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Politecnico di MilanoFacoltà di Ingegneria dell’Informazione

WIRELESS INTERNET

A. Redondi

Contact details

o Alessandro E. C. Redondin Assistant Professorn [email protected] Office: DEIB, 3rd floor, room 329n Tel: 02 2399 3403n Office hours: Monday, 10-11:30

A. Redondi: Wireless Internet 2

Course organization

o Wireless Internet (5 CFU)n Subpart of ‘Wireless Networks (10 CFU)’n Runs in parallel with Mobile Radio

Networks (5 CFU, Prof. Capone)

o Weekly schedulen Wednesday, room E.G.2, 8:30 – 10:00n Thursday, room E.G.4, 10:30 – 12:00


Class material and website

o Slides, papers and other material will be updated on the course web page:

http://www.antlab.polimi.it/ale-teaching/wireless-internet


Course program

o Technologies for wireless networksn WiFi(*), WiMax, Bluetooth(*)

o Network and transport layers for wireless networksn Mobile IP, TCP over wireless

o Ad hoc networksn Routing algorithms: DSR, AODV(*)

o Multiple access to radio channels

* lab activities / flipped classroomA. Redondi: Wireless Internet 5

Flipped classroom

o Some topics will be taught in flipped mode:n Students study available material on

their ownn Class time is used for practical activities

on the subjecto Examples:

n Active / passive wi-fi scanningn Bluetooth ranging and localization


Practical activities

o Practical activities using software and small code examples (mostly Python)

o Examples will be (mostly) based on Linux: it’s good to have a laptop with Linux (Ubuntu) installed if you want to try the examples

o Mac/Windows users may have troubles sometimes


Final Exam

o Written exam with open questions and exercises on the topics seen during lectures

o (Optional) Small project (+ 4 points)



WI-1Wireless Local Area Networks (WLAN)

Wireless InternetProf. Alessandro Redondi

Spectrum allocation



Spectrum allocation

o Spectrum is a scarce resourcen Regulation is fundamentaln Access priority to “dedicated”

applications (military, medical, etc..)n Most of the bandwidth are licensed (a fee

must be paid to national authorities)o Spectrum usage is regulated at

international level by:n Federal Communications Commission

(FCC) in Nord American European Telecommunications Standard

Institute (ETSI) in Europe


Non licensed bandwidths


Industrial Scientific and Medical (lSM) bandwidths

o Non licensed spectrum portions allocated around 900 MHz and 2.4 GHz (80 MHz band at 2.40÷2.48 GHz) for individual users communicationsn 2.4 GHz band is available “worldwide”n FCC allocates both 900 MHz and 2.4 GHz

bandwidthsn ETSI allocates only 2.4 GHz band (900 MHz

band in Europe is used for cellular systems like the GSM)

o Low cost o High interference


Usage rules of the ISM band

o Use of Spread Spectrum techniques (no longer required)

o Tight limits on the maximum transmitted powern Nord America: 1* W both at 900 MHz and

2.4 GHzn Europe (ERC/DEC/(01)07): 100* mW at

2.4 GHzo Strong limitations also on out of band

emissions* Limit on the EIRP


Bandwidths around 5 GHz

o In Europe ERC/DEC/(99)/23:n band at 5.2 (5.15-5.35) GHz for the

HiperLan systemn band at 5.4 (5.47-5.725) GHz for

HiperLan IIo In Nord America

n Banda UNII (Unlicensed National Information Infrastructure) 300 MHz between 5.2 and 5.8 GHz with quite loose constraints

o Limits on maximum power only


Pros/Cons bands at 5 GHz

o Few systems use the 5 GHz bandsn Lower interferencen Higher availabilityn Higher nominal transmission speed

o High frequency carriern Higher attenuation due to free space propagationn More transmission power requiredn Obstacles are more opaque n At the same transmission power, radio range is

shorter with respect to 2.4 GHz systemsn More Aps are required for covering an area

(approx 1.5 times)


UNII Band

o 300 MHz divided into three sub-bands of 100 MHz eachn “Low” 5.15-5.25 GHz, max power 50* mWn “Middle” 5.25 - 5.35 GHz , max power 250*

mWn “High” 5.725 – 5.825 GHz, max power 1* W

o Usage of the sub-bands:n Low/Middle: indoorn High: outdoor

* Limit on EIRP


5 GHz bands in Europe –Decision ECC/DEC/(04)08

o 802.11a was not allowed in Europao A variant known as 802.11h is

allowed; it has additional functionalities for:n Transmission Power Control (TPC)n Dynamic Frequency Selection (DFS)

o In detail:n 5.15 – 5.35 GHz: indoor use with max

power 200* mWn 5.47 – 5.725 GHz: indoor/outdoor use

with max power 1* W* limit on EIRP


Overview of Wireless networks

10 feet 100 feet 1 mile 10 miles

100 kbps

1 Mbps

10 Mbps

100 Mbps

3G Wireless~ 2GHz

BlueTooth2.4GHz

802.11a/g5.5GHz Unlicensed

802.11b2.4GHz Unlicensed

Peak Data Rate

Range2 mph 10 mph 30 mph 60 mph

$ 500,000

$ 1000

$ 100

$ 500

$ 100

$ 10

$/Cell $/SubHigh performance/price

High ubiquity and mobility

Mobile Speed


WLANs basicsMotivation and historyStandardizationCertification programs WiFiTM


IEEE 802.11 standard - history o Most of the wired local area networks are based on

the Ethernet technology (standardized by IEEE in 802.3)

o Even if it is no longer used, there is a medium access control part for the sharing of a common bus

o The IDEA for wireless LANs was that of replicating the same approach in the scenario of radio communications in local areas


Standardizationo WLAN standards are issued by the IEEE under

the 802 LAN/MAN standards committeeo The working group in charge of WLANs is 802.11

http://grouper.ieee.org/groups/802/11/

ApplicationPresentation

SessionTransportNetworkData Link

Physical

ISOOSI7-layermodel

Logical Link ControlMedium Access (MAC)

Physical (PHY)

IEEE 802standards

IEEE 802.11 Revisions

802.11kRRM

802.11rFast Roam

802.11a 54 Mbps

5GHz

802.11b11 Mbps2.4GHz

802.11dIntl roaming

802.11vNetwork

Management

802.11sMesh

802.11uWIEN

802.11y3650-3700MHz

ContentionBased

Protocol

802.11nHigh

Throughput(>100 Mbps)

802.11wManagement

Frame Security

802.11zTDLS

802.11pWAVE

802.11-1999

PHY

MAC

802.11-2012

802.11-2007

802.11aaVideo Transport

802.11aeQoS Mgt Frames

802.11ah<1GHz

802.11acVery High

Throughput6Gbps @ 5GHz

802.11aiFILS

802.11adVery High

Throughput6Gbps @ 60GHz

802.11afTV Whitespace

802.11-2003

802.11g54 Mbps2.4GHz

802.11eQoS

802.11iSecurity

802.11hDFS & TPC

802.11jJP bands



IEEE 802.11- Milestones

o Second half of the 80sn Proprietary technologies for LAN wireless

interconnections (mainly in Nord America).n Operation in the 900 MHz band

o 1991: IEEE starts standardizationn Strong push from manufacturers (Aironet)

o 1997: approval of first 802.11 standardn 802.3 LAN emulationn 3 physical layers at 1 and 2 Mb/s

o FHSS – Frequency Hopping Spread Spectrumo DSSS – Direct Sequence Spread Spectrumo Infrared


