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WiFi in K-12

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

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  1. WiFi in K-12 Design Considerations & Emerging Standards

  2. Wired vrs Wireless

  3. CSMA/CD • Carrier Sense Multiple Access / Collision Detect. • Practical limit on 802.3 Nodes per collision domain

  4. Carrier Sensing • Listen before you talk.

  5. Multiple Access All stations share (access) the common media.

  6. 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.

  7. Collision Detection

  8. Listen WHILE WE TALK

  9. How Collisions are Detected.

  10. 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.

  11. “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.

  12. Half Duplex (Ethernet on Hubs) • CSMA/CD • Send and Receive share the same bus Half Duplex Collision Domain

  13. Full Duplex (Ethernet on Switches) • Switches Required • Switches create 2 virtual bus’s per connection Collision Domain Collision Domain

  14. 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.

  15. 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

  16. 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.)

  17. RF Engineering

  18. RF • Radio Frequency • 2.6 Ghz, 5.2 Ghz

  19. Characteristics of RF • Knows no boundaries • Unprotected from outside signals • Distance Sensitive • Law of Inverse Square • Regulated differently in each country.

  20. Power Output Levels • More power = More Distance. • Sorta.

  21. Law of Inverse Square

  22. 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

  23. NO Overlap between Channels 1, 6, and 11. • All other channels, to bad!

  24. 5 Ghz Band • Relatively unused. • Less Interference. • More Available Channels • Shorter Wavelength = ½ theoretical coverage • Absorbed more readily by solid objects.

  25. 5 Ghz Channels

  26. Line of Sight Transmission

  27. 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

  28. 802.11 Standards

  29. Digital Spread Spectrum

  30. Frequency Hopping

  31. 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.

  32. 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

  33. Spatial Multiplexing

  34. MIMO (Spatial Multiplexing)

  35. Beamforming

  36. 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

  37. 40Mhz channels in 2.4 Ghz

  38. “Exposed Node Problem” Room 106 Room 108

  39. 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

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

  41. RF Bandwidth Conention

  42. Multiple Access Points

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