Download Module 3 WLAN Presentation

802.11
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 1
Equipment that has been tested to comply with the 802.11 standard is
said to be Wi-Fi certified (like Hi-Fi, but Wireless Fidelity).
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 2
Wireless LANs
License Free Spread Spectrum
The FCC has set standards under Part 15 of the Rules and Regulations for
equipment used in the 2.4 GHz band. (The exact spectrum is 2400 to
2483.5 MHz). If the equipment uses Spread Spectrum techniques, then
effective radiated transmit powers up to 64 watts can be used!
There are two types of Spread Spectrum techniques used: Frequency
Hopping Spread Spectrum (FHSS) and Direct Sequence Spread
Spectrum (DSSS). This enables many radios to operate in this band with
minimum interference – up to a point.
With FHSS, a data packet is first sent on a random channel in the band with
the next packet sent, after a pause of a few milliseconds, on another
random channel in the band. With 80 channels or more available (one
channel per MHz, e.g., 2401, 2402, 2403, etc) signals from multiple radios
"hop" around each other. This is how they can operate with other radios in
the same band with minimal interference.
DSSS radios operate on a fixed radio channel, but the signal is "spread" on
that channel by mixing the signal with a Pseudo-Noise (PN) code. This
spreading causes the radio signal with the data on it to occupy a much
wider band, and looks more like noise to receivers not designed to "despread " that signal.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 3
IEEE 802 Committees
802.0 SEC
802.1 High Level Interface (HILI)
802.2 Logical Link Control (LLC)
802.3 CSMA/CD Working Group
802.4 Token Bus
IEEE 802.11
IEEE 802.11a
IEEE 802.11b WiFi
802.5 Token Ring
802.6 Metropolitan Area Network (MAN)
802.7 BroadBand Technical Adv. Group (BBTAG)
802.8 Fiber Optics Technical Adv. Group (FOTAG)
802.9 Integrated Services LAN (ISLAN)
802.10 Standard for Interoperable LAN Security (SILS)
801.11 Wireless LAN (WLAN)
IEEE 802.11g
IEEE 802.15.1 Bluetooth
IEEE 802.11e
IEEE 802.11f
IEEE 802.11h
IEEE 802.11i Security 2004
802.12 Demand Priority
IEEE 802.15 TG2
802.14 Cable-TV Based Broadband Communication
Network
IEEE 802.15 TG4
IEEE 802.15 TG3
802.15 Wireless Personal Area Network (WPAN)
802.16 Broadband Wireless Access (BBWA)
RPRSG Resilient Packet Ring Study Group (RPRSG)
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 4
Radio Licenses NOT required in these bands: 2.4 GHz, 5 GHz
Direct Sequence Spread Spectrum
Frequency Hop Spread Spectrum
IEEE 802.11
Standard for WLAN operations at data rates up to 2 Mbps
in the 2.4 GHz ISM band. FHSS or DSSS modulation.
IEEE 802.11a
Standard for WLAN operations at data rates up to 54 Mbps
in the 5 GHz band. OFDM Modulation. Proprietary “rate
doubling" has achieved 108 Mbps. Realistic rating is 20-26
Mbps.
IEEE 802.11b
Wi-Fi™ or “high-speed wireless” 1, 2, 5.5 and 11 Mbps in
the 2.4 GHz band. All 802.11b systems are backward
compliant. Realistic rating is 2 to 4 Mbps. DSSS modulation.
IEEE 802.11g
802.11a backward compatible to the 802.11b 2.4 GHz band
using OFDM.
Orthogonal Frequency Division Multiplexing
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 5
ISM Band
The Industrial, Scientific and Medical (ISM) radio bands were originally
reserved internationally for non-commercial use of RF electromagnetic fields
for industrial, scientific and medical purposes.
The ISM bands are defined by the ITU-T in S5.138 and S5.150 of the Radio
Regulations. Individual countries' use of the bands designated in these sections
may differ due to variations in national radio regulations.
