 Network Concepts
 ^^^^^^^^^^^^^^^^
      NODE           A  bulletin board  system that calls  another
       bulletin  board  system (a hub)  in order  to
       transfer mail.  A node  doesn't receive its mail
       directly from any other BBS  (that is, no one
       calls it to transfer mail.)

      HUB            A  bulletin board system which may or may not
       call another hub system in  order to transfer
       mail,  but is called by one or more  nodes to
       transfer mail.

      REGIONAL HUB   A hub  which calls  another hub  in order  to
       transfer  mail.  It's  called a  regional hub
       because  when designing a  network, a  hub is
       usually placed  in each of the  major regions
       serviced in order  to minimize long  distance
       phone charges.

      For example :

      P&BNetHub
    /    \      \
         /        \      \
       /           \       \
   RUBYS          BBS1     BBS2
   /     \
        /        \
      BBS3      BBS4


 P&BNetHub is the NATIONAL HUB because it does not call anyone for mail.
 BBS1 and BBS2 are NODES, directly linked to P&BNetHub.  BBS3 and BBS4 are
 nodes connected to RUBYS.  RUBYS is a node of P&BNetHub and also a
 REGIONAL HUB since it calls P&BNetHub to transfer mail.

 Each hub that is a regional hub  has a HUB ID.  This ID is used by the
 regional hub to identify itself when it calls the hub "above" it to
 transfer mail.  The network shown above would consist of four nodes, one
 regional hub and one national hub (P&BNetHub).
