Redes Inalámbricas – Tema 2.C Wireless LANs: the IEEE 802.11 standards

REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA

Redes Inalámbricas – Tema 2.C Wireless LANs: the IEEE 802.11 standardsThe 802 wireless familyIEEE 802.11

The physical layer The MAC layer Quality of service: 802.11e MIMO: 802.11n Management tools




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103 IEEE 802 Active Working Groups and Study

Groups 802.1 Higher Layer LAN Protocols Working Group

Link Security Executive Committee Study Group is now part of 802.1 802.3 Ethernet Working Group 802.11 Wireless LAN Working Group 802.15 Wireless Personal Area Network (WPAN) Working

Group 802.16 Broadband Wireless Access Working Group 802.17 Resilient Packet Ring Working Group 802.18 Radio Regulatory TAG 802.19 Coexistence TAG 802.20 Mobile Broadband Wireless Access (MBWA) Working

Group 802.21 Media Independent Handoff Working Group 802.22 Wireless Regional Area Networks

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104

Historical notes

The IEEE Working Group for WLAN Standards was created in 1997: http://www.ieee802.org/11/index.shtml

Defines the MAC and 3 different physical layers that work at 1Mbps and 2Mbps: Infrared (IR) in base band Frequency hopping spread spectrum (FHSS), band de 2,4 GHz Direct sequence spread spectrum (DSSS), band de 2,4 GHz

IEEE Std 802.11b (September 1999): Extension of DSSS; Up to 11 Mbps

IEEE Std 802.11a (December 1999): A different physical layer (OFDM):

Orthogonal frequency domain multiplexing Up to 54 Mbps

IEEE Std 802.11g (June 2003) ...

PHYLayer

Infra-Red(IR)

5 GHz (OFDM)Orthogonal Frequency Division Multiplexing

2.4 GHz (DSSS)Direct Sequence Spread Spectrum

2.4 GHz (FHSS)Frequency Hopping Spread Spectrum

802.11 IR1 / 2 Mbit/s

802.11b / TGbHigh Data Rate Extension

5.5/11 Mbit/s

802.11b-cor1 / TGb-cor1Corrigendum MIB

802.11g / TGgData Rates >20 Mbit/s

802.11d / TGdRegulatory Domain Update

802.11 FHSS1 / 2 Mbit/s

802.11 DSSS1 / 2 Mbit/s

802.11a / TGaHigh Data Rate Extension

6/12/24 Mbit/sOptional 9/18/36/54 Mbit/s

802.11h / TGhSpectrum Managed

802.11a

WLANs Next Gemeration SCGlobalization &Harmonization

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105

Evolution of the IEEE 802.11 standard OFFICIAL IEEE 802.11 WORKING GROUP PROJECT TIMELINES

IN PROCESS - Standards, Amendments, and Recommended Practices http://grouper.ieee.org/groups/802/11/Reports/802.11_Timelines.htm

802.11p: Inter car communications Communication between cars/road side and cars/cars Planned for relative speeds of min. 200km/h and ranges over 1000m Usage of 5.850-5.925GHz band in North America

802.11s: Mesh Networking Design of a self-configuring Wireless Distribution System (WDS) based

on 802.11 Support of point-to-point and broadcast communication across several

hops 802.11r: Faster Handover between BSS

Secure, fast handover of a station from one AP to another within an ESS Current mechanisms (even newer standards like 802.11i) plus

incompatible devices from different vendors are massive problems for the use of, e.g., VoIP in WLANs

Handover should be feasible within 50ms in order to support multimedia applications efficiently

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106

Evolution of the IEEE 802.11 standard

Other interesting groups 802.11t: Performance evaluation of 802.11 networks

Standardization of performance measurement schemes 802.11v: Network management

Extensions of current management functions, channel measurements

Definition of a unified interface 802.11w: Securing of network control

Classical standards like 802.11, but also 802.11i protect only data frames, not the control frames. Thus, this standard should extend 802.11i in a way that, e.g., no control frames can be forged.

Note: Not all “standards” will end in products, many ideas get stuck at working group

Standards are available here: http://standards.ieee.org/getieee802/

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107

IEEE 802.11 and WiFi

Wi-Fi is a set of standards for wireless networks based on IEEE 802.11 specifications.

