Slide 8:

The slide shows that the holding times of the Internet Dial modem calls
are not exponentially distributed.  The graph plots the cumulative
holding time on the network as a function of the number of calls of
different durations (top two curves) for both an exponential
distribution as well as the network actuals.  The reason this call count
is not exponentially distributed is that there are more short calls than
exponential (primarily for checking email) as well as more very long
calls (some of which resulted from continuous calls that defeated the
network idle timer by periodically sending keep alive messages, but
nothing else). 

The lower two curves give the distribution as a percent of the total
holding time on the network that calls of different length represent. 
This shows how much of the total holding time the long calls represent
(25% of the holding time is represented by calls longer than 180
minutes).

The overflow resources referred to in the chart were regional and
national overflow modem pools set up during the rapid time to market
growth of the service.  These were used while we learned the exact per
city traffic distribution.  Today we do not use them any more, since we
have more accurate per city traffic growth trends.