In recent years they have also been used for license-free error-tolerant
communications applications such as wireless LANs and Bluetooth:
•900 MHz band (33.3 cm)
•2.45 GHz band (12.2 cm) (2.4 - 2.4835 GHz range)
•5.150-5.250 GHz, 5.250-5.350 GHz and 5.725-5.825 GHz bands
IEEE 802.11b wireless Ethernet also operates on the 2.45 GHz band and 802.11a
and 802.11g operate in the 5.xxx GHz bands. The use of the spectrum in the
bands 5.150-5.250 GHz, 5.250-5.350 GHz and 5.725-5.825 GHz for LE-LAN
devices is on the basis that such devices cannot claim protection from other
radio systems and cannot cause harmful interference into other radio services
in these bands. (LE-LAN = License Exempt LAN)
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 6
BlueTooth
Bluetooth® wireless technology enables links between mobile computers,
mobile phones, portable handheld devices, and connectivity to the
Internet.
Hardware that complies with the Bluetooth wireless specification ensures
communication compatibility worldwide.
Unlike many other wireless standards, the Bluetooth wireless specification
includes both link layer and application layer definitions for product
developers which supports data, voice, and content-centric applications.
Radios that comply with the Bluetooth wireless specification operate in the
unlicensed, 2.4 GHz radio spectrum(2.4 - 2.4835 GHz) ensuring
communication compatibility worldwide.
These radios use a spread spectrum, frequency hopping, full-duplex signal
at up to 1600 hops/sec. The signal hops among 79 frequencies at 1 MHz
intervals to give a high degree of interference immunity. Up to seven
simultaneous connections can be established and maintained.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 7
Standard
802.11
802.11a
802.11b
Data Rate
≤ 2Mbps
2.4GHz
≤ 54Mbps
5GHz
≤ 11Mbps
2.4GHz
802.11g
≤ 54Mbps
Bluetooth
Up to 2Mbps
2.45GHz
2.4GHz
Modulation
Scheme
FHSS or
DSSS
Pros/Cons
This specification has been extended into 802.11b.
OFDM
"Wi-Fi Certified." 8 available channels. Less potential for
RF interference than 802.11b and 802.11g. Better than
802.11b at supporting multimedia voice, video and largeimage applications in densely populated user
environments. Relatively shorter range than 802.11b. Not
interoperable with 802.11b.
DSSS with
CCK
"Wi-Fi Certified." 14 channels available. Not
interoperable with 802.11a. Requires fewer access points
than 802.11a for coverage of large areas. High-speed
access to data at up to 300 feet from base station.
OFDM >
20Mbps
DSSS + CCK
< 20Mbps
"Wi-Fi Certified." 14 channels available. May replace
802.11b. Improved security enhancements over 802.11.
Compatible with 802.11b.
FHSS
No native support for IP, so it does not support TCP/IP
and wireless LAN applications well. Best suited for
connecting PDAs, cell phones and PCs in short intervals.
Adaptive
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 8
802.11
A key technology contained within the 802.11 standard is Direct
Sequence Spread Spectrum (DSSS).
DSSS applies to wireless devices operating within a 1 to 2 Mbps
range. A DSSS system may operate at up to 11 Mbps but will not be
considered compliant above 2 Mbps.
The next standard approved was 802.11b, which increased
transmission capabilities to 11 Mbps.
Even though DSSS WLANs were able to interoperate with the
Frequency Hopping Spread Spectrum (FHSS) WLANs, problems
developed prompting design changes by the manufacturers. In this
case, IEEE’s task was simply to create a standard that matched the
manufacturer’s solution.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 9
802.11b
802.11b may also be called Wi-Fi™ or high-speed wireless and
refers to DSSS systems that operate at 1, 2, 5.5 and 11 Mbps.
All 802.11b systems are backward compliant in that they also
support 802.11 for 1 and 2 Mbps data rates for DSSS only. This
backward compatibility is extremely important as it allows upgrading
of the wireless network without replacing the NICs or access points.
