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

["Fred R. Goldstein" <fgoldstein@bbn.com>]

  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