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Politecnico di MilanoFacoltà di Ingegneria dell’Informazione
WIRELESS INTERNET
A. Redondi
Contact details
o Alessandro E. C. Redondin Assistant Professorn alessandroenrico.redondi@polimi.itn 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
A. Redondi: Wireless Internet 3
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
A. Redondi: Wireless Internet 4
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
A. Redondi: Wireless Internet 6
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
A. Redondi: Wireless Internet 7
Final Exam
o Written exam with open questions and exercises on the topics seen during lectures
o (Optional) Small project (+ 4 points)
A. Redondi: Wireless Internet 8
Politecnico di MilanoFacoltà di Ingegneria dell’Informazione
WI-1Wireless Local Area Networks (WLAN)
Wireless InternetProf. Alessandro Redondi
Spectrum allocation
A. Redondi: Wireless Internet 10
A. Redondi: Wireless Internet 11
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
A. Redondi: Wireless Internet 12
Non licensed bandwidths
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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
A. Redondi: Wireless Internet 14
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
A. Redondi: Wireless Internet 15
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
A. Redondi: Wireless Internet 16
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)
A. Redondi: Wireless Internet 17
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
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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
A. Redondi: Wireless Internet 19
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
Politecnico di MilanoFacoltà di Ingegneria dell’Informazione
WLANs basicsMotivation and historyStandardizationCertification programs WiFiTM
A. Redondi: Wireless Internet 21
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
A. Redondi: Wireless Internet 22
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
A. Redondi: Wireless Internet 23
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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
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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)
A. Redondi: Wireless Internet 26
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
A. Redondi: Wireless Internet 27
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
A. Redondi: Wireless Internet 28
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
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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
Politecnico di MilanoFacoltà di Ingegneria dell’Informazione
IEEE 802.11 standard
Architecture Physical layerMAC layer
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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
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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)
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Infrastructure BSS
BSS
Centralized interconnection
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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
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Extended Service Set (ESS)
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BSS
BSS
WirelessDistribution
System (WDS)
A. Redondi: Wireless Internet 38
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
A. Redondi: Wireless Internet 39
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
A. Redondi: Wireless Internet 40
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
A. Redondi: Wireless Internet 41
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
A. Redondi: Wireless Internet 42
Distribution System
o How can an IP packet go from router R to destination station?
CBA
R
AP2AP1
A. Redondi: Wireless Internet 43
Distribution System
o What does it happen when the station moves to another BSS?
A
R
AP2AP1
A
A. Redondi: Wireless Internet 44
Distribution System
A
R
AP2AP1
A
Politecnico di MilanoFacoltà di Ingegneria dell’Informazione
802.11Medium Access Control (MAC)
Channel accessError controlAddressing
A. Redondi: Wireless Internet 46
Functions and services of the MAC layer
o Channel accesso Error controlo Fragmentation and reassemblyo Power savingo Addressingo Framing
A. Redondi: Wireless Internet 47
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
A. Redondi: Wireless Internet 48
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?
A. Redondi: Wireless Internet 49
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
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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)
A. Redondi: Wireless Internet 51
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)
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DCF Access Mode
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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
A. Redondi: Wireless Internet 54
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
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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)
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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
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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
A. Redondi: Wireless Internet 58
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
A. Redondi: Wireless Internet 59
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
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Virtual Carrier Sense
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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
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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
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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)
Politecnico di MilanoFacoltà di Ingegneria dell’Informazione
Point Coordination Function(optional)
Contention free mechanism Real-time traffic support
A. Redondi: Wireless Internet 65
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
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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)
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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
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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
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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)
Politecnico di MilanoFacoltà di Ingegneria dell’Informazione
Syntax of 802.11MAC
Frame formatAddressing
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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
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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
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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
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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
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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
A. Redondi: Wireless Internet 77
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
Politecnico di MilanoFacoltà di Ingegneria dell’Informazione
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
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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
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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
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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
A. Redondi: Wireless Internet 83
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
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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)
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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
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How to select the BSS?o Is there an app for that?
o What about Wi-Fi off loading?
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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
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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
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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
A. Redondi: Wireless Internet 90
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
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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
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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
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Power Management: Example
A. Redondi: Wireless Internet 94
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
A. Redondi: Wireless Internet 95
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
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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)
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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
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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
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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)
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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
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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
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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!
