asterisk users - Nov 2004 - SIP phones, Asterisk and bandwidth

I have not studied SIP yet. 


I have an Asterisk server, and SIP phones somewhere on the globe, not locally 
connected to the Asterisk server.

What is the bandwidth I need to the Asterisk server?
I assume that only the registration is the bandwidth to the Asterisk server, 
while the phone call will be made from phone to phone directly.

How much bandwidth do I need to server as a PBX for 100 phones, 1000 
phones, ....  How many phones are "usually" on at the same time from
100%
registered phones?

I understand that if I operate a gateway, than all the bandwith of the voice 
stream will also go through this gateway, ...

bye

Ronald