wireless fundamentals v821 c may 08 ww eng ltr

8/10/2019 Wireless Fundamentals v821 C May 08 WW Eng Ltr

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ProCurve Wireless

Fundamentals GuideTechnical Training Version 8.21


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Rev. 8.21 i

Contents

Module 1: Wireless Network Technologies and Specifications

Objectives..............................................................................................................1-1

Discussion Topics..................................................................................................1-2

802.11 Standard ............................................................................................1-3Overall Spectrum ..........................................................................................1-4

802.11bFirst Widely Adopted Standard....................................................1-5

802.11aAdding Speed ...............................................................................1-6802.11gAdding Speed and Compatibility.................................................1-8

Approximate Spectral Placement of 802.11b/g Channels.............................1-9

Channel Boundaries ....................................................................................1-10Summary of 802.11a, b, and g Transmission Rates....................................1-11

Basic Rates..........................................................................................1-11Supported Rates..................................................................................1-12

Slot Time ............................................................................................1-12Protection............................................................................................1-12

Modulation for 802.11b, 802.11a, and 802.11g..........................................1-13

802.11b ...............................................................................................1-13802.11a................................................................................................1-14

802.11g ...............................................................................................1-14

802.11 Frame Types....................................................................................1-15Frame Types and Subtypes .........................................................................1-17

Management Frames...........................................................................1-17

Control Frames ...................................................................................1-17Data Frames........................................................................................1-17802.11hMeeting Regulations ..................................................................1-18

802.11hDFS ............................................................................................1-19

Soliciting Reports ...............................................................................1-19Changing Channels.............................................................................1-19

802.11hTPC ............................................................................................1-20

802.11nThe Next-Generation Wireless Standard ...................................1-21802.11nMIMO ........................................................................................1-22

802.11nFurther Advancements ...............................................................1-24

802.11nSummary of Improvements........................................................1-25802.11nBeyond Draft 2.0........................................................................1-26

Other 802.11 Standards...............................................................................1-27

Discussion Topics................................................................................................1-28

Ad-Hoc Mode .............................................................................................1-29Infrastructure Mode.....................................................................................1-30

In-Cell Relay Mode.....................................................................................1-31


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ProCurve Wireless Fundamentals

ii Rev. 8.21

Discussion Topics................................................................................................1-32BSS and BSSID ..........................................................................................1-33

ESS and SSID .............................................................................................1-34

WLAN.........................................................................................................1-35

Multiple BSSIDs and WLANs....................................................................1-37

Open Versus Closed Systems .....................................................................1-38Stitching It All Together .............................................................................1-40

Assigning Wireless Traffic to VLANs on the Wired Network...................1-42Discussion Topics................................................................................................1-43

Scanning and Beaconing.............................................................................1-44

Active Scanning..................................................................................1-44Passive Scanning ................................................................................1-45

Preparing to Connect ..........................................................................1-45

Getting Connected...............................................................................................1-46802.11 Authentication.........................................................................1-46

802.11 Association .............................................................................1-46

Supplemental Authentication..............................................................1-47Open-System Authentication...................................................................1-48Shared-Key Authentication.........................................................................1-49

Association..................................................................................................1-51

Supplemental Authentication......................................................................1-52Summary..............................................................................................................1-54

Module 2: Introduction to Radio Technologies

Objectives..............................................................................................................2-1

Discussion Topics..................................................................................................2-2

Radio Waves..........................................................................................................2-3Radio Wave Frequency.................................................................................2-4

Signal Propagation.................................................................................................2-6ObstructionsCauses of Signal Loss ...................................................................2-7Types of Signal Loss ............................................................................................. 2-9

Shadowing ............................................................................................2-9Multipath...............................................................................................2-9

Dropout...............................................................................................2-10

Phase Shift and Signal Strength ..................................................................2-11Overcoming Poor Reception.......................................................................2-12

Measuring Wireless Power..................................................................................2-13

Discussion Topics................................................................................................2-14

Calculating Effective Isotropic Radiated Power (EIRP).....................................2-15

Adjusting EIRP Affects Coverage ..............................................................2-17Cabling Cautions.........................................................................................2-18

Calculating Path Loss..........................................................................................2-19Real-World Path Loss .................................................................................2-21

Scattering Exponent............................................................................2-21

Major Obstructions in the Signal Path................................................2-22Antenna Type......................................................................................2-22


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Contents

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Calculating Received Signal Level (RSL) and Range ........................................2-23System Gain and Fade Margin ............................................................................2-25

System Gain........................................................................................2-25

Fade Margin........................................................................................2-26

Formulas Relating EIRP, Range, and Coverage Area.........................................2-27

Free Space...........................................................................................2-27Real-World Environments ..................................................................2-28

Discussion Topics................................................................................................2-30Data Rate Sets......................................................................................................2-31

Operating Modes.........................................................................................2-32

Mixed 802.11b/g Mode ......................................................................2-33802.11b Only ......................................................................................2-33

802.11g Only ......................................................................................2-33

802.11 Pure g Only.............................................................................2-33802.11a................................................................................................2-33

Other Factors Governing Capacity......................................................................2-34

Modulation Technology......................................................................2-34Device Capabilities.............................................................................2-34Limited Media and 802.11 Standards.................................................2-35

Interference.........................................................................................2-35

Discussion Topics................................................................................................2-36Basic Antenna TypesOmnidirectional and Directional...................................2-37

Antenna Basics ...................................................................................2-37

Omnidirectional Antennas..................................................................2-38Directional Antennas ..........................................................................2-38

ProCurve Omnidirectional Antennas..........................................................2-39

ProCurve Directional Antennas ..................................................................2-40

Diversity Antenna .......................................................................................2-41ProCurve Diversity Antenna.......................................................................2-42

Yagi Antenna ..............................................................................................2-43

ProCurve Yagi Antenna ..............................................................................2-44Summary of ProCurve Antennas ................................................................2-45

Summary of Regions Permitting ProCurve External Antennas..................2-46

Summary of ProCurve External Antenna Mounting Options.....................2-47Connector and Cable Types ........................................................................2-48

Connectors ..........................................................................................2-48

Cable...................................................................................................2-49

Terminator ..........................................................................................2-49Installing an External AntennaMounting the Antenna............................2-50

1. Plan the Installation ........................................................................2-50

2. Mount the Antenna .........................................................................2-51Installing an External AntennaConnecting It to an AP or RP.................2-52

3. Connect the Antenna to the AP or RP ............................................ 2-52

4. Configure the AP or RP Radio .......................................................2-53Summary..............................................................................................................2-54


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Rev. 8.21 1


IntroductionThis fundamentals guide is designed to help network engineers and network

professionals understand the foundational technologies and standards for all

wireless networks and the way they interact to enable communications overwireless media. You must understand these technologies and standards before you

attend the ProCurve Mobility Certification Training Course, which provides

hands-on training for the following ProCurve Networking Mobility InfrastructureSolutions:

ProCurve Access Point (AP) 420

ProCurve AP 530

ProCurve Wireless LAN System, which includes the following components:

ProCurve Wireless Edge Services Module

ProCurve Redundant Wireless Services Module

ProCurve Radio Ports (RPs)

Wireless services-enabled switch (such as the ProCurve Switch 5400zl

Series, ProCurve 8212zl Switch, or ProCurve Switch 5300xl Series)

Objectives

This fundamentals guide includes two modules: Module 1: Wireless Network Technologies and Specifications

Module 2: Introduction to Radio Technologies

After completing these two modules, you should be able to:

List the main features, advantages, and disadvantages of the 802.11a, b, and g

standards

Describe the format and roles of the three 802.11 frame types

Discuss the impetus behind and features of the 802.11n standard

Define the three different modes in which wireless networks can operate:

