Design Considerations & Emerging Standards
Carrier Sense Multiple Access / Collision Detect.
Practical limit on 802.3 Nodes per collision domain
Listen before you talk.
All stations
share (access)
the common
media.
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.
The first station to detect a collision sends a
32 bit ‘Jam’ signal.
◦ All stations stop sending for at least 9.6
microseconds
The two stations that caused the collision
then calculate a “Backoff Period” Before
retrying.
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.
CSMA/CD
Send and Receive share the same bus
Half Duplex
Collision Domain
Switches Required
Switches create 2 virtual bus’s per connection
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.
Waits for each frame to be ACKd
If ACK not received, Collusion Assumed
◦ Takes LONGER then CD.
◦ More devices -> More Collisions -> More Wait Time
Back to the rules of Shared Media
Each section of air is Shared Media
Each Channel is a segment (at a certain
point.)
Radio Frequency
2.6 Ghz, 5.2 Ghz
Knows no boundaries
Unprotected from outside signals
Distance Sensitive
◦ Law of Inverse Square
Regulated differently in each country.
More power = More Distance.
◦ Sorta.
LOTS of interference
◦ Devices operating in the 2.4 GHz range include:
Microwave ovens.
Bluetooth devices.
Baby monitors.
Cordless telephones.
Building Security Systems
NO Overlap between Channels 1, 6, and 11.
All other channels, to bad!
Relatively unused.
Less Interference.
More Available Channels
Shorter Wavelength = ½ theoretical coverage
Absorbed more readily by solid objects.
Pros
◦ More Bandwidth
◦ More Channels
◦ Less Interference
Cons
◦ Less Coverage Area
◦ Lower Penetration
The Drawbacks” of 5 Ghz actually HELP K-12
Uses Different Frequency Hopping to pack
more of the RF Space.
Therefore, the faster the network, the more
“attack surface” for interference it has.
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.
802.11ac MultiGbps via MU-MIMO
802.11n
◦ 20 Mhz Channels X 3 Streams = 300 MB
◦ 40 Mhz Channels X 3 Streams = 600 MB
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
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
Limit Association Rates
Try for uniform Device Radio Types
http://www.nojitter.com/post/240000795/rfinterference-when-things-go-bump-in-theair
Bluetooth hops across the 2.4 GHz 1,600
times a second,
Interference – What’s already there?
Building Construction – Brick Walls?
Area to Cover
Type of service
◦ (VoIP/Wifi?)
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
Dual Band Devices
◦ 802.11 abgn
◦ List “preferred” devices with abgn support
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