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OECG – Chapter 22

807 Words. Plan about 5 minute(s) to read this.

802.11a – wireless LAN specification.  Up to 54Mbps using OFDM in the 5GHz band.

802.11b – wireless LAN specification.  Up to 11Mbps using DSSS in the 2.4GHz band.

802.11g – wireless LAN specification.  Up to 54Mbps using OFDM in the 2.4GHz band.  Backwards compatible with 802.11b.

802.11n – wireless LAN specification.  Not yet ratified.  Will have high data rates beyond 100Mbps using multiple antennas to send/receive data.  (Parallel channels.)

active scanning – an 802.11 radio sending a probe request frame on each RF channel.  The radio cards determine what AP to associate with based on the signal stregth of the AP’s response to the probe request frame.

ad hoc mode – a wireless LAN with no AP.  Data flows happen from user to user directly.

AES – advanced encryption standard.  Part of the 802.11i standard.  Much stronger than TKIP and WEP.

association ID – the ID assigned to a radio card by an AP when the card associates.  This ID is used by power-save mode.

beacon – a wireless LAN frame sent by APs and radio cards.  Used so that stations can discover the wireless LAN and for coordination within the LAN for tasks such as power-save mode.

distributed coordination function – a method of carrier access in 802.11 wireless LANs.  CSMA/CD.

DSSS – direct sequence spread spectrum – a spread spectrum method that spreads RF using a longer code to represent a data bit.  Used by 802.11b.

DTIM interval – the number of beacons defining when multicast frames and sent over a wireless LAN.
FHSS – frequency hopping spread spectrum – a spread spectrum method that spreads RF using different frequencies in a defined hopping pattern.

fragmentation – in a wireless LAN, dividing a frame into fragments.  Used as a congestion management technique.
infrastructure mode – a wireless LAN built around an AP or multiple APs.  Wireless radio card frames are relayed only through the AP.  Users can roam from radio cell to radio cell created by each AP.

multipath – when parts of an RF signal travel different paths, resulting in parts of the signal arriving at the receiver at different times.  This can cause bit errors.

network allocation vector – a counter tracked by every wireless station.  This is set by a sender’s frame, in which the sending duration is included.  The NAV must be 0 before another station is eligible to send.

OFDM – orthogonal frequency division multiplexing – sends data over multiple subcarriers at the same time.  Not particularly bothered by multipath interferenced.  Used by 802.11a and g.

passive scanning – a station monitors RF channels, listening for beacon frames.  The signal strength of the sending APs is used to determine what AP the station will associate to.

point coordination function – a method of carrier access where the AP polls radio cards before they are allowed to send traffic.  Not used in the real world.

power-save mode – conserving battery power in mobile laptops with wireless radio cards.  An AP will buffer frames for cards in power-save mode.  The radio card in power-save mode wakes up in time to receive beacon frames from the AP.  The AP will notify stations of buffered frames via the beacon frame.  The station will wake up long enough to receive traffic.

RF channel – the frequency on which an AP and radio card are broadcasting.  You can set this to different frequencies, depending on the wireless specification.

RTS/CTS – request to send/clear to send.  A way of reducing collisions when 2 radio cards can hear the AP, but not each other.  Requires a sending station to send an RTS frame to the AP, and wait for the AP to send a CTS frame before transmitting.

SNR – signal to noise ratio – not an actual ratio so much as the difference between the power of signal and the power of “noise” on the frequency at a time.  The higher the SNR, the better the wireless LAN is able to perform.

spread spectrum – spreading power over a wide wide chunk of spectrum over time, reducing interference between transmitters.

SSID – service set identifier – a way to identify a particular wireless LAN.  APs and radio card must have the same SSID before they can associate.

TKIP – temporal key integrity protocol – part of the 802.11i standard.  Uses a key that automatically updates.  Stronger than WEP, weaker than AES.

transmit power – the strength of an RF signal.  Measured at the output of a radio card or AP in milliwatts, watts, or dBm.

WEP – wired equivalent privacy – uses an RC4 cipher based on a known shared key to encrypt data.  The key does not change, and its predictable nature make it susceptible to snoopers.

WPA – Wi-Fi Protected Access – a standard using bother TKIP and AES as ratified by the Wi-Fi Alliance.