IEEE 802.11- Milestoneso 1999: two new physical layers approved

n 802.11a from 6 to 54 Mb/s in the 5GHz bandn 802.11b from 5.5 to 11Mb/s in the 2.4GHz band

o 2003:n 802.11g (OFDM in the 2.4GHz band)n 802.11F (Inter Access Point Protocol)n 802.11h (radio resource management, channel

selection e power control)o 2004:

n 802.11i (network security)o 2005:

n 802.11e (New MAC with QoS support)o 2009:

n 802.11n (High Rate)


Recent Task Groups

o 802.11p: inter-vehicular communications (MAC/physical) –Wireless Access in Vehicular Environment (WAVE)

o 802.11s: mesh (Routing)o 802.11ac: very high rate up to 1 Gb/s

with dense modulation (256 QAM), MIMO (up to 8 spatial streams), wider channels (up to 80 and 160 MHz) –2014.

o WLAN Timeline


802.11 History

860 Kbps

900 MHz

1 and 2 Mbps

2.4 GHz

Proprietary

� 802.11 � 802.11a,b

� 802.11g

1988 1990 1992 1994 1996 1998 2000 2002

2.4 GHz

11 and 54 Mbps Up to 600 Mbps

Standards-based5 GHz

� IEEE 802.11Begins Drafting

2004 2008 2010

� 802.11n � 802.11i

� 802.11e


Wireless Ethernet Compatibility Alliance (WECA)

o Members: Apple, Broadcom, Cisco, Dell, Huawei, Intel, LG, Microsoft, Motorola, Nokia, Qualcomm, Samsung, Sony, Texas Instr., many others …

o Mission:n Guarantee interoperability among products

based on 802.11 technologyn Trademark Wi-Fi™ (Wireless Fidelity)

certifies 802.11 products http://www.wi-fi.org

n Promotion of Wi-Fi™ as global standard n Support of roaming

http://www.wifizone.org


Wi-Fi Certificationso Radio interfaces:

n 802.11a (2000)n 802.11b (2000)n 802.11g (2003)n 802.11n (2009)

o Security:n WiFi Protected Access (WPA), 2003n WiFi Protected Access 2 (WPA2), 2004

o QoS:n WiFi MultiMedia (WMM), 2004

o Power Saven WMM Power Save


IEEE 802.11 standard

Architecture Physical layerMAC layer


IEEE 802.11 Overview

o Requirementsn Single MAC able to support different physical

layersn Robustness to interference (internal and external)n Robustness to hidden terminal problem

o Standard (legacy 1997) specifiesn MAC sublayer n MAC management protocols and servicesn Physical (PHY) layers

o IR o FHSSo DSSS


Componentso Station (STA)o Access Point (AP)

n Functionalities of bridging wired/wirelesso BSS - Basic Service Set

n Independent BSS (IBSS): ad hoc architecturen Infrastructure BSS: infrastructure based access

o ESS - Extended Service Setn Set of Infrastructure BSS.n Set of access points interconnected by a:

o DS – Distribution System (not explicitly defined in the standard)n Wiredn Wireless (WDS)

Basic Service Set (BSS)

o Set of stations controlled by the same “Coordination Function” (logical function that manages the access to a shared radio channel)

o Similar to the concept of cell in mobile radio networks

o Two different types of BSS:n Infrastructure BSSn Independent BSS (IBSS)


Infrastructure BSS

BSS

Centralized interconnection


Independent Basic Service Set (IBSS)

IBSSA. Redondi: Wireless Internet 35

Distributed ad hoc mode

Extended Service Set (ESS)

BSS

BSS

Distribution System

Wired DSLayer 2 connectivity among different BSS


Extended Service Set (ESS)


BSS

BSS

WirelessDistribution

System (WDS)


Network services

o We have two categories:n Station Services

specific for the wireless interface

n Distribution Servicesspecific for the distribution system

Servizio TipoDistribution DSIntegration DSAssociation DSReassociation DSDisassociation DSAuthentication STDeauthentication STPrivacy STMSDU delivery ST


Network servicesService ST or DS DescriptionDistribution DS Frame delivery to destination in

Infrastructure modeIntegration DS Frame delivery outside the WN(Re/Dis) Association DS Establish (change/remove) the

AP which serves as gateway(De) Authentication ST Establish station identity or

terminate authenticationConfidentiality ST Protect against eavesdroppingMSDU Delivery ST Delivers data to final recipientTransmit Power Control ST Reduces interferenceDynamic Frequency Selection ST Avoids interfering with radar

operation


Distribution System

o Association procedure is equivalent to “plugging the ethernet network cable into the wall”

o A STA is associated to an AP onlyo An ESS is a layer 2 network, and therefore it is an

IP sub-network with its own addressing space

CBA

RAP2AP1


Distribution System

o The Access Point acts as a bridge (layer-2 switch)o It manages association tables that uses for the bridging

process o For example, ethernet frames received from the DS that

contain addresses of wireless STAs are forwarded to the wireless interface once transformed into 801.11 frames

Bridge

DS

AP

StationA

StationB

StationC

STA


Distribution System

o How can an IP packet go from router R to destination station?

CBA

R

AP2AP1


Distribution System

o What does it happen when the station moves to another BSS?

A

R

AP2AP1

A


Distribution System

A

R

AP2AP1

A


802.11Medium Access Control (MAC)

Channel accessError controlAddressing


Functions and services of the MAC layer

o Channel accesso Error controlo Fragmentation and reassemblyo Power savingo Addressingo Framing


Access to the physical medium

o Access to the channel is regulated by means of logical functions (coordination functions)

o In 802.11 there are two standard coordination functions:n Distributed Coordination Function (DCF)

o Similar to Etherneto Based on CSMA with backoff

n Point Coordination Function (PCF)o “collision free” approacho Based on “poll-response” paradigm


Error Control

o Error control is crucial in a “noisy” channel

o It is defined only for unicast transmissions (broadcast transmissions are unreliable)

o Based on positive acknowledge for each frame (“stop ‘n wait”)

o Based on timers

Frame

ACK

Is there ACK in Ethernet? Why?


Interframe spacing

o 802.11 standard defines several time intervals that regulate the access to the channel

o The basic channel access scheme is based on carrier sensing

Previous Frame


Interframe spacing

o Short Inter Frame Spacing (SIFS): n High priority transmissions can start after a SIFS

after previous transmissiono PCF Inter Frame Spacing (PIFS):

n Minimum time channel must be free before accessing it with the PCF mode

o DCF Inter Frame Spacing (DIFS):n This is the time before starting a transmission in

DCF modeo Extended Inter Frame Spacing (EIFS):

n Used after a transmission in case it cannot be decoded (like e.g. after a collision)


DCF Access Mode

o DCF allows the coordination in the access of stations without a central controller

o It can be used both in IBSS and in infrastructure BSS

o It is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)n Before starting a transmission a station listen to the

channel for a DIFS; if:o Chanel is free: the station transmitso Channel is busy: the station waits and start a

backoff procedureo Waiting time is measured in “slots” (time tick)


DCF Access Mode


Collision Avoidance with Backoff

o If channel is busy, each station willing to transmit waits a number of slots equal to DIFS + a random number between 1 and CW (Congestion Window)

o If during backoff the channel becomes busy, backoff counting stops and it is resumed only when channel becomes free again

o If two or more consecutive frames must be transmitted, backoff is used even if channel is free