Ad hoc

Infrastructure

In-cell relay (or wireless bridge)


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Rev. 8.21 1 1

Wireless Network Technologies andSpecifications

Module 1

Objectives

This module describes the 802.11 standards and other specifications that govern

todays wireless networks. After completing this module, you should be able to:

List the main features, advantages, and disadvantages of the 802.11a, b, and gstandards

Describe the format and roles of the three 802.11 frame types

Discuss the impetus behind and features of the 802.11n standard

Define the three different modes in which wireless networks can operate:

Ad hoc

Infrastructure

In-cell relay (or wireless bridge)

Differentiate between the following wireless networking terms:

Basic service set identifier (BSSID)

Wireless local area network (WLAN)

Service set identifier (SSID)

Explain the difference between an open system and a closed system

Explain how a station connects to an access point (AP), including associating

and authenticating


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1 2 Rev. 8.21

Discussion Topics

Rev. 8.21 3

Discussion Topics

Introduction to 802.11 standards

802.11b

802.11a

802.11g

802.11h

802.11n

Wireless network operating modes

Understanding wireless networks

Getting connected

Fundamentals Guide: 12

To ensure compatibility of hardware and software across vendors and platforms,

companies should select products that support industry guidelines for wireless

networks. Developed by the Institute of Electrical and Electronics Engineers, Inc.

(IEEE), these guidelines are collectively called 802.11 standards, or simply802.11. In everyday use and in this module, 802.11refers to the entire set of

wireless standards or specific subsets indicated by letters after the 11.

This section provides a brief overview of the 802.11 standards of todays wirelessnetworksin particular, 802.11b, 802.11a, and 802.11g. It also explains how

802.11h has brought 802.11a in line with regulations in regions such as Europe.

And finally, it discusses the emerging wireless standard, 802.11n, which may oneday become the predominant standard on enterprise LANs.


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Wireless Network Technologies and Specifications

Rev. 8.21 1 3

802.11 Standard

Rev. 8.21 4

802.11 Standard

Original 802.11 standard

Part of the 802 standards for LANs; adapts 802 standard forwireless LANs

Released in 1997

Provides guidelines for the Physical and Data Link layers

Advertised rate of 2 Mbps

2.4 GHz ISM band

802.11a, b, and g amendments

802.11a, b, and g change modulation to increase speed

802.11a changes frequency to decrease interference


The IEEE published the original 802.11 standard in 1997. An addition to the 802

family of standards, which define the functions of wired LANs, 802.11 defined the

Physical and Data Link layers of wireless networks. In other words, the original

802.11 standard adapted the well-understood LAN standards and technologies fora LAN that uses radio waves as its physical medium.

The original 802.11 standard specified radio frequencies in the unregulated

Industrial Scientific and Medical (ISM) band at 2.4 GHz. Its modulation technique

allowed data transmission rates of 2 Mbpsslow even by 1997 standards when

network users were accustomed to Ethernet speeds of 10 Mbps and more.

The 802.11 working group issued important revisions to the original standard,including 802.11b, 802.11a, and 802.11g. In these revisions, the working group

maintained its original focus on the Physical and Data Link Layers. The

802.11b, a, and g revisions made changes to radio modulation and demodulationtechniques to increase data speeds. And 802.11a added the capability to operate in

the 5 GHz band to avoid interference encountered in the comparatively crowded

2.4 GHz band.


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1 4 Rev. 8.21

Overall Spectrum

Rev. 8.21 5

Overall Spectrum

VLF LF MF HF UHF SHFVHF EHF Infrared Vi sibl e UV GammaX Cosmic

AM radio550 - 1700kHz

FM radio 88-108 MHzVHF TV 54-220 MHz

UHF TV460-600MHz

Remote controls100GHz-500THz

Light700THz - 1000THz

Medical X-ray

Indoor wireless 2.4 GHz, and 5 GHz

Cellular 800-900 MHzPCS 1.8-2 GHz

Cordless phones 900 MHz, 2.4 GHz, and 5 GHzTerrestrial microwave 118 GHz

Super high frequency


The chart above illustrates the spectrum of electromagnetic waves, pointing out the

frequency bands in which familiar devices operate. The band names are shown in

the middle stripe. More important than memorizing the names, however, is

understanding the place of wireless networks within the spectrum.Wireless networking devices, including the ProCurve Mobility Infrastructure

products, generally operate in the super high frequency (SHF) band. The diagram

illustrates other items that occupy this band, such as microwaves and cordless

phones.


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Rev. 8.21 1 5

802.11bFirst Widely Adopted Standard

Rev. 8.21 6

802.11bFirst Widely AdoptedStandard

Transmission speeds of up to 11 Mbps

2.4 GHz range

Widely adopted standardmost public hotspots operate on thisstandard

Inexpensive equipment

AP2.4 GHz

802.11b111 Mbps


The IEEEs 1999 revision to the 802.11 standard, 802.11b, operates in the 2.4 GHz

range and advertises transmission speeds of up to 11 Mbps.

Many vendors produced APs and wireless network interface cards (NICs) based on

the new standard. The products were then and have remained inexpensive; as aresult, many wireless networks use 802.11b equipment. Traditionally, this is the

standard used in most home-based wireless networks and public hotspots.

802.11b equipment operates in the 2.4 GHz range, which does not require special

licensingone reason this equipment is relatively cheap. However, 802.11bnetworks may incur interference from microwave ovens, some cordless phones,

and some wireless phones, which operate in the same band.

Note

Advertised rates are the maximum theoretical speeds at which devices

operating on a standard can transmit; the figure relates to radio technology,not actual throughput. For various reasons, actual rates experienced by an end

user will be significantly lower than advertised rates, and can vary widely

even within a session.


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802.11aAdding Speed

Rev. 8.21 7

802.11aAdding Speed


5 GHz rangelower interference, but higher cost

More channels than 802.11b

Less widely adopted; not compatiblewith 802.11b

AP

5 GHz

654 Mbps


The next revision to the base standard was 802.11a. Although 802.11a was

proposed first, 802.11b came to the market firsthence the order in which they

appear in this module and the wider prevalence of 802.11b.

802.11a increases the slow rates offered by 802.11b, achieving advertised speedsof up to 54 Mbps. 802.11a radios operate in the 5 GHz band. Because this band is

less crowded than the 2.4 GHz band, 802.11a-compliant wireless products

encounter less interference from other electronic devices. However, some radar,

HiperLAN devices, and wireless phones use the 5 GHz band. The generally lesscrowded band comes at a cost: the 5 GHz band is regulated, so 802.11a devices

tend to be more expensive.

Due to the nature of radio communication, the faster possible rates of 802.11a

come at the cost of range. Devices operating on this standard must be 25 to 50percent closer together than 802.11b devices to achieve their maximum speeds,

making 802.11a a more practical option when high throughput is more importantthan wide coverage.

802.11a radios also have a larger range of channels on which to operate, allowingyou to create more overlap between APs.


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Rev. 8.21 1 7

One drawback for 802.11a is its incompatibility with 802.11b devices, which havebeen widely adopted by both home and business users. Because of the earlier

popularity of 802.11b, these users are often reluctant to reinvest in the new

hardware required to take advantage of the greater speed offered by 802.11a-

compliant devices. Nevertheless, 802.11a devices remain a choice for companies

that decide increased throughput and decreased interference is worth theinvestment.


