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Transcript of Majors Maobile Computing
IEEE 802.11 WLANs (WiFi)Part II/III – System Overview and MAC Layer
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
Part II – WiFi System Overview
Comparison: Infrastructure vs. Ad-hoc networks
infrastructurenetwork
ad-hoc network
APAP
AP
wired network
AP: Access Point
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
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
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
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)
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
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
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
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
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
Part III – MAC Layer Overview
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
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)
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
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
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
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!
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
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
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
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
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
DFWMAC-Point Coordination Function (PCF)
PIFS
stations‘NAV
wirelessstations
Point Coordinator
D1
U1
SIFS
NAV
SIFSD2
U2
SIFS
SIFS
SuperFramet0
medium busy
t1
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
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.
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
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?)
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
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
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
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
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 ?
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
Digression to IP addressingand L3 vs L2 forwarding
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
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
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 ?
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 ?
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)
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
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
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