DIFS DIFS

backoff

backoff Remaining backoff

Station A

Station B

Station C


Backoff algorithm – CW

o The number of backoffslots is randomly selected in the interval [0, CW]

o The value of CW is set according to the following rules:n After a non successful

transmission CW := 2 (CW+1) – 1 (up to CWmax=1023 slots)

n After a correct transmission CW:=CWmin=31

63127

255

511

1023

Initial attemptFirst retransmission

Second retransmissionThird retransmission

Fourth retransmissionFifth retransmission

31


Error control in DCF

o Transmitting station can recover non successful transmissions with retransmission

o Error control is based on “positive acknowledgement” messagesn Each unicast frame must be acknowledgedn If acknowledge message is not received, frame

is retransmittedn There is a maximum number of retransmissions

per frameo Retry Counters

n Short Retry counter (for short frames)n Long Retry counter (for long frames)


Error control in DCF

o SIFS < DIFS, therefore ACK has priority over data frames

Ack

Data

Next MPDU

Src

Dest

Other

Contention Window

Defer Access Backoff after Defer

DIFS

SIFS

DIFS


Hidden Terminal problem

o Station A is hidden to station Co Collision can occur at a common receivero Collision can be persistent

collision

A

BC


Solution to Hidden Terminal problem

o 802.11 standard add physical carrier sensing a “logical carrier sensing”

o It is based on control overhead in the frames in which a Network Allocation Vector(NAV) is encoded

o The NAV indicates the duration of the ongoing transmission on the channel

o The stations that receive frames, refrain from transmitting for the time indicated in the NAV even if the physical carrier sense indicates the channel as free


Virtual Carrier Sense

o Ingredientsn Control frames (Request To Send, Clear

To Send)n NAV

source

destination

neighbors

RTS

DIFS

CTS

SIFS SIFS

DATA

SIFS

ACK

NAV (RTS)

NAV (CTS) Random Backoff

source

destination

neighbors

RTS

DIFS

CTS

SIFS SIFS

DATA

SIFS

ACK

NAV (RTS)

NAV (CTS) Random Backoff


Virtual Carrier Sense


Virtual CS and the Hidden Terminal problem

RTS

CTSCTS

C

B

A

o Station C receives the CTS from B and does not access the channel for the duration of the A-to-B transmission

o The hidden terminal problem is solved in most of the common cases


Drawbacks of the Virtual CS (1)

o The virtual CS creates the so called “exposed terminal” problem

o Resource reuse is limited

o Possible solutionsn Intelligent Scheduling n Frequency planning

C

A

BC

D

CTS

RTS


Drawbacks of the Virtual CS (2)

o System capacity reduction (Overhead due to control frames exchange)

o NAV efficiency depends on:n Channel characteristicsn Size of the data frames

o 802.11 standard defines a threshold (RTS_Threshold) on the size (D) of data framesn If D < RTSThreshold the NAV is not used

(transmission is not protected)n If D > RTSThreshold the NAV is used

(transmission is protected)


Point Coordination Function(optional)

Contention free mechanism Real-time traffic support


PCF (1)

o Channel access is managed by a “point coordinator” implemented in the AP

o PCF works only in centralized architectures (infrastructure BSS)

o Associated stations can transmit data frames after explicit grant from the “point coordinator”

o Similar to “token based” access mechanisms


PCF (2)

o Time is divided into periods governed by the DCF (contention based) and by PCF (contention free) that alternate

o Timing of the super frame is provided by the beacon frames transmitted by the AP

Super FramePolling (PCF) CSMA-CA (DCF)


PCF access procedure

o At the beginning of a Contention Free Period (CFP), the AP sends a beacon frame with the indication of the maximum duration of the CFP (CFPMaxDuration)

o All stations receive the beacon and set the NAV for a time equal to CFPMaxDuration (DCF inhibited)

o In a CFP transmissions follow the POLL/RESPONSE mechanism (with piggybacking)

NAV

Beacon Poll ST1

Frame from ST1CF ack

Poll ST 2ack ST 1

CFend

Frame from ST2CF ack

CFP

SIFS

SIFS

SIFS

SIFS

SIFS

PC

ST

Altre ST


CFP duration

o In case the contention on the channel is prolonged, the start of CFP can be delayed and its duration reduced

o The AP can stop the CFP (CF-End Frame)

frame

ACK

frame

ACK

Beacon

CFPMaxDuration

Expected start of the CFPActual start

SIFS

DIFS

SIFS


Comments on PCF

o PCF is not commonly used due to its complex management and ineffectiveness in managing real-time services n No limitation to transmissions durationn Delays in beacon frame transmissions

o Actually, no quality management mechanism can be implemented with PCF

o These problems motivated the standard evolution (802.11e)


Syntax of 802.11MAC

Frame formatAddressing


MAC Syntax

o The approach of the 802.11 standad is to define a common MAC able to support different physical layers

o With the goal of replicating the functionalities and services available with ethernet

o 802.11 MAC syntax is complexn High number of framesn Complex interpretation


Frame format: Frame Control Field

o Protocol Version: MAC version (legacy or 802.11e)

FrameControl

DurationID Addr 1 Addr 2 Addr 3 Addr 4Sequence

Control CRCFrameBody

2 2 6 6 6 62 0-2312 4

802.11 MAC Header

Bytes:

ProtocolVersion Type SubType To

DS Retry PwrMgt

MoreData WEP Rsvd

Frame Control Field

Bits: 2 2 4 1 1 1 1 1 1 1 1

DSFrom More

Frag


Type e Subtype

o The combination of these two fields indicate the frame typen Data (type=10)n Control (type=01)n Management (type=00)

Subtype bit

Frame type

0000 Association request

1000 Beacon

1011 Authentiction

Subtype bit

Frame type

1011 RTS

1100 CTS

1101 ACK

Subtype bit

Frame type

0000 DATA

0001 DATA+CF ack

0010 Data+CF poll

management control data

Duration Field (16 bit)

o Three possibilities:n NAV (last bit = 0): represents number of

microseconds that the medium is expected to remain busy

n CFP (00…0001 = 32768): used for stations who didn’t receive the beacon to set the NAV to a large value to avoid interfering with CF transmission

n PS-Poll frames: (last bits = 11) stations waking up from sleep try to retrieve buffered frames from AP



Addressingo Destination Address (DA): Address of the final

destinationo Source Address (SA): original source of the

frameo Receiver Address (RA): address of the receiving

wireless interfaceo Transmitter Address (TA): address of the

transmitting wireless interfaceso Basic Service Set ID (BSSID): BSS address

n Infrastructure BSS: MAC address of the APn IBSS: pseudorandom number


Addressing

o DA: Destination Addresso SA: Source Addresso TA: Transmitter Addresso RA: Receiver Address

Tipo di TX

ToDS FromDS Address 1

Address 2

Address 3

Address 4

IBSS 0 0 DA SA BSSID Not used

TO APInfra.

1 0 BSSID SA DA Not used

FROM APInfra.