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1 8 Rev. 8.21

802.11gAdding Speed and Compatibility

Rev. 8.21 8

802.11gAdding Speed andCompatibility

2.4 GHz range


Backward compatible with 802.11b

Incompatible with 802.11a

AP2.4 GHz

802.11b111 Mbps

802.11g654 Mbps


802.11g matches the higher speed of 802.11a but is compatible with legacy

802.11b equipment. That is, APs or radio ports (RPs) operating at 802.11g speeds

can transparently adapt to 802.11b stations in their coverage area and provide

access at 802.11b speeds. However, when 802.11g APs detect 802.11b stations orAPs in the vicinity, they adapt by increasing the slot time and decreasing the

transmission speeds for frame preambles as well as multicast and broadcast traffic.

Therefore, 802.11g stations in the coverage area will not operate at speeds thatusers may expect.

To guarantee higher throughput for 802.11g stations, you can configure 802.11g

devices to ignore legacy equipment in the vicinity.

802.11g-compliant radios operate in the 2.4 GHz band and advertise rates of up to

54 Mbps. Using the 2.4 GHz range, 802.11g offers a larger range than 802.11a,although stations must be closer to the AP to take advantage of the higher speeds.

802.11g devices are not compatible with 802.11a. Because 802.11g devicesoperate on a different frequency, they do not cause interference with 802.11adevices.


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1 10 Rev. 8.21

Channel Boundaries

Rev. 8.21 10

Channel Boundaries

802.11b/g channels


0dBrTransmit

Spectrum MaskUnfiltered slnx/x

fcfc 22 MHz fc +22 MHz

fc 11 MHz fc +11 MHz

30 dBr

50 dBr

802.11a channels

TransmitSpectrum

Mask

Unfilteredslnx/x

fcfc 20 MHz fc +20 MHz

fc 10 MHz fc +10 MHz

The 802.11b and 802.11g standards dictate that, at 11 MHz above and below any

one of the center frequencies in the 2.4 GHz band, the signal should be one-

thousandth the strength (30 dB lower) of the signal at the center frequency.

Similarly, while the 802.11a channel boundaries lie 20 MHz above and below thecenter frequency, the signal is significant only over a 20 MHz range around the

center frequency.

As with the 802.11b and 802.11g standards, the 802.11a allowed channels vary

depending on regulatory domain. For the 802.11a, b, and g standards, the FederalCommunications Commission (FCC) regulates wireless networks in the United

States, and in Europe the European Telecommunications Standards Institute

(ETSI) defines allowed sets of channels. Local regulatory bodies adopt one ofthese sets and may add some local exceptions or restrictions.

802.11a channels are spaced every 20 MHz because the 802.11a standard only

defines channels with numbers four apart. For example, the center frequency ofchannel 36 (5.18 GHz, derived by multiplying the channel number by 5 MHz andadding the result to the starting frequency for that channels class) is 20 MHz

below the center frequency of channel 40 (5.20 GHz). Therefore, 802.11a channels

do not interfere with each other in the way that 802.11b channels do.

An exception is that Japan does permit several more closely spaced channels.


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Rev. 8.21 1 11

Summary of 802.11a, b, and g Transmission Rates

Rev. 8.21 11

Summary of 802.11a, b, and gTransmission Rates

6, 12, 24(default)

6, 12, 24(default)

1, 2 (default),5.5, 11

1 (default), 2

6, 12, 24(default)

Basic Rates(Mbps)

Not applicable9 s6, 9, 12, 18, 24,36, 48, 54

802.11a

9 s

9 s

9 or 20 s(dynamic)

20 s

Slot Time

Disabled6, 9, 12, 18, 24,36, 48, 54

802.11g, pureg (not Wi-Fistandard)

Enabled1, 2, 5.5, 11, 6,9, 12, 18, 24, 36,

48, 54

802.11g, gonly (Wi-Fi

standard)

Enabled

Not applicable

Protection for802.11b

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

1, 2, 5.5, 11

Supported Rates(Mbps)

802.11b/gmixed mode(Wi-Fistandard)

802.11b only

Standard


The slide summarizes the different transmission rates supported by the 802.11a/b/g

standards.

Basic RatesAPs advertise support for one or more basic rates, which they use for:

Management frames

Broadcast frames

Multicast frames

To associate with the AP, stations must support all of these rates. (The process of

associating with an AP is discussed later in this module.) Therefore, an APoperating in 802.11b/g mixed mode advertises the lower rates of 802.11b. In

mixed mode, an AP supports both 802.11b and 802.11g stations.

An AP operating in 802.11g only or pure g mode advertises only the higher rates,so only 802.11g stations can associate with it. In 802.11g only mode, APs do notsupport 802.11b stations in the service area, but they protect against interference

from such stations. In pure g mode, APs ignore all 802.11 b stations, but they do

not provide any protection against interference from 802.11b stations. You shoulduse pure g mode only if you are certain no 802.11b stations are operating in range

of the AP.


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1 12 Rev. 8.21

Supported Rates

Supported rates are those that an AP will accept for stations unicast traffic.Various stations in the same basic service set (BSS) may use different rates; in

fact, a station may change rates throughout its association, depending on the

current signal-to-noise ratio (SNR).

Although Wi-Fis 802.11g only mode does not allow 802.11b stations to associate,its supported rates include 802.11b rates to protect against interference from

802.11b stations in the area. 802.11 pure-g, which is nota Wi-Fi standard, does not

permit any of the 802.11b rates.

Slot Time

The slot time dictates how long a station waits between detecting a transmission

and attempting to transmit a frame. (It is one of the parameters associated with the

Carrier Sense Multiple Access with Collision Avoidance [CSMA/CA] mechanism

necessary on half-duplex shared media.)

802.11a and 802.11g have higher rates, transmit frames more quickly, and useshorter slot times. In the presence of 802.11b devices, however, 802.11g devices

must use the longer slot time; otherwise, the 802.11g devices will not wait long

enough after detecting contention, and frames may collide.

Protection

Although 802.11b stations cannot detect transmissions that use 802.11gmodulation, 802.11b and 802.11g transmissions interfere with each other. 802.11b

stations, which are deaf to their 802.11g neighbors, can cause collisions. Protection

requires 802.11g stations to alert 802.11b stations to their transmissions by using802.11b modulation for the preamble. The 802.11b may also be required to clear

the medium by sending a clear to send (CTS) frame with a modulation that802.11b stations can detectin other words, at a lower 802.11b, rather than

802.11g, speed.

A primary difference between 802.11g only and 802.11 pure g mode is that 802.11

pure g disables protection, reducing overhead and increasing throughput.

However, even if your AP does not allow 802.11b stations to actually associate,

such stations can cause collisions by sending probe requests. (Stations use proberequests to locate an AP, as will be explained later in this module.) Therefore,

protection is required not only when 802.11b stations might connect to your

WLAN, but also if they might enter your APs coverage areaanother reason touse 802.11 pure g only in environments without any 802.11b stations at all.


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Rev. 8.21 1 13

Modulation for 802.11b, 802.11a, and 802.11g

Rev. 8.21 12

Modulation for 802.11b,802.11a, and 802.11g

802.11b

Direct-Sequence Spread Spectrum (DSSS)

Binary and Quaternary Differential Phase Shift Keying (DPSK)1 and2 Mbps

Complementary Code Keying (CCK)5.5 and 11 Mbps

802.11a

Orthogonal Frequency-Division Multiplexing (OFDM)

DPBSK6 Mbps

DPQSK12 Mbps

Quadrature Amplitude Modulation (QAM)-1624 and 36 Mbps

QAM-6448 and 54 Mbps

802.11g OFDM similar to 802.11a54, 48, 36, 24, 18, 12, and 6 Mbps

DSSS for backward-compatibility with 802.11b


As part of a Physical Layer standard, a modulation technique specifies how a

device encodes data into the signal. 802.11 standards use four types of modulation.

Describing modulation in detail is beyond the scope of this fundamentals guide;

however, you should understand that the different standards use differentmodulation and that these differences translate to different data sets and receiver

sensitivities.

