CSC 290e Mobile Computing

wireless networks (802.11 wrapup)

31 October 2001

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Last time:

Medium access control
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  we described the protocol for gaining access to the network.  It was not
  guaranteed fair, but the extensions increased the fairness.
  

Does not handle hidden terminals
  introduce a RTS - request to transmit (sent by sender) 
              CTS - clear to transmit   (sent by receiver)
  
Still does not guarantee a maximum access delay
  add point co-ordination function 
  cannot be used in ad hoc networks (there is no coordinator!)
  

MAC Management
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handle integration with wired, integration of multiple cells, synchronization
of stations, etc

Synchronization - functions to support finding a wireless LAN, synchronization
of internal clocks, generation of beacon signals
  
  quasi-periodic beacon - not transmitted if medium is busy
        used for updating local clock
   in infrastructure networks, access point sends beacon
      all other synch to ap
   in ad hoc networks, everyone tries to beacon, random backoff
      the "winning" beacon causes others to update their clocks

Power management - functions to control transmitter activity for power
conservation, e.g., periodic sleep, buffering, without missing a frame

  turn off transceiver when not needed
  periodically wake up to see if anything is waiting for you
               wake up at the *right* time, need synchronization
  infrastructure - AP buffers frames for sleeping nodes
                   AP includes traffic indication map with each beacon
                       - includes stations with waiting frames
                   multicast and broadcast packets are not buffered
                             cannot sleep until hear a broadcast beacon
  ad hoc - more complicated
           each station buffers data for sleeping nodes
           destination announced when all nodes are awake
           

roaming - functions for joining a network (association), changing access
points, scanning for access points

  need more than one access point to cover department
  for good quality: 10-20 m
  Steps for roaming:
    1. decide current quality is too low, 
    2. start scanning for other APs
       passive scanning: listen to medium for beacons
       active scanning: send a probe on each channel, and wait for response
    3. select best AP based on signal strength, send "association request"
    4. new AP answer with an "association response".  If response is
       successful, then the station has successfully roamed, else go to 2
    5. new AP updates with DS (distribution system) that it now handles
       the node.  

Management information base - all parameters representing the current state of
a wireless stations and an access point are stored within a MIB for internal
and external access.  

  can be accessed via standardized protocols such as the simple network
  management protocol (SNMP).
 
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802.11a - 5GHz band
          up to 20Mb/s

802.11b - 2.4 GHz
          11 Mb/s

WPAN - wireless personal area networks
       does not want to have separate standard, but need to address low cost,
       low power, plus small form factor
       e.g., bluetooth consortium

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