04 FWL System ArchitectureandDesign(II)

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WLAN System Architecture

and Design (II)

Fundamentals of Wireless LANs


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WLAN System Architecture and Design (II)

CT031-3-3-Fundamentals of Wireless LANs

Topic & Structure of the lesson

Wireless Interference

Interoperability and WLAN Backbones


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Learning Outcomes

At the end of this module,YOU should be able to:

Describe the factors contributing to Wireless Interference.

Describe the standard regulations concerning Wireless Interference.

Describe the relationship and relevance of the Industrial, Scientific,

And Medical (ISM) standards devices.

Describe the approaches for detecting Interferences.

Describe Wireless Interoperability and WLAN Backbone.


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Key Terms you must be able to use

If you have mastered this topic, you should be able to use the

following terms correctly in your assignments and exams:

Federal Communications Commission (FCC)

Industrial, Scientific, and Medical (ISM) Detecting Interference

Access Points (AP)

Radio Frequency (RF)

Frequency Hopping (FH)

Basic Service Set (BSS)

Extended Service Set ID (ESSID)


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Main Teaching Points



Detecting Interference Access Points (AP)

Radio Frequency (RF)

Frequency Hopping (FH)


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Introduction

This device complies with part 15 of the FCC Rules

Operation is subject to the following two conditions: (1) this

device may not cause harmful interference, and (2) this device must accept any interference received,

including interference that may cause undesired operation

Interference is typically the state of the signal you are

interested in while it's being destructively overpowered by asignal you are not interested in



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The FCC has a specific definition of "harmful

interference":

Part 2.1(c) Harmful interference -Interference which

endangers the functioning of a radio-navigation

service or of other safety services or seriouslydegrades, obstructs, or repeatedly interrupts a radio-

communication service operating in accordance with

these [International Radio] Regulations

Interference will be a factor in your deployment

The 2.4GHz band is a bit more congested than the

5.8GHz band, but both have co-users that you must

consider

Refer to the following figure:



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The following subsections describe users that you

may encounter while deploying 802.11 devices and

detail what interference mitigation may be possible

for each

Spectrum allocations for 802.11b and co-users

Part / Use Start GHz End GHz

Part 87 0.4700 10.5000

Part 97 2.3900 2.4500

Part 15 2.4000 2.4830

RF lighting 2.4000 2.4835

Part 18 2.4000 2.5000

Part 80 2.4000 9.6000

ISM - 802.11b 2.4010 2.4730Part 74 2.4500 2.4835

Part 101 2.4500 2.5000

Part 90 2.4500 2.8350

Part 25 5.0910 5.2500

U-NII Low 5.1500 5.2500

U-NII Middle 5.2500 5.3500

Part 97 5.6500 5.9250

U-NII High 5.7250 5.8250

ISM 5.7250 5.8500

Part 18 5.7250 5.87



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Devices that Fall into Part 15 of the ISM Band (2400 to

2483 MHz)

This includes unlicensed telecommunications devices

such as cordless phones, home spy cameras, andFrequency Hopping (FHSS) or Direct Sequence (DSSS)

Spread Spectrum LAN transceivers

You have neither priority over nor parity with any of

these users

Any device that falls into Part 15 must not cause harmful

interference to all other licensed and legally operating

Part 15 users

but it must accept interference from all licensed and

legally operating Part 15 users



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This is explicitly defined in 15.5:

15.5(b) Operation of an intentional, unintentional, or

incidental radiator is subject to the conditions that

no harmful interference is caused and that

interference must be accepted that may be causedby the operation of an authorized radio station, by

another intentional or unintentional radiator, by

industrial, scientific and medical (ISM) equipment,

or by an incidental radiator

Interference objections don't necessarily have to come

from a "Commission representative."

Operators of other licensed and non-licensed devices

can inform you of interference and require that you

terminate operation.