0 1 DA BSSID SA Not used

WDS 1 1 RA TA DA SA


AddressingAddr1: S2Addr2: S1Addr3: BSSIDAddr4: empty

S1

S2

S1

AP1SVR

Addr1: S1Addr2: AP1 (BSSID)Addr3: SVRAddr4: empty

S1

AP1SVR

Addr1: AP1 (BSSID)Addr2: S1Addr3: SVRAddr4: empty

S1

AP1

Addr1: AP2 (BSSID)Addr2: AP1Addr3: SVRAddr4: S1

SVRAP2


Network ManagementScanningAuthenticationAssociationPower ManagementSynchronization

Network Management

o Having no wires is great, but:n Medium is unreliablen There are no physical boundaries and

malicious user can take advantage of this

n Power consumption is critical when batteries are used

o 802.11 management features try to reduce the effects of these problems



Management procedures

o Scanning: discovery of available BSSso Authentication: of stations within a

BSS o Association: establish association

STA/BSSo Power Management: for low energy

state modeso Synchronization: distributed

procedures for physical layer synchronization


Scanning

o The goal is to discover available BSSs to which to connect

o It doesn’t exist in wired networks (you need to use a map of the building to find your network plug J)

o Scanning procedure is performed by the STA

o There are two possible optionsn Passive moden Active mode


Passive Scanning

o STA listens to the available channels in sequence

o And stores all beacon frames received

AP1

AP2

AP3

ch1

ch2

ch3

BSS found:BSS 1, AP 1, ch1BSS 2, AP 2, ch2BSS 3, AP3, ch3


Active Scanning

o For each available channel, the station uses Probe Request frames for soliciting the transmission of the beacon

o Probe Request can be both unicast and broadcast

AP1

AP2

Probe Req.Probe Resp.

Probe Resp.

PRQ

PRS

PRS

A A

backoff

DIFS DIFSSIFS SIFS

ST

AP1

AP2


Scanning Report

o At the end of the scanning phase, the station creates a report with a entry for each of the BSSs found

o Each entry indicatesn BSSID, SSID, BSSType (Infra. Vs Indep.)n Frequency of the beaconn Synchronization informationn Physical layer informationn Periodicity of DTIM frames (power

management)


How to select the BSS?o BSS selection is not standardizedo It is implementation dependento Most of the devices allow at least a manual selection


How to select the BSS?o Is there an app for that?

o What about Wi-Fi off loading?


Association

o Equivalent to plugging the cable in the network

o With the association procedure: n The AP stores station info in the

association data basen The STA can start using the services of

the Distribution Systemo 802.11 standard does not allow

multiple associations


Association (2)

o Procedure is started by the STAo Exchange of management unicast frames

(with link-layer ack)o If process is successful, AP assigns to STA a

unique Association ID (AID)

AP1

1 - Association Request:

2 - Association ResponseAID

Reassociation

o When a STA moves between two APs:n STA monitors signal quality from several

APs in the same ESS (e.g. polimi)n If a better AP is detected, a reassociation

procedure is startedo The reassociation process is very

similar to the association processo The reassociation requests to the new

AP contains the address of the old AP


Reassociation (2)o Old AP and new AP must exchange

information to:n Verify that an old association did existn Forward any buffered frame on the old

AP to the new APn Old AP terminates association

o This exchange of information is known as the Inter Access Point Protocol (IAPP)

o An IAPP (802.11F) is standardized by IEEE, but seldom used


Power Management

o Active or Continuous aware moden Always on, always ready to send and

receiven Power Management field is 0

o Power Save Moden Transceiver is shut downn Power Management bit is 1n AP will buffer all traffic for that AP

o Different actions if infrastructure or ad-hoc


Power Management:Traffic Indication Map (TIM)

o When station associates to BSS, it gets and AID

o When station enters into power save mode, AP starts buffering traffic

o When traffic is buffered, the AID of the stations that have traffic in buffer appears in the Traffic Indication Map (TIM) in the beacon frame (bitwise)n TIM lists all stations that have traffic

waiting


Power Management:Traffic Indication Map (TIM)

o Beacons are transmitted at a regular intervaln Target beacon transmission time (TBTT)

o Station can sleep for more than one beaconn How often a station wakes up is the Listen

Intervalo When station wakes up and checks the

beacon, it checks for its AID bit in the TIMn If its AID bit is set to 1, station sends a PS-Poll

frame to APn AP will then start sending buffered traffic

o Includes the more data field-1 means more datao When more data is 0, AP has no more traffic

n Will also remove AID from TIM


Power Management: Example


TIM interval

t

medium

accesspoint

busy

D

busy busy busy

T T D

T TIM D DTIM

DTIM interval

BB

B broadcast/multicast

station

awake

p PS poll

p

d

d

d data transmissionto/from the station

Power Management: Delivery Traffic Indication Message (DTIM)

o Used to wake up stations for broadcast and multicast trafficn Special type of TIM

o DTIM interval for how often the DTIM is transmitted with the beaconn All stations will wake for DTIM frame


Power Management: Ad-hoc Traffic Indication Message (ATIM)

o With an IBSS, there is no central APn Power save will work differently

o Stations will tell other stations they are in power save by marking the field 1n Other stations will then buffer traffic

o Periodically all stations will wake to check in for buffered trafficn Announcement traffic indication message

window (ATIM window)n Station will send other station a ATIM

frame to notify of buffered frames and prevent them to going asleep



Synchronization

o Infrastructure BSS:n Synchronization is managed by the APn AP includes its local clock information in

the beacon and Probe Response frameso Independent BSS:

n STAs synchronize to the clock of the IBSS initiator that is in charge of transmitting beacon frames

Spectrum management

o 802.11a was originally developed as a standard for the US market only

o In EU the 5GHz frequency were already allocated to other uses

o Adaptation mechanisms were standardized in 802.11h

o Two main features:n Transmit Power Control (TPC)n Dynamic Frequency Selection (DFS)


Transmit Power Controlo Required by EU to ensure that 5GHz

radio stay within regulatory power limits

o Other PROs:n Reduce consumption and interference

o Basic idea:n Hold transmit power to the lowest

possible leveln Maximum power possible is regulated by

law in each country and specified in the Country element of beacon frames


Transmit Power Control (2)

o Both AP and STA may change their transmission power on a frame-by-frame basis

o STA use Action frame, requesting how well the AP is receiving from the STA

o STA can increase or decrease power according to the response received by AP


Dynamic Frequency Selection

o EU regulations require that stations avoid interfering with 5GHz radar systems

o Basic operation:n Association requests includes a

Supported Channel information elementn AP may reject the association based on

the content of the information element (e.g. if the STA supports too few channels)


Dynamic Frequency Selection

o Radar scan:n Stations search for a radar signal on the

channel before attempting any transmission

n In infrastructure mode, the AP decides the channel to use based on the supported channels of STA and STA measurements.

n Channel may be changed by the AP using the Channel Switch Announcement information element



Authentication

o In the wireless environment transmission medium is shared

o Potentially any station can access the network

o Need for verification of the identity of the stations

o Access control


Authentication

o Needed before Associationo Two approaches to authentication

n Open System Authentication(mandatory): no constraint on access

n Shared Key Authentication (optional): authentication based on a secret shared key

o Open System Authentication is not really authentication!


