polaris.umuc.eduitsTLMNtlmn645tlmn645cl3fa02.ppt

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					   TLMN 645
Class 3, Fall 2002
Wireless Infrastructure
         Quality of Service
•   1.
•   2.
•   3.
•   4.
•   5.
Analog vs. Digital - Signals
                                            Analog


                             “noise”
                                            Digital



                        Digital “Cliff” Effect
  Quality
                                        Analog


            Distance from Transmitter
    Multiple Access




     2   3   ….   N
1
Number of channels needed?

      How large should
      N be?


      2    3     ….      N
  1
           Erlang Definition
• Used for voice traffic.
• Represents the amount of traffic intensity
  carried by a channel completely occupied
• Example: a channel occupied for thirty
  minutes (30 minutes) during an hour is
  passing .5 erlangs of traffic
             TRAFFIC AMOUNT
For example, if a group of user made 30 calls in one hour,
        and each call had an average call duration of 5 minutes, then
        the number of Erlangs this represents is worked out as follows:
           Minutes of traffic in the hour
                        = number of calls x duration
           Minutes of traffic in the hour
                        = 30 x 5
           Minutes of traffic in the hour
                        = 150
           Hours of traffic in the hour
                        = 150 / 60
           Hours of traffic in the hour
                        = 2.5
          Traffic figure = 2.5 Erlangs
     Trunking


                    Cell with N channels out
                               1
                               2
                              …N

We measure traffic intensity A as
A=  tm = Erlangs
where  average arrival rate (calls/hr)
tm =average message length time (hrs.)
     Typical Message Lengths
• Phone call?
• Dispatch?
• Data?
Probability of Blocking

   P( A, C ) 
   GOS (Grade of Service) 
    probability of blocking 
     AC
           C!
     C

    A
           k
               k!
    k 0




   C= number of channels needed
   A=total traffic offered in Erlangs
Source:
Rappaport p.49
         Blocking Probability
• Blocking probability, number of channels C
  and total traffic offered are all interrelated.
• Specify two and the third is uniquely
  determined.
Sample Problem
Source:
Rappaport p.49
             FDMA




Source:IEC
             TDMA




Source:IEC
             TDMA




Source:IEC
                 CDMA
• Spread Spectrum modulation
• multiple access
• transmission bandwidth much larger than
  information bandwidth
• rf (radio frequency bandwidth) is
  determined by a function other than the
  information being sent
             CDMA

              CDMA


Direct Sequence   Frequency Hopping
                        Slow
                        Fast
                 OFDM
Single carrier
approach




Multi carrier
approach
OFDM
     Data Multiple Access
• CSMA



• DSMA
Hidden Node
  TLMN 645 Paper Description
         Fall 2002
• Choose one of the wireless technologies
  addressed in the TLMN 645 course.
  Describe the technology from both the
  implementers (i.e.. What does it do
  technically) and user (what does it do for
  the user) perspective.
     Paper Description Slide 2
• Identify and describe two real case studies
  where the technology was implemented.
  Explain in detail how the implementations
  improved (or failed) the business or
  government organizational effectiveness.
• Explain the expected growth trend for the
  technology through the year 2005. Include
  specific information from the literature.

				
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posted:5/19/2013
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