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Users of 802.11b can interfere with each other even ifthey are on different channels, as the channels are

22MHz wide and spaced only 5MHz apart

Channels 1, 6, and 11 are the only channels that

don't interfere with each other. Refer to table below:


Channel Bottom (GHz) Center (GHz) Top (GHz)

1 2.401 2.412 2.423

2 2.406 2.417 2.428

3 2.411 2.422 2.433

4 2.416 2.427 2.438

5 2.421 2.432 2.443

6 2.426 2.437 2.448

7 2.431 2.442 2.453

8 2.436 2.447 2.458

9 2.441 2.452 2.463

10 2.446 2.457 2.468

11 2.451 2.462 2.473


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Industrial, Scientific, and Medical (ISM) Devices: Part 18

This is also an unlicensed service

Typical ISM applications include the production of

physical, biological, or chemical effects such as heating,

ionization of gases, mechanical vibrations, hair removal,and the acceleration of charged particles

Users of this band include ultrasonic devices such as

jewelry cleaners, ultrasonic humidifiers, and microwave

ovens. Medical devices, such as diathermy equipment

and magnetic resonance imaging equipment (MRI) also

use ISM, along with some industrial devices such as

paint dryers



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RF should be contained within the devices, but other

users must accept interference from them

Part 18 frequencies that could affect 802.11 devices are

2.400 to 2.500GHz and 5.725 to 5.875GHz

It is difficult to coordinate with the users of Part 18devices because they are unlicensed and may not

realize the impact their equipment has on 802.11

devices

Satellite Communications: Part 25

This part of the FCC's rules is applicable to the uplink or

downlink of data to and from satellites in Earth orbit



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One band that overlaps the U-NII band is reserved for

Earth-to-space communications at 5.091 to 5.25GHz

Within this spectrum, 5.091 to 5.150GHz is also

allocated to the fixed-satellite service (Earth-to-space),

for non-geostationary satellites on a primary basis The FCC is trying to decommission this band for "feeder"

use to satellites

Because satellite transmissions involve very narrow

aperture antennas pointing into the sky and relatively

high power, you are not likely to interfere with them

If you are near one of these installations, there is a very

slight chance they could interfere with you



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Detecting Interference A pre-site survey form should have a place to identify

any known RF systems that are used on the site

However, not all sites will have a single document or

even someone who knows all of the RF equipmentthat is in use

Most enterprise WLAN equipment has the capability

to look for other WLAN devices (usually referred to as

rogue AP detection), and some can even report other

interfering signals

The drawback to such features is that the APs for the

new WLAN need to be installed first



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If the installer is not aware of the other WLANs, he might

install an AP in very close proximity to an interfering

device

This will in turn require a relocation of the AP, possibly

affecting other AP locations as well It is vital to identify all other possible interference in the

site before starting any RF survey work (through a

walkabout survey, for example)

Even if the survey to be used is an automated survey, it

is a vital part of an installation to first look for and

identify any interfering signals

There are a number of methods to identify potential

interference



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The most accurate is to use a spectrum analyzer

Spectrum analyzers enable you to view the entire

spectrum, looking for signals that might not only be within

the frequency range of the intended WLAN system, but

could be near or at a frequency that could causeinterference

It is vital to the quality of interference detection to become

proficient with a spectrum analyzer

To locate any possible interference from some non-

802.11 transmitter), use a higher-gain antenna on the

analyzer, a peak hold function to capture any signals that

are on line for a short period of time, and proper

resolution and video bandwidth settings



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Existing WLAN devices represent another commonsource of interference

If the existing device uses a separate band (900 MHz, for

instance), then this should not cause an issue

However, it is still recommended to keep some minimumdistance between any two RF devices (minimum of 3

feet, or about 1 meter) even if they are on different bands

When installing a system in the same facility that has

competing RF on the same band, exercise extreme

caution during the installation to keep interaction to a

minimum



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For example, when adding an 802.11b or 802.11gsystem to a site that has an existing frequency-hopping

(FH) system, maintain a minimum of 10 feet (3 meters)

between the 802.11b or 802.11g and the FH system RF

components

Another common issue with regard to interference is the

rogue AP (that is, the AP that some employee has

brought in and put into the network without the consent of

the IT staff)

This type of AP can cause several issues, with the

number one being security (because rogue APs typically

do not conform to the IT security requirements)



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The second issue is interference with the properlyinstalled WLAN

If not identified before the walkabout portion of the

survey begins, it can cause missed packets and higher

noise-floor readings, which in turn might trigger the needfor another AP in that location

These devices should be "sought and destroyed" before

starting a survey

Some WLANs systems offer rogue AP detection utilities,

but require the WLAN to be fully installed and

operational before they can be used

Therefore these utilities are more for maintaining a

WLAN and identifying rogue APs in an operation WLAN

and not for use as part of a site survey



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Interoperability

Interoperability is also a concern when you are selecting

products

Make sure that any product you select is Wi-Fi certified (and

not just that they use the term Wi-Fi in their literature)