Open System Authentication

o The AP authenticate any STA that makes a request

o No controlo MAC Address Filtering can be used

n Painful to manage, MAC can be easily faked

AP1

Management Frame:From STA1Authentication Algorithm: 0

(Open System)Sequence Number: 1

Management Frame:From AP1Authentication Algorithm: 0

(Open System)Sequence Number: 2Status Code


Shared Key Authenticationo Two components:

n Mechanism of challenge/responsen Cryptography algorithm with private key (based on

WEP)(1)From STA,

Authentication: 1 (SKA)Sequence Number: 1

(2)Authentication: 2Sequence Number: 2Status Code: 0Challenge

(3)Authentication: 2Sequence Number: 3Challenge

(4)Authentication: 2Sequence Number: 4Status code

(1)

(2)

(3)(4)


Ciphering with WEP

o Ciphering algorithm with keystream based on RC4

o Keystream at 64 bits

IV (24 bit) WEP Key (40 bit)

Key Stream (64 bit)

XORPlain Text IVCRC Encrypted Text


WEP weaknesseso WEP reuses the same Key for all packets changing

only the IVn An AP with heavy traffic and 1500 bytes packets at

11 Mb/s consumes all the IV space in less than 5 hours

n A hacker is able to get in relatively short time messages coded with the same key and the same IV

n Possible statistical passive attacks and active attacks

o Vulnerabilities for integrity (CRC is linear and weak)n An attacker can easily change bits in the encrypted

packet and change corresponding bits in the CRCn The packet is valid for the receiver but has no sense


Security problems

o Authentication problemsn Only stations have to authenticate, not APsn The approach is vulnerable to attacks like man-

in-the-middle (a malicious AP can intercept authentication traffic)

o Privacy issuesn It has been shown that WEP can be violated in

relatively short time (Airsnort, WepCrack, etc..)

o Need for:n Robust authenticationn Advanced cryptography algorithms

802.11i


802.11io New security standard released in June

2004o Main characteristics:

n Authentication managed at upper layers (not at link layer)

n Introduction of new protocols/infrastructure for authentication

n Improvement of privacy and integrity mechanisms

o Wireless Protected Access (WPA1 and WPA2) from WiFi alliance

802.11i

o Authenticationn Protocol 802.1X

o Privacyn Temporary Key Integrity Protocol (TKIP)

o Based on RC4o Integrity check robust with Message

Integrity Check (MIC)o Key changes at each packet

n Counter Mode/CBC MAC Protocol (CCMP)o Based on AESo More robust than TKIP



Authentication 802.1X

o Based on the Extensible Authentication Protocol (EAP)

o Authentication entitiesn Supplicantn Authenticatorn Authentication Server

Networkresources

ControlledPort

UncontrolledPort

Supplicant Access Point

AuthenticationServer

Authenticator Extensible AuthenticationProtocol (EAP)


Extensible Authentication Protocol (EAP)

o Two-ways authentication is possibleo Include functionalities for key exchange

STA AP ASEAP Request

EAP Response IdentityAccess Request (EAP Request)

EAP Exchange

Accept/EAP-Success/Key Material

EAP-Success

Not in the standard 802.11iThe standard de facto is RADIUS

Key exchange


Step 1: Use RADIUS to push PMK from AS to AP

Step 2: Use PMK and 4-Way Handshake to derive, bind, and verify PTK

Step 3: Use Group Key Handshake to send GTK from AP to STA

ASAPSTA

PMK: Pairwise Master KeyPTK: Pairwise Transient KeyGTK: Group Transient Key


WPA certification programs

o WPA allows two different types of EAP n LightWeigth EAP (LEAP): CISCO proprietary, based

on passwordn EAP Transport Layer Security (EAP-TLS): based on

certificates n EAP Tunneled TLS (EAP-TTLS) n Protected EAP (PEAP)

Hybrid solutionsPassword + certficate


MAC Evolution for QoS support

802.11e


Issues with PCF

o No mechanism for traffic flow differentiationn Single packet queue at MAC layer

o Possible delays in super frame timingn Beacon transmission starting super frame

and CF period can be delayedo No control on transmissions

n A station that receives a poll frame from PC can transmit multiple frames or an arbitrary length frame


802.11e

o Flow differentiationn Each device has 4 queues for 4 traffic categories

o Introduction of Transmission Opportunities(TXOP)n At each transmission a maximum time is

assigned for transmission completiono Direct transmission among stations also in

infrastructure BSSo Usage of the Block ACK technique (single

ACK for “trains” of frames)


802.11e – Channel access

o Hybrid Coordination Function (HCF)o Two modes

n Contention based (EDCA, Enhanced Distributed Channel Access)

n Controlled access (HCCA, HCF Controlled Channel Access)


EDCA – Contention based access

o EDCA defines 4 Access Categories (AC) that correspond to 4 traffic typesn AC_VO: vocen AC_VI: videon AC_BE: best effortn AC_BK: background

o Each AC is characterized by different backoff parametersn AIFS[AC]: Interframe spacesn CWMin[AC]: minimum backoff windown CWMax[AC]: maximum backoff windown TXOPlimit[AC]: maximum transmission duration


Access Categories

o Multiple backoff entities in the same station.


EDCA example


HCCA

o HCCA basic approach is similar to PCFo The Hybrid Controller (HC) can decide

to poll a station transmitting a QoS CF-Poll frame or a data frame

o The HC can access to the channel after a PIFS, without backoff (high priority)

o Differences wrt PCF:n HC specifies a TXOPLimit for each traffic

categoryn Possible hybrid operation with alternate

contention and contention-free periods


HCCA example

o The HC can poll a station also during a contention period

Further improvements

o Block ACK: cumulative ACK for groups of consecutive frames (no longer only “stop ‘n wait”)n Overhead reductionn It works only with good quality channels

o Direct Link Protocol (DLP): protocol for the direct communication between STAs in infrastructure architecturesn Increased capacityn Complex operation (synchronization,

power saving, etc.)


Enhanced Power Save mode

o Although 802.11 PSM significantly alleviates the power consumption problem, a dependency between the downlink delay (AP to station) and the listen interval is introduced.

o Consequently, some listen interval values can result in downlink delays that are unacceptable for certain QoS-sensitive applications (like VoIP).


Automatic Power Save Delivery (APSD)

o IEEE 802.11e defines an enhancement of the 802.11 power save mode, Automatic Power Save Delivery (APSD)

o Two APSD modes are available: unscheduled APSD (U-APSD) and scheduled APSD (S-APSD)


U-APSD

o The main novel idea of U-APSD is to proactively poll the AP to request frames buffered instead of waiting for notification in the beacon frames

o Data frames or poll frames in the uplink direction can be used as triggers for starting a Service Period (SP)

o In U-APSD each Access Category can be configured separately

o A SP is ended by the reception of a frame with the End Of Service Period Flag (EOSP) set

o During a SP one or more data frames can be delivered up to a maximum value


U-APSD example


Example of U-APSD operation. U-APSD configuration: AC VO and AC VI both trigger- and delivery-enabled. AC BE and AC BK neither delivery- nor trigger-enabled (i.e., use legacy 802.11 power save mode).

Source: Daniel Camps, PhD thesis, UPC

S-APSD

o The main novel idea of S-APSD is that the AP can schedule the wake up time to receive frames buffered

o Wake period schedule is based on the Service Start Time (SST) and Service Interval (SI) parameters

o SST and SI are defined for each access category for which the S-APSD is activated


S-APSD example


S-APSD example of operation. S-APSD configuration: VoIP traffic stream in the downlink configured to use S-APSD with HCCA access mode, AC VI traffic in the downlink uses S-APSD with EDCA access mode, AC BE and AC BK configured to use legacy 802.11 power save mode.

