Majors Maobile Computing

45
IEEE 802.11 WLANs (WiFi) Part II/III – System Overview and MAC Layer

Transcript of Majors Maobile Computing

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IEEE 802.11 WLANs (WiFi)Part II/III – System Overview and MAC Layer

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Design goals for wireless LANs (WLANs)

Global, seamless operationGlobal, seamless operationLow power for battery use Low power for battery use No special permissions or licenses needed to use the spectrum foNo special permissions or licenses needed to use the spectrum for r building WLAN (vs. Billions of dollars spent for 3G networks building WLAN (vs. Billions of dollars spent for 3G networks spectrum worldwide) spectrum worldwide) Robust transmission technologyRobust transmission technologyEasy to use for everyone, simple management Easy to use for everyone, simple management

e.g. Simplified spontaneous cooperation at meetingse.g. Simplified spontaneous cooperation at meetingsProtection of investment in wired networks, e.g. compatible withProtection of investment in wired networks, e.g. compatible withEthernetEthernetSecurity Security

no one should be able to read my data, no one should be able to read my data, no one should be able to collect user profilesno one should be able to collect user profiles

Safety (low radiation)Safety (low radiation)Transparency concerning applications and higher layer protocolsTransparency concerning applications and higher layer protocols

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Part II – WiFi System Overview

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Comparison: Infrastructure vs. Ad-hoc networks

infrastructurenetwork

ad-hoc network

APAP

AP

wired network

AP: Access Point

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802.11 - Architecture of an Infrastructure network and terminology

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

Access Point (or Base Station)Access Point (or Base Station)station integrated into the wireless station integrated into the wireless LAN and the distribution systemLAN and the distribution system

Basic Service Set (BSS)Basic Service Set (BSS)group of stations using the same group of stations using the same radio frequency (channel)radio frequency (channel)

Distribution SystemDistribution Systeminterconnection network for interconnection network for multiple multiple BSSesBSSes to form one to form one logical network, the sological network, the so--called called

Extended Service Set (ESS) Extended Service Set (ESS) PortalPortal

Bridge/Gateway to other (wired) Bridge/Gateway to other (wired) networksnetworks

Distribution System

Portal

802.x LAN

AccessPoint

802.11 LAN

BSS2

802.11 LAN

BSS1Access

Point

STA1

STA2 STA3

ESS

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802.11 - Architecture of an ad-hoc (infrastructureless) network

Station (STA):Station (STA):terminal with access terminal with access mechanisms to the mechanisms to the wireless mediumwireless mediumIndependentIndependent Basic Basic Service Set (Service Set (IBSSIBSS):):group of stations using group of stations using the same radio frequencythe same radio frequencyNo Access Points, every No Access Points, every STA is equalSTA is equalDirect (singleDirect (single--hop) hop) communication within a communication within a limited range ; i.e. no limited range ; i.e. no multimulti--hop routinghop routing

802.11 LAN

IBSS2

802.11 LAN

IBSS1

STA1

STA4

STA5

STA2

STA3

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WLAN: IEEE 802.11bData rateData rate

1, 2, 5.5, 11 1, 2, 5.5, 11 Mbit/sMbit/s, depending on , depending on SNR SNR User data rate max. approx. 6 User data rate max. approx. 6 Mbit/sMbit/sdirect sequence spread spectrum direct sequence spread spectrum (DSSS) in physical layer(DSSS) in physical layer

Transmission rangeTransmission range300m outdoor, 30m indoor300m outdoor, 30m indoorMax. data rate ~10m indoorMax. data rate ~10m indoor

FrequencyFrequencyFree 2.4 GHz ISM (Free 2.4 GHz ISM (UnlicensedUnlicensed))--frequency bandfrequency band

Security standardsSecurity standardsWEP insecure, WPA, WPA2, WEP insecure, WPA, WPA2,

802.11i, SSID802.11i, SSIDCostCost

Adapter/ Access Point price keep Adapter/ Access Point price keep droppingdropping

