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Giuseppe Bianchi
1. Introduction to 1. Introduction to 802.11 Wireless LANs802.11 Wireless LANs
Giuseppe Bianchi
WLAN HistoryWLAN History? Early Wireless LAN proprietary products? WaveLAN (AT&T)
? the ancestor of 802.11? HomeRF (Proxim)
? Support for Voice, unlike 802.11? 45% of the home network in 2000; 30% in 2001, … ?% today? Abandoned by major chip makers (e.g. Intel: dismissed in april 2001)
? IEEE 802.11 Committee formed in 1990? Charter: specification of MAC and PHY for WLAN
? First standard: june 1997? 1 and 2 Mbps operation
? Reference standard: september 1999? Multiple Physical Layers
? 2.4GHz Industrial, Scientific & Medical shared unlicensed band » Legacy; 802.11b/g
? 5 GHz ISM (802.11a)? 1999: Wireless Ethernet Compatibility Alliance (WECA)
certification? Later on named Wi-Fi? Boosted 802.11 deployment!!
Giuseppe Bianchi
Why so much talking about of Why so much talking about of 802.11 today?802.11 today?
? 802.11: no more “just” a WLAN? Hot-spots?Where the user goes, the network is available: home, school, office,
hotel, university, airport, convention center…? Freedom to roam with seamless connectivity in every domain, with single
client device? May compete (complement) with 3G for Wireless
Internet access
Which of these two is the proper (closer) picture
of Wireless Internet andMobile Computing?
Which technology is most suited?
Giuseppe Bianchi
Some citationsSome citations
? “we look at [802.11 technology] as becoming a requirement. Some carriers see it as a threat while other says, ‘it could have something for me’”?Bruce Dale, vice president of UMTS/W-CDMA products at
Lucent (July 2002):
?Qualcomm: so sure of the coexistence of these technologies that it is intending to include 802.11b capability in its CDMA chipset during late 2003.
Giuseppe Bianchi
The global pictureThe global pictureWide Area
Local Area
Personal AreaLAN:
collection of secure “hot spot”
connections, providing broadband access to
the Internet
PAN:collection of secure
connections between devices in a
“very” local area
BT/802.11switching
WAN:everywhere outside of
the hotspots, where wireless Internet connection are
provided802.11/UMTS
switching
Bluetooth< 800 Kb/s ? 10 m
Mobile Broadband InternetIEEE 802.11 (b)
> 10 Mb/s ? 100 m
GPRS, 3G – UMTS< 400 Kb/s ? Kms
Giuseppe Bianchi
WLAN Market WLAN Market -- ProductsProducts
? year 2002: overall spending for Wi-Fi (802.11b) products has increased of 38% up to 2.3 billion dollars
?[source: Gartner Dataquest]? (15.4 million Wi-Fi cards, and 4.4 million Access Points have been sold).
? Projections: 5 billion dollars by 2006 [source: Gartner]
? end of 2003, 50% of the medium-large companies are expected to own a WLAN infrastructure
?(source: Gartner).