Source: Daniel Camps, PhD thesis, UPC


Physical layerFrequency Hopping Spread Spectrum(FHSS)Direct Sequence Spread Spectrum (DSSS)


The first 802.11 physical layer

o Shared radio channelo Able to operate in a unlicenced

spectrumo Highly varying interference

o Need for a physical layer robust to interference from other systems


Protocol Stack

o Physical Layer Convergence Procedure (PLCP): layer for adapting MAC frames to the physical layer transmission

o Physical Medium Dependent (PMD): transmission layer (modulation and physical layer signaling)

MAC

Physical Layer Convergence Procedure

Physical Medium Dependent

MAC LayerManagement

Entity

PHY Layer Management

Entity


First standard PHY

o Multiple physical layers (historical/political motivations)

o Three transmission modes:n Infrared (IR, obsolete)n Frequency Hopping Spread Spectrum 1-

2Mb/s (FHSS, used in special environments)

n Direct Sequence Spread Spectrum 1-2 Mb/s (DSSS, Wifi)


DSSS vs FHSSoBoth DSSS and FHSS have the goal of limiting

the impact of interference on the performance of the transmission system

oDSSS n Spreads signal energy on a larger bandwidth than

that of the original signaloFHSS divide band into sub-channels of 1MHz

eachn At every transmission, transmitter hops to a sub-

channel according to a predefined sequencen Different hopping sequences at stations are

orthogonal


DSSS

o Barker sequence for spread spectrum operation

BB nBnBspreadingspreading

bit

Codicedi spreading

chipbitS

DSSS DSSS (Direct (Direct Sequence Spread SpectrumSequence Spread Spectrum ))

bit

Codicedi spreading

chipbitS

bit

Codicedi spreading

chipbitS

DSSS DSSS (Direct (Direct Sequence Spread SpectrumSequence Spread Spectrum ))


DSSS

o Robust to interference peaks

Spreading

Despreading

signal

Interference

band

energy

band

energy


DSSS

o Spreading process is not used for multiplexing different signals like in CDMA

o All transmissions use the same spreading code (Barker sequence)


Physical channels (1)o The standard defines 14 channels spaced of

5 MHz starting form frequency 2.412 GHzo Not all channels are available worldwide

Country Channels availableUSA 1-11 (2.412-2.462GHz)Europe 1-11 (2.412-2.472GHz)Spain 10-11 (2.457-2.462 GHz)France 10-13 (2.457-2.472 GHz)Japan 14 (2.484 GHz)


Physical channels (2)

o Most of modulated signal energy in a 22MHz band

o It is not possible to use adjacent channel since they overlap11

MHz-11MHz

P

f

-30dBr

Canale 11 fCanale 6Canale 1


Modulationo Two types of modulations are defined

n Differential Phase Shift Keying (DPSK):o Data rate of 1 Mb/s, o 1 bit per symbol

n Differential Quadrature Phase Shift Keying (DQPSK):o Data rate of 2 Mb/s,o 2 bits per symbol

o For higher data ratesn Different modulation (HR/DSSS)n Different physical layer (802.11a/g)n Different TX-RX approaches (MIMO, 802.11n)


PLCP

o PLCP adds a further protection for interference and error controln Scramblingn Cyclic Redundancy Check

n PLCP structure varies in the different versions of the standard physical layers

Service

16

Sync SFD

bit 128

Length. HeaderCRC PLCP_SDU

8 168

Signal

16

1Mb/s DPSK 1Mb/s DPSK 1Mb/s DPSK o

2Mb/s QPSK


DSSS – ParametersParameter Value

Slot Duration

20us

SIFSDuration

10us

CW From 31 to 1023 slots

PreamblePLCP

144us

HeaderPLCP

48us

FrameMAC

From 4 to 8191 bytes


Physical layer evolutions802.11b HR/DSSS (standard since 1999)802.11a (standard since 1999)802.11g (standard since 2003)802.11n (standard since 2009)802.11ac (work in progress)


Physical layer evolutions

802.11g2.4 GHz – OFDM/CCK

54 Mbps

Proprietary� IEEE 802.11a/b

Ratified

802.11a5 GHz – OFDM

54 Mbps

802.11b2.4 GHz – CCK

11 Mbps

Jan’99 Jan’00 Jan’01 Jan’02 Jan’03 Jan’04


802.11b – HR/DSSS

o Modification to PMD: n New modulation systems for guaranteeing

a higher rate (up to 11Mb/s)o Modification to PLCP:

n New header and new preamble PLCPo Compatible with the legacy standard


PMD – Modulation CCK

o Modulation QPSK (Quadrature Phase Shift Keying) with spreading

o Transmission rate = 1.375 Msymbols/so Two data rate defined:

n 5.5 Mbit/s, 4 bits per symboln 11 Mb/s, 8 bits per symbol

Scrambler SplitterCode

Selector1.375Msps

DQPSKModulator

I

Q


New PLCP

Service

16

Sync SD

bit 128


8 168

Signal

16

1Mb/s DPSK 1Mb/s DPSK 1Mb/s DBPSK 2Mb/s QPSK5.5/11 Mb/s CCK

Service

16

Sync SD

bit 56


8 168

Signal

16

1Mb/s DPSK 2 Mb/s DPSK 2 Mb/s DQPSK 5.5 Mb/s CCK11 Mb/s CCK

Preamble Header PLCP_SDU


802.11b – Actual rate

Source: http://www.uninett.no/wlan/throughput.html


802.11a – The OFDM solution

o Motivation:n Solution to the level of congestion of the

2.4GHz bandn Need for higher data rates than 11 Mb/s

o Solutionn Use of the U-NII band (Unlicensed

National Information Infrastructure) around 5 GHz

n Use of OFDM modulation


Pros and Cons

o Prosn Higher data rates (up to 54 Mb/s)n Lower interference (less crowded band)

o Consn Lower coverage (worse propagation

conditions)n Higher energy consumptionn Not compatible with European rulesn Higher cost


OFDM – Basics

o OFDM coverts a bit flow at high rate into multiple flows at lower rate

o Different flows are multiplexed together on orthogonal carriers

o It allows efficient numerical techniques for modulation and demodulation (FFT/IFFT)

o Multi-carrier modulation schemeo Possible inference among adjacent

symbols


Spectrumo Symbol (in

transmission)

o Group of symbols in transmission

o More efficient w.r.t. classical FDM


OFDM

o Sub-carriersn Minimum unit in which spectrum is

dividedo OFDM symbol

n Symbol in transmission divided into N sub-carriers


Inter Symbol Interference

o Need for guard symbols OFDM (based on cyclic prefixes)

Delay Spread

IOSI

OS1 OS2 OS3

Transmission

Reception


Guard time

o Cyclic prefixes allow:n Preserving orthogonal carriersn Avoid ISI

o Guard time value depends on:n Maximum delay spread (4 times)

TosTg

Cyclic Prefix

OS 1 OS 2


OFDM in 802.11a

o Spectrum organized into 20MHz channels

o Each channel divided into 52 sub-carriers spaced of 0.3125MHz

o 48 data sub-carriers, 4 control sub-carriers

-26 2621-21 7-7 Carrier number

Carrier central frequency

-32 32


802.11a physical layer parameters

o Duration of symbol OFDM 4µso Guard time 0.8µso Useful symbol duration 3.2µs

o Interleaving, scrambling and coding used for protecting transmissions


802.11a transmitter

ModulatorInputBits Scrambler Coding Interleaver

OFDM Symbols syntetizer

IFFTDAC


Synchronization EqualizationFFT Demodulator

DeinterleaverDecodingDescrambler

ReceivedSamples

Data

802.11a receiver

Multiple Data Rates/Modes



PLCP 802.11a

Service

4

Rate Reserved Length. TailPLCP_SDU

1 166

Parity

12

BPSK, R=1/2 Coded based on selecteddata rate

Preamble12 symbols

Signal,1 symbol PLCP_SDU

Tail Pad

1

o Different with respect to PLCP 802.11 and 802.11b


802.11a - parametersParameter Value

Slot Duration

9us

SIFS Duration

16us

CW From 15 to 1023 slot

PreamblePLCP

16us

HeaderPLCP

4us

FrameMAC

From 4 to 4095 byte


802.11g

o Motivation:n Increasing data rate wrt 802.11b in the same ISM

2.5 GHz bandn Compatibility with 802.11b devices

o Background:n Two competing solutions proposed:

o PBCC, supported by Texas Instrumentso DSSS-OFDM, supported by Intersil

o Solutionn One mandatory physical layer (Extended Rate

Physical OFDM), basically the same of 802.11an Two alternative PHY optional (PBCC, DSSS-OFDM)