802.11b devices achieve the higher data throughput rate by using a
different coding technique from 802.11, allowing for a greater amount
of data to be transferred in the same time frame.
The majority of 802.11b devices still fail to match the 11 Mbps
throughput and generally function in the 2 to 4 Mbps range.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 10
802.11a
802.11a covers WLAN devices operating in the 5 GHZ transmission
band.
Using the 5 GHZ range disallows interoperability of 802.11b devices
as they operate within 2.4 GHZ.
802.11a is capable of supplying data throughput of 54 Mbps and with
proprietary technology known as "rate doubling" has achieved 108
Mbps.
In production networks, a more standard rating is 20-26 Mbps.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 11
802.11g
802.11g provides the same throughout as 802.11a but with
backwards compatibility for 802.11b devices using Orthogonal
Frequency Division Multiplexing (OFDM) modulation technology.
Cisco has developed an access point that permits 802.11b and
802.11a devices to coexist on the same WLAN. The access point
supplies ‘gateway’ services allowing these otherwise incompatible
devices to communicate.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 12
www.wi-fi.com
Excellent
Products listings
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 13
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 14
20-30% overlap
Access Points (APs)
91.44 to 152.4 meters
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 15
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 16
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 17
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 18
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 19
D-Link - 22Mbps 802.11b+ Wireless Router and PC Card Bundle (Refurbished) $49.99
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 20
When a source node sends a frame, the receiving node returns a positive
acknowledgment (ACK). This can consume 50% of the available bandwidth. This
overhead, combined with the collision avoidance protocol (CSMA/CA) reduces
the actual data throughput to a maximum of 5.0 to 5.5 Mbps on an 802.11b
wireless LAN rated at 11 Mbps.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 21
802.11g
Adaptive
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 22
802.11g
Adaptive
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 23
802.11b
Not Adaptive
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 24
The D-Link AirPlus DWL-800AP+ is
an enhanced 802.11b Wireless
Range Extender that can operate
as an Wireless Access Point or
Wireless Repeater.
D-Link - DWL-800AP+ - Enhanced Wireless 2.4GHz Range Extender $69.99
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 25
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 26
Electromagnetic
radiation
Netgear - WGR614 –
Wireless 54 Mbps Cable/DSL Router "G“ $69.99
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 27
Electromagnetic Radiation
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 28
Electromagnetic Radiation
In a transformer, the idea is to
hold the energy inside a
ferromagnetic material.
If there is no
ferromagnetic
material, the energy
will radiate.
Sep-03 ©Cisco Systems
Here the Tx (L1) & Rx (L2) coils are
closely coupled.
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 29
Electromagnetic Radiation
Here the Tx & Rx
coils …
… are farther
apart.
And the induced
voltage is less
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 30
Electromagnetic Radiation
At some point they are no
longer coils, they are
ANTENNAS.
And between them is an
electromagnetic field.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 31
Antenna Evolution
Primitive coil
antenna.
Dipole antenna.
Ground plane
Dipole antenna.
Floyd
Sep-03 ©Cisco
Systems
SemesterFifth
1 Version
Electronics
Fundamentals,
Circuits, DevicesCCNA
and Applications,
Edition 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 32
Electromagnetic Radiation
½ Wave
Length Dipole
+
Sep-03 ©Cisco Systems
E
H
The electric field (E), the
magnetic field (H), and the
direction of propagation (Z) are
all at 90 degree to each other
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 33
Electromagnetic Radiation
An Electromagnetic wave carries the data stream
between the Access Point and the Node.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 34
Radiation Patterns
Omni directional is used to cover an area.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 35
•There are some applications
where you do not want an omni
directional pattern.