Wi-Fi is a trademark of the Wi-Fi Alliance (formerly the Wireless Ethernet Compatibility Alliance), the trade organization that tests and certifies that equipments meet the IEEE 802.11x standards.

The main problem which is intended to solve through normalization is compatibility. This means that the user is assured that all devices having the seal Wi-Fi can work together regardless of the manufacturer of each.

A complete list of devices that have the certification Wi-Fi: http://certifications.wi-fi.org/wbcs_certified_products.php?lang=en.




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109

Spread Spectrum Transmission

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1010 Comparison of Wireless Modulation Schemes

FHSS transmissions less prone to interference from outside signals than DSSS

WLAN systems that use FHSS have potential for higher number of co-location units than DSSS

DSSS has potential for greater transmission speeds over FHSS Throughput much greater for DSSS than FHSS

Amount of data a channel can send and receive

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1011 Orthogonal Frequency Division Multiplexing

(OFDM) With multipath distortion, receiving device must wait until all

reflections received before transmitting Puts ceiling limit on overall speed of WLAN

OFDM: Send multiple signals at same time High number of low BW ‘modems’ are used, each on a different sub

channel The ‘slow’ sub channels are multiplexed into a ‘fast’ combined channel Error correction is done with FEC and bit stripping

Avoids problems caused by multipath distortion Used in 802.11a networks

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1012 Notion of a channel

Wireless communication is carried over a set of frequencies called a channel

Sig

nal P

ower

Thanks to: Mishra, Shrivastava, Banerjee, and Arbaugh, The University of Wisconsin, Madison

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1013 Channels in Wireless

Available spectrum is typically divided into disjoint channels

Fixed Block of Radio Frequency Spectrum

Channel A Channel B Channel C Channel D


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1014 Ideal Spectrum Usage

Use entire range of frequencies spanning a channel Usage drops down to zero right outside a channel

Channel A Channel B

FrequencyP

ower


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1015 Realistic Spectrum Usage

In reality, this is what communication circuits can achieve Results in inefficient usage of spectrum

Channel A Channel B

Real Usage

Wastage of spectrum


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1016 Realistic Spectrum Usage

Channel A Channel B

Real Usage

Wastage of spectrum

Is it possible to eliminate such inefficiencies ?


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1017 Define a new channel

Define a new channel as shown Overlaps with neighboring two channels Called a `partially overlapped’ channel

Channel A Channel B

Channel A’


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1018 Define a new channel

Channel A’ would interfere with both A and B Is it possible to get any gains from using A, A’ and B ?

Channel A Channel B

Channel A’


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1019 802.11b Channels

In the UK and most of EU: 13 channels, 5MHz apart, 2.412 – 2.472 GHz Each channel is 22MHz Significant overlap Best channels are 1, 6 and 11

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1020 An 802.11 Experiment

Can we use channels 1, 3 and 6 without interference ?

Ch 1 Ch 6Ch 3

Amount of Interference

Link A Ch 1

Link C Ch 6

Link B Ch 3


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1021 An 802.11 Experiment

Link A Ch 1

Link B Ch X

35 m

eter

s60

met

ers

Channel Separation

5210

Non-overlapping channels, A = 1, B = 6Partially Overlapped Channels, A = 1, B = 3Partially Overlapped Channels, A = 1, B = 2

Same channel, A = 1, B = 1

LEGEND

3

4

5

6

0 10 20 30 40 50 60Distance (meters)

UDP T

hrou

ghpu

t (M

bps)


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1022 IEEE 802.11b

Data rate 1, 2, 5.5, 11 Mbit/s, depending on

SNR User data rate max. approx. 6

Mbit/s Transmission range

300m outdoor, 30m indoor Max. data rate ~10m indoor

Frequency Free 2.4 GHz ISM-band

Security Limited, WEP insecure, SSID

Availability Many products and vendors

Connection set-up time Connectionless/always on

Quality of Service Best effort, no guarantees (unless

polling is used, limited support in products)

Manageability Limited (no automated key

distribution, sym. Encryption) Pros

Many installed systems and vendors

Available worldwide Free ISM-band

Cons Heavy interference on ISM-band No service guarantees Relatively low data rate

