the physical layer) and collision collision avoidance through random back-offavoidance through random back-off..

» The standard defines also two control frames:The standard defines also two control frames: RTS: Request To SendRTS: Request To Send CTS: Clear To SendCTS: Clear To Send

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IEEE 802.11 (XIII)IEEE 802.11 (XIII)» Broadcast data transfer (DCF)Broadcast data transfer (DCF)

t

medium busy

DIFSDIFS

next frame

contention window(randomized back-offmechanism)

slot timedirect access if medium is free DIFS

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

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

– if another station occupies the medium during the back-off if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness – during time of the station, the back-off timer stops (fairness – during the next phase this node will continue its timer from where it the next phase this node will continue its timer from where it stopped)stopped)

– if the medium is free for the duration of a Distributed Inter-if the medium is free for the duration of a Distributed Inter-Frame Space (DIFS), the station can start sendingFrame Space (DIFS), the station can start sending

27

IEEE 802.11 (XIV)IEEE 802.11 (XIV)

» Unicast data transferUnicast data transfer

DIFS

data

ACK

otherstations

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 receivers acknowledge after waiting for a duration of a

Short Inter-Frame Space (SIFS), if the packet was Short Inter-Frame Space (SIFS), if the packet was received correctlyreceived correctly

28

IEEE 802.11 (XV)IEEE 802.11 (XV)

» Sending unicast packets with RTS/CTS control framesSending unicast packets with RTS/CTS control frames

SIFS

DIFS

data

ACK

totherstations

receiver

senderdata

DIFS

defer access contention

RTS

CTSSIFS

SIFS

NAV (RTS)NAV (CTS)

– 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 (reservation determines amount of time the data packet needs the medium and the ACK related to it). Every node receiving this RTS now medium and the ACK related to it). Every node receiving this RTS now has to set its net allocation vector – it specifies the earliest point at has to set its net allocation vector – it specifies the earliest point at which the node can try to access the medium againwhich the node can try to access the medium again

– sender can now send data at once, acknowledgement via sender can now send data at once, acknowledgement via ACKACK

– acknowledgement via CTS after SIFS by receiver (if ready to acknowledgement via CTS after SIFS by receiver (if ready to receive)receive)

– Other stations store medium reservations distributed via RTS Other stations store medium reservations distributed via RTS and CTSand CTS

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Masters thesis

http://eeweb.poly.edu/dgoodman/fainberg.pdf

30

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CoverageCoverage

Figure 2.13Figure 2.13 A predicted coverage plot for three access points in a modern large lecture hall. (Courtesy of Wireless A predicted coverage plot for three access points in a modern large lecture hall. (Courtesy of Wireless Valley Communications, Inc., ©2000, all rights reserved.)Valley Communications, Inc., ©2000, all rights reserved.)

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IEEE 802.11 – enhancements (I)IEEE 802.11 – enhancements (I) IEEE 802.11bIEEE 802.11b

– 11Mbit/s11Mbit/s

– in 2.4GHz frequency bandin 2.4GHz frequency band

– widely usedwidely used

IEEE 802.11aIEEE 802.11a– offers up to 54 Mbit/soffers up to 54 Mbit/s

– 5 GHz band5 GHz band» Shading is much more severe compared to 2.4 GHz Shading is much more severe compared to 2.4 GHz

» Depending on the SNR, propagation conditions and the distance between Depending on the SNR, propagation conditions and the distance between sender and receiver, data rates may drop fastsender and receiver, data rates may drop fast

33

IEEE 802.11 – enhancements (II)IEEE 802.11 – enhancements (II) IEEE 802.11eIEEE 802.11e

– MAC enhancements for providing some QoSMAC enhancements for providing some QoS» No QoS in the DCF operation modeNo QoS in the DCF operation mode» Some QoS guarantees can be given only via polling using PCFSome QoS guarantees can be given only via polling using PCF» For applications such as audio, video, or media stream, distribution For applications such as audio, video, or media stream, distribution

service classes have to be providedservice classes have to be provided For this reason, MAC layer must be enhancedFor this reason, MAC layer must be enhanced