802.11g - characteristics

Parameter Value

Slot duration

9us o 20us

SIFSduration

10us (+6us of virtual extension)

CW From 15 to 1023 slot

PreamblePLCP

16us

HeaderPLCP

4us

FrameMAC

From 4 to 4095 byte


802.11g compatibilities

o 802.11g is able to detect 802.11b (short, long) and 802.11a preambles, carrier sensing is possible

o 802.11g uses 802.11b PHY in the exchange of RTS/CTS framesn Same data raten Same modulationn Same slot duration

o 802.11b devices are not able to receive 802.11g transmissions


Performance of different physical layers

o Throughput in Mb/s measured excluding overhead at MAC and physical layers

o Source: Broadcom

Distance 802.11b 802.11a 802.11gonly

802.11g/bRTS/CTS

802.11g/bSelf CTS

3m 5.8 24.7 24.7 11.8 14.7

15m 5.8 19.8 24.7 11.8 14.730m 5.8 12.4 19.8 10.6 12.7

45m 5.8 4.9 12.4 8 9.160m 3.7 0 4.9 4.1 4.2

75m 1.6 0 1.6 1.6 1.6

90m 0.9 0 0.9 0.9 0.9


Which standard to select?o Comparison parameter:

n Nominal data raten Rangen Capacity (number of available channels)n Costn Compatibility

Tecnology Rate Range Compatibility 802.11b

Capacity Costo

802.11b Medium High yes Low Low

802.11a High Low no High Medium

802.11g High High yes Medium Low


802.11n – High rate

o Standard since September 2009o Goal: reach very high data rateso Standardization approach:

n Modifications of the OFDM physical layern Modifications at MAC layer


How to increase the rate?

o Possible approaches:n Spatial multiplexingn Higher bandwidthn Higher modulation constellation sizen Higher code raten Lower guard time

Data Rate = 20M time samplessecond

channel spacing

⋅48 freq tones64 freq tonesguard band overhead

⋅6 coded bits

freq toneconstellation size

⋅3 info bits

4 coded bitscoding rate

⋅

64 freq tones80 times samples

guard interval overhead

= 54M info bits/second


802.11n parameters802.11a/g 802.11n Requirement Throughput

Scaling FactorChannel BW = 20MHzNumber of data subcarriers = 48

Channel BW = 20MHzNumber of data subcarriers = 48

Mandatory 1x

Channel BW = 40MHzNumber of data subcarriers = 108

Mandatory 2.25x

Number of Transmit Antennas = 1

Number of Transmit Antennas = 2

Mandatory 2x

Number of Transmit Antennas > 2

Optional (e.g. 3 and 4)

3x or 4x

Maximum Constellation Size = 64QAM

64-QAM Mandatory 1x

>64QAM (i.e. 128 or 256 QAM)

256QAM optional 1.16x (128-QAM)1.33x (256-QAM)

GI = 800nsTsymbol = 3200ns

GI / Tsymbol = 800ns/3200ns Mandatory 1x

GI / Tsymbol = 400ns/3200ns Mandatory 1.11x

Coding Rate 1/2, 2/3, 3/4 Mandatory 1x

7/8 Mandatory 1.167x


802.11n – Physical layer

o Multi-band PHY 2.4GHz, 5GHz and 4.9GHz (Japan)

o Main modifications:n MIMO – OFDM: space multiplexing of

different flowso 2 Antennas (mandatory)o 4 Antennas (optional)

n Wider channels:o 20MHz (mandatory)o 40MHz (optional)



o Other modifications:n Shorter guard times between OFDM

symbols (400ns mandatory in the 20MHz)

n Modulations up to 64QAMn Convolutional codingn Optimized coding for MIMO

o Data rate up to: !! 600Mb/s !!


802.11n – MAC layer

o QoS support: 802.11n standard incorporates 802.11e (new MAC)

o Additional functionalities like frame aggregation

o Extension of MAC Management entity for supporting advanced radio resource management functions

802.11ac802.11a 802.11b 802.11g 802.11n 802.11ac

Release Sep. 1999 Sep. 1999 June 2003 Oct. 2009 under development

Carrier freq. 5 GHz U-NII bands 2.4 GHz ISM band 2.4 GHz ISM band2.4 GHz ISM band, 5 GHz U-NII bands 5 GHz U-NII bands

Bandwidth 20 MHz 20 MHz 20 MHz 20 / 40 MHz 80 / 160 MHz

Data rate 6 to 54 Mbps 1 to 11 Mbps 6 to 54 Mbps

7.2 to 150 Mbps(up to 600 Mbps with 4 streams and 80 MHz channel)

500 Mbps(up to 8 Gbps with 8 streams and 160 MHz channel)

Access method OFDM FDMA, DS-CDMA OFDM, DSSS OFDM SDMA

Modulation BPSK, QPSK, 16 / 64QAM

DBPSK, DQPSK, BPSK, QPSK DBPSK, DQPSK, BPSK, QPSK,

16 / 64QAMBPSK, QPSK, 16 / 64 / 256 QAM

Coding

Forward error correction coding (convolutional; code rates 1/2, 2/3, 3/4)

11 chip Barker sequence, CCK, PBCC

CCK, PBCC Convolutional Coding, LDPC BCC, LDPC, STBC

MIMO stream 1 1 1 4 8


802.11aco Backwards compatible to 802.11a and

802.11n and coexistence with 11a and 11n

o Frequency band: 5 GHzo RF bandwidths: 20 MHz, 40 MHz,

80 MHz and 160 MHzo Modulation types: BPSK, QPSK, 16QAM, 64QAM,

256QAMo MIMO antenna

support: 2x2, 4x4, 8x8


802.11aco The 80MHz channel will consist of two adjacent, non-

overlapping 40MHz channels. o The 160MHz channels will be formed by two 80MHz

channels n adjacent (contiguous)n non-contiguous

140

136

132

128

124

120

116

112

108

104

100

6460565248444036IEEE channel #20 MHz40 MHz80 MHz

5170MHz

5330MHz

5490MHz

5710MHz

160 MHz



Mesh Networking

IEEE 802.11sCommercial solutions


Mesh Networking and 802.11

o Goalsn Extend the size of the Wi-Fi hot spot

through a mesh wireless infrastructuren Extend application scenarios of WLAN

technology to metropolitan networkso Solution

n Distributed infrastructuren Mesh networks of Infrastructure BSS

with APs connected through a wireless distribution systems


Example of Mesh Net

Mesh Point

Mesh Point

Mesh Point

Mesh Portal

Mesh Portal

Mesh AP

Mesh AP

STA STA STA STA STA

InternetInternet

Mesh Network

BSSBSS

Internet

The “true” Wireless Mesh Networks

o Mesh routers (MRs) manage routing automatically based on ad hoc network technologies

o Devices are flexible and self-configuring

o All or a subset of MRs act as access points (Wi-Fi hot spot)

o Some devices are the gateways to the Internet

meshgateway

meshrouter

meshclient

Access LinkBackhaulGateway Link

Advantages of WMNs

o Not only wireless access but also wireless infrastructure

o The only viable solution in most of the application scenarios

o Probably the best in all cases under the cost/flexibility perspective

WMNs deployment• Ease up

time-to-deployment

• Reduced CAPEX/OPEX


Application scenarios

o Residential access (competitor of WiMax)

o Officeso Public networks for internet access in

common spaces (usually outdoor)o Public safety networkso Military networks

Applications

o Large range of application scenarios where creating a wired infrastructure is not possible or cost effectiven Municipal wireless networksn Temporary networksn Industrial networksn Surveillance & Environmental monitoringn Traffic control and driver assistance