AvailabilityAvailabilityMany products, many vendorsMany products, many vendors

Quality of ServiceQuality of ServiceBest effort, no guarantees Best effort, no guarantees (unless polling is used, limited (unless polling is used, limited support in real products)support in real products)

ManageabilityManageabilityLimited (no automated key Limited (no automated key distribution, sym. Encryption)distribution, sym. Encryption)

Special Advantages/DisadvantagesSpecial Advantages/DisadvantagesAdvantage: many installed Advantage: many installed systems, lot of experience, systems, lot of experience, available worldwide, free ISMavailable worldwide, free ISM--band, many vendors, integrated band, many vendors, integrated in laptops, simple systemin laptops, simple systemDisadvantage: heavy Disadvantage: heavy interference on ISMinterference on ISM--band, no band, no service guarantees, slow relative service guarantees, slow relative speed onlyspeed only

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802.11: Channels, association802.11b: 2.4GHz802.11b: 2.4GHz--2.485GHz spectrum divided into 11 channels at 2.485GHz spectrum divided into 11 channels at different frequencies ; different frequencies ;

Actually quite few of the 11 channels are totally nonActually quite few of the 11 channels are totally non--overlappedoverlappedNetwork administrator chooses frequency for each APNetwork administrator chooses frequency for each APInterference possible: channel can be same as that chosen by Interference possible: channel can be same as that chosen by neighboring AP!neighboring AP!

Better to have frequency assignment planningBetter to have frequency assignment planningStation STA (or host): must first Station STA (or host): must first authenticateauthenticate and then and then associateassociate with with an AP before it can start transmitting dataan AP before it can start transmitting dataHow a STA get started ?How a STA get started ?

The STA scans channels, listening for The STA scans channels, listening for beacon framesbeacon frames containing containing the the ESSESS’’ss name (SSID) and the MAC address of the AP (BSSID)name (SSID) and the MAC address of the AP (BSSID)selects AP to associate withselects AP to associate withmay perform authentication before associationmay perform authentication before associationAfter association is done, STA will typically run DHCP to get IPAfter association is done, STA will typically run DHCP to get IPaddress in APaddress in AP’’s IP subnet (if STA does not have an IP s IP subnet (if STA does not have an IP addraddralready, or if it has moved to a new IP subnet)already, or if it has moved to a new IP subnet)

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Channel selection for 802.11b (non-overlapping)Europe (ETSI)

channel 1 channel 7 channel 13

2400 2412 2442 2472 2483.5[MHz]22 MHz

US (FCC)/Canada (IC)

channel 1 channel 6 channel 11

2400 2412 2437 2462 2483.5[MHz]22 MHz

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Other IEEE 802.11 Wireless LAN Standards802.11a802.11a

55--6 GHz range6 GHz rangeup to 54 Mbpsup to 54 MbpsDisadvantages: Disadvantages:

stronger shading due to higher frequencystronger shading due to higher frequencyUsing different frequency ranges than 802.11b => extra hardware Using different frequency ranges than 802.11b => extra hardware to support dual mode of 802.11a or 802.11bto support dual mode of 802.11a or 802.11b

=> Bad for backward => Bad for backward compatiblilitycompatiblility802.11g802.11g

2.42.4--5 GHz range5 GHz rangeBackward compatible with 802.11bBackward compatible with 802.11b

up to 54 Mbpsup to 54 Mbps802.11a and 802.11g use OFDM in physical layer802.11a and 802.11g use OFDM in physical layerAll use CSMA/CA for multiple accessAll use CSMA/CA for multiple accessAll support AP (baseAll support AP (base--station), (infrastructure) and adstation), (infrastructure) and ad--hoc network modeshoc network modes

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Operating channels for 802.11a / US

5150

channel40 48 52 56 60 6436 44

5180 5200 5220 5240 5260 5280 5300 5320 5350 [MHz]16.6 MHz

center frequency = 5000 + 5*channel number [MHz]