Giuseppe Bianchi
WLAN Enterprise Market SharesWLAN Enterprise Market SharesVendor
NIC Units (k)2001
Share2001 Vendor
AP Units (k)
Share2001
Agere 392 14,9% Cisco 63,2 12,3%Cisco 272 10,3% Linksys 61,8 12,1%Avaya 199 7,5% Agere 36,3 7,1%Buffalo Technology (Melco) 124 4,7% Buffalo Technology (Melco) 32,4 6,3%Enterasys 106 4,0% D-Link 28,0 5,5%3Com 95 3,6% Apple 25,2 4,9%Intermec 90 3,4% Symbol 23,2 4,5%Proxim 89 3,4% SOHOware 20,4 4,0%Linksys 83 3,1% Enterasys 20,3 4,0%Compaq 80 3,0% 3Com 19,5 3,8%Apple 75 2,8% Avaya 13,5 2,6%Dell 73 2,8% Compaq 13,0 2,5%Intel 55 2,1% Proxim 10,7 2,1%Siemens 48 1,8% Intel 10,4 2,0%Symbol 46 1,7% Intermec 10,4 2,0%SOHOware 46 1,7% IBM 9,6 1,9%IBM 46 1,7% SMC 9,4 1,8%Nokia 44 1,7% Toshiba 9,0 1,8%HP 41 1,6% Nokia 7,9 1,5%SMC 36 1,4% Netgear 7,3 1,4%Toshiba 34 1,3% HP 7,2 1,4%D-Link 32 1,2% Dell 7,0 1,4%Other 506 20,1% Fujitsu 6,2 1,2%
Nortel 5,0 1,0%Other 55,0 10,8%
Total 2.637 100% Total 511,8 100%Source: Cahners In-Stat Group Sep 2001
Giuseppe Bianchi
WLAN Market WLAN Market -- HotSpotsHotSpots U.S. Commercial Hotspots
2001-2002: exceeded expectations by 14%
1.500
2.000
2.500
3.000
3.500
4.000
4.500
2001 2002
Forecasted Actual
0
10000
20000
30000
40000
50000
2002 2004 2006
U.S. Hotspots growth (2002-2006)
Unique U.S. Hotspots
2003: 125.000 regular hotspot US usersEnd 2006: 9 Million regular hotspot US usersEnd 2006: 1 Billion dollars revenue predicted from HotSpot operation
Giuseppe Bianchi
PHYPHY--related task groupsrelated task groups? 802.11a: PHY for 5 GHz
?published in 1999?Products available since early 2002
? 802.11b: higher rate PHY for 2.4 GHz ?Published in 1999?Products available since 1999?Interoperability tested (wifi)
? 802.11g: OFDM for 2.4 GHz?Published in june 2003?Products available, though no extensive interoperability testing yet
? 802.11n: ??? (Higher data rate)?Launched in september 2003?Minimum goal: 108 Mbps (but higher numbers considered)?Support for space division multiple access and smart antennas?
Giuseppe Bianchi
PHY rates at a glance PHY rates at a glance
1, 2, 5.5, 11; 6, 9, 12, 18, 24, 36, 48, 54
2.4 GHzDSSS, HR-DSSS, OFDM
802.11g
6, 9, 12, 18, 24, 36, 48, 54
5.2, 5.5 GHzOFDM802.11a
1, 2, 5.5, 11,22, 33, 44
2.4 GHzDSSS, HR-DSSS, (PBCC)
"802.11b+" non-standard
1, 2, 5.5, 112.4 GHzDSSS, HR-DSSS802.11b
1, 22.4 GHz, IRFHSS, DSSS, IR802.11 legacy
Data Rates Mbps Frequency Band Transfer Method Standard
Giuseppe Bianchi
PHY rates at a glancePHY rates at a glance
Giuseppe Bianchi
802.11a Data rates802.11a Data rates
Giuseppe Bianchi
PHY distance/rate tradeoffsPHY distance/rate tradeoffs(open office)(open office)
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
Dis
tanc
e (m
)
54Mbps36Mbps24Mbps12Mbps6Mbps 11Mbps5.5Mbps1Mbps
2.4 GHz OFDM (.11g)
5 GHz OFDM (.11a)
2.4 GHz (.11b)
Giuseppe Bianchi
Coverage performance Coverage performance Cisco Cisco AironetAironet 350 Access Point350 Access Point
11 Mb/s DSSda ~30 a ~45 metri
5.5 Mb/s DSSda ~45 a ~76 metri
2 Mb/s DSSda ~76 a ~107 metri
Configurable TX power:50, 30, 20, 5, 1 mW(100 mW outside Europe)
Greater TX power, faster battery consumptions!