MuniWireless

o A new emerging application area of wireless networks

o Tremendous interest worldwideo Public utility services as

distinguishing featureo Local governments and authorities

as new actors for building up infrastructures and providing services

è Big citiesè Medium-small citiesè Solution to digital divide

Environmental Monitoring

o Integration of WMNs with sensor networks for remote sensing and control of large areas, buildings, bridges

o Support to communication of public security officers (police, fire dept, etc.) and rescue teams

Wireless MESH Network

Sensor & ActorNetwork

actorHuman agent

Multimediasensor

Wireless MESH Network

Sensor & ActorNetwork

Ad hoc network

Temporary networks

o Wireless connectivity for temporary eventsn Fairsn Expositionsn Sport eventsn Festivals

o Fast deployment/dismissal

o Easy configuration and management Power supply

with solar panels


Standardization

o The TG 802.11s had the goal of defining an Extended Service Set (ESS) for supporting broadcast/multicast and unicast service in multihop nets.

o Draft 3 approval: March 2009o Draft 12 approval: July 2011


802.11so Robust and efficient Routing:

n Mesh Topology Learning, n Routing and Forwarding

o Security:n Compatibility with 802.11x

o Flexibility of MAC layern Mesh Measurementn Mesh Discovery and Associationn Mesh Medium Access Coordinationn Support to QoS

o Transparent to upper layerso Compatible legacy devices


Protocol stack

IEEE802.11 a/b/g/j/n

Mesh Media Access Coordination Function

Layer 2 Mesh Routing and Forwarding

.11s Mesh Security

IEEE802.11 MAC

IEEE802.11P PHY

.11s Mesh Network

Measurement

IEEE802.11s Amendment

InternetworkingConfiguration/ Management

Interfaces

IEEE802.11 a/b/g/j/n

Mesh Media Access Coordination Function

Layer 2 Mesh Routing and Forwarding

.11s Mesh Security

IEEE802.11 MAC

IEEE802.11P PHY

.11s Mesh Network

Measurement

IEEE802.11s Amendment

InternetworkingConfiguration/ Management

Interfaces

o Modifications at MAC layero New routing layero Untouched physical layer

MAC functions

o Mesh Coordination Function (MCF), based on EDCA (802.11e)

o Optionally, MCF controlled channel access (MCCA)n Similar to HCCA, is a reservation-based

methodn STA reserve the medium for a certain

period of time (MCCAOP)n Mesh AP advertises MCCAOP via beacon,

including MCCAOP of neighboring AP


Discovery processo Stations in a Mesh BSS (MBSS) send

beacons and answer to probe requests

o The Mesh Profile is broadcastedn Mesh ID, identifies the MBSSn Mesh configuration element

o Path selection protocol usedo Path selection metric usedo Authentication protocol usedo Other information and capabilities

o Traditional beacons are independentA. Redondi: Wireless Internet 196

Peering

o Two stations “associate” to each others and become peer stations

o A mesh station can establish peering with multiple stations

o Management messages involved:n Mesh peering open framen Mesh peering confirm framen Mesh peering close frame

o Open frame contains stations capabilities and mesh ID


Peering (2)

o If stations capabilities and mesh ID are compliant to the mesh, a mesh peering confirm frame is returned

o The frame contains the ID of the local link created between the two peers and an AID which uniquely identifies the neighbor

o Process must be bidirectional n A offers, B confirm, B offers, A confirmn A offers, B offers, A confirm, B confirm


Peering (3)

o Peering can be terminated with a mesh peering close frame if:n When a station doesn’t hear the

neighbor for a certain configurable timen If the neighbor does not answer for a

certain amount of timesn It the neighbor mesh profile changes and

does not match the station’s one anymore (e.g., the neighbor stops to have a direct link to the DS)


802.11s MAC

o Modification to the data frame format


6-Address Scheme


802.11 STA to external STA


MeshAP

Meshpoint

Meshportal

Securityo Peering is a flexible process o There is the risk that a rogue station

would peer with a valid mesh oneo Authenticated Mesh Peering Exchange

(AMPE) is definedo Two possibilities:

n AMPE with 802.1X, relies on central authentication server (both mesh stations needs to be connected)

n AMPE with Simultaneous Authentication of Equals (common PMK)


Path Selection (Routing)

o Mesh stations using 802.11s must implement the Hybrid Wireless Mesh Protocol (HWMP) to find which path to take for a certain destination

o In most cases the destination of a 802.11 station is the DS, but may be any other MAC address reachable through the MBSS


HWMP

o Similar in goal to a routing protocol (not at the IP, but at the MAC layer)

o Combination of AODV and tree based protocol

o Nodes exchange messages for determining the best path:n Hop count: how many stations between

the local station and target destiationsn Metric: Airtime (combination of data rate

and bit error rate on a link)


HWMP - AODV

o Relies on Path Request (PREQ) and Path Reply (PREP) messages

o When a STA needs to discover a path for a destination it sends a PREQ to all neighboring mesh stations, with:n Originator and target(s) MAC addressn Sequence number, discovery ID, TTLn Hop count (starts 0, incremented by

every station)n Metric field


HWMP - AODVo A station receiving a PREQ forwards it

to its neighboring nodes, incrementing the hop count and updating the metric field (duplicates are eliminated)

o When the PREQ reaches the target, a PREP is produced and delivered back to the originator

o The process can be done dynamically (all vs all) but generates a lot of traffic


HWMP - AODV


A

C

B

D

PREQ 1, A:D, HC = 0

PREQ 1, A:D, HC = 0

HWMP - AODV


A

C

B

D

PREQ 1, A:D, HC = 1

PREQ 1, A:D, HC = 1

DEST A, HC = 1

DEST A, HC = 1

HWMP - AODV


A

C

B

D

DEST A, HC = 1

DEST A, HC = 1 DEST A, HC = 2

PREP 1, D:A, HC = 2

HWMP - AODV


A

C

B

D

DEST A, HC = 1


PREP 1, D:A, HC = 2

HWMP - AODV


A

C

B

D

DEST A, HC = 1


DEST D, HC = 2, VIA C

HWMP - treeo Root Mesh AP (generally mesh portal)

starts disseminating Root Announcement (RANN) messages containingn Root AP MAC addressn HWMP sequence numbern Interval and Time to Liven Hop count field and Metric field, modified

by each station forwarding the RANN


HWMP - tree

o Upon reception of a RANN, each station sends a PREQ to the root mesh STA via the station from which it received the RANN.

o The root mesh STA sends a PREP in response to each PREQ. Both forward and reverse path are created.


Applications

o OLPC (One Laptop Per Child)o Open802.11s



Solutions “Off the Shelves”

o Several companies produce mesh devices:n Motorola (MeshNetworksTM): MeshNetworks

Enabled Appliances (MEA)n Tropos Networks (802.11-compliant)n Nortel (802.11-compliant)

o All commercial solutions provide hardware and software (proprietary)

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