149 153 157 161

5725

channel

5745 5765 5785 5805 5825 [MHz]16.6 MHz

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IEEE standard 802.11

fixedterminal

Router

mobile terminal

Application

TCP

802.11 PHY

802.11 MAC

IP

802.3 MAC

802.3 PHY

802.11 MAC

802.11 PHY

LLCLLC

802.3 PHY

802.3 MAC

LLC

Application

TCP

802.3 PHY

802.3 MAC

IP

LLC

802.3 PHY

802.3 MAC

IP

LLC

L2 forwarding (aka bridging)

L3 forwarding (aka routing)

Access Point

Most consumer-focused home wireless routers merge the functions of the AP and Router

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IEEE 802.11 Operations

Question 1) In the protocol stack of the Question 1) In the protocol stack of the WiWi--FiFi client, there are two layers client, there are two layers whiwhichch has packet retransmission function (MAC layer and TCP layer). has packet retransmission function (MAC layer and TCP layer). Why we need two layers at the Why we need two layers at the WiWi--FiFi client doing the same reclient doing the same re--transmission function? transmission function?

Question 2) What should be the source address of the Ethernet pQuestion 2) What should be the source address of the Ethernet packet acket from AP to router?from AP to router?

InternetrouterAP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3

802.11 frame

R1 MAC addr AP MAC addr dest. address source address

802.3 frame

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Part III – MAC Layer Overview

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IEEE 802.11 Multiple Access Control (MAC)Avoid collisions: 2Avoid collisions: 2++ nodes nodes transmitting at same timetransmitting at same time802.11: CSMA 802.11: CSMA -- sense before transmittingsense before transmitting

dondon’’t collide with ongoing transmission by other nodet collide with ongoing transmission by other node802.11: 802.11: nono collision detection!collision detection!

difficult to receive (sense collisions) and transmitting at the difficult to receive (sense collisions) and transmitting at the same time same time because (TDD operation)because (TDD operation)

transmission and reception are at the same frequencytransmission and reception are at the same frequency=> the station=> the station’’s receivers receiver’’s will be overwhelming by its will be overwhelming by it’’s own transmitting s own transmitting

signals and thus cansignals and thus can’’t hear the relatively t hear the relatively ““weakweak”” signals sent by the signals sent by the othersothers

In general canIn general can’’t sense all collisions anyway, e.g. t sense all collisions anyway, e.g. The Hidden terminal The Hidden terminal problemproblem

=> Design Goal: => Design Goal: avoid collisions:avoid collisions: CSMA/CSMA/C(ollision)A(voidanceC(ollision)A(voidance))

AB

CA B C

A’s signalstrength

space

C’s signalstrength

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802.11 MAC CSMA/CA access method

medium busy

DIFSDIFS

next frame

contention window(randomized back-offmechanism)

tdirect access if medium is free ≥ DIFS slot time

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 free for the duration of an Interif the medium is free for the duration of an Inter--Frame Frame Space (IFS), the station can start sending (IFS depends on Space (IFS), the station can start sending (IFS depends on service type)service type)if the medium is busy, the station has to wait for a free IFS, if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random backthen the station must additionally wait a random back--off off time (collision avoidance, multiple of slottime (collision avoidance, multiple of slot--time) time) if another station occupies the medium during the backif another station occupies the medium during the back--off off time of the station, the backtime of the station, the back--off timer stops off timer stops (fairness)(fairness)

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802.11 MAC CSMA/CA access method (cont’d)