IEEE 802.11gIEEE 802.11g– offers up to 54 Mbit/soffers up to 54 Mbit/s– 2.4 GHz band2.4 GHz band– Benefits from the better propagation characteristics at 2.4 GHz Benefits from the better propagation characteristics at 2.4 GHz

compared to 5 GHzcompared to 5 GHz» Backward compatible to 802.11bBackward compatible to 802.11b

34

WLAN Roaming (I)WLAN Roaming (I)

Corporate Network

AccessPoint

A

Laptop A(with WLAN card)

Laptop B(with WLAN card)

Laptop C(with WLAN card)

Moving to Access Point B

Computer Computer Computer

WLAN Roaming

AccessPoint

B

Laptop D(with WLAN card)

Laptop E(with WLAN card)

Laptop C(with WLAN card)

Registering to Access Point B

Reassocia

tion

Req

uest

Reassocia

tion

R

esp

on

se

35

WLAN Roaming (II)WLAN Roaming (II) No or bad connection? Then perform:No or bad connection? Then perform: ScanningScanning

– scan the environment, i.e., listen into the medium for scan the environment, i.e., listen into the medium for beacon signalsbeacon signals or or send probessend probes into the medium and wait for an answer into the medium and wait for an answer

Reassociation RequestReassociation Request– station sends a request to one or several AP(s)station sends a request to one or several AP(s)

Reassociation ResponseReassociation Response– success: AP has answered, station can now participatesuccess: AP has answered, station can now participate– failure: continue scanningfailure: continue scanning

AP accepts Reassociation RequestAP accepts Reassociation Request– signal the new station to the distribution systemsignal the new station to the distribution system– the distribution system updates its data base (i.e., location the distribution system updates its data base (i.e., location

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

release resourcesrelease resources

36

WLAN Roaming (III)WLAN Roaming (III)

L2 handoverL2 handover– If handover from one AP to another belonging If handover from one AP to another belonging

to the same subnet, then handover is completed to the same subnet, then handover is completed at L2at L2

L3 handoverL3 handover– If new AP is in another domain, then the If new AP is in another domain, then the

handover must be completed at L3, due to the handover must be completed at L3, due to the assignment of an IP belonging to the new assignment of an IP belonging to the new domain – hence routing to the new IP.domain – hence routing to the new IP.

» Mobile IPMobile IP deals with these issues – more later deals with these issues – more later

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WLAN Security (I)WLAN Security (I)

Not so efficient compared with Ethernet security due to the nature of the medium Not so efficient compared with Ethernet security due to the nature of the medium

& the requirements of the users& the requirements of the users

Security mechanismsSecurity mechanisms

– Service Set Identifiers (Service Set Identifiers (SSIDSSID))

» Used to Used to name the networkname the network and provide initial authentication for each client and provide initial authentication for each client

– Wired Equivalent Privacy (Wired Equivalent Privacy (WEPWEP))

» Data encryption technique using shared keys and a pseudorandom number as an initialization Data encryption technique using shared keys and a pseudorandom number as an initialization

vectorvector

» 64-bit key level encryption BUT several vendors now support 128-bit key level encryption64-bit key level encryption BUT several vendors now support 128-bit key level encryption

– Also a Also a VPNVPN could operate on top of the WLAN providing increased security could operate on top of the WLAN providing increased security

IEEE developing new standardsIEEE developing new standards

– 802.11e (Enhanced Security, QoS)802.11e (Enhanced Security, QoS)

– 802.11i (Advanced Encryption Standard – 802.11i (Advanced Encryption Standard – AESAES))

– Requires physical replacement of Access Points and WLAN CardsRequires physical replacement of Access Points and WLAN Cards

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WLAN Security (II)WLAN Security (II)Late-2001

•IEEE 802.1X ratified

•Mutual Authentication•Key Management•WEP Encryption

•WEP Attacks go public

Q3 2003Mid-2002

• Rapid re-keying •802.1X authentication•WEP Encryption•WEP key tumbling via 802.1X key management