802.11b

802.11b uses Direct-Sequence Spread Spectrum (DSSS). DSSS modifies the data

stream with a pseudorandom stream of chips, spreading the signal out across thefrequency and also increasing the amount of transmitted data.

For the lower data rates, 802.11b modulates data using DSSS and the original

802.11s Differential Phase Shift Keying (DPSK).

DPSK actually encodes the data into the radio signal. It shifts the radio wavesphase a certain amount depending on the value of the data bits being transmitted.DPSK comes in two varieties:

Binary DPSK (DPBSK) defines only two phase shiftsone for 0 and one

for 1which are separated by 180o. This type is used for the 1 Mbps data

rate.

Quaternary DPSK (DQPSK) defines four phase shifts, one for 00, one for 01,one for 10, and one for 11which are separated by 90

o. This type is used for

the 2 Mbps data rate.


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1 14 Rev. 8.21

The larger separation between phase shifts for DBPSK means that a receiver candetect the shift more easily. This is why the receiver sensitivity can be quite low

for a 1 Mbps connection.

802.11b also enhances 802.11s speeds by adding a new modulation technique:

Complementary Code Keying (CCK).

With CCK, the chips that DSSS uses to modulate the stream are 8-bitcomplementary codes. The exact code depends on a symbol, which in turn

depends on data to be transmitted. The 11 Mbps data rate defines symbols that

include 8 bits of data, while the 5.5 Mbps data rate symbols only include 4 bits ofdata. For example, 11010101 translates to a specific symbol for 11 Mbps CCK.

802.11a

802.11a uses Orthogonal Frequency-Division Multiplexing (OFDM) instead ofDSSS. OFDM divides a channel into a number of subchannels. Each subchannel

transmits a separate data stream, increasing the total amount of data transmitted.

Like 802.11b, 802.11a can use DPSK to encode data. However, the OFDM datarate is increased six-fold to 6 and 12 Mbps from DSSSs 1 and 2 Mbps.

802.11a also supports Quadrature Amplitude Modulation (QAM), which encodes

more data into the radio wave by combining multiple waves.

Note again that the modulation techniques that enable the higher data rates also

require receivers to detect more subtle shifts in the radio wave. Thus the receiversensitivity must be greater, and the receiver cannot use a signal as low as the signal

it could use at a lower data rate.

802.11g

802.11g specifies OFDM for the higher data rates, but also requires support forDSSS to provide backward compatibility with 802.11b.


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Rev. 8.21 1 15

802.11 Frame Types

Rev. 8.21 13

802.11 Frame Types

The fundamental frame types and their formats have remainedthe same through all 802.11 revisions:

Management frames

Control frames

Data frames

Field names and length in bytes

MAC Header


2

FrameControl

0-2312

Framebody

6

Address 4

2

SequenceControl

46662

FCSAddress 3Address 2Address 1Duration/ID

The foundational Data Link Layer specifications of 802.11 have remained the

same throughout the revisions. These specifications include:

Frame types and formats

MAC mechanisms

Every frame transmitted on a wireless network must conform to 802.11 standards

for structure and format, regardless of the operational mode of the radio (802.11a,b, or g). The general structure of a wireless frame is shown in the slide.

802.11 defines three types of frames, each with its own functions and subtypes.

The three main types, described in more detail on the next slide, are management

frames, control frames, and data frames. A frames type is identified in the framecontrol field of the 802.11 frames Media Access Control (MAC) header.

Besides the frame type, the MAC header also contains address and media access

control information. The slide illustrates the fields for a MAC header. Not everyframe type transmitted will include all of the listed fields; for example, someframes types need include only the frame control field and the first address field.

When a frame comes from the wired network destined to a wireless station, the AP

processes the frame by removing the Ethernet header and adding the 802.11

header. It then forwards the frame toward the correct destination station, which itknows by MAC address and association ID.


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Rev. 8.21 1 17

Frame Types and Subtypes

Rev. 8.21 14

Frame Types and Subtypes

User data;payload fromhigher level

protocols

Data Frames

Request to Send (RTS)

Clear to Send (CTS)

Acknowledgement (ACK)

Power-Save Poll (PS-Poll)

Control Frames

Beacon

Probe Request

Probe Response

IBSS announcement trafficindication map (ATIM)

Disassociation

Deauthentication

Authentication

Association

Reassociation

Association Response

Reassociation ResponseAction

Management Frames


The slide lists various subtypes of frames under the three main 802.11 types.

Management Frames

Management frames establish and regulate the Data Link Layer connectionbetween APs and wireless stations. APs and stations are allowed to transmit

different types of management frames. For example, an AP can send beacons to

advertise a wireless network. A station can send an association request, and an APcan send an association response, allowing the station to connect to it.

Control Frames

Control frames regulate access to the Physical Layer transmission medium.

Request to Send and Clear to Send (CTS) frames reserve the medium for atransmission. Transmitting a frame across a wireless network can be uncertain;

acknowledgements (ACKs) let a wireless device know that its frame reached its

destination successfully.

Data Frames

Data frames contain higher-layer protocols such as specific applications and

TCP/IP functions. Transmitting data is of course the ultimate goal of a wirelessnetwork.


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1 18 Rev. 8.21

802.11hMeeting Regulations

Rev. 8.21 15

802.11hMeeting Regulations

Designed to provide regulatory compliance for 802.11a

Required in Europe

Defined two mechanisms for complying with regulations andimproving quality:

Dynamic frequency selection (DFS)

Transmit power control (TPC)


Another reason many vendors failed to adopt 802.11a, despite initial advantages of

higher speed and lower interference, was the difficulty of meeting varying

regulations, particularly in Europe. For example, the military often uses the 5 GHz

band, and governments obviously object to private parties interfering with theirradar.

802.11h defines two mechanisms for meeting regulations: DFS and TPC. Both

mechanisms help an AP adapt to changing circumstances, such as significant

interference. Besides ensuring that your wireless network meets all regulations,these mechanisms provide the added benefit of better management for the radio

medium.

Note

As of July 2007, the United States Federal Communications Commission

(FCC) requires DFS.


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1 20 Rev. 8.21

802.11hTPC

Rev. 8.21 17

802.11hTPC

TPC minimizes interference with satellites and manages power.

The AP solicits reports on:

Maximum and minimum transmit powers for each station

Current signal strength

The AP enforces a maximum transmit power.

Stations can raise power up to the maximum if the link margin fallstoo low.


TPC minimizes a wireless networks interference with satellite communications by

allowing you to configure a maximum transmit power for your network. This

maximum is regulated by the AP, which not only complies with the limit, but also

forces stations to transmit at or below this maximum.In addition to enforcing regulatory compliance, TPC helps conserve power, a

useful feature for stations with a limited battery power. The AP monitors the

network to ensure that power usage remains just over the level to maintain

adequate signal strength. If the current signal strength falls below the fade margin(a signal strength slightly above that at which the signal is lost), stations can raise

their power as far as necessary up to the allowed maximum.


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Rev. 8.21 1 21

802.11nThe Next-Generation Wireless Standard

Rev. 8.21 18

802.11nThe Next-GenerationWireless Standard

Next-generation wireless applications necessitate improved WLANdata throughput capabilities.

802.11n is designed to

Increase network speed and reliability

Extend the operating distance of wireless networks

802.11n may eventually become the dominant enterprise LANtechnology.

It specifies either the 2.4 GHz or 5 GHz frequency bands.

Provides backward compatibility for 802.11b/g/a devices


As next-generation wireless applications emerge, improved WLAN data

throughput capabilities are becoming essential. Even now, enterprise-class,

bandwidth-intensive applications such as Enterprise Resource Planning (ERP) and

Customer Relationship Management (CRM) systems, workgroup computingapplications, and some wireless backhaul applications require throughputs larger

than current 802.11 technologies can provide. Videoconferencing is an uncertain

proposition with 802.11g, which while offering a theoretical maximum throughputof 54 Mbps, enables real-world speeds of half that or less.