Sending Sending unicastunicast packetspacketsstation has to wait for DIFS before sending datastation has to wait for DIFS before sending datareceivers acknowledge at once (after waiting for SIFS) if the pareceivers acknowledge at once (after waiting for SIFS) if the packet cket was received correctly (CRC)was received correctly (CRC)automatic retransmission of data packets in case of transmissionautomatic retransmission of data packets in case of transmissionerrorserrors

t

SIFS

DIFS

data

ACK

data

DIFS

sender

receiver

otherstations

waiting time contention

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802.11 - competing stations - simple version

busy

DIFSboe

boe

boe

busy

bor

bor

DIFS

boe

boe

boe bor

DIFS

busy

busy

DIFSboe busy

boe

boe

bor

bor

station1

station2

station3

station4

station5 t

busy boemedium not idle (frame, ack etc.) elapsed backoff time

borpacket arrival at MAC residual backoff time

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802.11 - MAC layer (cont’d)PrioritiesPriorities

defined through different inter frame spaces (IFS)defined through different inter frame spaces (IFS)no guaranteed, hard prioritiesno guaranteed, hard prioritiesSIFS (Short Inter Frame Spacing)SIFS (Short Inter Frame Spacing)

highest priority, for ACK, CTS, polling responsehighest priority, for ACK, CTS, polling responsePIFS (PCF IFS)PIFS (PCF IFS)

medium priority, for timemedium priority, for time--bounded service using PCFbounded service using PCFDIFS (DCF, Distributed Coordination Function IFS)DIFS (DCF, Distributed Coordination Function IFS)

lowest priority, for asynchronous data servicelowest priority, for asynchronous data service

medium busy SIFSPIFSDIFSDIFS

next framecontention

tdirect access if medium is free ≥ DIFS

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To Further Avoiding collisionsIdea:Idea: allow sender to allow sender to ““reservereserve”” channel rather than random access of channel rather than random access of

data frames: avoid collisions of data frames: avoid collisions of longlong data framesdata framessender first transmits sender first transmits smallsmall requestrequest--toto--send (RTS) packets to BS send (RTS) packets to BS using CSMAusing CSMA

RTSsRTSs may still collide with each other (but theymay still collide with each other (but they’’re short)re short)BS broadcasts clearBS broadcasts clear--toto--send CTS in response to RTSsend CTS in response to RTSRTS heard by all nodesRTS heard by all nodes

sender transmits data framesender transmits data frame

other stations defer transmissionsother stations defer transmissions

Avoid data frame collisions completely using small reservation packets!

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Collision Avoidance: RTS-CTS exchange

APA B

time

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

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More Details on 802.11 MAC, akaDFWMAC (Distributed Foundation Wireless MAC)

Traffic servicesTraffic servicesAsynchronous Data Service Asynchronous Data Service (mandatory)(mandatory)

exchange of data packets based on exchange of data packets based on ““bestbest--efforteffort””support of broadcast and multicastsupport of broadcast and multicast

TimeTime--Bounded Service (Bounded Service (optional, seldom implemented by real products)optional, seldom implemented by real products)implemented using PCF (Point Coordination Function) implemented using PCF (Point Coordination Function)

Access methodsAccess methodsDFWMACDFWMAC--DCF CSMA/CA (DCF CSMA/CA (mandatorymandatory))

Basic collision avoidance via randomized Basic collision avoidance via randomized ““backback--offoff”” mechanismmechanismminimum distance between consecutive packetsminimum distance between consecutive packetsACK packet for acknowledgements (ACK packet for acknowledgements (NO ACK for broad/multicastsNO ACK for broad/multicasts))

DFWMACDFWMAC--DCF with RTS/CTS (DCF with RTS/CTS (optionaloptional))Distributed Foundation Wireless MACDistributed Foundation Wireless MACavoids hidden terminal problemavoids hidden terminal problem

DFWMACDFWMAC-- PCF (PCF (optional, seldom implemented by productsoptional, seldom implemented by products))access point polls terminals according to a listaccess point polls terminals according to a list

