Accept/Send Network Data Concurrently?

[lf11 at deletethistomailme.andonet.com]

This is a puzzle that has stumped me for some time--how can I 
make server that accepts incoming requests, replies to them, 
and sends data at timed intervals?

For example, a chat server that accepts incoming requests 
and forwards them to the appropriate user.  It also maintains 
a list of online users, and pings them periodically to see if 
they are still on.

Thanks!

 -lf