1. Introduction to 802.11 Wireless LANstwiki.di.uniroma1.it/pub/Reti_Avanzate/AA0910/WebHome/wlan-01.pdf · Nortel 5,0 1,0% Other 55,0 10,8% Total 2.637 100% Total 511,8 100% Source:

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!!

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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?

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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.

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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

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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

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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

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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?

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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

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PHY rates at a glancePHY rates at a glance

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802.11a Data rates802.11a Data rates

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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)

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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!

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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

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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)

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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

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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

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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 ?

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2. Wireless LAN Networks 2. Wireless LAN Networks and related Addressingand related Addressing

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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

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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

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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…

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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)

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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

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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

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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

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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)

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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

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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

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Addressing in a BSS?Addressing in a BSS?

X

AP

DA

SA

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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

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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

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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)

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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

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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)

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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…

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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.

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3. 802.11 MAC: CSMA/CA3. 802.11 MAC: CSMA/CADistributed Coordination FunctionDistributed Coordination Function

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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!!

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? 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

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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

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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?

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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)

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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

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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

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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

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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

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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

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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

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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…

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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)