Design Considerations & Emerging Standards
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Carrier Sense Multiple Access / Collision Detect.
Practical limit on 802.3 Nodes per collision domain
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Listen before you talk.
All stations
share (access)
the common
media.
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Each station must wait at least 9.6
microseconds between packets
◦ InterPacket Gap (IPG)
◦ Allows receiver to process packet
◦ Also allows everyone a chance to use the medium.
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The first station to detect a collision sends a
32 bit ‘Jam’ signal.
◦ All stations stop sending for at least 9.6
microseconds
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The two stations that caused the collision
then calculate a “Backoff Period” Before
retrying.
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There are a certain number of availble values
for the random backoff period.
Once networks get to about 30 devices, the
backoff periods become congested.
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CSMA/CD
Send and Receive share the same bus
Half Duplex
Collision Domain
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Switches Required
Switches create 2 virtual bus’s per connection
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802.11 is Half Duplex
◦ Tx and Rx uses SAME space
◦ A radio can not Transmit and Receive
Simultaneously.
◦ Therefore, Collision Detection is not an option.
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Waits for each frame to be ACKd
If ACK not received, Collusion Assumed
◦ Takes LONGER then CD.
◦ More devices -> More Collisions -> More Wait Time
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Back to the rules of Shared Media
Each section of air is Shared Media
Each Channel is a segment (at a certain
point.)
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Radio Frequency
2.6 Ghz, 5.2 Ghz
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Knows no boundaries
Unprotected from outside signals
Distance Sensitive
◦ Law of Inverse Square
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Regulated differently in each country.
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More power = More Distance.
◦ Sorta.
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LOTS of interference
◦ Devices operating in the 2.4 GHz range include:
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Microwave ovens.
Bluetooth devices.
Baby monitors.
Cordless telephones.
Building Security Systems
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NO Overlap between Channels 1, 6, and 11.
All other channels, to bad!
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Relatively unused.
Less Interference.
More Available Channels
Shorter Wavelength = ½ theoretical coverage
Absorbed more readily by solid objects.
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Pros
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◦ More Bandwidth
◦ More Channels
◦ Less Interference
Cons
◦ Less Coverage Area
◦ Lower Penetration
The Drawbacks” of 5 Ghz actually HELP K-12
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Uses Different Frequency Hopping to pack
more of the RF Space.
Therefore, the faster the network, the more
“attack surface” for interference it has.
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802.11a
802.11b
802.11g
802.11n
up to 54 Mbps in 5 Ghz Band
up to 11 Mbps in 2.4 Ghz Band.
up to 54 Mbps in 2.4 Ghz Band.
up to 600 Mbps via MIMO
◦ Technically supported in 2.4 GHZ.
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802.11ac MultiGbps via MU-MIMO
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802.11n
◦ 20 Mhz Channels X 3 Streams = 300 MB
◦ 40 Mhz Channels X 3 Streams = 600 MB
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802.11ac
◦ 80 Mhz Channels x 4 streams = 1.7 Gb
◦ 160 Mhz Channels X 8 Streams = 6.9 Gb
 (No chipsets yet bond 160Mhz)
Room 106
Room 108
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IEEE 802.11 RTS/CTS mechanism helps to
solve this problem only if the nodes are
synchronized and packet sizes and data rates
are the same for both the transmitting nodes.
When a node hears an RTS from a
neighboring node, but not the corresponding
CTS, that node can deduce that it is an
exposed node and is permitted to transmit to
other neighboring nodes
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Limit Association Rates
Try for uniform Device Radio Types
http://www.nojitter.com/post/240000795/rfinterference-when-things-go-bump-in-theair
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Bluetooth hops across the 2.4 GHz 1,600
times a second,
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Interference – What’s already there?
Building Construction – Brick Walls?
Area to Cover
Type of service
◦ (VoIP/Wifi?)
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Number of potential Clients
Total Bandwidth required
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5 GHZ Devices
802.11n or 802.11ac
One AP : Traditional classroom
One AP : 30 Students
802.1x authentication on WiFi
Limiting “casual” associations
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Dual Band Devices
◦ 802.11 abgn
◦ List “preferred” devices with abgn support