u8.2 802.11

u8.2 802.11

1. Modes

   • Infrastructure Mode
     • For wireless devices wanting to connect to an established WiFi infrastructure and to connect onwards to computers in a wired NW
     • Has at least 1 Access Point (AP)
       • This AP and its associated devices are know as a Basic Service Set = BSS
         • 2 or more BSS on a single subnet = Extended Service Set ESS
   • Ad-Hoc mode
     • Intended for wireless devices that want to communicate between themselves
       • IE: group of laptops or PC's
       • Also called Peer to peer
       • Have no need to connect on to a wired network to access resources
       • Known as an independent Basic Service Set = IBSS

2. Speed

   • Common speed is 100 mbps
   • Interference, crowded environment etc can cause speeds to drop
   • If performance is critical then wifi may not be the best choice

3. Range

   • Common range is 100 m
     • Under ideal conditions
     • Reduced by building walls, floors, any obstructions, interference from other sources or frequencies
     • Can be exceeded in some cases

4. Physical layers

   • 3 Alternative Physical layers for 802.11
     • Frequency Hopping Spread Spectrum (FHSS) in the 2.4 Ghz Industrial, Scientific and Medical (ISM) band offering 75 channels.
     • Direct Sequence Spread Spectrum (DSSS) in the 2.4 Ghz Industrial, Scientific and Medical (ISM) band offering 14 channels (11 channels in the US).
     • One diffuse infrared layer.
   • Only one physical layer for each standard has gained market acceptance = DSSS.
   • ISM bands are unlicensed and alow anyone to transmit in that band
     • As a result other devices also use that band, wireless phones, baby monitors
       • All these devices cause interference