Create CHAP credentials for additional trusted callers 

"How to Create a CHAP Credentials Database (Dial-in Server)"

Update the PPP configuration files to support CHAP 

"How to Add CHAP Support to the PPP Configuration Files (Dial-in Server)"

Set up CHAP authentication on the dial-out machines of trusted callers 

"Configuring CHAP Authentication for Trusted Callers (Dial-out Machines)"