IEEE 802.11

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IEEE 802.11
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Approved IEEE 802.11 Standards

• IEEE 802.11b
• 2.4 GHz
• 11 Mbps ( 6 Mbps real throughput)
• DSSS
• IEEE 802.11a
• 5 GHz
• 54 Mbps ( 31 Mbps real throughput)
• OFDM
• IEEE 802.11g
• 2.4 GHz
• 54 Mbps ( 31 Mbps real throughput)
• OFDM

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CSMA/CA

• Cant detect collisions so CSMA/CA attempts to
  avoid collisions
• When a node wishes to transmit it senses the
  medium
• If it is free for a time called the Distributed
  Interframe Space (DIFS) it transmits the packet
  and waits for an acknowledgement that the packet
  arrived intact
• If no acknowledgement is received the node will
  try again
• If the medium is busy the node generates a random
  backoff time and waits before trying again

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CSMA/CA

• In cases where collisions are likely the nodes
  use a method called point coordination function
• When ready to transmit and the medium is idle the
  node transmits a short frame called the Ready to
  Transmit (RTS) frame
• The access point then transmits Clear to Send
  (CTS) frame if it ready to receive
• Adds considerable overhead

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Transmission Issues

• Overhead
• CSMA/CA has considerable overhead
• Hidden Nodes
• Two nodes on opposites sides of an access point
  that cant detect one another but can transmit to
  the access point
• Move the nodes or any obstructions between them
• Reduce packet size reducing transmit time
• Use RTS/CTS
• Request to send (RTS) is sent before transmitting
• Clear to send (CTS) is sent to ok transmission
• Required when 802.11b and 802.11g nodes coexist

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Transmission Issues

• Near/far
• When nodes near the access point have higher
  power levels than nodes farther from the access
  point
• Adjust power levels
• Excessive error rates
• Reduce packet size until maximum throughput is
  reached
• Interference
• Distance

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802.11 Response to Weak Signals
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Effect of Increasing distance or interference on
802.11a
Wireless LANs operate at their highest data rate
only under ideal conditions. If the signal is
weak because of distance or interference, they
will drop to lower data rates.
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Frequency Channels

• The spectrum used by a wireless LAN is divided
  into channels
• Since 802.11a operates at a higher spectrum it
  has more channels
• 802.11b and g 83.5 MHz provides 3 channels
• 802.11a 300 MHz provides 12 channels

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Overlapping 802.11 b and g Channels
The frequency range allocated to IEEE 802.11b is
split into 11 channels. However, in order to
achieve the bandwidth it does, more than one
channel is necessary to achieve the highest
bandwidth. Only channels 1, 6 and 11 are far
enough apart to not interfere with one another.
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Access Point Coverage
In a building, an access points range will extend
from 50 to 150 feet though the lower end of this
range is more likely. With an omnidirectional
antenna, the coverage is spherical.
Overlapping cells use different frequencies.
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Access Point Placement
Placement of access points using the 3
802.11b non-overlapping channels on a single
floor. Can be difficult with only three
channels. 802.11a offers 12 non-overlapping
channels.
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Network Organization
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Infrastructure Mode
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Ad Hoc Mode
WLAN NICs transmit directly to one another.
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Designing the Network

• An RF site survey is useful to
• Determine background interference
• Determine range of access points
• The RF site survey will take into account the
  architecture of the building, number of users,
  power levels required and other factors to
  determine
• Number of access points
• Placement of access points
• Power levels required
• Placement of Channels

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RF Site Survey

• Site surveys can be conducted with spectrum
  analyzers
• Expensive and requires some skill
• Another option is to use a wireless NIC and
  software to look for access points
• Cheap and easy but doesnt catch radiation from
  other sources

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Site Surveys Required?

• Thin access points are cheaper so more can be
  deployed reducing the need for site surveys
• Radio frequency spectrum management (RFSM) tools
  automatically configure parameters such as radio
  channel and transmit power dynamically and can
  identify holes in coverage

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Spectrum Analysis of FM range
Each peak represents a different FM radio
station.
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Narrowband vs. Spread Spectrum Signals
Spread spectrum signals are spread across a
wide band of frequencies at very low power.
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Spectrum Analysis of DSSS Signal
Since the signal is low power it is difficult to
detect.
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DSSS Spectrum
Signal peaks briefly at different points.
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DSSS Spectrum
Analyzer can hold peaks over time so that full
signal is recorded over time.
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DSSS Spectrum with Interference
Effect of a microwave oven on the DSSS signal.
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Locating Access Points

• Data rate
• Nodes must be closer to access points and power
  levels must be high to achieve high data rates
• Noise
• Noise sources should be avoided if possible
• Signal
• Signal level must be high enough
• Signal to noise ratio
• Signal to noise ratio should be gt 20 dB
• Security
• Should try to limit signal to within premises
• Must be able to mount access points securely

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Securing WLANs

• Authentication
• Encryption
• Intrusion tools
• Structured Wireless-Aware Network Architecture
  (SWAN)

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Authentication

• Access points broadcast a Service Set Identifier
  (SSID) every 100 ms
• When a wireless NIC receives the SSID, it can
  request an IP address and access the network
• By default, the SSID is broadcast in the clear
  and access points require no proof that a NIC is
  allowed to use the network
• MAC filtering

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Encryption

• Data is encrypted using a randomly generated key
• Any encryption can be broken as long as you have
  sufficient examples of the encrypted data
• The longer the key, the more difficult it is to
  break the encryption
• By default, encryption is not configured on most
  access points
• To reduce the chance that the encryption is
  broken
• Use a long key
• Change the key often

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Wireless Equivalency Privacy (WEP)

• WEP uses a single shared key that stays the same
  unless it is changed manually on every device
• Key can only be 40, 64 or 128-bits long
• Key is combined with a randomly generated 24-bit
  initialization vector
• IV is transmitted in the clear

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Wi-Fi Protected Access (WPA)

• Stronger encryption than WEP
• TKIP for WPA
• AES for WPA2 (IEEE 802.11i)
• Use of 128-bit dynamic keys
• Authentication
• RADIUS server
• RADIUS server authenticates nodes and transmits a
  master key to the AP and node
• Master key is used to generate new keys for every
  frame transmitted

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Security Standards