Modem Contention Ratios

Hello There

I was wondering whether there is someone to shade some light here as I have to commit I am quite confused.

When looking into modem contention ratio 20:1, means 20 people sharing 1 modem on the exchange end.

As far as I am aware on the exchange end say AXE or DMS there are E1 ports and if you configure them as PRI ports they end into modem banks. 1 port=1 E1=30*64kbps channels=30modems

On the user end with a 56K modem and from a 64kbps capable line you would expect to connect to to a similar modem. Or it would mean every user would require one modem on the exchange end per connection. And this is where I am confused. From some reading I have done on modem contention ratios it is clearly stated 20:1 ratio translates to 20 people sharing 1 modem on the exchange end. But if you follow my initial assumption something like that would be impossible.

Could some shade some light?

much appreciated
-John