Giuseppe Bianchi
The 2.4 (wifi) GHzThe 2.4 (wifi) GHz spectrumspectrum
?2.4 GHz bandwidth: 2.400-2.483,5?Europe (including Italy):?13 channels (each 5 MHz)?- 2.412 – 2.417 –
- 2.422 – 2.427 –…… – 2.472
?Channel spectrum:?Non Overlapping Channels
2412 2437 2462
Giuseppe Bianchi
The 5.0 (OFDM) GHzThe 5.0 (OFDM) GHz spectrumspectrum
? US regulation:? 3 x 100 MHz channels?5.150-5.250 (40 mW)?5.250-5-350 (200 mW)?5.725-5.825 (800 mW)
? Operating 20 MHz channels?5.180, 5.200, 5.220, 5.240?5.260, 5.280, 5.300, 5.320?5.745, 5.765, 5.785, 5.805
20 MHz / 64 subcarriers = 0.3125 MHz per subcarriers (52 used)
Giuseppe Bianchi
Task groups e/f/iTask groups e/f/i
? 802.11e: QoS extensions (packet prioritization)? Currently in draft version 7.0; final version: end-year?? Technical details later
? 802.11f: Interoperation between APs? Working Practice Standard: june 2003? Inter-Access Point Protocol for roaming between APs of different vendors
? 802.11i: improved security mechanisms? Wi-Fi Protected Access? Strongly pushed by the Wi-Fi Alliance? 802.11i components:
? Advanced Encryption Standard algorithm? Extensible Authentication Protocol authentication schemes? Temporal Key Integrity Protocol
Giuseppe Bianchi
SpectrumSpectrum--related task groupsrelated task groups
? 802.11d: global harmonization working group? not really a standard? Different countries have different usable parts of the 2.4- and 5-GHz bands; ? Goal: create standards that satisfy different regulations, and will be approvable in as many
different countries as possible.
? 802.11h: adds features to 802.11a 5GHz ? To deal with Hiperlan-a ETSI European standard? To avoid interference with radar and satellite european services? Features:
? Power management? Dynamic channel/frequency selection
? 802.11j: equivalent to 802.11h, but for Japan available spectrum? 4.9- to 5.0-GHz
Giuseppe Bianchi
LessLess--known task groupsknown task groups
? 802.11c: how to implement media-access-controlbridging ? completed in 1997? useful only to people who design wireless LAN hardware
? 802.11k: measurement report? Recently activated? Goal: standardize the way 802.11a, b, and g networks report
measurements of radio and network conditions to other parts of the network stack and new applications.
? 802.11m: collection of maintenance releases for the whole 802.11 WG? used for internal IEEE housekeeping ?
Giuseppe Bianchi
2. Wireless LAN Networks 2. Wireless LAN Networks and related Addressingand related Addressing
Giuseppe Bianchi
Basic Service Set (BSS)Basic Service Set (BSS)group of stationsgroup of stations that can communicatethat can communicate withwith eacheach otherother
? Infrastructure BSS?or, simply, BSS?Stations connected
through AP
? Independent BSS?or IBSS?Stations connected in
ad-hoc mode
AP
Network infrastructure
Giuseppe Bianchi
FrameFrame Forwarding in a BSSForwarding in a BSS
AP
Network infrastructure
BSS: AP = relay functionNo direct communication allowed!
IBSS: direct communicationbetween all pairs of STAs
Giuseppe Bianchi
Why AP = relayWhy AP = relay function?function?
