Design Considerations & Emerging Standards

 Carrier Sense Multiple Access / Collision Detect.  Practical limit on 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 Collision Domain

 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.

 a up to 54 Mbps in 5 Ghz Band  b up to 11 Mbps in 2.4 Ghz Band.  g up to 54 Mbps in 2.4 Ghz Band.  n up to 600 Mbps via MIMO ◦ Technically supported in 2.4 GHZ.  ac MultiGbps via MU-MIMO

 n ◦ 20 Mhz Channels X 3 Streams = 300 MB ◦ 40 Mhz Channels X 3 Streams = 600 MB  ac ◦ 80 Mhz Channels x 4 streams = 1.7 Gb ◦ 160 Mhz Channels X 8 Streams = 6.9 Gb  (No chipsets yet bond 160Mhz)

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 IEEE 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 IEEE RTS/CTS

 Limit Association Rates  Try for uniform Device Radio Types

interference-when-things-go-bump-in-the- air

 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

◦ 5 GHZ Devices ◦ n or ac ◦ One AP : Traditional classroom ◦ One AP : 30 Students 802.1x authentication on WiFi Limiting “casual” associations

 Dual Band Devices ◦ abgn ◦ List “preferred” devices with abgn support

 Probes – frames advertise the WLAN network  Beacons – frames locate the WLAN network of the client  Authentication – access and entry into the WLAN network  Association – set up of a link between the AP and WLAN client

 War Drivers: Scanning devices are used to discover and exploit networks.  Hackers (Crackers): Where the hacker understands the complexity of the system and exploits its weaknesses.  Employees: End users make changes to the security of the system leaving it vulnerable to intrusion.

 PAN  LAN  MAN  WAN