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802.11 – DFWMAC RTS/CTSSending Sending unicastunicast packetspackets

station can send RTS with reservation parameter after waiting fostation can send RTS with reservation parameter after waiting for DIFS r DIFS (reservation determines amount of time the data packet needs the(reservation determines amount of time the data packet needs themedium) medium) acknowledgement via CTS after SIFS by receiver (if ready to receacknowledgement via CTS after SIFS by receiver (if ready to receive)ive)sender can now send data after SIFS, acknowledgement via ACKsender can now send data after SIFS, acknowledgement via ACKother stations store medium reservations distributed via RTS andother stations store medium reservations distributed via RTS and CTS CTS

t

SIFS

DIFS

data

ACK

data

DIFS

RTS

CTSSIFS

NAV (RTS)NAV (CTS)

sender

SIFSreceiver

otherstations

defer access contention

NAV = Network Allocation Vector ; to enable “Virtual Carrier Sensing”Virtual Carrier Sensing: the sender tells others how long it will be using the channel by setting the value of NAV => others don’t need to keep on sensing till then

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SIFS

DIFS

data

ACK1

otherstations

receiver

frag1

DIFS

contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

NAV (frag1)NAV (ACK1)

SIFSACK2

frag2

SIFS

Fragmentation: to send a Big data frame in multiple pieces

sender

t

Why Fragmentation ?•May due to L2 limit on maximum frame-size of data sent each time ; •The bigger a frame, the more vulnerable it is in getting a bit transmission error ; A single error-bit can ruin the whole frame and require re-transmission of the whole frame => better off to send/retransmit small fragments instead

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SIFS

DIFS

data

ACK1

Other 802.11bstations

802.11g receiver

802.11 b/g AP Burst 1

DIFS

CTS

SIFS

NAV (CTS)

NAV (burst 1)NAV (ACK1)

SIFS ACK2

Burst 2

SIFS

Burst Mode for mixing .11g/.11g clientsFor backward compatibility, the 802.11g packets have a mode thatFor backward compatibility, the 802.11g packets have a mode that support support 802.11b preamble. This is needed to allow other 802.11b clients 802.11b preamble. This is needed to allow other 802.11b clients to carrier sense to carrier sense channel busy while 802.11g clients are transmitting. channel busy while 802.11g clients are transmitting. However, this makes the actual throughput of 802.11g clients preHowever, this makes the actual throughput of 802.11g clients pretty low (despite tty low (despite 54Mbps physical transmission rate for payload) bec54Mbps physical transmission rate for payload) becauause of huge overhead se of huge overhead involved in the 802.11b preamble (56us / 128us).involved in the 802.11b preamble (56us / 128us).To allow more efficient mixing of 802.11b/g clients, a To allow more efficient mixing of 802.11b/g clients, a b/gb/g compatible AP can use compatible AP can use the RTS/CTS for bursting traffic to g clients. the RTS/CTS for bursting traffic to g clients.

b/gb/g AP sends CTS with NAV AP sends CTS with NAV trying to reserve the media for a specified trying to reserve the media for a specified period of time. period of time. ––> all devices (> all devices (b/gb/g) observe the NAV and remains silence.) observe the NAV and remains silence.b/gb/g AP then sends the 11g packets (with gAP then sends the 11g packets (with g--preamble) to the intended g preamble) to the intended g clients one by one. clients one by one. the gthe g--packet transmission has less overhead due to packet transmission has less overhead due to much shorter preamble. much shorter preamble.

t

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DFWMAC-Point Coordination Function (PCF)

PIFS

stations‘NAV

wirelessstations

Point Coordinator

D1

U1

SIFS

NAV

SIFSD2

U2

SIFS

SIFS

SuperFramet0

medium busy

t1

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DFWMAC-PCF (cont‘d)

tstations‘NAV

wirelessstations

point coordinator

D3

NAV

PIFSD4

U4

SIFS

SIFSCFend

contentionperiod

contention free period

t2 t3 t4

Note: Since SIFS < PIFS, if User 3 has something to send, it would have done so before the Point coordinator’s waiting period of PIFS is over ; not hearing from user 3 by then means the Point coordinator can safely proceed to User 4

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Synchronization using a Beacon (infrastructure)

beacon interval

busy

B

busy busy busy

B B BAccessPoint

mediumt

B beacon framevalue of the timestamp

•The AP broadcasts a “Beacon” message periodically which contains the reference time-stamp value ; •Each station adjusts/tunes its local clock using the value of the time-stampin the Beacon as the reference.