•You want a directional pattern.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 36
Adding extra elements will focus the pattern
Reflector
Element
Active
Dipole
Yagi
antenna
3 Directors
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 37
Making a parabolic antenna
Then you can add a Parabolic
Reflector to focus the
pattern even more.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 38
Parabolic antenna pattern
12 dB yagi gain becomes 24 dB for a
1 meter dish.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 39
Various Antenna designs
6 dB omni 2.4
Ghz
6dB indoor omni
2.4 Ghz
16 dB
Panel 2.4
Ghz
Sep-03 ©Cisco Systems
24 dB solid
dish 2.4 Ghz
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 40
Typical indoor directional WLAN antennas
D-Link - DWL-R60AT Indoor 6 dBi
Microstrip Antenna $34.99
D-Link Ant24-0801 8.5 DBI
Pico Cell Patch Antenna $139.99
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 41
Most suppliers will have a complete family.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 42
Low speed Cell phone
connections
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 43
Low speed Cell phone
Availability
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 44
Radio Waves
Computers send data signals electronically.
Radio transmitters convert these electrical signals to radio waves.
Changing electric currents in the antenna of a transmitter generates
the radio waves.
These radio waves radiate out in straight lines from the antenna.
However, radio waves attenuate as they move out from the
transmitting antenna.
In a WLAN, a radio signal measured at a distance of just 10 meters
(30 feet) from the transmitting antenna would be only 1/100th of its
original strength.
Like light, radio waves can be absorbed by some materials and
reflected by others. When passing from one material, like air, into
another material, like a plaster wall, radio waves are refracted. Radio
waves are also scattered and absorbed by water droplets in the air.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 45
Modulation Schemes
How does the bit stream become an
electromagnetic wave ?
The purpose of a radio is to convert
a baseband signal (bit stream) into a
modulated electromagnetic signal. A
modulation scheme is selected that is
appropriate for the particular
electromagnetic spectrum. For
Wireless LANs there are two main
issues…
•Interference
•Multi-path distortion
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 46
Types of Modulation
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 47
Modulation
The process of altering the carrier signal that will enter the antenna
of the transmitter is called modulation.
There are three basic ways in which a radio carrier signal can be
modulated.
•Amplitude Modulated (AM) radio stations modulate the height
(amplitude) of the carrier signal.
•Frequency Modulated (FM) radio stations modulate the frequency of
the carrier signal as determined by the electrical signal from the
microphone.
•In WLANs, a third type of modulation called Phase Modulation is
used to superimpose the data signal onto the carrier signal that is
broadcast by the transmitter. In this type of modulation, the data bits
in the electrical signal change the phase of the carrier signal.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 48
Signals and Noise on a WLAN
Narrowband is the opposite of spread spectrum technology. As the
name implies narrowband does not affect the entire frequency
spectrum of the wireless signal. One solution to a narrowband
interference problem could be simply changing the channel that the AP
is using. Actually diagnosing the cause of narrowband interference can
be a costly and time-consuming experience. To identify the source
requires a spectrum analyzer and even a low cost model is relatively
expensive.
All band interference can affects the entire spectrum range. Bluetooth™
technologies hops across the entire 2.4 GHz many times per second
and can cause significant interference on an 802.11b network. It is not
uncommon to see signs in facilities that use wireless networks
requesting that all Bluetooth™ devices be shut down before entering.
Leakage from a microwave of as little as one watt into the RF
spectrum can cause major network disruption. Wireless phones
operating in the 2.4GHZ spectrum can also cause network
disorder.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 49
Generally the RF signal will not be affected by even the most extreme
weather conditions.
However, fog or very high moisture conditions can and do affect
wireless networks.
Lightning can also charge the atmosphere and alter the path of a
transmitted signal.
The first and most obvious source of a signal problem is the
transmitting station and antenna type. A higher output station will
transmit the signal further and a parabolic dish antenna that
concentrates the signal will increase the transmission range.
In a SOHO environment most access points will utilize twin
omnidirectional antennae that transmit the signal in all directions
thereby reducing the range of communication.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 50
Wireless Allocations in Canada
This color means ‘Fixed Service’
5 Ghz Band
Sep-03 ©Cisco Systems
2.4 Ghz Band
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 51
5150-5250 MHz, 5250-5350 MHz and 5725-5825 MHz.
