Introduction to 802.11 Wireless LANstwiki.di.uniroma1.it/pub/Reti_Avanzate/WebHome/wlan.pdf ·...

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1 Giuseppe Bianchi Introduction to 802.11 Introduction to 802.11 Wireless LANs Wireless LANs Quote from Matthew Gast - 802.11® Wireless Networks The Definitive Guide – apr. 2005, 2nd edition At this point, there is no way to prevent the spread of Wi-Fi. In the years since the first edition of [his] book, wireless networking has gone from an interesting toy to a must-have technology. […] [Wireless networking] seems poised to continue its march towards the standard method of network connection, replacing "Where's the network jack?" with "Do you have Wi-Fi?" as the question to ask about network access. Giuseppe Bianchi WLAN history WLAN history Original goal: Deploy “wireless Ethernet” First generation proprietary solutions (end ’80, begin ’90): WaveLAN (AT&T) HomeRF (Proxim) Abandoned by major chip makers (e.g. Intel: dismissed HomeRF 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 Two operative Industrial, Scientific & Medical (ISM) shared unlicensed band » 2.4 GHz: Legacy; 802.11b/g » 5 GHz: 802.11a 1999: Wireless Ethernet Compatibility Alliance (WECA) certification Later on named Wi-Fi Boosted 802.11 deployment!!

Transcript of Introduction to 802.11 Wireless LANstwiki.di.uniroma1.it/pub/Reti_Avanzate/WebHome/wlan.pdf ·...

Page 1: Introduction to 802.11 Wireless LANstwiki.di.uniroma1.it/pub/Reti_Avanzate/WebHome/wlan.pdf · Introduction to 802.11 Wireless LANs Quote from Matthew Gast - 802.11® Wireless Networks

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Giuseppe Bianchi

Introduction to 802.11 Introduction to 802.11 Wireless LANsWireless LANs

Quote from Matthew Gast - 802.11® Wireless Networks The Definitive Guide – apr. 2005, 2nd edition

At this point, there is no way to prevent the spread of Wi-Fi. In the years since the first edition of [his] book, wireless networking has gone from an interesting toy to a must-have technology. […] [Wireless networking] seems poised to continue its march towards the standard method of network connection, replacing "Where's the network jack?" with "Do you have Wi-Fi?" as the question to ask about network access.

Giuseppe Bianchi

WLAN historyWLAN history� Original goal:

� Deploy “wireless Ethernet”� First generation proprietary solutions (end ’80, begin ’90):

� WaveLAN (AT&T)� HomeRF (Proxim)

� Abandoned by major chip makers (e.g. Intel: dismissed HomeRF 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� Two operative Industrial, Scientific & Medical (ISM) shared unlicensed band

» 2.4 GHz: Legacy; 802.11b/g

» 5 GHz: 802.11a

� 1999: Wireless Ethernet Compatibility Alliance (WECA) certification� Later on named Wi-Fi � Boosted 802.11 deployment!!

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WLAN data ratesWLAN data rates� Legacy 802.11

� Work started in 1990; standardized in 1997� 1 mbps & 2 mbps

� The 1999 revolution: PHY layer impressive achievements� 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)

� 2003: extend 802.11b� 802.11g: OFDM for 2.4 GHz

� Published in june 2003� Products available, though no extensive interoperability

testing yet� Backward compatibility with 802.11b Wi-Fi

� 2009: 802.11n (oct 2009)� MIMO, 2x bandwidth (40 MHz)� Practical: up to 144.4 mbps (but 600=150x4 possible)

� Ongoing: 802.11ac/ad (1gbps, 2011 - ???)

Standard Transfer Method

Freq. Band

Data Rates Mbps

802.11 legacy FHSS, DSSS, IR

2.4 GHz, IR

1, 2

802.11b DSSS, HR-DSSS

2.4 GHz 1, 2, 5.5, 11

"802.11b+" non-standard

DSSS, HR-DSSS, (PBCC)

2.4 GHz 1, 2, 5.5, 11,22, 33, 44

802.11a OFDM 5.2, 5.5 GHz

6, 9, 12, 18, 24, 36, 48, 54

802.11g DSSS, HR-DSSS, OFDM

2.4 GHz 1, 2, 5.5, 11; 6, 9, 12, 18, 24, 36, 48, 54

Giuseppe Bianchi

Why multiple rates?Why multiple rates?“Adaptive” (?) coding/modulation“Adaptive” (?) coding/modulation

Example: 802.11a case

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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

tan

ce (m

)

54Mbps36Mbps24Mbps12Mbps6Mbps 11Mbps5.5Mbps1Mbps

2.4 GHz OFDM (.11g)

5 GHz OFDM (.11a)

2.4 GHz (.11b)

Giuseppe Bianchi

Coverage performance Coverage performance Cisco Aironet 350 Access PointCisco Aironet 350 Access Point

11 Mb/s DSSfrom ~30 to ~45 mt

5.5 Mb/s DSSfrom ~45 to ~76 mt

2 Mb/s DSSfrom ~76 to ~107 mt

Configurable TX power:50, 30, 20, 5, 1 mW(100 mW outside Europe)

Greater TX power, faster battery consumptions!

