BRKEWN-2017 – Understanding RF Fundamentals and the Radio Design of 11n/ac Networks

Presenter: Fred Niehaus, Technical Marketing Engineer, Cisco Wireless Networking Group

Basic understanding of radio:

  • Every radio wave has a physical size (proportionate to its wavelength)
  • The lower the frequency, the physically longer the radio wave
  • Higher frequencies have shorter waves and as such, takes more power to move them greater distances
  • 5Ghz UNI 2 and UNI 2 Ext overlap with DFS (RADAR)
  • Dynamic Freq Selection (DFS) requires that if a radio is operating in UNI 2/UNI 2 Ext and it hears something that is not WiFi, it has to get off that channel

Antenna basics:

  • Directional or omnidirectional (360 degree)
  • Cisco color markings: blue: 5, black 2.4, orange 2.4 and 5ghz
  • Low gain omnidirectional antennas radiate in a beach ball pattern
  • Omni-direction dipoles in a pancake pattern
  • Yagis are directional; radiate more like a flashlight
  • Patch: multiple elements in a square form factor; radiates like a flashlight
  • Dipole: does not require a ground plate as the bottom half of the antenna is the ground plate
  • Monopole requires a (conductive surface) ground plate; monopoles are shorter than dipoles and often used on APs (because of the smaller size) and they can use the metal in the AP as the ground plate.
  • Azimuth: how wide; Elev(ation): how high

Antenna patterns

  • Azimuth antenna pattern is looking straight down on the antenna
  • Elevation pattern is looking side on

Multiparty diversity and beam forming:

  • If signal arrives at a receiver on both the high side of the wave and the low side of the wave, they cancel each other out (receiver effectively gets nothing)
  • Even if they don’t totally cancel each other, they reduce signal strength
  • Beam forming allows the signal to be best directed towards the client

Understanding 802.11n and 802.11ac:

  • Too much to write…

Mid-span power injectors:

  • AIR-PWRIN2/3 – pre 802.11n APs
  • AIR-PWRIN4 – 802.3af (15.4W) and pre-standrard 30W
  • AIR-PWRIN5 – 802.3af
  • AIR-PWRIN6 – 802.3af and 802.3at << recommended for all new enterprise deployments
  • For reference: AP-2800 powers at 26W; AP-3800 powers at 30W

Multi User MIMO:

  • Beam forming. That is able to focus the RF at a client while sending NULLs to other clients (simultaneously)

Installation and deployment:

  • Tons of mounting options for Cisco APs
  • Vanity covers to change the color of the AP (Terrawave or Oberon are vendors)
  • The minute you put an antenna near something metal, you make it directional” — Fred; don’t put your antennas near metal stuff (1 meter/3ft separation)

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