CSC 290e Mobile Computing

wireless networks (Bluetooth/IEEE 802.11)
29 October 2001

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Important announcements

Project 2: Part 1 due today!!!
  if you try to load multiple SlaveAgentOneHost, 
    start them on different ports!!

"midterm exam" next Monday.  
         Short, little->no studying required
         One page of notes allowed

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Extra information
  standard industry data rate: 115 kbit/s
  IrDA - infrared data association: standards up to 4Mbit/s
  VFIr - very fast Infrared: 16Mb/s

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Bluetooth

Short range wireless capability
  10m
  ~1MB/s

Applications
  data and voice access points - real time voice and data
  cable replacement (10m->100m)
  ad hoc networking

piconet - 1 master with 1 to 7 slaves
          master determines the frequency hopping sequence
          1600 hops per second - slot = 0.625 ms
          master grants permission to slaves to communicate
          FH-TDD-TDMA
            frequency hop
            time division duplex
              alternate between master and slave
            time division multiple access 
              because more than two devices share the medium
          packet lengths: 1,3,5 slots in length
            do not frequency hop in the middle of a packet
            afterward, skip ahead in frequency sequence to get back on track
          
          
scatternet - collection of piconets, with or without overlap
           each piconet on different freq. hop. seq.
           collisions occur if two piconets hop to same freq at same time
           
use power control to limit the range of the communication

to connect, master page for a device on each of the 32 "wake-up" carriers
  standby-by devices periodically listen for pages

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802.11

Goals:
  simple and robust WLAN 
  time-bounded and asynchronous services
  operate with multiple physical layers (e.g., ir, radio)
  support power management
  handle hidden nodes
  operate worldwide (2.4 GHz ISM band)

Physical layers are part of the specification
   IR or radio -- FHSS and DSSS (direct sequence spread spectrum)

Medium access control
  control medium
  three basic access mechanisms
    mandatory basic method: based on CSMA/CD
    option method avoiding hidden terminal
    contention free polling method for time bound service

  wait DIFS (DCF inter-frame spacing) - lowest priority for medium access
  if sense nothing, transmit immediately
  else, choose random backoff time within the contention window and delay
        for this period of time
        
  as soon as sense busy, wait again for DIFS

  but this is not fair.  Each node has the same chances for transmission
      to provide fairness, add backoff timer
      each node selects a random waiting time
        if a station does not get access to the medium, 
           it stops the backoff timer, 
              wait for idle for DIFS
              starts counter again
           when counter expires, access granted to medium
      this means that deferred stations do not choose new random number
        and since they are counting down, statistically they will have
        a lower value
  See example - mobile communications p175, figure 7.11