In response, the IEEE Task Group N (TGn) and the Wi-Fi Alliance (WFA) have

set goals for the next generation of WLAN performance. The emerging IEEE

802.11n standard is intended to increase network speed and reliability as well as toextend the operating distance of wireless networks. Although the standard will

probably not be ratified until early 2009, expectations are that 802.11n will easily

provide up to twice the range of 802.11g; and while the TGns goal is 100 Mbps

net throughput, the final proposal seems certain to offer many times that inmaximum configurations. As such, 802.11n may eventually become the dominant

enterprise LAN technology.

802.11n operates in either the 2.4 GHz or 5 GHz frequency bandsenabling it toprovide backward compatibility for 802.11a/b/g devices.

The purpose of this section is to explain the soon-to-be-released 802.11n standard

and how it will enable WLANs to support emerging media-rich applications.


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1 22 Rev. 8.21

802.11nMIMO

Rev. 8.21 19

802.11nMIMO

Spatial multiplexingsends multiple data streams in the samechannel to multiply data throughput

Signals from each transmitter reach the target receiver via a unique path.

The receivers in MIMO systems consistently process each multipathcomponent.


The 802.11n standard is the first to call for multiple-input, multiple-output

(MIMO) antenna design. MIMO algorithms in a radio chipset send data out over

two to four antennas. Signals from each transmitter can reach the target receiver

via a unique path, allowing for spatial multiplexingthat is, sending multipledata streams in the same channel to multiply the throughput of a single stream.

MIMO works best if these paths are spatially distinct, resulting in received signals

that are uncorrelated. Thus, while traditional 802.11 networks degrade in the

presence of multipatha propagation phenomenon by which multiple radiosignals reach receiving antennas by bouncing off of objects along the way

multipath helps decorrelate the 802.11n channels, enhancing the operation of

spatial multiplexing. The signals are recombined on the receiving side by theMIMO algorithmsdramatically improving wireless performance and reliability.

Traditionally, when reflections combine, they distort the signal at the receiver. The

two to four receivers in MIMO systems, however, consistently process eachmultipath component, thereby eliminating the mixture of out-of-phase componentsthat would normally result in signal distortion.

Because spatial-multiplexing techniques make receivers much more complex,

designers usually combine them with OFDM modulation schemes, which are more

efficient than other modulation schemes. The 802.11n OFDM implementationimproves upon the one employed in earlier standards, using a higher maximum

code rate and slightly wider bandwidth.


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Rev. 8.21 1 23

Space-time block coding, an 802.11n option, uses multiple antennas forredundancy to increase robustness. Another option is beamforming, which uses

multiple antennas as if they were parts of an array, forming a directional antenna

that directs a beam to increase range. The 802.11n PHY specification allows

beamforming when the number of transmit antennas exceeds the number of spatial

streams, or when the channel between the receiver and transmitter is known wellenough by the transmitter to enable it to send most of the signal energy in

directions that will benefit the receiver.


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1 24 Rev. 8.21

802.11nFurther Advancements

Rev. 8.21 20

802.11nFurther Advancements

Channel bonding: combines two adjacent 20 MHz channels into asingle 40 MHz channel

Bandwidth is more than doubled.

Draft 2.0 recommends that 40 MHz channels be used only in the 5GHzband.

Other capabilities designed to reduce overhead and enhance

throughput include:

Frame aggregation

Block acknowledgements

Reduced inter-frame spacing


While MIMO represents the most significant architectural advancement in

802.11n, the standard includes additional Physical Layer feature enhancements

designed to boost performance. The most notable improvement is support for 40

MHz radio channels, which have twice the theoretical capacity of existing 802.11radio channels. A technique called channel bonding combines two adjacent 20

MHz channels into a single 40 MHz channel. Bandwidth is more than doubled,

because the guard band between the two 20 MHz channels, used to avoidinterference between these channels, can also be removed when they are bonded.

802.11n can also operate using the standard 20 MHz channels; in fact, draft 2.0 of

the specification recommends that 40 MHz channels be used only in the 5GHz

band. As mentioned earlier, the 2.4 GHz frequency band has only three non-overlapping 20 MHz channels, and therefore, bonding two 20 MHz channels uses

two-thirds of the total frequency capacity.

802.11n also improves upon the standard at the Data Link Layer. New capabilitiesinclude frame aggregation, block acknowledgements, and reduced inter-framespacing, all designed to reduce overhead and enhance throughput.


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Rev. 8.21 1 25

802.11nSummary of Improvements

Rev. 8.21 21

802.11nSummary ofImprovements

144 Mbps with OFDM, 2x2,20-MHz channel width

25 Mbps with OFDMTypical transmitting datarate

600 Mbps with OFDM, 4x4,40-MHz channel width

54 Mbps with OFDMMaximum transmittingdata rate

OFDM; backward compatiblewith CCK and DSSS

OFDM; backwardscompatible with CCKand DSSS

Modulation schemes

Between two and fourOneNo. of transmitting orreceiving spatial streams

20 MHz, 40 MHz20 MHzChannel width

2.4 GHz, 5 GHz

802.11n Draft 2.0(approved March 2007)

2.4 GHz

802.11g

RF band

Feature


Todays 802.11n-compliant products can typically reach a throughput of 144

Mbps, assuming OFDM modulation, two transmitting and two receiving

streamsknown as a 22 configurationand a 20-MHz channel width. The

currently theoretical maximum throughput rate of 600 Mbps assumes OFDMmodulation, a 44 configuration, and a 40-MHz channel width.

Range is harder to quantify because it's affected by many variables, such as

barriers that could block the signal. However, todays 802.11n equipment based on

draft 2.0 of the specification typically delivers more than twice the range of802.11g equipment, at any given throughput speed.


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1 26 Rev. 8.21

802.11nBeyond Draft 2.0

Rev. 8.21 22

802.11nBeyond Draft 2.0

802.11n is still being refined.

There are an unprecedented number of options.

The mandatory sections are unlikely to change.

Changes would threaten interoperability with existing Draft 2.0 offerings.

May 2007certified for 802.11n Draft 2.0 program

Certification program began in June of that year.

WFA-certified 802.11n Draft 2.0 products should be firmware upgradeableto the final IEEE 802.11n standard.


As mentioned at the beginning of this section, 802.11n is expected to continue

undergoing refinement until early 2009. The unprecedented number of options

makes this fine tuning particularly necessary. The mandatory sections, however,

are unlikely to change at this point, because such changes might threateninteroperability with the consumer markets many Draft 2.0 product offerings.

In May 2007, after the TGn approved Draft 2.0, the WFA unveiled the Certified

for 802.11n Draft 2.0 program, and announced the first certified chip, card, and

box products, which form the test bed for certifying additional products. Theformal certification program began in June 2007. It is believed that WFA-certified

802.11n Draft 2.0 products will be firmware upgradeable to the final IEEE

802.11n standard.


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Rev. 8.21 1 27

Other 802.11 Standards

Rev. 8.21 23

Other 802.11 Standards

Improves Layer 2 roamingUnderdevelopment

802.11r

An emerging standard that will allow the use ofthe 3560-3700 MHz band in the United States

Underdevelopment

802.11y

Makes 802.11a compatible with Japanese radioregulations

Approved802.11j

Improves Layer 2 securityApproved802.11i

Provides quality of service (QoS) for wirelessnetworks

Purpose

Approved

Status

802.11e

Standard


The IEEE continues to issue revisions and updates to wireless networking

standards. The complete list is longer than the one in the slide, which shows the

802.11 standards you are most likely to encounter as an IT professional. More

information about each standard can be found at the working groups Web site:http://grouper.ieee.org/groups/802/11/

A timeline for the publication and approval of future standards is displayed at:

http://grouper.ieee.org/groups/802/11/Reports/802.11_Timelines.htm

Note

Whats in a letter? The case of the letter following the 11 is not random:

lowercase letters denote a revision to the original standard, while uppercase

letters indicate a standard that can stand on its own.


