Re: ISP phone ratio's and Interoffice trunkage for voice

[James Love <love@tap.org>]

  thanks, i understood some of this.  Now I take this comment to say that 
  company's measure calling by smaller units, something called 36 CCS's to 
  an hour, and that they assume a particular  distribution of calling 
  (Poisson distribution??)..   I'm not an engineer, so bear with me.  
  Now, if an Online caller makes call which are longer in duration than 
  phone calls (say one hour rather than 6 minutes), what does this mean?  
  Does this make the online calls more or less expensive (on a per minute 
  basis, holding total calling constant), than modem calls?  
  
    Also, what does it mean when you say switch bandwith is cheaper than 
  trnnkage bandwidth?  Doesn't everything have to go over a switch at some 
  point?  jamie
  
  On Fri, 12 Jul 1996, Fred R. Goldstein wrote:
  
  > At 02:38 PM 7/12/96 -0400, James Love wrote:
  > >I am informed that ISP's need one incoming line for each 8 to 20 
  > >subscribers, depending upon how high the level of service is.
  > >
  > >I am curious.  What is the buildout for interoffice trunkage for voice 
  > >calls, for residents?    If lower than 8 to 1, then it would seem that 
  > >calls to ISP's do not cause any congession problems, that don't exist 
  > >with voice calls.  Is there a hitch somewhere?  
  > 
  > It's not quite that simple.  The magic number used in this function is the
  > number of Erlangs of busy-hour traffic capacity, where an Erlang is one hour
  > of call time (or 36 CCS, if you prefer to read traffic tables that way).  
  > 
  > The "typical" resi voice telephone line might generate around 3 CCS, or 1/12
  > Erlang, distributed bewteen intra-switch and the various inter-switch
  > destinations.  Thus a thousand resi subscribers might generate 3000 CCS, or
  > around 83 Erlangs.  If all of this were carried over one trunk group, it
  > would need 106 channels in order to achieve 1% blockage, or 99 channels with
  > around 5% blockage.  (This from the "Poisson" tables.)
  > 
  > Of course the real world isn't like that.  Calls go every which way.  So
  > each trunk group is simply engineered to handle the demand it sees, and the
  > switch fabric is engineered for aggregate demand.  Switch bandwidth is
  > somewhat cheaper than trunk bandwidth, especially with recent-generation
  > digital COs. We're talking small fractional cents/minute.
  > 
  > Now an ISP will get different usage/subscriber depending upon, among other
  > things, the way it prices.  If it doesn't offer unlimited usage, then I'd
  > guess that typical peak usage will be under 3-4% of the subscriber base.  If
  > they offer unlimited usage options, some folks will be slow to hang up and
  > the number will be higher.
  > 
  > All it takes is a small percentage doing "7x24" to throw the averages off,
  > though!
  > 
  > Does this data usage affect telco?  Sure, but so do teenagers who, as is
  > well known, often grow telephone handsets as bodily appendages.  I doubt the
  > average web surfer is on more than they are.  But again that ignores the
  > 7x24 crowd; at least teenagers stop to eat, sleep, go to the mall, etc.
  > ___
  > Fred R. Goldstein      fgoldstein@bbn.com  
  > BBN Corp.              Cambridge MA  USA    +1 617 873 3850
  > 
  > 
  
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