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
Wireless Interference
Interoperability and WLAN Backbones
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
Wireless Interference
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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:
Wireless Interference
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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
Wireless Interference
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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
Wireless Interference
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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.
Wireless Interference
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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:
Wireless Interference
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
Wireless Interference
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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
Wireless Interference
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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
Wireless Interference
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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
Wireless Interference
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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
Wireless 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
Wireless Interference
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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
Wireless Interference
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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)
Wireless Interference
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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
Wireless Interference
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Interoperability and WLAN Backbones
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
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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
Interoperability and WLAN Backbones
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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
Interoperability and WLAN Backbones
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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
Interoperability and WLAN Backbones
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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
Interoperability and WLAN Backbones
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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
Interoperability and WLAN Backbones
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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
Interoperability and WLAN Backbones
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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
Interoperability and WLAN Backbones
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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
Interoperability and WLAN Backbones
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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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