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1 28 Rev. 8.21

Discussion Topics

Rev. 8.21 24

Discussion Topics



Ad hoc

Infrastructure

In-cell relay (bridging)


Getting connected


Now that you understand the standards that provide a common foundation for all

wireless networks, you will learn about the three basic types of wireless networks.


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Rev. 8.21 1 29

Ad-Hoc Mode

Rev. 8.21 25

Ad-Hoc Mode

Includes two or more stations Provides peer-to-peer connectivity Also called an independent basic service set (IBSS)

IBSS


An ad-hoc network includes two or more stations that communicate directly with

each other through wireless transmissions. Each station in an ad-hoc network

receives every packet transmitted; 802.11 specifies the CSMA/CA mechanism to

prevent loss of data due to simultaneous transmissions.Ad-hoc networks are sometimes referred to as IBSSs because they require no

attachment to a wired network. Inexpensive and easy to establish, such networks

are used most often for exchanging files in small meeting areas when access to the

wired network is not necessary or not possible.


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1 30 Rev. 8.21

Infrastructure Mode

Rev. 8.21 26

Infrastructure Mode

One AP and one or more stations All data passes through the AP. The AP connects to a wired network.

Wired network


Today, infrastructure mode is the most common deployment for wireless

networks. In this mode, stations do not communicate directly with one another.

Instead, an AP handles all communication between wireless stations as well as

controls the security and speed parameters for the network.In addition to connecting wireless stations to each other, the AP is connected to a

wired network. As the interface between the wired and the wireless network, the

AP receives wireless traffic from stations and forwards it on to the wired network.

Likewise, the AP receives and forwards traffic that is being sent from the wirednetwork to the wireless stations.

Rather than using APs in an infrastructure mode, you can use RPs. RPs are APs

coordinated through a device on the wired network. For example, the ProCurve

Wireless Edge Services xl Module can control multiple RPs.


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Rev. 8.21 1 31

In-Cell Relay Mode

Rev. 8.21 27

In-Cell Relay Mode

Also called a wireless bridge or wireless distribution system (WDS) Wireless link connecting two networks

Available only on the ProCurve AP 530 and ProCurve AP 520wl

ConnectedAP 530

Wired LAN inbuilding 1

Stations

Wirelessbridge

betweennetworks Connected

AP 530

Wired LAN inbuilding 2

Stations


In-cell relay mode is more commonly called bridgingbecause this mode connects

two or more segments of a network. (The segments can be different segments of a

LAN, unconnected wireless networks, or even separate BSSs that are part of the

same broadcast domain.)In-cell relay mode is also called a WDS. In typical infrastructure mode, APs

simply bridge traffic to wireless stations; the wired network provides the

distribution system for transmitting traffic from wireless stations to its ultimate

destination. With WDS, the wireless medium becomes a distribution system aswell, operating as if it were a wired infrastructure.

WDS is available on the AP 530 and AP 520wl.

WDS connections are most often deployed for two broad purposes:

An Ethernet connection is not readily available to connect an AP to the wired

network.

A company wants to connect two wired networks, but pulling cable betweenthe two buildings is not practical or even possible.

The slide demonstrates one such application: IT managers want to connect two

segments of the enterprise network located in different buildings. Because pulling

cable beneath the street is impossible, they opt for a WDS. An AP 530 is attachedto each LAN, and one radio on each is configured to act as a bridge. The other

radio can be disabled or may serve wireless stations.


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1 32 Rev. 8.21

Discussion Topics

Rev. 8.21 28

Discussion Topics




BSS and BSSID

ESS and ESSID

WLANs

Open versus closed systems

Relationship to virtual LANs (VLANs)

Getting connected


You will now learn how to define a wireless network more precisely.


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Rev. 8.21 1 33

BSS and BSSID

Rev. 8.21 29

BSS and BSSID

BSSAP

Wirednetwork

One AP and its stations compose a BSS.

Each BSS has a 48-bit identifier (usually the APs MAC address)called the BSSID.

The BSSID distinguishes one BSS from another.


Any one or more stations and their AP compose a BSS. (As mentioned earlier, an

IBSS does not have an AP because the stations connect to each other instead of to

a wired network.)

Each BSS has a unique, 48-bit identifier called the BSSID, which is usually theMAC address of the APs wireless interface (its radio). Every frame transmitted to

and from the stations in a BSS contains the BSSID in the frame header, identifying

the frame as belonging to a particular APs coverage area. Thus the BSSID

distinguishes the BSS from others and increases efficiency by allowing the AP andstations to ignore frames not belonging to their BSS.

The ProCurve AP 420 pictured in the slide supports one BSSIDwhich is the

same as the APs wireless interface MAC address. The AP 530 supports one

BSSID on each radio for each of its 16 WLANs.

When a new station joins a cell, it appends the APs BSSID to all frames as the

receiver address in the 802.11 header.


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1 34 Rev. 8.21

ESS and SSID

Rev. 8.21 30

ESS and SSID

W

irednetwork

AP

AP

AP

BSS 1

BSS 2

BSS 3

ESS

Two or more BSSs compose an extended service set (ESS). Each ESS is identified by an extended service set identifier (ESSID).

The ESSID is commonly called the SSID, or network name.


Several BSSs, each with their own BSSID specifying the AP, may belong to the

same ESS. That is, even though they may be spatially separate wireless networks,

they behave as if they are the same network.

This slide illustrates several BSSs composing one ESS. For ease of illustration, theBSSs are spatially separated, but they need not be. In actual wireless networks,

some overlap is desirable to enable roaming.

Each ESS has a unique, 48-bit identifier called the ESSID, which functions as the

networks name. Although ESSID is more precise, the industry commonly uses thegeneral term SSID to signify the network name. For example, the command line

interface (CLI) and the graphical user interface (GUI) for most APs use SSID.

Like the BSSID, the SSID is included in the 802.11 header of every frame

transmitted on a wireless network.


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Rev. 8.21 1 35

WLAN

Rev. 8.21 31

WLAN

A WLAN defines settings for an ESS. The SSID identifies the WLAN.

Like VLANs in Ethernet networks, WLANs divide stations intoseparate broadcast domains.

VLANs

VLAN 10 VLAN 20

WLAN 1 WLAN 1

WLAN 2 WLAN 2

SSID:wireless 20

SSID:wireless 20

SSID:wireless 10

SSID:wireless 10

WLANs


A

B

C

An ESS can also be called a WLAN, which defines various settings for the ESS

such as the SSID and security options.

WLANs on wireless networks can be compared to VLANs on Ethernet networks.

VLANs isolate users into separate broadcast areas. Even though users may connectto the same network devices, they are effectively in different networks. VLANs

are important both for managing user traffic and for maintaining security.

WLANs fill a similar purpose in wireless networks: they divide users into different

groups, steering each user toward the appropriate resources and access levels. Justas VLANs on a switch effectively transform the switch into several virtual

switches, WLANs on an AP effectively divide the AP into several virtual APs,

each providing a separate network connection to a group of mobile users.