?Management:?Mobile stations do NOT neet to maintain neighbohr
relationship with other MS in the area?But only need to make sure they remain properly associated to
the AP? Power Saving:?APs may assist MS in their power saving functions?by buffering frames dedicated to a (sleeping) MS when it is in
PS mode
?Obvious disadvantage: use channelbandwidth twice…
Giuseppe Bianchi
ExtendedExtended Service SetService Set
AP1
AP2 AP3 AP4
BSS1
BSS2 BSS3 BSS4
ESS: created by merging different BSS through a network infrastructure(possibly overlapping BSS – to offer a continuous coverage area)
Stations within ESS MAY communicate each other via Layer 2 proceduresAPs acting as bridgesMUST be on a same LAN or switched LAN or VLAN (no routers in between)
Giuseppe Bianchi
The concept of Distribution The concept of Distribution SystemSystem
AP1 AP2 AP3
MSs in a same ESS need to1) communicate each other2) move through the ESS
Distribution system (physical connectivity + logical service support)
Ethernet backbone: Distribution system medium (but DS is more than just a medium!!)
DS role: - track where an MS is registrered within an ESS area- deliver frame to MS
Giuseppe Bianchi
Association and DSAssociation and DS
AP1 AP2 AP3
Association
IAPP IAPP
DS implementation:- an AP must inform other APs of associated MSs MAC addresses- proprietary implementation ? no interoperability (must use APs from same vendor)- standardized protocol on the way (?): IAPP (802.11f)
- 802.11f Working Practice Standard: june 2003
Giuseppe Bianchi
WirelessWireless Distribution SystemDistribution System
AP1 AP2 AP3
DS medium:- not necessarily an ethernet backbone!- could be the 802.11 technology itself
Resulting AP = wireless bridge
Giuseppe Bianchi
AddressesAddresses
?At least three addresses?Receiving station?Transmitting station?BSS address?To make sure a frame is valid within the
considered BSS?For filtering purpose (filter frame within a
BSS)
Giuseppe Bianchi
BSSIDBSSID
?Address of a BSS?Infrastructure mode:?AP MAC address
?Ad-hoc mode:?Random value
» With universal/local bit set to 1
?Generated by STA initiating the IBSS
802 IEEE 48 bit addresses
1 bit = individual/group1 bit = universal/local
46 bit address
Giuseppe Bianchi
Addressing in an IBSSAddressing in an IBSS
FrameControl
Duration/ ID
Address 1DA
Address 2SA
Address 3BSSID
SequenceControl
Address 4--- Data FCS
SA = Source AddressDA = Destination Address
SA
DA
Giuseppe Bianchi
Addressing in a BSS?Addressing in a BSS?
X
AP
DA
SA
Giuseppe Bianchi
Addressing in a BSS!Addressing in a BSS!
AP
Distribution system
Frame must carry following info:1) Destined to DA2) But through the APWhat is the most general addressing structure?
DASA
Giuseppe Bianchi
Addressing in a BSS (to AP)Addressing in a BSS (to AP)
FrameControl
Duration/ ID
Address 1BSSID
Address 2SA
Address 3DA
SequenceControl
Address 4---
Data FCS
AP
Distribution system
DASA
BSSID
Protocolversion
Type
2 2
Sub Type To DS
FromDS
MoreFrag
Retry PwrMNG
MoreData
WEP Order
4 1 1 1 1 1 1 1 1
1 0
Address 2 = wireless TxAddress 1 = wireless RxAddress 3 = dest
Giuseppe Bianchi
Addressing in an ESSAddressing in an ESS
AP
Distribution System
DA
SA
BSSID
FrameControl
Duration/ ID
Address 1BSSID
Address 2SA
Address 3DA
SequenceControl
Address 4---
Data FCS
Protocolversion
Type
2 2
Sub Type To DS
FromDS
MoreFrag
Retry PwrMNG
MoreData
WEP Order
4 1 1 1 1 1 1 1 1
1 0
AP
DA
Idea: DS will be able to forward frame to dest(either if fixed or wireless MAC)
Giuseppe Bianchi
Addressing in a BSS (from AP)Addressing in a BSS (from AP)
FrameControl
Duration/ ID
Address 1DA
Address 2BSSID
Address 3SA
SequenceControl
Address 4---
Data FCS
AP
Distribution system
DASA
BSSID
Protocolversion
Type
2 2
Sub Type To DS
FromDS
MoreFrag
Retry PwrMNG
MoreData
WEP Order
4 1 1 1 1 1 1 1 1
0 1
Address 2 = wireless TxAddress 1 = wireless RxAddress 3 = src
Giuseppe Bianchi
From AP: do weFrom AP: do we reallyreally need 3 need 3 addresses?addresses?