The band is shared with some pretty noisy services.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 52
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 53
The modulation scheme must take this into consideration.
Spectrum efficiency is NOT an issue.
The use of the spectrum in the bands 5150-5250 MHz,
5250-5350 MHz and 5725-5825 MHz for LE-LAN devices is on
the basis that such devices cannot claim protection from other
radio systems and cannot cause harmful interference into other
radio services in these bands. (LE-LAN = License Exempt LAN)
1 W transmitter output power; a power spectral density of 17
dBm in any 1 MHz band; a maximum 4 W EIRP; and fixed, pointto-point LE-LAN devices operating in this band may employ
transmitting antennas with directional gain up to 23 dBi.
Note: Reference Antenna = an antenna measured at Central States
dBd = dB of Gain over a Dipole
dBi = dB of Gain over an Isotropic Radiator
EIRP is the equivalent isotropically radiated power. EIRP represents the total
effective transmit power of a radio, including gains that the antenna provides and
losses from the antenna cable. You must take all of these into account when
calculating the EIRP for a specific radio.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 54
The 2.4 Ghz Band is similar.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 55
Then there is the problem of Multi-path distortion
interference.
Which will distort
a nice clean pulse.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 56
Standard
802.11
802.11a
802.11b
Data Rate
≤ 2Mbps
2.4GHz
≤ 54Mbps
5GHz
≤ 11Mbps
2.4GHz
802.11g
≤ 54Mbps
Bluetooth
Up to 2Mbps
2.45GHz
2.4GHz
Sep-03 ©Cisco Systems
Modulation
Scheme
FHSS or
DSSS
Pros/Cons
This specification has been extended into 802.11b.
OFDM
"Wi-Fi Certified." 8 available channels. Less potential for
RF interference than 802.11b and 802.11g. Better than
802.11b at supporting multimedia voice, video and largeimage applications in densely populated user
environments. Relatively shorter range than 802.11b. Not
interoperable with 802.11b.
DSSS with
CCK
"Wi-Fi Certified." 14 channels available. Not
interoperable with 802.11a. Requires fewer access points
than 802.11a for coverage of large areas. High-speed
access to data at up to 300 feet from base station.
OFDM >
20Mbps
DSSS + CCK
< 20Mbps
"Wi-Fi Certified." 14 channels available. May replace
802.11b. Improved security enhancements over 802.11.
Compatible with 802.11b.
FHSS
No native support for IP, so it does not support TCP/IP
and wireless LAN applications well. Best suited for
connecting PDAs, cell phones and PCs in short intervals.
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 57
Spread Spectrum modulation schemes ease address
problems, each in their own way.
•DSSS Direct Sequence Spread Spectrum
•OFDM Orthogonal Frequency Division Multiplexing
•FHSS Frequency Hopping Spread Spectrum
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 58
FHSS vs DSSS
With FHSS, a data packet is first sent on a random channel
in the band with the next packet sent, after a pause of a
few milliseconds, on another random channel in the band.
With 80 channels or more available (one channel per MHz,
e.g., 2401, 2402, 2403, etc) signals from multiple radios
"hop" around each other. This is how they can operate with
other radios in the same band with minimal interference.
DSSS radios operate on a fixed radio channel, but the
signal is "spread" on that channel by mixing the signal with
a Pseudo-Noise (PN) code. This spreading causes the radio
signal with the data on it to occupy a much wider band,
and looks more like noise to receivers not designed to "despread " that signal.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 59
•DSSS Direct Sequence Spread Spectrum
The result is a string of chips.
•In DSSS individual pulses are increased to a much higher
frequency by multiplying them with a code that is unique to each
WLAN. All the stations know the code.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 60
•DSSS Direct Sequence Spread Spectrum
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 61
•DSSS Direct Sequence Spread Spectrum
DSSS has good interference rejection.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 62
OFDM
Orthogonal Frequency Division Multiplexing (OFDM) is a special form of multicarrier modulation, initially proposed in the 1970s. It is particularly suited for
transmission over a dispersive (i.e., frequency selective) channel.