Like a VLAN, a WLAN creates a broadcast domainwhich acts like one unifiednetwork regardless of the physical location of the hardware. The slide above shows

two APs that support two WLANs. As far as logical connectivity is concerned, theWLAN to which a station connects is more important than the stations location.For example, station A connects to WLAN 1 through AP 1, station B to WLAN 2

through AP 1, and station C to WLAN 1 through AP 2. Station A and station C are

connecting to the same WLAN. Station A and station B are connecting to differentWLANs, even though they are located side by side.

IT managers can exercise a great deal of control over wireless access through

carefully planned WLAN options.


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1 36 Rev. 8.21

Note

In much of the industry the term WLANis used interchangeably with wireless

network. While this use of WLAN is correct, the term wireless network canalso be used to describe all wireless components of a larger networkall the

APs, wireless stations, and wireless services-enabled switches, which might

together support multiple WLANs. In this module, WLAN describes the

broadcast domain defined by an SSID.


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Rev. 8.21 1 37

Multiple BSSIDs and WLANs

Rev. 8.21 32

Multiple BSSIDs and WLANs

When you create a WLAN, the AP maps it to a BSSID.

A BSSID can carry one or more WLANs.

WLAN 1

WLAN 2SSID:Guests

SSID:Employees

AP 530 Radio 1 MAC address:00:14:aa:aa:aa:20

16 BSSIDs (virtual MACaddresses):

00:14:aa:aa:aa:20

00:14:aa:aa:aa:21..

00:14:aa:aa:aa:2f

Maps WLAN 1to this BSSID

AP 530

AP 420 Radio MAC address:

00:0d:bb:bb:bb:10

Maps WLAN 2to this BSSID

Maps both WLANsto this BSSID

AP 420 WLAN 1

WLAN 2SSID:Guests

SSID:Employees


As indicated on the previous slide, your companys APs might support multiple

WLANs. Each AP carries traffic for these WLANs within its own BSS. An AP

might carry traffic for several WLANs within the same BSS, or it might provideseparate BSSs for separate WLANs. It all depends on how many BSSIDs the

AP has.

For example, the AP 530 supports up to 16 BSSIDs per radio. It can carry one

WLAN on each BSSID, for a potential total of 16 WLANs enabled on either orboth radios. (That is, even though the AP has 32 total BSSIDs, it supports only 16

WLANs with individual configurable settings. The settings for a WLAN on radio

1 are copied to that WLAN on radio 2.) When you enable a WLAN on a radio, theAP 530 automatically assigns it to the next available of that radios pool of 16

BSSIDs.

The AP 420, on the other hand, has a single BSSID and carries traffic for multiple

WLANs on that single identifier.

The ProCurve RP 210 has four BSSIDs, and the ProCurve RP 220 and ProCurveRP 230 have four BSSIDs per radio. Each BSSID can carry traffic for up to four

WLANs, for a total of 16 WLANs on each RP. (If you use the advanced mode

configuration, the dual-radio RPs 220 and 230 can support up to 32 WLANs.)


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1 38 Rev. 8.21

Open Versus Closed Systems

Rev. 8.21 33

Open Versus Closed Systems

Open systemSSID(s) advertised in beacon frames

Closed systemSSID(s) not advertised

AP

AP

SSID: publicwifi

SSID

:publicwifi

Closed SystemOpen System

Stations thatknow SSID

Station that doesnot know SSID

SSID

:


In an open system WLAN, such as a public hotspot, APs advertise their SSID

at regular intervals, basically inviting anyone with a wireless device to join

the WLAN.

Many APs operate in open system by default. However, as a modest securityfeature, IT managers can configure APs to operate in closed system mode,

disabling the automatic advertisement of the SSID.

In this case, users must already know the SSID in order to join the WLAN. If an

AP supports only closed system WLANs, stations within range may detect itsradio signal, but their client utilities will not display any available wireless

networks. To join a network, users must manually configure their wireless

configuration utility with the correct SSID.

As a security measure, a closed system will deter only the most casualunauthorized users and should not be considered a reliable protection against

attacks. More determined attackers can use wireless sniffers to detect the SSID,which, even in closed systems, is included in plaintext in the header of every dataframe.

Because the source address of the beacon frame, which is used to advertise the

SSID, is a BSSID, the number of SSIDs an AP can advertise depends on how it

implements BSSIDs. For example, the AP 420 supports only one BSSID, whichcan be mapped to multiple WLANs. As a result, the AP 420 can advertise only the

SSID for the primary WLAN. You can configure the primary WLAN to operate in

open or closed system mode, but all others must operate in closed system mode.


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Rev. 8.21 1 39

The RPs adopted by the Wireless Module can operate up to four SSIDs in opensystem mode (eight with advanced configuration on dual-radio RPs).

By contrast, the AP 530 can advertise all of its SSIDs because each one is carried

on a separate BSSID. Thus, for each WLAN, you can choose either open or closed

system mode.


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1 40 Rev. 8.21

Stitching It All Together

Rev. 8.21 34

Stitching It All Together

Wired network

BSSID:1713

BSSID:

1712

BSSID:1711

Wireless services-enabled switch

SSID:guest

WLAN 1WLAN 2

WLAN 3

SSID:private

SSID:mywireless

WLAN 1

WLAN 2

WLAN 3

WLAN 1


Enterprise wireless networks can comprise many smaller WLANs, each identified

by a unique SSID. Those WLANs, in turn, might include several APs, each with

its own BSSID for that WLAN.

This slide illustrates a simple network made up of two RPs, which support thesame three WLANs named guest, private, and mywireless. The WLANs do

not interfere with one another because the SSIDs separate traffic into broadcast

domains. (That is to say, the WLANs are logically separateactual transmissionsmay potentially interfere with each other because the collision domain is defined

by the radio, not the WLAN.) The RP in the foreground maps separate SSIDs to

separate BSSIDs, but this is not mandatory. All three SSIDs could in fact beassigned to just one BSSID.

When a user enters the RPs coverage area, his wireless client displays two

available wireless connections: mywireless and guest. (Because WLAN 2 is

designed for employee use only, it operates in closed system mode, and its SSID,

private, is not advertised.) The user would then choose an SSID and associatewith an RP beaconing that SSID.

The WLAN determines settings for the connection. For example, all stations in the

mywireless network authenticate in the same way, use the same securitysettings, and receive the same broadcasts. Once associated, a station can roam

from RP to RP and maintain its connection to the WLAN, probably without the

user even noticing the change.


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Rev. 8.21 1 41

Each time a station transmits a wireless frame, the station includes its current RPsBSSID and the WLANs SSID in the frames header. The RP receives the frame

and forwards it to the wired network accordingly, as described on the next slide.


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1 42 Rev. 8.21

Assigning Wireless Traffic to VLANs on the Wired Network

Rev. 8.21 35

Assigning Wireless Traffic to VLANson the Wired Network

On the wired side, the AP assigns a WLANs incoming traffic:

To the specified VLAN

To the user-based VLAN (overrides WLAN setting)

WLAN 2

WLAN 1

SSID:Employees

SSID:Guest

APSwitch

VLAN 10

WLAN 1 frame


As discussed earlier, an AP (or the Wireless Module) is the interface between the

wireless and the Ethernet network. When a mobile station needs to send traffic to a

device in the Ethernet network, it sends the traffic to its AP, which forwards the

traffic on the stations behalf.An important part of the APs role, therefore, is determining the VLAN in which

to forward incoming wireless traffic. ProCurve wireless products can make this

decision based on:

WLAN, by forwarding all traffic from a particular WLAN in the VLAN youchoose

The VLAN associated with the WLAN is somewhat like a ProCurve switchs

authorized VLAN. Any user allowed to connect to the WLAN is placed in

that VLANunless you have configured a user-based, or dynamic, VLAN.

