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OECG – Chapter 22

717 Words. Plan about 4 minute(s) to read this.

Infrastructure Mode Operation – the most common mode in use today. Infrastructure mode has these common elements:

  • Scanning – radio cards perform scanning to find APs.  Scanning is used to first discover an AP, and then happens from time to time thereafter to support roaming.  802.11 defines both passive and active scanning.
    • Passive scanning
      • A radio card will tune to every RF channel, listen for a bit, and note information discovered about each AP on a channel.
      • APs send beacon frames every 100ms on the RF channel configured by the administrator.
      • While the radio card is scanning on a channel, it will receive these beacon frames from the AP.
      • The radio card notes the signal strength of the beacon frame and proceeds to scan other channels.
      • Once scanning of all RF channels is complete, the radio card will decide what AP to associate to, usually the AP with the strongest beacon signal.
    • Active scanning
      • A radio card will send probe request frames on all RF channels.
      • An AP receiving the probe request will send a probe response.
      • The card deviced what AP to associate with based on information in the probe response frame.
    • On both active and passive scanning, the radio card may make decisions about which AP to associate with depending on other factors such as noise and utilization.
  • Connecting with a network
    • Open system authentication
      • The radio card sends an authentication frame.
      • The AP responds with (you’ll never guess) an authentication response frame.
    • Optional shared key authentication
      • WEP (easy to crack)
      • 802.1x is much stronger
      • Book does not mention (at least not here) WPA, WPA2, TKIP, etc.
    • Upon completion of the authentication handshake, the radio card sends an association request frame to the AP.  This frame includes a service set identifier (SSID) and the data rates supported by the card.  The SSID must match what has been configured on the AP.
    • The AP responds with an association response frame which has an association identifier (AID), a number representing the card’s association.
    • Now the card is “associated” and can begin to send data frames.
  • Data transfer
    • Data exchange is bidirectional.  All data to and from a radio card traverses the AP.
    • ACKs are required by the destination station for every frame, adding quite a bit of overhead when compared to ethernet.  This ACK requirement is due to the likelihood of a transmission problem on a wireless medium when compared with wired.
    • Frames sent with no ACK in response will be retransmitted, up to a certain number of times.  Usually an unACK’ed wireless frame will be retransmitted 3 to 7 times.
    • Data rates can be automatically shifted to help with problems of retransmission, etc.
  • Roaming
    • Radio cards will scan periodically to update the AP list.  Some cards will only scan the RF channels where they had previously discovered an AP, saving on bandwidth.  The card cannot transmit while scanning.
    • If the signal of the associated AP gets too weak, the radio card will perform a reassociation process.  This uses a reassociation from sent to the new AP, and a disassociation frame to the old AP.  Note that there’s no authentication frame required when reassociating.
    • If the old AP has data frames buffered, heading for the radio card that just disassociated, then the old AP will send those frames along to the new AP so that they can be delivered to the radio card.

Ad Hoc Mode Operation – no APs involved, so the radio cards themselves send beacon frames.

  • The radio card is placed into ad hoc mode.  If no beacon frames are received for a certain amount of time, the card will transmit beacon frames himself.
  • When a beacon frame is received, the radio cards will wait a random period of time.
  • If no one else sends a beacon in that period of time, the station will send a beacon.  Since the wait time before sending this particular beacon frame is random, one of the ad hoc cards will start transmitting beacon frames before anyone else.  Over time, all the 802.11 ad hoc stations will roughly equally share the responsibility of transmitting beacon frames.  This process keeps one stations sending beacon frames if someone falls off the network.
  • A PC in ad hoc mode can be used as a wireless to wired gateway (like an AP) with the right software.