In a multipath channel, most conventional modulation techniques are
sensitive to interference. OFDM is significantly less sensitive to interference,
because a special set of signals is used to build the composite transmitted
signal. The basic idea is that each bit occupies a frequency-time window
which ensures little or no distortion of the waveform. In practice, it means that
bits are transmitted in parallel over a number of frequency-nonselective
channels. Applications of OFDM are found in
•Digital Audio Broadcasting (DAB) and
•Digital Video Broadcasting over the terrestrial network: Digital Terrestrial
Television Broadcasting (DTTB). In the DTTB OFDM transmission
standard, about 2,000 to 8,000 subcarriers are used.
•Wireless LANs, 802.11a and 802.11g
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 63
OFDM Orthogonal Frequency Division Multiplexing
Direct
signal.
Original reflected
signal.
Longer reflected
signal.
In OFDM, a serial bit stream of 10 bits are converted into 10 parallel bits, each
of which modulates its own radio carrier. Each carrier is now carrying a bit rate
that is 1/10th the bit rate of the original. A reflected signal path needs to be 10
times longer to cause the same interference. Longer paths are more attenuated
so the strength of the interference is also less.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 64
OFDM Orthogonal Frequency Division Multiplexing.
Where does orthogonal come from ?
If the individual Radio Carriers are
separated by exactly the bit rate…
…then they will always be zero at the adjacent carrier
frequency and there will be no interference between them.
OFDM has good multi-path rejection.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 65
FHSS Frequency Hopping Spread Spectrum
FHSS also uses many frequencies, but only one at a time. The baseband jumps
around very rapidly from one frequency to the next according to a predetermined
pattern that is unique to each WLAN. Any interfering signal strong enough to
cause an error will only affect the particular packet on that frequency.
The transport layer would just resend that packet.
Sep-03 ©Cisco Systems
CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 66
Layer 2
WLAN authentication occurs at Layer 2.
It is the process of authenticating the
device not the user.
There three states….
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Layer 2 - Authentication
Unauthenticated and unassociated
The node is disconnected from the network and not
associated to an access point.
Authenticated and unassociated
The node has been authenticated on the network but
has not yet associated with the access point.
Authenticated and associated
The node is connected to the network and able to
transmit and receive data through the access point.
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Active scanning causes a probe request to be sent from the wireless
node seeking to join the network. The probe request will contain the
Service Set Identifier (SSID) of the network it wishes to join. When
an AP with the same SSID is found, the AP will issue a probe response.
Service Set Identifier (SSID)
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Passive scanning nodes listen for beacon management frames
(beacons), which are transmitted by the AP (infrastructure mode) or
peer nodes (ad hoc). When a node receives a beacon that contains the
SSID of the network it is trying to join, an attempt is made to join
the network. Passive scanning is a continuous process and nodes may
associate or disassociate with APs as signal strength changes.
Service Set Identifier (SSID)
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Authentication
Open Authentication.
This is an open connectivity standard in which only the SSID must
match. This may be used in a secure or non-secure environment
although the ability of low level network ‘sniffers’ to discover the
SSID of the WLAN is high.
Shared Key.
This process requires the use of Wireless Equivalency Protocol (WEP)
encryption. WEP is a fairly simple algorithm using 64 and 128 bit keys.
The AP is configured with an encrypted key and nodes attempting to
access the network through the AP must have a matching key.
Statically assigned WEP keys provide a higher level of security than
the open system but are definitely not hack proof.
Wi-Fi Protected Access (WPA).
Subset of 802.11i and backward compatible. Now mandatory for
certification. Usually requires a server.
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The payload of wireless and 802.3 frames is 1500 bytes; however, an Ether
frame may not exceed 1518 bytes whereas a wireless frame could be as large
as 2346 bytes. Usually the WLAN frame size will be limited to 1518 bytes as
it is most commonly connected to a wired Ethernet network.