Question: how to select transmission rate? (STA does not explicitly know its distance from AP)

More later (implementation-dependent ☺)

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WLAN NIC addressesWLAN NIC addresses

� Same as Ethernet NIC� 48 bits = 2 + 46

� Ethernet & WLAN addresses do coexist� undistinguishable, in a same (Layer-2) network

� role of typical AP = bridge» (to be precise: when the AP act as “portal” in 802.11

nomenclature)

802 IEEE 48 bit addresses

1 bit = individual/group1 bit = universal/local

46 bit address

AP

192.168.1.3200:0a:e6:f8:03:ad

AP

192.168.1.4300:06:6e:00:32:1a 192.168.1.52

00:82:00:11:22:33

C:>arp -a192.168.1.32 00-0a-e6-f8-03-ad dinamico192.168.1.43 00-06-6e-00-32-1a dinamico192.168.1.52 00-82-00-11-22-33 dinamico

Giuseppe Bianchi

Protocol stackProtocol stack

802.2 Logical Link Control

802.3MAC

802.3PHY

DATA LINK LAYERLLC sublayerLLC

MAC

802overview

&architecture

802.1management

&bridging

802.11 MAC……………

802.11FSSS PHY

802.11DSSS PHY

802.11aOFDM PHY

802.11bHR-DSSS

PHY

802.11gExtended

Rate PHY

DATA LINK LAYERMAC sublayer

PHYSICAL LAYER

802.11: “just” another 802 link layer ☺

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802.11 MAC Data Frame802.11 MAC Data Frame

PHY IEEE 802.11 Data 0 - 2312 FCS

Protocolversion

Type Sub Type info

2 2 12

Sub TypeTo DS

FromDS

MoreFrag

RetryPwr

MNGMoreData

WEP Order

4 1 1 1 1 1 1 1 1

FrameControl

Duration/ ID

Address 1 Address 2 Address 3SequenceControl

Address 4 DataFramecheck

sequence

2 2 2 40-23126666

Fragmentnumber

Sequence number

4 12

MAC header: - 28 bytes (24 header + 4 FCS) or- 34 bytes (30 header + 4 FCS)

DETAILS AND EXPLANATION LATER ON

Giuseppe Bianchi

EncapsulationEncapsulation

Identical To 802.3/LLC encapsulation

802.11 MAC frame: no “type” field (such as Ethernet II)!!LLC encapsulation mandatory

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Crossing a wireless bridge / 1Crossing a wireless bridge / 1

ETH/802.11bridge

802.11/ETHbridge

typeSRCDEST P

typeSRCDEST P

AAAA 03 00.00.00802.11 MAC header Type P

Giuseppe Bianchi

Crossing a wireless bridge / 2Crossing a wireless bridge / 2

ETH/802.11bridge

802.11/ETHbridge

AAAA 03 00.00.00LenSRCDEST Type P

typeSRCDEST P

AAAA 03 00.00.00802.11 MAC header Type PType P

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Why Ethernet Tunnel?Why Ethernet Tunnel?(just needed in very special cases: IPX, AARP)(just needed in very special cases: IPX, AARP)

AAAA 03 00.00.00LenSRCDESC Type

ETH/802.11bridge

802.11/ETHbridge

P

AAAA 03 00.00.00LenSRCDESC Type P

TypeSRCDESC P ?????Some protocols

MUST have this

Encapsulation:

-Novell IPX

(Type 0x8137)

- Apple-Talk ARP

(Type 0x80F3)

Giuseppe Bianchi

Handling 802.11 framesHandling 802.11 frames

� Ethernet-like driver interface� supports virtually all protocol stacks� Maximum Data limited to 1500

octets

� Frame translation� IEEE Std 802.1H

� IEEE 802.3 frames: translated to 802.11

� Ethernet Types 8137 (Novell IPX) and 80F3 (AARP) encapsulated via Ethernet Tunnel

» In general, any protocol listed in the “selective translation table” of the bridge

� All other Ethernet Types: encapsulated via RFC 1042 SNAP

� Transparent bridging to Ethernet

Platform Computer

PC-Card Hardware

Radio Hardware

WMAC controller withStation Firmware

(WNIC-STA)

Driver Software(STADr)

802.11 frame format

802.3 frame format

Ethernet V2.0 / 802.3frame format

Protocol Stack

BridgeSoftware

PC-Card Hardware

Radio Hardware

WMAC controller withAccess Point Firmware

(WNIC-AP)

Driver Software(APDr)

802.11 frame format

802.3 frame format

Ethernet V2.0 / 802.3frame format

Kernel Software (APK)

BridgeHardwareInterface

EthernetInterface

STA AP

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802.11 Network Architecture802.11 Network ArchitectureAnd related addressingAnd related addressing

Giuseppe Bianchi

Basic Service Set (BSS)Basic Service Set (BSS)group of stations that can communicate with each othergroup of stations that can communicate with each other

� Infrastructure BSS

� or, simply, BSS

� Stations connected through AP

� Typically interconnetted to a

(wired) network infrastructure

� Independent BSS (IBSS)

� Stations communicate directly

with each other

� Smallest possible IBSS: 2

STA

� IBSS set up for a specific

purpose and for short time

(e.g. meeting)

�That’s why they are also called ad hoc networks

AP

Network infrastructure

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Frame Forwarding in a BSSFrame Forwarding in a BSS

AP

Network infrastructure

BSS: AP = relay functionNo direct communication allowed!