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Synchronization using a Beacon in an Ad-hoc Network (i.e. No AP)

beacon interval

tmedium busy

B1

busy busy busy

B1

value of the timestamp B beacon frame

B2 B2

random delayFor each station:(1) Set a random delay timer ; (2) If a Beacon from another station is received before timer expires

Adjust local clock using time-stamp in the received Beacon as reference

else (i.e. no Beacon from others heard before timer expires)Upon timer expires, broadcast a Beacon using local clock to

generate the time-stamp ; (to be used by all other stations as time reference)

station1

station2

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Power management

Idea: switch the transceiver off if not neededIdea: switch the transceiver off if not neededStates of a station: sleep and awakeStates of a station: sleep and awakeTiming Synchronization Function (TSF) (see previous 2 pages)Timing Synchronization Function (TSF) (see previous 2 pages)

stations wake up at the same timestations wake up at the same timeInfrastructureInfrastructure

Traffic Indication Map Traffic Indication Map (TIM)(TIM)list of list of unicastunicast receivers transmitted by APreceivers transmitted by APWhen When unicastunicast receiver wakes up and hear it has incoming frames receiver wakes up and hear it has incoming frames buffered by the AP, it sends PSbuffered by the AP, it sends PS--poll to AP to ask for deliverypoll to AP to ask for delivery

Delivery Traffic Indication Map Delivery Traffic Indication Map (DTIM)(DTIM)list of broadcast/multicast receivers transmitted by APlist of broadcast/multicast receivers transmitted by APUnicastUnicast Polling mechanism does not work for Multicast/Broadcast Polling mechanism does not work for Multicast/Broadcast from AP ; instead, itfrom AP ; instead, it’’s each s each STASTA’’ss responsibility to wait up every responsibility to wait up every ““DTIMDTIM--intervalinterval”” to check for Multicast/Broadcast from APto check for Multicast/Broadcast from AP

AdAd--hochocAdAd--hoc Traffic Indication Map (ATIM)hoc Traffic Indication Map (ATIM)

announcement of receivers by stations buffering framesannouncement of receivers by stations buffering framesmore complicated more complicated -- no central APno central APcollision of collision of ATIMsATIMs possible (scalability?)possible (scalability?)

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Power saving with wake-up patterns (infrastructure)

TIM interval

t

busy

D

busy busy busy

T T D BB

p

d

d

DTIM interval

accesspoint

medium

station

T TIM D DTIM awake

d data transmissionto/from the station

B broadcast/multicast

p Power Saving (PS) poll

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Power saving with wake-up patterns (ad-hoc)

B1

ATIMwindow

awake

A transmit ATIM D transmit datat

B1

B2 B2

A

a

D

d

beacon interval

station1

station2

B beacon frame random delay

a dacknowledge ATIM acknowledge data

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802.11 - Frame format

TypesTypescontrol frames (e.g. RTS, CTS, ACK, PScontrol frames (e.g. RTS, CTS, ACK, PS--poll), management poll), management frames (ATIM, association, authentication, etc), data framesframes (ATIM, association, authentication, etc), data frames

Sequence numbersSequence numbersimportant against duplicated frames due to lost important against duplicated frames due to lost ACKsACKs

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

MiscellaneousMiscellaneoussending time, checksum, frame control, datasending time, checksum, frame control, data

