| OverviewRouting in Telephone Networks
				The national telephone network is structured as a three-level hierarchy:
				 
					Subscribers (telephones, modems, fax machines)
					
					Central offices
					
					Long distance offices (toll switching offices)
					
 
				A local exchange carrier (i.e. Bell Atlantic), provides
				local telephone services and manages many central offices
				interconnected with one-hop links.
				 | The Telephone Network  Figure 10:
					Subscribers are served by central offices, which are in turn linked at
					the core at toll-switching offices.
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 The Routing Algorithm Used:
					If a call's source and destination are within a central
					office, directly connect them.
					
					If the call is between central offices within a local
					exchange carrier, use a one-hop path between central
					offices.
					
					Otherwise, send the call to the appropriate 
					core (or cores).
					
 
				The only major decision in the selection of a path between two toll
				switches for a long-distance call is between a one-hop path and a
				two-hop path.  (Paths longer than two hops usually are not
				considered).
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