IBSS: direct communicationbetween all pairs of STAs

Giuseppe Bianchi

Why AP = relay function?Why AP = relay 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�Association = get connected to (equivalent to plug-in a wire to a

bridge ☺)

� 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 channel bandwidth twice…

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Addressing in IBSS (ad hoc)Addressing in IBSS (ad hoc)

FrameControl

Duration/ ID

Address 1DA

Address 2SA

Address 3BSSID

SequenceControl

Data FCS

SA = Source AddressDA = Destination AddressBSSID = Basic Service Set IDentifier

used for filtering frames at reception (does the frame belong to OUR cell?)format: 6 bytes random MAC address with Universal/Local bit set to 1

SA

DA

Giuseppe Bianchi

Addressing in a BSS?Addressing in a BSS?

X

AP

DA

SA

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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

Data FCS

AP

Distribution system

DASA

BSSID

Protocolversion

Type

2 2

Sub TypeTo DS

FromDS

MoreFrag

RetryPwr

MNGMoreData

WEP Order

4 1 1 1 1 1 1 1 1

1 0

Address 2 = wireless TxAddress 1 = wireless RxAddress 3 = dest

BSSID = AP MAC address

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Giuseppe Bianchi

Addressing in a BSS (from AP)Addressing in a BSS (from AP)

FrameControl

Duration/ ID

Address 1DA

Address 2BSSID

Address 3SA

SequenceControl

Data FCS

AP

Distribution system

DASA

BSSID

Protocolversion

Type

2 2

Sub TypeTo DS

FromDS

MoreFrag

RetryPwr

MNGMoreData

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 we really need 3 From AP: do we really 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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Giuseppe Bianchi

Extended Service SetExtended Service 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

Service Set IDentifier (SSID)Service Set IDentifier (SSID)

� name of the WLAN network � Plain text (ascii), up to 32 char

� Assigned by the network administrator� All BSS in a same ESS have

same SSID� Typically (but not

necessarily) is transmitted in periodic management frames (beacon)� Disabling SSID transmission =

a (poor!) security mechanism� Typical: 1 broadcast beacon

every 100 ms (configurable by sysadm)

� Beacon may transmit a LOT of other info (see example – a simple one!)

IEEE 802.11 wireless LAN management frameFixed parameters (12 bytes)

Timestamp: 0x00000109EAB69185Beacon Interval: 0,102400 [Seconds]Capability Information: 0x0015

.... .... .... ...1 = ESS capabilities: Transmitter is an AP

.... .... .... ..0. = IBSS status: Transmitter belongs to a BSS

.... .... .... 01.. = CFP participation capabilities: Point coordinator at AP for delivery and polling (0x0001)

.... .... ...1 .... = Privacy: AP/STA can support WEP

.... .... ..0. .... = Short Preamble: Short preamble not allowed

.... .... .0.. .... = PBCC: PBCC modulation not allowed

.... .... 0... .... = Channel Agility: Channel agility not in use

.... .0.. .... .... = Short Slot Time: Short slot time not in use

..0. .... .... .... = DSSS-OFDM: DSSS-OFDM modulation not allowedTagged parameters

Tag Number: 0 (SSID parameter set)Tag length: 4Tag interpretation: WLAN

Tag Number: 1 (Supported Rates)Tag length: 4Tag interpretation: Supported rates: 1,0(B) 2,0(B) 5,5 11,0 [Mbit/sec]

Tag Number: 6 (IBSS Parameter set)Tag length: 1Tag interpretation: ATIM window 0x2

Tag Number: 5 ((TIM) Traffic Indication Map)Tag length: 4Tag interpretation: DTIM count 0, DTIM period 1,

Bitmap control 0x0, (Bitmap suppressed)

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The concept of Distribution SystemThe concept of Distribution System� “Logical” architecture

component� Provides a “service”� DSS = Distribution

System Service

� Standard does NOT say how it is implemented� Specified only which

functions it provides � Association� Disassociation� Reassociation� Integration� Distribution

� Association/disassociation� Registration/de-registration of a STA to an AP

� Equivalent to “plugging/unplugging the wire” to a switch

� DS uses this information to determine which AP send frames to

� Reassociation� i.e. handling STA mobility in a same ESS!

� Distribution� An AP receives a frame on its air interface (e.g. STA 2)

� It gives the message to the distribution service (DSS) of the DS

� The DSS has the duty to deliver the frame to the proper destination (AP)

� Integration� Must allow the connection to non 802.11 LANs� Though, in practice, non 802.11 LANs are Ethernet

and no “real portals” are deployed

Giuseppe Bianchi

DS, againDS, again

AP1 AP2 AP3

Association

IAPP/proprietary IAPP/proprietary

Distribution system (physical connectivity + logical service support)

MSs in a same ESS need to1) communicate each other2) move through the ESS

� Typical implementation (media)� Switched Ethernet Backbone� But alternative “Distribution Medium” are

possible� E.g. Wireless Distribution System (WDS)

� Implementation duties� an AP must inform other APs of associated

MSs MAC addresses

� Standardization� From 1997: tentative to standardize an IAPP� Finalized as “working practice standard” in

802.11F (june 2003)� Nobody cared!

� Plenty of proprietary solutions� Must use APs from same vendor in whole ESS

� Current trends (2004+):� Centralized solutions (see Aruba, Cisco, Colubris)

� Include centralized management, too!� Current attempt: convergence to CAPWAP?