•IEEE 802.11i •802.1X authentication•Enhanced 802.1X key management•AES-based Encryption•Enhanced support infrastructure

Q1 2003

•Wi-Fi Protected Access

•802.1X Authentication•802.1X key Management•TKIP data protection

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OTHER WIRELESS OTHER WIRELESS TECHNOLOGIESTECHNOLOGIES

40

HIPERLAN – HIPERLAN – High Performance LAN (I)High Performance LAN (I)

The European Telecommunications Standards Institute The European Telecommunications Standards Institute (ETSI) standardized (ETSI) standardized HIPERLAN HIPERLAN as a WLAN allowing as a WLAN allowing for node mobility and supporting ad hoc and for node mobility and supporting ad hoc and infrastructure-based topologies.infrastructure-based topologies.

It is a wireless LAN supporting priorities and packet life It is a wireless LAN supporting priorities and packet life time for data transfer at 23.5 Mbittime for data transfer at 23.5 Mbit//s, including forwarding s, including forwarding mechanisms, topology discovery, user data encryption, mechanisms, topology discovery, user data encryption, network identification and power conservation network identification and power conservation mechanisms. mechanisms.

HIPERLANs operate at 5.1 – 5.3 GHz with a range of HIPERLANs operate at 5.1 – 5.3 GHz with a range of 50m in buildings at 1 W transmit power. 50m in buildings at 1 W transmit power.

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HIPERLAN – HIPERLAN – High Performance LAN (II)High Performance LAN (II)

The service offered by a HIPERLAN is compatible The service offered by a HIPERLAN is compatible with the standard MAC services known from IEEE with the standard MAC services known from IEEE 802.x LANs.802.x LANs.

The HIPERLAN Channel Access Control The HIPERLAN Channel Access Control mechanism was specifically designed to provide mechanism was specifically designed to provide channel access with priorities.channel access with priorities.

The CAC contains the access scheme The CAC contains the access scheme EY-NPMAEY-NPMA, , which is unique for HIPERLAN.which is unique for HIPERLAN.

42

HIPERLAN – HIPERLAN – High Performance LAN (III)High Performance LAN (III)

Elimination-yield non-preemptive priority multiple Elimination-yield non-preemptive priority multiple accessaccess ( (EY-NPMAEY-NPMA))– not only a complex acronym, but also the heart of the channel not only a complex acronym, but also the heart of the channel

access providing priorities and different access schemes.access providing priorities and different access schemes.– divides the medium access of different competing nodes into divides the medium access of different competing nodes into

three phases:three phases:» PrioritizationPrioritization: Determine the highest priority of a data packet ready to be : Determine the highest priority of a data packet ready to be

sent on competing nodessent on competing nodes» ContentionContention: Eliminate all but one of the contenders, if more than one : Eliminate all but one of the contenders, if more than one

sender has the highest current priority.sender has the highest current priority.» TransmissionTransmission: Finally, transmit the packet of the remaining node.: Finally, transmit the packet of the remaining node.

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HIPERLAN – HIPERLAN – High Performance LAN (IV)High Performance LAN (IV)

– The contention phase is further subdivided into an The contention phase is further subdivided into an elimination elimination phasephase and a and a yieldyield phase.phase.

– The purpose of the elimination phase is to eliminate as The purpose of the elimination phase is to eliminate as many contending nodes as possible. The result is a many contending nodes as possible. The result is a more or less constant number of remaining nodes, more or less constant number of remaining nodes, almost independent of the initial number of competing almost independent of the initial number of competing nodes.nodes.

– The yield phase completes the work of the elimination The yield phase completes the work of the elimination phase with the goal of only one remaining node. phase with the goal of only one remaining node.

44

HIPERLAN – HIPERLAN – High Performance LAN (IV)High Performance LAN (IV)

– The contention phase is further subdivided into an The contention phase is further subdivided into an elimination elimination phasephase and a and a yieldyield phase.phase.