User, by assigning a users traffic to a particular VLAN


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Rev. 8.21 1 43

Discussion Topics

Rev. 8.21 36

Discussion Topics




Getting connected

Scanning and beaconing

802.11 authentication

Association

Supplemental authentication


When you attempt to connect to a wireless network, a number of steps, usually

transparent to you, must occur before you can begin using the wireless link. This

section describes these stepswhich are the same whether you are connecting to

an AP or to an RP controlled by the Wireless Module.


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1 44 Rev. 8.21

Scanning and Beaconing

Rev. 8.21 37

Scanning and Beaconing

Is anybodyout there?

Beacon framesbroadcast at

regular intervalsStation detectsbeacon, begins

handshake process

AP

Im here. My SSID is ABCWireless.-OR-

Im here, but in a closed system.

AP

Active scanningstation probes

Passive scanningstation listens for beacons

SSID: publicwifi


When a station wants to connect to a wireless network, it must first know whether

an AP is within range, and if so, what WLAN or WLANs the AP supports. This

process of discovery is called scanning.

A station can scan for APs in two ways:

Active scanning

Passive scanning

Active Scanning

In active scanning (also called probing), stations send out probe request frames ona particular channel. APs within range operating on that channel respond with a

probe response frame containing information about their capabilities, data rates,

and so on. The slide illustrates this process in the upper exchange between station

and AP.


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Rev. 8.21 1 45

Passive Scanning

In passive scanning, stations listen for beacon frames from APs within range.Broadcast at regular intervals by APs, beacons are management frames containing

the following information:

Radio settings

Capabilities

SSID

Time stamps

Other data

Wireless stations within range detect the beacon frames and can then move to thenext step in the connection process. The slide illustrates beaconing and passive

scanning in the lower exchange between station and AP.

A station can listen for beacon frames on all supported channels. This type of

passive scanning is called sweeping.

Preparing to Connect

If multiple APs are within range, the station chooses which one to associate with

based on signal strength. At the same time, the station builds a table to keep track

of SSIDs and other connection data. If the station changes location, it can morequickly reconnect to another AP that supports the correct SSID using the data

compiled in the table.


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1 46 Rev. 8.21

Getting Connected

Rev. 8.21 38

Getting Connected

Open-System Authentication

Association

802.11 Authentication(required, 2 options)

Shared-Key Authentication

Network Access

Supplemental authentication


When a scan shows that APs are within range and wireless network access is

available, the station begins the process of joining the network.

802.11 Authentication

First, the station initiates a series of negotiations with the AP. These negotiations

are often referred to as a handshake, but in the 802.11 standards they are more

formally known as authentication.

The most basic option for 802.11 authentication is open-system authentication. If

Wired Equivalent Privacy (WEP) is enabled, stations can instead use shared-key

authentication to prove their legitimacy to the AP or Wireless Module.

802.11 Association

If the 802.11 authentication is successful, a station proceeds to a formal

association with the AP.


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Supplemental Authentication

The term authenticationcan mean different things in wireless networks, and youmust evaluate which meaning is intended when the term is used. In reality, 802.11

open-system authentication is more a pre-association handshake than actual

authentication: open-system authentication does not establish identity and

legitimacy as the term authenticationtypically implies. Shared-key authentication,which was developed to provide true authentication for wireless networks, is

flawed in many ways.

To overcome the limitations of open-system and shared-key authentication, many

WLANs require supplemental authentication after the 802.11 association. Thissupplemental authentication is recommended as a much more stringent protection

measure than those outlined in the 802.11 standard.


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Rev. 8.21 40


Authentication request

Authentication response+ challenge text

1

2

Encrypted challenge text

Authenticationsuccess or failure

3

4

AP

Stations must encrypt a challenge with the correct key toauthenticate.


A slightly more elaborate authentication exchange can take place if a WLAN is

configured to use WEP security.

NoteWLANs using WEP can also require open-system authentication. In fact, this

is the recommended option.

Shared-key authentication assumes that each device is already in possession of the

same key, enabling the device to encrypt and decrypt data contained in frames. In

order to join the network, the station must prove to the AP that it has the correctkey and should therefore be granted network access.

The following frames are exchanged for shared-key authentication:

1. The station issues an authentication request frame, containing the stations

MAC address and a value indicating shared-key authentication.2. The AP issues a response frame containing challenge texta 128-byte,

randomly generated data stream.


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3. Using the key it should already possess, the station encrypts the challengetext from the AP and sends it back.

4. Using the same key, the AP decrypts the challenge text received from thestation. If the decrypted challenge text matches that sent in the second frame,

the authentication is successful. The final frame in the exchange indicates

authentication success or failure.

If successful, the station may then proceed to the association process. As with

open-system authentication, the station is now authenticated but not yet associated

and cannot yet send data to the wired network.

Note

A WLAN that uses shared-key authentication cannot use the more secure

supplemental authentication of 802.1X after association. For the best possible

security, use open-system authentication in the pre-association stage and

supplemental authentication after association.


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Rev. 8.21 1 51

Association

Rev. 8.21 41

Association

Association request frame

Association response:Association ID (AID)

1

2

Stations cannot send data until associated.

Association follows authentication.

AP


If the pre-association authentication is successful, the station sends an association

request frame to the AP, which can accept or reject the request. If it accepts the

association, the AP assigns an AID to the station and allocates RAM and other

resources to the connection. The AP registers the station on the network so thatframes destined for the new station are sent to the correct AP for processing.

If no supplemental authentication is in place, the station is now authenticated and

associated and is a part of the network. The station is allowed to transmit data

frames, and the AP begins to process frames for it. The association remains activeuntil it is terminated by either party. Stations cannot associate with more than one

AP at a time. They can, however, roam and re-associate to a new AP in the

same WLAN.


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Rev. 8.21 42


Association comes up

Authentication exchange

1

2

Success or failure3

After associating, stations must authenticate to a network RADIUS

server.

The exact procedure depends on which supplemental authenticationmethod is used.

AP


RADIUSserver

2

2

3

3

When a WLAN requires supplemental authentication, another process follows

successful association before a user is granted network access. The station is still

authenticated and associated, but is not yet permitted to fully access the network.

The exact authentication process varies, depending on the selected method.Supplemental authentication is vital to wireless network security because it

ensures that only authorized users access the network. Pre-association

authentication may attempt to use the MAC address or shared keys to grant access

to the correct stations, but methods such as 802.1X more rigorously ensure thatonly legitimate users connect.

802.1X, one of the most common supplemental authentication methods, shuts

down the association until the user authenticates. 802.1X authentication is defined

by IEEE standards. Based on user credentials and digital certificates, thesestandards address security weaknesses in the original 802.11 standards.

This slide shows a general overview of 802.1X authentication:

1. The association between a station and AP opens.

2. The AP then helps the station authenticate itself to a network RemoteAuthentication Dial-In User Service (RADIUS) server. Depending on theauthentication method chosen, the station and the server might exchange

certificates, challenge text, encryption keys, or other security parameters.


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Rev. 8.21 1 53

3. When the process is finished, the RADIUS server sends an authenticationsuccess or failure frame, and the AP forwards it to the station. If

authentication is successful, the station also receives the encryption keys it

will use to decrypt received frames and encrypt transmitted frames.

The user perceives the successful completion of supplemental authentication as theinterface coming up or the wireless network being online, and he is now permitted

to access network resources appropriate for his credentials.

To improve network security, 802.1X authentication can be reinitiated at random

or regular intervals throughout a session, ensuring that encryption keys are

changed often enough to make interception and eavesdropping more difficult.


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Summary

Rev. 8.21 43

Summary

Main features of 802.11a, b, g, h, and n standards

Three operating modes for wireless networks

Logical architecture for wireless networks (BSSID, SSID, and WLAN)

Frames exchanged to enable communication between an AP and astation

Fu

wireless fundamentals v821 c may 08 ww eng ltr

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