Page 15: Introduction to 802.11 Wireless LANstwiki.di.uniroma1.it/pub/Reti_Avanzate/WebHome/wlan.pdf · Introduction to 802.11 Wireless LANs Quote from Matthew Gast - 802.11® Wireless Networks

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Giuseppe Bianchi

Addressing in an ESSAddressing in an ESSSame approach! Works in general, Same approach! Works in general,

even if DA in different BSSeven if DA in different BSS

AP

Distribution System

DA

SA

BSSID#1

FrameControl

Duration/ ID

Address 1BSSID#1

Address 2SA

Address 3DA

SequenceControl

Data FCS

Protocolversion

Type

2 2

Sub TypeTo DS

FromDS

MoreFrag

RetryPwr

MNGMoreData

WEP Order

4 1 1 1 1 1 1 1 1

1 0

AP

DA

(unprecise! No portal here) idea: DS will be able to forward frame to dest(either if fixed or wireless MAC)

Giuseppe Bianchi

Addressing in an ESSAddressing in an ESSSame approach! Works in general, Same approach! Works in general,

even if DA in different BSSeven if DA in different BSS

AP

Distribution System

DASA

BSSID#2

AP

DA

FrameControl

Duration/ ID

Address 1DA

Address 2BSSID#2

Address 3SA

SequenceControl

Data FCS

Protocolversion

Type

2 2

Sub TypeTo DS

FromDS

MoreFrag

RetryPwr

MNGMoreData

WEP Order

4 1 1 1 1 1 1 1 1

0 1

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Giuseppe Bianchi

Wireless Distribution SystemWireless Distribution System

AP1 AP2 AP3

DS medium:- not necessarily an ethernet backbone!- could be the 802.11 technology itself

Resulting AP = wireless bridge

Giuseppe Bianchi

Addressing within a WDSAddressing within 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 TypeTo DS

FromDS

MoreFrag

RetryPwr

MNGMoreData

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

Function To DS From DS Address 1 Address 2 Address 3 Address 4

IBSS 0 0 RA = DA SA BSSID N/A

From AP 0 1 RA = DA BSSID SA N/A

To AP 1 0 RA = BSSID SA DA N/A

Wireless DS 1 1 RA TA DA SA

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

802.11 MAC802.11 MACCSMA/CA Distributed Coordination FunctionCSMA/CA Distributed Coordination Function

Carrier Sense Multiple AccessCarrier Sense Multiple AccessWith Collision AvoidanceWith Collision Avoidance

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Giuseppe Bianchi

Wireless Medium UnreliabilityWireless Medium Unreliability

11 Mbps 802.11b outdoor measurements - Roma 2 Campus - roof nodes

Giuseppe Bianchi

Must rely on explicit ACKsMust rely on explicit ACKs

�Successful DATA transmission:

�ONLY IF an ACK is received

�ACK transmission provided by MAC layer

�Immediate retransmission» Don’t get confused with higher layer rtx

�DATA-ACK exchange:

�Also called two-way handshake

�Or Basic Access Mechanism

SENDER RECEIVER

DATA

ACK

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Possible errorsPossible errors� Three causes of insuccess

� PHY Error�Receiver cannot synchronize with transmitted frame

» preamble + SFD needed�or cannot properly read Physical Layer Control Protocol (PLCP) header

» PLCP header contains the essential information on employed rate» Without it receiver cannot know how to demodulate/decode received frame!

� CRC32 error�MAC frame (MAC Header + Payload) CRC failures

» The greater the rate, the higher the SNR required to correctly transmit

� ACK Error�Transmitter does not receive ACK

» ACK corrupted by PHY or CRC32 errors� It IS an error: though data frame was correctly received, transmitted does not know

» Introduce issue of duplicated frames at the receiver

PHY MAC header Payload FCS

Preamble SFD PLCP hdr

Giuseppe Bianchi

Wireless errorsWireless errors11 Mbps 802.11b/g OUTDOOR measurements - Roma 2 Campus - roof nodes

PHY errors CANNOT be reduced through automatic rate fallback mechanisms

An (apparent) paradox: 802.11b@11mbps outdoor outperforms 802.11g@6mbps !!!but it is NOT a paradox ☺ since most 802.11g errors are PHY (unrelated with rate)…

802.11b@11Mbps 802.11g@6Mbps

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Giuseppe Bianchi

Must forget Collision Detection!Must forget Collision Detection!

� One single RF circuitry� Either TX or RX…� Half-duplex

� Even if two simultaneous TX+RX: large difference (100+ dB!) in TX/RX signal power � Impossible to receive while transmitting

� On a same channel, of course

� Collision detection at sender: meaningless in wireless!� Ethernet = collision detection at sender� Wireless = large difference in the interference

power between sender & receiver!

� Collision OCCURS AT THE RECEIVER

STA

tx

rx

CA B

A detects a very low interference(C is far)no “collision”

B detects a disructive interference(C is near)collision occurs

Giuseppe Bianchi

Distributed Coordination Distributed Coordination Function BasicsFunction Basics

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Giuseppe Bianchi

802.11 MAC802.11 MAC

DISTRIBUTED COORDINATION FUNCTION

DCF(CSMA/CA)

POINT COORDINATION

FUNCTION

PCF(polling)

Intended for Contention-Free Services

Used for all other services, and used as basis for PCF

PCF: baiscally never user / supported!!