AP
Distribution system
DASA
BSSID
DA correctly receives frame, and send 802.11 ACK to … BSSID (wireless transmitted)
DA correctly receives frame, and send higher level ACK to … SA (actual transmitter)
Giuseppe Bianchi
AddressingAddressing within a WDSwithin a WDS
AP
Wireless Distribution System
SA
TA
AP
DA
FrameControl
Duration/ ID
Address 1RA
Address 2TA
Address 3DA
SequenceControl
Address 4SA
Data FCS
Protocolversion
Type
2 2
Sub Type To DS
FromDS
MoreFrag
Retry PwrMNG
MoreData
WEP Order
4 1 1 1 1 1 1 1 1
1 1
RA
Address 4: initially forgotten…
Giuseppe Bianchi
Addressing: summaryAddressing: summary
Wireless DS
To AP
From AP
IBSS
Function
SADATARA11
N/ADASARA = BSSID01
N/ASABSSIDRA = DA10
N/ABSSIDSARA = DA00
Address 4Address 3Address 2Address 1From DSTo DS
Receiver Transmitter
? BSS Identifier (BSSID)? unique identifier for a particular BSS. In an infrastructure BSSID it is the MAC address of the AP. In IBSS, it is
random and locally administered by the starting station. (uniqueness)? Transmitter Address (TA)
? MAC address of the station that transmit the frame to the wireless medium. Always an individual address.? Receiver Address (RA)
? to which the frame is sent over wireless medium. Individual or Group.? Source Address (SA)
? MAC address of the station who originated the frame. Always individual address. ? May not match TA because of the indirection performed by DS of an IEEE 802.11 WLAN. SA field is considered
by higher layers.? Destination Address (DA)
? Final destination . Individual or Group. ? May not match RA because of the indirection.
Giuseppe Bianchi
3. 802.11 MAC: CSMA/CA3. 802.11 MAC: CSMA/CADistributed Coordination FunctionDistributed Coordination Function
Giuseppe Bianchi
Wireless EthernetWireless Ethernet? 802.3 (Ethernet)? CSMA/CD?Carrier Sense Multiple Access ?Collision Detect
? 802.11(wireless LAN)? CSMA/CA ? (Distributed Coordination Function)?Carrier Sense Multiple Access ?Collision Avoidance
A B C
? Both A and C sense the channel idle at the same time ? they send at the same time.
? Collision can be detected at sender in Ethernet.
? Why this is not possible in 802.11?1. Either TX or RX (no simultaneous
RX/TX)2. Large amount of power difference in
Tx and Rx (even if simultaneous tx-rx, no possibility in rx while tx-ing)
3. Wireless medium = additional problems vs broadcast cable!!
Giuseppe Bianchi
? Large difference in signal strength; physical channel impairments (shadowing)? It may result that two stations in the same area cannot communicate
? Hidden terminals? A and C cannot hear each other? A transmits to B? C wants to send to B; C cannot receive A;C senses “idle” medium
(Carrier Sense fails)? Collision occurs at B.? A cannot detect the collision (Collision Detection fails).? A is “hidden” to C.
Hidden Terminal ProblemHidden Terminal Problem
BA C
Giuseppe Bianchi
802.11 MAC approach802.11 MAC approach?Still based on Carrier Sense:
?Listen before talking?But collisions can only be inferred
afterwards, at the receiver?Receivers see corrupted data through a CRC error?Transmitters fail to get a response
?Two-way handshaking mechanism to infer collisions?DATA-ACK packets
TX RX
packet
ACK
Giuseppe Bianchi
Channel Access detailsChannel Access details?A station can transmit only if it senses the
channel IDLE for a DIFS time?DIFS = Distributed Inter Frame Space
DIFSDATA
SIFS ACK
? Key idea: DATA and ACK separated by a differentInter Frame Space
?SIFS = Short Inter Frame Space? Second station cannot hear a whole DIFS, as
SIFS<DIFS
TX
RX
Packet arrival
What about a station arriving in this frame time?
