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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


Wired vrs wireless
Wired vrs Wireless


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.)



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 channels
5 Ghz Channels



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



Exposed node problem
“Exposed Node Problem”

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


So…..

  • Limit Association Rates

  • Try for uniform Device Radio Types


Rf bandwidth conention
RF Bandwidth Conention




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


Motion sensor
Motion Sensorhttp://www.nojitter.com/post/240000795/rf-interference-when-things-go-bump-in-the-air


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

  • Bluetooth hops across the 2.4 GHz 1,600 times a second,


Design considerations in wireless
Design Considerations in Wirelesshttp://www.nojitter.com/post/240000795/rf-interference-when-things-go-bump-in-the-air

  • 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 1:1http://www.nojitter.com/post/240000795/rf-interference-when-things-go-bump-in-the-air

  • 5 GHZ Devices

  • 802.11n or 802.11ac

  • One AP : Traditional classroom

  • One AP : 30 Students

    802.1x authentication on WiFi

    Limiting “casual” associations


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

  • Dual Band Devices

    • 802.11 abgn

    • List “preferred” devices with abgn support


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