– The purpose of the elimination phase is to eliminate as The purpose of the elimination phase is to eliminate as many contending nodes as possible. The result is a many contending nodes as possible. The result is a more or less constant number of remaining nodes, more or less constant number of remaining nodes, almost independent of the initial number of competing almost independent of the initial number of competing nodes.nodes.

– The yield phase completes the work of the elimination The yield phase completes the work of the elimination phase with the goal of only one remaining node. phase with the goal of only one remaining node.

45

BLUETOOTH (I)BLUETOOTH (I)

Bluetooth technology aims at so-called Bluetooth technology aims at so-called ad hoc ad hoc piconetspiconets, which are local area networks with a , which are local area networks with a very limited coverage and without the need for an very limited coverage and without the need for an infrastructure. infrastructure.

Needed to connect different small devices in close Needed to connect different small devices in close proximity without expensive wiring or the need for proximity without expensive wiring or the need for a wireless infrastructure.a wireless infrastructure.

Represents a single-chip, Represents a single-chip, low-costlow-cost, radio-based , radio-based wireless network technology.wireless network technology.

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BLUETOOTH (II)BLUETOOTH (II)

Up to now Bluetooth is not a standard like IEEE Up to now Bluetooth is not a standard like IEEE 802.11 or HIPERLAN, but it soon become a de-802.11 or HIPERLAN, but it soon become a de-facto standard – established by the industry and facto standard – established by the industry and promoted by the Bluetooth consortium.promoted by the Bluetooth consortium.

Bluetooth uses the license-free frequency band at Bluetooth uses the license-free frequency band at 2.4GHz allowing for worldwide operation.2.4GHz allowing for worldwide operation.

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BLUETOOTH (III)BLUETOOTH (III)

Physical layer:Physical layer:– A frequency-hopping\time-division duplex scheme is used for A frequency-hopping\time-division duplex scheme is used for

transmission with a fast hopping rate of 1,600 hops per second. transmission with a fast hopping rate of 1,600 hops per second. The time between two hops is called a slot, which is an interval The time between two hops is called a slot, which is an interval of 625of 625μμs, thus each slot uses a different frequency.s, thus each slot uses a different frequency.

– On average, the frequency-hopping sequence ´visits´ each hop On average, the frequency-hopping sequence ´visits´ each hop carrier with an equal probability.carrier with an equal probability.

– All devices using the same hopping sequence with the same All devices using the same hopping sequence with the same phase form a phase form a Bluetooth piconetBluetooth piconet..

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BLUETOOTH (IV)BLUETOOTH (IV)

– With transmitting power of up to 100 mW, Bluetooth With transmitting power of up to 100 mW, Bluetooth devices have a range of up to 10m (or even up to 100m devices have a range of up to 10m (or even up to 100m with special transceivers).with special transceivers).

– Having this power and relying on battery power, a Having this power and relying on battery power, a Bluetooth device cannot be in an active transmit mode Bluetooth device cannot be in an active transmit mode all the time.all the time.

– Bluetooth defines several low-power states for the Bluetooth defines several low-power states for the device.device.

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BLUETOOTH (V)BLUETOOTH (V)

– States of a possible Bluetooth device and possible States of a possible Bluetooth device and possible transitions:transitions:

» StandbyStandby mode: Every device which is currently not mode: Every device which is currently not participating in a piconet (and not switched off)participating in a piconet (and not switched off)

In this mode, a device listens for paging messages.In this mode, a device listens for paging messages.

» Connections can be initiated by any device which becomes Connections can be initiated by any device which becomes the the mastermaster..

This is done by sending This is done by sending pagepage messages if the device already knows messages if the device already knows the address of the receiver, or the address of the receiver, or inquiryinquiry messages followed by a page messages followed by a page message if the receiver’s address is unknown.message if the receiver’s address is unknown.