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1023 IEEE 802.11a

Data rate 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s,

depending on SNR User throughput (1500 byte

packets): 5.3 (6), 18 (24), 24 (36), 32 (54)

6, 12, 24 Mbit/s mandatory Transmission range

100m outdoor, 10m indoor E.g., 54 Mbit/s up to 5 m, 48 up to

12 m, 36 up to 25 m, 24 up to 30m, 18 up to 40 m, 12 up to 60 m

Frequency Free 5.15-5.25, 5.25-5.35, 5.725-

5.825 GHz ISM-band Security

Limited, WEP insecure, SSID Availability

Some products, some vendors

Connection set-up time Connectionless/always on

Quality of Service Best effort, no guarantees (same as

all 802.11 products) Manageability

Limited (no automated key distribution, sym. Encryption)

Pros Fits into 802.x standards Free ISM-band Available, simple system Uses less crowded 5 GHz band Higher data rates

Cons Shorter range

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1024 IEEE 802.11g

Ratified in June 2003 by the IEEE Standards Board standard preliminary draft submitted in December 2001;

Uses the 2.4 GHz band OFDM and codification PBCC

Backward compatibility IEEE 802.11b They can co-exist in the same WLAN

New transmission speeds: 6, 9, 12, 18, 24, 36, 48 & 54 Mbps

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1025 Examples of the physical parameters of a real

deviceal DATA SHEET of a Cisco Aironet 802.11a/b/g CardBus Wireless

LAN Client Adapter

http://www.grc.upv.es/docencia/cosas/products_data_sheet09186a00801ebc29.html

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1026 WiFi and health

RFR's biological effects are measured in terms of specific absorption rate (SAR) -- how much energy is absorbed into human tissue -- which is expressed in Watts per kilogram (W/kg). A dangerous level (by U.S. standards) is considered to be anything above 0.08 W/kg. Thus far, RFR measurements for Wi-Fi, both at home and abroad, are a minute fraction of emissions that could amount to this level. Wi-Fi, in fact, emits less than other common sources of RFR like microwaves and mobile phones. Since mobile phones were recently cleared as a potential carcinogen by a comprehensive, long-term study conducted by the Danish Institute of Cancer Epidemiology in Copenhagen, it seems very unlikely that devices emitting a lower (and less frequent) level could be more dangerous.

By Naomi Graychase, January 12, 2007 http://www.wi-fiplanet.com/news/article.php/3653711

More information: http://www.fcc.gov/oet/rfsafety/




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1028 Available architectures

Independent Basic Service Set (IBSS) is the simplest of all IEEE 802.11

networks in that no network infrastructure is required. As such, an IBSS is simply comprised of one or more Stations which communicate directly with each other.

Do not confuse it with ad hoc!! infrastructure Basic Service Set (BSS)

Components: Station (STA)

Access Point (AP)or Point Coordinator (PC)

Basic Service Set (BSS) Extended Service Set (ESS)

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1029 The MAC basics

CSMA/CA with binary exponential backoff

The protocol, at its minimum, consists of two frames: data and ack

Point CoordinationFunction (PCF)

Distributed Coordination Function (DCF)

MAC

Services without contention

Services with contention

DIFS DIFS

PIFS

SIFS

Contention window

defer access

busy medium

slot

The 5 timing values:• Slot time• SIFS: short interframe space (< slot

time)• PIFS: PCF interframe space (=SIFS+1slot)• DIFS: DCF interframe space (=SIFS+2slots)• EIFS: extended interframe space

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1030 DCF example

The backoff intervals are chosen within the contention window. That is in the interval [0, CW]

The CW can vary between 31 slots (CWmin) and 1023 slots (CWmax)

CW increases after a failed transmission and re-initialized after a successful transmission

data

waitB1 = 5

B2 = 15

data

wait

B1 = 25

B2 = 20

B1 and B2 are the backoff intervals in STA 1 and 2 CW = 31

B2 = 10

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1031 A couple of problematic configurations

Exposed nodeHidden node

A

B

C

A

B C

D

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1032 Hidden nodes situations

The obstacle prevents MU1 and MU2 from hearing one another

MU3 cannot hear MU1 or MU2 because of the distance

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1033 RTS/CTS mechanism

Based on the network allocation vector (NAV)