Giuseppe Bianchi
DIFS & SIFS in wiDIFS & SIFS in wi--fifi
?DIFS = 50 ? s?SIFS = 10 ? s
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Why Why backoffbackoff??DIFS
DATA
SIFS ACKSTA1
STA2
STA3
DIFS
Collision!
RULE: when the channel is initially sensed BUSY, station defers transmission;But when it is sensed IDLE for a DIFS, defer transmission of a further random time(BACKOFF TIME)
Giuseppe Bianchi
SlottedSlotted BackoffBackoff
STA2
STA3
DIFS
Extract random number in range (0, W-1)Decrement every slot-time ?
w=7
w=5
Note: slot times are not physically delimited on the channel!Rather, they are logically identified by every STA
Slot-time values: 20?s for DSSS (wi-fi)Accounts for: 1) RX_TX turnaround time
2) busy detect time3) propagation delay
Giuseppe Bianchi
BackoffBackoff freezingfreezing
?When STA is in backoff stage:?It freezes the backoff counter as long as the channel is
sensed BUSY?It restarts decrementing the backoff as the channel is sensed
IDLE for a DIFS period
DIFS DATA
SIFS ACK
STATION 1
DIFS
SIFS ACK 6 5
DIFS
Frozen slot-time 4BUSY medium
STATION 2DIFS
3 2 1
Giuseppe Bianchi
BackoffBackoff rulesrules? First backoff value:?Extract a uniform random number in range (0,CWmin)
? If unsuccessful TX:?Extract a uniform random number in range (0,2×(CWmin+1)-1)
? If unsuccessful TX:?Extract a uniform random number in range (0,22×(CWmin+1)-1)
?Etc up to 2m×(CWmin+1)-1
Exponential Backoff!CWmin = 31CWmax = 1023 (m=5)
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ThroughputThroughput vsvs CWminCWminP=1000 bytes
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RTS/CTSRTS/CTS
?Request-To-Send / Clear-To-Send?4-way handshaking ?Versus 2-way handshaking of basic access
mechanism? Introduced for two reasons?Combat hidden terminal?Improve throughput performance with long
packets
Giuseppe Bianchi
DIFSDATA
SIFS ACK
TX
RX
Packet arrival
RTS
SIFS CTS SIFS
RTS/CTS and hiddenRTS/CTS and hidden terminalsterminals
TX
RX
hidden
others
RTS
NAV (RTS)
RTS/CTS: carry the amount of time the channelwill be BUSY. Other stations may update a Network Allocation Vector, and defer TX
even if they sense the channel idle (Virtual Carrier Sensing)
CTS CTS
NAV (CTS)
(Update NAV)
data
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RTS/CTS and performanceRTS/CTS and performance
RTS/CTS cons: larger overheadRTS/CTS pros: reduced collision duration
Giuseppe Bianchi
RTS/CTS throughputRTS/CTS throughput
RTS/CTS convenient with long packets and large number of terminals (collision!);
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RTS/CTS more robust to number of users and CWmin settings
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Relation between IFSRelation between IFS
Busy Medium
SIFS
DIFS
Backoff Window
Slot Time
Defer Access Select Slot and decrement backoff as long as medium stays idle
DIFS
Contention WindowImmediate access when medium is idle >= DIFS
Next Frame
PIFS
PIFS used by Point Coordination Function- Time-bounded services- Polling scheme
PCF Never deployed
102331205010802.11b PHY
CWmaxCWminSlot Time (? sec)
DIFS (?sec)SIFS (? sec)
Parameters
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EIFSEIFS
DataACK
NAV
Sourcestation
Destination station
Other stations receiving Data frame correctly
Other stationsreceiving Data frameincorectly
DIFS
SIFS
EIFS
Back- o ff
Back- o ff
Back- o ff
Giuseppe Bianchi
Data FrameData Frame formatsformats
PHY IEEE 802.11 Data 0 - 2312 FCS
Protocolversion Type Sub Type info
2 2 12
Sub TypeTo DS
FromDS
MoreFrag Retry
PwrMNG
MoreData WEP Order
4 1 1 1 1 1 1 1 1
FrameControl
Duration/ ID Address 1 Address 2 Address 3
SequenceControl Address 4 Data
Framecheck
sequence
2 2 2 40-23126666
Fragmentnumber Sequence number
4 12
Time in microseconds. Update the NAV time in the
neighborhood
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Why NAV (i.e. protect ACK)?Why NAV (i.e. protect ACK)?