Giuseppe Bianchi

802.11 MAC evolution 802.11 MAC evolution (802.11e, finalized in december 2005)(802.11e, finalized in december 2005)

DCF

PCF(polling)

Intended for Contention-Free Services

Used for service differentiation (priorities)

All enhancements rely on DCF basic operation!

Dead ☺

HCF Controlled Channel Access

HCCA(scheduling)

Enhanced Distributed ChannelAccess

EDCA(prioritized CSMA)

Legacy

HYBRID COORDINATION FUNCTION

HCF

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Giuseppe Bianchi

Carrier Sense Multiple AccessCarrier Sense Multiple Access� Station may transmit ONLY IF senses channel IDLE for a

DIFS time� DIFS = Distributed Inter Frame Space

� Key idea: ACK replied after a SIFS < DIFS� SIFS = Short Inter Frame Space

� Other stations will NOT be able to access the channel during the handshake� Provides an atomic DATA-ACK transaction

DIFSDATA

SIFS ACK

TX

RX

Packet arrival

OTHERSTA

DIFS

Packet arrival

Must measure a whole DIFS

OK!

Giuseppe Bianchi

DATA/ACK frame formatDATA/ACK frame format

FrameControl

Duration/ ID

Address 1 Address 2 Address 3SequenceControl

Address 4 DataFramecheck

sequence

2 2 2 40-23126666

FrameControl

Duration/ ID

Address (RA)Framecheck

sequence

2 2 46

DATA frame: 28 (or 34) bytes + payload

Protocolversion

Type

2 2

Sub TypeTo DS

FromDS

MoreFrag

RetryPwr

MNGMoreData

WEP Order

4 1 1 1 1 1 1 1 1

ACK frame: 14 bytes – No need for TA address (the station receiving the ACK knows who’s this from)!!

Protocolversion

Type

2 2

Sub TypeTo DS

FromDS

MoreFrag

RetryPwr

MNGMoreData

WEP Order

4 1 1 1 1 1 1 1 1

0 1Type = Control (01)SubType = ACK (1101)1 1 0 1

Type = Data (10)SubType = Data (0000)

1 0 0 0 0 00 0

0 0 0 0 0 0 0 00 x

x x x x x xx x

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23

Giuseppe Bianchi

Grasping wiGrasping wi--fi (802.11b) numbersfi (802.11b) numbers

� DIFS = 50 µµµµs� Rationale: 1 SIFS + 2 slot-times

�Slot time = 20 µs, more later

PHY MAC header 24 (30) Payload FCS

� SIFS = 10 µµµµs� Rationale: RX_TX turnaround time

�The shortest possible!

� DATA frame: TX time = f(rate)� Impressive PHY overhead!

� 192 µs per every single frame� Total data frame time (1500 bytes)

� @1 Mbps: 192+12288= 12480 µs » PHY+MAC overhead = 3.3%

� @11 Mbps: 192+ 1117.1 = 1309.1 µs» PHY+MAC overhead = 16.%

� Overhead increases for small frames!� ACK frame: TX at basic rate

� Typically 1 mbps but 2 mbps possible…� ACK frame duration (1mbps): 304 µs

Preamble SFD PLCP hdr

128 16 48

1 mbps DBPSK

192 µs

(28+payload) [bytes] x 8 / TX_rate [mbps] = µs

PHY ACK 14

192 µs

DATA

ACK

112 µs

Giuseppe Bianchi

And when an ACK is “hidden”?And when an ACK is “hidden”?

SENDER RECEIVERSTA

1) Sender TX Receiver RX

STA defers

BUSY DETECT (DATA)

SENDER RECEIVERSTA

2)Receiver ACKs(after SIFS)STA cannot hear…

SIFSACK

STA STA TX!DIFS

SENDER RECEIVERSTA

3)STA tranmitsAnd destroys ACK!

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Giuseppe Bianchi

The Duration FieldThe Duration Field

FrameControl

Duration/ ID

Address 1 Address 2 Address 3SequenceControl

Address 4 DataFramecheck

sequence

2 2 2 40-23126666

0# microseconds1514131211109876543210

When bit 15 = 1 � NOT used as duration (used by power-saving frames to specify station ID)

DIFSDATA

SIFS ACK

OTHERSTA

Physical carrier sensing

NAV (data)

� Allows “Virtual Carrier Sensing”� Other than physically sensing the channel, each station keeps a Network

Allocation Vector (NAV)

� Continuously updates the NAV according to information read in the duration field of other frames

Virtual carrier sensing

Giuseppe Bianchi

And when a terminal is “hidden”?And when a terminal is “hidden”?

RECEIVER SENDERSTA

… this can be “solved” by increasing the sensitiveness of the Carrier Sense…Quite stupid, though (LOTS of side effects – out of the goals of this lecture)

SENDERSTA

… this can’s be “solved” by any means!

RECEIVER

� The Hidden Terminal Problem� SENDER and STA cannot hear each

other� SENDER transmits to RECEIVER� STA wants to send a frame

� Not necessarily to RECEIVER…

� STA senses the channel IDLE� Carrier Sense failure

� Collision occurs at RECEIVER� Destroys a possibly very long

TX!!