RTSDIFS+contention

CTSSIFS

data

ACKSIFS SIFS

DIFS

NAV (RTS)NAV (CTS)

source

destination

Other STA

defer access

Contention window

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1034 PCF: Point Coordination Function

• The beacons are used to maintain synchronization of the timers in the stations and to send control information

• The AP generates the beacons at regular intervals• The stations know when the next beacon will arrive

• the target beacon transmission time (TBTT) are announced in the previous beacon

Data+PollDATA+ACKBeacon

Data+PollACK

Station 2 sets NAV(Network Allocation Vector)

CF-End

PIFS SIFS SIFS SIFS SIFS

SIFS(no response)

PIFS

CP

PC

STA1

Contention Free Period CP

Data+Poll

SIFS

STA2 NAVReset

TimeSTA3 Station 3 is hidden to the PC, it does not set the NAV.It continues to operate in DCF.

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10

Frames structure35

• management (00)• control (01), • data (10), • reserved (11)

Types of addresses:

• Source address (SA)

• Destination Address (DA)

• Transmitter Address (TA)

• Receiver Address (RA)

• BSS identifier (BSSID)

SADATARA11Wireless DS

-DASARA = BSSID01To the AP

-SABSSIDRA = DA10From the AP

-BSSIDSARA = DA00IBSS

Addr. 4Addr. 3Addr. 2Addr. 1From DS

To DS

Función

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10

Addressing and DS bits36

SADATARA11Wireless DS

-DASARA = BSSID01To the AP

-SABSSIDRA = DA10From the AP

-BSSIDSARA = DA00IBSS

Addr. 4Addr. 3Addr. 2Addr. 1From DSTo DS

Función

Server

DA

DSRA (BSSID)

SA/TA

ClientAP

Server

SAAP

AP

TA

Client

RA

DA

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1037 Services

The IEEE 802.11 architecture defines 9 services Station services:

Authentication Deauthentication Privacy WEP Data delivery

Distribution services: Association generate a connection between a STA and a PC Disassociation Reassociation like association but informing the previous PC Distribution integration

Similar to plugging in and out in a regular network

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1038 State variables and services

State 1:unauthenticated,

unassociated

State 2:authenticated,unassociated

State 3:authenticated,

associated

Disassociation notification

Successful authentication Deauthentication notification

Successful authenticationor reassociation

Class 1, 2 & 3 frames

Class 1 & 2 frames

Class 1frames

Deauthentication notification

In a IBSS there is no auth. nor ass. Data service is allowed

A STA can be authenticated by several AP but associated only with one AP

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1039 BSSID y SSID

BSSID (Basic Service Set Identity) BSS: MAC address of the AP Ad-Hoc: 46 bits random number

SSID (Service Set ID) Known as the Network Name because it is basically the name that

identifies the WLAN Lenght: 0~32 octets

0: it is the broadcast SSID Used to distinguish WLAN among them The access points and stations who want to connect to a single WLAN

must use the same SSID

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1040 The Extended Service Set (ESS)

BSSAP

WLAN LAN

Inter-acces point protocol (IAPP)

Distribution System (DS)

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1041 IAPP and the Task Group f

Scope of Project: to develop recommended practices for an Inter-Access Point Protocol (IAPP) which provides the necessary capabilities to achieve multi-vendor Access Point interoperability across a Distribution System supporting IEEE P802.11 Wireless LAN Links.

Purpose of Project: ... including the concepts of Access Points and Distribution Systems. Implementation of these concepts where purposely not defined by P802.11 ... As 802.11 based systems have grown in popularity, this limitation has become an impediment to WLAN market growth. This project proposes to specify the necessary information that needs to be exchanged between Access Points to support the P802.11 DS functions. The information exchanges required will be specified for, one or more Distribution Systems; in a manner sufficient to enable the implementation of Distribution Systems containing Access Points from different vendors which adhere to the recommended practices

Status The 802.11F Recommendation has been ratified and published in 2003. IEEE 802.11F was a Trial Use Recommended Practice. The IEEE 802 Executive

Committee approved its withdrawal on February 03, 2006

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1042 Wireless Distribution System

IEEE 802.11, WDS means Multiple wireless “ports” inside the access-point, to wirelessly

interconnect cells (access-points connecting to other access-points) pre-IEEE 802.11, did not support WDS:

