I've started to encounter a problem connecting to my 3G Mobile Broadband with Vodafone Ireland using a Huawei E220 USB modem. It tries to connect and authenticate and then it disconnects and I get the Internet Connect message "Could not negotiate a connection with the remote PPP server."
The log includes the following:
Macintosh pppd[298]: IPCP: timeout sending Config-Requests
Macintosh pppd[298]: IPV6CP: timeout sending Config-Requests
iMac G5 20" (iSight),
Mac OS X (10.5.6),
iPod Photo Shuffle
The phone company do provide software that I installed initially. I then connect via System Services-->Network. Within that, connecting via the usb modem is just another option like Airport or Ethernet. A username and password are expected and then I press Connect.