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Giuseppe Bianchi

DIFSDATA

SIFS ACK

TX

RX

Packet arrival

RTS

SIFS CTS SIFS

The RTS/CTS solutionThe RTS/CTS solution

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)

Giuseppe Bianchi

RTS/CTS framesRTS/CTS frames

FrameControl

Duration/ ID

Address (RA)Framecheck

sequence

2 2 46

CTS frame: 14 bytes (same as ACK)

Protocolversion

Type

2 2

Sub TypeTo DS

FromDS

MoreFrag

RetryPwr

MNGMoreData

WEP Order

4 1 1 1 1 1 1 1 1

0 1Type = Control (01)SubType = CTS (1100)1 1 0 0

FrameControl

Duration/ ID

Address 1 (RA)Framecheck

sequence

2 2 46

RTS frame: 20 bytes

Protocolversion

Type

2 2

Sub TypeTo DS

FromDS

MoreFrag

RetryPwr

MNGMoreData

WEP Order

4 1 1 1 1 1 1 1 1

Type = Control (01)SubType = RTS (1011)

Address 2 (TA)

6

0 0 0 0 0 0 0 00 x

0 1 1 0 1 10 0 0 0 0 0 0 00 x

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Giuseppe Bianchi

RTS/CTS and performanceRTS/CTS and performance

RTS/CTS cons: larger overheadRTS/CTS pros: reduced collision durationESPECIALLY FOR LONG PACKETSLong � packet > RTS_Threshold (configurable)

TODAY higher rates � No more significant

Giuseppe Bianchi

Issues with “duration” readingIssues with “duration” reading

RX

TXC

�“Duration” field in MAC header�Coded at same rate as payload

�Must receive whole MAC frame correctly

11 Mbps tx

11 mbpsrange

5.5 mbpsrange

2 mbpsrange

1 mbpsrange

� C cannot read TX frame� No way to know duration value

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Giuseppe Bianchi

ACK may be hidden once again!ACK may be hidden once again!

RX

TXC

� C hidden from RX� Carrier sense remains IDLE during RX�TX ACK

� NAV could not be updated

� May transmit after a DIFS

� Destroying ACK!

ACK transm

Giuseppe Bianchi

EIFS = protect ACKEIFS = protect ACK

RX

TXC

�C cannot read data frame�CRC32 error

�Most of PHY errors

11 Mbps tx

11 mbpsrange

5.5 mbpsrange

2 mbpsrange

1 mbpsrange

� If planning to transmit:� No more after a DIFS

� But after a LONGER interval of time�Sufficiently long to protect

ACK transmission

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Giuseppe Bianchi

EIFSEIFS

Data

ACK

NAV

Source

station

Destination station

Other stations receiving

Data frame correctly

Other stations

receiving Data frame

incorectly

DIFS

SIFS

EIFS

Back-off

Back-off

Back-off

Giuseppe Bianchi

Why backoff?Why backoff?

DIFSDATA

SIFS ACKSTA1

STA2

STA3

DIFS

Collision!

RULE: when the channel is initially sensed BUSY, station defers transmission;THEN,when channel sensed IDLE again for a DIFS, defer transmission of a further random time (Collision Avoidance)

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Giuseppe Bianchi

Slotted BackoffSlotted Backoff

STA2

STA3

DIFS

Extract random number in range (0, W-1)Decrement every slot-time σ

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: 20µs for DSSS (wi-fi)Accounts for: 1) RX_TX turnaround time

2) busy detect time3) propagation delay

Giuseppe Bianchi

Backoff freezingBackoff freezing

�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

SIFSACK

STATION 1

DIFS

SIFSACK 6 5

DIFS

Frozen slot-time 4BUSY medium

STATION 2

DIFS3 2 1

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Giuseppe Bianchi

Why backoff between Why backoff between consecutive tx?consecutive tx?

� A listening station would never find a slot-time after the DIFS (necessary to decrement the backoff counter)

� Thus, it would remain stuck to the current backoff counter value forever!!

DIFS DATA

SIFSACK

S 1

DIFS6 5

DIFS

Frozen slot-time 4

BUSY medium

S 2

DIFS3

DATA

SIFSACK

DIFS

BUSY medium DIFS

Giuseppe Bianchi

Backoff rulesBackoff rules

�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!For 802.11b:

CWmin = 31CWmax = 1023 (m=5)

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Giuseppe Bianchi

Further backoff rulesFurther backoff rules

� Truncated exponential backoff� After a number of attempts, transmission fails and frame is dropped

� Backoff process for new frame restarts from CWmin

� Protects against cannel capture �unlikely when stations are in visibility, but may occur in the case of

hidden stations� Two retry limits suggested:

� Short retry limit (4), apply to frames below a given threshold

� Long retry limit (7), apply to frames above given threshold

� (loose) rationale: short frames are most likely generated bu realk time stations�Of course not true in general; e.g. what about 40 bytes TCP ACKs?

Giuseppe Bianchi

DCF OverheadDCF Overhead

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Giuseppe Bianchi

802.11b parameters (summary)802.11b parameters (summary)

PIFS used by Point Coordination Function- Time-bounded services- Polling scheme

PCF Never deployed

ParametersSIFS (µsec)

DIFS (µsec)Slot Time (µsec)

CWmin CWmax

802.11b PHY

10 50 20 31 1023

Giuseppe Bianchi

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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Giuseppe Bianchi

Example: maximum achievable Example: maximum achievable throughput for 802.11bthroughput for 802.11b

DATA SIFSACK

DIFS

backoffDATA

Cycle time

� Data Rate = 11 mbps; ACK rate = 1 mbps� Payload = 1500 bytes

MbpsThr

BackoffE

DIFSSIFS

T

T

ACK

MPDU

07.631050304101303

81500

31020231

][

50;10

3041/148192

130311/)150028(8192

=++++

×=

=×=

===⋅+=

≈+⋅+=

� Data Rate = 11 mbps; ACK rate = 1 mbps� Payload = 576 bytes

MbpsThr

BackoffE

DIFSSIFS

T

T

ACK

MPDU

53.33105030410631

8576

31020231

][

50;10

3041/148192

63111/)57628(8192

=++++

×=

=×=

===⋅+=

≈+⋅+=

REPEAT RESULTS FOR RTS/CTS � Not viable (way too much overhead) at high rates!