Three ports exist in one access-point (one Ethernet, and two wireless cells)

One wireless backbone extension can be made (using two radio modules in the access-point)

WDS allows: Extending the existing infrastructure with wireless backbone links Totally wireless system without any wired backbones, needed in

locations where large areas are to be covered and wiring is not possible

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1043 WDS examples

Bridging two wired networks

As a repeater to extend a network

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1044 Operational processes

Traffic flow - WDS operation

STA-1 STA-2BSS-A

BSS-B

Packet for STA-2

ACK

Packet for STA-2ACK

AP-1000 or AP-500

Avaya Wireless PC-Card

Association table

Bridge learn table

AP-1000 or AP-500

Avaya Wireless PC-Card

Association table

Bridge learn table

STA-1

STA-2 2STA-1

STA-2

STA-1

2STA-2

2

2

Wireless

Backbone

WDS Relay

WDS RelayPacket for STA-2

ACK

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1045 Linksys Wireless-G Access Point

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1057 Limitations of the MAC standard for QoS

DCF (Distributed Coordination Function) Only support best-effort services No guarantee in bandwidth, packet delay and jitter Throughput degradation in the heavy load

PCF (Point Coordination Function) Inefficient and complex central polling scheme Unpredictable beacon frame delay due to incompatible cooperation

between CP and CFP modes Transmission time of the polled stations is unknown

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1058 Overview of 802.11e

Task group e formed in Sep. 1999 and Approved in July 2005 Current version: IEEE P802.11e/D13.0 Backwardly compatible with the DCF and PCF

New QoS mechanism: HCF (Hybrid Coordination Function) Contention-based channel access

EDCA (Enhanced Distributed Channel Access)was Enhanced Distributed Coordination Function (EDCF)

Controlled channel access (includes polling)HCCA (HCF controlled channel access)

The station that operates as the central coordinator for all other stations within the same QoS supporting BSS (QBSS) is called the hybrid coordinator (HC). The HC reside inside an AP

A BSS that includes an 802.11e-compliant HC is referred to as a QBSS.

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1059 EDCA parameters for AC

4 access categories (AC), AIFS[AC] = SIFS + AIFSN[AC] * aSlotTime, AIFSN[AC] 2.

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1060 EDCA and AC Mapping

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1061 HCF: Hybrid Coordination Function

During CFP Poll STAs and give a station the permission to access channel Starting time and maximum duration of each TXOP are specified by the

HC During CP

Can use the EDCA rules HC can issue polled TXOPs in the CP by sending CF-Poll after a PIFS idle

period Controlled Contention

Allows STAs to request the allocation of polled TXOPsSTAs send resource request frames with the requested TC and

TXOP durationHC sends an ACK for resource request to the STA

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1062 HCF superframes

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1063 Performance

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1064 QoS: 802.11e and WMM™

WMM (Wi-Fi Multimedia) Prioritized QoS subset of 802.11e draft Widely accepted by 802.11e members Added to Wi-Fi certification in September 2004

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1065 WMM™ for Video

Source: Wi-Fi Alliance




Thanks to: Paul Young / Bernie Rasenberger

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1067 What is “Wireless N”?

802.11n is the long anticipated update to Wi-Fi standards. Ratified by IEEE in September 2009. “Pre-N” wireless devices were available prior to ratification

(Draft N) with speeds of up to 300Mb/s and range of up to 300 metres (300x300).

Increases channel utilisation through MAC aggregation (40MHz) and increased range & throughput through the use of MIMO (Multiple Input/Multiple Output) technology of 2+ antennas.

Will co-exist with 802.11b/g networks, but can degrade them because of channel overlap caused by MAC aggregation.

Same performance hit if you mix 802.11n clients with 802.11b clients, as you get with mixing 802.11g & 802.11b clients (OFDM).

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1068 What is “Wireless N”?

802.11n

Release Date November 2009Speed 300 MbpsThroughput 74 MbpsFrequency 2.4GHz &/or 5.0GHz

Range (outdoor) 250 meters

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1069 Why is it so fast?

Spatial multiplexing With spatial multiplexing, the stream of data is split between

2 antennae and reassembled at the receiver. More data goes through in the same amount of time than when using a single antenna.