FrameControl Duration Address

1Address

2Address

3Sequence

ControlAddress

4 Data CRC

2 2 6 6 6 62 40-2312bytes

Protocolversion Type Subtype To

DSMoreFrag Retry Power

MgmtMoreData WEP

2 2 4 1FromDS

1

Order

bits 1 1 1 1 1 1

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AcknowledgementAcknowledgement

Request To SendRequest To Send

Clear To SendClear To Send

FrameControl Duration Receiver

AddressTransmitter

Address CRC

2 2 6 6 4bytes

FrameControl Duration Receiver

Address CRC

2 2 6 4bytes

FrameControl

Duration(for

settingNAV)

ReceiverAddress CRC

2 2 6 4bytes

ACK

RTS

CTS

Special Frames: ACK, RTS, CTS

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Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3

802.11 frame

R1 MAC addr AP MAC addr dest. address source address

802.3 frame

802.11 frame: addressing

Why can’t 802.11 WLAN just use 2 addresses per frame like wired Ethernet ?

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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 (= MAC address of the AP)RA: Receiver AddressTA: Transmitter Address

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Digression to IP addressingand L3 vs L2 forwarding

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Review on Naming and Addressing and Translation Mechanisms

Given theGiven the DomainDomain--name name of the destination host,of the destination host, e.g. e.g. www.ee.ust.hkwww.ee.ust.hk, , the endthe end--host ask an Domain Name System (DNS) server to translate it host ask an Domain Name System (DNS) server to translate it toto an IP address, e.g. 128.83.50.156an IP address, e.g. 128.83.50.156This This IP address IP address is put into the destination IP address field of the is put into the destination IP address field of the packets sent to the destination hostpackets sent to the destination host ; this destination IP address is used ; this destination IP address is used for routing table lookup along the pathfor routing table lookup along the pathWhen the packet arrives at the When the packet arrives at the ““destination networkdestination network”” (the destination IP (the destination IP subnet), e.g. the LAN segment connected to the destination host,subnet), e.g. the LAN segment connected to the destination host, the the lastlast--hop router use the hop router use the Address Request Protocol (ARP) to translate Address Request Protocol (ARP) to translate thethe destination IP address destination IP address to theto the MAC address MAC address of the of the ethernetethernet network network interface card (NIC) on the destination hostinterface card (NIC) on the destination hostThis MAC address is used as the destination address to deliver tThis MAC address is used as the destination address to deliver the he Ethernet frame to the destination host Ethernet frame to the destination host via L2 forwardingvia L2 forwarding

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IEEE standard 802.11

fixedterminal

Router

mobile terminal

Application

TCP

802.11 PHY

802.11 MAC

IP

802.3 MAC

802.3 PHY

802.11 MAC

802.11 PHY

LLCLLC

802.3 PHY

802.3 MAC

LLC

Application

TCP

802.3 PHY

802.3 MAC

IP

LLC

802.3 PHY

802.3 MAC

IP

LLC

L2 forwarding (aka bridging)

L3 forwarding (aka routing)

Access Point

Most consumer-focused home wireless routers merge the functions of the AP and Router

Page 40: Majors Maobile Computing

802.11 frame: addressing

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3

802.11 frame

R1 MAC addr AP MAC addr dest. address source address

802.3 frame

Should be H1’sMAC addr.

Why can’t 802.11 WLAN just use 2 addresses per frame like wired Ethernet ?

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802.11: mobility within same subnet

L2 forwarder(bridge), e.g. an Ethernet Switch

AP 2

AP 1

H1 BBS 2

BBS 1

router

Same IPsubnet

H1 remains in same IP H1 remains in same IP subnet: IP address can subnet: IP address can remain sameremain sameSwitch: which AP is Switch: which AP is associated with H1?associated with H1?

selfself--learning: switch will learning: switch will see frame from H1 and see frame from H1 and ““rememberremember”” which which switch port can be used switch port can be used to reach H1to reach H1But what if there are But what if there are more than one switch more than one switch in the IP subnet ? in the IP subnet ?