Giuseppe Bianchi

DCF overhead (802.11b)DCF overhead (802.11b)

0 2000 4000 6000 8000

Tra nsmssion Time (use c )

Basic

RTS/CTS

Basic

RTS/CTS

DIFS Ave Backoff RTS+SIFS CTS+SIFS Payload+SIFS ACK

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Giuseppe Bianchi

DCF overhead (802.11b)DCF overhead (802.11b)

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

100

300

500

700

900

1100

1300

1500

1700

1900

2100

2300

Payload Size (Bytes)

Norm

alized T

hroughput

BAS-2M bps

RTS-2M bps

BAS-11Mbps

RTS-11Mbps

Giuseppe Bianchi

802.11 MAC 802.11 MAC extrasextras

Selected topics, and for BSS case only (no IBSS)Selected topics, and for BSS case only (no IBSS)

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Giuseppe Bianchi

Why FragmentationWhy Fragmentation

frame

Radioimpairment

frame

frag0 frag1 frag1 frag2 frag3 frag4 frag5

Radioimpairment

frag0 frag1 frag2 frag3 frag4 frag5

ACK

ACK

ACK

ACK

ACK

ACK

� High Bit Error Rate (BER) � increases with distance � The longer the frame, the lower the successful TX probability� High BER = high rtx overhead & increased rtx delay

� backoff window increase: cannot distinguish collision from tx error!!

Once again ☺: Fragmentation not Viable with “modern” 802.11 rates � not used

Giuseppe Bianchi

FragmentationFragmentation

� splits message (MSDU) into several frames (MPDU)� Same fragment size

�except last one� Fragmentation burst

� Fragments separated by SIFS�channel cannot be captured by

someone else� Each fragment individually ACKed

RTSsender

receiver

Other stations

t

DIFS SIFS SIFS

CTS

frag0SIFS

ACK

SIFS frag1

ACK

SIFS

NAV (RTS)

NAV (CTS)

NAV(frag0)

NAV(ACK)

� Each fragment reserves channel for next one

�NAV updated fragment by fragment

� Missing ACK for fragment x� release channel (automatic)

�ACK_Timeout much longer that SIFS!

� Backoff

� Restart from transmission of fragment x

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Giuseppe Bianchi

Fragment and sequence numbersFragment and sequence numbers

FrameControl

Duration/ ID

Address 1 Address 2 Address 3SequenceControl

Address 4 DataFramecheck

sequence

2 2 2 40-23126666

DATA frame: 28 (or 34) bytes + payload

Fragmentnumber

Sequence Number

4 12

� Fragment number� Increasing integer value 0�15 (max 16 fragments since 4 bits available)

� Essential for reassembly� More Fragment bit (frame control field) set to:

� 1 for intermediate fragments

� 0 for last fragment� Sequence Number

� Used to filter out duplicates� Unlike Ethernet, hede duplicates are quite frequent! � retransmissions are a main feature of the MAC

� Retry bit: helps to distingush retransmission� Set to 0 at transmission of new frame

� Set to 1 at retransmissions

prot

MoreFrag

Retry

1 1

typ subtypetods

frds

PS

+da

Wor

Giuseppe Bianchi

Power managementPower management� beacons:

� Periodically transmitted� Include timestamp

� To enable STA synchronization

� [….etc etc…]

� Every beacon includes a “Traffic Indication Map” (TIM)� Information element listing the stations for which UNICAST frames are buffered

�Bitmap!! (2008 bits = 251 bytes… transmission split over multiple beacons)� A station may then issue a PS-Poll control frame to enable transmission

� Instead of duration, PS-Poll contains AID (Association IDentifier of the STA: 1…2007)

� What about broadcast & multicast frames� transmitted only after beacons containing a DTIM (Delivery TIM)

� 1 DTIM every X beacon (X configurable)

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Giuseppe Bianchi

Power management Power management -- exampleexample

STA A

STA B

STA B has set ReceiveDTIM to false to minimize power consumption!…losing broadcast & multicast frames

Giuseppe Bianchi

Point Coordination Function Point Coordination Function

� Token-based access mechanism� Polling

� Channel arbitration enforced by a “point Coordinator” (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

� HCCA (802.11e) may be considered as PCF extension…

PHY

DCF

PCF

PCF deployed on TOP of DCFBackward compatibility

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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)

Giuseppe Bianchi

MultiMulti--rate operationrate operation

�Rate selection: proprietary mechanism!

�Result: different chipsets operate widely different

�Two basic approaches

�Adjust rate according to measured link quality (SNR estimate)�How link quality is computed is again proprietary!