A. Redondi: Wireless Internet 105
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
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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)
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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
A. Redondi: Wireless Internet 108
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
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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
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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
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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)
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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
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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
A. Redondi: Wireless Internet 115
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
Politecnico di MilanoFacoltà di Ingegneria dell’Informazione
MAC Evolution for QoS support
802.11e
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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
A. Redondi: Wireless Internet 118
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)
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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)
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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
A. Redondi: Wireless Internet 121
Access Categories
o Multiple backoff entities in the same station.
A. Redondi: Wireless Internet 122
EDCA example
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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
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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.)
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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).
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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)
A. Redondi: Wireless Internet 127
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
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U-APSD example
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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
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S-APSD example
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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
Politecnico di MilanoFacoltà di Ingegneria dell’Informazione
Physical layerFrequency Hopping Spread Spectrum(FHSS)Direct Sequence Spread Spectrum (DSSS)
A. Redondi: Wireless Internet 133
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
A. Redondi: Wireless Internet 134
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
A. Redondi: Wireless Internet 135
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)
A. Redondi: Wireless Internet 136
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
A. Redondi: Wireless Internet 137
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 ))
A. Redondi: Wireless Internet 138
DSSS
o Robust to interference peaks
Spreading
Despreading
signal
Interference
band
energy
band
energy
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DSSS
o Spreading process is not used for multiplexing different signals like in CDMA
o All transmissions use the same spreading code (Barker sequence)
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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)
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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
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Physical channels (3)
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Physical channels (4)
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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)
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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
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DSSS – ParametersParameter Value
Slot Duration
20us
SIFSDuration
10us
CW From 31 to 1023 slots
PreamblePLCP
144us
HeaderPLCP
48us
FrameMAC
From 4 to 8191 bytes
Politecnico di MilanoFacoltà di Ingegneria dell’Informazione
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)
A. Redondi: Wireless Internet 148
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
A. Redondi: Wireless Internet 149
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
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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
A. Redondi: Wireless Internet 151
New PLCP
Service
16
Sync SD
bit 128
Length. HeaderCRC PLCP_SDU
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
Length. HeaderCRC PLCP_SDU
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
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802.11b – Actual rate
Source: http://www.uninett.no/wlan/throughput.html
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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
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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
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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
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Spectrumo Symbol (in
transmission)
o Group of symbols in transmission
o More efficient w.r.t. classical FDM
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OFDM
o Sub-carriersn Minimum unit in which spectrum is
dividedo OFDM symbol
n Symbol in transmission divided into N sub-carriers
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Inter Symbol Interference
o Need for guard symbols OFDM (based on cyclic prefixes)
Delay Spread
IOSI
OS1 OS2 OS3
Transmission
Reception
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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
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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
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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
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802.11a transmitter
ModulatorInputBits Scrambler Coding Interleaver
OFDM Symbols syntetizer
IFFTDAC
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Synchronization EqualizationFFT Demodulator
DeinterleaverDecodingDescrambler
ReceivedSamples
Data
802.11a receiver
Multiple Data Rates/Modes
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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
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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
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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)
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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
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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
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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
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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
A. Redondi: Wireless Internet 172
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
A. Redondi: Wireless Internet 173
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
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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
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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)
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802.11n – Physical layer
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 !!
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802.11n – Physical layer
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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
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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
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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
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Politecnico di MilanoFacoltà di Ingegneria dell’Informazione
Mesh Networking
IEEE 802.11sCommercial solutions
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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802.11s MAC
o Modification to the data frame format
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6-Address Scheme
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802.11 STA to external STA
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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)
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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
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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)
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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
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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
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HWMP - AODV
A. Redondi: Wireless Internet 208
A
C
B
D
PREQ 1, A:D, HC = 0
PREQ 1, A:D, HC = 0
HWMP - AODV
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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
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A
C
B
D
DEST A, HC = 1
DEST A, HC = 1 DEST A, HC = 2
PREP 1, D:A, HC = 2
HWMP - AODV
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A
C
B
D
DEST A, HC = 1
DEST A, HC = 1 DEST A, HC = 2
PREP 1, D:A, HC = 2
HWMP - AODV
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A
C
B
D
DEST A, HC = 1
DEST A, HC = 1 DEST A, HC = 2
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
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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.
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Applications
o OLPC (One Laptop Per Child)o Open802.11s
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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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