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1070 Why is it so fast?

Support for 40Mhz Channels So far each 802.11b/g channel only used 20MHz of the

spectrum. With more spectrum available, more data can go through.

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1071 Wireless N is also more reliable

Through the use of Multipath we can achieve a more robust signal

Antennas cleverly combine the same signal which has travelled through different paths. Even if the environment changes and some of the signal is obstructed, enough can still go through.

This is how Wireless N achieves A ROBUST SIGNAL, less prone to interference and environmental changes.

TransmitterReceiver

Multiple copies of signal received

Adjusted and combined signals

Resulting signal

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1072 Deployment Considerations

802.11n can operate on 2.4 GHz or/and 5 GHz and is backward compatible with 802.11 a/b/g. Access Points can be set to support 11n only.

AP’s can be: single radio (2.4GHz only or 5GHz only) switchable dual radio

(switchable between 2.4GHz and 5GHz) concurrent dual radio

(operates 2.4GHz and 5GHz at the same time)

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1073 Deployment Considerations

When introducing 802.11n into existing 802.11a/b/g WLANs both bands (2.4GHZ and 5GHz) can have 802.11n enabled.

In case of dense AP architecture channel bonding for 2.4GHz should be disabled (set to 20MHz).

Or if there are other 2.4GHz networks in the area – disable channel bonding for 2.4GHz.

802.11n can be offered to throughput-critical clients only which support 11n: 5GHz band can be set as “11n only”. Leaving 2.4GHz for the rest of the clients which will not interfere with the critical data (802.11b/g/n).

dual-radio 802.11n AP

2.4GHz802.11b/g/n5GHz802.11n only

High Speed Wi-Fi802.11n client

Legacy mixed Wi-Fi

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1074 The ‘Sting’

Increased channel spectrum from 22Mhz to 40Mhz, using MAC aggregation techniques;

Consumes 2 of 3 non overlapping 2.4Ghz channels; Not an issue in “pure N” networks, but will cause issues in

hybrid networks; Uses OFDM, so if 802.11b clients on network performance

degrades for all users.




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1076 Wireshark / Ethereal

Wireshark is the world's foremost network protocol analyzer, and is the de facto (and often de jure) standard across many industries and educational institutions.

Wireshark development thrives thanks to the contributions of networking experts across the globe. It is the continuation of a project that started in 1998.

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1077 Wireshark Features

Deep inspection of hundreds of protocols, with more being added all the time

Live capture and offline analysis Standard three-pane packet browser Multi-platform: Runs on Windows, Linux, OS X, Solaris, FreeBSD,

NetBSD, and many others Captured network data can be browsed via a GUI, or via the TTY-

mode TShark utility Rich VoIP analysis Read/write many different capture file formats: Capture files compressed with gzip can be decompressed on the fly Live data can be read from Ethernet, IEEE 802.11, PPP/HDLC, ATM,

Bluetooth, USB, Token Ring, Frame Relay, FDDI, and others Decryption support for many protocols, including IPsec, ISAKMP,

Kerberos, SNMPv3, SSL/TLS, WEP, and WPA/WPA2 Output can be exported to XML, PostScript®, CSV, or plain text

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1078 Wireshark / Ethereal

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1079 Kismet

Kismet is an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system. Kismet will work with any wireless card which supports raw monitoring (rfmon) mode, and can sniff 802.11b, 802.11a, and 802.11g traffic.

Kismet identifies networks by passively collecting packets and detecting standard named networks, detecting (and given time, decloaking) hidden networks, and infering the presence of nonbeaconing networks via data traffic.

Some of the features Ethereal/Tcpdump compatible data logging Built-in channel hopping and multicard split channel hopping Hidden network SSID decloaking Graphical mapping of networks Manufacturer and model identification of access points and clients Detection of known default access point configurations Runtime decoding of WEP packets for known networks Over 20 supported card types

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1080 gKismet

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1081 Network Stumbler

Allows to save and export data in several different formats Supports GPS and the ability to store GPS information in

conjunction with other data

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1082

The graphical interface used is very intuitive and allows various types of analysis in a simple and direct form

Network Stumbler

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1083 Network Stumbler

Documents

Redes Inalámbricas – Tema 2.C Wireless LANs: the IEEE 802.11 standards