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BLUETOOTH (VI)BLUETOOTH (VI)

» To save battery power, a Bluetoth device can go To save battery power, a Bluetoth device can go into one of into one of three low power states if no data is ready to be sent:three low power states if no data is ready to be sent:

PARKPARK state: The device has the lowest duty cycle, and thus the state: The device has the lowest duty cycle, and thus the lowest power consumption. The device releases its MAC address, but lowest power consumption. The device releases its MAC address, but remains synchronized with the piconet. The device occasionally remains synchronized with the piconet. The device occasionally listens to the traffic of the master device to resynchronize and check listens to the traffic of the master device to resynchronize and check for broadcast messages.for broadcast messages.

HOLD HOLD state:state: The power consumption of this state is a little higher. The power consumption of this state is a little higher. The device does not release its MAC address and can resume sending The device does not release its MAC address and can resume sending at once after transition out of the HOLD state.at once after transition out of the HOLD state.

SNIFFSNIFF state:state: It has the highest power consumption of the low-power It has the highest power consumption of the low-power states. The device listens to the piconet at a reduced rate. states. The device listens to the piconet at a reduced rate.

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BLUETOOTH (VII)BLUETOOTH (VII)

STANDBY

inquiry page

connectedtransmit

PARK HOLD SNIFF

unconnected

connecting

active

low power

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BLUETOOTH (VIII)BLUETOOTH (VIII)

MAC layer:MAC layer:– Several mechanisms control medium access in a Bluetooth Several mechanisms control medium access in a Bluetooth

system.system.– First of all, one device within a piconet acts as a master, all other First of all, one device within a piconet acts as a master, all other

devices (up to seven) act as slaves.devices (up to seven) act as slaves.– The master determines the hopping sequence as well as the phase The master determines the hopping sequence as well as the phase

of the sequence.of the sequence.– All Bluetooth devices have the same networking capabilities, All Bluetooth devices have the same networking capabilities,

i.e., they can be master or slave. The unit establishing the piconet i.e., they can be master or slave. The unit establishing the piconet automatically becomes the master and controls medium access; automatically becomes the master and controls medium access; all other devices will be slaves.all other devices will be slaves.

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WLAN technologies – summary (I)WLAN technologies – summary (I)

The basic goals of all three LAN types (WLAN, The basic goals of all three LAN types (WLAN, HIPERLAN, BUETOOTH) are the provision of a HIPERLAN, BUETOOTH) are the provision of a much higher flexibility for nodes within a network.much higher flexibility for nodes within a network.

All WLANs suffer from limitations of the air All WLANs suffer from limitations of the air interface and higher complexity compared to their interface and higher complexity compared to their wired counterparts but allow for a new degree of wired counterparts but allow for a new degree of freedom for their users within rooms, buildings freedom for their users within rooms, buildings etc.etc.

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WLAN technologies – summary WLAN technologies – summary (II)(II)

The three technologies differ in some respects.The three technologies differ in some respects. HIPERLAN comprises many interesting features, is much HIPERLAN comprises many interesting features, is much

more powerful than IEEE 802.11, has a higher data rate more powerful than IEEE 802.11, has a higher data rate (23.5 Mbit(23.5 Mbit//s) but it is questionable if it will ever be a s) but it is questionable if it will ever be a commercial success.commercial success.

The 5 GHz band required for HIPERLAN is not available The 5 GHz band required for HIPERLAN is not available worldwide compared to the 2.4 GHz used for IEEE worldwide compared to the 2.4 GHz used for IEEE 802.11.802.11.

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WLAN technologies – summary WLAN technologies – summary (III)(III)

No standardization body has set up any No standardization body has set up any specification regarding Bluetooth.specification regarding Bluetooth.

The primary goal of Bluetooth is not a complex The primary goal of Bluetooth is not a complex standard covering many aspects of wireless standard covering many aspects of wireless networking, but a quick and very cheap solution networking, but a quick and very cheap solution enabling ad hoc personal communication within a enabling ad hoc personal communication within a short range in the license-free 2.4 GHz band. short range in the license-free 2.4 GHz band.