�Adjust rate according to frame loss�How many retries? Step used for rate reduction? Proprietary!�Problem: large amount of collisions (interpreted as frame loss)

forces rate adaptation

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Giuseppe Bianchi

Performance AnomalyPerformance Anomaly� Question 1:

� Assume that throughput measured for a single 11 mbps greedy station is approx 6 mbps. What is per-STA throughput when two 11 mbps greedy stations compete?

� Answer 1:� Approx 3 mbps (easy ☺)

� Question 2:� Assume that throughput measured for a single 2 mbps greedy station is approx 1.7 mbps.

What is per-STA throughput when two 2 mbps greedy stations compete?� Answer 2:

� Approx 0.85 mbps (easy ☺)

� Question 3:� What is per-STA throughput when one 11 mbps greedy station compete with one 2 mbps

greedy station?� Answer 3:

� ...

Giuseppe Bianchi

Understanding Answers 1&2Understanding Answers 1&2(neclect collision (neclect collision –– indeed rare indeed rare –– just slightly reduce computed value)just slightly reduce computed value)

STA 1 SIFS

ACKDIFS

backoff

Cycle time

STA 2 SIFS

ACKDIFS

Frozen backoff

� Data Rate = 11 mbps; ACK rate = 1 mbps

� Payload = 1500 bytes

][]2[]1[81500

][][

]2[]1[backoffEDIFSACKSIFSTDIFSACKSIFSTtimecycleE

payloadEThrThr

MPDUMPDU ++++++++×===

MbpsThr

BackoffE

DIFSSIFS

T

T

ACK

MPDU

3.3310)50304101303(2

81500

310202

31][

50;10

3041/148192

130311/)150028(8192

=++++×

×=

=×=

===⋅+=

≈+⋅+=

� Data Rate = 2 mbps; ACK rate = 1 mbps

� Payload = 1500 bytes

MbpsThr

BackoffE

DIFSSIFS

T

T

ACK

MPDU

88.0310)50304106304(2

81500

310202

31][

50;10

3041/148192

63042/)150028(8192

=++++×

×=

=×=

===⋅+=

≈+⋅+=

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Giuseppe Bianchi

Emerging “problem”: Emerging “problem”: longlong--term fairness!term fairness!

� If you have understood the previous example, you easily realize that

�802.11 provides FAIR access to stations�in terms of EQUAL NUMBER of

transmission opportunities in the long term!

�But this is INDEPENDENT OF transmission speed!

STA1 STA2 STA1 STA1STA2 STA2

Giuseppe Bianchi

Computing answer 3Computing answer 3

STA (2mbps) SIFS

ACKDIFS

Cycle time

STA 11SIFS

ACKDIFS

Frozen backoff

RESULT: SAME THROUGHPUT (in the long term)!!

!!!!!!39.1310)5030410(213036304

81500

][]2[]1[81500

][

][]2[]1[

Mbps

backoffEDIFSACKSIFSTDIFSACKSIFST

timecycleE

payloadEThrThr

MPDUMPDU

=+++++

×=

=++++++++

×=

===

DRAMATIC CONSEQUENCE: throughput is limited bySTA with slowest rate (lower that the maximum throughput achievable by the slow station)!!

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Giuseppe Bianchi

Performance anomaly into actionPerformance anomaly into action

Why the network is soooo slow today? We’re so Close, we have a 54 mbps and“excellent” channel, and we getLess than 1 mbps …

Hahahahahah!!Poor channel, Rate-fallbacked @ 1mbps ☺

Giuseppe Bianchi

EDCA operationEDCA operation

See details in: See details in: G. Bianchi, I Tinnirello, and L. ScaliaG. Bianchi, I Tinnirello, and L. ScaliaIEEE NETWORK Magazine July/Aug. 2005IEEE NETWORK Magazine July/Aug. 2005

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Giuseppe Bianchi

Multiple queuesMultiple queues

AC Best-effort

AC Best-effort

AC Back-ground

AC Back-ground

ACVideoAC

VideoAC

VoiceAC

VoiceVirtual collision HandlerVirtual collision Handler

AC Best-effort

AC Best-effort

AC Back-ground

AC Back-ground

ACVideo

ACVideo

ACVoice

ACVoice

Virtual collision HandlerVirtual collision Handler

Wireless Channel

AP�4 “Access Categories”

�Mapping the 8 priority levels provided by 802.1p

�Different parameters�Independently

operated�Collide (virtually) each other!

Giuseppe Bianchi

Differentiation methods in IEEE Differentiation methods in IEEE 802.11e EDCA802.11e EDCA

� Varying time to wait before channel access� Different size of AIFS (arbitrary inter frame space)

� Varying the size of contention windows� Different size of CWmin and CWmax

� Varying the amount of channel accessible time� Different duration of TXOP

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Giuseppe Bianchi

TXOP differentiationTXOP differentiation

�Effective since it changes the holding time of the channel for each station

�Does not affect collisions

Giuseppe Bianchi

CWmin differentiationCWmin differentiation

� Operates by changing the long-term fairness ratio� The sharing of resources

is inversly proposional to the employed CWmin value�A station with

CWmin/4 will have 4 transmission opportunities versus 1, in average

� Problem: small CWs increase collision level!

Large N = large amount of collisions = = less effective differentiation = penalty in overall thr

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Giuseppe Bianchi

AIFS differentiationAIFS differentiation

“Protected” slots for green

Light traffic

“Protected” slots for green

Heavy traffic

% of protected slots INCREASES