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WLAN Access
The basic access method for 802.11 is the Distributed Coordination Function
(DCF) which uses Carrier Sense Multiple Access / Collision Avoidance (CSMA /
CA). This requires each station to listen for other users. If the channel is idle,
the station may transmit. However if it is busy, each station waits until
transmission stops, and then enters into a random back off procedure. This
prevents multiple stations from seizing the medium immediately after
completion of the preceding transmission.
– 61 % OH @ 54 Mbps link sending
512 byte data => ~ 21 Mbps useful..
SIFS Short Inter-Frame Spacing
PIFS PCF Inter-Frame Spacing = SIFS + slot time
DIFS DCF Inter-Frame Spacing = SIFS + 2*slot time
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The hidden node problem
1.
2.
3.
4.
5.
6.
– 72 % OH @ 54 Mbps link
sending 512 byte data => ~
15 Mbps useful.
Sep-03 ©Cisco Systems
The station listens before it sends.
If someone is already transmitting, wait for a
random period and try again (as normal).
If no one is transmitting then it sends a short
message. This message is called the Ready To
Send message (RTS).
This message contains the destination address
and the duration of the transmission. Other
stations now know that they must wait that long
before they can transmit.
The destination then sends a short message
which is the Clear To Send message (CTS). This
message tells the source that it can send without
fear of collisions.
Each packet is acknowledged. If an
acknowledgement is not received, the MAC layer
retransmits the data. This entire sequence is
called the 4-way handshake as shown by figure 7
below.
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VPN - Using an integrated server VPN technology creates a tunnel on top of an
existing protocol such as IP. This is a Layer 3 connection as opposed to the Layer
2 connection between the AP and the sending node.
EAP-MD5 Challenge – Extensible Authentication Protocol is the earliest
authentication type, which is very similar to CHAP password protection on a
wired network.
LEAP (Cisco) – Lightweight Extensible Authentication Protocol is the type
primarily used on Cisco WLAN access points. LEAP provides security during
credential exchange, encrypts using dynamic WEP keys, and supports mutual
authentication.
User authentication – Allows only authorized users to connect, send and receive
data over the wireless network.
Encryption – Provides encryption services further protecting the data from
intruders.
Data authentication – Ensures the integrity of the data, authenticating source
and destination devices.
Power over Ethernet - Support staff can disable a PoE-enabled access point by
shutting off its power after detecting a breach of security.
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Wireless Security – Home users
1.Change the default SSID (network name).
2.Disable the SSID broadcast option.
3.Change the default password needed to access a wireless
device.
4.Enable MAC address filtering.
SSID (service set identifier) a 32-character unique identifier attached
to the header of packets sent over a WLAN that acts as a password
when a mobile device tries to connect to the BSS. The SSID
differentiates one WLAN from another, so all access points and all
devices attempting to connect to a specific WLAN must use the same
SSID. A device will not be permitted to join the BSS unless it can
provide the unique SSID. Because an SSID can be sniffed in plain text
from a packet it does not supply any security to the network.
An SSID is also referred to as a network name because essentially it is a
name that identifies a wireless network.
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END
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CCK
•Short for Complementary Code Keying, a set of 64 eight-bit code
words used to encode data for 5.5 and 11Mbps data rates in the
2.4GHz band of 802.11b wireless networking.
•The code words have unique mathematical properties that allow them
to be correctly distinguished from one another by a receiver even in
the presence of substantial noise and multipath interference.
•CCK works only in conjunction with the DSSS technology that is
specified in the original 802.11 standard. It does not work with FHSS.
•CCK applies sophisticated mathematical formulas to the DSSS codes,
permitting the codes to represent a greater volume of information per
clock cycle.
•The transmitter can then send multiple bits of information with each
DSSS code, enough to make possible the 11Mbps of data rather than
the 2Mbps in the original standard.
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