Station C receives frame from station TXStation C IS NOT in reach from station RXBut sets NAV and protects RX ACK
RX
TXC
Giuseppe Bianchi
Why EIFS (i.e. protect ACK)?Why EIFS (i.e. protect ACK)?
Station C DOES NOT receive frame from station TX but still receives enough signal to get a PHY.RXEND.indication error
Station C IS NOT in reach from station RXBut sets EIFS (!!) and protects RX ACK
RX
TXC
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4. DCF Overhead4. DCF Overhead
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FrameFrame formatsformats
FrameControl
Duration/ ID Address 1 Address 2 Address 3
SequenceControl Address 4 Data FCS
DATA FRAME (28 bytes excluded address 4)
FrameControl
Duration RA TA FCS
FrameControl
Duration RA FCS
RTS (20 bytes)
CTS / ACK (14 bytes)
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DCF overheadDCF overhead
min_
[ ]
[ ] / 2station
Frame Tx
E payload
E T DIFS CWS ?
? ?
_Frame Tx MPDU ACKT T SIFS T? ? ?
TxCTSPLCPCTS
TxRTSPLCPRTS
TxACKPLCPACK
TxMPDUPLCPMPDU
RTT
RTT
RTT
RLTT
_
_
_
_
/148
/208
/148
/)28(8
???
???
???
????
_Frame Tx RTS CTS MPDU ACKT T SIFS T SIFS T SIFS T? ? ? ? ? ? ?
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DCF overhead (802.11b)DCF overhead (802.11b)
0 2000 4000 6000 8000
T r a n s m s s i o n T i m e ( u s e c )
Basic
RTS/CTS
Basic
RTS/CTS
DIFS Ave Backoff RTS+SIFS CTS+SIFS Payload+SIFS ACK
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DCF overhead (802.11b)DCF overhead (802.11b)
00.10.20.30.40.5
0.60.70.80.9
1
100
300
500
700
900
1100
1300
1500
1700
1900
2100
2300
P a y l o a d S i z e ( B y t e s )
Nor
mal
ized
Thr
ough
put
BAS-2Mbps
RTS-2Mbps
BAS-11Mbps
RTS-11Mbps
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5. Point Coordination Function5. Point Coordination Function
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PCF vs DCFPCF vs DCF
PHY
DCF
PCF
PCF deployed on TOP of DCFBackward compatibility
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PCFPCF
? Token-based access mechanism? Polling
? Channel arbitration enforced by a “pointCoordinator” (PC)? Typically the AP, but not necessarily
? Contention-free access? No collision on channel
? PCF deployment: minimal!!? Optional part of the 802.11 specification? As such, almost never deployed? But HCCA (PCF extension in 802.11e) is getting considerable attention…
Giuseppe Bianchi
PCF frame transferPCF frame transfer
SIFS
Polling strategy: very elementary!!- send polling command to stations with increasing Association ID value…- (regardless whether they might have or not data to transmit)
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