Go to the Wi-Fi Alliance website (www.wi-fi.com) and view

the list of certified devices

This at least provides some basic level of interoperability

testing and certification Also be aware that there are several different Wi-Fi

certifications, such as 802.11a, 802.11b, 802.11g, security,

quality of service, and so on
http://www.wi-fi.com/http://www.wi-fi.com/http://www.wi-fi.com/http://www.wi-fi.com/


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The packages of newer Wi-Fi certified products include a

certification compliance label that lists the features supported

by the product (802.11a, 802.1b, WPA, QoS, and so forth)

WLAN Backbones

No matter how many wireless clients you intend to support,you will eventually need to "hit the wire" in order to access

other networks (such as the Internet).

There are a number of different kinds of physical devices you

can use to jump from wireless back to your wiredinfrastructure:

Access Point Hardware

APsare widely considered ideal for "campus" coverage



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They provide a point of entry to the wired infrastructure

that can be configured by a central authority

They typically allow for one or two radios per AP,

theoretically supporting hundreds of simultaneous

wireless users at a time They must be configured with an ESSID (Extended

Service Set ID, also known as the Network Name or

WLAN Service Area ID, depending on who you talk to)

It's a simple string that identifies the wireless network Many APs use a client program for configuration and a

simple password to protect their network settings

All hardware access points provide BSS master services

Most APs also provide a number of enhanced features



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External antennas (or antenna connectors), advancedlink status monitoring, and extensive logging and

statistics are now common on many APs

In addition, most access points provide two additional

security measures: MAC address filtering and closed

networks

With MAC filtering enabled, a client radio attempting

access must have its MAC address listed on an internal

table before it can associate with the AP

In a closed network, the AP doesn't beacon its ESSID at

regular intervals

This means that each client must know the ESSID ahead

of time



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APs are by far the most widely used devices for providingwireless services, particularly in corporate networks

Another class of AP is occasionally referred to as a

residential gateway (RG)

The Apple Airport, Orinoco RG series, and Linksys

WAP11 are popular examples of RGs

They are typically much less expensive than their

"commercial" counterparts

Many have built-in modems, allowing for wireless-to-

dialup access (which can be very handy, if Ethernet

access isn't available)

Most even provide Network Address Translation (NAT),

DHCP, and bridging services for wireless clients



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While they may not support as many simultaneous clientsas a high-end AP, they can provide cheap, simple access

for many applications

When configuring an inexpensive AP for bridged Ethernet

mode, you can still have a high degree of control over

what individual clients can access on the wired network

by controlling communications at a higher level

APs (that is, BSS masters) do not talk to each other over

the air

In order to have 802.11b BSS mode communications,

one device (e.g., an access point) must be a master, and

the other must be a client



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Peer-to-Peer (IBSS) Networking

Radios that are operating in IBSS mode can

communicate with each other without a hardware access

point if they have the same ESSID and WEP settings

This is particularly handy for setting up temporarywireless workgroups without an AP, or for building point-

to-point wireless connections

As stated earlier, any computer with an 802.11b card and

another network connection (usually Ethernet, dialup, or

even another wireless connection) can serve as a

gateway between the two networks



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There is one important constraint on using IBSS mode:although it is defined by the 802.11b standard, few client

cards actually interoperate well in the real world with

others using IBSS

While two radios of the same manufacturer (and of thesame firmware revision) generally work just fine, trying to

get a Cisco card to talk to a Proxim card in IBSS mode

(for example) is usually futile

With this in mind, why would you choose to use IBSS

mode rather than use an AP or the Host AP driver?

There are a couple of reasons



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If you happen to have two cards of the samemanufacturer and a couple of old computers, IBSS mode

is ideal if you want to create a fixed point-to-point

connection

Also, Host AP supports only a limited set of wirelesscardsif you already own a card that isn't supported,

you're out of luck

Finally, if you're using a laptop and need to exchange

data with another wireless user, IBSS is your only option

if you're out of range of an AP and can't run Host AP



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Quick Review Question


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Follow Up Assignment


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Summary of Main Teaching Points


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Q & A

Question and Answer Session


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04 FWL System ArchitectureandDesign(II)

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