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WiFi in K-12. Design Considerations & Emerging Standards. Wired vrs Wireless. CSMA/CD. Carrier Sense Multiple Access / Collision Detect. Practical limit on 802.3 Nodes per collision domain. Carrier Sensing. Listen before you talk. Multiple Access.

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wifi in k 12

WiFi in K-12

Design Considerations & Emerging Standards

csma cd
CSMA/CD
  • Carrier Sense Multiple Access / Collision Detect.
  • Practical limit on 802.3 Nodes per collision domain
carrier sensing
Carrier Sensing
  • Listen before you talk.
multiple access
Multiple Access

All stations share (access) the common media.

sharing the medium
Sharing the Medium
  • 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.
collision detection recovery
Collision Detection Recovery
  • 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.
backoff period
“Backoff Period”
  • 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.
half duplex ethernet on hubs
Half Duplex (Ethernet on Hubs)
  • CSMA/CD
  • Send and Receive share the same bus

Half Duplex

Collision Domain

full duplex ethernet on switches
Full Duplex (Ethernet on Switches)
  • Switches Required
  • Switches create 2 virtual bus’s per connection

Collision Domain

Collision Domain

cd on wifi
CD on Wifi?
  • 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.
csma ca
CSMA/CA
  • Waits for each frame to be ACKd
  • If ACK not received, Collusion Assumed
    • Takes LONGER then CD.
    • More devices -> More Collisions -> More Wait Time
wifi vrs wired conclusion
WifiVrs Wired Conclusion
  • Back to the rules of Shared Media
  • Each section of air is Shared Media
  • Each Channel is a segment (at a certain point.)
slide19
RF
  • Radio Frequency
  • 2.6 Ghz, 5.2 Ghz
characteristics of rf
Characteristics of RF
  • Knows no boundaries
  • Unprotected from outside signals
  • Distance Sensitive
    • Law of Inverse Square
  • Regulated differently in each country.
power output levels
Power Output Levels
  • More power = More Distance.
    • Sorta.
2 4 ghz band
2.4 Ghz Band
  • LOTS of interference
    • Devices operating in the 2.4 GHz range include:
      • Microwave ovens.
      • Bluetooth devices.
      • Baby monitors.
      • Cordless telephones.
      • Building Security Systems
5 ghz band
5 Ghz Band
  • Relatively unused.
  • Less Interference.
  • More Available Channels
  • Shorter Wavelength = ½ theoretical coverage
  • Absorbed more readily by solid objects.
5 ghz conclusion
5 Ghz Conclusion
  • Pros
    • More Bandwidth
    • More Channels
    • Less Interference
  • Cons
    • Less Coverage Area
    • Lower Penetration

The Drawbacks” of 5 Ghz actually HELP K-12

802 11 modes
802.11 Modes
  • 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 11 standards1
802.11 Standards
  • 802.11a up to 54 Mbps in 5 Ghz Band
  • 802.11b up to 11 Mbps in 2.4 Ghz Band.
  • 802.11g up to 54 Mbps in 2.4 Ghz Band.
  • 802.11n up to 600 Mbps via MIMO
    • Technically supported in 2.4 GHZ.
  • 802.11ac MultiGbps via MU-MIMO
802 11n 802 11ac
802.11n & 802.11ac
  • 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)

Theoretical Bandwidth

slide45

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

slide46
So…..
  • Limit Association Rates
  • Try for uniform Device Radio Types
slide52

http://www.nojitter.com/post/240000795/rf-interference-when-things-go-bump-in-the-airhttp://www.nojitter.com/post/240000795/rf-interference-when-things-go-bump-in-the-air

bluetooth
Bluetooth
  • Bluetooth hops across the 2.4 GHz 1,600 times a second,
design considerations in wireless
Design Considerations in Wireless
  • Interference – What’s already there?
  • Building Construction – Brick Walls?
  • Area to Cover
  • Type of service
    • (VoIP/Wifi?)
  • Number of potential Clients
  • Total Bandwidth required
district supplied 1 1
District Supplied 1:1
  • 5 GHZ Devices
  • 802.11n or 802.11ac
  • One AP : Traditional classroom
  • One AP : 30 Students

802.1x authentication on WiFi

Limiting “casual” associations

slide57
BYOD
  • Dual Band Devices
    • 802.11 abgn
    • List “preferred” devices with abgn support
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