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802.11 - RoamingWhen STA experienced No or bad connection (e.g. poor SNR) perfoWhen STA experienced No or bad connection (e.g. poor SNR) perform:rm:

ScanningScanningscan the environment,scan the environment,

Either passively listen into the medium for beacon signals or Either passively listen into the medium for beacon signals or Actively send probes into the medium and wait for an AP to Actively send probes into the medium and wait for an AP to answeranswer

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

ReassociationReassociation ResponseResponsesuccess: AP has answered, station can now participatesuccess: AP has answered, station can now participatefailure: continue scanningfailure: continue scanning

AP accepts AP accepts ReassociationReassociation RequestRequestsignal the new station to the distribution systemsignal the new station to the distribution systemthe 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 typically, the distribution system now informs the old AP so it can can release resources using yetrelease resources using yet--toto--be standardized Inter Access Point be standardized Inter Access Point Protocol (IAPP)Protocol (IAPP)

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802.11 - MAC management Task Summary

SynchronizationSynchronizationtry to find a LAN, try to stay within a LANtry to find a LAN, try to stay within a LANtimer etc.timer etc.

Power managementPower managementsleepsleep--mode without missing a messagemode without missing a messageperiodic sleep, frame buffering, traffic measurementsperiodic sleep, frame buffering, traffic measurements

Association/Association/ReassociationReassociationSTA becomes a member of a WLAN/LANSTA becomes a member of a WLAN/LANRoaming, i.e. change networks by changing access points Roaming, i.e. change networks by changing access points Scanning, i.e. active search for a networkScanning, i.e. active search for a network

Maintenance operations for AP, e.g.Maintenance operations for AP, e.g.System parameters, e.g. Channel #, configuration ;System parameters, e.g. Channel #, configuration ;AccountingAccountingActivity loggingActivity logging

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Some current IEEE 802.11 Standardization activities

802.11e: MAC Enhancements 802.11e: MAC Enhancements –– QoSQoS –– almost donealmost doneEnhance the current 802.11 MAC to expand support for applicationEnhance the current 802.11 MAC to expand support for applications with s with Quality of Service requirements, and in the capabilities and effQuality of Service requirements, and in the capabilities and efficiency of iciency of the protocol. the protocol.

802.11f: Inter802.11f: Inter--Access Point Protocol (IAPP)Access Point Protocol (IAPP)–– experimental best current experimental best current practice (not a binding standard) is available practice (not a binding standard) is available

Establish an InterEstablish an Inter--Access Point Protocol for data exchange via the Access Point Protocol for data exchange via the distribution system.distribution system.

802.11i: Enhanced Security Mechanisms 802.11i: Enhanced Security Mechanisms –– completed recently completed recently Enhance the current 802.11 MAC to provide improvements in securiEnhance the current 802.11 MAC to provide improvements in security. ty.

802.11n: High Throughput (100Mbps+) 802.11n: High Throughput (100Mbps+) –– ongoingongoing802.11r: Fast (Seamless) Handover support 802.11r: Fast (Seamless) Handover support -- ongoingongoing802.11u: 802.11u: InterworkingInterworking between 802.11 and nonbetween 802.11 and non--802.11 Wireless networks802.11 Wireless networks802.11s: Mesh Networking 802.11s: Mesh Networking –– ongoingongoing

Using a selfUsing a self--configurable wireless infrastructure within an ESSconfigurable wireless infrastructure within an ESS

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IEEE 802.11s – Mesh Network of APs

Distribution SystemDistribution System(DS)(DS)

802.11 ESS Mesh802.11 ESS Mesh

Mesh AP LinksMesh AP Links802.11 MAC/PHY802.11 MAC/PHY

(4(4--addr data frames)addr data frames)

ClientClient--toto--AP LinksAP Links802.11 MAC/PHY802.11 MAC/PHY

(3(3--addr data frames)addr data frames)

802.11 BSS802.11 BSS

802.11 ESS802.11 ESS